| Buhl |
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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| 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, |
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| Buhl |
| 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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| Nelson Pass |
| 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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