Troubles with new power supply

[soundNERD]

I had built a set of 2.1 speakers for my computer using 2 x 3886 for the satellites and 2 x opa549 bridged for the subwoofer.

I originally had two 16volt 10watt doorbell transformers powering it, and had very poor volume output.

Later, I found 2 transformers, both 120V to 15V and 7V. So, I used the 15V tap on both to power the circuit (using both the 15V and the 7V in series generated about +/-31VDC, which is one volt over the max for the 549s and caused them to operate poorly and get very hot.


This puts out about 3.5ADC (measured on the positive rail) which although much better than 10 watts, isn't exactly enough for full power, it still sounds much better.

There is one annoying problem, though. The 3886 amps are dead silent, but there is an annoying buzz coming from the sub box. I can't tell if it is the transformers, but I hear it from the speaker, and if I put my ear by where the amps are, I also hear an odd sound coming from the chips or some other part.

What is causing this? I don't remember it happneing before I put the speaker in the box (but I could be wrong), so could it have something to do with magnetic waves from the speaker and the transformers making that buzzing sound as a result?

Thank you for the help.

[mateo88]

The transformers might be hooked up out of phase.

[soundNERD]

Well, I have them sitting directly next to each other, and are in parallel connection to 120VAC. I checked, the wires connecting the two primaries are hooked to the same side of the transformers.

I was able to tighten the mounting screws on them so they don't shake as much when the hum, and that helped a lot.

But what I find odd is the 3886 amps are not affected by this, only the 549s.

[carlosfm]

That trafo is feeding 2xLM3886 and 2xOPA549?
This is what I understood from your post.
You should use a trafo for the OPA549s, and high capacitance on the PSU (yes, I hope you are not using it with 1000uf per rail).
The OPA549 is more noisy than the LMs.
The first time I used a regulated PSU with these chips was with my sub (2 // OPA549s).
The big woofer kept singing with 50hz noise, and I had 10,000uf per rail.
This is, of course, much more noticeable in a sub.
I made a regulated PSU at +/-29V with LM338s and the sub is dead silent.

[soundNERD]

Well, I have 10,000uf per rail for both the 549s and 3886s.

The 3886s are dead silent before putting my ear against the driver, but the sub is annoyingly loud.

When I took out the speaker, I found that the transformers are making much of the buzz noise, and the sub is making even more buzz.

Why should I use a seperate transformer for the 3886s and 549s? Not enough power?

[carlosfm]

The fact that the trafo is buzzing may indicate it's working on it's limits.
Is it ?

+/-31V DC is over the top for the OPA549s.
Using this kind of voltage with 4 chips you need a serious trafo, I believe it's not the case.

[soundNERD]

well, they shouldn't because that is even with no sound going through it.

also, +/-31 is not what I am using, more like +/-20.