Help Troubleshooting volume loss PLEASE

[sonolink]

Hi,

I recently finished an old project consisting of a tube preamp (AB763) inside a pedal, followed by Merlin's CabSim followed by a D.I. output based on an Elliott Sound circuit. The pedal has a instrument input, a mono output to connect it before an amp OR before a power amp (like in the FX loop return), and a TRS balanced output.

I first began building the unit only with the PSU components to check the voltages were right. Everything was fine (B+=190VDC, Heaters=6VDC).
I then added the AB763 components, input and jack output and tried it on an amp with a guitar. After some adjustments made following forum member Matador advice, it sounds great both before the amp or in the FX loop return.

I then added the DI output section and noticed that the sound was ok when using a LINE input in my MOTU soundcard (guitar -> pedal -> DI out -> souncard TRS in), but noisy when using a preamp input. Also, I noticed that after a few minutes of playing there was a loss in volume. If I unpower the pedal and power it back up the volume comes back and fades away after a couple of minutes...

I finally added the rest of the circuit (Merlin's CabSim) just in case this had to do with components missing on the board but the same thing happens.
Could this be some capacitor? Any ideas/suggestions please?

This is the schematic of the whole circuit:


Thanks a lot for your time and help
Cheers
Sono

 

[JohnRoberts]

I am not a tube guy, but the symptom sounds perhaps like a leaky capacitor altering the bias of some stage.

JR

 

[sonolink]

I am not a tube guy, but the symptom sounds perhaps like a leaky capacitor altering the bias of some stage.

JR

This morning I tried the pedal again in front of a guitar amp and everything sounds perfect.
So my guess is that the cause for the volume loss after a few minutes of switching it on and playing must be either in the DI output circuit or the CabSim.

I've also noticed that the Gain control is hissing when moving it in both setups (connected through the output mono jack to an amp or connected through the TRS Di Out to a souncard)

How can I check for a leaky capacitor? (assuming that's the only cause)
Do I have to lift one leg and check capacitance? Or should I check if there is or there's not DC on both sides? (to verify it's actually blocking it).
Are we talking all the caps or just Electrolytics?

Thanks a lot for your suggestions and help
Cheers
Sono

 

[usbdevice]

I finally added the rest of the circuit (Merlin's CabSim) just in case this had to do with components missing on the board but the same thing happens.
Could this be some capacitor? Any ideas/suggestions please?

Since the symptoms were present even before you added the CabSim part, the problem is most likely in the DI out circuit - if I understand correctly the problem only manifests on the DI out?
What strikes me as odd there is the polarity of C28. Unless there's phantom power present on the DI out, there's a reverse 4.5VDC present on that capacitor. I don't know whether that would cause your symptoms, but it might be worth trying to flip C28 around?

 

[Khron]

Unless there's phantom power present on the DI out, there's a reverse 4.5VDC present on that capacitor. I don't know whether that would cause your symptoms, but it might be worth trying to flip C28 around?

Or even better, series up a pair of double-value 50v caps, "just in case".

 

[sonolink]

Since the symptoms were present even before you added the CabSim part, the problem is most likely in the DI out circuit - if I understand correctly the problem only manifests on the DI out?

Thanks for your reply
I agree with you usbdevice and yes, the volume loss problem only manifests on the DI Out.

What strikes me as odd there is the polarity of C28. Unless there's phantom power present on the DI out, there's a reverse 4.5VDC present on that capacitor. I don't know whether that would cause your symptoms, but it might be worth trying to flip C28 around?

C28 and C29 are blocking caps supposed to protect from any phantom power incoming from a console/soundcard and R42/R43 bleeder resistors to discharge them. Wouldn't flipping C28 around ruin that purpose?

Or even better, series up a pair of double-value 50v caps, "just in case".

Sorry Khron, I don't follow what you mean...

 

[Khron]

Or even better, series up a pair of double-value 50v caps, "just in case".

Sorry Khron, I don't follow what you mean...

Granted, i wasn't quite as clear as i should have - instead of C28/29 being a single 10u/63v, use a pair of back-to-back 22u/50v (or 63v, whatever you can get).

Back-to-back meaning connecting, say, the positives together, and using the negatives as the "new" capacitor's terminals (what is now more or less a bipolar/non-polarized capacitor).

Reply #3 here:
https://www.eevblog.com/forum/begin...a-single-polarised-cap/msg2524410/#msg2524410

 

[sonolink]

Ok, I'll try that!
Maybe before doing that I can test the circuit WITHOUT the caps to make sure the volume loss isn't caused by them?

How can I check for a leaky capacitor? (assuming that's the only cause)
Do I have to lift one leg and check capacitance? Or should I check if there is or there's not DC on both sides? (to verify it's actually blocking it).
Are we talking all the caps or just Electrolytics?

I'd still like to learn how to check for a leaky cap please

Cheers
Sono

 

[Khron]

Maybe before doing that I can test the circuit WITHOUT the caps to make sure the volume loss isn't caused by them?

Why would you want to risk potentially toasting / upsetting even more components?

 

[usbdevice]

C28 and C29 are blocking caps supposed to protect from any phantom power incoming from a console/soundcard and R42/R43 bleeder resistors to discharge them. Wouldn't flipping C28 around ruin that purpose?

You are of course correct, and I should have been clearer. The flipping (and maybe even replacing it - I don't know if the 4.5VDC across it could have damaged it) of the capacitor is only to see if that eliminates the problem. Khron's suggestion of using 2 back-to-back capacitors is the solution then if you will.
The circuit was most likely designed with a bipolar power supply in mind, in which case the capacitors C28/C29 as they are would be fine - there would be 0VDC (ideally) on the output of the opamp.

And I would also not test it without those capacitors, there would be 4.5VDC on the output, in the worst case you'll damage the input circuitry of your interface - unlikely, but why take the risk indeed.

 

[JohnRoberts]

I've also noticed that the Gain control is hissing when moving it in both setups (connected through the output mono jack to an amp or connected through the TRS Di Out to a souncard)

are you describing a "scratchy" pot that makes scratchy noise when turned? This is a symptom of DC voltage across the pot.

How can I check for a leaky capacitor? (assuming that's the only cause)
Do I have to lift one leg and check capacitance? Or should I check if there is or there's not DC on both sides? (to verify it's actually blocking it).
Are we talking all the caps or just Electrolytics?

If you measure DC voltage where it should be 0V, that suggests a leaky capacitor.

Thanks a lot for your suggestions and help
Cheers
Sono

I repeat, I am not the tube guy here.....

JR

 

[sonolink]

Why would you want to risk potentially toasting / upsetting even more components?

The flipping (and maybe even replacing it - I don't know if the 4.5VDC across it could have damaged it) of the capacitor is only to see if that eliminates the problem. Khron's suggestion of using 2 back-to-back capacitors is the solution then if you will.
[...] And I would also not test it without those capacitors, there would be 4.5VDC on the output, in the worst case you'll damage the input circuitry of your interface - unlikely, but why take the risk indeed.

I was just thinking to discard parts. I was saying to TEST the circuit without the protection in a controlled environment and check if the volume loss happens or not, to determine if it's caused by those caps or not. That's all...BUT, as usbdevice points out, if the circuit will spit out DC without those caps I'll just directly try Khron's suggestion.

So if I understand correctly the idea behind the back to back caps is to completely block DC either way, whereas as it is now it will only block incoming DC from the TRS output. Is that right?

are you describing a "scratchy" pot that makes scratchy noise when turned? This is a symptom of DC voltage across the pot.

It makes a noise similar to "air"....I reslly don't know how to describe it better. It's not crackling or scratching (like guitar or amp pots when they're old), it's much softer.
I'll measure voltages and report back

If you measure DC voltage where it should be 0V, that suggests a leaky capacitor.

That's the answer I reckoned and needed John. Thanks!


I'll report back asap with more results!
Thanks for your help
Cheers
Sono

 

[sonolink]

Hi again

So I replaced the electrolytic caps each with a pair of 22uF/50v back to back as indicated, but unfortunately that didn't really change anything...

I have recorded an audio hoping that it will be more helpful with diagnosing the problem.
The setup is guitar -> pedal input -> pedal Direct Out -> Motu soundcard line Input -> Logic X

The pedal tone controls are at noon, Gain and Master cranked almost all the way up, Cab volume all the way up. You will hear a distorted (on purpose) guitar at first that will fade away after about 20 secs. After that, you will hear the scratchy sound of the Gain pot (I know I said it was hissing but I was wrong, sorry). Then you will hear a pop (disengaging the effect), the bypassed guitar, another pop (engaging the effect) and again the lifeless guitar without any volume or strength...

This is the audio:

View attachment PK79-01.mp3

Obviously, the pedal has several issues, related or not, but I really don't know where to begin...Any suggestions/ideas please? Your help is very much appreciated

Thanks
Cheers
Sono

 

[usbdevice]

Hi Sono, thanks for the recording, that makes things a little clearer.
Did you also get to measure any voltages? The gain pot to me sounds like it has DC across it. You could measure from terminal 3 to ground and see if there's any voltage present.
Another thing I noticed while browsing the schematic (while still keeping in the back of my head that the volume drop occurs only on the D.I. out) is that you're using three halves of 2 different TL072, the fourth is missing from the schematic. What did you do with it? Is it unused? If so, is it terminated properly?
Texas Instruments has an article explaining it here: How to Properly Configure Unused Operational Amplifiers In your case connect the non-inverting input of the unused half to 4.5VDC and the inverting input to the output of the op-amp. I don't know if it's a problem in your build but it's good practice anyway.

 

[sonolink]

Thanks a lot for your reply usbdevice

Did you also get to measure any voltages? The gain pot to me sounds like it has DC across it. You could measure from terminal 3 to ground and see if there's any voltage present.

I did. Here goes:

Gain (cranked all the way up): p1=0v p2=-0.12v p3=-0.15v
High (cranked all the way up): p1=0v p2=-0.26v p3=-0.26v
Mid and Bass: p1=0v p2=0v p3=0v
Cab-EQ and Cabsim Vol: p1=4.3v p2=4.3v p3=4.3v

IC1:
p1=4.5v
p2=4.5v
p3=3.9v
p4=0v
p5=4.2v
p6=4.2v
p7=4.2v
p9=9v

IC2:
p1=4.5v
p2=4.5v
p3=4.2v
p4=0v
p5=0.8v
p6=0.8v
p7=0.8v
p9=9v

Another thing I noticed while browsing the schematic (while still keeping in the back of my head that the volume drop occurs only on the D.I. out) is that you're using three halves of 2 different TL072, the fourth is missing from the schematic. What did you do with it? Is it unused? If so, is it terminated properly?

Actually no. The pins 5, 6 and 7 of the opamp are left unconnected.

Texas Instruments has an article explaining it here: How to Properly Configure Unused Operational Amplifiers In your case connect the non-inverting input of the unused half to 4.5VDC and the inverting input to the output of the op-amp. I don't know if it's a problem in your build but it's good practice anyway.

So the correct way would be like so? (correction is at the top right end)


I'll try to implement those connections on the board and report back.

BTW, I don't know if this is relevant, but I substituted both TL072s with NE5532s because I ran out of 72s.....

Thanks again for all your help
Cheers
Sono

 

[Khron]

Gain (cranked all the way up): p1=0v p2=-0.12v p3=-0.15v
High (cranked all the way up): p1=0v p2=-0.26v p3=-0.26v

That 250pF cap's starting to sound suspicious. 100-260mV of DC leaking through is questionable, to say the least. Weirder still that it seems to be negative..?

That being said, this would be the point where i whip out the signal tracer - could be as simple as a TS jack going into an instrument input in your interface, a 100n/400V dc-blocking cap and a 1meg to the sleeve of the cable. You'll want to macgyver yourself a couple of probes, one way or another; the ground one could be an alligator clip, so you only need to focus on maneuvering one probe.

Then i'd feed a sinewave into the input, and follow the signal along the way (tube grid 1, tube plate one, tonestack out, tube grid 2, tube plate 2 etc), and see where it starts going wonky.

 

[sonolink]

That 250pF cap's starting to sound suspicious. 100-260mV of DC leaking through is questionable, to say the least. Weirder still that it seems to be negative..?

That being said, this would be the point where i whip out the signal tracer - could be as simple as a TS jack going into an instrument input in your interface, a 100n/400V dc-blocking cap and a 1meg to the sleeve of the cable. You'll want to macgyver yourself a couple of probes, one way or another; the ground one could be an alligator clip, so you only need to focus on maneuvering one probe.

Then i'd feed a sinewave into the input, and follow the signal along the way (tube grid 1, tube plate one, tonestack out, tube grid 2, tube plate 2 etc), and see where it starts going wonky.

I bought this some time ago and I've used it with several pedal builds:
https://www.musikding.de/Signal-Tracer-Injector-Pen-V62-kit

But I guess I should be careful using it with this build because of high voltage, or build another one as you say, right?

Cheers
Sono

 

[Khron]

But I guess I should be careful using it with this build because of high voltage, or build another one as you say, right?

Very much so - ideally you'd want that first capacitor right after the tip-pin to be a 400V one minimum. Which it very likely isn't, in the stock build.

Or, well, with careful and judicious probing, you could still get away with that thing as it is, but "at your own risk"...

 

[usbdevice]

So the correct way would be like so? (correction is at the top right end)

Exactly.

BTW, I don't know if this is relevant, but I substituted both TL072s with NE5532s because I ran out of 72s.....

I don't think it is.

That 250pF cap's starting to sound suspicious. 100-260mV of DC leaking through is questionable, to say the least. Weirder still that it seems to be negative..?

I agree. And weirder still, the voltages at Gain P3 and High P2 should be the same, but according to your measurement they are not.
If there is an easy way for you to disconnect these two pins from each other, High P2 and Gain P3, and then measure again at both of these nodes, what voltages do you get then? [edit] And are they stable? [/edit]

Lenny

 

[sonolink]

Thanks to both for your help
I'm going to mod my audio probe and take the measurements when I'm back home after the studio. I will report back this evening! Thanks again!
Cheers
Sono