In article , Iain M Churches
wrote:

"Don Pearce" wrote in message
...

First the capacitors. Go through many samples with a meter to make
sure they are as far as possible equal in value. Install them in a
switcher box so they can be changed without delay.

OK. The service technician at the studio has promised to select with a
bridge carefully matched samples from those we give him.

I would wish to know the details of the measurement. Frequencies used (if
sinewave), distortion levels, signal levels, measurement system
calibration, etc. Ideally, I'd wish to know values measured across the
audio band, and get a match to a tight level.

If you are using a single-frequency cap bridge, you may need to consider
the effects of series ESR, etc. Given the points made elsewhere about
the dc you may also find it useful to measure with dc applied, and perhaps
even before/after use to see if this has had some effect!

Afraid I am quasi-paranoid about such things as I've spent 20+ years
working on precision measurement systems for people like the NPL 8-

Data only becomes information when you know how it was produced. :-)



Check the whole system to make sure that when a change is made the
output level remains the same.

Yes of course;.) We can see the output levels (sample and hold) on the
console meter bridge.

I think I'd like to know things like the frequency and phase response with
each cap in circuit, again with a fair degree of precision.


Do you think that we should let the subject know that a change had been
made? (it would be simple to rig a cue light for this purpose) I think
it would be better if the subject were asked to detect the change
without any visual warning of when and if it had taken place.

It may be better to give a signal when a change *may* have been made. Then
sometimes change, sometimes not, and sometimes change there-and-back
swiftly (in case of things like a 'click' or contact cleaning). However as
indicated elsewhere, my preference is ABX as opposed to AB, with the
listener choosing as and when to switch.

At any point in the test, allow the subject to ask to hear either of
the capacitors identified, to verify impressions of difference.

We planned to give the subject a cue button with which he could, on the
fly, send a TC marker when he hears a change. This will alow us to roll
back to the same point to recheck.

The tester should determine the order of switching just before the
test with thirty coin tosses. He should write these down and follow
his list.

Thirty coin tosses? We shall be spending most of our time in the dimly
lit control room on our hands and knees looking for 1 EUR coins:-))

Use a psudo-random generator to give you a set of 'one time pads' of 1's
and 0's. Then choose a pad sheet just before each session, burn it after
the session, and use a different one next time. :-)

We could use a random numbers generator to pick from three numbers, and
feed these to the desk automation.

Erm... how are you planning to do the switching?

Slainte,

Jim

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