Carver PM350 stereo power amp

[Brad1]

Friend has a Carver PM350 amp. It was shutting down after a while, I thought maybe because the fan was running very slowly from being dirty and covered in greasy gunk and dust. I removed the fan and motor, disassembled, cleaned and lubed, and now that works fine.

I noticed that it turns on and works fine, sounds good, until switched off.

THUMP!

YIKES! It's a LOUD one. I called him and he said, "Yeah, forgot to tell you I always unplugged the speaker before powering it off, because of that". The volume level makes no difference. As mentioned, there is no turn-on thump.

I put a voltmeter across the speaker out terminals, set to DC. It idled at pretty close to 0 volts, and when I switched it off, it jumped quickly to ~50V, then slowly diminished.

Ideas, anyone? Can't find a schematic to download.

Thanks,

Brad1

[Enzo]

Turn off thump in solid state amps is extremely common. The amplitude of the thump is limited to the power supply voltage. SO in other words, the thump is no larger than the peak audio. If your speakers can handle the peak audio, they are fine with the thump too.

[Brad1]

Thanks Enzo.
I realize that many amps thump, but thought this one was a bit extreme. I didn't know if there was something I could do to minimize it, other than just unplugging the speaker.

There's an easily-removable blank back plate right next to the speaker posts. I wonder if I should just pop a double-pole/single-throw switch in there to make it easy to disconnect. If possible, got a recommendation...for the power going through this?

BTW, this has been being used in bridged mode. He had been using this as part his bass rig, of which I loaned him an Ampeg SVPro preamp, and my ACME Low B4 cab. I didn't realize it was thumping like that, but I'm glad to know he's at least been trying to protect my speakers!

Thanks,

Brad1

[Enzo]

Switches are rated for the amount of current and voltage they can safely SWITCH, not as a rule what they can carry. SInce you are not likely to turn the speakers off in mid note, I don;t think switch ratings are an issue. The average full size toggle should be sufficient.

[bob p]

Good advice from Enzo. But that goes without saying -- he knows a lot and he even helped me repair my first Carver amp. I've repaired a few Carvers since then.

I don't have any tech data for the PM-350, but if that amp is what I think it is, then its a late model variant of the PM-1.5/PM-1200 type amps.

IMO any DC going to your speakers is not a good thing, but I think its safe to say that soft thumping is such a common problem with many of the Carver designs that people consider it normal and just learn to live with it. Some of the Carver amps had relays built into the protection circuits which effectively masked the thumping problem, while other designs without relays have nothing to hide the thumps. The thumping should be soft and subtle. In that case something it's benign and nothing to worry about. If its loud, or if its getting louder, then its pointing to a correctable problem. For this reason I don't particularly like the amps with relays, as the relays tend to make you unaware of a developing problem as it becomes progressively worse.

Some of the Carver amps like the 1.0t had TSBs issued about the thumping problem. But that amp is a different animal.

With the 1.5 type amps, shutoff thumps that are quiet and unobjectionable are normal. If they become loud then they are indicative of an evolving problem with your amp. It would be very helpful to know the Z of your load, whether or not you are seeing any changes in the status lights at power-down when the thump occurs, and if lowering the Z of your load makes the problem worse.

I don't have a schematic or a repair manual for your amp, but I can tell you this -- in my experience the most common causes for the annoying thump at turn off on one of the 1.5 variant amps is caused by:

1. A defective op-amp at the front end. The TL 081 on the amp boards and the 072 on the input card are notorious. Replace them.
2. Leaky drivers / pre-drivers. Replace them.
3. Uneven collapse of the +/- 12V supply. Recap your 20-30 year old amp. Its way beyond overdue.

That last point is an important one. The bi-value caps that Carver used on the low voltage rails were absolute junk and they're not getting any better with age.

[bob p]

One other thing to consider is whether or not its going to be worth your time to dive into some of the Carver amps. I've repaired quite a few of them for my Carver collection. Having done so I'm of the opinion that many of the Carver designs should not be restored and should just be thrown onto the scrap heap, as they're getting to the age where they're just not worth restoring. Here's why:

As much as I hate to say it, they're not the highest fidelity amps. They produce nasty commutation noise that my ears find objectionable. They're Class B amps that were designed to run cold, so that they would be cheap to produce. They used a high parts count of cheap parts, and a low parts count of expensive parts. This made them cheap to produce. On a Carver, most of the manufacturing cost went into a really nice, masculine looking box.

Compare a Carver to the REAL HiFi amps that Carver challenged back in the day --

HiFi Amp:
Class A, simple circuit, lots of heat, massive heatsinks and transformers. Expensive to build.

Carver:
Class B, complicated circuit, little heat, not much in the way of heatsinking, small transformers. Cheap to build.

Early in their service life, the Carver wins because its cheap to build and the circuit is reliable. After 20 years pass, the high parts count works against the Carver, and the complex circuit is more likely to fail. Its also more difficult to repair.

Add to that the fact that some of the Carvers were designed very poorly from a servicability standpoint. Some of the PM amps and the Cube are such nightmares to work on that I won't waste my time on them.

My advice then, would be to think long and hard about committing a lot of time into fixing a Carver, especially if you're not already familiar with fixing Carver designs, or you don't have tech data, or if the amp's layout isn't one of the pancake PCBs that's easy to work on. You don't want to get stuck working on a tarbaby -- in the end it won't be worth the effort.

BTW, on the subject of fans -- if the fan has gotten dirty enough that it squeals and needs to be cleaned, then its already ruined. Cleaning it will buy you some time, but IME you're much better off just tossing the old fan out and replacing it with a new one.

Good luck.

[Brad1]

Thanks Bob, good stuff. I just wish I could find the schematics for the thing. I did install a switch to disconnect the speakers before powering down, and that'll do for a stop-gap measure until it either 1) blows up completely, or 2) gets fixed properly. But, hard to figure out without a schem, and hard to know if it'll be worth continuing to mess with until I know what's wrong.

It does seem to work just fine when used. Not excessively noisy, and plenty of decent, punchy, clean sound for the bass guitar. It took a 3-hour pounding yesterday at an outdoor gig in high-humidity and nearly 100 degrees.

One other clue that might narrow down symptoms: It only pops when using the power switch to switch off. If the power is just turned off through the rack-mount power distribution, it goes quietly.

Again, thanks,

Brad1

[Mike C]

Hello

I used many of the pro Carver amps when they were the "it" to have. They did sound good, light weight and high power....for the time. As mentioned they were stuffed full of components and had a tight window of allowed component tolerance. I used the PM600, PM900, PM1200 and PT1250 amps. When the QSC Power Light 2 series came out I switched them all over and went to Crown MA3600's on the lows.

The PM350 was an earlier amp in the Carver line up, gray front, no output level metering only signal presence and clip indicator if I remember correctly.

You mentioned that when you turn off the amp via the rack power strip that is does not thump. The power switch on the Carver amps do not actually disconnect the main AC power they only activate the triac that controls the switching "magnetic field supply"

Mike Caldwell
http://www.mikecaldwellaudioproductions.com

[Steve Conner]

On a Carver, most of the manufacturing cost went into a really nice, masculine looking box.

Me thinks, gut it and use the box for a new amp...