Spectral DMA-50 distorted output

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BlownRx7

Member
Joined 2004
#1
So a friend of mine cleaned out his garage and gave me this amp. Only problem is that the left channel runs pretty clean at very low volume but gets distorted as volume is increased. The right channel is distorted all the time and has very low output. Any ideas where to start looking. The boards seem pretty simple but I have no expertise and no schematic so looking for any advice.
BTW, there are a grand total of 6 electrolytic caps in the entire amp: 4-330uF caps on the driver board and the two big ps caps
TIA
 
#7
re DMC-10 and DMA-50 from the designer

1audio said:
In my experience those amps are pretty durable. The output transistors (Hitachi's) are nearly bulletproof. If it is oscillating the output network would heat up and the heatsinks will get hot but no other bad side effects. It was designed to deliver full power to 500 kHz. If the components were unable to handle that then we would have lots of smoke in testing.

The problems with that amp are: first, thermal cycling will cause the connections between the outputs and the circuit board to get flakey. They may need to be resoldered. Second the bias may have drifted. Its open loop so it should be checked. I don't remember the settings.

FET failures are easy to spot, the gate current goes up. If there is a measurable voltage across the gate resistor the device is blown. However if its working and doesn't have offset it should be fine. The heatsinks will run warm.

The preamp is even simpler. The output protection circuit is its problem. Just remove the LM339 and it will sound better. As usual thats a fix for the earlier preamp that was not necessary for this generation.
Click to expand...

took me a while to find this, hope it helps

regard
james
 
#9
I have the schematics in a binder, never scanned.

The good part is that the circuit is pretty simple.
The first step is to do the usual troubleshooting methods, check for shorted devices, visually inspect for bad solder connections. look for broken/cracked resistors.

Then bring it up on a variac slowly and observe the line current draw AND the DC offset.
That needs a scope. And a signal, sine or triangle for input...

The amp ought to be stable DC wise by about 35-40vac of line voltage, I would expect.

IF nothing is blown up, and both channels seem to amplify, WITHOUT DC offset, then check the bias... the bias pot may be open and the devices more or less cut off - or maybe someone got in there and turned the pots. (entirely possible).

Fwiw, that is an amp with a bandwidth, iirc of ~1mHz!!

If you give up on it, I would buy the chassis from you, just to muck around with it...

_-_-bear
 
#10
mlloyd1 said:
can you share any good, detailed photos of the internals?
i always liked looking at Spectrals DMA products that Demian had a hand in ...

mlloyd1
Click to expand...

Here are some photos.
I had removed the board to look for burn spots and look at the solder joints and everything looked ok. I will go back and check a little more closely.

I do not believe the unit was ever opened prior to me so everything (including the pots) are undisturbed.
I don't have a scope so I can't look at waveforms (don't know if I would know what I'm looking at either...)

Bear,
If you can scan the schematic, I would be most appreciative. I will send a pm.
 

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