Alan Rutlidge wrote:
>
> "Trevor Wilson" <trevor@[EMAIL PROTECTED]
> wrote in message
> news:67gaftF2jg0mhU1@[EMAIL PROTECTED]
> >
> > "Alan Rutlidge" <don't_spam_me_rutlidge@[EMAIL PROTECTED]
> wrote in message
> > news:4811dac0$0$1603$5a62ac22@[EMAIL PROTECTED]
> >>
> >> 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.
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