No problem.
Here’s my take on what I think the problem was. Apart from the faulty Zener and a couple of other things, you clearly had some oscillation issues related to the feedback circuitry. I think that may have been caused by the modern transistors and opamps you fitted being faster than the old one. Simply put, slow (as in limited bandwidth) components put a brake on oscillations. So when you switch to faster ones, it might become more prone to oscillation. The purpose of C520 is to limit the bandwidth of the amplifier and to modify the relationship between gain and phase to eliminate the risk of oscillation. 100pF is a rather large value. It MAY affect your treble performance a little bit. You can try to reduce it to 68pF or maybe 47pF if you feel the high end of the frequency spectrum is lacking, but be careful!
Here’s my take on what I think the problem was. Apart from the faulty Zener and a couple of other things, you clearly had some oscillation issues related to the feedback circuitry. I think that may have been caused by the modern transistors and opamps you fitted being faster than the old one. Simply put, slow (as in limited bandwidth) components put a brake on oscillations. So when you switch to faster ones, it might become more prone to oscillation. The purpose of C520 is to limit the bandwidth of the amplifier and to modify the relationship between gain and phase to eliminate the risk of oscillation. 100pF is a rather large value. It MAY affect your treble performance a little bit. You can try to reduce it to 68pF or maybe 47pF if you feel the high end of the frequency spectrum is lacking, but be careful!
I belive you are correct. This morning i tested to se if i could install the 33pf again and with it installed it oscillates, put back 100pf and i'm safe again. I dont have any value inbetween in stock so i need to get me some.
47,56 and 68 pF on it's way. I feel i want it to be stable with as high bandwitdth as possible. But that sucker seems to be a deal breaker.
47,56 and 68 pF on it's way. I feel i want it to be stable with as high bandwitdth as possible. But that sucker seems to be a deal breaker.
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Thank you, I haven’t popped a bottle just yet but soon! Actually I want to thank you all for guiding me true this and having patience with me! I will let you guys know how it goes and what value I ended up with using.
And there is more fun to come because I have another Poa 6600 to repair but I have so much knowledge now to have my hopes up. And it will be fun🙂
Hello and hope all is good with you all! So my son just arrived last week so I haven’t got time to finish the last thing things.
Had some time over today so I started to mess around with what value of C520 I should use and seems 100pf is best. I ramped up the bulb to 200w and set the bias to 2mV and then shut I down and let the amp rest for some time. When I came back and powered the bulb started to flashing, after powering on and off 2-3 times the amp start to behave good again.
Do you guys thinks this behavior is related to the DBT limiting the inrush for the amplifier to power on correctly when I start adjusting the bias or? turning the trim pod back to zero I have no power on problems.
Edit. Also after amp is warm I can power on direct to wall with no problems.
Had some time over today so I started to mess around with what value of C520 I should use and seems 100pf is best. I ramped up the bulb to 200w and set the bias to 2mV and then shut I down and let the amp rest for some time. When I came back and powered the bulb started to flashing, after powering on and off 2-3 times the amp start to behave good again.
Do you guys thinks this behavior is related to the DBT limiting the inrush for the amplifier to power on correctly when I start adjusting the bias or? turning the trim pod back to zero I have no power on problems.
Edit. Also after amp is warm I can power on direct to wall with no problems.
@njswede
So I was able to set bias and all was working fine I left I on for several hours with no problem and <30mV dc on speaker terminals so i connected speaker and played some music and fine for 10min until it popped and now something is shorted, 6.3A fuses blown. Need to investigate what got blown. 😞
So I was able to set bias and all was working fine I left I on for several hours with no problem and <30mV dc on speaker terminals so i connected speaker and played some music and fine for 10min until it popped and now something is shorted, 6.3A fuses blown. Need to investigate what got blown. 😞