Thanks for clarification.
So If so called sweet spot is around 13-16V then why standard version has only 10 V ? Is there something other which should be considered ?
Whatever power supply voltage you end up with it is easy for you to calculate and install the resistors for whatever cascode voltage you want. The ones on the schematic are going to work for a wide variety of ps voltages. You can adjust to taste. The cascode improves performance of the front end but it also protects the Jfets from over voltage. This allows you to us the 74/170 parts in higher power amplifiers.
OK, thanks! The temp rise it pretty big now, about 28 degC, after playing music for a few hours. This is NOT a summer time amp, unless you have serious AC going.
As for DC offset, one channel has 6 mV.
The other has 74 mV. This gets adjusted tomorrow. Funny, it was the better one on the bench yesterday, as it stabilized to 0.1 mV ( no RCA plugs). I hope that it's not due to oscillation with a load on the rca. That will also have to be checked again. RCA out, vs RCA in.
The last think I did today before playing was measure distortion and adjust bias on the
channel that now shows 6mV. I may have done that with the analyzer's RCA attached. I don't recall.
As for DC offset, one channel has 6 mV.
The other has 74 mV. This gets adjusted tomorrow. Funny, it was the better one on the bench yesterday, as it stabilized to 0.1 mV ( no RCA plugs). I hope that it's not due to oscillation with a load on the rca. That will also have to be checked again. RCA out, vs RCA in.
The last think I did today before playing was measure distortion and adjust bias on the
channel that now shows 6mV. I may have done that with the analyzer's RCA attached. I don't recall.
I'm only concerned if the numbers are vastly different between shorting plugs in and out. As I understand, that means the amp is oscillating, and I do not want that.
On the bench with the plugs out, I saw 0.1mV offset on both channels for an extended period. I should have shut it down and put the plugs in at that time to verify, but I felt it was already not oscillating.
If not oscillation, I assume that it is due to the rising operating temp, as the ambient temp rises from the heat being pumped into the room by the amp. This can be rectified to some degree.
I'm a bit uneasy given the history of this build. It gets checked later tonight.
All that being said, it sounds absolutely terrific. For sure its in the top 3 amps I've had in my system ever... and one of those is dazed2's F5T build ( 4 box monoblock system with power supplies, vastly more power, boutique parts throughout. ) The other was a loaner McCormack DNA-500, and I think this F5T sounds better.
On the bench with the plugs out, I saw 0.1mV offset on both channels for an extended period. I should have shut it down and put the plugs in at that time to verify, but I felt it was already not oscillating.
If not oscillation, I assume that it is due to the rising operating temp, as the ambient temp rises from the heat being pumped into the room by the amp. This can be rectified to some degree.
I'm a bit uneasy given the history of this build. It gets checked later tonight.
All that being said, it sounds absolutely terrific. For sure its in the top 3 amps I've had in my system ever... and one of those is dazed2's F5T build ( 4 box monoblock system with power supplies, vastly more power, boutique parts throughout. ) The other was a loaner McCormack DNA-500, and I think this F5T sounds better.
BigE: First congrats on getting your amp up and running again! 
Not shorting the inputs, or not having some load connected (eg: the impedance of a preamp output), leaves the inputs floating around and open to pick up any extraneous signals in the air. Shorting the inputs just stabilizes them. The small offset your measuring is almost surely the result of a difference in temperature from when it was adjusted and after being allowed to fully heat up rather than any oscillation.
The offset will change slightly from day to day depending on the ambient temp changes. At this point you need to run it for a bit letting everything 'burn in' then making very tiny changes and monitoring. What you're seeing is a pretty small offset. I would try to adjust it slightly lower, trying to get it under 50mv, but, as was mentioned it will wander a bit depending on the season/ambient temp/humidity, etc..
*If you measure an offset with the preamp connected then make sure to check the preamp output for any offset also. The amp will try to amplify any DC on it's inputs.
I biased my V3 mono blocks at 0.7A (350mv across source resistors) at ~20c ambient (+/- 47v rails) and they've never gone over 55c on the warmest days. Occasionally as the seasons change I'll check and tweak if necessary but I haven't needed to yet. When I turn mine on cold there's a ~200+ mv offset which eventually goes down to ~10-20mv when fully warmed up. It's a natural function of being DC coupled with no servo control and there's no need to worry. Burn in, make final adjustment then listen & enjoy.
TJ
Not shorting the inputs, or not having some load connected (eg: the impedance of a preamp output), leaves the inputs floating around and open to pick up any extraneous signals in the air. Shorting the inputs just stabilizes them. The small offset your measuring is almost surely the result of a difference in temperature from when it was adjusted and after being allowed to fully heat up rather than any oscillation.
The offset will change slightly from day to day depending on the ambient temp changes. At this point you need to run it for a bit letting everything 'burn in' then making very tiny changes and monitoring. What you're seeing is a pretty small offset. I would try to adjust it slightly lower, trying to get it under 50mv, but, as was mentioned it will wander a bit depending on the season/ambient temp/humidity, etc..
*If you measure an offset with the preamp connected then make sure to check the preamp output for any offset also. The amp will try to amplify any DC on it's inputs.
I biased my V3 mono blocks at 0.7A (350mv across source resistors) at ~20c ambient (+/- 47v rails) and they've never gone over 55c on the warmest days. Occasionally as the seasons change I'll check and tweak if necessary but I haven't needed to yet. When I turn mine on cold there's a ~200+ mv offset which eventually goes down to ~10-20mv when fully warmed up. It's a natural function of being DC coupled with no servo control and there's no need to worry. Burn in, make final adjustment then listen & enjoy.
TJ
I think you should relax a bit about the offset changes when the shorting plug are out. Generally speaking you should never run an amplifier with the inputs open. It is an invitation for RF pickup or allowing huge hum voltages to get into the amplifier if you happen to touch the input or a cable connected to it that is open on the other end. When it is connected to a source the source impedance is generally low enough to short out minor RF interference. You also would typically have shielding on your input cable. Good practice is to turn off the amplifier before you connect or disconnect the input and don’t run it without a source or shorting plugs connected to the inputs.
These F5 amps have more of a tendency for the offset to drift with temperature changes than, for instance, an Aleph J. So, if the offset changes slowly over a period of time as the amp warms up it is normal. Mine will start at 75mv or so and drop to maybe 15mv in the first few minutes before slowly stabilizing below 10mv. Line voltage changes and ambient temperature changes will cause it to move around a bit as well.
If your amp is oscillating you should be able to see AC voltages at the output, providing you have a meter or scope that has the bandwidth necessary. This would be a much more reliable indicator of stability issues than changes in DC offset with the shorting plugs in or out.
I have not had a huge number of amps in my system but the F5V3 monoblocks are very special. They continue to impress me and are going to be hard to beat.
One caution. If you included the source diodes in your build be very cautious about upping your bias. Resist the temptation to get a few more class A watts. Bias set to high and the amp will self destruct.
I did not put them in my build. With 4 sets of outputs it does not make a big difference in power output unless you are running 2 ohm or lower loads, which most of us will never do.
The weirdest thing just happened..... I was starting the amp up to bias with no shorting plugs. I put two meters in place, on the same polarity to read the bias voltage. At this time the DC offset read 6mV and 1 mV. Then, I read the above posts and thought ok, I will power down and put the plugs in.
I turned it back on, the speaker protection decided it was OK to switch on the load. Then a minute later, my slow charge circuit turned on allowing the power to fill through the thermistor. The meter on the channel that read 74mV last night started flashing, and the speaker protection went on.
So I turned it off, removed the shorting plugs and turned it back on again.
No problem this time. The DC offset is again at 6 mv and 2 mv.
This is troublesome.
I turned it back on, the speaker protection decided it was OK to switch on the load. Then a minute later, my slow charge circuit turned on allowing the power to fill through the thermistor. The meter on the channel that read 74mV last night started flashing, and the speaker protection went on.
So I turned it off, removed the shorting plugs and turned it back on again.
No problem this time. The DC offset is again at 6 mv and 2 mv.
This is troublesome.
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The amp was connnected to the variable outputs of a Benchmark DAC1 USB. The DC offset on the outputs of the benchmark is around 0.1 mV at any volume
The meters on the amp output boards of the suspicious channel ( both on + polarity ) read 0.305 and 0.289 mV. THe DC offset is dropping without shorting plugs, now at 5.0 mV.
I am not sure what do do next.....
I have a digital scope that goes to 100 MHz ( rigol 1000E)
The meters on the amp output boards of the suspicious channel ( both on + polarity ) read 0.305 and 0.289 mV. THe DC offset is dropping without shorting plugs, now at 5.0 mV.
I am not sure what do do next.....
I have a digital scope that goes to 100 MHz ( rigol 1000E)
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Put it back in place, without speakers attached.
The offending channel jumps to about 82 mV. Turning it off, swapping the interconnects, and it jumps to 75 mV when amp is first turned on.
Now I turned the meter to AC, and it read about 0.5mV, same as the other channel.
When turning it back to VDC, it reads 0.3 mV.
ANOTHER BUM METER?????
I just bought a new one.... it reads 63mV on start up.
The right channel is stable. It is the left that is odd.
The offending channel jumps to about 82 mV. Turning it off, swapping the interconnects, and it jumps to 75 mV when amp is first turned on.
Now I turned the meter to AC, and it read about 0.5mV, same as the other channel.
When turning it back to VDC, it reads 0.3 mV.
ANOTHER BUM METER?????
I just bought a new one.... it reads 63mV on start up.
The right channel is stable. It is the left that is odd.
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And finally, the DC offset is at 63mV, but the P channel board that was measured to set the bias is at the same value... so the N channel is now conducting less, which means the N channel bias must have dropped.
I changed the P channel trimmer to no effect.
Perhaps the N channel trimmer should be changed?
I changed the P channel trimmer to no effect.
Perhaps the N channel trimmer should be changed?