NEW low gain Bluesman pedal build report

[Ice-9]

BluesMan Build

I have just finished designing and building what I am calling the "BluesMan"
I have been looking to build a low gain booster type pedal that at it's maximum gain will break up and drive an amp nicely and decided to go down the route of using Jfets.

A little background and thanks is necessary first to the people that inspired this project.

Over the last couple of years I have played around with circuits like the BSIAB by Ed Guidry and Boosters by Jack Orman and have learned quite a bit from these along with R.G's excellent write up on Mu-Amps and SSRP with noiseless biasing.

Also having listened to quite a bit of Gary Moore's live playing recently I have been inspired by his sound, the way he can get a huge sustained dirty sound that on listening closer is actually quite clean and very sensitive to picking. Of course this also comes from playing cranked up amps at very loud levels.

I hope this effect lives up to what I intended it to do, I have also build into the PCB a couple of optional components that can be added to change it to a higher gain drive, and also will note some other component values that could be altered to change the character of the effect.

Enough waffle first the schematic.



It's based on SSRP noiseless biasing , and is a two stage gain circuit. The first gain stage around Q2 I have kept quite clean with very little signal breakup, but a bit of a signal boost which in turn drives the second gain stage around Q4.

The first stage gain can be altered by adjusting the values of R9 and C5, originally C5 was omitted altogether keeping the sound very clean, adding a 22n cap there gave a nice boost to the level without too much signal clipping and also not allowing the signal to get muddy.

The second stage gives the circuit a bit more clipping and can be altered to suit taste by either adjusting R13 or by adding a capacitor in parallel with R13 (the cap is not drawn on the schematic but has a place on the PCB numbered C13).

Next there is a BMP type tone section which has a good range which also has a place for an additional capacitor (C12) which is not placed on my original circuit but experiment here if you feel like it. This is followed by a 10K 1n low pass filter to the level pot.  

I haven't added a protection diode into the schematic or PCB as I include these on my main bypass/switch jack PCB. So if anyone builds this then maybe add your preferred reverse polarity protection.

Component layout



PCB


Copper pour PCB version


After building this I am happy with the results it offers a good boost to drive an amp but still give control and feel when pick hard/soft, it's also nicely sensitive to volume control adjustments. Of the 2 guitars I tried out one has a very hot bridge pickup and this gave a great boost and clipping to the point of rock while the lower output pickups gave a just breaking up blues type of drive.

Although not to far away from all the other jfet designs I do think it has its own sound and place.

All jfets used were 2n5457 but others could be tried.

[Ice-9]

And here are a couple of the finished pedal, i slipped with the screen print so it blurred a bit

[euronymous0001]

can't seem to find the input. 

[Ice-9]

The input is at the juction of C2 /R4 . Oversight on the way I draw my schematics. I have updated the schematic so it shows the input.

[amptramp]

The Mu-amp or SRPP (Shunt Regulated Push Pull) stage sent me back to the (online) books, specifically the Tubecad journal.  This is one of the best amplifier resources on the net.  The geofex page showed a sample source resistance of 1K and it will be good if you have the lowest gain 2N5457 (1000 micromhos).  The value of the upper source resistor (R8 and R12) should be 1/Gm if you follow this derivation on page 8:

http://www.tubecad.com/may2000/page8.html

for the circuit shown on page 9.  (It pays to start at the beginning of the article, on page 1 or you will get lost.  The pages are short, so it is not hours worth of reading.)  Since the 2N5457 can have a transconductance between 1000 and 5000 micromhos, the circuit will behave differently depending on the specific device used, so one build may not sound quite the same as another.  The range of values for R8 and R12 would be 200 to 1000 ohms.  The SRPP stage was patented in 1943 (so it has expired) but did not get much use in audiophile equipment until the 1990's.  As the article shows, it is optimized for one load impedance, but behaves reasonably well with some drive imbalance even if this is changed.  This also applies to the BSIAB and other SRPP circuits.

[Ice-9]

Thanks for the link Amptramp, I will give this whole article a read to gain some more understanding of these circuits.
sorry everyone for the mistake as i called the stages SSRP instead of SRPP, I blame it on a drink too many and not paying attention to my typing .