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Old 17th May 2010, 06:23 PM   #221
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Since I want to have an input selector (relay based), I am inclined to go with the buffer as the first stage, followed by volume pot and then the gain stage, which should have a sufficiently low output impedance to drive the interconnect to the power amp.
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Old 17th May 2010, 06:48 PM   #222
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by Jen-B View Post
Hi Andrew, you missed my underlying point,
I did not miss your underlying point.
I agree with and did not quote the parts of the post that were not controversial.

The problem was the advice you gave and I quoted, about locating a buffer in front of a pot.
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Old 17th May 2010, 06:52 PM   #223
AndrewT is offline AndrewT  Scotland
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Originally Posted by Samuel Jayaraj View Post
I want to have an input selector (relay based), I am inclined to go with the buffer as the first stage, followed by volume pot and then the gain stage, which should have a sufficiently low output impedance to drive the interconnect to the power amp.
no.

Gain stage first if some of your sources need it, then pot/attenuator, then buffer to feed the interconnects.
The pot then has the opportunity to attenuate the noise created in the gain stage.


If any of your sources cannot drive the cables and the relay audio pins and the pot then fit a buffer or gain stage at the sources that are incapable.
Then the pre-amp will not need any gain stage, just a buffer.

If the gain stage is guaranteed stable with gains of +0dB, +6dB and +12dB, it would be nice to have a switchable gain stage, for 1times, 2times and 4times. That covers almost every source you are likely to want to connect up.
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Last edited by AndrewT; 17th May 2010 at 06:55 PM.
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Old 17th May 2010, 08:23 PM   #224
Jen-B is offline Jen-B  United Kingdom
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Originally Posted by AndrewT View Post

...the parts of the post that were not controversial.

The problem was the advice you gave...
Controversial? Me?

Problem advice? Me?

Well Andrew you sound like a frustrated engineer in a meeting where he thinks everything must be done his way! Good luck with that!

(And I bet you really want to reply to this post justifying your position...)
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Old 18th May 2010, 03:31 AM   #225
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I sure don't intend to start a controversy here. Both approached have their merits and compromises. Let's give it a while. I'll come back with what I find out in the practical situation. Thanks again.
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Old 31st May 2010, 07:30 AM   #226
vzs is offline vzs  Europe
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Hi! As I can not find in local shops IRF9620 nor IRFP044 can I use IRF9610 and IRF610 for both positive and negative supplies as in the schematics of Salas at #84. Did someone used those?
Thanks!
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Old 31st May 2010, 12:53 PM   #227
juma is offline juma  Serbia
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Originally Posted by vzs View Post
...can I use IRF9610 and IRF610 ...
They'll work all right.
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Old 9th June 2010, 06:05 PM   #228
BFNY is offline BFNY  United States
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Quote:
Originally Posted by Samuel Jayaraj View Post
Since I want to have an input selector (relay based), I am inclined to go with the buffer as the first stage, followed by volume pot and then the gain stage, which should have a sufficiently low output impedance to drive the interconnect to the power amp.
hate to tread into the muck, but, why not.

I can see reasons for doing it the way you suggest. Not all sources have low output impedance, but are happy driving a reasonable load, i.e. 100-200k. And this load will determine the low frequency cutoff point in combination with the source's output coupling cap, almost always located after the output stage.

It's not always convenient to add a buffer to the output of the source. Especially in a case where a friend may bring over a classic and pricey vintage source made 40 or 50 years ago. This would include reel to real and cassette decks, FM tuners, phono pre-amps, etc.

So why not just use a 100k or 250k pot?

The higher thermal noise related to the higher pot resistance is one reason, probably a minor one.
The pot will have max. output impedance at the midpoint -The output impedance is the parallel combination of R1 and R2, i.e. Zout = (R1 * R2) / (R1 + R2). So for a 250k pot, Zout will be 62.5k, and driving the next stage, a jfet gate.
So what?
Jfets don't like to be driven by high source impedance due to their non-linear input capacitance. This point was discussed in the original B1 thread. The effect can apparently be heard, and certainly measured. So a low value pot, of say, 25K or less is preferred.

Anyway, this leads a logical approach, to me at least, where one may wish to have a line stage with high input impedance R to ground (say 250k), then buffer (b1 type) driving a lowish value volume pot (10 to 25k), which is then followed by either another jfet buffer (b1) or jfet gain stage / buffer.
If one use low gain, and low noise parts (BF862) in the gain stage, noise should not be an issue.

As mentioned, there are many ways to do this. Unless one assigns real world values for components involved, the "do's" and "don'ts" really have little meaning, at least to me. Everything is relative.

Bob
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Old 9th June 2010, 06:14 PM   #229
Zen Mod is offline Zen Mod  Serbia
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cascoded jfet stage is good for all these issues
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Old 9th June 2010, 07:07 PM   #230
AndrewT is offline AndrewT  Scotland
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BNFY,
are you aware of the stray capacitances in your system and what effect they could have when they interact with your source and load resistances/impedances?
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