BSIAB2 hum problem

[rody82]

I've built a BSIAB2 recently. I love its tone, there's only one problem. There's a VERY noticable
hum... I've tried changing the trannies. Mine has the J201's and one 5457, and its unmodded.
Tried to shield some wires (input and output), tried to re-bias it...
I know that my guitar is a bit noisy, i can hear that with a ross comp, or a high gain pedal like
the ThunderChief, but this is much more louder and annoying.
The hum is almost constant, it doesn't get quieter when i turn down the vol, or gain.
Instead, the pitch of the hum varies a bit with vol, gain, and even with the guitar's vol..
Hums with batteries too...
Any idea?

[petemoore]

  There are probably other ways to 'categorize' hum...
  Here are the two types which can be "nomenclated"...[IME]
   The kind that occurs with the guitar unplugged.
  The other kind.
  The other kind has been 'investiged' using another guitar, I have some with double shielding I use to A/B test, and some circuits, [especially high gain ones]even with the quietest guitars, make 'some' noise at high gain settings.

 

[rody82]

Yep, but this one is some inner noise. At first i didnt house the project, and thought this problem would go away with the metal enclosure, but its still there... Its too much LOUD and very very noticable when i dont play!

[powerplayj]

I have had the same experience with my BSIAB2.  The hum only comes with high gain and isn't dependent so much on volume.  I just dismissed the hum as something that comes along with a high gain circuit.  However, with as many people who have built this circuit on this forum you would think that more people would have responded to your post with the same problem.

[Joe]

Try a 100uF cap across the power rails, even with a battery.

[rody82]

I just dismissed the hum as something that comes along with a high gain circuit.

Ok, high gain pedals are noisier, but i'm sure that what i hear is not normal...

Try a 100uF cap across the power rails, even with a battery.

There's one in the schematic already...

I'll try to tidy up the wiring, hope it will work...

[powerplayj]

Try a 100uF cap across the power rails, even with a battery.

Joe, could you clarify where to put the cap?

Thanks,

Jason

[johngreene]

I'd put a 100uF cap closer to the 1n914 diode. Can anyone explain what purpose that diode performs? I would also be careful about the wire going to the input of the board and going to the drive pot. Keep these separated as much as possible. Since the pitch of the hum changes with volume and tone controls it's likely an oscillation. Try moving the wires around while listening to the hum to see if you can find the 'hot' spot. Something that has a dramatic affect on the hum. Changes are that is what will need to be more carefully routed.

--john

[Fret Wire]

http://www.generalguitargadgets.com/diagrams/bsiab_2_sc.gif
Reverse polarity protection (in series, with a .5-.7v voltage drop, desired or not).

[johngreene]

Ok, I guess so but with a IDss of 3mA and a series resistor, I wouldn't think you could harm the FETs even with reversed polarity supply. Seems a waste of headroom....

--john

[Fret Wire]

Personally, I'd put the diode in parallel, but I don't want to nitpick a nice sounding ckt like this.

[Joe]

Nevermind, there's already a 100uF cap there. Hehehe.

[johngreene]

There is but it's on the opposite side of the board and the Miniboosters are connected between it and the power input 'protection' diode. When I first looked at it (the layout) I saw a half-wave rectifier with the filter cap on the opposite side of the load. I said to myself, "self, this can't be a good thing" and suggested moving the filter cap to the other side of the board and went on to other possible ways that the signal could oscillate. The question I asked about the diode was more for myself to go back and look into it further when I reviewed my post and I had the minibooster schematic open and the Sweet Thing schematic open and RG's minibooster page open and didn't see the diode and right about then, Hellboy was getting the crap beat out of him in a den full of Hellhounds and I hit 'post' before finishing my thought. 

Since the problem is described as a hum that changes pitch, I was thinking it's some kind of oscillation. And since there have been many people who have built this circuit successfully, the problem is probably either wiring or assembly related. If there are long wires, the power supply filter cap location pops out to me as a potential problem area and the fact that the output of the first stage (drive pot) is physically right next to the input of the stage. When you are being an 'arm chair troubleshooter' all you can do is point out everything that could be a problem. I don't see it as nitpicking but rather just pointing out potential troublespots.

However, in re-reading the post and seeing that the hum didn't go away with the volume control turned down..... I'd say check the grounding on the output volume pot or output jack.

--john

[rody82]

Well, thank you for your replies.
tried an additional 100uF at top of the pcb, not much change...
I've just resoldered some points, and tried some 2-3nF film caps between ground and drive lug 2.
That helped a bit, reducing noise at the cost of a little high end.
Its better now, doesnt change pitch, fades away with vol down.   
Now, compared to my Thunderchief, the hum and noise level is almost identical...
I think its time to open up my guitars and check the wiring and shielding 

[Fret Wire]

When you are being an 'arm chair troubleshooter' all you can do is point out everything that could be a problem. I don't see it as nitpicking but rather just pointing out potential troublespots.
john

Exactly. I hope you didn't take that wrong John. I meant Me (Fret Wire/Pete), when I made the nitpicking statement. I haven't earned the right, since I don't have such a widely accepted, nice sounding dist. (I like my BSIAB II) as Ed's. My comment wasn't of much debugging value, just me stating my preference of polarity protection diodes in parallel not series. I didn't mean it as you were nitpicking.

[johngreene]

Fretwire: no worries. I was just trying to explain my thin post prior. I envy RG in his ability to consistently convey complete thoughts in writing. I do better with pictures. 

[wampcat1]

My $.02, I built one for personal use and noticed the same thing except the hum would change with the volume knob on the guitar. I noticed that when I put a buffer right at the input the oscillation disappeared. Rather than trouble shoot, I just kept it like that 

Don't know if that helps or not, but it's worth a shot - you might throw a boss pedal or something right before it (turned off) and see what happens.

Take care,
Brian

[petemoore]

the hum would change with the volume knob on the guitar
  Good point, IIRC the BSIAB II does plenty of gain, almost as much as with guitar full on, with the knob [on mine] rolled back to about 8.8 or so, less noise is often the case like this with high gain circuits for me, plenty of gain and less noise, I use HB pickups...perhaps a pregain would help the noise in the same way [?]/
  Another trick is Mu Amp noiseless biasing, nice because you only need to use 1 large resistor per MA stage, coming off the common divider. Kind of a hassle though if you're PCB, or already perfed one. Don't really know how much difference it would make, it is as high gain as I'd want to go with Mu Amps, [third MA stage would be rediculous, I've tried 3 seriesed MA's] so it would be a good testbed IMO for whether noiseless biasing would make a difference.