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Old 23rd October 2012, 02:27 AM   #631
Art M is offline Art M  United States
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Default 1Kohm V.S 10Kohm

In a very general sort of way, how much worse would performance be if I used
10Kohm controls and scaled the requisite component values ? Often I prefer to use
what parts I have at hand.
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Old 23rd October 2012, 02:40 AM   #632
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Now if one end of the pot is connected to the wiper the situation will be different,..............
I request's you to please explain, What will be the situation here? Both, when (1k resistor shunts the pot as well as when two pairs of 560 ohms shunting the pot).

What's wrong with using, 560 ohms pair with 10K pot, if its ok in every way?

Please, do consider, that i want to use a linear pot.

Thanks.

Last edited by noddy55; 23rd October 2012 at 02:44 AM.
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Old 23rd October 2012, 02:52 AM   #633
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Incidently, my preamp that uses this relay attenuator is my favourite for sound quality. It has a single non-inverting gain stage after the attenuator using a discrete opamp that is based on the Figure 3.33 circuit in your Small Signal Audio Design book, but with the addition of a current mirror in the input stage and some tweaking of bias currents etc.
Can you please tell me the actual brand, model and make of your favourite preamp?

If it's your self made then, can you please, post its schematic? I feel it's worth building one to feel the magic.

I would love to hear the sound you love.

Thanks.
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Old 23rd October 2012, 06:43 AM   #634
Mooly is offline Mooly  United Kingdom
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Quote:
Originally Posted by Art M View Post
In a very general sort of way, how much worse would performance be if I used
10Kohm controls and scaled the requisite component values ? Often I prefer to use
what parts I have at hand.
The noise factor of the preamp will be slightly worse (slightly as in on paper). A real benefit is the loading on the opamps is reduced substantially.

Dougs older "1996 precision preamp" was more complex than this one and used the higher impedance values. It was ultra quiet too. You won't hear any difference noise wise.
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Old 23rd October 2012, 06:46 AM   #635
Mooly is offline Mooly  United Kingdom
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Quote:
Originally Posted by noddy55 View Post
I request's you to please explain, What will be the situation here? Both, when (1k resistor shunts the pot as well as when two pairs of 560 ohms shunting the pot).

What's wrong with using, 560 ohms pair with 10K pot, if its ok in every way?

Please, do consider, that i want to use a linear pot.

Thanks.
Not sure who you are quoting here but didn't we cover this in post #600
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Old 23rd October 2012, 07:41 PM   #636
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Quote:
Originally Posted by Art M View Post
In a very general sort of way, how much worse would performance be if I used
10Kohm controls and scaled the requisite component values ? Often I prefer to use
what parts I have at hand.
The noise performance would be significantly worse. Exactly how much worse I cannot say without doing the experiment.

The whole essence of the preamplifier design is that it shows how to use 1K pots without resorting to the complications of discrete circuitry to drive the low impedances. To build it with 10K pots is to really miss the point.
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Old 24th October 2012, 07:40 PM   #637
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Not sure who you are quoting here but didn't we cover this in post #600
Please, see post 605 by Mr.kevinkr.

Thanks.
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Old 25th October 2012, 06:48 AM   #638
Mooly is offline Mooly  United Kingdom
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Originally Posted by noddy55 View Post
Please, see post 605 by Mr.kevinkr.

Thanks.
See the last part of the reply in kevinkr's post... "the situation will be different, but the taper resulting from such a connection would not be the most useful I could think of."

So it's OK apart from the "taper" or law of the pot. What that means is that rotating the pot a few degrees will not give the same change in output voltage on the wiper as a pot with no resistors connected to the wiper. You would find all the "action" happens very close to each end of the rotation.
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Old 25th October 2012, 09:26 AM   #639
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Hi Noddy55
Off-topic, but if you want to understand and minipulate potentiometer law (taper) with external resistors, try this article, which covers most issues. Potentiometers (Beginners' Guide to Pots)
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Old 3rd November 2012, 06:34 AM   #640
owdeo is offline owdeo  Australia
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Quote:
Originally Posted by noddy55 View Post
Can you please tell me the actual brand, model and make of your favourite preamp?

If it's your self made then, can you please, post its schematic? I feel it's worth building one to feel the magic.

I would love to hear the sound you love.

Thanks.
Well as I said it's basically Self's discrete opamp circuit as published in his book. Although most of the component values I ended up with are different I think I would still be breaching copyright by posting the schematic as the topology is the same. I recommend getting a copy of the book - it's excellent.

The circuit is basically a Lin toplogy - degenerated LTP input with current mirror, simple common emitter VAS (I tried both darlington and cascode enhancements and found they made the sound worse), and single-ended emitter follower output stage with constant current source (again I found complementary push-pull sounded worse). I suppose the differences in sound quality I heard with the different configurations came down to the distribution of harmonics, but I have no way of measuring distortion that low. I could see it in Spice though.

Would Mr Self object if I posted my simulation schematic here given the component values are different? IIRC he had the circuit free to view on his old website so maybe he won't mind. If he doesn't I will post it.
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