Need help with Engineer's Thumb compressor build

[LoonDawg]

No worries, it was a link rather than an embedded picture since it wasn't wanting to work for me

[merlinb]

The trouble is, few people are familiar with that layout, so even after positing voltages it takes ages to figure out what part of the circuit they're referring to. It would be better if you posted the voltages in the circuit diagram rather than the layout. It's also a bit of a rats nest, which doesn't make it any easier. 

[LoonDawg]

I totally understand but since I am fairly new to this whole thang, I can't really read a circuit diagram very well. It's alright, it was a learning experience and although the pedal didn't turn out exactly like I planned, it actually does something this time.

[tubegeek]

One of the subjects I teach in electronics lab (my day job) is the various types of diagrams and what their purposes are.

A schematic diagram differs from a layout diagram, and Merlin's comment above is an excellent illustration of how they differ.

Essentially, a layout diagram exists for the purpose of showing HOW a circuit can be built, so it is mostly a resource for construction and assembly of a device.

A schematic, on the other hand, is meant to explain the functionality of a circuit, how signal flows from input to output, what the power supply connections are, which subcircuits are connected to each other for what purpose, etc. A schematic can be a resource for building a device, certainly, but it also allows for a more analytical understanding of HOW the device operates.

Either type can be a useful resource for troubleshooting a project, but they are helpful in different ways. For example, looking at a layout diagram might reveal areas where a short-circuit (due to components being close together, say) could be likely. A schematic, on the other hand, when compared with measured voltages, could help make it clear that an op-amp input was grounded when it shouldn't be or something of that nature. That could be caused by the same problem as the one the layout diagram revealed, but it would make it obvious in a totally different way.

Ideally, you want to have access to both when you're stuck with something that doesn't work. And also, ideally, you want to learn how to understand each type. It's a very cool thing to be able to build up so many effects from the many layouts, veros, PCB transfer graphics, and all the other valuable resources that are available now for us to work from. But when the dung hits the impeller, it's important to understand what the schematic tells you, in order to understand the proper functioning of the circuit and the differences and similarities between various circuits.

I'm not sure my little rant is so helpful in this case, sorry. Call it pedantry for its own sake, I suppose. The one thing I can say that may be helpful is I'd encourage you to keep at it and be patient - a good compressor is well worth building and most people really like the Engineer's Thumb that have built it, so a working one would surely be nice to have. For that to happen, you may have to set it aside until you have more knowledge - I know I've been in that position, and eventually my knowledge caught up and I was able to fix something I hadn't understood well enough when I started the project.

In fact, I remember getting in a similar bind as you are in - I wanted to make a phono preamp that would convert to a line preamp by switching out the phono EQ components, but I hadn't bargained on the fact that the circuit didn't want to work properly with the wiring running to and from the switch that I had added.

I was able to solve that problem a while later - once I learned what was the likely problem - by simplifying the switching wiring. So maybe that applies here.

[tubegeek]

OK, so after typing all that pedantic ranting, I suppose I ought to try and help in a more concrete way. Here's what *I* would do if *I* were trying to fix this thing:

I printed out the vero layout, the vero layout with your voltages, and the schematic.

Next step would be to match up the two, and see if we can populate the schematic with the measured voltages. Fortunately, if we can do that properly, we have THE ACTUAL DESIGNER OF THE CIRCUIT HIMSELF who seems to be willing to take a look, which will be MUCH better than me taking wild guesses at that point. So that's good news. I'll see how far I get with this. I'll try and explain how I do it, too.

I can't really read a circuit diagram very well. It's alright, it was a learning experience...

Your learning experience is over when WE say it's over!



Hang in there, help is on the way.....

[LoonDawg]

haha, thanks so very much everyone. I'm not giving up yet

[tubegeek]

For me, step 1 is, number the IC pins on the vero, find IC pinouts for TL072 and LM13700, locate power pins for each IC. Check + and - voltage connections.

+ connections:
Vero coordinates k12, a10 and h21, all should be 9.47 ** why is a10 a little low? ??bad solder joint?? ??bad measurement?? DOUBLE CHECK HERE **
- connections:
Vero coordinates d7, n10, h18, all should be 0 **OK**

OK, the next part is a little less systematic. Looking at the IC pin voltages, one thing popped out at me:

** IC1/Pin3 is not at VBias. It's also .5V different from coordinate C3 (representing threshold3 and also one end of capacitor C1) which is directly connected to IC1/Pin3. These voltages cannot be different if all is well, and since that point involves off-board wiring, that's a question that merits investigation. IC1/Pin3 should be at the same DC voltage as IC1/Pin2 and IC1/Pin1 - that's one of the "laws of op amps." **

The Threshold pot should not have a DC voltage difference between any two of its pins: they should all be at VBias, so roughly 4.7V. I say this because the only things connected to that pot are TL072 op amp inputs, which is a JFET op amp whose inputs draw negligible current, and VBias, which sets these DC voltages explicitly. Also a capacitor which does not draw DC current. So for there to be a voltage drop of half a volt across the Threshold pot means something is drawing current through it.

My bet is that C1 is short circuit or that there is a solder bridge causing the same short. And also something is wrong if there is any difference between c3 and c7. Check that out please?

Trouble in the Threshold setting would tend to screw up the operation of a compressor, so I like my chances. And the + connection on IC1 being off (a10), that all points to an issue around IC1 as far as I can tell.

[merlinb]

** IC1/Pin3 is not at VBias. It's also .5V different from coordinate C3 (representing threshold3 and also one end of capacitor C1) which is directly connected to IC1/Pin3. These voltages cannot be different if all is well, and since that point involves off-board wiring, that's a question that merits investigation. IC1/Pin3 should be at the same DC voltage as IC1/Pin2 and IC1/Pin1 - that's one of the "laws of op amps." **

That's normal. Pin 3 is a high impedance point, so the average voltmeter will load it down and give a false (low) voltage reading.

[tubegeek]

That's normal.

<scratching head then>

[StephenGiles]

I agree entirely with tubegeek, it's vital to follow the schematic when building on vero - if only to see a map of how the parts connect together. As I have frequently said, I never have used a layout, prefering to connect on vero as I go and test each section before moving on to the next.

[GibsonGM]

I agree entirely with tubegeek, it's vital to follow the schematic when building on vero - if only to see a map of how the parts connect together. As I have frequently said, I never have used a layout, prefering to connect on vero as I go and test each section before moving on to the next.

Yes...the vero is irrelevant, if you're following the schematic.  All that matters are the voltages from *here to here*.    I personally think they hinder a person's growth and understanding....far too "plug and play".  But, if they are used in conjunction with the schematic, they could have some purpose in creating interest, getting people in the game.     Just that they then have to face a real letdown when it's debugging time!

This pedal probably has like 1 solder bridge somewhere, but the vero 'setup' is making it very hard to track down /

[rankot]

OK, so I built this compressor once again, but now I have a problem with it.

It does compress (or better say, limit, cause I don't hear it's amplifying decay of the tone), but it seems to be lacking some bass and it always distorts the tone. I tried to find help in the original ETC thread, but was asked to move troubleshooting somewhere else, to avoid clutter there. So it seems better to continue this thread, for all the future ETC troubleshooters to easily find (possible) answers. This is what Rob told me:

Now I removed everything I had extra and reverted my ET to issue 4 stock version. Even tried to use LM13700 from another batch, and still the same: sound is distorted when playing low stings or chords and also the tone is different - like there is less bass. I really can't figure this out. I have another ET that I built few years ago and it simply works as expected - tone is not altered, bright switch does what it shall do, all the pots (even Threshold) do their job. And it compress like heaven. This new one simply f*cks the things up.



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Now I removed everything I had extra and reverted my ET to issue 4 stock version. Even tried to use LM13700 from another batch, and still the same: sound is distorted when playing low stings or chords and also the tone is different - like there is less bass. I really can't figure this out. I have another ET that I built few years ago and it simply works as expected - tone is not altered, bright switch does what it shall do, all the pots (even Threshold) do their job. And it compress like heaven. This new one simply f*cks the things up.

It's a good strategy to start from the basics.

I'd be lifting R10 and driving C5 from a pot wired from 0V to 4.5V.  As you adjust the pot you should measure the voltage between C5 and 4.5V, the check the same voltage appears across R12,  then confirm the current "V across 1K"/1k  = 2 * "V across R7" / 220 = 2 * "V across R9" / 220.    Adjusting the pot should control the level.  It will be maximum when the C5 voltage is near 4.5V and minimum when the C5 voltage is near 0V (maybe limit the lowest voltage to about 1V).   That will at least confirm the is gain is being controlled.      If the added pot is set to high gain and the ratio pot is set to max then you will get very high gain, probably enough to cause clipping.   You should be able to just what is clipping due to high gain settings and distortion even low gain settings.

After that I'd repeat the test but instead wire the added pot to IC "B"'s + input.

If you get that far perhaps the problem is dynamic in that there's something wrong with the filtering C4 and C5.

The weird symptom is the loss of bass.  Which can only come from C3 at the input and C3 at the output.    There's two C3's  so if you have 10nF for the output cap it's going to cut the bass for sure.

After doing what he proposed, this is what happens with a pot: signal is distorted if amp bias voltage is lower than 0.78V, from there to 4.3V it's fine, after it goes over 4.3V it is distorted again (becomes almost square wave in both cases). I don't know if this is normal for LM13700? I have checked PCB and my schematic, it all seems to be fine, but the damn thing isn't working...

Now I removed everything I had extra and reverted my ET to issue 4 stock version. Even tried to use LM13700 from another batch, and still the same: sound is distorted when playing low stings or chords and also the tone is different - like there is less bass. I really can't figure this out. I have another ET that I built few years ago and it simply works as expected - tone is not altered, bright switch does what it shall do, all the pots (even Threshold) do their job. And it compress like heaven. This new one simply f*cks the things up.

[rankot]

One more thing - I have used BC307 instead of BC327, it is also PNP, but I didn't have 327 at hand. Could this be the problem?

[Rob Strand]

Just for reference I'm linking the schematic.

After doing what he proposed, this is what happens with a pot: signal is distorted if amp bias voltage is lower than 0.78V, from there to 4.3V it's fine, after it goes over 4.3V it is distorted again (becomes almost square wave in both cases). I don't know if this is normal for LM13700? I have checked PCB and my schematic, it all seems to be fine, but the damn thing isn't working...

I'm assuming you are driving opamp C with the variable voltage?   
Also  I'm assuming as you varied the pot voltage from 0.78V to 4.3V the gain of the circuit decreased?    The fact the voltage on opamp C changes the gain the gain means the LM13700 is working and responding to the control current on pins 1 and 16.

The distortion with the opamp C voltage at 4.3V may or may not be normal.  It all depends on how you drove the audio input.  If you put in very large signal and the ratio pot was high then it would be normal to distortion and by reduce the ratio pot it would reduce the gain and allow larger input signal before clipping.     Basically you have two ways to control the gain:  the ratio pot and the DC voltage on opamp C.     I guess I wouldn't be 100% about letting that one go by without knowing why it's clipping.

If you want, you can try the DC voltage on IC B.  It should have similar behaviour.

If we assume all that is working then the other weirdness is  bass loss.     I don't see how that can happen so easily.      So one obvious one is the opamp is faulty and this loading down the input cap C3.    The other way would be that your 4.5V rail is varying when you drive strong signal into the unit.   You might be able to see that on the Oscilloscope.

What version of the Engineer's thumb did you build before?   The issue 4 has the output buffer.    The output buffer has two Vbe's (1.2V)  loss in swing for one polarity.  It's possible to compensate by increasing R3 to 120k (and technically you should increase R12 to maybe 1.2k).   

One more thing - I have used BC307 instead of BC327, it is also PNP, but I didn't have 327 at hand. Could this be the problem?

The transistor is not *very* critical in that circuit.

[merlinb]

If you are using the compression indictor LED (D8) I suggest you temporarily short it out. The circuit can be sensitive to what type of LED you use, and the value of R23.
Short out the LED for now, and get the circuit working. Then when you know everything is working OK, you can un-short it and see if the LED or R23 needs adjusting.

[rankot]

Just for reference I'm linking the schematic.

After doing what he proposed, this is what happens with a pot: signal is distorted if amp bias voltage is lower than 0.78V, from there to 4.3V it's fine, after it goes over 4.3V it is distorted again (becomes almost square wave in both cases). I don't know if this is normal for LM13700? I have checked PCB and my schematic, it all seems to be fine, but the damn thing isn't working...

I'm assuming you are driving opamp C with the variable voltage?   
Also  I'm assuming as you varied the pot voltage from 0.78V to 4.3V the gain of the circuit decreased?    The fact the voltage on opamp C changes the gain the gain means the LM13700 is working and responding to the control current on pins 1 and 16.

Exactly, I was driving opamp C like you suggested. But the gain increased with voltage. I measured voltage on positive input of C and watched output signal on E/F. But distortion is the same even below or above the mentioned range. Within range, signal level increase follows the control voltage, but when it goes above 4.3V, is suddenly jumps into the full saturation, just as when it is below 0.78V.

The distortion with the opamp C voltage at 4.3V may or may not be normal.  It all depends on how you drove the audio input.  If you put in very large signal and the ratio pot was high then it would be normal to distortion and by reduce the ratio pot it would reduce the gain and allow larger input signal before clipping.     Basically you have two ways to control the gain:  the ratio pot and the DC voltage on opamp C.     I guess I wouldn't be 100% about letting that one go by without knowing why it's clipping.

If you want, you can try the DC voltage on IC B.  It should have similar behaviour.

If we assume all that is working then the other weirdness is  bass loss.     I don't see how that can happen so easily.      So one obvious one is the opamp is faulty and this loading down the input cap C3.    The other way would be that your 4.5V rail is varying when you drive strong signal into the unit.   You might be able to see that on the Oscilloscope.

What version of the Engineer's thumb did you build before?   The issue 4 has the output buffer.    The output buffer has two Vbe's (1.2V)  loss in swing for one polarity.  It's possible to compensate by increasing R3 to 120k (and technically you should increase R12 to maybe 1.2k).   

One more thing - I have used BC307 instead of BC327, it is also PNP, but I didn't have 327 at hand. Could this be the problem?

The transistor is *very* not critical in that circuit.

I assumed transistor is not critical at all, but had to mention. I will try other things soon.

[rankot]

If you are using the compression indictor LED (D8) I suggest you temporarily short it out. The circuit can be sensitive to what type of LED you use, and the value of R23.
Short out the LED for now, and get the circuit working. Then when you know everything is working OK, you can un-short it and see if the LED or R23 needs adjusting.

I'm using it, so I'll try without it.

[befey]

The circuit can be sensitive to what type of LED you use, and the value of R23.

Can you talk about why this is the case?

[Rob Strand]

but when it goes above 4.3V, is suddenly jumps into the full saturation, just as when it is below 0.78V.

I can see it might misbehave at 0.78V but suddenly jumping for the 4.3V case looks like it's worth investigating.
If as you approach 4.3V the gain increase then it just happens to hit the clipping point of the opamps or buffer then that's "normal".
You can't change the peak to peak clipping points  as that's fixed by the supply and opamps.     If you input a relatively small signal somewhat below the clipping point, and perhaps set the ratio pot for minimum gain,  the signal itself should not cause clipping.  Now if when you adjust the control voltage to 4.3V it still clips then clearly the clipping has something to do with the control voltage and not the signal levels.   That would point to some thing wrong.

[merlinb]

Within range, signal level increase follows the control voltage, but when it goes above 4.3V, is suddenly jumps into the full saturation, just as when it is below 0.78V.

That sounds correct.

Can you talk about why this is the case?

While fiddling with things I have witnessed some distortion on very small signals, and shorting the LED fixed it. Possibly it was because, when not compressing, little or no current flows in the LED and therefore its dynamic resistance was high, meaning the Vref supply was higher-impedance than normal, which would introduce some unwanted feedback between the various Vref connections. However, I was also doing other things at the same time, so it may have been a red herring. I only mention it here to simplify rankot's troublshooting situation.