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John Curl's Blowtorch preamplifier part II
John Curl's Blowtorch preamplifier part II
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Old 23rd September 2009, 01:37 PM   #1461
PMA is offline PMA  Europe
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John Curl's Blowtorch preamplifier part II
If that of Courage is allowed, then I hope I can post the circuit:

(if not, let me know, please)
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Old 23rd September 2009, 01:48 PM   #1462
jan.didden is online now jan.didden  Europe
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Quote:
Originally Posted by PMA View Post
If that of Courage is allowed, then I hope I can post the circuit:

(if not, let me know, please)
One question I have to this circuit is the gain balance. I guess the gain is set by the ratio of R20/R21 to R19.
Beyond maximizing dynamic range, I guess there's no special requirement to match R20 to R21, am I correct?

jd
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Old 23rd September 2009, 01:51 PM   #1463
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Thanks PMA, that should be enough for intelligent discussion. The values are on target, and make sense. This is the closest approximation to the BT yet, and I suspect that it can be improved, as well, with a little constructive discussion. As you know, people have already taken the early schematics and made them into commercial products, so putting this here, isn't going to give away anything.
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Old 23rd September 2009, 01:55 PM   #1464
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John Curl's Blowtorch preamplifier part II
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Originally Posted by janneman View Post
One question I have to this circuit is the gain balance. I guess the gain is set by the ratio of R20/R21 to R19.
Well yes, the value of R19 would change the gain. R20 and R21 are chosen to convert current output to voltage source with 1k output impedance.
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Old 23rd September 2009, 01:56 PM   #1465
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I think that at least 1% tolerance resistors should be used, but perhaps this is an area where better matching might bring improvement. What do you think, PMA? I'm thinking of a small value bulk metal trimpot between the 1K loads and wiper to ground.
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Old 23rd September 2009, 02:04 PM   #1466
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John Curl's Blowtorch preamplifier part II
For true balanced output, it would be good to set precisely and individually, I would say.
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Old 23rd September 2009, 02:05 PM   #1467
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Quote:
Originally Posted by john curl View Post
I think that at least 1% tolerance resistors should be used, but perhaps this is an area where better matching might bring improvement. What do you think, PMA? I'm thinking of a small value bulk metal trimpot between the 1K loads and wiper to ground.
What kind of improvements could we then expect?

jd
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Old 23rd September 2009, 02:07 PM   #1468
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Originally Posted by PMA View Post
For true balanced output, it would be good to set precisely and individually, I would say.
Ahhh we cross posted. OK, I know that, but would that be worth while to do?
What's the advantage to balancing the outputs to say 0.1% of each other, instead of say 1%?

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Old 23rd September 2009, 02:07 PM   #1469
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Who knows as of yet?
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Old 23rd September 2009, 02:10 PM   #1470
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Quote:
Originally Posted by PMA View Post
If that of Courage is allowed, then I hope I can post the circuit:

(if not, let me know, please)
In fact is a schematic that I sent to courage before stopping to help people who wanted to clone it rather than learning.
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