MyRefC build guide

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Greetings all

After about a year of procrastination, I've finally completed the My_Ref amp. Worked perfectly on first power on and sounds great. I realise that I am late to the party and people have probably moved on to other interests, but I would like to thank Siva (Linuxguru) for making the boards available and getting me started and Dario and Tom for their advice. I would also like to thank the other knowledgeable people on this forum who contributed to the many threads on this amp - Reading your advice, opinions and experiences gave me the confidence to embark on the project.
I'm now looking forward to years of listening pleasure.

Regards

Fred
 
Sorry for dragging up an old post - I can't believe it's been nearly 6 years since I first started this project. The bits have been languishing in a box all this time but I have finally found some time and populated all of the boards from Uriah's original BOM for my 5 channel amp. I decided to build one 2 channel amp and another 3 channel.

The problem is that having assembled the 2 channel box I have a hum when nothing is connected to the input. It is equally as loud on both channels but drops considerably when I connect a phono cable. I only have to connect the central conductor for the noise to drop and it drops further when the source starts to play a track.

I have tried an alternative mains cable to the strip connector after the on/off switch which made no difference. I also tried a different phono connector with a spare Obligatto cap connected to the PCB with a twisted pair, again, no difference.

Connecting the copper body of the Obliggato caps to earth does make it significantly quieter but does not eradicate it.

I'd like to figure out what the problem is so that I can fix it and not make the same mistake for the 3 channel.

Any suggestions?

Thanks
 

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Sorry for dragging up an old post - I can't believe it's been nearly 6 years since I first started this project. The bits have been languishing in a box all this time but I have finally found some time and populated all of the boards from Uriah's original BOM for my 5 channel amp. I decided to build one 2 channel amp and another 3 channel.

The problem is that having assembled the 2 channel box I have a hum when nothing is connected to the input. It is equally as loud on both channels but drops considerably when I connect a phono cable. I only have to connect the central conductor for the noise to drop and it drops further when the source starts to play a track.

I have tried an alternative mains cable to the strip connector after the on/off switch which made no difference. I also tried a different phono connector with a spare Obligatto cap connected to the PCB with a twisted pair, again, no difference.

Connecting the copper body of the Obliggato caps to earth does make it significantly quieter but does not eradicate it.

I'd like to figure out what the problem is so that I can fix it and not make the same mistake for the 3 channel.

Any suggestions?

Thanks
You don't have a metal/conductive chassis/enclosure.
Therefore your Protective Earth (PE) should go to the biggest piece of metalwork in there. Bolt the PE directly to each heatsink. Don't go through a connector. That increases the risk of a broken PE.

Becaue you don't have an enclosure you have effectively two monoblocks inside your wooden box. A monoblock is VERY easy to get hum free, because there is no connection to another channel.

Leave the two inputs open and measure the output Hum+Noise on the 199.9mVac scale of your DMM, then measure the offset voltage of both channels on the 199.9mVdc scale.
Post those measurements here.

Then add an input shorting plug to both inputs and re-measure the offsets & H+N Post them here.
 
Thank you both. I have grounded the heatsink and it is indeed much quieter.

Andrew, do you mean connect the PE straight from the back of the IEC onto the heatsinks? Should I leave the PE connected to the PCB?

Also, I need to do some reading about taking the measurements that you suggested, basic I know, but it's been a while since I first started this project. My DMM has a 200mVac & 200mVdc scale (as well as a flat battery:eek:). Is this suitable?

Thanks again
 
You can leave an extra PE to PCB, but you must make sure that you are not creating a loop in either channel.

The PE is what protects you from a catastrophic failure in the Mains wiring.
It prevents any part from being LIVE by blowing the mains fuse very quickly.

What metal parts are you likely to touch?
The Input RCA barrels and the Output Terminals?
Anything else?
These should connect to the protected heatsinks (or big metalwork).
The PCB tracks could be so thin that they rupture before the Mains fuse. Or have a significant impedance such that they slow down the blowing of the Mains fuse.

Yes the 200mVac and 200mVdc scales are the ones you should use. But start with higher scales first to find out what voltages might be on the test points.

Finally:
how does the heat get out your wooden box?
Two channels of 3886 could have 10W of dissipation (look at the datasheet for 1W of output for a 3886)
 
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Ok, thanks for that. I now have a very nice sounding 2 channel amp :)

It's hard to see from the pictures, but there are holes cut out of the bottom directly under the heat sinks and there are also adjacent holes in the top so things should stay nice and cool.

I've managed to build the next box with the three channels in, but I've hit another problem.

One of the boards is ok but the other two are not. They both have the same fault. When I power on, the LED doesn't light and the relay doesn't click but after a while the heat sink gets quite hot. Also the mains bulb tester initially glows brightly then dims to a very faint glow, not quite going out.

I've checked all the components and also looked for dry joints but can't find any visible difference between the good and bad boards.

The voltage on AC1 and AC2 is 27V but I have nothing on D2 and D3.

It seems strange to have exactly the same symptoms on two boards.

I don't really know where else to look.

Any help would be much appreciated
Thanks
 
disconnect all three amplifiers from the PSU.
plug in the remainder to the Bulb tester and check voltages and polarities.
Switch off.
Discharge the PSU.

Attach one amplifier.
Power on via the bulb tester.
Check voltages.
Switch off
Remove amplifier
Discharge PSU

Repeat for each amplifier.

Finally, add the proved amplifiers, one at a time, to the PSU, checking via the bulb tester each time.
 
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Andrew, thanks for taking the time to respond.

I've now got a good and a bad amp set up on my bench using a mains bulb tester, I've printed off the schematic - which I don't fully understand - and I'm doing some A/B comparison voltage checks using a live ground as you mentioned - thanks :eek:.

Good amp:
AC1 = 24.4 VAC
AC2 = 24.3 VAC
D2 anode 25.2 VAC
D1 anode 24.9 VAC
D1 & D2 cathode 18.2 VDC
D1 & D2 cathode 7.9 VAC
R14 out = 12.5 VDC
R22 drops from 12VDC to 1.8VDC
R21 drops from 12VDC to 1.3VDC

Bad amp:
AC1 = 24.3 VAC
AC1 = 24.6 VAC
D2 anode 24.7 VAC
D1 anode 24.7 VAC
D1 & D2 cathode 30.8 VDC
D1 & D2 cathode 0 VAC
R14 out = 30.2 VDC
R22 stays at 28.3 VDC across it
R21 is open circuit


Not sure how to interpret this.
Thanks
 
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