Need help debugging Engineer Thumb Compressor

[nguitar12]

Hi everyone recently I finish my Engineer Thumb Compressor unfortunately it didn't work as it suppose to be. This is the version I build and I use the external threshold mod. I use my personal layout to build the circuit.





How my pedal didn't works?

1. It come out a dry signal without any compression (with Ratio pot all the way down)
2. Threshold pot doesn't make changes on the sound. Ratio pot act as gain. Volume function properly. So basically it it now a overdrive pedal.

Some debug that I have already done

1. I am sure the power section and the input gain section (U1A) is build correctly I barely understand this circuit. All the power pin of the IC have correct voltage and the guitar signal pass through.
2. I understand that U1B U2A and U3A act as a signal controlled variable resistor and control the gain of U1A hence even out the volume (however I don't understand how its work in detail.)
3. The second photo highlighted in green are surely have no problem.

Can someone please help me to debug the side chain state by state? (U1b>U2a>U3a). For example. How to check the voltage to ensure each state is working? Thanks in advance.

[PRR]

The tops of the 1uFd caps should sit at +4.5V with no signal, and go lower with signal.

The end of the 1K that connects to BC327 should do the same thing.

> U1B U2A and U3A act as a signal controlled variable resistor

Not quite.

U1b catches the negative peaks of the input signal, compared to +4.5V, and stores that voltage on the 1uFd caps.

U2a transfers that voltage through BC327 to the 1K resistor from +4.5V.

When there is no signal, there is no current through this 1K.

When there is signal, there is current through this 1K, proportional to input signal.

This current must flow from the other side of BC327, from the control port of the LM13700.

When LM137 has no control current, it is "dead". (Here there is a small idle current through the 1Meg to keep the LM13700 just-barely-awake.)

When LM137 does get control current, it passes signal.

Signal from LM13700 is applied to U1a in such a way as to oppose the input signal.

So for no-signal or small-signal, U1a just amplifies a lot.

For large-signal, LM13700 signal reduces the effective amplification of U1a.

You would think you could learn something by looking at LM13700's control port, but you can't; and careless poking this pin can blow the LM13700. The voltage there is small and does not change enough to measure meaningfully.

[nguitar12]

Thanks for your detail explanation on this circuit. 

The tops of the 1uFd caps should sit at +4.5V with no signal, and go lower with signal.

The end of the 1K that connects to BC327 should do the same thing.

I can only get 4.2v at the first 1uF cap. It go down to around 3.0v with a hard strum (humbucker pick up). I got 4.58v (bias voltage) at the point between 1k and BC327. The voltage will drop to around 3.5v with hard strum as well.

Any idea how I can further debug my circuit state by state?

Any thought on ratio pot becoming a distortion pot?

[nguitar12]

Can someone please help me out on this? I spent lot of time in this circuit and I put high expectation on it. I really want to have it working.

[Keppy]

Any thought on ratio pot becoming a distortion pot?

The ratio pot controls how much the feedback loop of U1A is affected by U3A. Turning it up increases the impact of U3A. That makes me think that U3A might be distorting. Check the voltages on the input and output pins, and double check the resistor values on those pins.

Also, did you remember in your layout to terminate the unused half of U3? Leaving those pins unconnected can cause unwanted consequences.

What would be most helpful is posting the voltages from all IC and transistor pins as described in the debugging page in the top menu. You'll tend to get more responses here if you post complete information. As Paul said, be careful poking the bias pin of the LM13700, or don't even try to meter that one. It's at 1.2-1.4v no matter what, and destroys the chip if subjected to >2mA of current.

[tomdg]

Appreciate this was 3 years ago but thought I'd post as I had exactly the same symptoms and actually found the problem!

My problem was that I'd got the wrong transistor - it should be a BC327 (PNP) and I had a BC337 (NPN).

If it can happen to me, it might happen to someone else so sharing just in case

Definitely more likely than this being any help to the OP

[PRR]

Welcome. Thanks for the report.

[rankot]

I'm having a problem with my build of ET. Sound's getting distorted when Threshold is past noon. Could anyone post voltages on LM13700 pins?

Vcc: 8.75V

• 1.285
• 0
• 4.57
• 4.57
• 4.05
• 0
• 0
• 0
• 0
• 0
• 8.75
• 4.05
• 4.57
• 4.57
• 0
• 1.285

All controls are maxed, no signal at input. I used 100k instead of 1M for Threshold pot, could it be the problem? When tried 1M (plus 200k between Thresh CCW and Vref, as in V3 schematic), distortion is always present, but the sound is much less distorted than with 100k Thresh pot. Voltages shown with 1M pot.

[merlinb]

I used 100k instead of 1M for Threshold pot, could it be the problem?

No, the value can't cause distortion. 
Voltages around LM13700 seem OK (although pin 5/12 possibly a little low, hmm). Why haven't you posted the voltages around the TL074?

[rankot]

I used 100k instead of 1M for Threshold pot, could it be the problem?

No, the value can't cause distortion. 
Voltages around LM13700 seem OK (although pin 5/12 possibly a little low, hmm). Why haven't you posted the voltages around the TL074?

I will in the morning, just returned from the very exhausting business trip
But when I simulate the circuit, I get 3.94 at pin 5 and 1.43 at pin 1. Other pins in simulation are similar to measurements (4.49 instead of 4.57V).

[rankot]

These are the voltages on 5532:

• 6.64
• 4.04
• 3.63
• 0.00
• 3.57
• 4.23
• 8.72
• 6.64

TL072:

• 4.05
• 4.05
• 3.63
• 0.00
• 3.70
• 4.04
• 3.48
• 8.72

[rankot]

Schematic I used:

[merlinb]

Schematic I used:

These are the voltages on 5532:
    1. 6.64

Where are you getting your 4.5V from? I can't see it on the schematic. If it's not properly buffered this could be the source of the distortion.
You meant 5534 rather than 5532, right? 6.64V is pretty low for the power pin. Sounds like you have a dropping resistor in the power supply that is too large for that IC. Also it's best to use a 100k threshold pot with the 5534 (not 1Meg) because it has significant bias current unlike TL07x.

[rankot]

Where are you getting your 4.5V from? I can't see it on the schematic. If it's not properly buffered this could be the source of the distortion.
You meant 5534 rather than 5532, right? 6.64V is pretty low for the power pin. Sounds like you have a dropping resistor in the power supply that is too large for that IC. Also it's best to use a 100k threshold pot with the 5534 (not 1Meg) because it has significant bias current unlike TL07x.

You're right, it is 5534, so pin 1 is not connected anywhere, and I have 8.72V at pin 7, which is Vcc.

This is how I get Vref - now I see I forgot to add some big capacitance after the transistor:

[merlinb]

This is how I get Vref -

Hmm, that makes me suspicious. Use an opamp.

[PRR]

>> This is how I get Vref ............
> Hmm, that makes me suspicious. Use an opamp.

The lack of pull-down bothers me.

It's also eating as much power as the rest of the board.

If an op-amp is not handy, I see a Brute-Force Vref on the same holes.


D11 and Q3 may be left on the board or removed, makes no real difference. 255 Ohms and 100uFd "ought" to suck-up all current in Vref well.

[rankot]

OK, I'll try with this simple divider.

[rankot]

Tried to add 10uF capacitor in parallel with my Zener-based Vref, also tried what PRR proposed (removed Zener and transistor, added another resistor and capacitor), but whatever I do the result is the same - there is some distortion when I strum strong on chords. Single string sound OK. So it seems it's really not compressing? Could it be fake LM13700 or something else? Maybe I should have use IC sockets

After using PRR's divider, this is what I have at LM13700 pins:

• 1.27
• 0.00
• 4.21
• 4.21
• 3.75

If I turn Threshold to 0, pin 1 is at 1.18V.

I have another ET I built long ago which works fine (clearly compress on high strumming instead of distorting), I will try to compare voltages etc. with this one, although this one is built using different schematic.

[rankot]

Hmm, that makes me suspicious. Use an opamp.

Is it OK to use NE5532 - one half as amplifying op amp, another one as a voltage follower for Vref?

[merlinb]

Is it OK to use NE5532 - one half as amplifying op amp, another one as a voltage follower for Vref?

Yes that's fine