New Fet preamp schematic

[Gilles C]

I just noticed this new project on Red Circuits.

http://www.redcircuits.com/Page120.htm

18-30V... he's never been affraid using higher voltage 

Gilles

[trevize]

it's a clever use of fet. using mu amp stages intead of simple gain stages you have self biasing circuit and you're not afraid of changing voltage. I suppose it would be also possible to have a rotary selector for different voltages, to modify headroom.

[markm]

So, with 9 volts I suppose this would be a bit grittier sounding 

[Stompin Tom]

I've noticed it before and thought about making it my preamp for a chipamp... but I wonder if it sounds good? I wasn't able to find any sort of a review of it on the site... It's like he never even built it... Unless I'm missing something.

[petemoore]

  LArge C1 and C7 for Mu Bassiness.
  Small [compared to other Mu's] bias divider resistors [100k].
  I wonder if people designing dual Mu circuits, prefer other 'classic mu bias' to RG's 'noiseless biasing' [two really big divider R's between V+/- with a 'big resistor to each Mu bias transistors gate, or they haven't seen it.
[four resistors are used, with either method of biasing 2 Mu amps].

[Gilles C]

So, with 9 volts I suppose this would be a bit grittier sounding 

Right...

I've noticed it before and thought about making it my preamp for a chipamp... but I wonder if it sounds good? I wasn't able to find any sort of a review of it on the site... It's like he never even built it... Unless I'm missing something.

From what I think or suppose, he builds at least one, because he always knows how much current the circuit uses, and a few more details like that.

He also likes to know when someone built one of his circuits with success. He also likes to have someone test a circuit before he publishes it on his site, but I don't know if he does that for every circuits he designs.

He's the only one who could answer that for sure...

One thing I know for sure, is that if you email him for details, he will answer you.

I would like to know how this preamp circuit compares in sound with the other circuits already built by people here on this forum.

Gilles

[Gus]

I see two possable issues with 2n3819's   They are only hints

Specs are all over the place for a 3819 lot of hand tweeking

People overlook the d to g voltage problems with fets like the 3819.

[Gilles C]

Hummm...I don't remember having problems with FETs in this configuration.

I even played with the Mini-Booster circuit many years ago when Jack first published it, and one of the things that I like was being able to vary the supply voltage without having to change any part.

Pete is right though about using the 3 resistors for the bias instead of 2 large ones. I don't see that used very often.

Gilles

[JHS]

I think the Randall marshall-type FET-preamp is the better way to go, runs on 33V but 27V is fine too (9V supply and charge pump with diode/cap voltage multiplier will work fine, for 33V this cascade has to much loss).

BTW:
both circuits are preamps. If you want to use it as an OD-stompbox it must be modified, different tonestack values and maybe a high cut filter (app 5-6kHz) before the output.

JHS

[Gus]

Here is a story for you.  One of my hobbys is buildng condenser microphones tube and solid state.

I once bought 1,000 2n3819s so far out of the maybe 500 I tested 4 that have readings that will give  good operating points for the circuit.

Yes you can make fets work by adjusting things but the input and output swings change and with the 3819 more than 8 or so volts gate to drain increases leakage and noise.

Now take your j201 better gate to drain voltage before problems and MUCH tighter specs.

[Gilles C]

Good points JHS.

Nice story Gus. But do you mean you bought 1000 FETs for a circuit without testing some of them before?

Now I know why you seem to don't lke this FET. You have a good reason.

What I did when I began building circuits with FETs is I bought a few items of many numbers. I then went back to the store to buy a batch of the one that I found was working the best with the circuits I was building. But I admit I never built enough circuits to buy 1000 FETs all at the same time. 100 FETs is enough for me for quite a while.

The first circuit I built with FETs was the Mini-Booster, and it worked with all my FETs, but gain was different. The circuit that made me choose the FETs I liked was the Fet Muff I think.

Btw, I never used 2N3819s, so I would have used J201s to try that circuit. Maybe I just got lucky with the FETs I used.

Gilles

[Threefish]

Hi all,

I noticed this preamp last week and posted a query over at ssguitar (hi Stompin Tom), and got encouragement to try but no builders so far. I'm still considering it as a preamp for my lm1875 chipamp, either that or a higher voltage Professor Tweed. I have a 22.5v rail in my chipamp, so I have access to that higher voltage. I'm after a mainly clean or that just-on-the-edge sound, or at least a usable range for when I'd like to play a bit louder clean (as loud as I can get with the ~20w from an lm1875 anyway). The buffer at the output looks like a good idea. I'll be having a go at this over the next few weeks, so I'll post results.

I've queried the redcircuits guy in the past about the 10w combo on the site, and like Giles C said, he does answer. I haven't bugged him about this though, as I don't have easy access to 2N3819's, and from what I've read here, they don't sound worth trying to me. I think I'll perf the circuit with sockets and try different FETs I usually use (MPF102, 2N5484, 2N5485). I realise the results will be different to the published circuit or may not work at all.

[Dragonfly]

...you could probably get decent results by using a 9v battery with a charge pump, like a maxim1044....that'd get you 17-18v, which should work fine....


just a thought...someone may have a better idea though...
   AC

[Gilles C]

I have too many outputs on my Spyder style Power Supply. 9V+9V=18V+9V=27V...

I also have a 30vdc PS I made from a wall-wart for a Tube Shaka Bradah. Everybody should have a 30Vdc power supply these days 

I got tired of seaching for batteries at home while I was using an ajustable power supply at work. I find a multi 9V PS fun to use at home now.

Btw, a reason I would find this preamp useful is to test different Tone stacks. I have to try a Marshall style of tone control someday. Or any other tone controls I could calculate with the Duncan TSC program anyway.

I'll be waiting for reports because I am sure I won't be able to build it soon.

Gilles