B22 causing oscillation?

[ivant]

I've just completed a gainclone kit and while it works fine on its own with an ipod or 0404usb, it does not work with a B22 as preamp. The B22 somehows "drives" the gainclone into oscillation whenever the volume knob is at fully clockwise or anticlockwise position. Theres no problem when the knob is inbetween. Any ideas?

 

[ivant]

Maybe the B22 output is abit noisy at the extremes of the volume pot? Though i don't hear anything with headphones.

 

[NelsonVandal]

How do you know the amp is oscillating? Are you sure it's the Gainclone that's oscillating (or misbehaving in some way)?

Where is the volume control? Only on the B22 or both the B22 and the Gainclone? Is it a 2, 3 or 4 channel B22?

If you have a volume control on the Gainclone I can understand why it's oscillating at midpoint, but not if you only have a volume control on the B22.

Can you show us a schematic of the Gainclone? Do you use a zobel or inductor on the output? Do you use any compensating feedback capacitors? What values for feedback resistors do you use? Is it inverting or non-inverting? Do you use a filtering cap on the input? What's the value of the input resistor? Do you use a PCB or is it built on a perfboard or is it hardwired?

 

[linuxworks]

Quote:

Originally Posted by ivant /img/forum/go_quote.gif Maybe the B22 output is abit noisy at the extremes of the volume pot? Though i don't hear anything with headphones.

ONLY when you have ground problems!!

tell me about it.....

I fought ground problems for weeks (it seemed) on my b22.

likely that's it. grounding is not obvious (sometimes it takes insight) and it can make or break your noise level on the b22.

I once had that exact problem, nice in the middle but noisy on the ends. I also had it the other way - noisy in the middle and nice on the ends. both times it was slight tiny ground currents in loops.

before you 'mount' a b22, it makes sense to prototype it and try things before you finalize the wiring and grounding. this is a complex project and needs to be handled as such.

 

[ivant]

Not sure if this is ocsillation but i get over excursion on the speakers (Loud pop) . I measure about 1.5vdc and 4.5vac at the outputs of the gainclone at this point and thats regardless of speakers being connected or not. Output of B22 measures ok. Though a multimeter probably issn't the best tool in this situation. The B22 is a 3 channel build with only 2 channels + signal ground used for the preout. The gainclone is a lm3886 kit from chipamp.com. Wired as per the manual. Zobel is part of the kit. Feedback capacitor Ci is also used. No input capacitor.

 

[ivant]

Quote:

Originally Posted by linuxworks /img/forum/go_quote.gif ONLY when you have ground problems!! tell me about it..... I fought ground problems for weeks (it seemed) on my b22. likely that's it. grounding is not obvious (sometimes it takes insight) and it can make or break your noise level on the b22. I once had that exact problem, nice in the middle but noisy on the ends. I also had it the other way - noisy in the middle and nice on the ends. both times it was slight tiny ground currents in loops. before you 'mount' a b22, it makes sense to prototype it and try things before you finalize the wiring and grounding. this is a complex project and needs to be handled as such.

I too suspect the problem to be from the B22 as other sources don't seem to exhibit the same problem with the gainclone. So you reckon grounding is more likely the problem than say... interference on the signal wiring?

 

[n_maher]

Have you checked the DC offset of your sources? What's the gain on the beta22?

 

[ivant]

No dc offset at the output of b22 and the source. Gain of b22 at default of 8x

 

[johnwmclean]

On the same subject,
active ground (3 board) VS 4 board (balanced only)
is one version likely to be more troublesome with grounding issues?
Sorry to the OP for sidewinding the thread...

 

[amb]

Quote:

Originally Posted by johnwmclean /img/forum/go_quote.gif active ground (3 board) VS 4 board (balanced only) is one version likely to be more troublesome with grounding issues?

One is not any more "troublesome" than another. Just somewhat different.

 

[NelsonVandal]

So, is there a volume control on the Gainclone? Is there an input cap on the Gainclone? There are many versions of Gainclone out there. Could you please show the schematic or link to it.

If there's no input cap on the Gainclone, it could be the B22 that causes the DC-offset and is unstable. If so you could try to add a zobel on the B22's output.

 

[ivant]

Quote:

Originally Posted by NelsonVandal /img/forum/go_quote.gif So, is there a volume control on the Gainclone? Is there an input cap on the Gainclone? There are many versions of Gainclone out there. Could you please show the schematic or link to it. If there's no input cap on the Gainclone, it could be the B22 that causes the DC-offset and is unstable. If so you could try to add a zobel on the B22's output.

No volume control on the gainclone. No input cap. No measurable dc offset on the B22. Zobel already installed at the B22 output.

Link to gainclone schematics

 

[amb]

ivant, please describe in detail about how you wired all the grounds, both in the β22 and in the gainclone (I assume that they are in separate cases). Is the β22 poweed by a σ22? Is that also in a separate enclosure?

Perhaps posting clear pictures of everything would help.

 

[ivant]

B22
Preout is located at the center of the rear panel. All signal ground connected to a point somewhere under the cable mess on the right of the chassis. Ground is not switched at the input selector.

GC

 

[amb]

I can't tell how you did the chassis grounding. Are the input and pre-out jacks isolated from the case? How is the IEC's AC ground hooked up? Is there a ground loop breaker? How does the ground wires go from the input jacks to the volume pot and to the b22 PCBs? Is the headphone jack isolated from the chassis?

Maybe a hand-drawn diagram might help.