Classic Pioneer M 25 amplifier. What to do?

[Randy]

Ok, I was reading Soundhounds amplifier thread and it made me curious. I started doing some searching again and found some good info on this amplifier. It is supposed to remain in class A mode until 30 watts. From I read in SH balanced/differentail amp thread, this seems very good.

So i have one of these laying around in my office.

Here is some info I found.
"M-25 Power Amplifier — In Pioneer's literature on the M-22, they state that the M-22 would be capable of 150 watts per channel if it was configured as a class B amplifier. The M-25 is the result of engineers running that experiment. But since class B amplifiers have crossover and notch distortion, plus have a hard time delivering large transients, the M-25 was designed to run in class-A mode at lower power levels, and automatically switch to class-AB operation as power demands increased. To provide large current transients, Pioneer invented a whole new type of transistor called the Ring Emitter Transistor, where many low power transistors were etched into the silicon die and ran in parallel. To top it off, the M-25 was the first component that I saw in person that had gold-plated connectors to ensure years of noise-free connections. The M-25 was rated at 120 watts per channel with no more than 0.008% distortion. Retail price $1200. "



I have had it for several years and never did anything with it. I have thought about putting it upstairs and running my Klipsch mains off of it. But, I recall reading somewhere that if one of these amps had been sitting around for a long time you should have it checked out before hooking it up to any speakers otherwise it could cause damage.

Soundhound, IG, or any others, do you have any ideas. The info that is out on the net about this amp makes it seem like a beast. What do i do?

 

[Randy]

Oh ya, I also have one of these.

RG-2 Dynamic Expander — Another external sound processor, the RG-2 was designed to make music sound more alive by expanding the dynamics of the music. It did this by making the low volume parts even lower, and the high volume parts even louder. This compensated for the fact that tape and record albums could not record the full dynamics of live music, and the music had to be compressed to fit on these media. Expanders were supposed to undo the compression. The RG-2 was an entry-level component. DBX really defined this market, and the DBX 3BX and 4BX expanders were the state of the art at the time. Both the SR and RG units can be rack mounted on their own using JA-R104 rack kits. I chose to mount them together using the JA-R102 kit that is designed for the cassette decks. Available 1979 to 1981, $230.

 

[Botch]

Randy, I have no advice on this classic equipment for you, but I wanna visit your office!

 

[soundhound]

From the looks of the heatsinking, the claim of 30 watts in class A is reasonable. With normal listening, the amp would spend probably 90% of its time in class A before switching to class A/B.

If this amp is really from the late 1970s, its design might have high amounts of TIM distortion which was just becoming identified as a byproduct of high levels of global negative feedback in early transistor amplifiers. The only way you can tell is to listen to it. If it sounds OK, then your're golden.

 

[Rope]

I gots one each of these ones, but only if it makes me Kool as Randy, other wise, I don't gots them

Rope

 

[Randy]

Have no fear Rope. It makes you every bit as cool as me. But, if you talked to my wife or kid they would tell you that is not all that impressive. :angry-cussingblack: :angry-cussingblack:

 

[Randy]

From the looks of the heatsinking, the claim of 30 watts in class A is reasonable. With normal listening, the amp would spend probably 90% of its time in class A before switching to class A/B.

If this amp is really from the late 1970s, its design might have high amounts of TIM distortion which was just becoming identified as a byproduct of high levels of global negative feedback in early transistor amplifiers. The only way you can tell is to listen to it. If it sounds OK, then your're golden.

Any problems you see with firing it up and giving it a try? I am curious because as I mentioned, I read online somewhere that if these amps have not been used for a long time they should not be hooked up before being gone through by a Tech. And,if tha tis true, where woiuld I find such a Tech??

 

[Randy]

Randy, I have no advice on this classic equipment for you, but I wanna visit your office!

Botch, my office is in my basement and it is kind of a mess, but you are welcome to visit anytime you like. :text-welcomeconfetti:

That amp has been sitting on top of one of my filing cabinets for years. I have never had it hooked up.

 

[PaulyT]

Do you have a cheapo pair of speakers around to test it on?

 

[DIYer]

Any problems you see with firing it up and giving it a try? I am curious because as I mentioned, I read online somewhere that if these amps have not been used for a long time they should not be hooked up before being gone through by a Tech. And,if tha tis true, where woiuld I find such a Tech??

I think the device is called Variac which allows you to power up slowly. http://www.variac.com/staco_Variable_Tr ... er_Map.htm

 

[Randy]

Do you have a cheapo pair of speakers around to test it on?

I could probably dig some up. That is a good idea, but I was more worried about the ensuing fire. :violence-torch: I cannot begin to imagine the riddicule I would receive if i had to call the boys down at the station to come put out a fire in my house. Especially since they are all tired of covering my shifts since I broke my leg.

 

[Randy]

Any problems you see with firing it up and giving it a try? I am curious because as I mentioned, I read online somewhere that if these amps have not been used for a long time they should not be hooked up before being gone through by a Tech. And,if tha tis true, where woiuld I find such a Tech??

I think the device is called Variac which allows you to power up slowly. http://www.variac.com/staco_Variable_Tr ... er_Map.htm

HMMMM, interesting. I need to make some phone calls to some of the shops around St. Louis.

Thanx for the help so far guys.

 

[soundhound]

Any problems you see with firing it up and giving it a try? I am curious because as I mentioned, I read online somewhere that if these amps have not been used for a long time they should not be hooked up before being gone through by a Tech. And,if tha tis true, where woiuld I find such a Tech??

You should slowly bring up the voltage with a variac, which can be purchased either online or at an electronics store. The variac does not need to be rated to power the amplifier anywhere near full power since once the voltage is brought up and left stable for perhaps 15 minutes, it is safe to then unplug the variac and plug the amp into the house AC. The main problem is that the power supply capacitors need to be formed after sitting unused for so long. If this is not done, the sudden shock could cause these capacitors to explode.

Variacs make fantastic light dimmers once you're done with it. In fact, that is the preferred method of dimming lights in studios because triac light dimmers inject and radiate high frequency crap which can be picked up by sensitive electronics (like your receiver). Using a variac solves that problem. All of the lights in my room with the exception of a couple are run off variacs.

 

[Randy]

Any problems you see with firing it up and giving it a try? I am curious because as I mentioned, I read online somewhere that if these amps have not been used for a long time they should not be hooked up before being gone through by a Tech. And,if tha tis true, where woiuld I find such a Tech??

You should slowly bring up the voltage with a variac, which can be purchased either online or at an electronics store. The variac does not need to be rated to power the amplifier anywhere near full power since once the voltage is brought up and left stable for perhaps 15 minutes, it is safe to then unplug the variac and plug the amp into the house AC. The main problem is that the power supply capacitors need to be formed after sitting unused for so long. If this is not done, the sudden shock could cause these capacitors to explode.Variacs make fantastic light dimmers once you're done with it. In fact, that is the preferred method of dimming lights in studios because triac light dimmers inject and radiate high frequency crap which can be picked up by sensitive electronics (like your receiver). Using a variac solves that problem. All of the lights in my room with the exception of a couple are run off variacs.

OH BOY!!! I'll bet that could be impressive. I htink I will look into this Variac thing you guys speak of.

Could I plug it in outside and leave it on for 15 minutes or so to tell if the capacitors are still good?

 

[soundhound]

You gradually bring up the voltage being fed to the amp to 120 VAC over a period of 30 sec to a minute, then leave it at 120 VAC for about 15 minutes.....if the amp survives this, then you can use it normally from the wall AC outlet.

 

[DIYer]

Could I plug it in outside

As long as it's not raining.

 

[DIYer]

then leave it at 120 VAC for about 15 minutes.....if the amp survives this, then you can use it normally from the wall AC outlet.

I guess transistor amp can be run without load?

 

[Randy]

then leave it at 120 VAC for about 15 minutes.....if the amp survives this, then you can use it normally from the wall AC outlet.

I guess transistor amp can be run without load?

I am assuming that is the case. It is the capacitors that SH said could potentially explode and once you plug it in, even without a load, the capacitors should charge.

Am I wrong?

 

[soundhound]

then leave it at 120 VAC for about 15 minutes.....if the amp survives this, then you can use it normally from the wall AC outlet.

I guess transistor amp can be run without load?

I am assuming that is the case. It is the capacitors that SH said could potentially explode and once you plug it in, even without a load, the capacitors should charge.

Am I wrong?

Transistor amplifiers can be safely run without a load.

 

[Randy]

Thank you very much SH. I will hopefully get a chance to take it outside and plug it in for a while and see how it acts. Can't do it now, as we just got several inches of snow.