distortion in one channel

[randytsuch]

Hi guys,
So I finally finished my gainclone, one channel sounds good, the other channel is very distorted.

I am building a non traditional gc. It is fully dual mono, after the AC input switch. It has two fuses, two 18 VAC, 225VA Plitron torroids, 4 bridges with Mur860’s, and 4 Jensen 10,000 uf caps. The PS section is in the front of my gc, with the IEC and power switch on the front side.

I am using the LM3886, inverted design with 220K feedback resistors, and 10K input resistors. IC’s have a 1,000 uf blackgate right at the chip, for each rail. I have no pot in my GC, 10k input is tied to the RCA jack. No input decoupling cap either. My real system uses a Transformer Volume control, so I am not worried about DC on the GC input. All resistors are soldered directly to the chip, for very short signal path. Have power and signal star grounds, for each channel. Power wiring uses 12 gauge THHN wire.

The PS rails looked fine, checked them with a meter before connecting them up, and they are giving me around 26VDC.

I also checked out the amps, after I hooked up the resistors, with a lab supply and a signal generator. Put a sine wave in, and got a bigger sine wave out, gain was around 15.

So, I hooked everything up. For this testing, I used an external CDROM drive, connected to a resistor/pot to ground for volume control, which connects to the GC.

I hooked up an 8 ohm resistor to the first channel, powered up the GC, and checked the signal out with a scope. Everything looked fine, so I hooked up an old speaker to it. Sounds good, I was pretty happy at this point.

So, I tried the other channel, and on the scope it looked good, but when I hooked up the speaker, sound was really distorted.

So now, I have to go into troubleshooting mode. Did not have time yesterday to really look at the cause of the distortion. I might hook up both 3886’s to the same PS today, but I don’t really think it’s the PS, but something with the 3886 and it's connections. I will check it out closely, and see if I find anything there.

Randy

 

[randytsuch]

Hi all,
I fixed it.
Only problem (to me) is I don’t really know what I did to fix it.

I ended up disconnecting the chips from the PS section, so I could unscrew the heatsinks from the base of my GC. Makes it easier to work on it. Added some more wire, so I could hook up the PS with the IC’s with the thing apart, so I could work on it, then power it up.

So, first I tried hooking up the bad side to the other PS. No change. Then as I described about, I disconnected things, laid them out, and reconnected it. Still had the distortion.

Then, I disconnected the two blackgates I have right at the chip. I also noticed the wire connected the two v+ pins looked like it was not making good contact with one of the pins. I connected that, and things work. After that, I disconnected the two V+ pins, and it still worked.

Then I hooked up the caps, one at a time, and it still worked. So, my GC now seems to work fine, except other then removing and reinstalling those two caps, I didn’t do anything. I also looked for shorts before doing anything, and could not find any.

The only thing I could think of is that I did have to move one of those caps slightly when I screwed the heatsink/IC’s into the baseplate. Might have caused a short that I missed when I looked.

So tonight, I screw everything back together, put it together in a little more reliable condition, and will start burning it in. Have one 8 ohm resistor from RS, will pick up one more, and let it run for awhile in the garage.

Randy