From Newsgroup: sci.electronics.repair

I was asked by a friend if I could sort out a problem that had struck
his highly-specialised professional record-transcription equipment. On
quiet notes the Right channel was slightly low in output but louder
notes produced violent blasts of distortion; the left channel was
unaffected.

The problem seemed to be originating in a rack with a Prism A/D
converter, a couple of Cedar de-noisers and a Prism D/A converter.
By-passing the Cedar produced no improvement, neither did a substitute
A/D converter. The fault was definitely coming from the D/A converter; swapping analogue leads around showed that the fault stayed with Right
analogue output.

The Prism is a complex piece of kit and, in working order, is probably
worth as much as a small secondhand car, so I approached it with some trepidation. There was no service data available anywhere, not even a
circuit diagram, so I had to deduce what I could from the board layout.
The tracks were almost invisible and there may have been multiple layers
to the board - it wasn't obvious.

Working from the chip pinouts, I established that there was a large DC
offset on the Right balanced XLR output connector. It was symmetrical
about earth, so it probably originated before the balanced output stage.
There were four stereo D/A converters on the board, they appeared to be arranged with two chips (four channels) to the Right and two chips
(another four channels) to the Left. One of the Right channel outputs
was clipping hard against the rails.

Looking at the data sheet for the Philips TDA1574 D/A converter chip, I discovered that there is an op-amp built-in for each analogue output,
with its gain set by an external resistor. Philips suggest a value of
13k for this resistor as the op-amp forms part of a filter. On the
board I found the relevant resistor ...and it was open-circuit!

Changing the resistor was a bit fiddly but the equipment is now working properly again. A failed resistor is a rarity these days, so this was
quite a surprise (and a big relief in view of the value of the
equipment).

The question arises: "Why go to all the trouble of putting the outputs
of four D/A converters in parallel?" The answer appears to be that
Prism achieved an incredibly low noise figure by this method. Every
time you parallel a pair of signals the coherent signal increases by 6dB
but the noise (which is not coherent) only increases by 3dB. Every
doubling produces a 3dB improvement in the signal-to-noise ratio, so
Prism gained a 6dB advantage.

I don't know whether they also staggered the clock pulses to increase
the frequency of the clock ripple on the outputs (to give better
filtering), but it wouldn't surprise me if they did.
--
~ Liz Tuddenham ~
(Remove the ".invalid"s and add ".co.uk" to reply)
www.poppyrecords.co.uk
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From Newsgroup: sci.electronics.repair

In article <1rfbd8b.1v7wb2ln5mtkiN%liz@poppyrecords.invalid.invalid>,
Liz Tuddenham <liz@poppyrecords.invalid.invalid> wrote:

Looking at the data sheet for the Philips TDA1574 D/A converter chip, I >discovered that there is an op-amp built-in for each analogue output,
with its gain set by an external resistor. Philips suggest a value of
13k for this resistor as the op-amp forms part of a filter. On the
board I found the relevant resistor ...and it was open-circuit!

Changing the resistor was a bit fiddly but the equipment is now working >properly again. A failed resistor is a rarity these days, so this was
quite a surprise (and a big relief in view of the value of the
equipment).

Nice catch! It's really fun to be able to repair and recover an expensive piece of kit with what amounts to a zero-parts-cost repair. Diagnosis and labor are the big costs here, of course.

Or, to quote from the Congress of Wonders:

Captain Quirk: "OK, Smock, what's the situation with the computeer?"
Mr. Smock: "Captain, it's at least $57,000,000 worth of damage."
Captain Quirk: "$57,000,000?!? AARGH!"
Mr. Smock: "Well, it's actually only one tube which costs 15 cents,
but the service charge is enormous out here in space... especially
considering that we're half a billion miles off course."

The question arises: "Why go to all the trouble of putting the outputs
of four D/A converters in parallel?" The answer appears to be that
Prism achieved an incredibly low noise figure by this method. Every
time you parallel a pair of signals the coherent signal increases by 6dB
but the noise (which is not coherent) only increases by 3dB. Every
doubling produces a 3dB improvement in the signal-to-noise ratio, so
Prism gained a 6dB advantage.

I believe it also can also lower the distortion by a similar amount.
Many of the TDA1xxx DACs are of an R-2R ladder design, and variations
in the matching of the on-chip resistors result in some amount of
nonlinearity in the digital-to-analog-current function, hence
distortion. The mismatches are slightly different in every chip, and apparently there's enough cancellation between them when run in
parallel to make it worthwhile.

There are a number of audiophile-DIY projects out there, which parallel
as many as 8 of these R-2R DAC chips to gain this benefit. The ones
I've seen don't seem to have any clock-shifting circuitry - they drive
all of the latch inputs in parallel.

The TDA1574 seems to be an FM-tuner IF chip, not a DAC? Typo in the
chip number? The TDA1547 is a DAC, but it's a single-bit converter
rather than an R-2R ladder... one would get the noise-reduction benefit
by running several in parallel, but since this sort of DAC doesn't have
an R-2R ladder inside there would be no resistor mismatching to conquer.


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