Problem with M-500

Today, after almost 30 years of daily use, I noticed a "problem" with my M-500.
Perhaps one of you fine gentlemen have come across this issue before.

I was cranking my system pretty hard (0db+) when my main speakers shut off.
Startled by this, it took me a few seconds to get out of the recliner.
About the time I got to the gain control, the mains roared back to life.
As I stood there watching the VU meters on the M-500, the mains shut off again.
I lowered the gain a "click" or two and waited.
After a few seconds the mains came back on and stayed on for the rest of the CD.

Now I recently made a change to my system based on a comment from TNRabbit.
For years I powered each satellite via it's own M-500(t) with both amps bridged in mono.
The change was to power both satellites using only the one M-500 running in stereo.
(I made no change to my subs which are each powered by a single M-1.5t.)

So, after the CD finished playing I decided to switch the M-500 for the M-500t.
I went back to the same CD I was listening to earlier and set the gain to 0db.
I continued to raise the gain intermittently to +3db without incident.
Just for the heck of it I switched back to the M-500 and set the gain to 0db.
It played fine for about 5 minutes before it started cycling off and on again.

Any ideas about what the problem is and what it will cost to fix it?
Is the M-500 worth repairing or should I look for another M-500t?

Thanks in advance for your assistance,

You just encountered the overcurrent safety feature of the M-500. I doubt if anything is wrong with it. I'm somewhat confused by your use of the term satellites. I thought these were your mains that were powered by the M-500.

Super Satellite is Legacy's name for my main speakers.
Perhaps they call them satellites because they are small compared to the Focus or Whisper.
They roll off around 80Hz. I cross them over to the subs at 100Hz.

I've driven this amp this hard, and harder, many times in the past.
If there isn't anything wrong with the M-500, why has this never happened before?

EDIT: I switched back to driving each "satellite" with it's own M-500(t).
The problem M-500 still shuts after running for about 5 minutes at 0db.

Bill, would he be encountering the phenomenon wherein the amp is only using the upper rail?

Does your number of 0db represent what is also know as "wide open" or in some languages "all knobs to the right?

Rainman

The M-500 and the M-500t are different in a number of ways. Of particular interest here is their sensitivity. The M-500t requires 2V to drive it to 200W (0dB). The M-500 only requires √2V (1.414V) to drive it to 200W. So the M-500t is linear whereas the M-500 is quadratic in their sensitivity slopes (both amps require 1V to reach 100W or -3dB). So, unless you were listening at unequal volumes on each speaker, you couldn't have been running BOTH at 0dB.

Another likely problem is that your meters are off. I've only worked on one M-500(t) whose meters were set even close to accurate (I've worked on 6 so far). I use a big signal generator that will generate signals for everything from audio to MHz for aligning FM tuners. I set the output with a DVM at precisely at 1V or 2V (or √2V) to align the meters.

Thanks BillD. You are correct about the meters. I fed both channels with the same constant signal (interchannel noise from my tuner) and the left meter both my amps read 2-3db lower than the right. How hard is it to adjust the meters to track better? I know I won't be able to zero them but at least I can make them equally sensitive. Is there a pot on the back of each meter?

I was not aware of the higher sensitivity of the M-500 vs the M-500t, but I came up with a solution. When I bought my M-500t I purchased a 2nd for my youngest brother. I called him this morning and found out that he's using his to power a small system in his bedroom which he never drives to high levels. He is willing to swap with me so I can have 2 amps with the same sensitivity.

I packed up my M-500 and sent it to him this afternoon. I should have his M-500t by this weekend. I'll let you know if it makes a difference.

Does your number of 0db represent what is also know as "wide open" or in some languages "all knobs to the right?

The meters on the M-500 respond quickly to changes and do not have a peak hold function so I'm kinda guessing about where the needles are "peaking".

That said, they reach a peak of (roughly) 0db with the preamp gain set about halfway. (5 on a scale of 10)

Before I borrowed my neighbors signal generator, I used to use a PC with tone generator software. I wasn't able to get 2V out of it, but if you put it into a preamp, you can. Feed it 1000 Hz at 1V and you should get 100W on both amps. I hooked my meter up to the RCA plug tip and sleeve (if grounded, or chassis if not, to get an accurate measure). Because the curve on each meter is different (I'm guessing that the M-500t is 100V=W, whereas the M-500 is 100V²=W), the only point they have in common is when V=1 Volt. There, both outputs are 100 Watts.

Sensitivity.jpg (66.86 KiB) Viewed 552 times

Inside the M-500(t) there are two adjustment pots (actually there are 5, but only two you are interested in). They are on the amplifier board (to your right as you are looking at the amp from above with the meters toward you) toward the front right of the unit paired together. There are a couple toward the back left and right corners, also. Don't touch those, they are bias adjustments). When you have two identical amps, just adjust them to be the same. The spec is 1V in equals 100 Watts out. I usually check there and at 200W (which makes sure the top rail is engaged).

Thanks for the "how to" BillD. I'll give that a try this weekend.

I think the change just shows the emergence of the CD. Source output voltages rarely exceeded 1½ volts before CD players came out, so the M-500s nonlinear output curve made sense. Now, CD players could output up to 5V, and amplifier design needed to change to accommodate that much input. Indeed, you need to be careful with older pre-amps so as not to clip their input op-amps with the source signal.

Great information here, Bill. This answers a couple questions I always had about the differences between my M500 and my M500t. Many thanks.

Hi zumbini,
Your amp is well worth repairing, never doubt that. Make sure the technician is good, not cheap!

You may have other problems there, the amp should be checked out.

Hi Bill,
I have to comment that an amplifiers output must be linear with input level changes. If not, the amp will create distortion. I suspect that the instrument monitoring the signal is not linear, depending on the brand. If you are taking the output level from the meters on the amplifier, these are generally not that accurate and may very well show non-linearity.

An amplifier should amplify the input signal by a constant (it's gain). If not, something is broke.

-Chris

Well, Chris, I don't know any way for two amps to be at 100W at 1V input and then one of the amps be 200W at 2V input and the other to be 200W at 1.414V input without one being non-linear, do you? I may not be an electronic wizard, but I do know mathematics.

I did make the assumption that both amps produce 100W with one volt input (it indicates that's how you set the meters). If that isn't so, then both amps could be linear. The M-500t would be at 100W at one volt, and the M-500 would be at 100√2 (141.4W) at one volt.

The other cause could be that the sensitivity spec published in the manuals is incorrect.

Your amp is well worth repairing, never doubt that. Make sure the technician is good, not cheap!

BillD overhauled and upgraded my M-500 several months ago and I am VERY HAPPY with the results.
At the moment he is doing the same work on one of my M-500t's with the other M-500t to follow shortly.