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Re: Benson Preamp

Posted: 18 Aug 2019, 14:15
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

Re: Benson Preamp

Posted: 18 Aug 2019, 19:10
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.

Re: Benson Preamp

Posted: 19 Aug 2019, 07:55
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.

Re: Benson Preamp

Posted: 19 Aug 2019, 14:51
by temol
It may be cheaper to use charge pump than buy MMBFJ201 transistors :)

T.

Re: Benson Preamp  [traced]

Posted: 19 Aug 2019, 17:19
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 183 times
BensonPreampLayoutSolderSide.pdf
(3.67 KiB) Downloaded 166 times
BensonPreampLayoutSolderSideMirrored.pdf
(3.67 KiB) Downloaded 169 times
BensonPreampWamplerOutputCircuitryLayout.JPG
BensonPreampLayoutWamplerModComponets.pdf
(20.64 KiB) Downloaded 186 times
BensonPreampLayoutWamplerModSolderSide.pdf
(3.76 KiB) Downloaded 154 times
BensonPreampLayoutWamplerModSolderSideMirrored.pdf
(3.75 KiB) Downloaded 155 times
SprintLayout6 file contains both versions:
BensonPreamp.zip
(40 KiB) Downloaded 203 times

Re: Benson Preamp

Posted: 20 Aug 2019, 04:04
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!

Re: Benson Preamp

Posted: 20 Aug 2019, 04:27
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.

Re: Benson Preamp

Posted: 22 Aug 2019, 07:39
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.

Re: Benson Preamp

Posted: 04 Sep 2019, 01:24
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.

Re: Benson Preamp

Posted: 04 Sep 2019, 07:44
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?

Re: Benson Preamp

Posted: 04 Sep 2019, 09:45
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.

Re: Benson Preamp

Posted: 05 Sep 2019, 01:48
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

Re: Benson Preamp

Posted: 05 Sep 2019, 08:53
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.

Re: Benson Preamp

Posted: 07 Sep 2019, 02:39
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.

Re: Benson Preamp

Posted: 07 Sep 2019, 02:43
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

Re: Benson Preamp

Posted: 07 Sep 2019, 06:21
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.

Re: Benson Preamp

Posted: 13 Sep 2019, 18:36
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/

Re: Benson Preamp

Posted: 13 Sep 2019, 18:56
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.

Re: Benson Preamp

Posted: 13 Sep 2019, 19:05
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

Re: Benson Preamp

Posted: 16 Sep 2019, 08:22
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.