Benson Preamp  [traced]

General documentation, gut shot, schematic links, ongoing circuit tracing, deep thoughts ... all about boutique stompboxes.
bmxguitarsbmx
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Post by bmxguitarsbmx »

andy-h-h wrote:Any suggestions as to what capacitors should be used to get the same response using a 1M pot instead of 2M or a dual 1M? :hmmm:

Thanks.
Scale all resistors by 0.5, all capacitors by 2. I do mean all resistors and capacitors in this network, including gain pot.

knottypine
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Post by knottypine »

atalas wrote:
knottypine wrote:Is there a suggested voltage for Q1?

Pedal sounds pretty nice in front of my Music Man with Bass, Mid, and Treble set to 5
~5V, or if you have a bunch of J201, set the source resistance to 12K and swap JFETs until you get one sit at 5V at the source. This will achieve a closer sonic result than just changing the source resistance to achieve 5V for whatever JFET is in there now.

Could someone mock up a stripboard layout with a trimmer for Q1? ✌️

atalas
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Post by atalas »

knottypine wrote:
atalas wrote:
knottypine wrote:Is there a suggested voltage for Q1?

Pedal sounds pretty nice in front of my Music Man with Bass, Mid, and Treble set to 5
~5V, or if you have a bunch of J201, set the source resistance to 12K and swap JFETs until you get one sit at 5V at the source. This will achieve a closer sonic result than just changing the source resistance to achieve 5V for whatever JFET is in there now.

Could someone mock up a stripboard layout with a trimmer for Q1? ✌️
You could just bodge in a trimmer instead of the 12k. Maybe use short little jumpers, or even make it an external control.

chasbenson
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Post by chasbenson »

Bias at 4v....5v sounds like shit

Manfred
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Post by Manfred »

Try a value of 14k for the drain resistor of Q1 that gives a drain voltage off about 5.9 Volts.

atalas
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Post by atalas »

chasbenson wrote:Bias at 4v....5v sounds like shit
~5V is what I measured on the unit I had for Q1. Sounded alright to me.
Cool thing about a trimpot is it let's people decide for themselves what "sounds like shit." :thumbsup

FlyingWild
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Post by FlyingWild »

I put a 2K2 in series with a 25K trim pot for Q1 and this gives me a bias voltage range of 3.18v - 8v. I'm using SMT J201.

Thank you chasbenson for providing the biasing voltages, and for clarifying aspects of this circuit. It's a very compelling design to play, especially with all the shades you can get just with the use of the guitar volume knob.

Manfred
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Post by Manfred »

Manfred wrote:Try a value of 14k for the drain resistor of Q1 that gives a drain voltage of about 5.9 Volts.
I found that value of 14k not by hearing but by measurement the output of the Q1-stage using an oscilloscope.
At this value I achieved the maxium headroom the clipping starts at obout 1.5 Vpp input voltage,
thereby the clipped signal shape is very symmetrical.
I put a 2K2 in series with a 25K trim pot for Q1 and this gives me a bias voltage range of 3.18v - 8v. I'm using SMT J201.
I agree with you, it therefore makes sense to replace the resistor bei a potentiometer to balance out the variations of the J201 Fets.

FlyingWild
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Post by FlyingWild »

Hello Manfred,

I dug out my oscilloscope this morning and a 5.2V drain voltage gives me symmetrical clipping, but as this adjustment was done via the trim pot I can't say what the resistance value is. And to be fair my oscilloscope skills are a little limited, I used the tone generator in the oscilloscope to give me a sine wave, set the output level low so there is no clipping of the sine wave after Q1 then increased the tone generator output until I noticed one half starting to flatten, then adjusted the trim pot in for Q1 until both halves of the sine wave started to flatten at the same time. I've just discovered the triangle wave form is perhaps easier to detect early clipping.

The question I'm left with is Mr. Benson's design intended to be that Q1 provides maximum head room, or is his intention that there be some asymmetric clipping at Q1?

Manfred
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Post by Manfred »

FlyingWild,
I dug out my oscilloscope this morning and a 5.2V drain voltage gives me symmetrical clipping, but as this adjustment was done via the trim pot I can't say what the resistance value is. And to be fair my oscilloscope skills are a little limited, I used the tone generator in the oscilloscope to give me a sine wave, set the output level low so there is no clipping of the sine wave after Q1 then increased the tone generator output until I noticed one half starting to flatten, then adjusted the trim pot in for Q1 until both halves of the sine wave started to flatten at the same time. I've just discovered the triangle wave form is perhaps easier to detect early clipping.
I did it the same way.
The trim pot setting can be measured using an Ohmmeter.
The question I'm left with is Mr. Benson's design intended to be that Q1 provides maximum head room, or is his intention that there be some asymmetric clipping at Q1?
This Jfet amp circuit is inspired by a tube amp circuit, therefore, I presume that the output signal of the Q1 stage should be remain undistorted for a wide input level range
and the clipping is being generated by the follwing stages.

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