F5 power amplifier

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rade95

Member
Joined 2010
6L6 said:
750 ohm? That's a big difference. I haven't heard of anybody using such a high value... Is it still working properly? Just curious. :)
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I haven't powered up the amp yet after the FB resistors change. Perhaps 750 ohm is too high, but I would like to understand the impact of this change on the amp before I power it up and whether there is a danger of blowing something up if I leave the bias as it was before. What value would make sense? Is there a formula for calculating gain with and without feedback? The original amp sounds great, but I don't think it has enough gain. I am sure other people have increased the gain somehow, other then cranking up the +/- PS voltage. Anyone?
 
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permaneder

Member
Joined 2009
rade95 said:
I haven't powered up the amp yet after the FB resistors change. Perhaps 750 ohm is too high, but I would like to understand the impact of this change on the amp before I power it up and whether there is a danger of blowing something up if I leave the bias as it was before. What value would make sense? Is there a formula for calculating gain with and without feedback? The original amp sounds great, but I don't think it has enough gain. I am sure other people have increased the gain somehow, other then cranking up the +/- PS voltage. Anyone?
Click to expand...

Gain is: log (Rnfb + Rsource)/ Rsource*20

log (750 + 10)/10*20 = 37.6dB
 
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rade95

Member
Joined 2010
6L6 said:
Change the (4) feedback resistors to 220 ohm 3W. That seems to be the magic number to get it to a 'normal' gain.
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Hi Jim, I remember us talking about it. Will try 220 ohm x 4 instead of original 100 ohm x 4. Increasing JFETs source resistors should decrease the gain, according to the formula so I'll leave them @ 10 ohm. Any idea how would this change affect the bias? FB was briefly discussed on page 1112 of this thread. thanks
 
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rade95

Member
Joined 2010
jameshillj said:
There are a few dedicated gain stages developed just for the purpose of driving the amp to full power output with a standard signal input - "preamp for F5 thread", I think.
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I think this is the thread you are referring to:
http://www.diyaudio.com/forums/pass-labs/125893-preamp-ideas-f5-3.html

But, I don't like the idea of pre-amping the F5 for more gain. I already have a pre-amp that can drive 8-10W out of a stock F5. I will stick with changing FB resistors to 220 ohm, like Jim suggested and leave it like that. Then I'd rather design a new amp with a BJT driver stage, bump up PS to +/- 50-60V and push F5 to class AB. This could produce 50W with a single MOSFET pair or approx. 100W with 2 MOSFET pairs. Have the circuit ready. Oh and I am replacing the IRFs with Toshiba K1058/J201 laterals. More to come...
 
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