That Corp VCAs are great but expensive. Vactrols are getting hard to find, are expensive, inconsistent from unit to unit, and have limited control over dynamics timing. JFETs are tricky to use, and require careful selection and trimming. OTAs are noisy (unless you put them in the feedback loop), and even LM13700s are over $2 each at Tayda these days.
As an exercise I set out to see if I could design a high quality guitar compressor with a handful of penny transistors: no op amps, no vactrol or LDR, no JFETs, no VCA, no OTA. The PMC (Poor Man's Compressor) is the result.
It's well known that the transconductance of a long tail pair of bipolar junction transistors is proportional to the current flowing through them. By modulating this current you can vary the transconductance (and hence gain) and thereby build a current controlled amplifier (CCA). This, along with some current mirrors, is basically what you find inside an operational transconductance amplifier (OTA) such as the LM13700.
(https://i.postimg.cc/dZy8DR67/Revision-A.png) (https://postimg.cc/dZy8DR67)
TR1 and TR2 are a complementary feedback pair (CFP) configured as a unity gain buffer with high input impedance. TR5 and TR6 are the long tail pair transistors which form the heart of the CCA. You can only insert about 100mVpp signal in this configuration before distortion becomes a problem so you have to pad the guitar signal down. TR7, TR8 and TR9 are a Wilson current mirror to correctly bias the output of the LTP and allow for reasonable voltage swing at the output. C9, R10, R21 and C14 form a simple emphasis / de-emphasis circuit to reduce the noise (hiss) of the transconductance stage. I also run the LTP at pretty high current compared with most OTAs which helps reduce noise a bit further. The output of the CCA is buffered by TR15 and TR16. TR13 is a half wave peak detector which turns on when the output voltage starts to rise to around one diode drop. This pulls down the drive to TR12, reducing the current into current mirror transistors TR11 and TR10, reducing the tail current flowing in the LTP and hence lowering the gain. TR3 and TR4 provide voltage smoothing to reduce the effect of noise in the power supply. It will improve performance if you match (TR5, TR6), and (TR8, TR9) at least somewhat. I just used transistors all off the same reel and it worked fine.
Attack time is controlled by R16 and C15. Release time is controlled by R17 and C15. I tweaked the values by ear to get it to sound sweet when played with a strat. I measured the attack time around 10ms and release time just over 200ms which is about right for a guitar compressor in my experience. You could in theory make these adjustable but I doubt it will sound any better than with the stock values. For use with a bass guitar you might want to increase C15 to 22uF or maybe more.
Conceptually the circuit topology is similar to a Dynacomp, but with the CA3080 replaced with discrete transistors, and the side chain only being half wave rectified. The design was certainly inspired by the simplicity and beauty of the Dynacomp.
The circuit is laid out with all components mounted on the PCB, including the pots, sockets, LED and footswitch (it is a true bypass pedal and uses the 3PDT switch from Tayda). This makes it really easy to assemble and test, although you do have to solder little extension wires to get the pots to sit at the right height to mount it in the case. The PCB is designed to fit into a 1590BB case. I ordered 5 boards so I have 4 spare, email me jonny@recklesstechnology.com with your name, address and contact details if you would like one for $5 plus postage. When I run out I could maybe share the Gerbers if someone wants to get more boards made.
Overall I am pretty happy with the sound of this little compressor. It's got plenty of squish, can do the chicken picking thing pretty well, and sounds really clean and quiet.
(https://i.postimg.cc/qh4jC5YP/IMG-20200622-220137.jpg) (https://postimg.cc/qh4jC5YP)
(https://i.postimg.cc/f38ks3x6/IMG-20200622-220113.jpg) (https://postimg.cc/f38ks3x6)
(https://i.postimg.cc/Y4fxkx9b/IMG-20200622-220517.jpg) (https://postimg.cc/Y4fxkx9b)
(https://i.postimg.cc/ThPcMgXJ/IMG-20200623-121313.jpg) (https://postimg.cc/ThPcMgXJ)
(https://i.postimg.cc/hQGs27KM/PCB-layout.png) (https://postimg.cc/hQGs27KM)
Further details and bill of materials can be found in my shared Google Drive:
https://drive.google.com/drive/folders/1_N7u3hYvdd_FXDAGF_orDp-hgn8lYr03?usp=sharing (https://drive.google.com/drive/folders/1_N7u3hYvdd_FXDAGF_orDp-hgn8lYr03?usp=sharing)
Listen to it here:
Hah! You have more likes than posts! Congrats!
Like the unit - nice job! Nice playing, too!
Cheers!
Larry S.
Clean design.
Why is the point in the center 5.47V instead of 4.06V? 0.28mA unbalance? TR15 base current and meter-loading would pull the other way. Does TR6 want a 100r base resistor to match TR5? Or just stray tolerances?
Does Vdd need that much filtering? Looks like only the R13 leg does. R4 is filtered. All other paths are through collectors.
Quote from: PRR on June 23, 2020, 06:26:11 PM
Why is the point in the center 5.47V instead of 4.06V? 0.28mA unbalance?
Probably due to a mismatch between TR5 and TR6. I didn't bother to match them, I just pulled 2 off the reel. Tail current is 7mA (sustain was at full) so a 0.28mA imbalance is plausible. It didn't seem to affect performance.
You sure you ota is drawn right Jonny ? TR7 ain't part of no Wilson mirror here ...
Quote from: Eb7+9 on June 23, 2020, 09:53:38 PM
You sure you ota is drawn right Jonny ? TR7 ain't part of no Wilson mirror here ...
100% certain.
ok, I'm used to the four device version ...
https://en.wikipedia.org/wiki/Wilson_current_mirror (https://en.wikipedia.org/wiki/Wilson_current_mirror)
You can make one with 4 transistors but the "regular" Wilson mirror has 3.
Obviously in this circuit it's done with PNP rather than NPN.
I automatically go for the improved version everytime I guess ...
good sounding design ... what's the headroom limit on your circuit (with comp off) ?!
I can play with a Seymour Duncan JB bridge pickup which has 16k coil resistance and puts out about 2.5Vpp when you hit the strings hard (think Malcolm Young), and it's just on the edge of breakup.
good stuff ... me likes the way you combine bringing ota input signal low and dropping common mode bias to two diode drops above ground to give ota output some swing room
Would it be possible to substitute the bc546s and bc556s with bc547s and bc557s?
I can never tell Wilson from Widlar from Winona, but as you guys already sorted, this is the Original Wilson.
(https://i.postimg.cc/PLfsTnw7/Reckless-Wilson-42.gif) (https://postimg.cc/PLfsTnw7)
I wondered if it needed 2 devices for economy or 4 for perfection and realized it isn't worth the brain-pain.
Quote from: 11-90-an on June 24, 2020, 12:06:30 AM
Would it be possible to substitute the bc546s and bc556s with bc547s and bc557s?
Yes, any similar low noise transistors like that should be fine. BC549 and BC559 might actually be a bit quieter than the ones I used. Just make sure they're from the same manufacturer, and preferably the same batch (or match them yourself).
so I'll just ask out of curiosity ... why a Wilson mirror here anyway ??!
QuoteOverall I am pretty happy with the sound of this little compressor. It's got plenty of squish, can do the chicken picking thing pretty well, and sounds really clean and quiet.
You did a nice job on that. Very bold step doing a DIY OTA with off the shelf BJTs.
Very nice, Jonny! You are really plumbing the possibilities in compression. (Funny you mention THAT--I am in the final stages of throwing together a +-12v Carl Martin-alike using one of the last THAT4301 DIP packages on the planet and a pcb I found on a European site. But I don't expect it to replace yours, which I got working again and remains my fave for Strat.) Wondering if perhaps the OG synth designers did it your way, I stumbled across the following discrete CA3080 article which folks may find of interest: https://www.elektormagazine.com/labs/discrete-otas-for-synth-diy-elektor-formant-upgrades (https://www.elektormagazine.com/labs/discrete-otas-for-synth-diy-elektor-formant-upgrades)
Quote from: rutabaga bob on June 23, 2020, 05:25:02 PM
Hah! You have more likes than posts!
Aaaand one.
QuoteProbably due to a mismatch between TR5 and TR6. I didn't bother to match them, I just pulled 2 off the reel. Tail current is 7mA (sustain was at full) so a 0.28mA imbalance is plausible. It didn't seem to affect performance.
That could be due to the unequal base impedances on TR5 and TR6. Perhaps adding a 100R in series with the base of TR6 would make things balance better.
(I'm assuming the test voltages are with the compression pot set to minimum resistance, the offset will reduce when the pot is opened up.)
Quote from: Rob Strand on June 24, 2020, 03:53:51 AM
QuoteProbably due to a mismatch between TR5 and TR6. I didn't bother to match them, I just pulled 2 off the reel. Tail current is 7mA (sustain was at full) so a 0.28mA imbalance is plausible. It didn't seem to affect performance.
That could be due to the unequal base impedances on TR5 and TR6. Perhaps adding a 100R in series with the base of TR6 would make things balance better.
(I'm assuming the test voltages are with the compression pot set to minimum resistance, the offset will reduce when the pot is opened up.)
Yes I measured it with the sustain at maximum, i.e. zero ohms across RV1 and about 7mA flowing in the tail of the LTP. It gets better with less sustain.
If you think about the offset I am seeing, the bias point is higher than expected, adding 100 ohms in series with the base of TR6 would actually make it slightly worse in this case since it will make TR6 turn off a tiny bit more. It would also increase the noise floor slightly - this topology is
very sensitive to base resistance, I find even 100 ohms is barely acceptable.
I think it's more likely small mismatch in VBE of the LTP transistors. In any case it doesn't matter much. :icon_biggrin:
Quote from: Eb7+9 on June 24, 2020, 02:01:01 AM
so I'll just ask out of curiosity ... why a Wilson mirror here anyway ??!
I found that with a regular 2 transistor current mirror the output impedance wasn't high enough, so the bias point moved around with input signal level, causing control breakthrough and audible thumping. When I upgraded to a 3 transistor Wilson mirror it fixed this issue. I didn't try the 4 transistor mirror. Maybe I should have. :icon_cool:
IMHO, you shouldn't try 4 transistors 'cause 3 BJTs current mirror delimits Poor - Wealthy man margins.. :icon_biggrin:
Nice project! Thanks for sharing!
Best regards, Jack
Quote from: jonny.reckless on June 24, 2020, 04:20:33 AM
I found that with a regular 2 transistor current mirror the output impedance wasn't high enough, so the bias point moved around with input signal level, causing control breakthrough and audible thumping. When I upgraded to a 3 transistor Wilson mirror it fixed this issue. I didn't try the 4 transistor mirror. Maybe I should have. :icon_cool:
your ota is driving 5k(ac) to ground at the output - it's not a Zout issue // a plain-jane (non-cascoded) discrete ota will more than do here ... i'm not asking for nothing as it's obvious why your bias (down stream) would drift wildly ... it's a similar situation to the other engineer'ed limiter in this forum that includes an ota ... hint: where luck plays a big, sometimes disastrous, role
QuoteIf you think about the offset I am seeing, the bias point is higher than expected, adding 100 ohms in series with the base of TR6 would actually make it slightly worse in this case since it will make TR6 turn off a tiny bit more. It would also increase the noise floor slightly - this topology is very sensitive to base resistance, I find even 100 ohms is barely acceptable.
I think it's more likely small mismatch in VBE of the LTP transistors. In any case it doesn't matter much
Yes, the offset goes the wrong way for adding a resistor to TR6 - I only eyeballed the asymmetry. It could be imbalance in the mirror since that also needs to match Vbe's. A common balancing act is to add emitter resistors to the mirror (provided they match better than the Vbe's). Like you say the imbalance doesn't seem to matter, sounds fine.
I always like your designs because you aren't afraid to keep the impedances low to keep the noise down.
Quote from: Eb7+9 on June 24, 2020, 05:49:52 PM
your ota is driving 5k(ac) to ground at the output - it's not a Zout issue // a plain-jane (non-cascoded) discrete ota will more than do here ... i'm not asking for nothing as it's obvious why your bias (down stream) would drift wildly ... it's a similar situation to the other engineer'ed limiter in this forum that includes an ota ... hint: where luck plays a big, sometimes disastrous, role
I don't know what you mean, can you explain? The bias doesn't drift wildly, it does move a little with input signal level but it's not audible. The Wilson mirror is there for a reason: the performance was inferior without it. You need fairly good static and dynamic current mirroring in this topology to avoid thumps and audible artifacts.
Quote from: Eb7+9 on June 24, 2020, 05:49:52 PM
your ota is driving 5k(ac) to ground at the output - it's not a Zout issue // a plain-jane (non-cascoded) discrete ota will more than do here ... i'm not asking for nothing as it's obvious why your bias (down stream) would drift wildly ... it's a similar situation to the other engineer'ed limiter in this forum that includes an ota ... hint: where luck plays a big, sometimes disastrous, role
Which one?
Sorry if this is a very idiotic queston but...
Are Vdd and +9v the same in this circuit? If so, then why are there 4 decoupling caps, 2 100nf and 2 100uf? Or can they just be replaced with 1 100nf and 1 100uf to lower part count? :o
Quote from: 11-90-an on June 25, 2020, 08:02:34 AM
Are Vdd and +9v the same in this circuit?
No they don't..!!
(you can realize it either by measurents taken or by V
BE drop on TR3/TR4 Emitter follower (Sziklai pair)
Quote from: antonis on June 25, 2020, 08:22:49 AM
No they don't..!!
(you can realize it either by measurents taken or by VBE drop on TR3/TR4 Emitter follower (Sziklai pair)
So do I connect anything to the VDD...? :icon_question:
Or is it just a label that has no significance on breadboard? (Sorry for my stupidity) :(
It's never a stupidity matter .. :icon_wink:
You probably don't undestand well overal circuit function..
Quote from: 11-90-an on June 25, 2020, 08:27:19 AM
So do I connect anything to the VDD...? :icon_question:
Or is it just a label that has no significance on breadboard? (Sorry for my stupidity) :(
There are no stupid questions, only stupid answers 8)
9V is the supply rail that comes into the pedal from your PSU. This supply powers the side chain transistors TR12 and TR13 and their associated components. Then there is a filter / smoother comprising TR3 and TR4 which generates VDD. VDD is about 0.7V lower than 9V, but has much less noise and ripple on it. VDD is used to power the rest of the circuit which is much more sensitive to noise. When you are playing clean it's not usually a problem but if you put a compressor in front of a drive pedal for example, the noise starts to become really obvious, and I wanted to keep it as quiet as possible, in terms of both hiss and hum. You could certainly build the unit without the extra VDD power supply and it would work but it will be a bit noisier depending on how high quality your power supply is. If you are running from batteries this will likely be fine. I use a Voodoo labs pedal power and that is also pretty clean but some of the cheaper 9V adapters generate a lot of high frequency switching ripple and 120Hz hum on their outputs, sometimes 10 - 20mV or even more which will adversely affect a design like this without extra filtering. Feel free to experiment!
Quote from: jonny.reckless on June 25, 2020, 05:49:15 PM
There are no stupid questions, only stupid answers 8)
9V is the supply rail that comes into the pedal from your PSU. This supply powers the side chain transistors TR12 and TR13 and their associated components. Then there is a filter / smoother comprising TR3 and TR4 which generates VDD.
Sorry to bother you Jonny, but how exactly this transistor pair filters the noise? I'm really curious to learn this!
Quote from: jonny.reckless on June 24, 2020, 09:42:20 PM
You need fairly good static and dynamic current mirroring in this topology to avoid thumps and audible artifacts.
exactly ...
I've got your "Reckless Pair" idea down to 9 transistors
similar to yours, it's exhibiting no static or dynamic level shifting at the output
I'm also playing with scale-able input gain - for gtr/bass headroom
---
note:
using op-amp buffers gives the signal path a much higher cmrr
(with psu ripple way less of a an issue) and there's less noise overall ...
the ota is just a standard four-device structure ...
achieving close-to-ideal one sided ota performance
---
btw, where's the half-wave side chain from ? ... I've seen it before
> how exactly this transistor pair filters the noise?
Emitter follower with an R-C filter to the base. Instead of raw supply crap you have filtered supply crap.
An elaboration: the Sziklai (https://en.wikipedia.org/wiki/Complementary_feedback_pair) pair to allow smaller capacitor (or just because he uses the Sziklai everywhere else).
Also because any stray impedance in the filter will let Vdd bounce with the beat which may add thump. In this form the output impedance is very low.
okk... BB'd the PMC and it works nicely... :)
(anyone has any chicken pickin riffs? ::))
Quote from: rankot on June 25, 2020, 06:18:02 PM
Quote from: jonny.reckless on June 25, 2020, 05:49:15 PM
There are no stupid questions, only stupid answers 8)
9V is the supply rail that comes into the pedal from your PSU. This supply powers the side chain transistors TR12 and TR13 and their associated components. Then there is a filter / smoother comprising TR3 and TR4 which generates VDD.
Sorry to bother you Jonny, but how exactly this transistor pair filters the noise? I'm really curious to learn this!
In essence it's a capacitance multiplier! (The capacitance located at the base of the transistor)
(https://i.postimg.cc/k2hMDpS4/Poor-Man-s-Compressor.png) (https://postimg.cc/k2hMDpS4)
Redrew the schematic for people (Like me) who get dizzy with all the intersecting-lines-that-aren't-connected-to each-other... :icon_cool:
Quote from: 11-90-an on June 26, 2020, 10:54:26 AM
(https://i.postimg.cc/k2hMDpS4/Poor-Man-s-Compressor.png) (https://postimg.cc/k2hMDpS4)
Redrew the schematic for people (Like me) who get dizzy with all the intersecting-lines-that-aren't-connected-to each-other... :icon_cool:
Yes, that looks correct to me :)
Quote from: 11-90-an on June 26, 2020, 08:13:26 AM
okk... BB'd the PMC and it works nicely... is there a way to make it 'tighter'? :o
(anyone has any chicken pickin riffs? ::))
I am not sure what you mean by "tighter" sorry, can you explain?
Quote from: Eb7+9 on June 25, 2020, 07:12:53 PM
btw, where's the half-wave side chain from ? ... I've seen it before
Dynacomp, Ross compressor, Boss CS-2, DOD FX80, you name it, they all pretty much did the side chain like that with some minor variations. I just didn't bother with the phase inverter and full wave peak detector; you don't actually need it unless you are building a super fast attack limiter, and I was trying to keep component count down. As I mentioned in the original post, the whole design was heavily inspired by the circuit topology of the Dynacomp.
i was wondering about values ... attack is still pretty fast in the original dyna, which were populated half wave, the sink device has no current limiting resistor there (yours does that with 470r) ... the release time is longer in the original dyna with 150k/10u versus your 22k/10u ... otherwise identical in structure
was just curious // well done!
My suspicion is that the very wide range of playing styles and expectations mean that eventually you "must" support variable time constants.
(The LA-2a is the exception to prove the rule.)
And C12 is also part of the time-action. 0.1u seems small to drive 1k and a naked bass. The first person to complain "tighter" should tack a 10uFd across C12.
Me myself, I would not be content with a half-wave, but that's my technician background.
Quote from: PRR on June 27, 2020, 01:53:52 AM
Me myself, I would not be content with a half-wave, but that's my technician background.
I really don't think it's necessary for standard guitar compression where you want an attack time of 10ms to 20ms typically. The Dynacomp did it that way and everybody just seemed to copy it without trying it out any other way. I experimented with a full wave sidechain and in my opinion the difference in sound and feel was pretty negligible, and it adds to the cost and component count so went against the "Poor man's" minimalist spirit.
The Engineer's Thumb also only has a half wave sidechain and that sounds great.
Quote from: jonny.reckless on June 27, 2020, 04:20:03 AM
I experimented with a full wave sidechain and in my opinion the difference in sound and feel was pretty negligible ...
no doubt ...
there's been lots of talk about the dyna side-chain on this forum over the years
... it keeps coming up
original 70's dynacomps had boards populated for full-wave operation
but an intentionally missing jumper made them half-wave ...
see note [3] in one of my early hand scribbles
http://www.lynx.net/~jc/dynacompLayout.gif (http://www.lynx.net/~jc/dynacompLayout.gif)
whoever at MXR came up with the dyna thought it might be better to go FW "as the old tube designs did"
or at least have the option to use it, otherwise that stuff wouldn't have been populated in
but ...
the fact that it was left disengaged suggests a few things :
(i) obviously MXR couldn't put out a pedal design with a 600mV output clamp
(ii) had they not been perfectly pleased w HW operation you can bet your dollars that circuit would never have been manufactured ... at least as is
somebody overlooked a critical performance aspect of the design at first
realized it, had doubts ... edged bets (just in case)
and in the end were forced to ditch ... the rest is history
the only drawback some see with this side-chain architecture is its lack of potential for controlled and independent variability ... seems the best way is to use it is as was done here, with its generally fast enough attack and properly slowed down release // invariably leading to a streamline design
so i've been curious... what does this do?
(https://i.postimg.cc/D8BbhZK4/Screen-Shot-2020-07-01-at-7-07-15-PM.png) (https://postimages.org/)
wondering why this network appears twice in this schem... ???
Quote from: 11-90-an on July 01, 2020, 07:09:14 AM
so i've been curious... what does this do?
(https://i.postimg.cc/D8BbhZK4/Screen-Shot-2020-07-01-at-7-07-15-PM.png) (https://postimages.org/)
wondering why this network appears twice in this schem... ???
Read first - wonder after reading..
"
C9, R10, R21 and C14 form a simple emphasis / de-emphasis circuit to reduce the noise (hiss) of the transconductance stage."
As for "double appearance", differential stages consist of
two inputs..
Quote from: antonis on July 01, 2020, 08:22:24 AM
Quote from: 11-90-an on July 01, 2020, 07:09:14 AM
so i've been curious... what does this do?
(https://i.postimg.cc/D8BbhZK4/Screen-Shot-2020-07-01-at-7-07-15-PM.png) (https://postimages.org/)
wondering why this network appears twice in this schem... ???
Read first - wonder after reading..
"C9, R10, R21 and C14 form a simple emphasis / de-emphasis circuit to reduce the noise (hiss) of the transconductance stage."
As for "double appearance", differential stages consist of two inputs..
wait what... i didnt even notice that.... gah :-X :-[ :icon_redface:
so basically they are filters... right?
found this...
(https://i.postimg.cc/DmR8Ty5B/Screen-Shot-2020-07-01-at-8-54-46-PM.png) (https://postimg.cc/DmR8Ty5B)
> basically they are filters...
Mr Reckless' plan makes a series resistor which is 2.2k for lows and (2.2k||1k)= 0.68k for highs. Unloaded this does nothing. But you snipped-out the (important) load, so we expect highs to be 3X or 4X as strong as lows. (And we expect to find the inverse response after the processing.)
Basically it's a first order shelving treble boost before the long tail pair (emphasis), and a complementary first order shelving treble cut after it (de-emphasis). This keeps the overall signal frequency response of the pedal flat, but reduces the amount of hiss generated by the LTP, which can be quite a problem if you put a compressor in front of a drive pedal for example. It also makes the clipping transients you get on note attack sound a bit snappier.
Did some experimenting on this and discovered that adding a 100k pot and a 22uF cap in parallel with C12 makes a "gain reduction" control. It controls how low the gain reduction can go.
Now for the questions... :-\
1. What is the minumum value of R17 before blowing the transistors up? (Don't wanna risk some trannies... :icon_mrgreen:)
2. Would adding a "ratio" control be putting a pot on pins 1 and 2 of TR13? If not, where? :icon_neutral:
As long as Base resistors (R16 & R18) are kept in place, neither TR12 nor TR13 could be blown up..
But there should be a limit for half-wave peak detector to be practically functional..
(by zeroing R17 value, you will turn TR12 into a crass Emitter follower.. )
(https://i.postimg.cc/N2Y6STf8/image.jpg) (https://postimg.cc/N2Y6STf8)
Changing the circuit like this should make it a full-wave peak detector... right?
(And would 20Ω be sufficient for no blown-up trannies?)
And if anyone has an idea for a ratio mod, please share...
This looks OK to me, although I would question the benefit of doing a full wave sidechain. I experimented with a full wave side chain like the dynacomp but, at least to my ears, the difference was negligible.
Quote
Now for the questions... :-\
1. What is the minumum value of R17 before blowing the transistors up? (Don't wanna risk some trannies... :icon_mrgreen:)
2. Would adding a "ratio" control be putting a pot on pins 1 and 2 of TR13? If not, where? :icon_neutral:
You can probably go down to 10k on R17, anything less than that and the release time will be so fast you'll get distortion on low notes due to gain modulation. You might want to go the other way, maybe a 10k resistor in series with a 100k pot for R17 to increase the release time. I always set release time pretty fast on a compressor; I like the snap it gives me. Most other guitar compressor designs have a bigger release time constant than this (200ms) for example the engineer's thumb and dynacomp are both in the 800ms range.
I don't think this would be easy to adapt to variable ratio due to the simple peak detector nature of the side chain. Maybe a small resistor in the emitter of TR13? Perhaps 100 ohms or so would soften the knee a bit. It's really a simple cheap circuit; I would probably use a THAT2181 if I was going to build a full 5 knob compressor 8)
SAME GREAT TASTE, NOW ONLY HALF THE FAT 8)
I realized I could have laid this out a bit smaller. I did a revision B PCB layout which fits in a 1590B enclosure. Still uses PCB mounted pots, sockets and switch for ease of assembly and test.
Details here: https://drive.google.com/drive/folders/1_beVxacCYdQylqaJsPzDPl9SqTk5rp4o?usp=sharing (https://drive.google.com/drive/folders/1_beVxacCYdQylqaJsPzDPl9SqTk5rp4o?usp=sharing)
(https://i.postimg.cc/k6c1sfD1/IMG-20200722-231119.jpg) (https://postimg.cc/k6c1sfD1)
(https://i.postimg.cc/V5fHvQH3/IMG-20200723-204259.jpg) (https://postimg.cc/V5fHvQH3)
(https://i.postimg.cc/Q9XSF1Sp/Capture.png) (https://postimg.cc/Q9XSF1Sp)
(https://i.postimg.cc/T5vdj2Lq/pcb.png) (https://postimg.cc/T5vdj2Lq)
Let me know if you would like a PCB. I also uploaded the Gerbers and NC drill file to the Google drive if you want to make your own board from this layout.
Just wondering jonny.reckless, why didn't you put the peak detector at the input side like the engineer's thumb? It would probably give adjustable threshold, attack and ratio... and sustain... So adding volume/level to that, you have a 5-knob compressor! :icon_mrgreen:
Finally.. a legit mod! ;D
For a "threshold" control, place a 100k pot in between C12 and R18... :icon_biggrin:
i'm afraid that's all... :P
Quote from: 11-90-an on July 23, 2020, 07:44:24 AM...Changing the circuit like this should make it a full-wave peak detector...
Except the phase splitter is practically a cripple, can't swing the lower Q into conduction. The upper Q works, but it is back to half-wave.....
(https://i.postimg.cc/PPvKwjbw/PMC-full-wave-42.gif) (https://postimg.cc/PPvKwjbw)
Quote from: PRR on July 24, 2020, 07:33:22 PM
Quote from: 11-90-an on July 23, 2020, 07:44:24 AM...Changing the circuit like this should make it a full-wave peak detector...
Except the phase splitter is practically a cripple, can't swing the lower Q into conduction. The upper Q works, but it is back to half-wave.....
(https://i.postimg.cc/PPvKwjbw/PMC-full-wave-42.gif) (https://postimg.cc/PPvKwjbw)
So I need to change the emmiter resistor value...right?
And also, jonny.reckless, why did tou use a transistor configured as a diode rather than a diode itself? Diodes are cheaper... :icon_lol:
> change the emmiter resistor value...right?
Wrong. To preserve AC balance you change both resistors and come right back to the same near-bottomed condition.
> Diodes are cheaper...
Not always. And if you buy transistors by the scoop, a stray diode is more expensive.
"Recovery" mod.... ;D
Scrapped R16 and replaced R17 with a 10k resistor and 100k pot with a 22uF cap in parallel to smooth out some volume loss... I have no oscilloscope so I can't measure the attack/release times...
Basically starting to look like more of a dynacomp... :icon_cool:
EDIT: Putting a 10nF cap in parallel with the 1k output resistor works too... :icon_mrgreen:
QuoteAnd also, jonny.reckless, why did tou use a transistor configured as a diode rather than a diode itself? Diodes are cheaper... :icon_lol:
To make the peak detector input impedance symmetrical. You can use a 1N4148 there and it will work, but the turn on voltage for negative and positive half cycles will be different which affects the sidechain slightly. I pay about a penny for a 1N4148 or a BC546 so there's no difference in cost for me. The total component cost of the poor man's compressor (neglecting the footswitch and case) is less than $2 hence the name.
Quote from: 11-90-an on July 24, 2020, 04:20:49 AM
Just wondering jonny.reckless, why didn't you put the peak detector at the input side like the engineer's thumb? It would probably give adjustable threshold, attack and ratio... and sustain... So adding volume/level to that, you have a 5-knob compressor! :icon_mrgreen:
This type of peak detector doesn't work well with a feedforward topology. The design was inspired by the simplicity and beauty of the Dynacomp, hence just 2 knobs for sustain and level. I've designed several feedforward circuits including both revisions of the Reckless diode compressor, but that wasn't my design intent here. The goals were low cost, simplicity and a nice squish. The engineer's thumb is a wonderful design, but it's a fundamentally different architecture. If you're interested, I did an engineer's thumb redux variant a few years ago, featuring lower noise and the inclusion of a flexible noise gate. That has 8 knobs and a switch 8)
https://www.diystompboxes.com/smfforum/index.php?topic=97123.msg1129089#msg1129089 (https://www.diystompboxes.com/smfforum/index.php?topic=97123.msg1129089#msg1129089)
Ok.. Makes sense... ;D
I tried swapping the transistor with a diode and there was a bit of a difference. :icon_eek: The transistor sounded better.... :icon_biggrin:
the engineer's thumb redux that you did sounds amazing, thig is the lm13700 is pricey... :icon_eek:
An interesting variation to the dynacomp full-wave is the one used on the Eden WT400,
https://elektrotanya.com/eden_wt400_pre_sch.pdf/download.html#dl
See Q5 and Q6. I did a slightly more balanced version at some point. IIRC I added another transistor somewhere so only base currents flowed. As it stands one polarity has base current and the other emitter.
does anyone have an enclosure drill template for the rev B? since everything is PCB mounted, not much wiggle room so i'd like to play it safe.
update, i threw the gerber files in a viewer and measured it out.
not sure if anyone else has the original PCB, but here's what i found:
the top edge is 53mm above center.
the pins for the pots are 8mm below the center, and 15mm to either side of the center. note that the actual pot drill holes are ~16.5mm above the pins.
the jacks are centered 23mm below the center.
the 3pdt is 43mm below the center.
the LED is at 16, -43.