Haven't measured it, after the new feedback resistors were put in. It was 2-3mV before. Bias was 1.2A.
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. 🙂
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?
Change the (4) feedback resistors to 220 ohm 3W. That seems to be the magic number to get it to a 'normal' gain.
The way this has been done is to increase F5,6,7, and 8 to 150 or 220 ohm.
I would turn the pots all the way down and set the voltages from scratch to avoid any magic smoke.😱
I would turn the pots all the way down and set the voltages from scratch to avoid any magic smoke.😱
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Hi, In your formula if you do 10 * log [(750+10)/10*20] then you get 38dB. Thanks anyway.
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
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.
I would turn the bias down or off, then replace the feedback resistors. I wouldn't try to keep the same bias setting when changing the components.
Re-bias and see if it comes up properly... It most likely will be fine. 🙂
Re-bias and see if it comes up properly... It most likely will be fine. 🙂
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...
More feedback and gain discussions on page 1154, for those interested...newbies like myself.
Really, so if I send a link with the page number: http://www.diyaudio.com/forums/pass-labs/121228-f5-power-amplifier-1154.html
This will surely take you to page 1154, but you may not see post # 11531 at the beginning? I am using Safari 602.
My user settings on this forum are set for 5 posts per page so I can view pages easier on my phone. Thus page numbers are meaningless. When I click on your link I see post #5766 which is about half of #11531.
I think if you do something like this. It will take you to the exact post and not a virtual page.
http://www.diyaudio.com/forums/pass-labs/121228-f5-power-amplifier-2307.html#post2853766
http://www.diyaudio.com/forums/pass-labs/121228-f5-power-amplifier-2307.html#post2853766