Parasound D/AC 1000 issue

[Pure_Brew]

Hi,
When I turned on my stereo last night, I had a whiny high pitched sound coming out of both speakers at a level close to the sound of background music-fairly loud. It was not effected by input or balance. The sound changed a bit with volume and tone.
First I thought - great - my preamp is toast. After systematically powering down, restarting and unplugging different devices/rca connectors, I found that the sound went away with the DAC's RCA connectors unplugged from the preamp.
Next thing I did, which I hadn't tried before is unplugging the power cord from the DAC while the system was on. The sound went away immediately, and stayed gone went I plugged it back in. Keep in mind, the sound comming through the system was the same with the DAC turned on or off.
Finally, I had it all on for a few hours, playing music, everything sounding just right. When I turned the system off, I had first turned the Amp off.
Well, the sound came back, when I snapped the amp off, during the period of the amp depleting its power supply for a few seconds.
Given all this, I think there is something wrong with the DAC, and whatever it is, power and/or power fluctuations seems to freak it out.
This is all plugged into a very basic monster type power strip.
What do you think the problem might be with this, and what might I want to look at first? worth fixing?

Thanks in advance.

[Bill Fuss]

Wow, no replies at all. If you like it I would go ahead and recap the entire unit. A lot of older equipment stays powered up even when the switch is off. I have two players like that.

[Pure_Brew]

Thanks for the reply. After about a week of use (the dac had been recently purchased). I haven't had this problem again.

What I did is swap out the amplifier, since the problem seemed to rear its ugly head when powering the amp up/down. I tried 2 different amps and ran the system alot. So far, no more noise. This makes some sense now, since the preamp really didn't effect this loud noise, on any given input or otherwise.

I suppose this doesn't prove the dac is 100% problem free, but what weirdness! The old amp seems to mess with the dac, then the dac, inturn, causes the amp to make this noise.

[Kurt von Kubik]

It seems as if your DAC cannot keep locked on the SPDIF signal during powerline transients.
Actually I think this is a not a severe problem, it might be solved with a small capacitor over the powerswitch on your amp, or replacement of the powerswitch, because it can be weared out.

On the other hand, it also seems as if attenuation of incoming noise on the PSU of your DAC is insufficient. Transients or ripple should not reach the digital circuits, which themselves also attanuates incoming noise in the powerrails.
Maybe the signal onto which the DAC is supposed to lock, is a bit fluffy, and the recieverchip already has problems with locking to the signal, thus the easy loss of it.

Do you know which reciever chip is used in your DAC?
Maybe this can help disclosing the origin of your problem.

[Pure_Brew]

I haven't opened up the DAC yet (well, not lately), and I'm not entirely sure which chip I should be looking at, but I can locate the DAC chips which I believe are PCM63's/J grade.

The amp I was using is still suspect. It's not a common design (not common to me). The power switch acts only as a small signal to a huge 40A Open Chassis relay - which makes a nice blue flash/snap across the terminals. This is normal operation on the PS AUDIO 200c as described by the manufacturer circa 1986ish. It also is a direct coupled amplifier and must be tested prior to hooking up loudspeakers. Four indicator lights on the front panel will show if there is any DC coming from the preamp, which would be multiplied 30x if present. The amp has 2 inputs, one for DC, one for AC. The AC is capacitor coupled, the DC is not. I had it hooked up with the DC input only, but I suppose I could have tried the AC input, as suggested if "DC problems" arise. This amp is also recommended to stay on 24/7.

[Kurt von Kubik]

I think the blue flash is what causes your trouble, or at least the somewhat oversensitiviness of it.
The reciever chip might be on the edge of locking, and then the transient through the mains supply or through the common ground plane makes it shatter.

One solution might be to isolate the DAC from the rest of the system. In studios they use transformers, which I cannot recommend.
Another could be to install a soft start circuit in your amp, which to me seems to be the right solution to your problem.
A third way could be to leave the power amp on 24/7/365, but in these climatic troublesome times, you might be better of looking for a more environmental freindly solution.
A fourth way could be to install a capacitor parallel to the relay, this might work with a bit of luck.

Good luck finding your way.

[Pure_Brew]

I appreciate the input. I suppose I could just use something like a component size power strip/surge protector to power on/off the DAC after the amp.

Your suggestion about a soft start circuit sounds reasonable. Another poster, which I haven't been able to get in touch with, swapped out that open relay for a high quality solid-state relay due to noise issues (coil/transformer hum-which this amp is notorious for).In the end this solved his problem. So based on your recommendation, and other potential issues, this solution is low cost and a GO. Worth a shot imo.

It seems like it would be an easy swap, but I would most likely have to have a local Tech do this for me. There's a thermal protection circuit from the heat sinks to the relay, and I can't quite see/understand how the new style relay should be wired. I might try a post again in another forum.

Thanks again.