Hello!
I'm designing a circuit in which I use a transistor to remotely pass/mute an audio signal by sending a voltage to it. I want the cleanest possible version of the signal when it's passed. I can't use a relay because I will be switching very frequently: think square-wave tremolo.
I'm currently thinking of using a 2N4123, because it sounded best out of the transistors that I have, but I'm sure there's something better?
Thanks for the help!
Maybe if you post the circuit the gurus here may have some excellent suggestions.
Personally, when I think tremolo, my mind goes to vactrols.
^ yes please.
I'm thinking ea tremolo.
Square wave in an ea trem might not go so well .
I have an idea though.
Hi Rich,
are you thinking about the EA Tremolo but with the FET as on-off switch in the audio path instead of controlling the gain? I had that on the breadboard once, sounds great! Highly recommended. I think I used a J201 or 1N5457.
Cheers,
Andy
Thanks for the replies! It's not a muting effect, so perhaps square wave tremolo was a bad comparison. A lot of tremolo gets a better throbbing sound by changing the tone of the signal, but I don't want that. I just want to be able to abruptly mute an audio signal on command. Nothing gradual – a vactrol would be much too slow.
My circuit already does this fine with the transistor that I use, but I was wondering if anyone could recommend a transistor that might sound cleaner and more transparent than the 2N4123. There's no point sharing the circuit, as it's as basic as these things get: audio-in on the collector, audio-out on the emitter, and the controlling voltage to the base (through a resistor). I'm also fine tweaking the circuit for a PNP or other transistor if it has a cleaner sound.
So if its just a pure voltage controlled on/off switch you are looking for, have a look at the bypass switching of Boss or Ibanez/Maxon pedals. I think they mostly use the 2SK30A for this role, but some others have also been used over the years. I've also seen the J112 used for similar jobs, which may be easier to find. When used correctly, a JFET in such a position should not introduce any audible sound coloration.
EDIT: And as always, there is an excellent source for this type of thing from one of the usual suspects, R.G.'s Geofex:
http://www.geofex.com/article_folders/tstech/tsxtech.htm
Also tells me that the 2SK118 is also used. Whereas electrosmash lists a 2SK44SP:
https://www.electrosmash.com/tube-screamer-analysis
Plenty of choices.
Andy
Quoteit's as basic as these things get: audio-in on the collector, audio-out on the emitter, and the controlling voltage to the base (through a resistor).
The success of such a circuit depends strongly on what is driving it and where it's driving from. Basically you will get blatty distortion when it is supposed to be turned off (failure to mute) when you get signal levels above ~0.6V. When turned on, the resistor to base shows up as load impedance to ground. Furthermore you introduce DC bias into the signal path through the base/gate resistor in the case of a JFET or BJT. A MOSFET would be the best bet for what you are doing, but you still have the body diode that will turn on. It might work ok if you do back-to-back MOSFETs (like 2N7000). Still depends on 0 DC bias on each side, and its effectiveness and amount of distortion depends on load impedance.
As a bare minimum a JFET could be used with the reverse diode in the gate (similar to the typical bypass switches) provided your control voltage can go negative, to maybe -3V or -4V when off, and to +3V to +4V when on. DC blocking and buffering is preferred since you can control input and output impedances.
I think buffered active bypass circuits as suggested by others will be the best design.
The JFET is a tried-and-true way to do this. When working against >1M load impedance the nonlinear effects of current through the effective <100 ohm pass-impedance will result in distortion far below what any amp or recording console can achieve.
Put in 1 BJT emitter follower stage and you have introduced 100x as much distortion as the pass-FET, just to get an idea how unimportant the transistor part selection is.
As implied by responses from others on this thread, the way you are using the transistor is far more important that the specific transistor.
+1 on suggestions for active buffered bypass. If you are really looking for clean pass-through sound use op amp buffers.
> There's no point sharing the circuit, as it's as basic as these things get: audio-in on the collector, audio-out on the emitter, and the controlling voltage to the base (through a resistor).
Bad attitude. ALL the details matter.
{As Transmogrifox says} "Audio-in on the collector" ignores the impedance of whatever is driving it, a very important factor. Also how big the audio is, which matters less until you get break-through. So DO sketch-up ALL the details.
That transistor has no special magic. "Same" transistors are produced in one pot and sold under hundreds of part-numbers. To my eye it is a lowish-spec 2N2222. Are you really getting different results with different small BJTs?
Digitech uses 2N4125's in the XP series units, with J113 jfets for audio switching duties. This is the PNP complementary to the 2N4123. The transistor handles the voltage switching, while the jfet handles the audio path. This is along the lines of what Transmo and Fancy described.
(https://s26.postimg.org/jdeodb4s5/Untitled.jpg) (https://postimg.org/image/jdeodb4s5/)
Quote from: bartimaeus on September 19, 2017, 02:14:08 PM
I just want to be able to abruptly mute an audio signal on command. Nothing gradual – a vactrol would be much too slow.
Granted, a vactrol in some cases may be slow (depending on the vactrol model and the application), but in the case of an opto such as an H11F1, 45us on/off time is pretty damn fast. Not quite as fast as me getting out of doing yard work, but unless your ears can blink faster... :icon_wink:
Quote from: PRR on September 19, 2017, 11:46:59 PM
> There's no point sharing the circuit, as it's as basic as these things get: audio-in on the collector, audio-out on the emitter, and the controlling voltage to the base (through a resistor).
Bad attitude. ALL the details matter.
"He had his immortality inadvertently thrust upon him by an unfortunate accident with an irrational particle accelerator, a liquid lunch, and a pair of rubber bands. The precise details are not important because no one has ever managed to duplicate the exact circumstances under which it happened, and many people have ended up looking very silly, or dead, or both, trying."
Douglas Adams about Wowbagger the Infinitely Prolonged in The Hitchhiker's Guide to the Galaxy
The cleanest passed signal is going to be with some form of FET. FETs are inherently a bar of semiconductor doped to be a resistor, and then having that resistance decreased (for JFETs) or increased (most MOSFETs) by a voltage on the gate. That is, FETs are inherently most like nondistorting resistors if you can drive them properly.
Bipolars can and have been used as switches, but they are full of funny junction effects when not funny on or fully off, and have offset voltages and other complications. But if what you want is full on or full off, they can be used. They tend to work best as shunt switches, "shorting" signals to ground when paired with a series resistor. A great deal of Japanese stereo equipment up through the 90s and early 2000s use a lot of shunt switches.
Metal switches are hard to drive, bounce, and are slow. LDRs are slow and also have some built in distortion, although it's small.
A combination device is perhaps a good choice. The H11F1/2/3 is an LED+photoFET, and although there are issues with distortion on larger signals, the on and off are fast, and completely isolated as well as clean. I've always wanted to use power MOSFETs for speaker switching, but avoided them because of the funny drive requirements and some questions about actual linearity. That's probably over. Two series MOSFETs get around the issues with the body diodes conducting in one direction, and listening reports say that they don't distort in a noticeable way. One IC version of this is the TLP222G, being an LED and two MOSFETs on the output. They listen well, and are fast and easy to drive. I'm thinking about incorporating them in the next version of the boards that implement the complete replacement for the preamp in the Vox Beatle amp.
I can make more specific recommendations if needed, but some of the JFETs already mentioned will work fine. A bit of caution: you have to drive the gate to a more negative (foir N-channel) voltage than Vgsoff plus the signal size to keep them turned off. Otherwise you get the signal modulating the gate-source partly off, and distortion.
A long channel, high Vgsoff device will make a better variable resistor, a short channel, low Vgsoff device an easier to drive switch.
The advice you got on switching times is good and ought to be heeded. If you use an instant switch, like metal contact or fast-driven FET or bipolar, you >> CAUSE << switching transient clicks as the switch causes a vertical part of the signal to instantly appear when the switch makes or breaks. Slowing this change down to few tens of milliseconds still sounds "instant" to the ear, but doesn't have these little clicks.
So, in the phase-shifter world, there are adaptations intended to play off various properties against each other. One of those is the use of a network around the FETs used as variable resistors, to reduce distortion. You can find that in a Korg phaser and the venerable MXR Phase 45.
(http://www5b.biglobe.ne.jp/~houshu/synth/PhaseFet0205.GIF)
Can such a network (illustration 2 or 4) be used to diminish distortion when a FET is used for switching?
Thank you very much for the replies, everyone!
I would like this circuit to be functional as a standalone mute box that I could plug a variety of sources into. It'd be nice to be able to use a eurorack gate signal, for example. I'm looking for this to work with both instrument and line-level signals, but instrument-level signals are the priority.
A JFET + reverse diode solution (as used in BOSS/Ibanez/Maxon bypass circuits) looks simple and useful, but it looks like they're non-compatible with a gate signal that only goes positive.
Also, if I have DC-bias on the output when using a JFET, can't I just stick a capacitor on the output to filter out the bias?
Quote from: R.G. on September 20, 2017, 12:56:28 PMBipolars can and have been used as switches, but they are full of funny junction effects when not funny on or fully off, and have offset voltages and other complications. But if what you want is full on or full off, they can be used. They tend to work best as shunt switches, "shorting" signals to ground when paired with a series resistor. A great deal of Japanese stereo equipment up through the 90s and early 2000s use a lot of shunt switches.
Using the transistor as a shunt switch rather than a series switch isn't something that I'd considered, but it seems a lot more sensible for this application. I'm going to mock one up on my breadboard to try it out. I do have one (amateur) question: if I mult the signal prior to the series resistor, will shunting this to ground also mute the other multed versions of the signal?
Quote from: digi2t on September 20, 2017, 10:53:04 AMGranted, a vactrol in some cases may be slow (depending on the vactrol model and the application), but in the case of an opto such as an H11F1, 45us on/off time is pretty damn fast. Not quite as fast as me getting out of doing yard work, but unless your ears can blink faster... :icon_wink:
I hadn't realized that opto's can go so fast! The H11F1 is now looking like a very good option. I'm familiar with Vactrols from eurorack modules, where they're often used to in VCA's to give a natural sounding decay of ~100 ms. Since instrument-levels are a priority, and I'd be willing to accept the potential for distortion when using line-level signals. The price of a H11F1 is not ideal, though, running at about $3 compared to $0.50 for a lot transistors.
By the way, I wasn't trying to say that the details were unimportant, but rather that there were no more details to give. Here's a Kicad mockup of my original idea:
(https://68.media.tumblr.com/ff423cfdbf2331cdada2212defdc66a6/tumblr_owl6ixowkp1u8kzyzo1_540.png)
If low distortion is your primary goal, and you want a BJT as a switching element, here's a project that Rod Elliot has put together that should be able to be tweaked to work for you (note that it is operating as a shunt switch): http://sound.whsites.net/project147.htm (http://sound.whsites.net/project147.htm) Note that the specific transistor used is not that critical, given sufficient base current to really switch it on. There are transistors designed to be worked this way, but I only know of one model currently available (2SD2704K) and it is only available in surface mount.
In fact, he has a whole page on (shunt) muting circuits at http://sound.whsites.net/articles/muting.html (http://sound.whsites.net/articles/muting.html). Since this is slanted towards power on/off muting use in amps and the like, much of it will not apply to you, and there are some other details he leaves out too, like that there are CMOS analog switches (DG201/411 etc.) available that can operate at voltages up to 44V. If you want options, there are options.
Transistor muting is done all over the place as Jonnyeye says above. Digital effects use it to mute while you change program and everything jumps to new settings which could cause all kinds of horrible noise. Hi-fi's do it to quieten un-selected sources.
In principle...
First, the input is AC coupled if necessary to make sure there is no DC voltage.
Then a series resistor, 1k usually enough followed by a larger pull down resistor to ground reference it.
Then the Collector of an NPN is connected to the signal.
The Emitter connects to ground.
The mute control drives the Base via a resistor.
The Collector is also connected to the output via another capacitor.
When mute control volts is high enough, the transistor turns on and clamps the signal out to ground.
As muting can happen anytime, it can click when the audio signal is not at or near 0v, causing a rapid voltage change to 0v. The mute control can be made a little more gentle with an RC time delay added to slow the switching speed.
In practice, x2 muting transistors, like this (note there's no coupling capacitors or pull-down as it assumes a ground referenced signal path)...
(http://sound.whsites.net/articles/muting-f3.gif)
BTW, the reversed second muting transistor emitter/collector is not an error. I think it's for symmetry & protection against large signal swings. By coincidence, I have an old Philips Hi-fi to repair that uses this 2 transistor scheme but does not bother reversing the 2nd transistor, instead they are different transistors! 1st is BC548B and 2nd is BC337-40!
The MC14007 data sheet from ON Semiconductor shows a 2-input analog multiplexer in Figure 1 that can be used as an audio switch. The nice thing about it is the control lines are not buffered so you can slow them down to minimize switch popping.
https://www.onsemi.com/pub/Collateral/MC14007UB-D.PDF
If you run the device from the 9 volts and bias an inverting op amp at 4.5 volts, you can feed the op amp without using up the voltage excursion limits because the switch is held at 4.5 volts. Of course, it may pay to have some resistance in series with the inverting input for stability so the capacitance to ground of the MC14007 does not cause a feedback lag that would promote oscillation.
An SSR also can be used as a mute part. Some time ago I was playing with some VO1400s and they do the job fine. Low capacitance, transparent, no change in sound, can handle up to 60V. If used in tube amps a zener clamp is recommended though.
The state of the art would be the modern solid state switches.
I'd say the switching JFETS like J201 and the like would be best easy to get single component,
they also tend to be low resistance.
BJT's used like AnotherJim mentioned are OK but probably not as clean as JFETs.
Most component MOSFETs have terribly high capacitance (and hence switching feed-through).
Quote from: bartimaeus on September 20, 2017, 01:54:12 PM
A JFET + reverse diode solution (as used in BOSS/Ibanez/Maxon bypass circuits) looks simple and useful, but it looks like they're non-compatible with a gate signal that only goes positive.
Also, if I have DC-bias on the output when using a JFET, can't I just stick a capacitor on the output to filter out the bias?
Maybe. You're dealing with instrument level signals, on the order of 100mV peak. Any minor DC shifts that show up will be heard as clicks or thumps. A far better solution is to force the input side of the switch to a DC level with a capacitor and a resistor to a bias level, and do the same on the output. That guarantees that if some of the control signal happens to feed through the control pin to the switch itself, it is held in check by the DC level on the in and out pins. What would be best would be to make the "DC bias" be 0V by using a P-channel JFET, and use a 0V gate level for on, a higher voltage for turning it off. It is reverse logic, but then inverters are cheap.
Quoteif I mult the signal prior to the series resistor, will shunting this to ground also mute the other multed versions of the signal?
A BJT is only half of any shunt switch. A resistor is the other half. Each shunt switch will have a series resistor of its own and a shunt transistor. You also have to make the resistor value be much larger than the source impedance of the signal source so the signal source is not loaded down by the shunt switches (which is what I think you meant) but also much larger than the on resistance of the shunt switch. So if your transistors have an on resistance maybe 10 ohms, your series resistor can be anything bigger than 10K for 40db muting. 60db muting takes 100K, just on the voltage divider rule. In this hypothetical situation, the source would have to have a source impedance lower than 10K in the 60 db case and lower than 1K in the 40db case. If there are multiple outputs, any number of which can be muted, that drives down the allowable source output impedance by the number of outputs. You may need a buffer or multiple stages of shunt muting. Or both.
That seems like it makes series muting the better answer. Series muting - which works by making the path from the signal source to the next circuit go as open circuit/high resistance as possible, has its own set of flaws, not least of which is high thermal noise and easy introduction of control feedthrough.
QuoteThe price of a H11F1 is not ideal, though, running at about $3 compared to $0.50 for a lot transistors.
This is only an issue if you're making lots of them. The box it goes in will likely be more than that. And you're overpaying for transistors. BJTs are available for US$0.07 to 0.10.
Quote from: anotherjim on September 20, 2017, 03:45:28 PM
In principle...
First, the input is AC coupled if necessary to make sure there is no DC voltage.
Then a series resistor, 1k usually enough followed by a larger pull down resistor to ground reference it.
Then the Collector of an NPN is connected to the signal.
The Emitter connects to ground.
The mute control drives the Base via a resistor.
The Collector is also connected to the output via another capacitor.
BTW, the reversed second muting transistor emitter/collector is not an error. I think it's for symmetry & protection against large signal swings. By coincidence, I have an old Philips Hi-fi to repair that uses this 2 transistor scheme but does not bother reversing the 2nd transistor, instead they are different transistors! 1st is BC548B and 2nd is BC337-40!
I think the circuit does several things. One is to use two series stages of muting to get a better "off" muting as I mentioned above. Another is that I think the reversed transistor is for two things. BJTs have an offset voltage on the collector caused by the forward voltage of the base-emitter. It's normal to put a BFC in series with the collector of BJTs used like this to isolate the offset voltage. And it's possible that there is some cancellation of any control voltage feedthrough this way. Maybe.
BJTs have a quirk that in general they can clamp to lower resistance when used backwards, using the 'alpha' of the transistor instead of the 'beta'. An NPN is still an NPN if you swap emitter and collector, but now the "collector" junction is heavily doped and low resistivity and as a side effect has low gain and low BVceo. The new base-"emitter" is now backwards to what gives high gain, high linearity, high frequency response, etc., but that doesn't hurt the collector-base resistance much.
The Japanese semiconductor industry used to and may still make specialized transistors for shunt muting.
Quote from: Rob Strand on September 20, 2017, 05:12:43 PM
The state of the art would be the modern solid state switches.
I'd say the switching JFETS like J201 and the like would be best easy to get single component,
they also tend to be low resistance.
BJT's used like AnotherJim mentioned are OK but probably not as clean as JFETs.
Most component MOSFETs have terribly high capacitance (and hence switching feed-through).
It takes more design work to make a BJT squeaky clean, but as Japanes hifis demonstrated, it can be done, with good results, if you use shunt switching.
Single component MOSFETs do have higher gate capacitances, but if you are slowing down the signal a bit anyway and perhaps doing shunt switching, that makes a great place to use the gate capacitor for the slowing. If you're using discrete MOSFETs for series switching, the connection that puts both sources in series and then floats the sources and gates with an isolated switching control introduces the same signal, but offsetting, on the setup, so you get first order cancellation.
As I mentioned, the integrated LED to dual MOSFET signal switches have become impressive, and frankly, this may be the OP's best choice. Except that they're US$2.50 to $3.00 each too.
Ah, well. Good, fast, cheap. Pick any two.
QuoteIt takes more design work to make a BJT squeaky clean, but as Japanes hifis demonstrated, it can be done, with good results, if you use shunt switching.
Yes it's good enough. Even if a small tick got through no one would care as long as it's not the big *bang* at power-up you would get without them.
Quote
Single component MOSFETs do have higher gate capacitances, but if you are slowing down the signal a bit anyway and perhaps doing shunt switching, that makes a great place to use the gate capacitor for the slowing. If you're using discrete MOSFETs for series switching, the connection that puts both sources in series and then floats the sources and gates with an isolated switching control introduces the same signal, but offsetting, on the setup, so you get first order cancellation.
The capacitance tends to couple the switching into the signal path. If you slow things down externally, like with the RC network that BOSS use, you might get away with it. The capacitance is *much* higher than a JFET you are never going to match performance. I pretty much avoid using (component) MOSFETs to switch signals. It makes me cringe when I see it - their characteristics (because of their chip size) aren't set-up for that job.
If you plan to stick with the bare-bone one-bjt muting circuit, you can increase the bjt's base isolation by simply adding a small-signal (fe 1N4148) diode in series with the existing base-drive (i.e. "mute") leg (at VOLTAGE/GATE-IN in above schem).
Everyone has made it quite clear that the choice of transistor matters much less than the circuit designed around it. This makes sense to me, but I am a bit surprised that there isn't superstition about transistor use like there is about capacitors, etc. But maybe there is in other applications?
Quote from: jonnyeye on September 20, 2017, 02:42:19 PMIn fact, he has a whole page on (shunt) muting circuits at http://sound.whsites.net/articles/muting.html (http://sound.whsites.net/articles/muting.html). Since this is slanted towards power on/off muting use in amps and the like, much of it will not apply to you, and there are some other details he leaves out too, like that there are CMOS analog switches (DG201/411 etc.) available that can operate at voltages up to 44V. If you want options, there are options.
Even though it's not totally directed towards my goal, I REALLY wish I had read this initially. Super helpful!
Quote from: anotherjim on September 20, 2017, 03:45:28 PM
Transistor muting is done all over the place as Jonnyeye says above. Digital effects use it to mute while you change program and everything jumps to new settings which could cause all kinds of horrible noise. Hi-fi's do it to quieten un-selected sources.
...
In practice, x2 muting transistors, like this (note there's no coupling capacitors or pull-down as it assumes a ground referenced signal path)...
(http://sound.whsites.net/articles/muting-f3.gif)
I tested out a shunt switch with a single BJT, but found that it did not sufficiently attenuate the signal. Looks like I needed to use a pair instead... If all of this extra circuitry is required to make a BJT work, I'd rather build a shunt switch with a JFET (assuming that I only need one JFET instead of multiple BJT's).
Quote from: Rob Strand on September 20, 2017, 05:12:43 PM
I'd say the switching JFETS like J201 and the like would be best easy to get single component,
they also tend to be low resistance.
BJT's used like AnotherJim mentioned are OK but probably not as clean as JFETs.
Why exactly are JFETS cleaner than BJT's? I assume that this only applies when they're used as series switches, not shunt switches.
Quote from: amptramp on September 20, 2017, 04:17:23 PM
The MC14007 data sheet from ON Semiconductor shows a 2-input analog multiplexer in Figure 1 that can be used as an audio switch. The nice thing about it is the control lines are not buffered so you can slow them down to minimize switch popping.
https://www.onsemi.com/pub/Collateral/MC14007UB-D.PDF
If you run the device from the 9 volts and bias an inverting op amp at 4.5 volts, you can feed the op amp without using up the voltage excursion limits because the switch is held at 4.5 volts. Of course, it may pay to have some resistance in series with the inverting input for stability so the capacitance to ground of the MC14007 does not cause a feedback lag that would promote oscillation.
I'm a bit confused as to the benefit of this device over a couple of BJT or JFETS, since it contains several transistors and seems like overkill.
Quote from: R.G. on September 20, 2017, 08:01:12 PM
Quote from: bartimaeus on September 20, 2017, 01:54:12 PM
A JFET + reverse diode solution (as used in BOSS/Ibanez/Maxon bypass circuits) looks simple and useful, but it looks like they're non-compatible with a gate signal that only goes positive.
Also, if I have DC-bias on the output when using a JFET, can't I just stick a capacitor on the output to filter out the bias?
Maybe. You're dealing with instrument level signals, on the order of 100mV peak. Any minor DC shifts that show up will be heard as clicks or thumps. A far better solution is to force the input side of the switch to a DC level with a capacitor and a resistor to a bias level, and do the same on the output. That guarantees that if some of the control signal happens to feed through the control pin to the switch itself, it is held in check by the DC level on the in and out pins. What would be best would be to make the "DC bias" be 0V by using a P-channel JFET, and use a 0V gate level for on, a higher voltage for turning it off. It is reverse logic, but then inverters are cheap.
That DC extra filtering sounds quite doable. But could you please explain further about inverting the JFET to accept 0V as on? I don't quite understand.
Quote from: R.G. on September 20, 2017, 08:01:12 PM
QuoteThe price of a H11F1 is not ideal, though, running at about $3 compared to $0.50 for a lot transistors.
This is only an issue if you're making lots of them. The box it goes in will likely be more than that. And you're overpaying for transistors. BJTs are available for US$0.07 to 0.10.
BJT's go for pennies, but JFET's seem to go for around $0.50 even on Tayda. And for a circuit this simple, I could easily fit it in an old tin.
Quote from: bool on September 21, 2017, 09:57:40 AM
If you plan to stick with the bare-bone one-bjt muting circuit, you can increase the bjt's base isolation by simply adding a small-signal (fe 1N4148) diode in series with the existing base-drive (i.e. "mute") leg (at VOLTAGE/GATE-IN in above schem).
Thanks for the tip!
Also, does anyone have any recommendations for a through-hole alternative to the VO1400? None of Vishay's other SSR's seem to have similar on/off times: http://www.mouser.com/catalog/catalogusd/648/2006.pdf
Quote from: bartimaeus on September 21, 2017, 10:03:28 AM
I am a bit surprised that there isn't superstition about transistor use like there is about capacitors, etc. But maybe there is in other applications?
Oh, yes, very much so. It is likely that the reason you aren't hearing magic-transistor blather on this topic is that shunt muting and JFET switching are not used in the old-time "magic" pedals. Otherwise we'd hear the same stuff about only transistors made in Sunnyvale in 1968 on thursdays by a team of Icelandic virgins being any good.
Quote
I tested out a shunt switch with a single BJT, but found that it did not sufficiently attenuate the signal. Looks like I needed to use a pair instead... If all of this extra circuitry is required to make a BJT work, I'd rather build a shunt switch with a JFET (assuming that I only need one JFET instead of multiple BJT's).
Be very wary about the results from any one circuit test. As you were noting the circuit matters. I was trying to point to this by the comments on how much attenuation you get from any series resistor and BJT pair. If you don't pick a low on resistance BJT, it will not attenuate enough. The series resistor and BJT you happened to use may not be well suited to the source and the load conditions, and so would and probably were not optimal. It takes some thought, calculations, and then getting the right parts for what those turn up to say whether the whole class of BJT attenuators is usable or not.
The series resistor and shunt switch make a "volume control" with the BJT to ground being the bottom portion of the volume control If you can't make that be zero ohms, you have the "volume control" still a little bit above fully off. There are no semiconductor switches that get down to metal-contact switch resistances, although big MOSFETs come close. I was trying to explain that the attenuation depends on both the BJT "on" resistance and the resistor, and that the resistor is constrained by other things than just the BJT, and that led to the possible need for multiple stages.
As for "all this extra circuitry", be careful what you ask for. You're comparing a fully developed two stage attenuator with your idea that you can do it with just one transistor. This circuit includes two transistors to feed the two bipolars with a clean on off signal of just the right amount and size. A single BJT **can** be made to work with just a resistor to feed it a signal on its gate. Exactly **how well** it works depends on (a) how good "good enough" is, and how much you have to add to the basic, trivial circuit to get to your "good enough". It also depends in this case on which BJT you pick. As I said, some BJTs are better than others. It's just not widely known what to select for. JFETs need a bias voltage on drain and collector, generally capacitor isolation on each end, usually a diode, resistor, and a cap for each FET, and then some kind of circuit to feed it a gate voltage; the first two bipolars in that circuit are this "circuit to feed it a gate voltage.
Quote from: Rob Strand on September 20, 2017, 05:12:43 PM
Why exactly are JFETS cleaner than BJT's? I assume that this only applies when they're used as series switches, not shunt switches.
As I described in my note, JFETs are inherently resistors made skinnier or fatter by the action of the voltage on the gate; they're variable resistors. BJTs are inherently diodes, and diodes are inherently non-linear. BJTs as a switch rely on their diode-ish-ness being hidden by their low relative resistance compared to other things in the circuit, and always retain a tiny bit of diode conduction distortion. This is least when they are off - infinite nonlinear resistors are almost as linear as infinite linear resistors - or fully on. It's between worse and horrible in the middle, the degree of worseness being proportional to how big the signal across it is.
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I'm a bit confused as to the benefit of this device over a couple of BJT or JFETS, since it contains several transistors and seems like overkill.
(1) It is a bunch of matched MOSFETs in one package that can be wired up various ways.
(2) It is a bunch of matched MOSFETs that can be strapped to be a complementary MOSFET analog switch and use the inherent linearity of the MOSFET as a variable resistor without having the problem of the body diode intrude.
(3) It's cheap, around US$0.50 new, maybe much less surplus.
You seem to be confusing more transistors with too complex and overkill. You're hearing a lot of people tell you that using just one transistor may not do what you want. If it takes more stuff to do what you really want, you're stuck with more stuff not being overkill. Also, overkill isn't necessarily bad. If overkill makes what you want to get done be trivally easy for the solution to do well, then it's not necessarily overkill. In a manufacturing situation, it's reasonable to design right down to the fewest of everything. In making one or two, using more parts to do it easily and well isn't that bad a choice.
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That DC extra filtering sounds quite doable. But could you please explain further about inverting the JFET to accept 0V as on? I don't quite understand.
There are two kinds of bipolars, NPN and PNP. They are the same in how they work, only the voltages have to be reversed. They're kind of mirror images. Likewise, there are two kinds of JFET, N-channel and P-channel which are mirror images in terms of how they operate. For N-channel devices, letting or making the gate be the same voltage as the source leaves the drain-source channel at its lowest resistance, and making the gate be some volts -negative- compared to the source makes it higher resistance up to effectively turned completly off. A P-channel FET is a low resistance when the gate is at the same voltage as the source, and turns off when the gate is pulled -positive- compared to the source.
So for N-channel, you have to attach the drain and source to some fixed voltage and turn it off by pulling the gate negative. For circuits with a positive power supply, you have to pull the drain and source up from zero by enough to let the JFET turn off if you pull its gate to ground. For a P-channel JFET, it is normally conducting if the gate is at the same voltage as the source, but turns off if you pull the gate positive. I've always thought that JFET switching in a positive voltage circuit would best be done with P-FETs. This allows you to make the "bias voltage" for the drain and source be 0V, or ground. The turn-off voltage then become yanking the gate up to the power supply. It is more expeditious, and doesn't force some restrictions on the bias voltage. The comment on "inverting" was just that the logic of the gate voltage is inverted, from off being zero volts and on being high for the N-FET to on being zero volts and off being high with the P-FET.
Frankly, I think that all the industrial switching with JFETs would be with PFETs if they weren't more expensive per device. This is another case of the right device being a tiny bit more expensive and not favored for reasons that don't matter to us one-off guys.
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BJT's go for pennies, but JFET's seem to go for around $0.50 even on Tayda. And for a circuit this simple, I could easily fit it in an old tin.
Yep. What you're telling us is that in fact, the absolute cost of the parts is a critical design decision to you in this matter. A whole lot of your comments here are asking for fewer parts and cheaper parts. I was there once, when I was a starving student, and another dollar for parts was a dollar I didn't have to feed myself and my wife. Dollars were worth about five times as much back then, but the concept is the same. I don't know if your situation is even comparable in any way, or if you're just being careful with your money for your own reasons. Over the years, I accumulated more money to buy parts, and also have a fairly large stock of leftover parts, so I am in a condition where how well a circuit works and looks is getting more important relative to the bare minimum parts number and costs.
For whichever reasons though, you are making the parts cost be a minimum be a design criteria. That's fine, but you need to realize that you're doing that.
Other people have other criteria. You may want to read "Effects Economics 101" at geofex.com. Preferring an old tin as an enclosure over a commercially made box is another way of saying that the cost of the box is more important to you than how the finished box looks and how durable it is. There are other consequences of making the design rule be "cheapest and fewest parts". Again, that's fine, but do consciously recognize you're doing it, and state it that way.
Quote from: R.G. on September 21, 2017, 01:06:28 PM
Quote from: bartimaeus on September 21, 2017, 10:03:28 AM
I am a bit surprised that there isn't superstition about transistor use like there is about capacitors, etc. But maybe there is in other applications?
Oh, yes, very much so. It is likely that the reason you aren't hearing magic-transistor blather on this topic is that shunt muting and JFET switching are not used in the old-time "magic" pedals. Otherwise we'd hear the same stuff about only transistors made in Sunnyvale in 1968 on thursdays by a team of Icelandic virgins being any good.
Yeah.... like reverb tanks...
(http://www.stevehouse.com/bucket/OC_Electronics_2.jpg)
QuoteWhy exactly are JFETS cleaner than BJT's? I assume that this only applies when they're used as series switches, not shunt switches.
I'd say JFETs are cleaner than BJTs because with a BJT the DC level on the switched terminals changes causing a glitch. There is no DC with JFET (with the reverse diode gate circuit like Boss/Ibanez use).
JFETs are clean because:
- The gate is largely isolated from the signal (drain/source).
- They have low capacitance from the gate to the drain/source.
- They have a variable resistance characteristic.
When you switch from one signal to another the two signals might not join-up and this
discontinuity causes a glitch. By slowly blending between the signals the join is smoother
which makes it less audible.
JFETs do all the right things. You can also get fairly low resistance.
[One caveat: For the case of muting, where you want to kill the signal, the BJT can provide more attenuation than a JFET as the effective resistance is (/can be) lower. This is outright suitability to the specific problem, nothing to do with glitches and cleanness.]
It's common when using the BJT as a muting device to put a BFC* in series with the collector. This isolates the circuit being muted from the inevitable offset voltage at the collector. This itself is a problem, because the cap must be "large" to have a reactance that's compared to the BJT on resistance, which is quite low. This is commonly several hundred microfarads. It's another PITA with the BJT muting scheme that is often unappreciated until the last minute.
I'm surprised no one has mentioned the obvious: use two JFETs, one series and one shunt after it. You feed them the correct gate voltages and bingo, you have a really, really off "off-switch" and a much better/lower on resistance than with a shunt mute only.
This violates the OP's design rule to use only one transistor, but you get really good results. It generally requires opposed gate signals, as you want the series off when the shunt is on, and vice versa, but the application of an N-channel series device with a P-channel shunt device works well and only a single polarity of control signal.
*Big Freaking Cap
QuoteIt's common when using the BJT as a muting device to put a BFC* in series with the collector. This isolates the circuit being muted from the inevitable offset voltage at the collector. This itself is a problem, because the cap must be "large" to have a reactance that's compared to the BJT on resistance, which is quite low. This is commonly several hundred microfarads. It's another PITA with the BJT muting scheme that is often unappreciated until the last minute.
The BFC gets rid of the *steady-state* DC but it doesn't get rid of the glitch. There is a DC *change* on the BJT before and after the muting; it is like injecting a step signal. One component of DC is the (non-ohmic) saturation voltage, the other component is the ohmic drop due to the base-current. At line levels, like on CD player outputs, the DC level is small compared to the signal but as the signal level drops it becomes more of an issue.
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I'm surprised no one has mentioned the obvious: use two JFETs, one series and one shunt after it.
Yes, that will fix it!
You also see this configuration in modulators and synchronous rectifiers - often so the following stage sees 0V with low source impedance.
Yes with BJT switching there can be a little (couple mV's) DC leak from the base to collector; but that can be minimized with some attention to the base "mute" signal current.
The BJT base has to saturate for efficient muting and the associated Vbe voltage would leak through via the B-C "diode"; but if the signal source impedance is sufficiently low; practically a high-enough beta BJT usually shouldn't cause too much DC leaks/troubles to get it working.
Getting the "mute" signal right would be one place too look at when designing and optimizing the simple circuit. But this usually boils down to selecting the right BJT ... and the right base resistor. And as I suggested, putting a small-signal diode there to isolate the BJT base when the "mute" logic signal is at "low" (iow, "play" position).
In some, like CE-1 chorus.
(http://experimentalistsanonymous.com/diy/Schematics/Chorus/Boss%20CE-1.gif)
See Q12 for a J-FET shunt mute, But, note it is controlled by a negative envelope follower to ensure the gate of the N-channel can definitely pinch-off the FET.
The Heathkit AJ-1510A FM tuner (the first digital tuner) used diodes wither forward or reverse biased to block or pass signal. They used a set of diodes to ground and a set in series with the signal to get the degree of cutoff they needed.
In the MC14007 CMOS implementation I suggested earlier, you could connect on input to the signal and the other to the Vcc/2 ground so as well as the megohms of resistance in series with the signal, you could get ~100 ohms to ground. This should be good for about 80 db of signal suppression in cutoff.
Yes, the BJT muting scheme requires careful attention to both the transistor and the drive signal to it, as well as the source impedance, the series resistor that is the other half of the attenuator, and the load impedance it drives. The nature of the drive signal is important because of the leakage of the B-C junction; that junction has to be forward biased to get good saturation, as the base can't be any lower than 600-700mv, and the collector has to be significantly lower than that to be saturated to a low voltage, which is what you want for low "resistance". So you have to carefully tinker the base drive for both low saturation resistance and low feedthrough.
Then there is the issue of the transistor. In general, low saturation voltages are antithetical to high beta at equal current levels. Something to do with needing low resistivity silicon for low saturation voltages and that not being very good for high beta. That is old information and silicon has moved on to much better levels of refinement these days, so it may have been fixed. But I suspect that it's only been fixed for new-designed transistors special to the purpose. There's not a huge market for BJT switches these days, given how good FETs have become. Probably best is finding the types that were actually used in the Japanese hifi stuff back in the day. They had the incentive to design transistors to do that job.
Again, not that it can't work - but a pair of JFETs, well designed, will do better.
Thanks again for all of the replies!
Quote from: R.G. on September 21, 2017, 01:06:28 PMQuote
BJT's go for pennies, but JFET's seem to go for around $0.50 even on Tayda. And for a circuit this simple, I could easily fit it in an old tin.
Yep. What you're telling us is that in fact, the absolute cost of the parts is a critical design decision to you in this matter. A whole lot of your comments here are asking for fewer parts and cheaper parts. I was there once, when I was a starving student, and another dollar for parts was a dollar I didn't have to feed myself and my wife. Dollars were worth about five times as much back then, but the concept is the same. I don't know if your situation is even comparable in any way, or if you're just being careful with your money for your own reasons. Over the years, I accumulated more money to buy parts, and also have a fairly large stock of leftover parts, so I am in a condition where how well a circuit works and looks is getting more important relative to the bare minimum parts number and costs.
For whichever reasons though, you are making the parts cost be a minimum be a design criteria. That's fine, but you need to realize that you're doing that.
I apologize for being unclear about this. I'm lucky enough that I can currently afford the more expensive options listed above, especially since none of them are astronomically-priced vintage parts. I started this thread looking for a swap-in replacement for a circuit that was accomplishing it's purpose, if with coloration. I ended up trying to keep the circuit basic for simplicity-of-design reasons more than anything else – to see how much I could improve signal transparency with an equally simple schematic. As you surmised, I didn't quite realize these limitations which I was putting on the design.
Quote from: R.G. on September 21, 2017, 01:06:28 PMQuoteThat DC extra filtering sounds quite doable. But could you please explain further about inverting the JFET to accept 0V as on? I don't quite understand.
So for N-channel, you have to attach the drain and source to some fixed voltage and turn it off by pulling the gate negative. For circuits with a positive power supply, you have to pull the drain and source up from zero by enough to let the JFET turn off if you pull its gate to ground. For a P-channel JFET, it is normally conducting if the gate is at the same voltage as the source, but turns off if you pull the gate positive. I've always thought that JFET switching in a positive voltage circuit would best be done with P-FETs. This allows you to make the "bias voltage" for the drain and source be 0V, or ground. The turn-off voltage then become yanking the gate up to the power supply. It is more expeditious, and doesn't force some restrictions on the bias voltage. The comment on "inverting" was just that the logic of the gate voltage is inverted, from off being zero volts and on being high for the N-FET to on being zero volts and off being high with the P-FET.
Frankly, I think that all the industrial switching with JFETs would be with PFETs if they weren't more expensive per device. This is another case of the right device being a tiny bit more expensive and not favored for reasons that don't matter to us one-off guys.
Thank you very much for explaining this!!! For my particular muting application, P-Channel JFET's seem like a good compromise between BJT's and SSR's. I ordered a couple P-Channel JFET's to test out.
Quote from: R.G. link=topic =118655.msg1105440#msg1105440 date=1506013588QuoteI'm a bit confused as to the benefit of this device over a couple of BJT or JFETS, since it contains several transistors and seems like overkill.
(1) It is a bunch of matched MOSFETs in one package that can be wired up various ways.
(2) It is a bunch of matched MOSFETs that can be strapped to be a complementary MOSFET analog switch and use the inherent linearity of the MOSFET as a variable resistor without having the problem of the body diode intrude.
(3) It's cheap, around US$0.50 new, maybe much less surplus.
You seem to be confusing more transistors with too complex and overkill. You're hearing a lot of people tell you that using just one transistor may not do what you want. If it takes more stuff to do what you really want, you're stuck with more stuff not being overkill. Also, overkill isn't necessarily bad. If overkill makes what you want to get done be trivally easy for the solution to do well, then it's not necessarily overkill. In a manufacturing situation, it's reasonable to design right down to the fewest of everything. In making one or two, using more parts to do it easily and well isn't that bad a choice.
Quote from: amptramp on September 22, 2017, 12:21:00 PMIn the MC14007 CMOS implementation I suggested earlier, you could connect on input to the signal and the other to the Vcc/2 ground so as well as the megohms of resistance in series with the signal, you could get ~100 ohms to ground. This should be good for about 80 db of signal suppression in cutoff.
It seemed like overkill to me because even some of the more complex solutions offered here don't utilize it's full capabilities. That said, I do see how it would simplify the transistor options, and it does seem like an economical solution.
Quote from: Rob Strand on September 21, 2017, 06:23:21 PM
QuoteWhy exactly are JFETS cleaner than BJT's? I assume that this only applies when they're used as series switches, not shunt switches.
I'd say JFETs are cleaner than BJTs because with a BJT the DC level on the switched terminals changes causing a glitch. There is no DC with JFET (with the reverse diode gate circuit like Boss/Ibanez use).
JFETs are clean because:
- The gate is largely isolated from the signal (drain/source).
- They have low capacitance from the gate to the drain/source.
- They have a variable resistance characteristic.
When you switch from one signal to another the two signals might not join-up and this
discontinuity causes a glitch. By slowly blending between the signals the join is smoother
which makes it less audible.
JFETs do all the right things. You can also get fairly low resistance.
[One caveat: For the case of muting, where you want to kill the signal, the BJT can provide more attenuation than a JFET as the effective resistance is (/can be) lower. This is outright suitability to the specific problem, nothing to do with glitches and cleanness.]
This is a very clear explanation, thank you for the help! So it seems like a JFET is much better as a series mute/switch, since the gate is isolated and you get no glitches, while a BJT is better for a shunt mute/switch, since it offers more attenuation and the audio signal doesn't flow through the transistor.
Do I need to worry about isolation of the gate/base in the case of shunt switches? It seems like yes, but you can add extra circuitry to minimize it, which is why several BJT with supporting architecture is a solution used in many Japanese products?
Quote from: R.G. on September 22, 2017, 12:33:05 AM
I'm surprised no one has mentioned the obvious: use two JFETs, one series and one shunt after it. You feed them the correct gate voltages and bingo, you have a really, really off "off-switch" and a much better/lower on resistance than with a shunt mute only.
This violates the OP's design rule to use only one transistor, but you get really good results. It generally requires opposed gate signals, as you want the series off when the shunt is on, and vice versa, but the application of an N-channel series device with a P-channel shunt device works well and only a single polarity of control signal.
I find this solution very cool/interesting, since it mixes techniques, but if we're talking about dual-transistor solutions then I don't quite understand its benefit over using a pair of shunt or series switches, or even just a single shunt switch (which seems sufficient for many effects)?
Quote from: anotherjim on September 22, 2017, 11:01:01 AM
In some, like CE-1 chorus.
(http://experimentalistsanonymous.com/diy/Schematics/Chorus/Boss%20CE-1.gif)
See Q12 for a J-FET shunt mute, But, note it is controlled by a negative envelope follower to ensure the gate of the N-channel can definitely pinch-off the FET.
I'll try building something similar with a P-channel JFET once the ones I ordered arrive. They seem to get by with a single shunt switch. These old BOSS/Ibaneze/Maxon circuits all have slightly different support architecture for the switches, so I need to start figuring them out.
Quote from: R.G. on September 22, 2017, 12:22:19 PM
Again, not that it can't work - but a pair of JFETs, well designed, will do better.
Sure will do much better but with fets you have to tinker with the gate drive logic vs tweaking two resistors for the bjt.
For fets, iirc D.Self had a series/shunt circuit block in one of his books.
Quote from: bool on September 23, 2017, 08:57:38 AM
Sure will do much better but with fets you have to tinker with the gate drive logic vs tweaking two resistors for the bjt.
Yep. For better results you have to work a little harder, or smarter, or both. The trick is to work the least harder for the most better. That's always a personal tradeoff, or an outgrowth of the "rules" you set yourself for the project.
Plus you have to take into account the rest of the circuit. You're at the edge of that point, but not quite on it. Generating the control signal nearly always means using whatever is most expedient in the rest of the circuit. So for the two BJT case, you have to use two resistors and two diodes, and generate a switched high voltage. Pretty easy.
For the series-shunt JFET version, you have the option of using a NFET and a PFET as the series and shunt elements, two resistors and two caps for voltage positioning of the NFET, and the >same< gate voltage on each one. The different polarities of the FETs do the inverted logic for you.
It takes one more cap than the BJT case, and one less cap than the BJTs if you do wind up not achieving acceptable feedthrough with carefully tinkered base drive.
Which is better? As always, it depends on your scoring criteria.The FETs will cost more. The BJTs will do a less total attenuation than the JFETs. Either version may or may not have more thinking required for getting the control signal right, and whether you already have a simple way to generate the control signal in the rest of your circuit already is unknown until you have the rest of the circuit defined.
Like the answer to most interesting design questions, the answer to this issue is - it depends.
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For fets, iirc D.Self had a series/shunt circuit block in one of his books.
Yes, he did. I believer it was in his small-signal audio book.
The most hardcore and "audiophile" way to do it would be to use a small relay.
Higher cost than any of the semiconductor, but you'd get the bragging rights. (IF and only IF you choose one that has gold-plated contacts).
To make things manageable space-wise; you could use some small DIP8 relay. Also you can wire it in series/shunt config if the relay is a spdt/dpdt type.
Fets always made me a little un-easy; personally I wouldn't use them without some buffer amp after the attenuator. Bjts, otoh, I've seen them at the very end of the signal chain (output); with say "audiophile" computer audio cards, and I used them as a simple kludgy "bolt on" to an existing circuit as well - I feel they're a little sturdier.
I'm under the impression that the mute was needed for some kind of chopper/stutter effect?
JFET's (any FET) too have signal/gate interaction if you aren't careful.
1: If signal can wiggle the source voltage, it can change the gate-source voltage, turning it away from the intended switch state.
2: The signal can cause forward bias of the gate-source/drain diode. As with BJT base, this can have an extra diode in the gate for more headroom. Gate can be supplied via a reversed signal type diode. The diode reverse leakage is enough for the gate to see the control voltage - everything else sees 2 back to back diodes and the control path is effectively blocked from the signal.
(https://www.neatcircuits.com/pix/fetsw1.gif)
No matter how perfect the muting action, it can make an audible click when signal is present if there's an abrupt switching action. As already mentioned, a little slewing of the control signal can give a short fade to smooth out a glitch. Up to 20ms will still be heard as a fast switch without causing a click.
With series/shunt, it can be a little tricky to get the right slope of the control. The series and shunt happening at the same time speeds up the switch between signal and silence. Also, each FET may have overlapping thresholds. The result of that is a very short span of the control slope to get a fade in or out effect.
I tried this with 4007 MOSFETs. P for series and N for shunt with a common gate control should make this easy, well...
Once you find a working compromise between signal amplitude & the reference voltage to keep switch states unaffected by signal, getting a smooth, glitch free signal fade proved a complete PITA. It turned out to be easier to use only an N channel as shunt.
When you factor all the pieces that you need for a simple shunt jfet version and a bjt version, and with a RC smoother for a click suppression; you get to approx 3x resistors, 1 diode (for gate/base isolation), one cap and one transistor.
In case of a fet; you will probably use a ceramic/poly cap in 100nf's range, with a bjt you will use a cap in 10uf's range. If smt; both circuits are going to be almost exact size (if you use a small tantalum in case of bjt; but there are also small ceramics in 10uf's range).
R.G., did you see my post above addressing many of your points? I think you may have confused me with user bool – no worries if so!
Quote from: anotherjim on September 24, 2017, 02:39:06 PM
I'm under the impression that the mute was needed for some kind of chopper/stutter effect?
Something in that ballpark, yep! Which is why a standard relay wouldn't work: I'm worried about its life-expectancy.
Quote from: anotherjim on September 24, 2017, 02:39:06 PM
(https://www.neatcircuits.com/pix/fetsw1.gif)
It seems I'd only need half of that circuit, since I'm not switching between inputs. Looks like this one would need 2x resistors, 2x caps, 1 diode, and 1 transistor. But does this have sufficient click suppression? I don't see any series resistors after the switch, although I know that those elec caps will help.
If using a combo of series and shunt introduces issues with synchronization, I'd really rather not bother. I also still don't quite understand why the combo would be necessary when muting a single signal? Does it just offer the most possible attenuation?
If you are wondering how to slew the transition ON/OFF time know all that is needed is an RC circuit. I implemented this on a Decimator that was giving me popping issues past the 12:00 position. (See mimmotronics.com/er8)
QuoteIf using a combo of series and shunt introduces issues with synchronization, I'd really rather not bother. I also still don't quite understand why the combo would be necessary when muting a single signal? Does it just offer the most possible attenuation?
JFET on-resistance isn't low enough for a total kill shunt unless used against a much higher series resistance from the input. Having a series FET cut off the input gives that higher input resistance. Think voltage divider. It can be necessary to have 2 shunts in series to get closer to silence.
I only posted the above audio switch scheme as an example of the reversed gate isolation diode trick.
I had a little epiphany today while re-reading this thread.
R.G. brought up using two JFETs as an L-pad (one series and one shunt) to ensure very good muting. This has a new set of problems; either you use one P-channel and one N-channel and have simple control circuitry (but getting the right devices is more difficult) or you use two N-channel (and have to build control circuitry). Here is where my eureka moment comes in: A CMOS SPDT switch is an L-pad, if wired correctly. How do? Here are two options:
(https://s26.postimg.org/mzmos3ijt/simplecvmute.png)
(The chip layout was stolen shamefully from Maxim's datasheet)
When CV is low, the switch connects IN to OUT (with a small and somewhat non-linear resistance) and opens OUT to GND (or rather connects it with a very large resistance). So the input voltage passes through essentially unaltered. When CV is high (mute), there is a very large resistance between IN and OUT, and OUT is also connected to GND through a low resistance. This ensures that the signal is always attenuated greatly in the mute position, no matter what impedance you are working into.
Note that +-12V is just a suggestion; it can run on any bipolar power supply from (say) +-5 up to +-15V (check the datasheet), but this changes the threshold voltage for switching if wired as shown. However, you do have the option of adjusting the CV threshold voltage (as alluded to in the image); by bringing the VL pin to any voltage up to V+ (a resistive divider should work), the threshold is set to 2/3 of the voltage on pin 5 (VL). The DG419 is not exactly a cheap device but should be about $2 in unit quantities. If that seems too rich for one 8 pin device, or if bipolar power is not available, then check out the following:
(https://s26.postimg.org/ddt4bsre1/cheapcvmute.gif)
(Sorry RG)
Again, 9V is a suggested voltage to run this from, but anything from 5 to 20V will work (although the threshold voltage changes again with applied voltage, and there is no easy way to alter it here). This version is conceptually identical to the previous version, except that we need to set up a local ground Vr for this circuit to do its thing (the DG419 version side-steps this because of its bipolar supply). The two resistors to generate Vr are equal value, anything from 10k to 100k should be fine. Distortion may also be somewhat higher than the DG419 version due to the reduced working voltage and the different specification/construction of the underlying devices, but is likely to be tolerable in most cases. Total parts count is about $1 or so, although you could replace the electrolytic in and out caps with film if you have a disdain for electros.
(Note that these are untested (I don't even have a DG419 in my stock!) and may need some tinkering to work properly. No refunds on free advice.)
I did think of 4053, however...
Size seems critical to the OP.
Buffered control input means you can't slew the switch action. An RC would only introduce a time delay before it switches fast. Same goes for the DG series switches (didn't know they were still available).
> anything from 5 to 20V will work
You are pushing luck with 20V on an 18V-rated chip.
> the DG series switches (didn't know they were still available).
Many of the DG-series switches were used in huge quantity in factory and hospital gear. Some of them will be available for a long time to come.
Is it possible to ramp the control voltage on a TLP222 like it is on an H11F1, or will this simply delay the time it takes to turn on very quickly?
It seems not, at least not perfectly...
http://stompville.co.uk/?p=423
Quote from: tempus on September 28, 2017, 11:19:07 AM
Is it possible to ramp the control voltage on a TLP222 like it is on an H11F1, or will this simply delay the time it takes to turn on very quickly?
One more reason to go with the CD4007 or MC14007 - the control voltages are brought out to pins and are not buffered. You can delay turn-on and turn-off and tailor the R-C networks or use diodes to accelerate either turn-on or turn-off as the requirements require.
Could we perhaps use the idea in that TLP222 of inverse series MOSFET channels with, say, x2 2N7000's? This puts the parasitic substrate diodes of the FETS in opposition. Solves that problem with single MOSFET's?
So you'd make a simplistic (yet convoluted) mosfet-relay ...
That could be done; but I think you'd have to come up with a clever-ish bi-phase switching scheme if you wanted to use a series-shunt config (with added RC smoothing)...
Is was thinking the on resistance could be low enough, you might get away with just a shunt.
QuoteOne more reason to go with the CD4007 or MC14007 - the control voltages are brought out to pins and are not buffered. You can delay turn-on and turn-off and tailor the R-C networks or use diodes to accelerate either turn-on or turn-off as the requirements require.
But the H11F1 or TLP222 give you the advantage of isolation from the control signal. In the article quoted, the author found that he couldn't effectively ramp up the control voltage on the 222 - is there a way to tell this from the device's datasheet or is it only ascertainable through trial and error?
As a side note, the CD4007 is an attractive option, with 3 switches in a 0.63 package, provided it can be set up to switch noiselessly. I've tried to use BS170s for audio switching, but they distort as a result of the protection diodes. Will this not be an issue with this device?
Well now, in the CD4007, it is a bit different with those pesky diodes...
(http://www.d.umn.edu/~sburns/EE2212Spring2014/Experiment5MOS_files/image002.jpg)
(http://www.circuitsgallery.com/wp-content/uploads/2014/01/IRFZ44-Power-mosfet-pin-out.png)
A MOSFET is formed on a bulk/substrate which ends up with that diode effect across the channel. You can call it a protection diode but it's really a parasitic that can't be avoided. It isn't necessarily suitable for sole flyback protection from a big inductive load.
Anyway, having more connections, the 4007 MOSFET's turn the substrate diodes to the nearest power rail. Signal would have swing beyond the power supply rails to clip those diodes.
I have used the 4007 successfully as voltage controlled resistor and signal switch and so have many others. I think its use becomes a good idea when all of its parts can be found a use. Like JFET's, variations in thresholds means you need to tweak the relationship between control level and signal bias to get the right control response with every build. I personally do not find this onerous. For self & boutique builders having to do a set-up should not be a deterrent.
With the photo-mosfet, it seems linear control is more difficult. Possibly the gates are so sensitive that they switch fully almost the moment the LED emits some photons?
That's why I thought you might at least borrow the series N-channel MOSFET idea from the TLP222, since you then have access to the gates. For an RC slew control, the last gate that switches on fully affects the shunt mute and the first gate that switches off removes it.
Finally! Some love for the 4007. If you make one input the signal and the other the Vcc/2 ground, this circuit:
(https://wiki.analog.com/_media/university/courses/electronics/a28_f12.png?w=600&tok=0d584b)
shows you how to make a switch with series and shunt switching elements. You can add R-C lag networks from pin 6 to pin3 and pin 13 and 8 to pin 10. Pin 13 is the drain of the upper transistor and pin 8 is the drain of the lower transistor and 8 and 13 are connected externally. The economics of dealing with IC's is skewed; you get six MOSFET's for the price of one and the CD4007 / MC14007 IC is available anywhere CMOS is sold.
Quote from: anotherjim on September 30, 2017, 10:37:19 AM
Is was thinking the on resistance could be low enough, you might get away with just a shunt.
I think not only that, but if you pulled the both sources down with a high-mohm bleed resistor down, you could simplify the feed the gates control circuit immensely. You would just need to "isolate" the both gates with a two high-mohm resistors and feed them via a "nice" RC constant. (for the complete circuit; five resistors, one cap and two mosfets).
Otoh, good luck gents - I had a working noise gate with a series-connected BF960 (!) and a 555/741 control circuit in 80's - and it wasn't exactly easy to make it work "as expected". Fets are bitches to get right, but they do the job when you get them to behave.
x2 series 2N7000 shunt does work. "Linear" control for fade is narrow, just about 1v - 2V relative to ground. During fade the channel diode cancellation is lost, since (I suppose) one or other MOSFET is bypassing it's own diode, so there is distortion during the fade. This would probably be ok in guitar use with a reasonably fast fade. I would expect TLP222 would suffer from this also.
Isolation from signal (ground referenced by 1M) when the mute is off appears good, but I didn't feed it a large test signal.
Channel On resistance isn't low enough for a good mute against a 1k input resistor. 10k is better. Output probably ought to be buffered although 10k is ok into normal Hi-Z inputs, we don't always have those.
To get a better fade window, imho you'd need "perfectly matched" mosfets. At least that's what I "ass"-umed when I simulated something along these lines. Meaning that in the world of computer models this will have some desirable performance, but when there's real-world device mis-match involved, the linear "fade window" will become much narrower.