Infinity EVO: a new universal guitar preamp

[DSV]

Hi all!

After getting a degree and thus having more time for DIY, I decided to redisign Infinity, the Universal Guitar Preamp described here:
http://www.diystompboxes.com/smfforum/index.php?topic=74219.0

The goals were to achieve the same tonal versatility, and at the same time solve a few issues with the previous model:
Offer true clean sounds;
Reduce the number of controls and make them more intuitive;
Limit the amount of jfets in use, in order to eliminate the need for selectim them or using trimpots.

The result is a preamp more geared towards vintage sounds (fender, early marshall) but at the same time be capable of delivering metal distortions. Here are some samples:
Fender, bridge pickup : http://www.4shared.com/audio/Rw6nr1bq/fender_bridge.html
Fender neck pickup : http://www.4shared.com/audio/6AUbqY-F/fender_neck.html
Marshall, bridge pickup, then neck : http://www.4shared.com/audio/nTPDdXLm/marshall.html
Mesa, bridge pickup, one of my best riffs : http://www.4shared.com/audio/PsZgzhaY/mesa.html

The cabinet simulator and post-distortion equalizing (5 bands used) are provided by my Zoom G9.2tt, which is then linked directly to the PC. guitar is a Yamaha Pacifica 412, out of tune and with a broken string ...  The transition between clean and dirstortion is obtained through the volume control of the guitar.

The current build (input filters + distortion section) is based on 3 opamps and has 1 control: Infinity, which shapes the distortion from fender to marshall to mesa. A gain and volume control will be implemented, too. I think I'll add a compressor pre-distortion in order to refine the sound, a bit weak on the high strings. A compressor should also be added after the distortion, in order to get roughly the same output when fiddling with the guitar volume control (a bit like the autoclean function of Rockman gear). Finally, the cabinet simulation of the zoom G9 will be studied through a white noise analysis and its frequncy response reproduced (as well as the 5 band eq simplified).

... and a schematic will follow soon 

[Arn C.]

Just Freakin' Awesome!   Wow, great job man!

Peace!
Arn C.

[Brymus]

I just ran through your original thread and I was really impressed.
I do hear an improvment in your present version,even thought the first one was fantastic.
I am seriously considering building it, but am going to wait to see the new schematic first.
Do you attribute this to the EQ and cab sim of the Zoom ?
I have been wanting to build something like your Infinity project,it would be very convienant to play through my PC amp while recording directly.
VST's have come a long way but are still lacking to analog filtering and distortion.
The tough part would be matching the DI sound to a live sound
Me and my drummer collaborate via recordings, and the live sound of us together is never as good.

[stringsthings]

looking forward to the results ! 

[Processaurus]

I think I'll add a compressor pre-distortion in order to refine the sound, a bit weak on the high strings. A compressor should also be added after the distortion, in order to get roughly the same output when fiddling with the guitar volume control (a bit like the autoclean function of Rockman gear). Finally, the cabinet simulation of the zoom G9 will be studied through a white noise analysis and its frequncy response reproduced (as well as the 5 band eq simplified).

For the high strings being weak, consider some careful pre-distortion filtering instead of a compressor, to get the balance right.  That's one of the important choices, across the board, with distortion pedal design, for instance the rat and TS9 have pretty radical bass rolloff in the initial gain stage.  The Rangemaster, (just a simple Hi pass filtered booster)for instance, was originally sold to solve just that problem of the hi strings sounding weak compared to the lows.  One could even do it scientifically, by figuring out the Hz of the note where the boosting should start.

A compressor after a distortion isn't nearly as useful as before, because distortion IS compression, in its crudest form, ie, it remove dynamics by clipping off transients.  Compressing something without dynamics won't have much effect.

[DSV]

Thanks for the comments, and here is the schematic:


@ Brymus
It should be possible to reproduce the response of the Zoom through analog circuitry: there is no apparent compression/distortion when using the filters. I think the improvement comes form:
less gain
better pre-distortion filtering, which has both variable frequncy and Q
good compression of the sound when being at the transition between clean and distortion

@ Processaurus
I know about pre-distortion filtering ... it took me 3 months to fine tune the Infinity control
Weak was a bad term. Shrill is more like it, with a too sharp attack and not enough sustain. I tested some compressor configurations, and it would be difficult to find a simple compromise for all the settings. I'll play with:
clipping devices (imho 3mm LEDs tend to add some compression).
high frequency filtering (> 10kHz), which is not yet implemented.
better gain distribution between stages.
A post-distortion compressor is imho necessary: gain is pretty low, so the dinamics of the signal are still very strong, especially for Fender-like sounds. A 20dB range + ratio around 5-6:1 should be enough. It will also help with the aforementioned problem for high notes. Only downside is noise and, of course, getting it right.

[rotylee]

any changes to pcb previously posted?

[Gus]

The 2nd stage(1st gain stage as marked on the schematic) does not work as you have the filter drawn.   It is a gain of 1 until the RC set filter starts to work.  The input opamp need some protection. Also look at books about input pairs and how to keep them balanced.