Super low volume in muff overdrive clone. Unsure of cause.

[jdoughty]

Ok, so my son and I just did our first stomp box together and it's a simple muff overdrive clone https://www.pedalpcb.com/product/minimuffin/

However on completion, we get next to no sound unless I really hammer on my strings.

Video: https://youtu.be/Daj4D3rGYd4

Could this be from replacing the diodes with leds? I didn't have the germanium ones, but otherwise it's all to spec... I think.

[iainpunk]

https://www.diystompboxes.com/smfforum/index.php?topic=29816.0

have you read one of the sticky posts on 'building your own stompbox', called DEBUGGING?

start there

cheers, Iain

[blackieNYC]

No, swapping LEDs for the 1N34s in that design will still work. Less distorted but fine. 

[PRR]

> from replacing the diodes with leds?

No; that would just be loud.

Have you looked for bad solder joints, loose connections? After that: we can't SEE electricity so we use Voltage Measurements to verify happy DC Voltages. As Iain says.

[jdoughty]

I'll start looking for bad joints

In the mean time, IC pins:

• -7
• -6.5
• -6.4
• -6.8
• -6.3
• -6.9
• -6.5
• -7.5

-8 at pos battery connect point

[jdoughty]

LED1
A (anode, the non-band end) = -7
K (cathode, the banded end) = -7

LED2
A = -7
K = -7

[jdoughty]

[jdoughty]

Ugg I keep forgetting something. I meant to ask if any of those solders look particularly aggregious?

[11-90-an]

For a start, clip all those soldered wire and component leads shorter.

And the readings... uh there seems to be something a bit wrong with how you're getting the voltages.

[jdoughty]

Yeah, I need a new clipper...

Good news! The one of the IC pins wasn't soldiered properly! We're up and running!

Little dude is going to be excited in the morning!

[jdoughty]

Thanks for all the help. I'll check into a how to vid on the voltages for next time!

[j_flanders]

it's a simple muff overdrive clone https://www.pedalpcb.com/product/minimuffin/

https://www.pedalpcb.com/docs/MiniMuffinFuzz.pdf
Compare to EHX Muff Fuzz

They lowered the input resistor, and thus input impedance, from 100k to 1k.
Thereby also increasing the gain from 4,7 to 470!

And just like all the other incorrect schematics online, they too lack the feedback resistor, parallel to the diodes or LEDs, in the second opamp.

Not really a clone (IMO) and with these changes it won't sound anything like an original opamp version of the EHX Muff Fuzz/Overdrive.

[Mark Hammer]

Since I have been negligent in welcoming new members/builders of late, I'll try to improve my batting average by saying "Welcome" and saluting you for introducing your son to the world of DIY via a pedal circuit.  I just hope you won't mind if he starts wearing all black and eye-liner, now that he has fuzz.   

As PRR/Paul notes, using a pair of LEDs will be both louder and somewhat less distorted.  Insomuch as that may be what you're going for, it's not an "error" as much as a choice.  However, if you wouldn't mind getting a little more grind from the circuit, using a pair of simple silicon diodes, like 1N914, 1N4148, or 1N4001, etc., will strike a balance between sizzle and output level.  Not that loud is bad, but when a pedal is too loud, the range of usable volume-control settings becomes restricted and "twitchy".

Note that the original was in a tiny box that plugged directly into the guitar, (as the Dan Armstrong boxes did, though some had the plug going into the amp, as below) leaving no room for a Gain/Drive control, so the gain was fixed.  I doubt you'll be boxing it up in similar fashion, so if you'd like to be able to get harder and softer overdrive, I suggest you make the gain of the first op-amp variable.  You could do that be either replacing R3 (470k) with a 22k resistor and 500k pot (wired as variable resistor) in series, OR by inserting a 50k pot (also wired as variable resistor) in series with R2 (1K).  Either way will let you vary the gain from mild overdrive to max.

[Vivek]

As PRR/Paul notes, using a pair of LEDs will be both louder and somewhat less distorted. 

Either way will let you vary the gain from mild overdrive to max.

Since there is no feedback resistor between output and inverting input on IC1.2,

and Open loop gain is very very high,

I dont understand how replacing with LEDS make the sound somewhat less distorted.

and I dont understand how lowering the gain of the first stage will allow mild to wild distortion.

[Mark Hammer]

As PRR/Paul notes, using a pair of LEDs will be both louder and somewhat less distorted. 

Either way will let you vary the gain from mild overdrive to max.

Since there is no feedback resistor between output and inverting input on IC1.2,

and Open loop gain is very very high,

I dont understand how replacing with LEDS make the sound somewhat less distorted.

and I dont understand how lowering the gain of the first stage will allow mild to wild distortion.

"Clipping" occurs when the diodes begin to conduct, providing negative feedback from the output to the input of the op-amp.  LEDs have a forward voltage (hence "clipping threshold") about 3x higher than regular silicon diodes, so they don't start conducting until the output of IC1.2 comes close to +/-1.5V.

HOW does it come close to that tipping point?  By the gain of IC1.1 bringing the signal level up.  My suggested mods allowed for the gain of IC1.1 to be the same as stock (if either the additional input resistance from the suggested pot is reduced to zero, or the feedback resistance of the 500k pot is increased), but also permit reducing it from there.

Make sense?

Somewhere around 1972 or so, I was playing in a band and we had a gig in a school gymnasium, with the usual cinder-block walls.  For some reason I can't recall, I was in possession of TWO borrowed Muff Fuzzes, of the sort that plugged directly into the guitar.  I plugged them in series into the guitar, and the result into the amp.  I suspect the school custodian and principal are still wondering "What IS that noisy echo I hear from the basement or locker rooms?"  almost 50 years later. 

[antonis]

Since there is no feedback resistor between output and inverting input on IC1.2,
and Open loop gain is very very high, I dont understand how replacing with LEDS make the sound somewhat less distorted.
and I dont understand how lowering the gain of the first stage will allow mild to wild distortion.

Closed-loop gain (when Diodes are forward biased) is almost unity, for IC1.2 output greater than Diodes forward voltage drop..
The last stands even for incoming signal of few μV amplitude, due to very high open-loop gain..
I mean, practically, IC1.2 ouput never exceeds Diodess forward voltage drop..
That voltage drop value is the maximun pedal output amplitude, for Vol pot set all the way up, and for LEDs is higher than Si or Ge diodes..

BY lowerin the first stage gain, you make undistorted signal waveform (+/- 4.5V ideally) less "square" so more sinusoidal-alike part of signal comes into 2_nd stage..


edit: I hate some faster typing guys, like Mark..!! 

[11-90-an]

Since there is no feedback resistor between output and inverting input on IC1.2,

and Open loop gain is very very high,

I dont understand how replacing with LEDS make the sound somewhat less distorted.

and I dont understand how lowering the gain of the first stage will allow mild to wild distortion.

I'm not sure, but from what I can get from reading some stuff here in this forum, it depends on the forward voltage drop* of the diode, as the diode won't conduct when voltage is less than the forward voltage drop (which if using 1n4148s, should be 0.6V-0.7V, leds sometimes reach 1V-3.8V or something, don't really know the values... and germaniums should be around 0.2-0.3...)* and so the diode acts like some sort of limiter* that clamps the voltage down on it's frwrd vltge drop (I'll just call it Vf), so higher Vf would be louder-signal-passing-through-without-distortion/limiting... hopefully makes sense..

I'm also not sure but for me you will get different "intensities" of distortion by controlling the gain of the 1st stage because again, you are bashing the signal on the artificial headroom generated by the Vf of the diodes... so increasing gain would make it "easier" for the diodes to conduct, so less intensity of strumming/picking the instrument will be required to reach distortion... hopefully makes sense again...

* I'm not sure so please correct me if I'm wrong and tell me if I got it slightly correct too...

Since I have been negligent in welcoming new members/builders of late, I'll try to improve my batting average by saying "Welcome" and saluting you for introducing your son to the world of DIY via a pedal circuit.  I just hope you won't mind if he starts wearing all black and eye-liner, now that he has fuzz.   

I came upon this yesterday...

(oh and OP I think you can show this to your son and he would know these tv show characters... )

edit: i hate some faster typing guys like Mark AND Antonis...

[Vivek]

I'm sorry I still dont understand the explanations.

To me, the explanation makes sense only when there is a feedback resistor on the second Opamp.

I see it this way :

There is no feedback resistor on the second Opamp. It's gain will be extremely huge when the diodes are not clipping.

Suppose we have Si diodes,

The 2nd IC will clip when there is 0.65 V on the diodes

Suppose open loop gain is 200,000

That means that any input signal for second IC over 0.00325 mV will be clipped.


But the input signal for 2nd Opamp is the output of IC1 which has a gain of 470

Hence it means that any signal to the pedal that is more than 0.000007 mv will be clipped

Suppose the input signal is 100 mV peak, we are clipping it at 1/(14 Million) of its peak


If we do these calculations for LED, we get that a 100 mv signal will be clipped at 1/(3.7 Million) of its peak


Both are very extreme clipping and essentially square waves

Hence both will sound similar



Suppose we reduce gain of the first stage to 47 and we still have LED, then we clip a 100mv signal at 1/370,000 of its peak.

That too is very extreme clipping and essentially a square wave and will sound same as the first stage being clipped at 1/3.7 Millionth of its peak


So way I look at it

Changing to LED will not make it less distorted

Changing gain from 470x to 47x will not make it less distorted

since gain of next stage is 200,000 without a feedback resistor. There is no perceptible difference in a wave clipped at 1/14 million of its peak and one clipped at 1/4 millionth of its peak, except volume


Am i seeing it wrong ?

[j_flanders]

Since there is no feedback resistor between output and inverting input on IC1.2,

Apart from the technical discussion, the original did have a feedback resistor, and a capacitor.
We have a couple of threads on this particular subject.

[Vivek]

Yes I see the earlier comments make sense if there was a feedback resistor on the 2nd IC



But without the feedback resistor,

Changing diodes = no perceptible difference except volume

Changing gain of the first IC from 470 to 47 = no perceptible difference at all