Welcome, Guest. Please login or register.

July 24, 2021, 10:57:39 AM

Login with username, password and session length

Recent Posts


Author Topic: hybrid preamp idea  (Read 8835 times)


  • Legendary
  • ******
  • Posts: 2184
  • Chip Points: 289
    • View Profile
    • Australian Valve Amps
Re: hybrid preamp idea
« Reply #15 on: October 11, 2014, 06:35:54 PM »
Quote from: phatt
Hi Roly, Gee you have a sharp memory. 8|

(heh heh) not bad in the face of advancing senility.    :loco

Quote from: phatt
I figured I'd better not post the circuit

Patents are in the public domain.  It's a major reason why some companies don't patent their developments.

Some interesting tones there.   :dbtu:
If you say theory and practice don't agree you haven't applied enough theory.


  • Elite
  • ****
  • Posts: 280
  • Chip Points: 40
    • View Profile
Re: hybrid preamp idea
« Reply #16 on: October 11, 2014, 10:44:59 PM »
The diodes in the feedback loop of a non-inverting opamp has an interesting characteristic. The output contains the clipped signal that is formed across the diodes with the original signal added in without distortion. Is this why the Tube Screamer is so popular?

J M Fahey

  • SSGuitar Global Mod
  • Legendary
  • ****
  • Posts: 4142
  • Chip Points: 429
    • View Profile
Re: hybrid preamp idea
« Reply #17 on: October 12, 2014, 02:59:51 AM »
Among other things,yes.
Signal is not that destroyed, a little of the original one is still present.

Also, the louder you play (which makes it fuzzier) , you also increase the amount of clean signal mied in.

No Tube sound by any means but results are quite acceptable.

It also cuts bass and low mids below some 700Hz (4k7/.047 crossover) before clipping, and cuts highjs (buzz) afterwards.

Notice that when Tone is set on brightest , it's actually flat because treble boost compensates treble cut ... and loses the fight above some 3kHz ; while on bassiest it heavily cuts mids and highs.

Not NASA technology but a well tweaked pedal.

By contrast, the MXR Dist+ , the other late 70's classic, clips fully and has no tone control, although it still has the same pre distortion low cut .

Anyway they are only half the equation, both became famous as front ends for tube amps: Stevie used a TS pushing an already overdriven Fender, 70s and 80s British heavy rockers (think Judas Priest, Iron Maiden, most others) used MXR (they called them the meat and potatoes pedal) to further push balls to the walls Plexi and JCM800 Marshalls .

In both cases, the final distortion comes from overdriven tube power amps.

Many kids who read too many Forums bought them, plugged them into practice SS amps and were disappointed, that gave birth to a boutique pedal industry which actually delivers 1%  of what it promises ... because the old pedals were very good, just sometimes poorly used.

The same MXR pedal giving glorious British Metal tones was used by Jerry Garcia, driven by a fat Gibson (read farty, results with a Strat would have been acceptable) into a clean Fender .... with ultrabright JBL speakers.

Sonic results remind me of a visit to the dentist the day he run out of anaesthesia and his drill was particularly bad setup :(

It's not the size of the tool what matters but how you use it.


  • Legendary
  • ******
  • Posts: 2204
  • Chip Points: 272
    • View Profile
Re: hybrid preamp idea
« Reply #18 on: October 25, 2014, 06:16:31 AM »
Another update,,
I have built the circuit into a box and it will get a live gig test next week which is the real test.  wish me luck ;)

For the build, I've used the transformer output with Led torch for this first build but deleted the limiter circuit as it's not really needed. (The BD139 and associated bits) You still the diodes for rectification and that works fine.

Meantime I've been tinkering with the circuit still on the test bench so here is what I've found.
Depending on what LED you use you may not need the transformer so best to test this out as not all Led's are equal.

This circuit is a delicate balance between the light intensity and the type of LDR as well as the value of Trim 1 which is the gain set for U4. Tweaking of VR3 and trim 1 will define the limiting points. IF trim 1 is large it will pump and sound like an awful cheap compressor.

My advice, setup a test circuit with trim pots to find the best settings. You can delete buffers U1 and U3 but the hiss does increase.

If you make R7 & R8 trim pots it will allow you to experience how important it is to have some resistance before clipping diodes, low values here will increase the distortion but it will become harsh.
So try it you will be surprised at how much effect it has on the sound produced.

This is well worth reading for those who wish to understand clipping diodes in circuits.

I doubt this circuit will ever compete with a real Valve setup but it certainly improves the feel of the interface between the player and the instrument/amp.

I should mention that I'm using this *after* a dedicated preamp so the input is low if you want to use this after a distortion pedal or whatever then just change R2 = 1Meg and R5 = 1k.

To setup the input sensitivity, set guitar volume half way and use VR1 so that the Led just starts to flicker on big strums then you use the guitar volume to control the dynamic feel.

Posting the built version and the modifications on the test circuit..
(when the sun comes out I'll take some photo's of the build)
have fun, Phil.