Fairfield Circuitry - The Barbershop (gut)

lama00 · Post by **lama00** » 14 Jun 2010, 20:26

My pedal works.. but the sag control is strange: when i'm from 2 hours till the end, the volume decrease.. and when I'm with the pot on 8 hours the volume is a little higher.

When I look at the demos on youtube it's not like that.. I tried to find out what's wrong but I find anything..

Any ideas ?

JMFreak · Post by **JMFreak** » 16 Jun 2010, 10:47

I'm experiencing similar results with the Sag pot. With the volume constant, the Sag at minimun (full counterclockwise) yields the most output. I get a gradual reduction in volume as I turn it clockwise. Full clockwise on the Sag gives zero output.

Is this likely due to the bias of the J201s?

Excuse my ignorance, but to check the bias, do I just set Sag at minimum and check the voltage on the Drain of both jfets?

And would bias at 2/3 supply mean I'm looking for 6ish volts at the Drain of both?

For info, I built this from ibodog2's layout. Many thanks to all for making this happen. What I've got so far does sound real good!

Steve.

fairfield · Post by **fairfield** » 17 Jun 2010, 16:24

I wire my SAG the other way around, meaning it's at its loudest fully clockwise. It makes no difference as the pot is linear, I just prefer having all knobs adding volume when turned clockwise.

The bias should indeed be measured when the SAG is essentially 0 ohms, loudest. And yes 2/3 supply is around 6 volts at the drain. If you can turn the knob enough to completely cut out the signal, it's almost certainly the bias of the J201s.

JMFreak · Post by **JMFreak** » 17 Jun 2010, 16:55

Cheers guys, appreciate it. Will grab a handful of 201s and get to work.

lama00 · Post by **lama00** » 17 Jun 2010, 20:47

fairfield wrote:I wire my SAG the other way around, meaning it's at its loudest fully clockwise. It makes no difference as the pot is linear, I just prefer having all knobs adding volume when turned clockwise.

The bias should indeed be measured when the SAG is essentially 0 ohms, loudest. And yes 2/3 supply is around 6 volts at the drain. If you can turn the knob enough to completely cut out the signal, it's almost certainly the bias of the J201s.

So What do you propose for the wiring?

I did this vero:

You mean I should also replace the J201 ?

Yc for your help

JMFreak · Post by **JMFreak** » 02 Jul 2010, 10:09

Well, tried approx. 30 J201s, sourced from a few different suppliers, and just can't get the bias of Q1 close enough to 6v, nearest is 6.4v.

The two lowest voltage J201s I found bias at 6.4v and 6.7v respectively in Q1. So I have 6.4v in Q1 and the other in Q2, which then yields 6.03v, so guess Q2 is ok.

With this arrangement, the sound seems off with the SAG (now wired correctly) under 12 o'clock, and it's almost inaudible at lowest settings.

Surely you don't have this much trouble finding J201s that will bias around 6v. Does this seem like a build issue?

Could it be anything to do with the 2n2222? I picked one of average gain from the few I had. Is there a specific gain range I should be looking for?

If my only option would be to put in a trim pot to replace one of the resistors, am I right in thinking it would be the 8.2k for Q1?

Thanks again.

atreidesheir · Post by **atreidesheir** » 08 Jul 2010, 11:32

It is certainly reassuring to read Mr. Fairfield's intimate knowledge of his own circuit.
I am impressed.
It also gained him my business. I bought (and built) a Barbershop. They are both fantastic.
I appreciate his respect for knowledge and pragmatism as well.

Cheers

fairfield · Post by **fairfield** » 09 Jul 2010, 04:48

ibodog2 is right, it's easier to tune the resistors to the JFET. Just pick a J201 and try different resistors in the 7.5k to 12kohm range. You should be able to get somewhere between 6-7 V. No need to be more precise, it'll sound good.

There's no real right way to build this, I chose a set of components to get a certain repeatability. The circuit is so simple, modifying it is quite straightforward. You could get a pretty nice fuzz by just swapping out some caps and resistors...

I appreciate your business atreidesheir, thanks for your support.

Now, let's see some builds.

ToneKarma · Post by **ToneKarma** » 03 Oct 2010, 10:38

Okonomiyake wrote:My only issue is I don't have anything resembling the polarity protection jfet (VP3203). Can someone recommend an alternative that might be more commonly accessible? Or, if I promise not to plug the wrong polarity power supply in, could it be omitted altogether? Thanks much.

I was wondering about this issue too, as I find the VP3203 hard to find in Europe too and I am not really inclined to spend €20 p&p on Mouser´s overpriced stuff (for outside US orders that is) just to get one tranny.

How about substituting the VP3203 with a BS250 ? That should work alike.
Maybe change the gate resistor from 2M to 1M?

What do you think?

Cheers

RnFR · Post by **RnFR** » 04 Oct 2010, 03:08

bs250 should work as well.

ToneKarma · Post by **ToneKarma** » 04 Oct 2010, 10:16

RnFR wrote:bs250 should work as well.

Just found an older article on the Geofex website covering exactly this issue (I should have gone there before...

).

R.G. Keen writes that his BS250 were in fact BSP250 (TO-92), that are no longer available.
A Geofex-reader notified him that the BS250 measured a voltage drop of 250mV @ 20mA.
Too much to be of a good use ...

So, R.G. suggests the "infamous" VP3203 as a substitute.
Furthermore he writes that IRFD9024 (4-pin hex layout) and the FQPF11P06 are also great replacements.

The good thing :
RS Components lists the IRFD9024 (order no. 541-0547), which can be easily incorporated in a vero layout of the Barbershop.

One more thing on the Barbershop, Guilleaume :
I love the sound of your design, fantastic work !!
I also love the enclosure design with the bare aluminium and the stampings. Very distinctive (you´re not into classic/vintage cars, are you ?).

Cheers,
TK

ToneKarma · Post by **ToneKarma** » 04 Oct 2010, 10:21

ToneKarma wrote: A Geofex-reader notified him that the BS250 measured a voltage drop of 250mV @ 20mA.

Sorry, mistake ... it was 150mV @ 20mA
Still not favourable.

TK

fairfield · Post by **fairfield** » 14 Oct 2010, 15:49

I like vintage stuff in general, especially when it's dirty, rusty and decrepit. Just bought my first bike, a '72 Honda CB350. It's been sitting in a shed for some 20 years and now it's back on the road.

The BS250 should be fine. Its "Rds on" is around 10 ohms, my Barbershops pull 5mA with a high efficiency LED. That's 50mV. Even if you lost 150mV it would be acceptable. Hell, just replace the whole thing with a series diode and your good to go. Most adapters put out 9.6V, as if they wanted you to drop it down to 9V. If you're building this pedal, you'll have to play around with the bias on both gain stages to get the sound the way you like it. Then you stick in the Sag pot which turns the whole thing on its head. 9.6V vs 9V is the least of your worries.

RnFR · Post by **RnFR** » 14 Oct 2010, 16:21

yeah, 150mV isn't that bad. I think RG was just trying to get the drop down as low as he possibly could. remember a series Si diode is going to drop 600 mV. if you want less, you can use a Ge or Shottkey as stated earlier for 300mV. so, if you look in that light, you are still doing extremely well with the BS250.

RnFR · Post by **RnFR** » 14 Oct 2010, 16:38

fairfield wrote: Most adapters put out 9.6V, as if they wanted you to drop it down to 9V.

indeed, they do! most major manus use a series Si diode as it's the cheapest. hence the 9.6 volt supply. I've come to decide that those few extra mV are only worth it when trying to get as clean as possible. anything else, and the difference is negligible.

ToneKarma · Post by **ToneKarma** » 15 Oct 2010, 09:19

fairfield wrote:I like vintage stuff in general, especially when it's dirty, rusty and decrepit. Just bought my first bike, a '72 Honda CB350. It's been sitting in a shed for some 20 years and now it's back on the road.

The BS250 should be fine. Its "Rds on" is around 10 ohms, my Barbershops pull 5mA with a high efficiency LED. That's 50mV. Even if you lost 150mV it would be acceptable. Hell, just replace the whole thing with a series diode and your good to go. Most adapters put out 9.6V, as if they wanted you to drop it down to 9V. If you're building this pedal, you'll have to play around with the bias on both gain stages to get the sound the way you like it. Then you stick in the Sag pot which turns the whole thing on its head. 9.6V vs 9V is the least of your worries.

Thank you Guilleaume and RnFR.
Of course it is not necessary and it may appear nitpicking, but it´s a useful addition to the/any circuit.

The CB350 is a nice and reliable bike (and easy to maintain too), but my bike days are over and I have drifted to old things with four wheels. BTW, the CB350 is the successor of the bike that Robert M. Pirsig rode on his travels that lead to the book "Zen and the art of motorcycle maintenance" ... a book about quality of life.

Cheers

jwpartain1 · Post by **jwpartain1** » 29 Jan 2011, 02:40

Hello, been checking out this pedal for a while, and I'm buying the parts right now to build it. From what I see Smallbear doesn't carry a 15V Zener, is it required? If so, where else could I get one (it's hard for me to follow mouser's parts and know what's the good stuff and what's not [n00b]). Could I use a 12V Zener? (1 watt)

jwpartain1 · Post by **jwpartain1** » 29 Jan 2011, 02:58

Also, are 4.7 uf 16v caps OK at c2 and c5? Thanks!

MicMicMan · Post by **MicMicMan** » 29 Jan 2011, 11:52

jwpartain1 wrote:Hello, been checking out this pedal for a while, and I'm buying the parts right now to build it. From what I see Smallbear doesn't carry a 15V Zener, is it required? If so, where else could I get one (it's hard for me to follow mouser's parts and know what's the good stuff and what's not [n00b]). Could I use a 12V Zener? (1 watt)

12v will be fine, you just won't have as much headroom possibilities (with greater voltage powers) as with a 15v.
Let's say that with a 15v zener, you'll be able to plug the pedal using a 18v power unit, and have some benefits from it, while with the 12v zener, any voltage above, say 15v, will be useless. I think it's not a big deal.

jwpartain1 · Post by **jwpartain1** » 29 Jan 2011, 13:31

Cool, I appreciate it! And the 4uF value at C2 and C5, it'll just let more bass through? I'm just guessing, ordered the parts last night though! Super excited.

Fairfield Circuitry - The Barbershop (gut) [traced]