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Old 10th June 2006, 09:45 AM   #1
Buhl is offline Buhl  Denmark
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Default About output pot. value and gain on BoZoS..

Hello Group
Having just finished a larger update on my X-BoSoZ, that included X-tra large heatsinks for the mosfets and a new 10Kohm alps blue aouput pot, I noticed that my gain have been reduced quite a bit from before when I used a 4,7Kohm - I.e. double the value - is this to bee expected ???

I use 124 ohm for the gain resistor, and this gives me about 20dB gain without the X-resistors, that have the value of 37Kohm - I am afraid to go lower as NP spoke of potential trouble with lower gain resistors, causing the need for matching of the FETS - but with the X-resistors reducing gain - should i just go lover - to 50 or 60 orh, or should use a higher value for the X-resistor. I judge the circuit to be about unity gain as it is now, and my Pearl MM pre and tuner would like at least 10 dB of gain...


Cheers !
Hans
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Old 10th June 2006, 10:23 AM   #2
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Hi,

I think Nelson's comment must have meant that higher gain might create unpleasant level of distortion. If your X-feedback resistor is sized properly to find the “gain and acceptable distortion”, the size of gain resistor (of your 124) could go down even to zero. Isn’t the X-resistor to reduce the distortion . . . ?

Hope this info will help.

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Old 10th June 2006, 07:44 PM   #3
Buhl is offline Buhl  Denmark
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Quote:
Originally posted by jh6you
Hi,

I think Nelson's comment must have meant that higher gain might create unpleasant level of distortion. If your X-feedback resistor is sized properly to find the “gain and acceptable distortion”, the size of gain resistor (of your 124) could go down even to zero. Isn’t the X-resistor to reduce the distortion . . . ?

Hope this info will help.

Regards,

Well, Sir Pass writes that the only cause that justifies matched devices is when the R15 resistor coupling the sources is close to zero, giving a gain of 50 (34dB) - this is proberly due to distortion as you write
Funny that he also writes that low value output pots will reduce the gain a bit - I got the opposite. I also changed the Xresistors from 39K to 37K when i changed the pot, because the only 39 ohm I had was metal wire wound, that I suspect is worse than my current carbon resistors.... I will tear it a part tomorrow, and try to put in higher value X-resistors, and mabye reduce R15...... fun, fun, fun....


Cheers !
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Old 12th June 2006, 10:01 PM   #4
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Quote:
Originally posted by Buhl
Funny that he also writes that low value output pots will reduce the gain a bit - I got the opposite.
I think you're talking about two different resistors. The R15
which couples the Sources of the Mosfets will increase gain
as the value declines, but as it approaches 0 you will depend on
matching of the Mosfets to keep the DC current values of both
devices equal.

At the output resistance to ground, usually in the form of a pot,
is used to set the output gain also, and since that resistance
works as a shunt, a lower value pot will decrease the maximum
gain.
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