Help needed for BZLS: low freq oscillation

[EJ]

Dear all,

I tried to build a balanced zen line stage several weeks ago, and now
it's almost finished. However, there seems to be some low frequency oscillation
because each time I connected the bzls to my power amp for test( a simple diy
LM3886) after the power transient has settled down, the bass cone kept thumping
forward and backward furiously about seven or eight times per second (both channel
). I thought it was some kind of layout problem, but I tried Mr.Pass's original
layout but omitted the negative input and output related parts and the oscillation
didn't go away. Now I have no idea what else I can do.

Here are some more detail about my modification to my circuit:
1. IRF630 instead of IRF610 for Q1 and Q2
2. Omitted R9, R11, P3, C1 (negative output)
3. Shorted R14 (negative input)
4. Used two 3kohm/2W in parallel instead of the Source resistors
5. Used two 1.5kohm/1W in parallel intead of the Drain resistors
6. used 400ohm instead of R15 and P5 (between Sources of the
transistors)
7. The rail voltages are about +/-58V. All pin voltages of the
transistors are just the same as Mr. Pass described in his article.
8. The biasing current is somewhat smaller that usual, which is
about 65mA each chanel.
9. I don't have an oscilloscope, but I can see the output of this
circuit drifting with my multimeter (input grounded)

Can anybody kindly give me some advice about what is wrong about my circuit
? Thank you!!

EJ.

 

[nar]

I suggest you to construct the whole thing again with respect of all NP's design points.The balanced version is really helpfull to hook up a balanced input amp.Did you try to plug it to another material like headphones amp to see if the oscillation still here?

Every mos must have his own 221R series with the gate,and at the input you MUST leave the small resistors series signal,and parallel to gnd after cap.

I had no problem like this with mine.So PLEASE do observe all NP's construction details once again.

Did you check the oscillation wouldn't come from the PSU.To do so ,use a cap series on the supply rails and a 100k resistoe after it parallel to ground,that on the 2 rails and hook up high sensivity headphones.You will hear all PSU noise,so you can check if the problem is not the PSU.If not,do another mainboard and cable all elements like Nelson stated;Should be no more problem afterwards.

Regards

 

[EJ]

I have found the problem

It's no oscillation at all.

As I read through the article NP wrote about bride of zen, he mentioned that the power ripple will couple into the output and thus cause noises. This could also happen to my BZLS since I omitted the balanced part. So I changed R16 and R17 to 33ohm, C3 and C4 to 4700uF to further filter the ripple, and it worked! The thumping of my speaker is still there, but not that hard now.
I will now try some other kind of voltage regulator. Hope this will help, because I really like the sound of BZLS very much.