FET preamp Observations and potential enhancements

[glasman]

Been testing various front end preamps and have done quite a bit of testing with the FET preamps.

For points of reference here are the test conditions.

Vcc = 18V
Vd = 9v (biased adjusted to provide Vcc/2)
2N4416A used (fwiw, the 2N5486 gives exactly the same results and performance at about a 1/10 the cost). I did not check the noise performance of the two FETs and this could be the deciding factor.

Basic circuit performance (HAD original circuit).

Midband gain ~ 19dB
LF -3db point = 80Hz
HF response shows a dropoff starting at about 1kHZ and drops by -1dB at 5khz and remains flat to 100khz. this HF dropoff is created by the HF preemphasis (100pf||1Meg) on the front end of the preamp. Removing the 100pf is not a solution to the dropoff as it limits BW to under 10kHZ.

If it were me...... I would make the following changes....


1. Change the 100pf to a 470pf. This flattens the HF response.
2. Change the 4.7uf source bypass cap to at least 10uf or higher this extends the low end response.

[Bob-I]

I've been using a BYOC triboost on my pedal board. For those who don't know this pedal it's got 3 clean boosts, FET, OP-Amp and Germ Rangemaster type. There are some really cool tones to be found with these boosts, decidedly non-Dumble but very cool.

Thx for the analysis Gary.

[markusw]

Thanks a lot for sharing your observations!

Peace,

Markus

[Structo]

I've been using a BYOC triboost on my pedal board. For those who don't know this pedal it's got 3 clean boosts, FET, OP-Amp and Germ Rangemaster type. There are some really cool tones to be found with these boosts, decidedly non-Dumble but very cool.

Thx for the analysis Gary.

Thanks Gary, I will build an FET board one of these days to complete my ODS clone and your notes will help.

Bob, I was going to build a TriBoost a couple years ago but never did.
I did build a BYOC Tube Screamer with some of the tweaks and a BYOC Fuzz Face.

How long ago did you build the TriBoost?

Good quality kits those are.

[Bob-I]

How long ago did you build the TriBoost?

Good quality kits those are.

Maybe 2 years. I mostly use it with my little Princeton Reverb but it works well with the Dumble's too.

[Dr d]

Thank you Gary. Your topic is very timely!!!

[martin manning]

Good stuff, thanks! Is your source resistor about 2K? The value of the bypass cap to get the full low-end would depend upon that, right? OTOH, maybe a little hi-pass on a booster isn't altogether bad?

[glasman]

Good stuff, thanks! Is your source resistor about 2K? The value of the bypass cap to get the full low-end would depend upon that, right? OTOH, maybe a little hi-pass on a booster isn't altogether bad?

Based on a Vcc of 18V.

For the 2N5484 device I ended up with a 2.74K resistor
On the 2N4416A it was closer to 2.2K.

For my final design, I used a Vcc of 16.5V as I will be testing the preamps in two different amps, one with a 15V supply and one with an 18V supply (splitting the difference), in that case a 3.3K was the best compromise.

Yes it is be possible to roll off the bottom a bit, but for my testing, I wanted the preamp to be as flat (and hopefully transparenet) as possible. I am comparing the FET to a OPAMP design that I put together (single supply design). I am testing several different opamps in the design, OPA134, OPA604 etc etc.

Gary

[jaysg]

I am comparing the FET to a OPAMP design that I put together (single supply design). I am testing several different opamps in the design, OPA134, OPA604 etc etc.

I've built a number of op amp boosters. I can always hear the coloration no matter which op amp I've tried. The compromise is to make it an always on device, so that it's switched from unity gain to whatever, so you can adjust the color out....how to put this...it's less obvious when it's always on.

[Structo]

Gary what is your goal for this input?

IOW, is it best suited for an piezo pickup on an acoustic or other lower impedance devices?

[glasman]

Gary what is your goal for this input?

IOW, is it best suited for an piezo pickup on an acoustic or other lower impedance devices?

The goal is to provide an additional gain stage before the first tube stage. I have a customer that wants a little extra push into the front end of the amp for his violin and a couple of other instruments. Nothing new, TR did this a few years ago, I have been sitting on the board designs since 2007 and just now getting around to testing them.

My goal is to not have a phase reversal from the stage, that is the reason why I am also evaluating opamp circuits.

Both designs (FET and Opamp) are on a 1.5" x 1.5" boards with switching capability.

[Structo]

OK, so this is cascading with the first tube stages?

Do you have it on a relay?

[67plexi]

Gary, this is a plug and play solution that works great for me and has the most amazing mid boost.
No coloration of the input signal. Adjustable input and output level, channel switching.
http://www.tonebone.com/tb-switchbone-using.htm

Steve.

[glasman]

OK, so this is cascading with the first tube stages?

Do you have it on a relay?

Yes and yes. It can be placed on the footswitch if so desired.

Gary

[glasman]

Gary, this is a plug and play solution that works great for me and has the most amazing mid boost.
No coloration of the input signal. Adjustable input and output level, channel switching.
http://www.tonebone.com/tb-switchbone-using.htm

Steve.

Cool idea, but probably not what my customer is looking for at this time. But I will forward the link to him.

Gary