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Hi Pierre, did you install the caps in the shunt reg? Sorry I do not remember if you mentioned this earlier or not..... Also, would be interesting to know the frequency of the oscillations. Did you measure the supply rails?
The regulators should be stable without load, so that should make no difference. As a alast resort, you may want to change C105 / C205 for "bad" capacitors with a high ESR, or put 1Ohm resistors in series - if these caps are too good that can cause oscillation as well.
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Yep, caps are installed. I tried various values between 680 and 3300 pF.
Oscillation frequencies are around 5 MHz with a 60+ MHz overlay.
Attachments
Supply rails oscillate too, with greater amplitude (4x) than the regulated outputs.
Hi Pierre,
try lower values for the compensation caps. In my setup (D45H11/D44H11/FQPF11/FPQF20, BC327/337 around 420 cause I didn't have any lower, SK170) I have to use 47pF/47pF on the outer BC's (Q104/204) and on the other board 47pF (positive) 147pF (negative) to get rid of the oscillations. I'm very suprised that there is only a small margin of compensation wich is successful. in three positions no comp and everything over 47pF (only had 100pF in my box) didn't work.
I didn't see any difference on the scope if I change the value of the inner caps - so I use 2,2nF, which I have at hand.
Maybe the following information is also useful for you: I have to use D45/D44. With SC5171/SA1930 (which I use before cause they are isolated) I also have had oscillations with similar amplitudes like you (~100mV ppk) and I#m not able to cure them.
galac
p.s. be sure to have no moving air or fan near the board when you scope it.
Hi Galac
Thank you for your detailed advice. Much appreciated. I will try lower cap values along the lines you suggest and report back.
Regards - Pierre
Try to test the CCS on it's own, take a 10 ohm resistor from the + or - 18V output to ground and measure current and see if there is still oscillation. If there is oscillation then the problem is in the CVS/CCS (top half) else the problem is in the shunt (bottom half) of the supply. While testing leave the pre-amp disconnected.
where did you find this information?
100mV across 10r amounts to a dissipation of 1mW
1.1V across 10r gives a dissipation of ~120mW
1.1V across 10r gives a current of 110mA
there is conflicting information on what the correct size of the output inductor.
I went for 27T to match the 10uH requirement.
0.5uH is more suited to very low impedance loads, like 8ohms speaker.
4mm former is very small. Larger diameter gives larger inductance.