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Eli Duttmann's phono preamp voltages
Eli Duttmann's phono preamp voltages
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Old 20th August 2019, 03:51 PM   #41
gregas is offline gregas  Canada
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My plan is to use some Veroboard and solder the Mosfet and the diode on it.
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Old 20th August 2019, 05:13 PM   #42
wiseoldtech is offline wiseoldtech  United States
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Speaking about Phono Preamps...... and Veroboard... I just finished making the Project 99 - Sallen-Key Subsonic Filter by Rob Elliot to add onto a Harman Kardon 330A receiver. I wanted it to be as compact as possible, to mount on the reverse side of the phono preamp mounting panel.

I've made and installed these filters on several things so far with excellent results. It effectively removes any audio below 20 Hertz, which is unneeded anyway - turntable rumble, record warp issues, and bass speaker feedback issues. You can't even notice any audible difference in the audio, and it helps eliminate that power-robbing subsonics in the amplifier.
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Old 12th September 2019, 04:34 AM   #43
gregas is offline gregas  Canada
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Hey guys,
So I finally finished building this phono preamp.
It sounds great.

I'm using a switching power source as a temp power supply. I mentioned it earlier. I'm getting a buzz that sounds like a bee. I believe it's coming from the power supply, so this power supply will only be temporary.

I will build the one recommended on this thread for this phono stage.

This is my first build from scratch, and I'm very happy with the results.
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Old 12th September 2019, 01:00 PM   #44
Eli Duttman is offline Eli Duttman  United States
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IIRC, fellow Torontoian kodabmx is experienced with SMPS powered tube circuitry. Perhaps he can help you get rid of that buzz.
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Old 13th September 2019, 12:53 AM   #45
gregas is offline gregas  Canada
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Thank you, I'll see if he has some advice for my situation.
For now I'm just enjoying the phono stage as is.
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Old 18th September 2019, 04:08 PM   #46
kodabmx is offline kodabmx  Canada
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Eli Duttmann's phono preamp voltages
So I've met up with Greg now. His 12V supply was too small and wouldn't start. I had a 6A version which started into the load. There was still motorboating. I added a 2k/100uF RC filter to the supply which solved the issue when using both SMPS, one for heaters, and the other for B+. When he got it all home and tried just the single supply, it was motorboating again.

The circuit is drawing 30mA @ 250V. This seems high to me (my 8 tube phono only needs 50mA). Maybe Eli can comment? I've never built a hybrid. It would also motorboat without the tubes, just the MOSFETs, before I added the RC filter.

Last edited by kodabmx; 18th September 2019 at 04:12 PM.
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Old 18th September 2019, 05:50 PM   #47
Eli Duttman is offline Eli Duttman  United States
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30 mA. seems large. The 12AX7 sections should draw between 900 μA. and 1 mA. each. The buffering FETs draw approx. 2 mA. each. So, 30 mA. is easily 3X what should be measured. There's a "leak" to be tracked down.
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Old 19th September 2019, 12:35 AM   #48
kodabmx is offline kodabmx  Canada
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Eli Duttmann's phono preamp voltages
Hopefully Greg and I can meet again soon. I didn't even open the phono amp. I modified the external power supply. But a 2k resistor that I added is dropping ~60V. Thanks for the info, Eli. I guess we'll have to open it up and do some useful measurements... Any reason you didn't design this using a depletion MOSFET like DN2540?
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Old 19th September 2019, 01:46 AM   #49
PRR is offline PRR  United States
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> a 2k resistor that I added is dropping ~60V

That's absurd and obscene.

It smells like a 10K resistor slipped in where should be a 100k.

Since even a shorted-load 12AX7 can't pull many mA, it also suggests the wrong value may be on a MOSFET?

The other possibility is a reversed mis-rated, or bad filter cap. However 7 Watts in a small can should "expose itself" (and its guts) in a few minutes (if the power supply holds it up long enough to burst).
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Old 19th September 2019, 03:31 AM   #50
Eli Duttman is offline Eli Duttman  United States
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Quote:
Any reason you didn't design this using a depletion MOSFET like DN2540?
DC coupling an enhancement MOSFET to a tube's plate is a total "no brainer". You know the voltage on that plate and, therefore, the voltage on the gate. You know the B+ rail voltage and, therefore, the voltage on the drain. The source sets up a little below the gate, in order for the necessary forward biasing to be present. The difference between gate and source can be ignored. Pick a safe drain current and use Ohm's Law to calculate the load resistor's value.

The capacitances of the ZVN0545A are tiny. Even a wimp, like the 'X7 triode, has no problem driving them.

The "gotcha" in this topology is the fact that FETs are instant on and tubes need time to start conducting. A Zener diode clamp between gate and source keeps the FET safe, until tube conduction starts.
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