Need help troubleshooting my MicroAmp variant

[WickedBlade]

Hello,

I would like to ask the help of the knowledgeable people here.
I have 'designed' and built an MXR Micro Amp variant that is mostly working but it does have issues that I need to fix.
So first, the design. It is a MicroAmp based on the Tonepad schematics, but since I had a dual opamp, I figured I would like to use the other half of the opamp to make an impedance buffer.
That part (the buffer) is inspired by AMZ's SuperBuff design but with only one opamp.

So here's the schematics I came up with.


I made a layout that is inspired by Tonepad's layout for the 'stereo microamp', since basically I only had to remove some components from one of the MAmps to get a layout for the buffer.
I etched the PCB using the toner transfer method (it's not the first time I do this BTW).
As far as components go, I used metal film resistors all the way, the opamp is an OPA2134 (I tested with a TL072 too), the polarized capacitors are all Panasonic FC, and the other caps are Panasonic SMF, except for the 51pF one which is actually a 47pF Silver Mica.
That is, as I said, mostly working. But there are issues. First of all, I am aware that when one effect is working, the other has its input basically unwired, so that may be part of the problem. I should mention that I know that it's generally bad to leave an opamp unused in a dual/quad opamp chip (I read Analog Devices' RAQ "What shall we do with an unused op-amp?")


As for offboard wiring, I used a 3PDT, where one pole is connected to the jack input and goes either to the mamp or the buffer input, another is for the jack output (wired in a similar manner) and the third pole is connected to ground and goes either to a LED+resistor or is left unconnected, so that the LED lights up only when the mamp is working. So I was hoping to have a buffer when the switch is off and a MAmp when it's on.



The first issue I noticed was that when the pedal was in buffer position, if I set the MAmp knob to the maximum gain position I had a high pitch noise. When I started decreasing the gain, the noise pitch went up, until at about 3 o'clock where it became inaudible (probably too high pitched for my ears). The first thing I thought of is that it was the op amp oscillating (because of the unwired input), so to test that, I wired the remaining pin of my 3PDT to the MAmp circuit input, so that when the buffer is on, the MAmp receives a grounded input. That has gotten rid of the squeal, so I'm guessing that I was correct in surmising a problem with the opamp.

The other issue is that if I plug the pedal when it is in buffer position, I get no sound at first, and need to activate the MAmp and go back to the buffer for it to work. Conversely, if I unplug the power feed but keep the guitar and amp plugged, I get no sound at first, but a few seconds later some sounds creeps back to the amp, but that might be normal due to the schematics, I don't know. I think the pedal did this before I wired the last pin of the switch, BTW, but cannot be sure.

So, I figure that I should at least find a way to get the unused input of the circuit properly grounded to avoid oscillation. I have thought about it and cannot think of a clever wiring scheme that would allow this and still keep the LED display. At worse, I think I'll rewire the pedal so that the buffer is always active, and the MAmp is put in series with that, using the 3PDT switch in a classic 'true bypass scheme' (that is, the MAmp would be true bypassed, but being in series with a buffer, the pedal on the whole would act as a buffer). I'd appreciate any suggestion here.

But mostly, what I would like is to understand what is happening here. I had thought that it was okay to leave the unused inputs unwired because there was a resistance to ground in the circuit anyway, so I was hoping that it would suffice, but apparently not. So could someone explain this to me please? And tell me if the "warm up" behavior (maybe the caps charging or something) is normal or something I should look more into?

Thanks a lot for your time and help!

[MarcoMike]

Hi, i might suggest to reduce the 10M resitor "feeding" Vr on the buffer... I know that 10M should work on the microamp, but once I had a no-signal problem as you report, which was solved reducing that resistor... it's not a difficoult thing to try...
regarding the switch wiring... grounding input is not a problem... just keep searching here for switching wirings, you'll find it!

[WickedBlade]

Thanks.

Could you or someone else explain to me why this 10M value was not the best choice and why 2.2M is better (and if it's not too much to ask, why it works nonetheless on the MicroAmp)? My understanding was that no current was going to come from there anyway, so the actual value didn't matter much as the potential would be the same on both sides of the resistor. Seems I was totally wrong, then.
Any explanation please?

[MarcoMike]

so, you mean, it does work now??

[WickedBlade]

Sorry, I wasn't clear. I have yet to try anything, as I am not home right now. So I don't know if that will work.

[Quackzed]

Could you or someone else explain to me why this 10M value was not the best choice and why 2.2M is better

it's because we have 2 references for voltage, +9v and ground. so the signal exists between these 2 extremes. at the input, the signal has a 2.2m to ground, therefore if you use a 2.2m resistor to +9, the signal will be at rest, 1/2 way between 9v and ground, able to swing positive and negative as it is referenced to 1/2 of 9v or 4.5v. the 2 resistors form a voltage divider, so if you use a 2.2m to ground and a 10m to 9v the signal will be biased at @ 1/12 of 9v or @ .75vish... which will distort one side of the signal sooner, as its closer to ground, than if its biased in the middle of the 2 references for voltage at 4.5v
... it will clip asymmetrically sooner as is, and will be cleaner and louder without distortion with 2 2.2m's...

[PRR]

> why this 10M value was not the best choice

10Meg is awful large to bias the inputs of an old-school opamp like 1458.

> My understanding was that no current was going to come

Opamps have input bias currents. 1458 is specified 0.2uA to 1uA. In 10Meg this means 2V to 10V drop from VB to the chip pin. 2V is no big deal. "10V" can't happen in a 9V world; the chip will just bottom-out.

> why it works nonetheless

Most 1458 at reasonable temperatures will be closer to 0.2uA bias current and will work OK.

2Meg will cover larger bias current yet will not hurt the guitar signal.

> the other has its input basically unwired

It realy should have a DC path, typically the Meg resistor. It should also have a low-impedance path for AC/Audio/Radio or it will pick-up garbage and make trouble. Typically you ground the far end of the coupling cap.

[WickedBlade]

Sorry for the late answer, but I had to locate some 2.2M resistors.

First of all, thanks to all who answered for their input. I have changed both 10M resistors to 2.2M resistors, but unfortunately, this didn't change anything as my issues go. Not better, not worse.

I still have the issue that, upon firing up, the buffer part of the circuit takes time before outputing anything. At first I get no sound but sputtering noise, and after a few seconds (I'd say 5 or 6 seconds) I start to hear things and then everything is fine. The MicroAmp side works like a charm from the start.
Please note that the buffer takes as much time to warm up whether I start with the footswitch in one position or the other, and even if I flip it repeatedly.

I have re-read my schematics and compared with AMZ's original design and couldn't find any notable difference apart from my use of a 22uF cap at the voltage divider instead of the 10uF one AMZ uses. Could that be related?

This warmup time, for some reason, made me think about caps. So I checked the orientation of the caps, and they all seem fine.
Next, I measured the voltage at the pins of the 10uF cap (it's on the output stage of the buffer circuit) using my DMM set to DC voltage meter. To my surprise, it looks like the voltage starts from 0 and very progressively ramps up to about 3.3V.
When I measure at the .1uF cap pins (input stage of the circuit), I get ~3.3V almost immediately. I have tried swapping the opamp with another, and this changes nothing.

So unless someone points out a glaring error in my schematic, I am thinking that I got a bad 10uF cap. So my next step is, I am going to try another 10uF cap to see if it makes things better.

In the meantime, any suggestion or observation is welcome. I would really appreciate your help.

Thanks !

[PRR]

The 10uFd which is paralleled by a 0.1uFd?

Take out the 10uFd. Does that pass signal? "Right away"?

Is there enough bass response? If so, done. 0.1uFd is often enough in a guitar chain.

If not, put the 10uFd back but change the 2Meg after it to 10K-100K.

I think the buffer is working but the 2Meg+10uFd=22 second charge-up time is holding the next stage/box input way positive for several seconds.

[WickedBlade]

Hi PRR,

it seems that you were right: I took your advice, or near enough: your observation about the charge up time made so much sense to me that I went straight to swapping the 2.2M output resistor with a 10k resistor.
My thought was that, it's supposed to be a buffer, so it should have a low output impedance, right? Isn't a 2.2M resistor defeating that? Wouldn't 10k (or less) be better?

So I swapped the output resistor, and voilà, it's now working fine. So first of all, a big thank you to all who helped, and especially to PRR.

But now I am wondering. Isn't AMZ's designed flawed? It is supposed to be a buffer, so it should have a low impedance output. I believe the 2.2M goes against the grain there. Am I right or am I missing something?

BTW I put a 10k resistor for two reasons:

• it is what the MicroAmp side uses, so I thought I should have a similar output impedance
• I have read that a guitar pickup has a similar impedance, so this way the buffer should drive other pedals in a similar manner to if the guitar was plugged straight in, without buffer

Any thoughts?

Thanks!

EDIT: it occurs to me that I may be calculating the output impedance wrongly, and that the 'output' resistor has nothing to do with it. On the contrary, the 10k resistor may be weakening the input impedance of the next pedal in line (because it's basically in parallel with the next pedal input resistor). Any correction of my way of thought appreciated!

[Gurner]

EDIT: it occurs to me that I may be calculating the output impedance wrongly, and that the 'output' resistor has nothing to do with it. On the contrary, the 10k resistor may be weakening the input impedance of the next pedal in line (because it's basically in parallel with the next pedal input resistor). Any correction of my way of thought appreciated!

You've pretty much got it...that output resistor doesn't set the output impeance...the opamp does (& it's very low). there's a general obsession with pull down resistors round these parts - they aren't always necessary (it depends on the follow on stage & whether you are switching stuff in/out a lot)). Re 'weakening' the impedance of the follow on stage ...being a pedant now & then I'd have used the word reducing, but in this instance impacting the impedance of the follow on stage is not critical ...becuase your follow on stage input impedance is almosty certainly not going to be lower than the output impedance of your buffer opamp.

[PRR]

> should have a low output impedance, right? Isn't a 2.2M resistor defeating that?

Ponder "parallel resistors".

Or parallel roads, parallel ditches.

The 2Meg is like my driveway, long and narrow and rutted. Traffic flows like 1/10 of a real road.

The opamp output is about 1 ohm, plus the 100 ohms added (series) to it. That's like a 10-lane freeway.

Let's say I open a pedal mega-store. Crowds flock to it. The driveway won't do. I build a 10-lane freeway next to it. In effect I have 10.1 lanes (10 good lanes and a rut). Is that any different from just 10 good lanes? Not enuff to matter.

The 2Meg or 10K does not matter to output impedance when parallel to 1+100 ohms from the chip.

101||2Meg= 100.99 ohms
101||10K= 99.99 ohms

"Some" pull-down is often needed to drain-off stray DC charge or leakage on the capacitor. As your experience shows.

And electrolytic caps have much more leakage than film caps.

[WickedBlade]

Thanks guys.

I get the idea. My problem is that too often I think in terms of voltage when I should think in terms of current. I'm probably not alone, what with the signal being a question of voltage.
Anyway, when I start thinking about current, it gets easier (just as it did when I started thinking of star grounding and stuff like that).

Thanks for your help again. Now I'm off to build my next pedal, a script Phase 90 clone.