Boss slow gear built

[jimbob]

Zeta- Anything new with this one?

[zeta55]

Zeta- Anything new with this one?

Not really. I'm still running it with the 2SK30A but have not had the time to play it so much.

/Krister

[Morocotopo]

Wouldn´t it be nice to mod this for a Attack - Decay - Sustain - Release controls (an envelope generator, sort of)? Or at least, to add a Release control?

Morocotopo

[9 volts]

I've had another look at mine, the problem seems to be that the signal ripples when fading out and distorts when a chord is played. I'm using a 2sk30 from small bear. Does anyone have any advice? Thanks

[Paul Marossy]

I sure wish I could get mine to work. I replaced all of the 1uF caps this weekend since some of the ones I used were "reclycled". No change. I've tried all kinds of different FETs, different zener diodes, nothing seems to work. Ugghhh!! 

I did make some amount of progress, though. At least now, with a certain FET, the signal level is really low -  compared to getting absolutley nothing before. Do I need to try smaller zener diodes than a 5.6V zener? I just don't get it... 

[zeta55]

As for the box I made, I've used BC549 for all the trannsistors. The fet as mentioned earlier in my thread. The OP is a 741, and the zener a 5.6V.
The only thing that differs is that I used my own circuitboard layout.

/Krister

[R.G.]

The magic parameter on the FETs in these boxes is Vgsoff. That is the voltage that the JFET takes to turn off the channel entirely.

The Japanese transistor makers use a grading system on their JFETs of testing them and marking the devices with color code. On the 2SK30(a) there are four or five of these, I believe. US type numbers tend to use a different number for different Vgsoff devices, like the 2N5484/5485/5486 parts, although the "buckets" for Vgsoff and Idss are bigger.

For and pedal that depends on the characteristics of the JFET to fade the sound linearly, I would select JFETs like I do JFETs for phasers by measurement. The point of those zeners and trimpots is to ensure that the source and drain of the JFET are sitting at a voltage bigger than the Vgsoff of the FET and that the gating signal pulls the gate to just a bit more than Vgsoff, not a whole lot more. In this circuit, you want a JFET with Vgsoff of about 3-4V and a zener just a bit bigger so you can adjust the bias voltage down to a bit over Vgsoff.

[Paul Marossy]

The magic parameter on the FETs in these boxes is Vgsoff. That is the voltage that the JFET takes to turn off the channel entirely.

I understand that part. My problem is that the FET nevers seems to turn on, so to speak - it sounds like a volume pedal on its lowest setting where only a little bit of signal gets thru. Is that behaviour also controlled by Vgsoff? 

[R.G.]

Yes, to a degree. Also by the other stuff in a circuit.

Here's what happens inside a JFET. When the gate is open circuit, the channel between the drain and source is just a path of doped silicon with some resistance. In switch-optimized JFETs this resistance is the Rdson number, and is generally from maybe ten ohms to a few hundred ohms. As you reverse bias the gate, the depletion region of the gate-channel diode actually protrudes a certain distance into the channel. The depletion region is an area swept clean of charge carriers by the electric field, so the available area of the channel to conduct is actually reduced, just like squeezing on a garden hose. This reduces the available conductive area, and since resistance is R = rho*L/A where rho is the conductivity of the channel material, you have reduced A but left rho and L the same, so the resistance goes up. The resistance can go up to many megohms, but only down to the bare channel resistance either with an open gate, or with the gate tied to the source - that is, Vgs=0.

Gate open is actually better for conduction than Vgs=0 because there are voltage drops along the channel resistance that change the voltage between gate region and channel as the the I*R voltage varies along the length of the channel. This is also why the channel is not a perfectly linear resistor, by the way. With the gate held at a voltage, the signal voltage along the channel adds and subtracts from the real Vgs at each point, so the signal self-modulates the channel resistance.

But I digress.

For JFET switching you only want two states, on and off, and a smooth variation in between to kill clicks. This is best done by holding the JFET source and drain at some fixed voltage, and switching the gate drive voltage between two voltages, one much lower (for N-channel) than Vgsoff and one higher, then putting a diode between the gate and the control voltage. The diode lets you pull the control voltage higher than the channel (drain and source) DC voltages and reverse bias the series diode. This lets the gate be open circuited when the diode is reverse biased and the conduction is smoother in the "on" state. When you pulll the diode down past Vgsoff, the diode is forward biased and pulls the gate down with it to lower than Vgsoff, and the gate makes the channel resistance very high, so the switch is off.

The ideal Vgsoff for effects JFETs is 3 to 4V. This lets you hold the drain and source at half of an 8.0-9.5V battery voltage, pull the gate diode high to open the gate, and pull it down to ground for fully turning the JFET off. You want as big a Vgsoff as you can comfortably switch because it's the ratio between Vgsoff and the signal level that determines distortion in the "on" state if you don't do other measures to reduce it. So Vgsoff should be as big as you can get compared to the signal and still switch reliably within the confines of the available power supply.

The Slow Gear is a very slow switch. If  you're never getting it turned on, then one of a few things are happening.
1. the gate may not ever be getting up high enough to let Vgs go to 0 and have the full channel conductivity available. That would keep the channel resistance high.
2. the series resistance of the signal path may be high, so the JFET may be turning on and that not making much difference.
3. the usual: solder, wiring, pinout, copper or solder threads, wrong part values, etc. in other places than the JFET.
4. other stuff I didn't think of.

[Paul Marossy]

Thanks for the tips RG. Later on this week I will go over it (again) with a fine tooth comb. 

[gaussmarkov]

 

[9 volts]

I've built both versions of the slow gear and used different transistors both of them (except the 2sk30) and got the same results. My fet turns on turns off nd off, but the sound distorts when I play chords and the linear fade out of the note is rippled and fuzzy....so to some extent the fet is actually working.
I was under the believe that the voltage to the 2sk30 should be 5.2 volts. I'll experiment with the voltage. Thanks for the great reply RG.
ps  I may try another fet to see if the sound is any different. Maybe I fried mine pulling it in and out it in....

[343 Salty Beans]

If it helps, my first foray into effects in general was the ME-50B (the B is for bass...that's right, I'm a bassist, what's it to ya, wanna fight about it???  ). It had a slow gear, and the manual noted that the notes will intermodulate and fade in and out if you play a chord. Sure enough, it did. It's actually kind of a neat ripple. Try knocking on your bass/guitar while sustaining a chord...you'll get something like a random, sawtooth tremolo.

[9 volts]

Single notes do the same thing...it's not oscillation it sounds more like a dying power supply. I also have to have all the controls flat out to get this responce. ( it sounds blatty on the fade out)

[343 Salty Beans]

play with the bias like you mentioned above. Are you sure you're not underpowered? Are you using a 9v or a wall wart? if it's all splatty and distorty, it sounds like maybe the circuit is starving. Measure voltages at all the +9v and +4.5v areas on the schem. If they're not right, maybe you accidentally subbed a wrong resistor or gota mislabeled one or something.

ah, debugging is such tedious work.

[Paul Marossy]

I think my problem has been that the variability of FETs has been working against me. I guess I need to go thru what I have on hand and see if I can come up with one that has suitable characteristics for the Slow Gear circuit.

[9 volts]

I've checked all my resistors and they are correct. The fuzziness seems to be worse. I'm wondering if a capacitor may be faulty...The 2sk30a seems to work. It sits at 5.4v and drops to around 3.7v when a note is played. I'll also do some voltage checking to the rest of the transistors.
What would the symptoms of faulty capacitors be?

[Zero the hero]

I built both old and new version of the available layouts of this project, and they both worked as soon as plugged to amp / guitar.
Thanks for the great post RG, things are getting clearer and clearer about how this circuit works.
On my site i suggested to adapt the zener value depending on what FET you're using, so the second Slow Gear I've built uses a BF245.
Make sure that the envelope follower is working by measuring voltage at the end of its section (positive side of C6 on my schem http://topopiccione.atspace.com/pjimages/BossSG1Sch.gif ) by measuring it during silence and as soon as you struck a chord.

I noticed that both my SG-1 doesn't work with my hollow-body Ibanez artcore. Mark Hammer said that one possible cause of this was the different harmonic content of this guitar (full of basses), and the subsequent bad behaviour of the envelope follower. I had no time to investigate more, however.

[Paul Marossy]

I  built both old and new version of the available layouts of this project, and they both worked as soon as plugged to amp / guitar.

Yeah, that's what really bugs me about this one. I usually have a circuit work fine immediately when testing, but not this time. And I have even used J201s in my Phase 45 clone, which aren't supposed to work in that circuit... 

[R.G.]

Paul: I suspect that there's a forehead-slapping Doh! in there waiting for you to find it, perhaps not the FET or not just the FET. Is the audio signal full sized before it gets into the FET?

Others: The ADSR question is one that I've played with several times. ADSR is something the synth guys do to make a continuous-level signal sound real. A guitar signal is an AR signal - it has only attack and release, at least noticeably. ADSR is the most general case, for an instrument like a horn where there is an attack blip that decays, the tone sustains as long as you blow, and releases quickly when you quit.

You can make a guitar be quasi-constant volume by either clipping the devil out of it or with a compressor. Once you've done that, you can impose ADSR on top of that relatively level tone and reshape the dynamics some. I would approach this by
(a) making up a good constant level chain with a compressor, perhaps some clipping.
(b) making a "gate" sidechain to derive a logic signal that tells you when the signal is above X level
(c) using the gate to fire an ADSR which modulates the constant level signal in (a)

It's complicated, but it gets you closer to the more-generalized reshaping that the discussion has been sniffing at.