Benson Preamp  [traced]

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Benson Preamp

Postby atalas » 24 Jul 2019, 22:53

Hello FSB,
Been lurking as a guest for a while, got lots of great tips and info vicariously. About time I contributed.
A friend loaned me their Benson Preamp, so I'm tracing it for y'all. Here's what I got so far.

bensonpreamp-5.JPG


bensonpreamp-4.JPG


benson preamp.jpg


Really hard to measure capacitance on SMD, if anyone has tips I'm all ears.
I'm fairly certain I got the first two caps correct, the others I can't get a good reading.
Also if someone wants to recommend how to redraw the tone stack, it's a bit too "swastika-ey" for me right now.
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Re: Benson Preamp

Postby Manfred » 25 Jul 2019, 08:56

A common capacitance meter do not read the real capacitance because there is a impedance on the measurement clamps.
In some cases the impedance contains resitors and/or LCR-networks in parallel too.
I use the Peak-Atlas LCR45 - LCR Impedance Meter that could measures the real and imaginary part.
Together with the measure freqency it is possible to calculate the capacitance from the imaginary value.
But you have to be careful, because it works not always especially if there are other LCR-circuit in parallel,
with just one resistor in parallel it mostly works well.
In case of LCR-circuits in parallel there remains only an estimation after separate measurement of these circuits.
lcr45-userguide-en.pdf
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Re: Benson Preamp

Postby vtguedes » 25 Jul 2019, 13:47

Thanks for the tracing!

To my eyes it looks like the tone stack has something in common with the James tonestack from the Tone Stack Calculator website:

James Tonestack.JPG


That would give us the idea of the value of a couple of capacitors.

The capacitor in paralel with the 1k resistor I'd test 680n based on Runofgroove Thunderchief schematic. Also based on the same schematic I'd try 2n2 on the last capacitor, in paralel with the master potentiometer.

I guess coupling caps should be somewhere between 22n and 100n, I'd start with 47n and breadboard it.

I'll try to get this breadoarded this weekend :horsey:
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Re: Benson Preamp

Postby atalas » 25 Jul 2019, 14:06

Manfred wrote:A common capacitance meter do not read the real capacitance because there is a impedance on the measurement clamps.
In some cases the impedance contains resitors and/or LCR-networks in parallel too.
I use the Peak-Atlas LCR45 - LCR Impedance Meter that could measures the real and imaginary part.
Together with the measure freqency it is possible to calculate the capacitance from the imaginary value.
But you have to be careful, because it works not always especially if there are other LCR-circuit in parallel,
with just one resistor in parallel it mostly works well.
In case of LCR-circuits in parallel there remains only an estimation after separate measurement of these circuits.
lcr45-userguide-en.pdf


Gotcha, cool. Can't really swing buying this thing though.
I'm thinking that just trying to come up with educated guess values for the caps and do listening tests is the right path here?

vtguedes wrote:Thanks for the tracing!

To my eyes it looks like the tone stack has something in common with the James tonestack from the Tone Stack Calculator website:

James Tonestack.JPG


That would give us the idea of the value of a couple of capacitors.

The capacitor in paralel with the 1k resistor I'd test 680n based on Runofgroove Thunderchief schematic. Also based on the same schematic I'd try 2n2 on the last capacitor, in paralel with the master potentiometer.

I guess coupling caps should be somewhere between 22n and 100n, I'd start with 47n and breadboard it.

I'll try to get this breadoarded this weekend :horsey:


Yep, 100% agree, I was just messing with the calculator last night.
What I can say through listening, the bass control is subtle.
Yeah I'm seeing a lot of similarity with the thunderchief as well.
I'm gonna double check the connections on the 470p/470K network before Q2.
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Re: Benson Preamp

Postby vtguedes » 25 Jul 2019, 14:59

Still on the tone stack theme, RoG (again) Supreaux Deux bottom switch mod also looks a lot like the bass tone control.

My guess is that the first capacitor on the bass control don't let much bass through it and then the bass potentiometer acts like a bypass to the capacitor. The second capacitor on the bass control should be something like 470pF.
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Re: Benson Preamp

Postby Manfred » 25 Jul 2019, 15:18

Gotcha, cool. Can't really swing buying this thing though.

You don't have to do this, I do not receive commission for. :D
It should be only a hint regarding messurement.

I'm thinking that just trying to come up with educated guess values for the caps and do listening tests is the right path here?

Trial and error always a practical methode.
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Re: Benson Preamp

Postby pietro_moog » 25 Jul 2019, 15:55

It looks like a Wampler Plexi Drive, kind of
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Re: Benson Preamp

Postby atalas » 25 Jul 2019, 17:27

vtguedes wrote:Still on the tone stack theme, RoG (again) Supreaux Deux bottom switch mod also looks a lot like the bass tone control.

My guess is that the first capacitor on the bass control don't let much bass through it and then the bass potentiometer acts like a bypass to the capacitor. The second capacitor on the bass control should be something like 470pF.


bensonpreamp-6.JPG


Got it on breadboard now. I haven't done much breadboarding, but I suddenly understand the desire for one of those contraptions with mounted pots and jacks.
I've used 1nf for the bass cap, seems to work similarly to the original. Like I said, the bass control is subtle, at least with my tele. It reminds me of the bass control on some matchless amps I've played.
I've got 10n in for the treble control, but it isn't quite right. On the original, there's a audible change across the entire sweep, but on breadboard only the last 20% affects the treble. Seems like a lot of builders are using 25K for this pot which would make sense to me, but the benson definitely is a 250K.

Other cap values I'm using are 22n for coupling, 1n for the bypass, 470pf for the gain cap, and 2.2nf for the master cap.
I'm gonna modify the schematic to add component numbers.

Manfred wrote:
Gotcha, cool. Can't really swing buying this thing though.

You don't have to do this, I do not receive commission for. :D
It should be only a hint regarding messurement.

I'm thinking that just trying to come up with educated guess values for the caps and do listening tests is the right path here?

Trial and error always a practical methode.


Cool, yeah I'm A/Bing it with the benson now.

pietro_moog wrote:It looks like a Wampler Plexi Drive, kind of


Sure does :hmmm:
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Re: Benson Preamp

Postby atalas » 25 Jul 2019, 17:48

Sorry for the double post, here is the schematic updated with component numbers.

benson preamp.jpg
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Re: Benson Preamp

Postby Manfred » 26 Jul 2019, 08:32

Really hard to measure capacitance on SMD, if anyone has tips I'm all ears.


BTW, have you measured the capacitances using a capacitance meter?
Even if the read values are not 100% correct they could be of help to find the appropriate values.
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Re: Benson Preamp

Postby atalas » 26 Jul 2019, 13:50

Manfred wrote:
Really hard to measure capacitance on SMD, if anyone has tips I'm all ears.


BTW, have you measured the capacitances using a capacitance meter?
Even if the read values are not 100% correct they could be of help to find the appropriate values.


Sort of, I tried using my multimeter but could only get readings of which I was really confident on C1 and C2. The rest my multimeter just wouldn't read.
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Re: Benson Preamp

Postby vtguedes » 26 Jul 2019, 14:18

Maybe you could try comparing capacitance readings between the original board and your breadboarded circuit?
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Re: Benson Preamp

Postby atalas » 26 Jul 2019, 15:04

vtguedes wrote:Maybe you could try comparing capacitance readings between the original board and your breadboarded circuit?


I hear what you're saying, and I'll try again, but my meter literally would not give readings for some of those caps. As in, the display doesn't change from touching nothing, to touching the contacts on the caps.
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Re: Benson Preamp

Postby atalas » 26 Jul 2019, 19:54

Had some luck recording some capacitor values.
I was able to measure repeatedly ~300pf for C3 and ~490pf for C4. I tried those in circuit and it seems to work similarly to the original.
I measured 31nf for C8. I had 33nf which I tried in circuit.
I think I have C5 at 1.5nf, but I'm less certain about this. I have 1.2nf in there now.
Still can't measure the others.

I recorded some samples to see where we're at:

Soundcloud link

The four samples described in order are:
1) Benson, bass, treble, and gain on full
2) Clone, bass, treble, and gain on full
3) Benson vs clone back to back, still full everything (starts on benson)
4) Benson vs clone back to back, but reduced gain on benson to match clone (starts on benson)

So right off the bat, the Benson has more distortion, more sustain. The drains are all biased closely (~5V for Q1, ~4.2V for Q2 & Q3) and both had fresh 9V batteries (~9.6V).

Interested to hear your thoughts on the samples, EQ-wise.

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Re: Benson Preamp

Postby FlyingWild » 26 Jul 2019, 22:11

This is looking quite exciting!

Any code numbers on the FETS?
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Re: Benson Preamp

Postby atalas » 26 Jul 2019, 22:48

FlyingWild wrote:This is looking quite exciting!

Any code numbers on the FETS?


Code is "P1HYE" which according to this site is SST201 from Temic Semi.

I'm using Fairchild J201 through hole.

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Re: Benson Preamp

Postby Manfred » 27 Jul 2019, 10:01

hi atalas,

could you please measure the voltages on the source pins and also the set values of the trimming potentiometers.
I have doubts as to the accuracy of the read value to C5.
With a value of 1.5 nF the roll-off frequency will then between 300 to 400 kHz.
C7 and C9 should be measurable If the treble potentiometer ist set to the maximum value.
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Re: Benson Preamp

Postby atalas » 27 Jul 2019, 14:15

Manfred wrote:hi atalas,

could you please measure the voltages on the source pins and also the set values of the trimming potentiometers.
I have doubts as to the accuracy of the read value to C5.
With a value of 1.5 nF the roll-off frequency will then between 300 to 400 kHz.
C7 and C9 should be measurable If the treble potentiometer ist set to the maximum value.


Q1 source: 0.366V
Q1 drain: 5V

Q2 source: 0.295V
Q2 drain: 4.1V
Q2 Trim resistance: 5.7K

Q3 source: 0.365V
Q3 drain: 4V
Q3 Trim resistance: 3.9K

I was able to get more solid readings with max resistance to ground on C7 and C9 as you said, and the values make sense. My meter read 24nF for C7 and 2.2nF for C9.

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Re: Benson Preamp

Postby pamaz » 29 Jul 2019, 16:06

hello Guys.
First of all, many thanks to you for this effort. i was hoping to find one of these benson pre to trace , but you have been much faster than me!
Still i have not started a proto, but as a general suggestion, I would try to increase C5 value, as listening to the samples, it seems that the main difference i can hear from the pc ( no headphones on hand just now). is that the clone is
lacking on midbass and a little bit of gain.
Increasing C5 , you 'll get a larger mid low response and a larger gain at the same time. I cannot point out a precise value, but I would expect to get a considerable effect with something in the range of 220- 470 nf .
Thanks again.

Paolo
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Re: Benson Preamp

Postby atalas » 29 Jul 2019, 17:16

pamaz wrote:hello Guys.
First of all, many thanks to you for this effort. i was hoping to find one of these benson pre to trace , but you have been much faster than me!
Still i have not started a proto, but as a general suggestion, I would try to increase C5 value, as listening to the samples, it seems that the main difference i can hear from the pc ( no headphones on hand just now). is that the clone is
lacking on midbass and a little bit of gain.
Increasing C5 , you 'll get a larger mid low response and a larger gain at the same time. I cannot point out a precise value, but I would expect to get a considerable effect with something in the range of 220- 470 nf .
Thanks again.

Paolo


To my ears, the benson definitely has more gain or distortion (not sure what the right term is here), and is brighter. The clone is fatter, doesn't cut quite as well.
I'm thinking the gain/distortion discrepancy should be sorted first, it might be that the brightness(or other perceived EQ differences) is just a byproduct of that.
I've tried a 1u cap in for C5, without much audible change. What other ways should I try to increase the gain/distortion? I know selecting JFETs for Idss and Vp is a thing, should I be doing that here?
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