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A practical question
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Building my nephew's guitar amp raised a question for me. Looking at
fender schematics I noted their test criteria (and measured voltages)
were with all pots 'centered'. Not a problem for the guitar amp as I
used the first rule of engineering: plagiarize.
It did make me wonder what position on the volume knob people design
their 'Hi-Fi' power amps around. Nominal power (whatever that means)
at 'mid scale'? Max power at pot maxed? And vs what input level?
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If there's a volume control on a *power amp* at all, it's there
simply to pad down the input. The specs are given at a given
voltage input level (eg: "Sensitivity 1.5V input for 35 W output",
etc) to the amp itself.
Guitar amps are different as having the pots maxed typically would
have the amp clipping at the input levels a guitar pickup would
put out, and frankly, few people give a hoot about the numerical
distortion specs of a guitar amp. The pots are centered so with a
typical signal, you can check the AC signal level at various points,
without the signal being clipped and giving erroneous readings on
the kind of AC voltmeter they expected techs to use.
Probably something like a Simpson 260, often on old amp schematics
there will be a note on the diagram as to what kind of meter was used,
like "Measured with 5,000 ohm per volt meter" or "Measured with VTVM",
in some cases there may be a note that one needs to use a VTVM to get
accurate readings.
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Makes for a significant gain issue as 'mid scale' to full open is
20dB. Of course, there's also 16-20dB of content above the nominal
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17dB for the JIS A taper ( 15% ) actually.
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I get conflicting data with some saying 9%. others 10%, and 15%. At
any rate, the question is basically the same whether it's 20dB or 17.
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Depends on the pot taper or how the attenuator is set up, mid
scale might be -30 dB. Still, it's irrelevant to measuring
power amp specs unless it's internal rather than simply padding
down the input, which would only be useful if the power amp in
question amp has no overall negative feedback.
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316mV CD input level too.
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What 316 mv ? CD output FSD should be 2V. That is 16dB above 316mV it's true.
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As I said *nominal* (I.E. program) level.
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Yes, FS. But unless it's a rock band one doesn't expect every note and
nuance to be FS. Not much of a dynamic range, eh?
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CDs, and digital in general, are(is) a unique beast in there's both a
hard limit (ain't no more bits past FS) and a discrete digitizing
minimum (ain't analog) and you get the best 'specs' when you do it at
FS. (A one bit error over FS looks better than a one bit error at any
lower level.)
But that isn't the way program content, nor listening, works. You have
a nominal program content level with peak transitions, and lower
passages. I.E. dynamic range.
2V FS derives from nominal program level being 316mV ("consumer" line
level -10dBV=316mV vs Professional +4dBu=1.23 V vs the original,
canonical, 0dBm=1mW into 600 Ohms/0dBu 775mV). The rest is peak
headroom.
At any rate, I still have the original question: what levels and at
what knob settings to design around.
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