I have built many F5's over the years but never had a problem like this. On the latest build one channel is acting differently. When I power on cold the offset is around -180mV on this channel, after a minute -150mV after 5min around -100mV, after 30min -50mV. Finally after 2 hours 0mV, then after 3 hours +10mV, after 4 hours +20mV and still rising. I have built multiple identical stereo F5's in identical enclosure and never before this much offset on startup and the offset rise has stopped after an hour or two.
The question is should I be worried? And what could cause this? I have already resoldered all solder joints but that did not help.
Boards and all parts including jfets are official from diyaudiostore.
The question is should I be worried? And what could cause this? I have already resoldered all solder joints but that did not help.
Boards and all parts including jfets are official from diyaudiostore.
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Your board layout may not cooperate, but press the flats face to face, joined with thermal paste
and a tie wrap, or else thermal epoxy them together. See this board layout for Q1 and Q2.
https://www.diyaudio.com/archive/bl...390-gb-f5-guide-pcb-version-2-f5_v2_front.jpg
and a tie wrap, or else thermal epoxy them together. See this board layout for Q1 and Q2.
https://www.diyaudio.com/archive/bl...390-gb-f5-guide-pcb-version-2-f5_v2_front.jpg
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Thanks, I'll try this. They are thermally coupled with thermal paste between them and tightly tied together with a tie wrap
Good point rolandong. I just thought that if there is something wrong now it might get worse over time. I am driving tweeters directly without any dc protection with this amplifier (LX521.4) and would like to get as little dc as possible even before fully settled.
use series capacitor with tweeter
they are too fragile for even most benign transition behavior** of DC coupled amp
**power up/down
cap must and can be pretty larger than needed for xover point, to avoid phase disturbance
they are too fragile for even most benign transition behavior** of DC coupled amp
**power up/down
cap must and can be pretty larger than needed for xover point, to avoid phase disturbance
I don't want to use capacitor there. I think it is going backwards in fully active setup. But I might use relay protection/delay during power on/off.
that's your choice, which way you're going to protect tweeter
but , there is no choice do you need or not to protect it
but , there is no choice do you need or not to protect it
Close look at photos suggests one thermistor may not be touching mosfet washer. Could that be related to problem?
The correct answer is no. -180 mv to start and 50 mv hot is not a big issue.
Most likely you should consider lowering the value of the resistor in series
with the thermistor on the positive side so that it is a little more effective,
and readjusting the bias/offset. Lower it just a little bit, say 2.2K down
to 1.5 K and see what you get.