Humming GC

[fireman]

Last night, I put the 2 rectifier boards and the 2 amp boards together. Today, after coming back from work, I finished wiring the boards. It looks like this:



Last night, when I put the rectifier board together, I got a reading somewhere around 30V DC out of it (under no load). Since I am using a Chia Yu 21 V transformer, that looks pretty fine to me.

I decided to leave the rectifier board attached to the amp board so when came time to wire the two together, I just used jumpers across the boards as can be seen here.



With no sources attached to the input (well I had my Alps 50k pot wired but that's not a source anyway), I turned it on. For one or two seconds, there was no sound and then it started humming. Not very loud (maybe as loud as a person's voice) but it hummed nonetheless After a few seconds, I turned it off and when I approched my hand near the rectifier, I could feel that the diodes were hot, really hot I also notice that the toroid also becomes quite warm to the touch.

I tried the second board with the same result. One or two seconds of silence followed with the dreaded hum. I guess I did the same mistake twice. I separated the rectifier board from the amp board on one channel to see if it would make any difference but it didn't.

I also tried shorting the input but ended up with the same result.

I've had a tough week so maybe I'm just tired and not seeing the obvious. I'm gonna close up shop right after I finish this message and call it a night. Hopefully, tomorrow after reading your answers I'll be able to find the problem and start listening to some music.

Here is the bottom of the boards before you ask. This one had not been cleaned with rubbing alcool before I took the picture.



I haven't checked the DC offset of the amps since I don't want to fry the rectifier nor the toroid. It sure looks like there's a short somewhere. Can you guys help me find the problem ? Pleaaaase !!!

[Jean]

if you disconnected the bridge board from the chip board and the diodes still get hot, then you have a problem with diodes in the bridge. you might have also damaged the chip too if the voltage or polarity wasn't correct.

i woudln't connect the bridge board to the amplifier board until you get it fixed to the point where it doesn't get hot, and you have correct voltage on the output of both rails.

did you check the toroids before connecting to the bridge ?
when you solder the bridge you can check for correct continuity (sp) with ohm meter to make sure the isn't a SHORT on the AC side of the bridge(s).

good luck

[Vikash]

I've read somewhere that keeping the rectifier board so close to the amp may also induce hum. Not related to the the diodes and transformer heating up though.

Jean, people are obeying your signature over this side

[fireman]

Originally posted by Jean
if you disconnected the bridge board from the chip board and the diodes still get hot, then you have a problem with diodes in the bridge. you might have also damaged the chip too if the voltage or polarity wasn't correct.

That is not a problem. When everything is attached together, the diodes become hot in a matter of seconds. I disconnected the bridge board and let it run for 10 minutes already and it's still cold. The toroid is also cold.

i woudln't connect the bridge board to the amplifier board until you get it fixed to the point where it doesn't get hot, and you have correct voltage on the output of both rails.

That's what I figured out and it's also why I didn't try to measure DC offset at the output. I was afraid I might blow something.

did you check the toroids before connecting to the bridge ?
when you solder the bridge you can check for correct continuity (sp) with ohm meter to make sure the isn't a SHORT on the AC side of the bridge(s).

I did. Since I want to build monoblocks, I have two. I tested both of them. Those Chia Yu toroids were originally center tap trannies. I unwrapped them and split the center tap so I now have dual secondaries.

Toroid #1: 24.6 VAC and 22.3 VAC
Toroid #2: 24.0 VAC and 22.6 VAC

When I connect a bridge board to one tranny (in this case toroid #2), I get 29.6 VDC on V- PG- and 28.5 VDC on V+ PG+. As I said in my first post, there seems to be a short on the amp board. But what puzzles me most is that I get the same behaviour with both boards...

Originally posted by Vikash
I've read somewhere that keeping the rectifier board so close to the amp may also induce hum. Not related to the the diodes and transformer heating up though.

Yeah, I read that too yesterday after I noticed something wasn't working properly. I have since separated one of the bridge board from the amp board to see if it made a difference. When I get this humming and overheating problem taken care of, I'll let you know if it makes a difference.

[jan.didden]

Originally posted by fireman
[snip]When I connect a bridge board to one tranny (in this case toroid #2), I get 29.6 VDC on V- PG- and 28.5 VDC on V+ PG+. As I said in my first post, there seems to be a short on the amp board. But what puzzles me most is that I get the same behaviour with both boards... [snip]

Just to be sure, and probably make a fool of myself, but I assume that the +V is +28.5 and the V- is -29.6 , right....?

Jan Didden

[fireman]

Originally posted by janneman



Just to be sure, and probably make a fool of myself, but I assume that the +V is +28.5 and the V- is -29.6 , right....?

Jan Didden

I put the red lead on V+ and the black on PG+ and got 28.5 VDC. Then I put the red lead on PG- and the black lead on V- and got 29.6 VDC. Since PG+ and PG- should be the neutral (or reference), I should have put the red lead on V- and the black lead on PG- right ? I would then get -29.6 VDC.

Sorry for the confusion. I'm not used to working with a bridge board. I can read schematics but I have actually never built something like this before.

[Peter Daniel]

Originally posted by fireman

Since PG+ and PG- should be the neutral (or reference), I should have put the red lead on V- and the black lead on PG- right ? I would then get -29.6 VDC.

Right.

[fireman]

I finally tracked my problem down. When I split the center tap of my Chia Yu trannies, I color coded the shrink wrap on the solders in order to remember which is which

Well guess what ? I didn't look at the color code when I wired the trannies to the connector blocks and screwed up !

I have now taken the liberty to correct this problem and now the dreaded hum is gone. I only briefly tested one amp and it works great !!! I have only a small problem. I tried controlling the volume with my 50K Alps pot. I have to turn it almost half the way before I can hear something. But when I do hear something it is quite loud. I haven't tried turning it further for now. I might try it tomorrow to see if the volume level increases.

Gotta go. I have to go pick my sister up at the airport.

[fireman]

I did more tests this morning. The DC offset of the amps are 10.2 mV and 5.4 mV I am glad this thing is finally working. Now all I have to do is find a way to make my volume control work.

As I stated earlier, I have to turn my pot almost half the way around before I start hearing something. And then, it is quite loud and stays about the same until I get it all the way around.

Picture of the pot here and here.

Any ideas on what's going on here ?

[jackinnj]

if it's a Canadian GC, it's probably humming "Rose Marie", just turn down the Nelson Eddy.