Stealing an amps distortion channel for pedal use

[gutsofgold]

I am working on copying the distortion channel of a Fender Frontman Reverb amp for pedal use. I got me a schematic and I believe I have found the distortion channel on it. Please check the image below... portion in red.



Looks like one half of an opamp setup with variable gain (GAIN KNOB) fed into the other half of the opamp that has diodes as clipping in the negative feedback loop. This is then run into the overall VOLUME KNOB.

So... how'd I do? Just making sure I have good understanding here before I dive into the breadboard.

[MikeH]

Don't know if it's just me, but no image is showing up

[anchovie]

It's not just you.

[CynicalMan]

Don't know if it's just me, but no image is showing up

It's not just you.

I can see it. 

The first opamp stage does look like a gain stage but there's also some filtering going on. It has a very low input impedance, which means you would need a buffer preceding it. Q2 should be removed as it is only for switching. The op amps are running on +/- 15V, so, if you're running it on 9V (+/-4.5V) it will sound different. Other than that, it looks OK.

[anchovie]

I can see it. 

It's showing for me now. Maybe Imageshack was being weird.

[cab42]

I am in a process of doing the same thing with the overdrive channel from my Guyatone Flip 2000 amp.

Its has been quite a long process, it has been on my breadbord since february 2008, but a few months ago I got the correct transistors (2SK30) so I hope to finish it before my next gig in august

Regards

Carsten

[gutsofgold]

Don't know if it's just me, but no image is showing up

It's not just you.

I can see it. 

The first opamp stage does look like a gain stage but there's also some filtering going on. It has a very low input impedance, which means you would need a buffer preceding it. Q2 should be removed as it is only for switching. The op amps are running on +/- 15V, so, if you're running it on 9V (+/-4.5V) it will sound different. Other than that, it looks OK.

It's a big image so give it a minute please. I did notice that and I figured I'd steal the tube screamer in and out buffers for it. Not sure whats going on with that filtering so I think I'll skip it and add filtering in as needed. As long as I am on the right trail with the hole variable gain opamp stage feeding into the opamp stage with clipping diodes in neg. feedback then I am confident to put it on breadboard.

Thanks man!

[gutsofgold]

Are D1 and D2 used for clipping? or otherwise? I can see that D25, D26, D3, and D4 in the feedback loop of stage 2 are used as such, not sure on D1 and D2 though.

[Renegadrian]

...I hope to finish it before my next gig in august

August?! Man, you got time to reharse!!! I couldn't stand such a long time to be on stage!!!

[WGTP]

I have been meaning to post to this thread.  I tried this several years ago when the design came up.  Sounds pretty good as I recall.  Drew it up in some designs.  I think the idea is that the feedback loop clips at lower setting and then the diodes to ground get going as it is turned up.  Also checkout Mark Hammers Chaos and RoseyRay.   

http://www.urinals.com/gallery/main.php/v/WGTP/AlteredMuff.jog.HTML

http://www.urinals.com/gallery/main.php/v/WGTP/Diagram.GIF.HTML?g2_image ViewsIndex=1

[puretube]

I have been meaning to post to this thread.  I tried this several years ago when the design came up.  Sounds pretty good as I recall.  Drew it up in some designs.  I think the idea is that the feedback loop clips at lower setting and then the diodes to ground get going as it is turned up.  Also checkout Mark Hammers Chaos and RoseyRay.   

http://www.urinals.com/gallery/main.php/v/WGTP/AlteredMuff.jog.HTML

http://www.urinals.com/gallery/main.php/v/WGTP/Diagram.GIF.HTML?g2_image ViewsIndex=1

this...
cleaner?

[grapefruit]

I could be missing something, but from what I understand about op amps if you assume Q2 is switched fully on (zero ohms) then D1, D2 are connected directly to the inverting input of the op amp. Wouldn't this mean that the signal will never reach the threshold voltage of the diodes because the op amp will try to keep its inputs at the same voltage??

I know that there will be some on resistance for the FET but surely the design isn't relying on this.

Cheers,
Stew.

[PRR]

> if you assume Q2 is switched fully on (zero ohms) then D1, D2 are connected directly to the inverting input of the op amp. Wouldn't this mean that the signal will never reach the threshold voltage of the diodes

Good catch. Yes, D1 D2 are NOT distortion diodes (that is the job of D3 D4 D25 D26).

D1 D2 are there so that when Q1 is "open", and U2-A is slapping big swings, Q1 doesn't become turned-on and conduct signal. We see this in large JFET-matrixed mixing consoles (Steve Dove explains the idea).

[WGTP]

Hmmmm.  Here is the one I was referring to.  The post by ahermida.  No jfet in the diagram.  There is some discussion on down the thread about it.  Also, lots about Mosfets as clippers.  

http://www.diystompboxes.com/smfforum/index.php?topic=38581.20

IIRC the clipping threshold is pretty high (around 2v), slightly more than just the LED's would be.  I'm assuming that the 220k/SI diodes in parallel make the clipping softer than the LED's would be by themselves.  The 1N4448 diodes are about the same as the std. 1N4818 diodes typically used.  The diodes to ground before the feedback loop circuit could be switched in/out, as well as, the 220k/SI diodes. 

[bancika]

Just curious, why would you want the sound, frontman amps aren't considered to be very good.

[Joe Hart]

Any updates on this??
-Joe Hart

[MikeH]

Just curious, why would you want the sound, frontman amps aren't considered to be very good.

I've found that many small solid-state fender amps have great sounding distortion, but I thinks it's usually due to the 'fender champ' effect, meaning low wattage amps with small speakers break up really nicely, and generally don't get loud enough for you to notice how bad they can sound (heh heh heh).  I have a fender squire 15 practice amp that I've had for a really long time, and I always thought (still do really) that the distortion sounds absolutely glorious.  Tight, crunchy, super-saturated with harmonics.  I swear. No lie.  I decided I wanted that sound for my live rig, so a friend and I went through all of the trouble to put a proper line out on it, and properly disconnect the speaker, so that it would just be a preamp.  Then I ran it through my v4.  Pure fizzle crap.  Sounded awful.  Tried on the twin.  Even worse.  I don't know what it is, but those small amps play a good trick on your ear, and that sound just doesn't translate to stage volume for whatever reason.