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DIY Stompboxes => Building your own stompbox => Topic started by: jdoughty on November 24, 2020, 06:57:00 PM

Title: Super low volume in muff overdrive clone. Unsure of cause.
Post by: jdoughty on November 24, 2020, 06:57:00 PM
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.
Title: Re: Super low volume in muff overdrive clone. Unsure of cause.
Post by: iainpunk on November 24, 2020, 07:09:44 PM
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
Title: Re: Super low volume in muff overdrive clone. Unsure of cause.
Post by: blackieNYC on November 24, 2020, 09:11:04 PM
No, swapping LEDs for the 1N34s in that design will still work. Less distorted but fine. 
Title: Re: Super low volume in muff overdrive clone. Unsure of cause.
Post by: PRR on November 24, 2020, 11:13:19 PM
> 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.
Title: Re: Super low volume in muff overdrive clone. Unsure of cause.
Post by: jdoughty on November 25, 2020, 12:24:29 AM
I'll start looking for bad joints

In the mean time, IC pins:

-8 at pos battery connect point
Title: Re: Super low volume in muff overdrive clone. Unsure of cause.
Post by: jdoughty on November 25, 2020, 12:27:51 AM
LED1
A (anode, the non-band end) = -7
K (cathode, the banded end) = -7

LED2
A = -7
K = -7
Title: Re: Super low volume in muff overdrive clone. Unsure of cause.
Post by: jdoughty on November 25, 2020, 12:33:37 AM

(https://i.postimg.cc/hfSLzjPT/PXL-20201125-053142789.jpg) (https://postimg.cc/hfSLzjPT)

(https://i.postimg.cc/svwpprv8/PXL-20201125-053051231.jpg) (https://postimg.cc/svwpprv8)

(https://i.postimg.cc/VJvXgVtN/PXL-20201125-053045342.jpg) (https://postimg.cc/VJvXgVtN)
Title: Re: Super low volume in muff overdrive clone. Unsure of cause.
Post by: jdoughty on November 25, 2020, 12:34:44 AM
Ugg I keep forgetting something. I meant to ask if any of those solders look particularly aggregious?
Title: Re: Super low volume in muff overdrive clone. Unsure of cause.
Post by: 11-90-an on November 25, 2020, 01:00:09 AM
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.
Title: Re: Super low volume in muff overdrive clone. Unsure of cause.
Post by: jdoughty on November 25, 2020, 01:25:40 AM
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!

(https://i.postimg.cc/pys5pB9Y/PXL-20201124-015027345.jpg) (https://postimg.cc/pys5pB9Y)
Title: Re: Super low volume in muff overdrive clone. Unsure of cause.
Post by: jdoughty on November 25, 2020, 01:26:25 AM
Thanks for all the help. I'll check into a how to vid on the voltages for next time!
Title: Re: Super low volume in muff overdrive clone. Unsure of cause.
Post by: j_flanders on November 25, 2020, 04:02:24 AM
it's a simple muff overdrive clone https://www.pedalpcb.com/product/minimuffin/
Quote from: pedalpcb.com
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.

Title: Re: Super low volume in muff overdrive clone. Unsure of cause.
Post by: Mark Hammer on November 25, 2020, 10:06:28 AM
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.   :icon_lol:

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.
(https://lh3.googleusercontent.com/proxy/q5mPpfVqNNV7-fH5-88FNslo0MW1OHV9xCNO350ghnINhGitUVLnmI5_uJId363SJVqU-Y06e_w9oXM06HefWZYwFfRH-hpNTA0n)
Title: Re: Super low volume in muff overdrive clone. Unsure of cause.
Post by: Vivek on November 25, 2020, 10:23:44 AM

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.



(https://i.postimg.cc/9MfL13ms/Fuzz.jpg) (https://i.postimg.cc/9MfL13ms/Fuzz.jpg)



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.

Title: Re: Super low volume in muff overdrive clone. Unsure of cause.
Post by: Mark Hammer on November 25, 2020, 11:13:19 AM

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.



(https://i.postimg.cc/9MfL13ms/Fuzz.jpg) (https://i.postimg.cc/9MfL13ms/Fuzz.jpg)



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.  :icon_lol:
Title: Re: Super low volume in muff overdrive clone. Unsure of cause.
Post by: antonis on November 25, 2020, 11:25:14 AM
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 2nd stage..


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

Title: Re: Super low volume in muff overdrive clone. Unsure of cause.
Post by: 11-90-an on November 25, 2020, 11:30:09 AM
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... :icon_eek:

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.   :icon_lol:

I came upon this yesterday... :icon_lol:



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

edit: i hate some faster typing guys like Mark AND Antonis... :icon_lol:
Title: Re: Super low volume in muff overdrive clone. Unsure of cause.
Post by: Vivek on November 25, 2020, 11:35:41 AM
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 ?
Title: Re: Super low volume in muff overdrive clone. Unsure of cause.
Post by: j_flanders on November 25, 2020, 11:54:26 AM
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.

Title: Re: Super low volume in muff overdrive clone. Unsure of cause.
Post by: Vivek on November 25, 2020, 12:00:34 PM
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
Title: Re: Super low volume in muff overdrive clone. Unsure of cause.
Post by: antonis on November 25, 2020, 12:04:10 PM
To me, the explanation makes sense only when there is a feedback resistor on the second Opamp.

Consider Diode incremental resistance as an almost short...
(that turns second amp into a buffer..)  :icon_wink:

Title: Re: Super low volume in muff overdrive clone. Unsure of cause.
Post by: Mark Hammer on November 25, 2020, 12:19:30 PM
Antonis, your response could be a haiku if it was but one word shorter.  :icon_biggrin:

Vivek, I find the way to understand op-amps is that they really really REALLY want to amplify at their maximum gain-bandwidth product.  In our case (audio frequencies) that is often in the vicinity of 60-80db.  The way to adjust gain is to feed or restrict negative feedback from the output, generally by bleeding it off.  When we put a cap in the feedback loop, we are essentially giving a "pass" to lots of negative feedback for high frequencies, but less negative feedback for whatever lies below the corner frequency set by the cap value.

The metaphor I like to use is that of a car whose gas pedal is welded and bolted to the floor, and the only way to control the speed of the vehicle is via the brake pedal.  Less brakes = more speed.

What Antonis notes is correct.  Below the forward voltage, the diodes allow for the negative feedback to be blocked.  But get near the forward voltage, and as the diodes conduct, that negative feedback begins to flow, reducing the gain, near instantly....clipping!

Conceivably, a smoother sound, coming from a more seamless transition between maximum open-loop gain and the reduced gain, could be achieved by having a resistor in parallel with those diodes.
Title: Re: Super low volume in muff overdrive clone. Unsure of cause.
Post by: nocentelli on November 25, 2020, 12:29:22 PM
For anyone still searching in vain for suggested values for the resistor and cap in parallel with the diodes, 100k and 470p are suggested in this thread:

https://www.diystompboxes.com/smfforum/index.php?topic=117223.0

It is bewildering that virtually every single schematic on the opamp muff fuzz is wrong: i know that analog guru posted a correct, traced (1977) version at "the other place" a number of years ago. He has since pulled every schematic of his from the web, so i'll check my pc to see if I downloaded it at the time.
Title: Re: Super low volume in muff overdrive clone. Unsure of cause.
Post by: Vivek on November 25, 2020, 12:33:46 PM

Vivek, I find the way to understand op-amps is that they really really REALLY want to amplify at their maximum gain-bandwidth product.  In our case (audio frequencies) that is often in the vicinity of 60-80db.  The way to adjust gain is to feed or restrict negative feedback from the output, generally by bleeding it off.  When we put a cap in the feedback loop, we are essentially giving a "pass" to lots of negative feedback for high frequencies, but less negative feedback for whatever lies below the corner frequency set by the cap value.

Still, I maintain,

If there is no feedback resistor and capacitor on OpAmp 2, then

Changing diodes = no perceptible difference except volume

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




Title: Re: Super low volume in muff overdrive clone. Unsure of cause.
Post by: willienillie on November 25, 2020, 01:14:27 PM
I didn't read the thread, sorry if this is already solved.

(https://i.postimg.cc/QVwq7h7s/PXL-20201125-053045342.jpg) (https://postimg.cc/QVwq7h7s)
Title: Re: Super low volume in muff overdrive clone. Unsure of cause.
Post by: 11-90-an on November 25, 2020, 01:18:03 PM
Changing diodes = no perceptible difference except volume

Changing diodes affects clipping characteristic, and “tone” as some people call it...:icon_wink:

I didn't read the thread, sorry if this is already solved.

(https://i.postimg.cc/QVwq7h7s/PXL-20201125-053045342.jpg) (https://postimg.cc/QVwq7h7s)


Indeed, it is solved...  :icon_mrgreen:
Title: Re: Super low volume in muff overdrive clone. Unsure of cause.
Post by: Vivek on November 25, 2020, 01:25:46 PM
Changing diodes = no perceptible difference except volume

Changing diodes affects clipping characteristic, and “tone” as some people call it...:icon_wink:



Generally yes/maybe

But not in the situation where there is no feedback resistor.


Please be kind to analyse the situation with no feedback resistor  and guide me in case I misunderstood.
Title: Re: Super low volume in muff overdrive clone. Unsure of cause.
Post by: Vivek on November 25, 2020, 01:31:30 PM
If there is a feedback resistor setting gain to 200

input of 100mv peak

without diodes, the signal would have become 20V peak

si diode will clip that at 0.65 ie at 3.25%

LED will clip that at 2.5V ie at 12.5%


Big difference in tone due to clipping the input at different places (also, more volume from LED)



Suppose no feedback resistor on 2nd Opamp

Signal would have become 9 Million Volts

Clipping that at 0.65V is 0.000000%
Clipping that at 2.5V is 0.0000000%


No difference in tone due to clipping the input at same place effectively (also, more volume from LED)
Title: Re: Super low volume in muff overdrive clone. Unsure of cause.
Post by: j_flanders on November 25, 2020, 02:08:29 PM
For anyone still searching in vain for suggested values for the resistor and cap in parallel with the diodes, 100k and 470p are suggested in this thread:
Should be the other way round: 100pF and 470k
Best to read this thread instead, from this post on:
https://www.diystompboxes.com/smfforum/index.php?topic=123951.msg1174407#msg1174407
Title: Re: Super low volume in muff overdrive clone. Unsure of cause.
Post by: 11-90-an on November 25, 2020, 02:10:01 PM
So I BB’d the circuit just now and here are my observations...
the diodes used where either 1n4148s or blue LEDs

- this circuit oscillates better than all the LFOs I wanted to oscillate but never worked (prolly a bb problem)
- from wiring a 50k pot in series with the 1k, dialing it to least resistance yields the most oscillation free-sound while the other extreme oscillates like crazy. Vivek’s speculation about opamp1 gain not affecting overall distortion  was correct, however there was a teeny “distortion characteristic” change... or maybe I’m hearing things since it’s 3am and I have to lower amp volume to not disturb family... :icon_mrgreen:
- leds yielded a thinner sounding fuzz while the 1n4148s have a bit more bass...
Title: Re: Super low volume in muff overdrive clone. Unsure of cause.
Post by: iainpunk on November 25, 2020, 02:22:34 PM
i believe i read somewhere that this mini muff fuzz uses germanium transistor leakage to set the gain in the 2nd gain stage, that's why at least one of the diodes should be germanium or they have to have a large value resistor in parallel.

cheers
Title: Re: Super low volume in muff overdrive clone. Unsure of cause.
Post by: antonis on November 25, 2020, 03:14:17 PM
Antonis, your response could be a haiku if it was but one word shorter.  :icon_biggrin:

OK, Mark.. :icon_wink:
(I revoke the word "almost"..)

:cheers:
Title: Re: Super low volume in muff overdrive clone. Unsure of cause.
Post by: antonis on November 25, 2020, 03:31:27 PM
If there is a feedback resistor setting gain to 200
input of 100mv peak
without diodes, the signal would have become 20V peak
si diode will clip that at 0.65 ie at 3.25%
LED will clip that at 2.5V ie at 12.5%
Big difference in tone due to clipping the input at different places (also, more volume from LED)

Suppose no feedback resistor on 2nd Opamp
Signal would have become 9 Million Volts
Clipping that at 0.65V is 0.000000%
Clipping that at 2.5V is 0.0000000%
No difference in tone due to clipping the input at same place effectively (also, more volume from LED)

Now, suppose a couple of Ohms feedback resistor (forward dynamic resistance) for Diodes and a couple of tens Ohms for LEDs..
(bot non-linear but of different slope..)

Make your calculations as above and let us know your result..
Title: Re: Super low volume in muff overdrive clone. Unsure of cause.
Post by: iainpunk on November 25, 2020, 04:01:25 PM
If there is a feedback resistor setting gain to 200
input of 100mv peak
without diodes, the signal would have become 20V peak
si diode will clip that at 0.65 ie at 3.25%
LED will clip that at 2.5V ie at 12.5%
Big difference in tone due to clipping the input at different places (also, more volume from LED)

Suppose no feedback resistor on 2nd Opamp
Signal would have become 9 Million Volts
Clipping that at 0.65V is 0.000000%
Clipping that at 2.5V is 0.0000000%
No difference in tone due to clipping the input at same place effectively (also, more volume from LED)

Now, suppose a couple of Ohms feedback resistor (forward dynamic resistance) for Diodes and a couple of tens Ohms for LEDs..
(bot non-linear but of different slope..)

Make your calculations as above and let us know your result..
i guess you are pointing towards the fact that after the clipping threshold is reached, the gain is 1?
that only works for non inverting gain, this unit inverts. the voltage at the inverting input node is always equal to the voltage at the non-inverting node (unless the op amp itself is clipping), this restrict the output being more than Vbias +or- Vfdiode
also the order of magnitude of that stages input resistance is so large that the dynamic resistance of diodes is negligible.

cheers, Iain
Title: Re: Super low volume in muff overdrive clone. Unsure of cause.
Post by: PRR on November 25, 2020, 04:13:57 PM
  • -7
  • -6.5
  • -6.4
  • -6.8
  • -6.3
  • -6.9
  • -6.5
  • -7.5
-8 at pos battery connect point

Hint for "All": when you have a 9V battery, you "expect" some parts to be spanning a large fraction of 9V. Specifically an op-amp will (in our world) have one leg on Zero, one leg on 9V, and other legs in-between.

Here EVERY pin is within about one volt span. That could be right but is very odd. All the numbers are near 9V (actually the 8V of the apparently half-dead battery). Where is the ZERO we expect??

Spoiler: JD reported that "one of the IC pins wasn't soldiered properly!" If it was one of the Power pins, that matches the wrong readings correctly.
Title: Re: Super low volume in muff overdrive clone. Unsure of cause.
Post by: antonis on November 25, 2020, 04:29:48 PM
the voltage at the inverting input node is always equal to the voltage at the non-inverting node (unless the op amp itself is clipping), this restrict the output being more than Vbias +or- Vfdiode

also the order of magnitude of that stages input resistance is so large that the dynamic resistance of diodes is negligible.

Never said anything different... :icon_smile:

I'm trying to make Vivek alter his consideration from "no feedback resistance - no NFB" to "short feedback resistance - 100% NFB"..
Title: Re: Super low volume in muff overdrive clone. Unsure of cause.
Post by: PRR on November 25, 2020, 05:11:02 PM
...If there is no feedback resistor and capacitor on OpAmp 2, then
Changing diodes = no perceptible difference except volume
Changing gain of the first IC from 470 to 47 = no perceptible difference at all

Agree.

At least for an ideal diode. Some diodes leak *approximately like* a resistor. This is of course unpredictable and should not be relied on.

The open-loop gain falls with frequency. Highs boosted less than lows.

The signal will rise easy to a MAX which depends of the diode. A part-volt for common diodes, a couple volts for LEDs. Once it hits max it flat-tops, further increase of signal make hardly any increase of output.

- this circuit oscillates better than all the LFOs I wanted to oscillate but never worked

Amplifiers oscillate and oscillators don't.

With no gain-limit resistor this may be more gain than anybody can handle without squeal.
Title: Re: Super low volume in muff overdrive clone. Unsure of cause.
Post by: Fancy Lime on November 25, 2020, 05:15:39 PM
Yepp, one more thread turned into the realization that diodes are not ideal. In fact, in this arrangement, you get forward and reverse leakage currents in parallel, which can be quite substantial but is rarely specified in datasheets. The leakage current reduces gain in the same way a parallel resistor would, only less predictively, which can be fun for diy but I would not recommend it for a commercial pedal. Generally Ge diodes tend to have rather a lot of leakage, no news there. If you need high leakage Si diodes, low-voltage Zeners or high current diodes are usually leakier than dedicated switching diodes.

Cheers,
Andy
Title: Re: Super low volume in muff overdrive clone. Unsure of cause.
Post by: 11-90-an on November 25, 2020, 11:05:05 PM
With no gain-limit resistor this may be more gain than anybody can handle without squeal.

This may be an understatement... :icon_lol:

A strange thing that happens (I mentioned it before)... upon lowering gain of the first opamp stage from around 470 to 4.7, the squeal becomes LOUDER when using leds, but when using 1n4148s, the inverse is true..(lower gain, low squeal)

I do gotta say, when it isn't squealing, it sounds quite nice...  :icon_mrgreen:
Title: Re: Super low volume in muff overdrive clone. Unsure of cause.
Post by: Vivek on November 25, 2020, 11:45:37 PM
If there is a feedback resistor setting gain to 200
input of 100mv peak
without diodes, the signal would have become 20V peak
si diode will clip that at 0.65 ie at 3.25%
LED will clip that at 2.5V ie at 12.5%
Big difference in tone due to clipping the input at different places (also, more volume from LED)

Suppose no feedback resistor on 2nd Opamp
Signal would have become 9 Million Volts
Clipping that at 0.65V is 0.000000%
Clipping that at 2.5V is 0.0000000%
No difference in tone due to clipping the input at same place effectively (also, more volume from LED)

Now, suppose a couple of Ohms feedback resistor (forward dynamic resistance) for Diodes and a couple of tens Ohms for LEDs..
(bot non-linear but of different slope..)

Make your calculations as above and let us know your result..


Thanks,

Forward voltage of diode decides WHERE will the diode start clipping.

So all above calculations are the same

Internal resistances and capacitances will decide HOW the clipping will take place

But in the situation with no feedback resistor to limit the gain, we are either chopping the wave at 1/ 14 million with Si or 1/4 million of its peak with LED. Both are very extreme clipping.

Then minute differences in internal resistance of diodes , especially if in the 10 to 100 ohm range, will make no perceptible change at all

Basically, the clipping is so severe in both cases, we get almost pure  square waves while considering / ignoring diode resistance WHEN THERE IS NO FEEDBACK RESISTOR.

The only difference i see between different diodes is volume.

I did a LTSPICE FFT analysis comparing Harmonic content of 9 million Volt peak signals chopped at 0.65V versus 2.5V.

It's same frequency content, except the 2.5v clip has higher volume. That was with ideal diodes.

Now suppose we considered diode resistance, the 0.65 could have become something like 0.65001 maybe.

So instead of clipping the wave at 1/14 millionth of the peak
, we will clip at 1/13.999999 millionth of the peak.

No perceptible difference.

Title: Re: Super low volume in muff overdrive clone. Unsure of cause.
Post by: Vivek on November 25, 2020, 11:56:22 PM
To me, the explanation makes sense only when there is a feedback resistor on the second Opamp.

Consider Diode incremental resistance as an almost short...
(that turns second amp into a buffer..)  :icon_wink:

Yes

Diode was considered as an almost short when it is clipping

The main point is

When is the diode clipping ?

If there is no feedback resistor, the diode is clipping when that Opamp input signal is more than 0.65/200,000 volts peak

That is very extreme, severe clipping.

There is no difference between two very extreme clippings

Clipping a 100mV signal at 0.65/200,000 V has same frequency analysis as

0.65001/200,000 with diode resistance considered

2.5/200,000 with diode changed to LED (except the volume difference


So I feel, WHEN THERE IS NO FEEDBACK RESISTANCE on 2nd stage ,

Changing gain of first stage from 470 to 47 changes nothing

Changing diodes does not change tone, it only changes volume. If that volume is normalised, there is no change at all

Title: Re: Super low volume in muff overdrive clone. Unsure of cause.
Post by: Vivek on November 26, 2020, 01:34:00 AM


I'm trying to make Vivek alter his consideration from "no feedback resistance - no NFB" to "short feedback resistance - 100% NFB"..

In general, I love changing my opinion based on new knowledge

But the above was already considered

A) No feedback resistor and diodes not conducting = very very high gain (open loop gain)

B) No feedback resistor and diodes leaking a bit = very high gain


C) No feedback resistor and ideal diodes clipping = signal clipped at Vf diode / gain ie maybe 0.65/200,000 V

D) No feedback resistor and non-ideal diodes clipping = signal clipped maybe at 0.65001/200,000 V ie no real difference than ideal diode


What am I missing out on ?



Title: Re: Super low volume in muff overdrive clone. Unsure of cause.
Post by: Fancy Lime on November 26, 2020, 02:19:42 AM
@Vivek
You are missing the diode leakage. Real diodes act as their own parallel resistor. Well, actually more like a constant current device with a series resistor but that also reduces the gain, just not in a linear fashion. Using different diodes will give different gains. Don't expect much useful information in the datasheets on that, though.

Hope that helps,
Andy
Title: Re: Super low volume in muff overdrive clone. Unsure of cause.
Post by: Vivek on November 26, 2020, 03:03:43 AM
You are right !!!

At 0.5V forward bias, a IN4148 appears to be similar to a 5000 ohm resistor

at 2V negative bias, the 1N4148 appears to be similar to 400 Mega Ohm resistor


The theory is right !!!!


The devil is in the details.

So how does that affect our situation when there is no feedback resistor in the loop

I would say a 400 Mega Ohm resistor can safely be ignored in places with 2 Volt signals.

On the other hand, the internal series resistance will create a extremely small difference in the clipping characteristics due to the extremely high gain of that stage.

So both people are right

Those who state that when there is no feedback resistor, the parallel resistors of a diode come into play. Maybe the 400Mega ohm equivalent resistor changes the total gain from 200,000 to 180,000

maybe the series resistor inside the diode changes the output from 0.65V to 0.650001 Volt

and they are theoretically right !!!!!


and those who say that the series and parallel resistors of a diode does not matter when there is no feedback resistor are right too, since the changes between ideal diodes and real diodes in this situation could possible be of the order of 0.00001%


Sure a 400 Meg resistor will make a difference when its in parallel to a 5000 ohm resistor !!

Is it significant ?

Title: Re: Super low volume in muff overdrive clone. Unsure of cause.
Post by: iainpunk on November 26, 2020, 08:43:03 AM
Quote
At 0.5V forward bias, a IN4148 appears to be similar to a 5000 ohm resistor

at 2V negative bias, the 1N4148 appears to be similar to 400 Mega Ohm resistor
i had to check my old school notes and my numbers on the 4148 are a bit different (comments are loosely translated):
0,63V -> below 100 ohm
0,6V FWD -> 3,682 ohm
0,5V FWD -> 18,574 ohm
0,4V FWD -> 25,336 ohm
0V -> more than 1M, less than 2M (bad DMM keeps fluctuating)
-1V -> 9M      \
-2V -> 17M     } constant-ish current sauce
-3V -> 25M    /

they are 2 measurements i did combined in one table, the 0 and negative measurements were done at home as homework, the others are done at school with $800 DMM's, so there is a difference in accuracy

cheers, Iain
Title: Re: Super low volume in muff overdrive clone. Unsure of cause.
Post by: Vivek on November 26, 2020, 08:51:24 AM
I got my data from

https://2n3904blog.com/1n4148-reverse-bias-leakage-current/

But

17M ohms parallel to 5K is same as 400M parallel to 5K


Hence I still believe that when there is no feedback resistor, the effects of internal resistances of diodes can be ignored.
Title: Re: Super low volume in muff overdrive clone. Unsure of cause.
Post by: amz-fx on November 26, 2020, 11:16:32 AM
The corrected schematic for the Muff Fuzz is on my page at:

http://www.muzique.com/schem/index.html

Best regards, Jack
Title: Re: Super low volume in muff overdrive clone. Unsure of cause.
Post by: Vivek on November 26, 2020, 02:02:31 PM
Thank you, Sir Jack !

Title: Re: Super low volume in muff overdrive clone. Unsure of cause.
Post by: iainpunk on November 26, 2020, 02:51:49 PM
I got my data from

https://2n3904blog.com/1n4148-reverse-bias-leakage-current/

But

17M ohms parallel to 5K is same as 400M parallel to 5K

Hence I still believe that when there is no feedback resistor, the effects of internal resistances of diodes can be ignored.
no they can't be ignored, the pedal was designed with germanium transistors, which have anywhere from 10k to 500k leakage resistance, this sets the pre-clip gain of the 2nd stage lower and makes the specific Ge diodes more important for the sound. and using Shockley diodes doesn't do the trick, at least 1 diode should be Ge so the leakage can set the gain.

cheers, Iain
Title: Re: Super low volume in muff overdrive clone. Unsure of cause.
Post by: antonis on November 26, 2020, 03:23:49 PM
Hence I still believe that when there is no feedback resistor, the effects of internal resistances of diodes can be ignored.
no they can't be ignored,
Yes, they can be ignored..
(although I stand for the opposite..)

Who's coming next..?? :icon_wink:
Title: Re: Super low volume in muff overdrive clone. Unsure of cause.
Post by: jdoughty on November 26, 2020, 08:44:46 PM
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

Hahaha, I can say, there is definitely no shortage of clipping in this pedal.

https://m.facebook.com/story.php?story_fbid=10164752297695221&id=903655220 did a post to my Facebook showing it off! It's a rough pedal, but it is fun :)

I'll inform my child he requires eye liner. I don't think he'll mind 🤣
Title: Re: Super low volume in muff overdrive clone. Unsure of cause.
Post by: Vivek on November 27, 2020, 12:03:28 AM
Really great idea to get the son involved !

It's wonderful bonding time.
Title: Re: Super low volume in muff overdrive clone. Unsure of cause.
Post by: Vivek on November 27, 2020, 12:05:19 AM
Are diode models in LTSPICE accurate enough to model leakage currents and effective internal resistances in Diodes ?