GC problem: voltage dropout on negative rail

[scone]

I've just finished my 100W gainclone which uses 2 LM3886 chips in parallel per channel. It sounds nice when it works, which is not always. I've found that at very very moderate (almost quiet) listening levels it behaves perfectly fine. However, as soon as the volume is raised a tad the negative voltage rail drops rapidly from 31V to around 15V.

I am using a version of Carlos' regulated PS. The first time I built the amp I used Carlos' supply and observed the behavor I mentioned above. I blamed the power supply and assumed that the 5A LM338 voltage regulators were not able to cope with the current spikes. I completly rebuilt the PS and added a current dumping bypass transistor (TIP2955). Otherwise the power supply is the same as carlos' original design. The transistor is connected to the PS the same way as shown on the bottom of this page . I used a .5 ohm resistor to bias the transistor. Now I've got a supply which should be capable of 15A at 31V, and yet the negative voltage rail still drops at around the same listening level as before. I kinda think the problem may be with my amplifier board, but I can't figure out what I've done wrong.

Has anyone observered this kind of behavior before? Anyone know what I can do to fix this?

Here is a jpeg of the top copper layer of the amplifier board:

 

[scone]

and here's a jpeg of the bottom layer:

 

[moamps]

Hi,

The PCBs look fine from what I can tell. To pinpoint the problem, could you answer the following:
a) What's the transformer's power rate?
b) What's the AC voltage of the secondaries?
c) What's the DC voltage on rectifiers' capacitors?
d) Have you also noticed a drop in the rectifier's capacitor voltage when the regulator's output voltage drops?
e) Have you tried to throw out the regulator to see how the amp would behave then?

A couple of pics would also be nice.

Regards,
Milan

 

[scone]

ok, so the problem is fixed for the most part, but I am still not sure what caused it. I assumed the problem was still with the PS so I tweaked a few things. I changed the resistor that biases the transitor to 3 ohms on the negative voltage rail so the transistor opens up sooner. The negative voltage rail now sits pretty comfortably at -30.8V and only drops a few mV during peaks. The transistor on the negative rail now does more of the voltage regulators work This fixes the dropouts on the negative rail.

I guess the problem might have been this... The voltage regulator was not fast enough to keep up with the changing load, perhaps because it did not have enough headroom (pre-regulated DC rails were 35/-35) and perhaps 4V of breathing room is insufficient?) However that doesn't explain why this only happens to the negative rail and not the positive.

I also thought it might have something to do with the fact that the + terminal of the negative half of the PS is tied to ground after the voltage regulator. Maybe if the AC ground is dirty that could cause fluxuation in the voltage? I'm not really sure, but perhaps the PS would have been better off with a negative voltage regulator on the negative side instead of two positive regulators? I'll post here if I ever figure out what the problem really was.

Also next time I'm going to build it with pots to change the voltage across the regulator. The voltage rails are not quite even at +31.5 and -30.8. The strange thing is they sit at +31.5 and -31.3 if I don't hook the amplifier boards up. Only then does the voltage on the negative rail drop to -30.8. ?? It's not really an issue right now, the DC offset across the speakers is only 1-4 mV, and the amp sounds great.

 

[scone]

oh, and to answer yours questions Milan:

a) What's the transformer's power rate?

500VA (overkill... i know)

b) What's the AC voltage of the secondaries?

27.6 +- a few mV for both

c) What's the DC voltage on rectifiers' capacitors?

35V

d) Have you also noticed a drop in the rectifier's capacitor voltage when the regulator's output voltage drops?

I didn't check that because it's very hard to get a multimeter in the gap.

e) Have you tried to throw out the regulator to see how the amp would behave then?

Actually, I let the magic smoke out of one of my transistors and created a nice 3ohm short across the voltage regulators in/out terminals. The negative rail sat at -35V after that. Worked quite well I did eventually replace the transistor...

 

[digi01]

I guess the problem is around the liner regulator.Full output current not available at high input-output voltages.
You can try to reduce the regulator's output voltage(raise the voltage drop of the regulator chips of +/- rails).

ZANG

 

[Upupa Epops]

Probably low beta of regulation transistor - main part of output current still " pull " VR.