Benson - Preamp  [traced]

General documentation, gut shot, schematic links, ongoing circuit tracing, deep thoughts ... all about boutique stompboxes.
User avatar
Nocentelli
Tube Twister
Information
Posts: 2222
Joined: 09 Apr 2009, 07:06
Location: Leeds, UK
Has thanked: 1152 times
Been thanked: 954 times

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
modman wrote: Let's hope it's not a hit, because soldering up the same pedal everyday, is a sad life. It's that same ole devilish double bind again...

User avatar
temol
Solder Soldier
Information
Posts: 200
Joined: 25 Feb 2017, 20:58
Has thanked: 23 times
Been thanked: 213 times

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.

User avatar
Manfred
Tube Twister
Information
Posts: 1937
Joined: 04 Apr 2009, 23:42
Has thanked: 1671 times
Been thanked: 1344 times

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.

User avatar
temol
Solder Soldier
Information
Posts: 200
Joined: 25 Feb 2017, 20:58
Has thanked: 23 times
Been thanked: 213 times

Post by temol »

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

T.

User avatar
Manfred
Tube Twister
Information
Posts: 1937
Joined: 04 Apr 2009, 23:42
Has thanked: 1671 times
Been thanked: 1344 times

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 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

User avatar
knottypine
Information
Posts: 8
Joined: 12 May 2012, 15:02

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.

User avatar
atalas
Information
Posts: 36
Joined: 24 Jul 2019, 21:22
Has thanked: 8 times
Been thanked: 27 times

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.

User avatar
Manfred
Tube Twister
Information
Posts: 1937
Joined: 04 Apr 2009, 23:42
Has thanked: 1671 times
Been thanked: 1344 times

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.

User avatar
knottypine
Information
Posts: 8
Joined: 12 May 2012, 15:02

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.

User avatar
FlyingWild
Information
Posts: 30
Joined: 28 Oct 2018, 21:01
Has thanked: 9 times
Been thanked: 7 times

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?

User avatar
modman
a d m i n
Information
Posts: 4890
Joined: 19 Jun 2007, 16:57
Has thanked: 4394 times
Been thanked: 2131 times

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

User avatar
knottypine
Information
Posts: 8
Joined: 12 May 2012, 15:02

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

User avatar
atalas
Information
Posts: 36
Joined: 24 Jul 2019, 21:22
Has thanked: 8 times
Been thanked: 27 times

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.

User avatar
knottypine
Information
Posts: 8
Joined: 12 May 2012, 15:02

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.

User avatar
knottypine
Information
Posts: 8
Joined: 12 May 2012, 15:02

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

User avatar
atalas
Information
Posts: 36
Joined: 24 Jul 2019, 21:22
Has thanked: 8 times
Been thanked: 27 times

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.

User avatar
Sm4sh
Information
Posts: 13
Joined: 07 May 2015, 09:03
my favorite amplifier: At the moment my THR10
Completed builds: Booster: AC/RC Boost, Echoplex, EP Booster,
OD: Euphoria, Plexi Drive, KOT, Super OCD, Angry Charlie, BB Preamp, Klon, 808, , Morning Glory, 808, Lighspeed, Timmy, Eleven OD, OD820, Palisades,
Distortion: RAT, Red Llama, El Rey Dorado, Purple Plexi, Grey Channel
Fuzz: Velvet Fuzz, Sunface Ge and Si, Fuzz Face, Soul Bender, Pharao Fuzz, Dirt Transmitter, Fuzz Factory, Big Muff Triangle, Fallout Cloud
Delay: Deep Blue Delay, Disaster Transport Jr
Tremolo: Magnalous Lunevibe,
Compressor: Bearhug
Location: France
Has thanked: 18 times
Been thanked: 2 times

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/

User avatar
plush
Cap Cooler
Information
Posts: 641
Joined: 08 Dec 2015, 09:29
Location: Moscow, Evil Russia
Has thanked: 36 times
Been thanked: 172 times

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.

User avatar
Sm4sh
Information
Posts: 13
Joined: 07 May 2015, 09:03
my favorite amplifier: At the moment my THR10
Completed builds: Booster: AC/RC Boost, Echoplex, EP Booster,
OD: Euphoria, Plexi Drive, KOT, Super OCD, Angry Charlie, BB Preamp, Klon, 808, , Morning Glory, 808, Lighspeed, Timmy, Eleven OD, OD820, Palisades,
Distortion: RAT, Red Llama, El Rey Dorado, Purple Plexi, Grey Channel
Fuzz: Velvet Fuzz, Sunface Ge and Si, Fuzz Face, Soul Bender, Pharao Fuzz, Dirt Transmitter, Fuzz Factory, Big Muff Triangle, Fallout Cloud
Delay: Deep Blue Delay, Disaster Transport Jr
Tremolo: Magnalous Lunevibe,
Compressor: Bearhug
Location: France
Has thanked: 18 times
Been thanked: 2 times

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

User avatar
andy-h-h
Breadboard Brother
Information
Posts: 112
Joined: 22 Jul 2017, 03:54
Location: Brisbane, Australia
Has thanked: 23 times
Been thanked: 42 times
Contact:

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.

Post Reply