His Master's Noise: A Thoroughly Modern Tube Phono Preamp - Page 14

21st April 2010, 12:00 PM
SY diyAudio Moderator Join Date: Oct 2002 Location: Chicagoland Blog Entries: 1	His Master's Noise: A Thoroughly Modern Tube Phono Preamp After more than 25 years of faithful service, it seemed that it might be time to redo my phono system. After all, I like to think that I've picked up a few tricks in the intervening years... The old system consisted of a VPI HW17-II, a Linn Ittok LVII tonearm, and a Troika cartridge. The Troika was... Click here to read the entire article Last edited by Variac; 2nd April 2011 at 12:17 AM.

23rd August 2012

diyAudio Moderator

Drew, I can't seem to find an inductance and DCR spec for your cartridge. Do you know it offhand?

My drawings were done with ExpressSCH freeware. There's better out there, I just am incompetent.

If there's a sucker born every minute, where do the rest of them come from?

23rd August 2012

PET-240

Contributor

diyAudio Member

Hey Sy,

Not offhand, will ask Mr Gregory....

Will have a geezer at ExpressSCH, they don't look incompetant, they work, that's enough!

Thanks again!

23rd August 2012

PET-240

Contributor

diyAudio Member

Sy,

Will advise when I receive response, have also asked why capacitance has no effect.

Now to find libraries for tubes etc for expressSCH.

Thanks for the heads up!

8th September 2012

jackinnj

diyAudio Member

I wonder if the filament regulator input bypass capacitors (C12,16 0.1uF) aren't better connected to the Chassis ground, rather than the heater common.

1st November 2012

rrrs

diyAudio Member

Hi SY, just completed your phono amp using DIY-tech boards! Was able to pack all together in single chassis including the power transformers; with shorted inputs it is perfectly quiet; a bit of hum when connected to the cartridge; have to try twisting the wires from input RCA to the board.

Really impressed with the sound, thanks for making this project public!
As well, thanks to Jack for making the PC boards!

I am using SAE 1000E HOMC cartridge, no input transformer, just 47K resistor on input.
SAE spec is: 2.5mV output, Load impedance: 47K (non-critical), Load capacitance: non critical; it was apparently made by Coral.

Two things I found strange:
1) From 4 D3a tubes, with CCS adjusted at 20mA (there is 19.8V drop across 1K plate resistor) I got huge plate voltage variations between the tubes: 140V, 163V, 179V and 199V! Did you maybe get simmilar experience? Should I buy some more D3a’s to be able to find two that are more similar?

2) My 160B+ is spot on, at 160.5V, but second B+ instead of 260V is 210V; it is all the same on both channels; my unregulated HV is at 339V. Do you think I should tweak the 260V regulator to get closer to 260V or 210 should be fine?

2nd November 2012

diyAudio Moderator

Yeah, there's a lot of variability in D3a, but 163 and 179 isn't too bad.

I'd want to get the B+ up a bit, maybe 220-230V minimum.

If there's a sucker born every minute, where do the rest of them come from?

2nd November 2012

rrrs

diyAudio Member

Thanks, will get my B+ up, closer to 260, as you sugest. I checked all resistors in 260V regulator and they are alll correct; really strange I am getting only 210, especially since 160 regulator is spot on.
Would you mind sugesting resistor values to adjust to get up from 210; I guess R25 and R26 would set the output voltage, but please, if you could help a bit with right way to calculate actual values.

2nd November 2012

diyAudio Moderator

You might want to look closely at the error amp transistor and the voltage reference. Could you measure the voltages at EBC of both transistors on the oddball channel?

The circuit functions as a basic feedback amp. The reference diode is the noninverting input, the two resistors from output to ground form the feedback voltage divider. I don't remember their numbers offhand, but let's say that the upper one is R1 and the lower one is R2. Vout(R2/(R1 + R2)) = Vref + 0.7V (the Vbe drop of the error amp).

If there's a sucker born every minute, where do the rest of them come from?

2nd November 2012

rrrs

diyAudio Member

OK, thanks, so it makes sense:

On 160V regulator I got 160V(8.2K/(160K+8.2K))=Vref+.7V; Vref comes to 7.1V; very close to nominal 6.9V for LM329.

On 160V regulator I got now 210V(8.2K/(220K+8.2K))=Vref+.7V; Vref comes to 6.84V; very close to nominal 6.9V for LM329.

In order to get 260 I could adjust R2 to 6.6K to get 260V(6.6K/(220K+6.6K))=Vref+.7V; V ref comes to 6.87V; very close to nominal 6.9V for LM329.

I will parallel my 8.2K with 51K I got here; this should do the trick!

2nd November 2012

rrrs

diyAudio Member

Sorry, did a type, second equasion was supposed to be:

On 260V regulator I got now 210V(8.2K/(220K+8.2K))=Vref+.7V; Vref comes to 6.84V; very close to nominal 6.9V for LM329.

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