Spot the Design Error

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** That pair act as Vbe multiplier and buffer to bias the Darlington stage.

TR115 is mounted on the heatsink.

No issues there.

** THD is low ( between 0.01% and 0.002% )and vanishes into noise at the sub 1 watt level.

..... Phil

Been ages..could be very wrong, but methinks that problem lies with the TR110, TR115 circuitry. And if so, then the "published specs" did not mention crossover distortion.

R155 and R156 seem to be there to monitor the residual current at low voltage swings for class B operation.

There are a whole lot of other resistors tied together with them, feeding the bases of TR113 and 114, which would presumably be sensing the voltage developed.

It looks like an incoherent mess, as if somebody wanted to use the same resistors to drive a current limiting system as well.

--
Bill Sloman, Sydney

I guess this is not your cup of tea. Or moose piss.

No, those resistors provide what is call dual slope current limiting. The current is monitored but the point at which it activates changes with the instantaneous voltage level of the output.

What this does is effectively detect a short at lower voltage yet still allows larger currents to flow at higher output levels.

I am not going on to someone who is my superior in the field. Just run through a few keywords, dual slope, current, audio, limiter. It's out there.

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** ROTFL !!

A poster here used the handle "Moosefet".

Dunno if he is still about, with a new name.

.... Phil

Phil Allison wrote in news: snipped-for-privacy@googlegroups.com:

Maybe the heat sinks are supposed to be electrically isolated from each other and they are not on the chassis?

That is a guess.

C125 is too small

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** The TO3s area all on a large finned heatsink.

The TO220s have individual ones on the PCB.

The design flaws are discoverable from examining the schematic.

" However, within the power stage is a flaw that makes the amp misbehave terribly. Another bad flaw involves the thermal switches. "

Obviously, the misbehavior is not evident all the time or Yamaha would never have sold any.

.... Phil

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** No issue there, the output stage is quite stable.

You need to know about good power amp design in some detail.

The thermal switch issue is pretty glaring though.

.... Phil

"misbehave terribly" is a bit broad and subjective but too much detail on the particular symptoms gives the game away I suppose

It's something that they could have not had happen without spending any (or very little more money), yeah? That is to say it's surely a design flaw vs a cost-cutting measure that they figured wouldn't bother with because it didn't happen too often.

Equipment is sold with no protection from plugging an AC adapter "wall wart" into a DC jack all the time but that can be a "design flaw" or cost-cutting measure depending on how you look at it, the mfgr probably does save 50 cents and also f*ck you dummy learn to read the labeling.

** It would.

If nobody gets close, in a couple of days I will post the symptoms.

But you can nail the thermal switch one, right now.

..... Phil

Second guess is I think something's maybe not right with TR111, TR112, D110, D111 usually the base of the driver transistor is connected to the junction of the bias transistor network and temperature compensation diode.

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** Yep.

** Cost a few cents in parts fix. ** Bet Yamaha's designer had no idea it could happen at all.

.... Phil

I'd think that C117 (and possibly C116) should be more like 25V instead of 16V.

I'll assume that the "E" on the input ground is physically tied to the "E" (common) on the other side of the amp...

John :-#)#

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** TR110 is the bias transistor with TR115 the temp sensor.

FYI:

TR115 is a TO126 device, so easy to bolt down.

TR110 is TO92 type.

..... Phil

Was this amp designed to pair with the NS-10s? The joke about the NS-10s is that every studio had a pair because they sounded so unremarkable that if a mix sounded good on them it would sound good on anything.

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** Well spotted - but that is a typo in the schematic.

The boost rail is 21V DC so I'm sure the caps fitted to real amps were 25V types.

.... Phil