Benson Preamp  [traced]

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Nocentelli
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Post by Nocentelli »

knottypine wrote:Could I use 2N5457 in lieu of J201?
May be worth trying

knottypine wrote:How about BS170 MOSfet?
Probably won't work
brownwhopping wrote:How can I learn by reading threads an making circuits, when some day I can see a lawsuit or somebody beat me in the face for that?

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temol
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Post by temol »

Here's some Ltspice simulation screenshots.
Comparison between J201 and 2N5457. Drain resistor values adjusted to get the same voltages as with J201.

Frequency response
benson1.jpg
Output sine.
benson2.jpg
benson3.jpg
Sine at drain of Q1. 2N5457 in green.
benson4.jpg
Sine at drain of Q2. 2N5457 in green.
benson5.jpg
Sine at drain of Q3. 2N5457 in green.
benson6.jpg
T.

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Post by Manfred »

I tried to adapt the circuit values in order to get the same stage gains using the 2n5457 Fets.
The adaption of the drain and source resistors is only possible when the supply voltage is 18 Volts.

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temol
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Post by temol »

It may be cheaper to use charge pump than buy MMBFJ201 transistors :)

T.

Manfred
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Post by Manfred »

BensonPreampWamplerOutputCircuitry.jpg
I renamed the components of the Wampler Output circuit.

I made these Layouts according the componet names in the schematics.
BensonPreampLayout.JPG
BensonPreampLayoutComponets.pdf
(20.41 KiB) Downloaded 56 times
BensonPreampLayoutSolderSide.pdf
(3.67 KiB) Downloaded 47 times
BensonPreampLayoutSolderSideMirrored.pdf
(3.67 KiB) Downloaded 39 times
BensonPreampWamplerOutputCircuitryLayout.JPG
BensonPreampLayoutWamplerModComponets.pdf
(20.64 KiB) Downloaded 52 times
BensonPreampLayoutWamplerModSolderSide.pdf
(3.76 KiB) Downloaded 39 times
SprintLayout6 file contains both versions:
BensonPreamp.zip
(40 KiB) Downloaded 61 times

knottypine
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Post by knottypine »

Because I am a bit impatient I went ahead and wired one up with what I had so buckle up...

I didn’t have a:

250pf so I used a 300pf

470pf so I used a 680pf

J201 so I used 2N5457 (Q2 & Q3 biased at 4V)

2M so I used a 1M for the bass pot

What I’ve observed is....

Decent output volume

Very subtle (if any) Bass response

Treble control seems to work in reverse (fully clockwise the sound is “dampened”, moving it counter-clockwise “brightens” the sound)

Drive is pretty distorted even on the lowest settings / starts to squeal past 12 o’clock

I have on order the correct values/replacements so I will implement and report back.

I am very excited at the potential of this circuit and I appreciate this forum and everyone involved.

Cheers!
Last edited by knottypine on 20 Aug 2019, 04:30, edited 2 times in total.

atalas
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Post by atalas »

temol wrote:It may be cheaper to use charge pump than buy MMBFJ201 transistors :)

T.
More importantly, it may make other, more readily available JFETS viable.

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Post by Manfred »

I tried different drain and source resistors at supply voltages greater than 9 Volts.
It is at least 30 Volts requiererd to achieve the stage gain and the clipping behaviour using the 2N5457 Jfet.

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Post by knottypine »

Checking in!

Switched Bass from 1M to 2M

Just replaced the 2N5457’s with J201’s

Q3 & Q2 are biased at 4v

Q1 measured at 6v

Also I had to rearrange the legs on the on the J201 to DSG (not DGS) to get Output. So I am not sure if the pin out on the strip board layout is correct

Overall more usable gain and more noticeable bass.

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Post by FlyingWild »

Hi Knttypine,

Does it still squeal when the drive is turned up beyond 12 O'clock?

Pedal PCB has leg 3 of the treble pot connected to the 22nF cap, 2 to ground and 1 not connected, is this how you had yours wired up?

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Post by modman »

Manfred wrote:I tried to adapt the circuit values in order to get the same stage gains using the 2n5457 Fets.
The adaption of the drain and source resistors is only possible when the supply voltage is 18 Volts.
We are back home!
Image
http://runoffgroove.com/sd.html

However, modding it to Wampler Plexithingie should not be the subject of this thread.
Also, whatever the builder says, don't believe him until you seen traces and values.
Please, support freestompboxes.org on Patreon for just 1 pcb per year! Or donate directly through PayPal

knottypine
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Post by knottypine »

FlyingWild wrote:Hi Knttypine,

Does it still squeal when the drive is turned up beyond 12 O'clock?

Pedal PCB has leg 3 of the treble pot connected to the 22nF cap, 2 to ground and 1 not connected, is this how you had yours wired up?

I got it to stop squealing once I used the right value on the 250pf and 470pf caps.

Kinda curious why my bass and treble boost when the knobs are CCW

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Post by atalas »

knottypine wrote: Kinda curious why my bass and treble boost when the knobs are CCW
Reverse your lug 1 and 3 connections on your pot, and the jumpers if you installed them.
modman wrote:
Manfred wrote:I tried to adapt the circuit values in order to get the same stage gains using the 2n5457 Fets.
The adaption of the drain and source resistors is only possible when the supply voltage is 18 Volts.
We are back home!
[ Image ]
http://runoffgroove.com/sd.html

However, modding it to Wampler Plexithingie should not be the subject of this thread.
Also, whatever the builder says, don't believe him until you seen traces and values.
We actually started with traces, and eventually got measured values. Benson only came in a made a few small clarifications at the end.
The schematic I last posted should be accurate.
But you're not wrong, and as I stated earlier, nothing revolutionary in this pedal.

knottypine
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Post by knottypine »

Reverse your lug 1 and 3 connections on your pot, and the jumpers if you installed them.
- I did exactly this and the sweep functions as it should. Thank you.

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Post by knottypine »

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

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Post by atalas »

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.

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Post by Sm4sh »

Hey !
@Bugg, I'm little confused here.
Why did you use a dual gang pot for the bass on your layout ?
https://www.pedalpcb.com/product/sobpreamp/

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Post by plush »

Sm4sh wrote:Hey !
@Bugg, I'm little confused here.
Why did you use a dual gang pot for the bass on your layout ?
https://www.pedalpcb.com/product/sobpreamp/
1M dual pots are way easier to get, than single 2M.
When stacked, dual 1M emulates single 2M.

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Post by Sm4sh »

plush wrote:
Sm4sh wrote:Hey !
@Bugg, I'm little confused here.
Why did you use a dual gang pot for the bass on your layout ?
https://www.pedalpcb.com/product/sobpreamp/
1M dual pots are way easier to get, than single 2M.
When stacked, dual 1M emulates single 2M.

Right!!! :slap:

Thank you! :thumbsup

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andy-h-h
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Post by andy-h-h »

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.

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