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Mod Squad Prism CD Player output too high

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dave_n_s

Active Member
I've got a Mod Squad (Philips-based) CD player being prepared for sale for our hospice thrift shop on my bench and having repaired a couple of power issues to get it working, it's generating an audio output of something like 8V peak to peak at the highest, overdriving my bench receiver into distortion.

There's no clipping showing on the scope at the CD player outputs and no front panel or internal output level adjustment.

In the absence of documentation can anyone cast any light on this please.

The whole point of the Mod Squad components was their fine quality audio stages, so clearly something amiss.

Bench and brain are suitably equipped to troubleshoot most things, but diving in to find a source of excessive gain without a circuit is not a good idea

thanks!
 
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Both channels, or just one?

Both channels would suggest a design or consistent build error; wrong component in a buffer opamp stage, perhaps?
 
It's both channels. I can conceive of a supply rail to the custom audio card causing a common issue, but without any technical info, it would be a significant amount of work and I'm in fund-raising mode here. Lots of other stuff in the donation queue for much simpler refurbs.
 
dave_n_s said:
I can conceive of a supply rail to the custom audio card causing a common issue,
Click to expand...

The only supply rail I can think of that might cause that problem would be the DAC Vref. The gain of op-amps should be set by the feedback resistor ratio, and be independent of supply voltage. The only way in which an op-amp supply has an effect on the output level is via clipping in the event of excursion exceeding the rails (and the op-amp's ability to drive near the rails. You're not getting clipping.

Find the DAC chip Vref* & see if it is compatible with the spec.
Find the DAC chip Vout and measure the swing & see if it is compatible with the spec & Vref.
Find the output buffer op-amp and check the feedback ratio (look at the feedback resistor divider between Vo/V-/gnd, depending on configuration).
Check that DAC chip Vout * op-amp feedback ratio = measured output swing at the o/p connectors
Change the feedback ratio to set the desired o/p swing.
It is likely that the buffer amp also includes filter components; you'd need to make sure your changes to the feedback resistors to set the reduced gain didn't change the filter break points.

* it may also be an Iref
 
By the way, there's a Stereophile review of the Mod Squad Prism II where they noted that it had very high output levels, which makes sense as the company's preamp was a passive unit.
 
dave_n_s said:
I've got a Mod Squad (Philips-based) CD player being prepared for sale for our hospice thrift shop on my bench and having repaired a couple of power issues to get it working, it's generating an audio output of something like 8V peak to peak at the highest, overdriving my bench receiver into distortion.

There's no clipping showing on the scope at the CD player outputs and no front panel or internal output level adjustment.

In the absence of documentation can anyone cast any light on this please.

The whole point of the Mod Squad components was their fine quality audio stages, so clearly something amiss.

Bench and brain are suitably equipped to troubleshoot most things, but diving in to find a source of excessive gain without a circuit is not a good idea

thanks!
Click to expand...

That sounds just like my CAL Alpha DAC when I first got it. The output was way high and distorting with my preamp. I called CAL and they said the reason the output was high was to allow it to drive passive preamps. All it needed was a resistor change for the voltage divider at the output. They offered to do it for free and pay shipping both ways. The board was silk screened so I ask which resistors needed changing and did it myself. You might look around near the output and see if there is a voltage divider and measure there.

Good luck
 
I'd prefer it as-is for my own purposes as most active preamps ime are disappointing, but it would be more "universal" if the output was lowered to a compromise level.
 
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