Carvin Legacy with opamps

[mirosol]

After wasting hours with my Leegazzy build, i started to wonder if we could do the same to that circuit what ROG did with English Channel. Feels like a job i'm not comfortable drawing on my own, so i'd like to get your input.

English channel schem: http://www.runoffgroove.com/englishchannel.png
Britannia schem: http://www.runoffgroove.com/britannia.html
and
Legacy schem: http://gr1ng0.tripod.com/Leegazzy/Calvin_Layupe.jpg

So the goal would be to have the sound and frequency response of a Carvin Legacy preamp, but made with opamps.
+m

[Frank_NH]

I couldn't read your schematic - is this the one?

http://bmamps.com/Schematics/carvin/carvin_legacy_vl100_vl212.pdf

If it is, you should be able to do a FET version like other ROG circuits for the lead channel preamp. If you come up with a circuit, I could try to create a LTSpice simulation of it (I did this for the Britannia recently and it was interesting to play around with).

[mirosol]

Don't know what happened to the original, so i'm posting it here to preserve it.

There is the Leegazzy layout on the main page, but i'm not the only one who's had great issues with it. That's the reason i thought the simulation circuit should be revised.

[Frank_NH]

Thanks for the schematic.  It looks like it follows the Lead Channel for the schematic I posted above.  Interesting with the parallel caps and all.  I don't see anything really unusual about the circuit as it looks to be like a Dr. Boogey style JFET gain stage cascade, except there is no buffer before the tone stack.  Hmmm...no buffer.  Did you audio probe the drains of the JFETs?  Does it lose power after the tone stack?

[Frank_NH]

Ran this in LTSpice. I left off the presence control as it should impact the output too much.  As far as I can see, it seems to work OK.  Lots of gain (no surprise).  I don't think you need the 47uF bypass cap in the last JFET stage (10uF is probably enough).  The modeled J201 JFETs biased at drain resistances from 15K to 33K, so 50K trimmers should be enough.

In the image below, the green signal is a 0.1 V, 800 Hz input signal, and the red is the output.  As you can see, lots of gain, certainly well above unity.

It may help to put a simple JFET source follower buffer in front of the tone stack like the Dr. Boogey.

Hope this helps.

[mirosol]

Not that much. The issue is that i simply couldn't get the circuit working in real life no matter what FETs i had in there. I've written a comment on the layout comments.

You have the LTspice file in store? Now you should replace the JFETs with two TL082s or a TL084 in a manner that the frequency response and the gain remains the same. If you come up with a schematic for that, i'll lay it out asap.
+m

[Frank_NH]

I can play around with my LTSpice model, but the main problem is developing op amp stages that act like FETs (or tubes) when they overdrive.  I suppose that's been a "holy grail" of op amp based distortion boxes since day one.  Despite that you could put in soft clipped op amp gain stages, with the gains matching (approximately) the JFET stages (which I can calculate from my JFET spreadsheet).  It may sound good - or not!