I was able to bias both channels, so the pots do work. DC offset can be adjusted when changing bias levels.... touchy, but it works.
6.8mA and 7.3mA for Id.
The jFETs are passing expected current.
Where does the difference of 0.5mA go?
Into the NFB?
Where else?
The jFETs are passing expected current.
Where does the difference of 0.5mA go?
Into the NFB?
Where else?
This may be a start up anomaly in a very few builds.
Can anyone think of an explanation?
Can anyone remember who posted the alleged problem?
Did they solve it?
Still, no sound.
Measuring the rca cable at the preamp shows 46.5K, nominally 47.5K, and center pin tests connected to Q1.1 and Q2.1 gate. Measuring Q1 and Q2 by directly hooking the gate pin on the transistors shows 1K both sides.
Measuring the rca cable at the preamp shows 46.5K, nominally 47.5K, and center pin tests connected to Q1.1 and Q2.1 gate. Measuring Q1 and Q2 by directly hooking the gate pin on the transistors shows 1K both sides.
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Found it!!!
Bad connection right at the speaker terminal. Somehow, DC was getting through, but not music. Is that weird or what?
Now to attack the noise issue -- step one is to home run the speaker return to the power supply. But that will happen next week. Enough for now
THANKS TO ALL THAT HELPED.
Especially that trick to measure Id Buzz.
AndrewT:
The other 0.5 ma could be going through P3 as well....
Bad connection right at the speaker terminal. Somehow, DC was getting through, but not music. Is that weird or what?
Now to attack the noise issue -- step one is to home run the speaker return to the power supply. But that will happen next week. Enough for now
THANKS TO ALL THAT HELPED.
Especially that trick to measure Id Buzz.
AndrewT:
The other 0.5 ma could be going through P3 as well....
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BTW: The amp is dead quiet, until the DCB1 is plugged in.... There is a phono section attached as well, all with ground loop breakers. There is a turn off thump when the Technics SP10 power supply is turned off.
OH, and there are ground loop breakers on the phono and DCB1.
OH, and there are ground loop breakers on the phono and DCB1.
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If the phono section is single ended power and DC blocked to protect the next stage then it MUST generate a large DC pulse on shut down.
The only way to avoid this is to mute the output of the phono section on start up and shut down.
You cannot expect subsequent sections to know that the phono is changing ON/OFF or OFF/ON.
The only way to avoid this is to mute the output of the phono section on start up and shut down.
You cannot expect subsequent sections to know that the phono is changing ON/OFF or OFF/ON.
AndrewT, it was not the phono section, but the power supply for the TT!
I have to fix some connections and wire the amp up properly. I was rather cavalier in not twisting.... afteralll, how bad can it be with really short runs of wire right? Well it's bad.
Moreover, the PCB pads/traces are very fragile when the vias are gone and you solder only to one side of the board.
Newbie mistakes, by a newbie. 😱
Yes, but it should recover once warmed up - that's what the thermistors near the mosfet are for.
hmm, I let the amp warm up and biased to ~300mV at 0vDC offset. Now if I let it cool and turn it on the offset starts at ~360mV or more and an offset somewhere near 100mV. It then drifts down as the amp warms.
Source resistor question
The F5Turbo BOM specifies a Vishay 71-RW79U1R00F resistor as the source resistor. Looking at the specs I find that the 71-RS02B1R000FE70 is identical and for half the price except for the Military designation. Any advantage to still getting the RW version?
I also find that Vishay sells these in a non-inductive version 71-NS02B1R000FE70.
Any advantage to getting these instead especially since the price is competitive with the above two?
The Mills MRA 5 and Caddock 930 and Fukushima Futaba MPC also seem to be quite popular here. Any opinions?
FWIV I listen to classical primarily.
Thanks.
Nash
The F5Turbo BOM specifies a Vishay 71-RW79U1R00F resistor as the source resistor. Looking at the specs I find that the 71-RS02B1R000FE70 is identical and for half the price except for the Military designation. Any advantage to still getting the RW version?
I also find that Vishay sells these in a non-inductive version 71-NS02B1R000FE70.
Any advantage to getting these instead especially since the price is competitive with the above two?
The Mills MRA 5 and Caddock 930 and Fukushima Futaba MPC also seem to be quite popular here. Any opinions?
FWIV I listen to classical primarily.
Thanks.
Nash
I would go for the Fukushima MPC74's. They are 5W and very good for audio...
Used by respected members indeed.
Used by respected members indeed.
I know you used 2x .47ohms in your F5 T build. I would like 0.5ohms total so 2x 1ohm which I cannot find in MPC74.
Do you feel it is OK to use one .47ohm 5watt resistor MPC74 instead? I could also put a cooling case on it if needed. That way I would be close to the .5ohm that I need.
Can you suggest a place to buy them? Burklin?
Thanks. Nash
Yup, so around 0.9 Watt... I bought them here: https://www.conrad.nl/nl/metaalfilm...4-5-w-mpc74-5-w-047-ohm-5-1-stuks-443893.html
Ok since the max voltage I will see will be .65V, max power is about 0.9W.
So it is fine to use just one MPC74 0 .47ohm 5W resistor per Mosfet instead of the two in the schematic?
Walter, I found it difficult to go thru the conrad website since it is not in English. Dont know whether they ship to the US?
Thanks.
Nash
Does anyone here have Fukushima MPC74's for sale??
There is Schuro on ehay selling MPC71 5 watters. They are in Germany but appear to ship to the US.
http://stores.ebay.co.uk/Schuro-Elektronik-GmbH/MPC71-5W-/_i.html?_fsub=18660682
http://stores.ebay.co.uk/Schuro-Elektronik-GmbH/MPC71-5W-/_i.html?_fsub=18660682
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