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Old 14th February 2007, 07:28 AM   #1
TDWesty is offline TDWesty  Canada
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Default Help, new PSU blowing fuses

I built my BrianGT chip amp tonight, using an old 100VA Hammond EI transformer for testing. It is 25V CT, and after building the PS and wiring it up, I saw 37V+ and 37V- and expected. Fine.

So then I went on wire up the amp boards, taking care to get everything connected correctly. Fired it up with no signal, and immediately got a loud buzz from the transformer and blew a 2A (fast) fuse. Hmm. Checked everything again for shorts on the chassis (boards are each mounted on a single brass standoff screwed into the chassis, but nothing else is touching. Tried it with just one amp connected, and blew a 3A fast fuse. I then tried the PS alone (as I had when testing), and blew 5A slowblow fuse (rather quickly...)

I can't understand how a PS which worked fine and yielded expected DC voltages could just start blowing fuses like this. I've double checked caps and diodes for correct placement and all seems fine.
----
I think I found the problem on the PS. The jumper I have to connect the CT of the transformer appears to have made contact with a diode pin on the bottom side. I trimmed the diode a bit more and the PS now starts without blowing a fuse.

However, wiring one amp back in still produces a buzz at power on - I killed it before the slow blow fuse goes. I can't see any shorts, and I've cleaned the board with alcohol.

Ok, now I'm out of fuses. I give up for tonight anyway. :-(
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Old 14th February 2007, 04:28 PM   #2
TDWesty is offline TDWesty  Canada
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Ok, I think I found my problem here

This describes it exactly. So I need to use the method explained in the older docs for the 3875 kit here.

But the PS boards are a little different in the current 3886 kit I have. Could someone please confirm this method will work with the newer PS boards?

Thanks!
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Old 15th February 2007, 02:54 AM   #3
TDWesty is offline TDWesty  Canada
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I was able to compare the BrianGT old version PS board with the new version, and get it to work with my CT transformer, and no blown fuses! :-)

Wired the amps back in and it still works! DC measured 100mv on one side at power on but quickly went down to 15mv after a few seconds. The other side measured 7V (yikes), but again dropped to 15mv in a few seconds.

I wired it up to my speakers, again checking for DC first, and detected some hum. Wired in my 3020 as preamp, and got more hum. Hmm. Decided to have a quick listen anyway, and it sounded not bad. After 4-5 minutes the heat sinks (smallish P3 style) were hot to the touch, so I powered it off.

Next to locate the source of the hum - I know this is a common topic, so I'll search the archives.

That's it for now, I'm getting the evil eye, so that's all I can get away with on Valentine's Day! ;-)
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Old 15th February 2007, 05:32 PM   #4
AndrewT is online now AndrewT  Scotland
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Hi,
I don't know what size your heatsink is but a pair of 3875 dissipate about 7 to 8W.

2C/W gives a deltaT>=16Cdeg and add on ambient and you end up at a sink temp of about 40degC and Tc~=45degC. That is quite hot.
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Old 16th February 2007, 01:54 AM   #5
TDWesty is offline TDWesty  Canada
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They are aluminum sinks from a P3 or Celeron CPU, about 2" square by 1" deep. They were definitely hotter than 40C, I'd say closer to 70C, as they were too hot to touch for more than a few seconds. I plan on trying some earlier P3 slot1 sinks that are about 2.5x the size, as soon as I can find a matched pair.

Still trying to resolve my hum problems before I try it again.

Thanks.
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Old 16th February 2007, 06:43 AM   #6
TDWesty is offline TDWesty  Canada
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Still trying to reduce the hum in my briangt GC amp. I've read several threads about grounding, including this one, and have tried every grounding arrangement I can think of, and still get terrible hum as soon as an RCA cable is connected, even if the cable is not attached to the preamp. Hum is nearly gone with no cable connected. The only combination which gets rid of hum with RCA input connected is to ground the input SG to the chassis ground, which also appears to short out the amps and activate protection, since output is dead until I power off/on the amp to reset.

I've done everything as per BrianGT's stereo instructions, with the exception of using a single CT transformer, which required taking half of the rectifiers out of the PS board (as per earlier 3875 intructions).

I have this ground setup currently:

mains GND -> chassis
CT -> PG+ & PG- on PS board -> chassis
CHG on each amp -> chassis

The above are on a single ground post, in the order shown (mains on bottom). I have tried it without the second line, but this made not difference.

This is the PS setup I have: page 15 of the 3875 briangt board here.

I have tried some of the suggestions in the first thread I referenced above, but they either made no difference, or shorted out the amp outputs, so I am clearly misunderstanding something. Brian's guide makes the grounding sound so simple, so I wonder if my non-standard CT arrangement is the problem?
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Old 16th February 2007, 09:50 AM   #7
AndrewT is online now AndrewT  Scotland
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Hi,
disconnect all the audio grounds from the chassis.
maintain the safety ground connection to the chassis.

Create a floating audio ground for all the returns and grounds that the audio side needs. This can be an isolated tag strip bolted to chassis, or a terminal strip , or simply a 3mm nut and set screw for all the 3mm solder tags on the ends of each of your many ground wires.

Keep the RCA ground at the side most remote from the disconnecting network that links to the safety ground.
I place mine in strict order:- signal, speaker, Zobel (if separate from speaker), PCB power, centre tap, PSU 0v, disconnecting network. Some experimentation in the order of attaching each of these to the bolted floating ground may improve the noise performance.
The RCA may go to either the PCB signal ground or to the floating ground but not both.

When you have the floating ground installed but without the disconnecting network check that your multimeter indicates open circuit between safety earth and audio ground.

If you build a multichannel amp then I recommend a separate audio ground for each channel, each with it's own disconnecting network to safety ground. This is different from usual chipamp advice, so more room for experimentation.
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Old 17th February 2007, 06:34 PM   #8
TDWesty is offline TDWesty  Canada
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Andrew,

Thanks for the help, but I still can't figure this out. I think I understand the need to keep SG and PSU grounds separate, but I can't see how this can be achieved with the chipamp.com boards I have. On the amp board, SG, PG and CHG are all tied together on the board traces. So how do I provide separate SG and PG ground leads to the chassis?
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Old 17th February 2007, 07:00 PM   #9
TDWesty is offline TDWesty  Canada
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Please disregard previous... I found the source of the hum - my input RCA wiring was reversed.

Too many late nights I guess. Hum level is now tolerable for basic listening, and I will the suggested disconnecting network to get rid of the remaining noise.
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Old 17th February 2007, 08:21 PM   #10
AndrewT is online now AndrewT  Scotland
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Oh! a PCB with combined power and signal ground.
You need to follow BrainGT's instructions for minimising hum.
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