[Patrick Turner]

Alan Rutlidge wrote:

"Trevor Wilson" wrote in message
...

"Alan Rutlidge" wrote in message
...

So what you are implying is that a transistor tester, capable of reading
current gain and Vbe is beyond the budget or the average electronics tech
and beyond his / her competency to use? Pull the other one Trevor, it's
got bells on it.

**No. That's not what I'm telling you. The problem is acquiring the sheer
number of devices, in order to obtain suitable matches. Not only that, but
Peter matches his large signal devices at 1 Amp. Few techs bother testing
at more than a few ma.

So what's going to happen if one of my output devices fails and there are no
matching spares?
I guess the whole lot of the output devices would need to be replaced. So
the customer bears the cost of the other undamaged devices and the labour
cost associated with that? Show me the economics in that rationalisation.

**All transistors in the amplifier stages are matched to within 1% for
hFE and Vbe. The resultant number is written on the top of the output
devices.

Oh forgive me Trevor. Just lifted the lid on my ME850 Hi-Cap. Lots of
5% tolerance parts around the output devices.

**You may care to note several things:
* There are lots of 1% and 2% (not 5%) resistors around the output
devices.
* 2% resistors typically measure better than 0.5% off the shelf.
* The Emitter resistors are matched to within 1% tolerance. As are the
other WW resistors.
* The only other 5% resistors are in non-critical sections.


Makes perfect sense now. Equip the device with 1% transistors and
surrounding circuitry has 5% components.

**Wrong.

In which case it makes even more sense that a service manual outlining these
critical components be made available.


What a ****ing laugh. Obviously
both you and PS haven't heard that saying - "A chain is only a good as
its weakest link." Also the WW resistors in the output stage have a PTC
so the resistance will change with temperature / power dissipation.

**Points:
* The resistors are matched.
* The resistors all drift by the same amount.
* The amount of drift is around 300 parts per million/oC.
* The resistors are rated, at maximum power dissipation, to drift less
than 1%.
* At 120 Watts (continuous) the dissipation of the resistors will be
approximately 0.12 Watts each.
* Temperature rise will be approximately 3oC under these conditions.
* At the rated drift, that corresponds to less than 0.01%
* At 220 Watts (continuous - 4 Ohm load) the dissipation of the resistors
will be approximately 0.44 Watts each.
* Temperature rise will be approximately 15oC under these conditions.
* That corresponds to less than 0.05% change in resistance.

Of course, under normal use, the fan will keep temperature change to less
than that. Additionally, The figures quoted are for continuous power
levels, not regular music.



Without a circuit diagram it isn't hard to guess that there are low value
(0.51 ohm) resistors in each leg of the emitter circuits of the output
devices. Matching or no matching (within reason), the currents flowing
in the emitter circuits will be effectively shared between parallel
output devices in this type of circuit configuation. It's no rocket
science secret. The technique is commonly used by nearly every competent
class A/B SS output stage.

**Not quite. ME connect Collectors to the load. This is a far less common
method. However, you are correct: It is not rocket science. Peter has
never claimed as much.

Read the Cherry and Hooper book on amplifier design. Published in the
early 70's it's essential reading for anyone contemplating SS amplifier
design. My God it even mentions the V-FET and MOSFETS.

As for ME output device matching. By that I assume all the MJ15026
devices in any one channel would have the same "batch test" number and
likewise all the MJ15024 devices would have their own?

**Maybe, maybe not. Batch numbers don't tell you much. Peter matches the
devices, based on their MEASURED parameters, not what the manufacturer may
claim.

I wasn't refering to the transistor manufacturer's batch numbers but the
numbers hand written on the top of the devices (presumably by PS or one of
his former employees).

I'm not saying PS doesn't attempt to match his devices within reason.
Just I think 1% matching is overkill and it is unnessecary if one selects
an output device with reasonably tight specifications to begin with.

**Your assumption would hold water, IF ME amplifiers were conventional,
high Global NFB designs. They're not. To perform properly, the outputs,
drivers and pre-drivers MUST be carefully matched.

[Anecdote] Back in the 1970s, when I was service manager for Marantz, I
noted that several of their high end models, when serviced, had to be
fitted with carefully matched devices. The matching was around 20-25% of
Hfe. MUCH closer than regular transistor manufacturer matching. At one
time, I ran short of matched devices. I attempted to use a set of
unmatched devices to get my customer's amp going. I could not allow it out
of the workshop. Distortion went through the roof. From a typical figure
of around 0.05% (20Hz-20kHz) at full power, the figure rose to in excess
of 0.5% at mid-band frequencies. A clearly audible figure. And, I might
add, that this was a more or less conventional, high global NFB design.


In that case, I'd say the basic design had some serious shortcomings.

Cheers,
Alan

BTW, thanks for answering the questions.

I never read what Trevor says about ME amps or the way transistors are
matched
because its all empty sales talk and utter BS.
Being highly technical myself, what I never see is exactly what
procedures are used
with ME amp productions, and the more Trevor raves, the deeper he digs
the
hole he's in.

Trevor won't prepare a website of his own which creates any confidence
and complete with references to schematics shown.


Nothing Trevor says really explains anything to any kind of
satisfaction.
He's a repairman who has never designed or built an amp in his life.

I'd need to hear a few pages from Peter Stein with evidence to proove it
all.

But before that, workshop manuals are required.

DON'T BUY ME AMPS UNLESS FULL SERVICE MANUALS ARE AVAILABLE.

Patrick Turner.