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Old 11th April 2005, 01:02 AM   #1
RogerG is offline RogerG  United States
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Default yamaha ns m-50

My ex-wife has a yamaha natural sound M-50 amp. It has low sound output but is also aged and needs some attetion in general.
I've not been able to find any info for it. Yamaha doesn't even list an owners manual/power specs on their site. I have removed the cover and pcb. There are some bad solder joints but nothing else looks! wrong. I sent a request to yamaha for a schematic and circuit values. Maybe I'll get that much so I can start checking things.
I noticed that it has only 2 pair of outputs per channel. But the board is drilled and printed for 4 pair. Higher power version maybe? If so, what could it take to bring the power up a bit. It's supose to be 120wrms/8ohm. Could 150+ be done without totaly re-engineering the amp. Oh yeah, it has 2 heatsinks. About 2.75-3.0"*5.5-6.0" and are also predrilled for adding outputs.
This site kicks a**!

Roger
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Old 11th April 2005, 02:24 AM   #2
anatech is offline anatech  Canada
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Hi Roger,
Order a service manual. They are not that expensive from Yamaha (in Canada anyhow). The other holes are for a larger unit, don't worry about it.
-Chris
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Old 12th April 2005, 12:09 AM   #3
RogerG is offline RogerG  United States
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I ordered a service manual today. Yamaha discontinued it but I was able to find a supplier online. I'll have it tomorrow. That will help. In cases I've worked on, low output has either been bad solder joints or bad outputs. Having picked this one up from ebay, it may have been beat on before I got it. So I will throughly test it.

As to the question of the outputs. I looked them up and they're a fairly low power device(this unit is only 120wpc/8ohm). Making it give big power is not my thought. But as I understand both ohm's law and current flow in parallel, adding the addition 2 pairs to each channel should give an increase in current flow(sum of all resistances/number of parallel circuits/voltage=current flow)(amp*volts=wattage). Correct?

So buy adding the extra outputs I should see some power increase, plus an improvment in over stability, right?

Of course, this is all academic untill I get the manual and see what the power supply will put out and the rail voltage is. As for the case layout; the heatsinks are made for 4 pair of outputs/channel. If it needs, there's room for a fan, also.

I'm not going for double the power or even a 50% gain. But pushing over 150w/8ohms would be nice.

Roger
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Old 12th April 2005, 01:08 AM   #4
anatech is offline anatech  Canada
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Hi Roger,
Of course there is always the thermal fuse in the power transformer. Best bet is to leave it stock power-wise. Upgrade the outputs if you want.
-Chris
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Old 12th April 2005, 04:33 AM   #5
RogerG is offline RogerG  United States
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Quote:
Originally posted by RogerG
But as I understand both ohm's law and current flow in parallel, adding the addition 2 pairs to each channel should give an increase in current flow(sum of all resistances/number of parallel circuits/voltage=current flow)(amp*volts=wattage). Correct?

So buy adding the extra outputs I should see some power increase, plus an improvment in over stability, right?
Thanks for your replys, Chris. I understand what your getting at with the thermal fuse. Well, I think I do anyway. But am I correct in the above quote?

If so, how can I figure the power increase?

If not, what am I not understanding about it?

Please explain! If I'm wrong, I need to learn why so I won't make mistakes from thinking this is how it works. Thanks

Roger
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Old 12th April 2005, 10:22 PM   #6
anatech is offline anatech  Canada
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Hi Roger,
Depends on current sense and available drive current. Protection may roll you back to the same peak current output. You will get a lower dynamic resistance that may or may not improve the sound.
If you do get higher current, the transformer may open. Yamaha when through this problem back then because the transformer was sized a little small. The core temp ran a bit high.

Anyhow, Yamaha is a funny breed. Sometimes they make a minor circuits overly complicated and they somes do strange things in the amp stage. Even though you may have more output headroom, the protection circuits may force the output back to the design maximum. Math has no bearing on this since the limit is artificial.

What I would do is fix it up and enjoy it within it's intended limits. If you get more, then fine.

-Chris
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Old 13th April 2005, 05:11 PM   #7
RogerG is offline RogerG  United States
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Thanks for the feedback, guys!

I tested all my outputs and they are shot. The service manual came yesterday. I ran the test on the p/s, bias and, offset. Right now, the only thing set correctly is the offset. Bias is high! Calls for 10mA, messuring 22mA.

The manual doesn't have specs for the p/s. So I guess I will recondition it to factory specs and be happy with it. The outputs are toshiba's. I looked all over for someone who had them. Turns out www.mcm.com has a lot of the toshiba's in stock.

It has a pair of settings called "zero distortion". I'm guessing that is the feedback circuits? Anyhow, thanks again!

BTW; if you have trouble finding out of print service manuals for audio elcetronics, www.servicemanuals.com has a ton of that stuff.

Roger
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Old 13th April 2005, 09:51 PM   #8
K-amps is offline K-amps  United States
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Roger,

Anatech has a lot of experience in repairing amps, heed what he says...

However personally I am "Tim the toolman" kind of person, I also make those funny grunting noises when I go for more power, oo, oo, oo.

I fully understand the ants in your pants and you "really" want to toy with it so here are my thoughts:

Increasing current in the OP stage will not get you more power unless you use a lower impedance.

If your amp does have current limiting, you can rig up additional OP devices such that they do not load up the current sense resistors on the stock pair. This way you can get additional current from the amp while keeping the current limiting operational as intended on the stock pair (I am assuming you will add more parallel pairs as you have the space for it.)

Overall the power is limited by the heatsinks and transformer assuming your OP devices can handle it.

Also assuming you will drive the amp 8 ohms and not 4 ohms, you can squeeze an additional 30%-50% power at 8 ohms, but don't try the same for 4 amps or you will blow something else in the amp.

So if you want 30-40% more power at 8 ohms and you are willing to paralell more OP devices, here are a couple of options:

1) Install a larger Trafo of same voltage but better regulation.
2) Using the same Trafo, jack up the voltage by 15% (not more) of the amp using a variac on the AC mains side. However this option is risky unless you know what you are doing because you need to ensure that the power caps and active devices are not run over spec or they will blow... Also know that jacking the voltage up will cause additional heat in the circuits (by the way I am assuming you know that as soon as you jack up the voltage, you need to re-bias the amp) because any regulators, resistors will be running over design parameters.

Now, is this all worth it.... probably not. Will you try it anyway... perhaps but know that by doing this you can blow the amp up.

We have all done stupid things (in my case I continue to push the envelope), you deserve your share too.

Cheers!

K-amps


PS if you are not sure if you want to ruin a perfectly good amp, remind yourself that this is your"EX"-wife's amp.
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Old 14th April 2005, 12:34 AM   #9
anatech is offline anatech  Canada
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Hi Roger,
Check carefully in the ZDR circuits. I almost forgot about those (actually, I did!). I have had to relpace the odd ZDR transformer. Check the primary for shorted windings.
Told ya Yamaha did wild & wooley things now & again. Basically a decent amp (consumer people) and sounded okay. Kinda like a Dodge 340 cid engine from the 70's .... don't hot rod these.

K-amps,
Don't egg an "Ex" on. It's just not safe . Could be fun though!

-Chris
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Old 14th April 2005, 12:54 AM   #10
RogerG is offline RogerG  United States
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Thanks guys!

I think I'll leave this alone. I have new outputs coming, but no extras for this amp. I'll set everything back up at factory specs and give it back to her. She might be a little nicer to deal with for minute or two!

Yeah, I'd love to push this one some. oo oo oo!! But, it just doesn't seem like much of an idea. Now, the Ampzilla! oo oo oo oo!!! If only there were a way.
Btw, you guys know of anyone needing some MPQ6002? Looks like I'll have to order 25 to get the pair I need. Oh, well. They're cheap!

Roger
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