MXR Distortion+ breadboard diode question

[coldbringer13]

I am breadboarding a MXR distortion+ based on the Beavis Audio layout but I keep running into a problem with the diodes (at least I think its the diodes).    When I place the 1N914 diodes in the designated positions the volume fades when the note is sustained.  I removed the one of the diodes and it works great/sounds great - better response with the potentiometers and no fading.   

has anyone attempted this and noticed this problem?  I have re-laid the breadboard several times and i have been constantly reviewing it looking for a mistake.  Any insight or tips would be greatly appreciated.


https://web.archive.org/web/20130401090013/http://beavisaudio.com/bboard/projects/bbp_MXRDistortionPlus_Rev_1_1.pdf

[GiovannyS10]

Man, i had a problem like this when i tried to build a Dist+ with silicon diodes. But i used 1N4148. But my problem was not with the volume, but with the gain. The bass strings distort few and the treble distorts a lot. My advice is, use germanium diodes. Works so much better. I used a couple of 1n270. You can use 1n34a too. You will feel the difference on the sound 

But only for troubleshoting, can you post pics of your build? You changes the resistors values? Maybe you grounded something wrong... I can't see much, i never worked with 1n914 before.

Hope help, good day!

[coldbringer13]

I'll get some better pictures when I get home this evening.  I'll also try the different diodes - thanks for the tip.

I haven't changed any of the resistors.  Perhaps I'll fool around with that as well.

[ElectricDruid]

1N914 diodes are pretty much identical to 1N4148. You won't be able to tell them apart.

Assuming the rest of the circuit is ok, the diode choice will affect the output level, since the signal will be clipped at the forward voltage of the diodes. This varies widely from 0.6V or so for silicon diodes like the 1N914/4148 up to more than a couple of volts for some LEDs. The way they turn on/off is different too, so you might find some choices are more or less "spattery" when a note decays away.
The other crucial component in this circuit is R7, the 10K resistor. This softens the clipping of the diodes and effectively makes them turn on slowly not quickly, giving a more rounded waveform. There's a good page on this on the ESP site:
http://sound.westhost.com/articles/soft-clip.htm

HTH,
Tom

[GiovannyS10]

Assuming the rest of the circuit is ok, the diode choice will affect the output level, since the signal will be clipped at the forward voltage of the diodes.

Yes, i tried to say it! When i made my Dist+ i tried some diodes. Few seems me too weird, but i really like the 1n270 sound.

The other crucial component in this circuit is R7, the 10K resistor.

Yes i was thinking about this too, because i asked about resistors...

But if it is not a diode/resistor problem, by now i don't know what can be.

[coldbringer13]

Not sure this will help but i appreciate the advice.  I am going to try switching out diodes as well as the 10K resistor. 

https://www.dropbox.com/s/uir006ba8mgz9lk/breadboard2.JPG?dl=0

https://www.dropbox.com/s/vdwpk7mybsh05i5/breadboard%201.JPG?dl=0

[Quackzed]

it might do this if you accidentally had the diodes in series like if the diodes and vol pot wasnt grounded on one side...i'd  check the grounds near those components ..

[MrStab]

it might do this if you accidentally had the diodes in series

kinda what i'm thinking, sounds like the signal falling below forward voltage as it decays. does the volume go fully-silent when you turn it all the way down, or can you still faintly hear it?

as well as double-checking that the connections to the strips at the side are correct as per the diagram, make sure each group of side-holes does indeed connect to each other internally! make sure they ALL have continuity to ground. if they don't, then the signal would go through the volume pot and loop back upon itself through D1, D2 and C6. as the signal on one side of the Volume pot will have more resistance than the other, the diodes wouldn't simply be shorted out.

...although, if this were the case and the breadboard's side pins aren't all connected, you wouldn't get any power on the opposite side. unless they're only connected in groups of 3 columns. in which case R5 isn't grounded either.

been a while since i've debugged so it's possible i've just confused myself and am talking through my behind. i don't really know what this scenario would sound like.

[coldbringer13]

I made it work last night.  I triple checked placement and connections  - everything was fine.  I replaced the 10K resistor with a 2.7K and it works.  Success!!!   next step is to play around with different diodes and see what works and sounds the best.

Thanks for all you help.