Keep me from throwing this BSIAB2 in the trash...

[Barcode80]

i spent all night soldering up a BSIAB2, with shielded ins and outs, and i'm getting nothing but a muffled bassy clean out of it. i used a 50k trim instead of 100 and  a 50k tone instead of a 100. would these likely be the culprit?

[mars_bringer_of_war]

A 50k tone should have no bearing but the 50k trimmer is quite possibly the issue. You may not be able to get the bias right with a 50k.

[aron]

Measure voltages.... I think it's a wiring mistake. I believe it should bias up with a 50K trimmer; at least if you are using a J201.

[Barcode80]

i'm using 3 2n5457's and 2 j201's. i'll try to take voltages and report back.

[Barcode80]

okay, i used gaussmarkov's layout for the pcb. here is a snapshot of it, with my voltages typed in.

[96ecss]

Don't throw it in the trash! You'll love it when you get it sorted out. I've built 2 and they both sound great. I'll check the voltages on mine and post them for you when I get a chance later today. I will say this though. When I built my first bsiab2, I didn't have a 100K trimmer so I used a 50K. It would not bias properly. I got a 100K the next day and replaced the 50K and it biased right up.

Dave

[DWBH]

One simple question concerning the Bsiab: If I replace the J201's with 2N5457's do I have to put my hands on the trimpot? When do I know when its position is correct?

[96ecss]

One simple question concerning the Bsiab: If I replace the J201's with 2N5457's do I have to put my hands on the trimpot? When do I know when its position is correct?

The trimpot is for setting the Drain voltage of Q5. According to JD Sleep from GGG, set it to around 4 Volts. He set his at 3.4V, I set mine closer to 4V. Set it at 4V, then fine tune by ear. The 2N5457's will give you less gain. I have all J201's in both of mine but I'll be switching one to the original schem with 3 2N5457's and 2 J201's. 

Dave

[DWBH]

Thanks. 

[petemoore]

  You could diddle with the 50k and get it to set bias, but with a stop resistor.
  It's hard to say when a Jfet 'decides' to bias, 50k usually does it but I've had Drain's that insisted on having say 87k drain resistor...
  So...either using a pot/resistor offboard to find the resistance value which biases the jfet, then using fixed value of same there...
  Or just putting in a 39k resistor series with the 50k pot for the 'stubborn' jfet..
  Or just using the 50k pot and finding another Jfet that biases correctly with Drain resistor of <50k.
  I guess we're talking about the '5th' transistor, as the BSIAB gain stages are Mu amps...self biasing.

[Barcode80]

i was able to bias the Q5 to 4 volts with the 50k quite easily. that's why i'm not sure where the prob is.

[audioguy]

There was a big error on that layout pertaining to caps 12 and 13. If you downloaded the project file a week ago it is VERY muffled.... Pauls package says that those 2 are .022, However GGG has them as .0022. I swapped them and the thing sounds great!

I hope this helps!

Audioguy

[Barcode80]

Oooh! maybe that's it. i'll try it when i get home...

[audioguy]

I guess on paper those errors aren't 'big'... but when you plug it in- thats a different story.
Good luck with it. I built one as part of a high gain pedal testing weekend.... which I'll post some reviews on sooner or later.

Audioguy

[gaussmarkov]

I guess on paper those errors aren't 'big'... but when you plug it in- thats a different story.
Good luck with it. I built one as part of a high gain pedal testing weekend.... which I'll post some reviews on sooner or later.

Audioguy

yes, i'm sorry about that.  audioguy pointed out the error on june 5 and i corrected it the same day.  i regret the confusion
and wasted time.  thanks to audioguy for being on the ball and finding the mistake!

[Barcode80]

anyone have any input on those voltages?

[gaussmarkov]

anyone have any input on those voltages?

where was your gain pot set when you took those?  at minimum, i hope.  otherwise i guess there is
something wrong with Q5.  maybe a solder bridge to that island of grounded copper that R1 and D1
are connected to?

[petemoore]

anyone have any input on those voltages?
  Yes, I have a tough time cyphering D/G/S and whether it's a gain stage or buffer stage I'm looking at...PCB mazes take some sorting out.
  If you could type them out like
  battery = 9.2v
  Q1 [gain stage]
  D. 4.8v
  G  .8v
  S  1.2v
  Q2 [Source follower]
  D 9.19v
  G 2.v
  S 4.v
  It'd be easier to spot a misbiased transistor.

[Barcode80]

anyone have any input on those voltages?
  Yes, I have a tough time cyphering D/G/S and whether it's a gain stage or buffer stage I'm looking at...PCB mazes take some sorting out.
  If you could type them out like
  battery = 9.2v
  Q1 [gain stage]
  D. 4.8v
  G  .8v
  S  1.2v
  Q2 [Source follower]
  D 9.19v
  G 2.v
  S 4.v
  It'd be easier to spot a misbiased transistor.

okay, i'll do that when i get home too.

anyone have any input on those voltages?

where was your gain pot set when you took those?  at minimum, i hope.  otherwise i guess there is
something wrong with Q5.  maybe a solder bridge to that island of grounded copper that R1 and D1
are connected to?

gauss, i can't remember if my gain pot was minimized or not, i will check that as well. at least it is good to have a specific problem people seem to know about, instead of a "help, no sound when effect is engaged" problem

[petemoore]

  Yupp...lotsa places for stuff to be 'off'..
  The audio probe and V measurements certainly open up new avenues/narrow down the alternatives.
  Being two Mu amps, the first stage [2 transistors] should have signifigant boosted output. Same with the second stage [probably depends on a gain or whatever pot setting though...easier to 'read' what's up with circuit flow [a divider/volume type control set low/CCW will of course attenuate/shunt all signal to ground]...you should get an output that is a boosted [and slightly to heavily distorted as you pass the 1rst/2nd stages] version of the input at each stage.