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Soundhaspriority
Guest
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 Posted: Fri Feb 23, 2007 12:06 am Post subject: mike preamp gain control devices |
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Grace Designs, http://www.gracedesign.com/support/tech_paper2006.pdf, state
that
"as music signal flows through a potentiometer, the potentiometer's
temperature modulates, causing its resistance to modulate. As the value of
resistance changes, so does the amplifier gain, resulting in dynamic
distortion. Most conductive plastic potentiometers have temperature
coefficients of +/-1000ppm/ºC, which can cause considerable signal
degradation at high amplifier gains. By comparison, a metal film resistor
has a typical temperature coefficient of +/-50ppm/ºC, which represents more
than an order of magnitude improvement in thermal stability."
How significant is this effect?
As a side note, the Grace gain block is apparently the same Burr-Brown IC as
the inexpensive Midiman DMP-3, an INA163. Grace surrounds this three buck
chip with very expensive components. To what significance is this? For both
products, the quoted performance specs are identical, the specs of the chip
itself.
If the specs tell all, the implication is that a DMP-3 has the same sound as
a Grace preamp.
Bob Morein
Dresher, PA
(215) 646-4894 |
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Serge Auckland
Guest
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| Posted: Fri Feb 23, 2007 12:48 am Post subject: Re: mike preamp gain control devices |
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Soundhaspriority wrote:
| Quote: | Grace Designs, http://www.gracedesign.com/support/tech_paper2006.pdf, state
that
"as music signal flows through a potentiometer, the potentiometer's
temperature modulates, causing its resistance to modulate. As the value of
resistance changes, so does the amplifier gain, resulting in dynamic
distortion. Most conductive plastic potentiometers have temperature
coefficients of +/-1000ppm/ºC, which can cause considerable signal
degradation at high amplifier gains. By comparison, a metal film resistor
has a typical temperature coefficient of +/-50ppm/ºC, which represents more
than an order of magnitude improvement in thermal stability."
How significant is this effect?
As a side note, the Grace gain block is apparently the same Burr-Brown IC as
the inexpensive Midiman DMP-3, an INA163. Grace surrounds this three buck
chip with very expensive components. To what significance is this? For both
products, the quoted performance specs are identical, the specs of the chip
itself.
If the specs tell all, the implication is that a DMP-3 has the same sound as
a Grace preamp.
Bob Morein
Dresher, PA
(215) 646-4894
|
If I've done my sums right, this is what I reckon:-
The potentiometer's temperature coefficient is 1000ppm or 0.1%
Let's assume that the pots value is 10kohms. Let's further assume that
the pot has a 1watt power handling, and that will provide a temperature
rise of 50 degrees. Finally, let's assume the pot is working at a level
of 1 volt of signal.
Doing the sums, 1 volt across 10kohms is a power level of 100 microwatts
That means that the pot will be heated by 50 microdegrees.
This means that the resistance will change by 50x10^-9, or a totally
negligible amount. So, whatever the improvements in using a fixed
resistor, it is barely measureable, let alone audible. Factor in the
thermal capacitance of the pot, and the whole thing is an exercise in
futility.
If the quoted specs are identical, and in each case the spec is below
the threshold of audibility, then yes, they will all sound the same.
Cost of components has no bearing on the sound. It is of course possibly
that the expensive one is in a nicer case, has better quality of
connectors, and could well give more prode of ownership, but sound
differences? no.
S. |
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