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Replacement for Parasound HCA-500?

PaulyT

PaulyT

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So, @Flint, one of the HCA-500 amps you sold me with your DIY speakers quite a few years ago (5+? I don't even remember) is apparently failing, just not producing the output of its twin. Other folks: these are used to power my active crossover L/R mains, two amps with two channels each.

I still have that HCA-750 that was also part of that purchase, that I'm not using because I never got around to installing the center channel, and I'm using the Sony 4-channel amp for the two rear surrounds. However, I'm hesitant to install a different amp for L vs. R in the mains, as symmetry is important.

Any suggestions on a replacement? Or, really, a pair of replacements? Because these are for active crossover speakers where subwoofers are handling the LF, I don't need high power. Just a good - and hopefully not overly expensive - matching pair of 2-channel amps.
 
If only someone would invent a place where you could take electronics that need to be repaired rather than casting them aside.....
 
JeffMackwood said:
If only someone would invent a place where you could take electronics that need to be repaired rather than casting them aside.....
Click to expand...
Eh... I'll look into that, after I see if there's anything I can do myself. But these things are pretty old and not super expensive to begin with. Just checking out my options.
 
I've got some extra amps laying around, pauly. Let me look at what's there when I get home.
 
>>>is apparently failing, just not producing the output of its twin<<<

What do you mean by "not producing the output of its twin? Is the volume lower? Is the measured maximum power less than it used to be? Is the sound distorted?
 
The sound just seems muffled. I had it all set up with careful measurements so that output for L/R was balanced. Lately I've been noticing that everything seems to be heavy - but not completely - on the right, so I fiddled around a bit and the output of the left channel is definitely lower from qualitative tests (e.g. by ear where I can hear that it's quieter for sounds that should be roughly centered). I'm assuming for the moment that it's the amp and not some other component, or the speaker itself. The weird thing is, when I turn off the power switch on the amp, it doesn't immediately go quiet, it takes ~30s or so for it to wind down, gradually fading out. Compared to the other amp, which quiets immediately when I turn it off.
 
The fact that the "good" amp silences immediately when switched off and the "bad" amp takes awhile to die down indicates that these amplifiers probably use physical relays just before the speaker output terminals. These relays are supposed to protect your speakers in the event of failure of the output stage. This type of "protection" was popular in the 80s and to some degree the 90s. The problem is that relays don't protect the speakers, especially when there is an output transistor failure. What happens is that the relay's contacts "fuse together" and the relay does not open when the amplifier is turned off. Therefore what you hear is sound while that amplifier's power supply dying down, and there will still be sound coming from the amp until the power supply voltages discharge to zero volts. The "good" amplifier's relay immediately mutes the sound when the amplifier is turned off.

Now the nasty bit. What all this means is that you quite possibly have a bad output transistor(s) in that amp, which caused the over-current which fused the relay contacts in the first place. That unfortunately also might mean that there was high DC voltages going to the speaker for an extended period of time, and you might have a blown out driver (it's voice coil is probably open). The fact that the volume balance is far off from what it should be strongly suggests a blown out driver in one of the speakers, and this was probably caused by a defective output transistor in the amp.

You mention that the sound is "muffled". That might mean that the amplifier output very high DC voltages which overwhelmed the blocking capacitor in the tweeter's crossover network, and the voice coil of the tweeter was burned out.

At least, that's where I would start looking if I were tasked to sort out a repair of the situation.

You can do a quick test for a DC problem with the amp by disconnecting the inputs and speakers, turn the amplifier on, and then measure for DC voltage on the speaker output terminals. It should be well below 25 millivolts or so.

Just as a reference, power amplifiers usually do not "loose volume" when problems arise. The gain structure of the amp is what it is, and it is usually not going to change; if there is a volume loss, it is usually accompanied by very high distortion, such as when the current sources in the pre-driver stages become defective.
 
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Ok, thanks, I'll do some more checking.
 
Ok, good news: the speaker itself is ok; I set up RTA on the right speaker (with the working amp), and then swapped the left speaker onto that channel, without changing anything else. The speakers measure identically.

Now I'm gonna futz with the amp a little, see what I can discover.
 
PaulyT said:
Ok, good news: the speaker itself is ok; I set up RTA on the right speaker (with the working amp), and then swapped the left speaker onto that channel, without changing anything else. The speakers measure identically.

Now I'm gonna futz with the amp a little, see what I can discover.
Click to expand...

Thats good news
 
Put the HCA-750 in place of the malfunctioning HCA-500, tuned it up via RTA and all looks fine. I can certainly live with that for now, but I know the lack of symmetry is going to bother me in the long run. ;)
 
@rammisframmis - I measured the outputs as you suggest, with the amp on but disconnected, and I see maybe 3-5 mV.

You're probably right about the relay. With the working amp, there's an audible click when you turn it on/off - you can hear the relay (dis)engaging. With the broken amp, no such sound. Actually, when I first got these amps, after Flint had them in storage for years, and then shipped them out to me, at least one of the Parasound amps wouldn't power on, I remember I actually opened it up and sprayed some contact cleaner inside the relays and that got things going again. Maybe I can re-apply that solution. But I'm a little hesitant to start experimenting with the good speakers... guess I'll go dig up one of the spares, I still have some lying around somewhere.
 
PaulyT said:
Put the HCA-750 in place of the malfunctioning HCA-500, tuned it up via RTA and all looks fine. I can certainly live with that for now, but I know the lack of symmetry is going to bother me in the long run. ;)
Click to expand...
There is no significant audible lack of symmetry using two amps so similar. I would still get two identical amps, but in the meantime, I wouldn't be bothered by it in the slightest.

I mean, if one of the amps was solid state and the other tubed, you might notice it.
 
:laughing: Yeah well if anything it's the visual symmetry in my front rack... but then again, I've said many times I don't generally give a shit what things look like!
 
PaulyT said:
@rammisframmis - I measured the outputs as you suggest, with the amp on but disconnected, and I see maybe 3-5 mV.

You're probably right about the relay. With the working amp, there's an audible click when you turn it on/off - you can hear the relay (dis)engaging. With the broken amp, no such sound. Actually, when I first got these amps, after Flint had them in storage for years, and then shipped them out to me, at least one of the Parasound amps wouldn't power on, I remember I actually opened it up and sprayed some contact cleaner inside the relays and that got things going again. Maybe I can re-apply that solution. But I'm a little hesitant to start experimenting with the good speakers... guess I'll go dig up one of the spares, I still have some lying around somewhere.
Click to expand...

Do not use contact cleaner on relay contacts. You burnish the contacts if you do anything at all. Better still, you need to replace the relay.

The reason you don't see relays any more is that they didn't work; the contacts could freeze and if the amp had a problem, the DC would go to the speakers with not other protection. Relays were a monumentally stupid solution to protection. Nowadays we use opto isolators which trigger a microcontroller (at least here at ATI) with the added benefit that we can also monitor for HF oscillation and other nasty stuff.

I've seen amps with relays which fried woofers more times than I wish to count, including one of my own surround speakers.

If you measured 3-5 millivolts, then there is probably no problem, at least at the time you measured the amp. There is probably something wrong in the diff-amp section, current sources, pre-drivers or possibly a lower voltage power supply rail, but honestly I'm surprised you had any problem with something like a power amp at all; they should last forever.

As an aside, I've seen the symptoms of low level caused by something as simple as a corroded RCA input jack. I cleaned the jack's interior barrel with the stick of a Q tip dampened with alcohol and the problem was solved.
 
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