The problem turned out to be P1 trimpot that was stuck in a dead-short position. Replaced the pot and the channel is happy.Good day friends. What would be the cause of gross distortion on an F3 if all the parts check out? P1 and P2 does not resolve the distortion no matter how I adjust them. The LU1014 has me worried.
Modifying F3 feedback
So, I hear some screeching from one channel of my F3 when testing for hum under a light load (headphone hum detector, ~18 ohm load on the 'LOUD' setting), but not a very light load (~166 ohms, headphone hum detector on the 'Quiet' setting). I don't know what it is but it's got a decay to it like a time constant. I'm thinking it's derived from the Aleph CCS, and it modulates whether the inputs are open (high decaying screech) vs. loaded with a source (duller deep pulses). I wonder if this is so-called oscillation, and it's got me wondering about the feedback circuit, as well as a gate stopper on the JFET.
It seems that film caps are preferred in the signal path. I don't know if the feedback loop is considered signal path or not, but the 33pF cap I'm using for position C9 is mica. The Aleph 30 build specs a 10pF mica 'compensation cap' in a similar position.
I can't tell if C9 is functioning as a high pass or low pass filter. It's described in the Zen v9 document as providing 'compensation that makes the square wave look good' which reads to me like it's meant to reduce ringing/oscillation, so low pass? If I wanted to reduce the amplifier's bandwidth to try and get rid of the screech/oscillation, what is the R to use in the RC filter calculation here? is it the 47.5K of R2 or the gate resistance of the JFET? Or something else? If a higher capacitance value here is worth trying, what value should I try? I have some nice WIMA 1uF film caps.
Edit: adding the screech
https://drive.google.com/file/d/1UnW5DdtIkgtUE1Z98Konm8cvsFy_dtgK/view?usp=sharing
So, I hear some screeching from one channel of my F3 when testing for hum under a light load (headphone hum detector, ~18 ohm load on the 'LOUD' setting), but not a very light load (~166 ohms, headphone hum detector on the 'Quiet' setting). I don't know what it is but it's got a decay to it like a time constant. I'm thinking it's derived from the Aleph CCS, and it modulates whether the inputs are open (high decaying screech) vs. loaded with a source (duller deep pulses). I wonder if this is so-called oscillation, and it's got me wondering about the feedback circuit, as well as a gate stopper on the JFET.
It seems that film caps are preferred in the signal path. I don't know if the feedback loop is considered signal path or not, but the 33pF cap I'm using for position C9 is mica. The Aleph 30 build specs a 10pF mica 'compensation cap' in a similar position.
I can't tell if C9 is functioning as a high pass or low pass filter. It's described in the Zen v9 document as providing 'compensation that makes the square wave look good' which reads to me like it's meant to reduce ringing/oscillation, so low pass? If I wanted to reduce the amplifier's bandwidth to try and get rid of the screech/oscillation, what is the R to use in the RC filter calculation here? is it the 47.5K of R2 or the gate resistance of the JFET? Or something else? If a higher capacitance value here is worth trying, what value should I try? I have some nice WIMA 1uF film caps.
Edit: adding the screech
https://drive.google.com/file/d/1UnW5DdtIkgtUE1Z98Konm8cvsFy_dtgK/view?usp=sharing
Attachments
More fun. I can measure 8mV of potential difference between either channel's circuit ground and safety/earth ground, so I'm inferring I have a substantial ground loop. Would the addition of a ground lift (the pair of anti-parallel diodes shown in the schematic below) between the PSU ground of the power supply and earth ground be an acceptable mitigation?
Edit: should add, this is 8mV of DC, with the positive end on circuit ground and negative end on earth ground. I don't recall measuring for AC.
Edit: should add, this is 8mV of DC, with the positive end on circuit ground and negative end on earth ground. I don't recall measuring for AC.
Attachments
Hi
Not really a specific question for the F3 but it’s my first time working with a UMS PCB/Heatsink, I’m building an F3 from the nice CSample pcb, Issue; I’m used to always centering my MOSFET so the heat spread evenly across the entire heatsink surface. Should I go through the trouble of installing my MOSFET far apart like on my picture or will it be ok spacing them only 40mm like on a UMS pcb/heatsink? I have a feeling that I’ll have some hot spot if I don’t spread them evenly... is it paranoias??
Thanks
Eric
Not really a specific question for the F3 but it’s my first time working with a UMS PCB/Heatsink, I’m building an F3 from the nice CSample pcb, Issue; I’m used to always centering my MOSFET so the heat spread evenly across the entire heatsink surface. Should I go through the trouble of installing my MOSFET far apart like on my picture or will it be ok spacing them only 40mm like on a UMS pcb/heatsink? I have a feeling that I’ll have some hot spot if I don’t spread them evenly... is it paranoias??
Thanks
Eric
Attachments
