BSIAB2 squeal

demym · September 25, 2009, 04:14:21 PM

Hi to all,

i have built a BSIAB2 on veroboard, using this layout:

I've used the same parts shown in the layout , apart from:

C3a - i used a polarized cap here
C13 and C14 - 1nF instead of 2n2
C3b - 470nF instead of 220nF

Also changed some resistors, but they are very very close to the original values (for example, 100k instead of 82k in R14).

The pedal works, when i play i get what i think should be the sound of a BSIAB2 (very good sound indeed). But when i turn gain over 60/70% i get a continuous high pitch squeal when not playing.

I have read some posts inherent to using shielded cables. But before trying that (i'm not using that kind of cable for the input right now), i just wanted to be sure that the difference in some of the cap values would not be the cause of the squealing..

I tried also using all 2N5457 in place of the J201s, and this way the squeal is much less evident (but also the sound of the pedal is different, less gain i would say)...

Any other things that i can try to defeat the squeal, without which the pedal would be perfect ??

I'm still a noob, i built a Thor (sounds right and without the squeal), the beginner projects, and i am now working onto the JFET Legacy emulation.

Thx in advance for your help

Have a nice time

Ripthorn · September 25, 2009, 04:19:04 PM

The BSIAB is higher gain than the Thor and will oscillate (squeal) when at higher gain values. The mu-amps in the BSIAB provide a ton of gain. Changing input and output caps will just change how much bass is let through or bled off and won't change the oscillation issue. Shielded cables are a good idea. Also, try to keep any wires that carry the amplified/distorted signal away from any of the input wires, as this helps feed the oscillation.

demym · September 25, 2009, 04:24:35 PM

Thanks,

input and output are cabled as shown in the vero layout, so they are on opposite sides...this was what you meant ? Will try with the shielded cables and let you know.

I've read that only input needs to have a shielded cable, is this correct ? Or even for pots should i use shielded cable ?

Thanks once very much

demym · September 28, 2009, 10:28:40 AM

Hi all, it's me once...

So i have put shielded cable on the input jack, but the BSIAB2 still squeals.... There are only two situations in wich it squeals:

1) Drive pot over 60% circa
2) 100k trimpot open for more than 5%; i keep it open less than 5%; this way i got very low volume, and less drive than biasing the trim at 4.5V... but at least it doesn't squeals....

I'm beginning to believe that this is not only a 'RF' problem; in fact, i've also shortened all the wires apart from shielding input jack wires.. is there some measurement i can do to debug this circuit ? I would really like to enclosure it, as the sound coming out is very very good, apart from squeal, obviously.....

the next thing i can do on my ignorant side is to change the pots (could be that some of them is broken...even if they work correctly as expected).. also, the gain pot is a linear one (it's marked 470B), whereas the schematic says that the three pots should be audio/log

By the way, without the horrible squeal, this would be my best sounding self made pedal...

Thanks in advance

demym · September 28, 2009, 10:59:38 AM

Just an update... if i put my Tillman JFET preamp in front of the BSIAB2, it doesn't squeal anymore, at any setting...

It's good news, but i would like the BSIAB2 to not squeal on his own... also, i have a very high ground noise... could it be because it's not yet metal enclosed ?

Thanks, sorry for the many questions, but if this pedal would work correctly, i would definitely consider myself stable onto the DIY road, after 3 working projects (Tillman, Thor and BSIAB2) projects... a very big push for the future..

Have a nice time

shadowmaster · September 28, 2009, 11:48:35 AM

How are you powering up your BSIAB?

Unregulated power supply is a no-no for my BSIAB as it squeals like hell. I've been using a well-filtered regulated power supply for it.

I can dial in a gain and tone of 10 but can only reach around 50% of the volume before it goes into self-oscillation. I would need to turn the gain down then if I need to turn my volume above 50%. If not playing my guitar, it doesn't produce any hum if I'm muting the strings. It feedbacks when strings are not muted due to its high-gain nature. My guitar has an HSH pickup configuration and is most quiet if used with my bridge pickup. Has hum if used with other pickups or pickup settings but not to the point of annoying me. I just mute it with my hands if I'm not playing and everything's OK.

Mine doesn't use shielded wires and was in a plastic enclosure before but still behaves the same way even when it was rehoused in an aluminum one.

Used GGG PCB layout pattern and all parts were stock. My Vox ACTV4 is loving it.

Hope this helps.

jacobyjd · September 28, 2009, 11:50:31 AM

I used the gaussmarkov layout for mine, and it sounds great w/ no oscillation--I've cranked it all the way to push my tube amp w/ great results...sounds like something's up with yours.

demym · September 28, 2009, 01:25:12 PM

I am powering it with a 9V battery, no other power source used.

My guitar is an HSH configuration (Ibanez RG550).

Is the Gauss a vero layout ?

This is my first time with vero (had non success with perfboard), and i see vero as the right choice for me (i'm not for etching at all, and perfboard gets too intricated to me)..

The fact that with the Tillman in front doesn't squeal has to do something with input impedance ? I've checked the 1M resistor at the start of circuit (R1) and it seems ok... Tillman has a 3.3M i think... do you think that putting a higher value resistor in R1 could do the trick ? Just blattering, as i'm pretty i noob.

Thanks anyway folks.

jacobyjd · September 28, 2009, 01:31:32 PM

Quote from: demym on September 28, 2009, 01:25:12 PM
I am powering it with a 9V battery, no other power source used.

My guitar is an HSH configuration (Ibanez RG550).

Is the Gauss a vero layout ?

This is my first time with vero (had non success with perfboard), and i see vero as the right choice for me (i'm not for etching at all, and perfboard gets too intricated to me)..

The fact that with the Tillman in front doesn't squeal has to do something with input impedance ? I've checked the 1M resistor at the start of circuit (R1) and it seems ok... Tillman has a 3.3M i think... do you think that putting a higher value resistor in R1 could do the trick ? Just blattering, as i'm pretty i noob.

Thanks anyway folks.

No, the GM layout is PCB--I had a nice etch done by John Lyons for mine. I was mainly just chiming in to say that any oscillation means there's something up. The circuit itself is good that way. It could be a lead dress issue, or maybe a problem with the layout, but of those things, I'm not sure.

demym · September 29, 2009, 10:19:23 AM

I've tried anything, rewired the pots, rewired the inputs, scraped dust from the vero traces, but ntohing, it still squeals...

I think i'm going to order the PCB, but that was not my intention at first, as i thought that veroboard could be optimal to me... Because the BSIAB2 sounds so good (without my squeal obviously), i will take the pcb from GGG. And hope that that will not squeal..

A little bit of delusion for not being able to track down the problem and solve it...

Thanks to you all, anyway

jacobyjd · September 29, 2009, 10:29:15 AM

did you sub any parts?

petemoore · September 29, 2009, 10:36:37 AM

I would guess maybe the gain got too high somehow, for a non-oscillatory breadboard build anyway.
I've read BSIAB oscillation threads, it is 2 high gain Mu amps, the gain of course depending on the Jfets...and...

liquids · September 29, 2009, 11:17:10 AM

Quote from: demym on September 28, 2009, 10:59:38 AM
Just an update... if i put my Tillman JFET preamp in front of the BSIAB2, it doesn't squeal anymore, at any setting...

It's difficult to answer, but this seems to indicate that inverting the phase (which is what the tilman is doing) helps the issue...

The BSIAB, if I'm not mistaken, has the output 'out of phase' with the input. This means that from beginning to end, it 'inverts' the signal, which helps reduce oscilation again, that is if I'm not mistaken...

Do you run anything else on in the signal chain that might be inverting your signal and hence returning the signal to 'in phase'?

If you have the time and patience, can you build an op-amp based buffer that inverts the signal, run it before/after and see if that has similar effect?

If so...line them all up...get the BSIAB to squeal. turn one the tillman...squeal should subside...then also turn on the inverting op amp buffer...squeal should return...etc.

liquids · September 29, 2009, 11:21:21 AM

Also, if you are afraid of losing gain...why not swap the J201s with another pair. Maybe that set is particularly gainy.

I find the BSIAB to have WAY too much gain, and most who like it stock also find it more useful with lower gain settings...but maybe the J201s you have are particularly hot, and another pair will solve the issue while assuring you that you have the 'right sound' with J201s in there.

Also, have you DMM'd the voltages on Q5, and messed with the your trimmer? Does turning down the tone pot reduce the squeal?

demym · September 29, 2009, 12:27:52 PM

So, good news:

i changed Q3, Q4 and Q5 to 2N5457, and the pedal stopped squealing ! I have a little less gain than before, but it's not bad because the gain with all J201s (at least, the ones that i had) was really to hot for real playing situation (i test circuits at home, and cannot get too loud, and the all J201s gain at the reharsals would be really to much).. I also changed one of the transistor sockets (that seemed to have on of his holes not free..).

I'll stick with J201 in Q1 and Q2 ad 2N5457 in Q3, Q4, Q5. So happy now ! Going to order an enclosure ! My first real enclosured pedal (the Tillman is into a plastic self made enclosure)

Thanks to all of you for your precious help !

Have a nice time

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BSIAB2 squeal