The entire point of P1 is to adjust the DC offset.
The amp cannot be DC stable with R16 (the feedback resistor) removed. Do not remove R16.
The amp cannot be DC stable with R16 (the feedback resistor) removed. Do not remove R16.
Sorry for the confusion. I mentioned lifting R16 as a temporary step in setting P1 as described by Mark and others:
Note that I hadn't actually done any of this ^^^ procedure yet when I took the measurements above (and blew fuses).
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hmmm... nothing looks incorrect.
Replace the fuses. Make sure the input + and - are shorted. Power it up and dial P1 to zero DC offset.
Replace the fuses. Make sure the input + and - are shorted. Power it up and dial P1 to zero DC offset.
Ok thanks! I wonder if I should parallel fuses temporarily to have some headroom (80mA -> 160mA) and do one side at a time. I have 8 more but I'd hate to run out -- not a size I was able to source locally. The next size up I have is 500mA slow blow!
BTW, short only +IN to -IN or also to GND?
BTW, short only +IN to -IN or also to GND?
Ok, I did one channel, got it bouncing around 0mV (+/-5mV). The other side continues to blow fuses so I'm not able to adjust P1 to see if it's the issue. Kind of a chicken/egg problem. I'm wondering if 80mA T fuses are too small (one per 15VA transformer on 230VAC).
I used the smaller output devices and Q8 and Q10 are hot. Not smoking hot of course, but you get uncomfortable with a finger touching either one for more than a few seconds.
I used the smaller output devices and Q8 and Q10 are hot. Not smoking hot of course, but you get uncomfortable with a finger touching either one for more than a few seconds.
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I got both sides adjusted finally, but I actually witnessed one of my fuses partly blow and then keep going (little smoke inside and then no flash). I've left it running because I know it's going to blow the rest of the way next time I turn it on!
So it seems like either my fuses are undersized, or I've got something else wrong, possibly related to Q8/Q10/Q20/Q25 being hot (measured 63C/145F with my multimeter's super cheap and probably not accurate temp probe).
What size fuses are people using (dual mono or otherwise)? I know it depends on the PSU and transformer, but I just want to get a ballpark idea. I thought 80mA slow blow (for 15VA/230VA = 65mA) would be enough overhead!
So it seems like either my fuses are undersized, or I've got something else wrong, possibly related to Q8/Q10/Q20/Q25 being hot (measured 63C/145F with my multimeter's super cheap and probably not accurate temp probe).
What size fuses are people using (dual mono or otherwise)? I know it depends on the PSU and transformer, but I just want to get a ballpark idea. I thought 80mA slow blow (for 15VA/230VA = 65mA) would be enough overhead!
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Subwoofer Output
What is the proper way to wire a second RCA output for a powered subwoofer with this pre-amp? I plan to incorporate a switching headphone jack too, so I am thinking the signal goes to the switching headphone jack, then to the main RCA output and Subwoofer output. I just don’t understand the details of setting up the sub output properly so that everything plays well together.
The power supply is the Pete Millet design that is so popular on this thread (thanks 6L6 for that suggestion). The amplifier I am using is the Pass ACA (a single one in stereo mode now, to be in dual mono block mode in the future). The subwoofer I will be using is a cheap, but effective, Polk Audio PSW-10.
Thanks for any light you folks can shed on the subject.
What is the proper way to wire a second RCA output for a powered subwoofer with this pre-amp? I plan to incorporate a switching headphone jack too, so I am thinking the signal goes to the switching headphone jack, then to the main RCA output and Subwoofer output. I just don’t understand the details of setting up the sub output properly so that everything plays well together.
The power supply is the Pete Millet design that is so popular on this thread (thanks 6L6 for that suggestion). The amplifier I am using is the Pass ACA (a single one in stereo mode now, to be in dual mono block mode in the future). The subwoofer I will be using is a cheap, but effective, Polk Audio PSW-10.
Thanks for any light you folks can shed on the subject.
Nothing wrong with Q8, 10, 20, 25. They are running class A and run hot, yes so hot they are uncomfortable to touch.
I put small heatsinks on all four.
You just need a bigger fuses.
Thanks. I think I'll try doubling the fuses to 150mA or so.
I have leftover aluminum sinks lying around that were from taller ones cut in half. I might find a way to attach them to these devices, or maybe even better, switch to the alternate devices since they are already in a package that can have sinks bolted on.
Cheers!
Here are the heatsinks I used: Mouser part#532-575200B00
Don't forget the other changes if you switch to the large outputs.
Where exactly did you place the fuses?
Are those power supplies shunt type?
If not shunt, what is the voltage drop across those series transistor regulators?
Please see my photo in post #1636. I'm not sure what is the name for the type of PSU the VRDNs are. They're basically rectifiers and capacitors with some de-noising section (and a lot of other neat stuff). So not shunt type(?).
There is one 80mA fuse between each transformers and the mains block. I am inclined to agree with people that are suggesting the fuse is under rated (slightly). When it starts without blowing a fuse, it will run indefinitely. But I could see on one of the switch ons a fuse that partly blew (made smoke inside the glass but didn't blow completely).
I need to place an order for some CL60 NTCs tomorrow (for floating the PSU ground) so I might order a few extra to limit inrush current. And I'll order some 125mA fuses. Maybe even bump up my 15VA transformers to 30VA (and 250mA fuses), because why not? 🙂
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CL60 will not warm sufficiently, to reduce its resistance. Its cold resistance will be in series with the mains feed at all times.
You have a lot of capacitance on those power supply PCB's, and that is causing the fuses to smoke/blow.... most likely.
You could check if the transformers are ringing... but you need an oscilloscope.
You have a lot of capacitance on those power supply PCB's, and that is causing the fuses to smoke/blow.... most likely.
You could check if the transformers are ringing... but you need an oscilloscope.