Born Again Vinylista Phono Stage

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izzy wizzy
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#121 Re: Born Again Vinylista Phono Stage

Post by izzy wizzy »

You'll be short of your gain requirement d3a triode/5687 but thre are other options as Mark says.

d3a pentode/5687 50dB gain

Something to consider, mentioned before. These hi gm valves as triodes have huge miller which for a MM could be an issue? From what I understand/read, miller is usually in practice more than calculated.
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#122 Re: Born Again Vinylista Phono Stage

Post by IslandPink »

Yes, good point.
I did have a scheme sketched out for D3a pentode first valve, with around 4k load. I think I got stalled thinking about how to supply the G2 quietly, that was before Nick's reference phono.
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#123 Re: Born Again Vinylista Phono Stage

Post by izzy wizzy »

Imho, many get fixated on the screen supply coz you can hear the cap. You can hear caps everywhere so it's not something I worry about. I don't roll caps so maybe I have no experience here.

It may become "a thing" coz you can bias the whole phono without caps so maybe that's why folk get animated about it to the point if being put off.

Just lie back and think of the gain and low C ;)
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Nick
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#124 Re: Born Again Vinylista Phono Stage

Post by Nick »

Yep, simple to use as a pentode. I just used a potential divider (maybe 300k in total) with cap across lower resistor to set low noise voltage or even better a LND150 as a CCS as the upper resistor., then use a 500v mosfet as a source follower off that voltage to give the low impedance into g2.

You will need low noise b+ though, but that’s easy enough to achieve.
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#125 Re: Born Again Vinylista Phono Stage

Post by izzy wizzy »

My G2 is low rent; a series 47k to G2, 22u to ground.

I might have some room for improvement here which could be fun.
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#126 Re: Born Again Vinylista Phono Stage

Post by RhythMick »

izzy wizzy wrote: Sat Aug 21, 2021 11:00 pm You'll be short of your gain requirement d3a triode/5687 but thre are other options as Mark says.

d3a pentode/5687 50dB gain

Something to consider, mentioned before. These hi gm valves as triodes have huge miller which for a MM could be an issue? From what I understand/read, miller is usually in practice more than calculated.
An option I plan to try with the D3a is that described by Merlin Blencowe on page 186 of his "Designing high fidelity valve preamps" book called "Low capacitance mode". Pentode strapped with anode grounded and acting as shield, g3 to cathode, g2 becomes effective anode. This would limit the current to about 4mA so loses some of the drive into the RIAA but the lower miller might make this option interesting.

Mentioning in case it's of interest. About 3rd on my list of things to try when work releases its grip.
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#127 Re: Born Again Vinylista Phono Stage

Post by Cressy Snr »

RhythMick wrote: Sun Aug 22, 2021 8:48 am
izzy wizzy wrote: Sat Aug 21, 2021 11:00 pm You'll be short of your gain requirement d3a triode/5687 but thre are other options as Mark says.

d3a pentode/5687 50dB gain

Something to consider, mentioned before. These hi gm valves as triodes have huge miller which for a MM could be an issue? From what I understand/read, miller is usually in practice more than calculated.
An option I plan to try with the D3a is that described by Merlin Blencowe on page 186 of his "Designing high fidelity valve preamps" book called "Low capacitance mode". Pentode strapped with anode grounded and acting as shield, g3 to cathode, g2 becomes effective anode. This would limit the current to about 4mA so loses some of the drive into the RIAA but the lower miller might make this option interesting.
Mentioning in case it's of interest. About 3rd on my list of things to try when work releases its grip.
@Mick
We must be telepathic.
That’s exactly what I’m looking at right now.
1E7507CA-22BC-45FF-901B-5CE929FF7FC1.jpeg
1E7507CA-22BC-45FF-901B-5CE929FF7FC1.jpeg (109.06 KiB) Viewed 2885 times
Looking at the D3a triode curves with the 1W max screen grid dissipation, the valve could be operated at 75V on the screen and 10mA current, giving a dissipation of 0.75W, -750mV bias and reasonable drive current. Would Just need a loadline through those points to find the plate resistor required.

Ref:
Circuit Lab Report Number 2 (1950) Grounded Plate Type 6Au6 Triode Connection for Preamplifier Use Radiotronics, 142, April, pp45-7
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#128 Re: Born Again Vinylista Phono Stage

Post by Nick »

Looks like a solution in search of a problem to me. If you actually need low input capacitance I would have thought a cathode coupled would have been better.
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#129 Re: Born Again Vinylista Phono Stage

Post by Cressy Snr »

Is there any reason not to put a cathode follower before the first valve, like you would in front of a passive tone control circuit. It would certainly sort out the ‘MM cartridge”problem, but you’d then be looking at an input coupling cap to get your bias to work. Nevertheless it could be a good move with an MM cart, or maybe not. There must be some reason why not.
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izzy wizzy
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#130 Re: Born Again Vinylista Phono Stage

Post by izzy wizzy »

Cressy Snr wrote: Sun Aug 22, 2021 10:41 am Is there any reason not to put a cathode follower before the first valve, like you would in front of a passive tone control circuit. It would certainly sort out the ‘MM cartridge”problem, but you’d then be looking at an input coupling cap to get your bias to work. Nevertheless it could be a good move with an MM cart, or maybe not. There must be some reason why not.
A pentode has low input capacitance. Wired as a triode, it has less gain and high input capacitance.

Not quite sure where the problem here is?

To use triodes with low input capacitance would lead you to a cascode for the gain however then you have little drive for the EQ. That leads to a high Z EQ and the loss of dynamics that go with it plus lack of drive for the 2nd stage. You could always stick a cathode follower in there to drive a lower Z EQ. That uses 3 triodes to do the work of 1 high performance pentode.
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#131 Re: Born Again Vinylista Phono Stage

Post by Cressy Snr »

izzy wizzy wrote: Sun Aug 22, 2021 12:13 pm
A pentode has low input capacitance. Wired as a triode, it has less gain and high input capacitance.

Not quite sure where the problem here is?

To use triodes with low input capacitance would lead you to a cascode for the gain however then you have little drive for the EQ. That leads to a high Z EQ and the loss of dynamics that go with it plus lack of drive for the 2nd stage. You could always stick a cathode follower in there to drive a lower Z EQ. That uses 3 triodes to do the work of 1 high performance pentode.
There isn’t a problem as such. It’s part of the generalised anxiety disorder I suffer from. People with GAD will often wait indefinitely and waffle on rather than do what is necessary. I’m normally well in control of it these days, but it can rear its head over the silliest things sometimes. I’ll shut up now and come back when I’ve done something constructive with the D3As when they arrive.

Thanks for the advice chaps.
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Nick
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#132 Re: Born Again Vinylista Phono Stage

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Cressy Snr wrote: Sun Aug 22, 2021 10:41 am Is there any reason not to put a cathode follower before the first valve, like you would in front of a passive tone control circuit. It would certainly sort out the ‘MM cartridge”problem, but you’d then be looking at an input coupling cap to get your bias to work. Nevertheless it could be a good move with an MM cart, or maybe not. There must be some reason why not.
Well I did suggest a cathode coupled first stage, so that is in effect a cathode follower before the first valve, though not sure why that would need a coupling cap. (imagine a long tail pair, but put a cap to ground from the input valve anode, so its gain is unity at AC, so no Miller. Just tried simming it with a ecc83, shows about 5pf input capacitance which seems about right

You seem to have invented a problem (‘MM cartridge”problem) and then set about trying to find a solution for it.

As Izzy has said, the simple solution to triode miller capacitance was and is a pentode.
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#133 Re: Born Again Vinylista Phono Stage

Post by Paul Barker »

Mark has already trod that path successfully and suggested it somewhere in a recent thread.
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#134 Re: Born Again Vinylista Phono Stage

Post by IslandPink »

Actually I never tried it in my phono ( came close) but I have always liked pentode drivers in a power amp, I currently run 4P1L ( DHP ) into KR300B, I don't feel any need to tinker with that.
I know Steve didn't like pentode drivers in a power amp, in the end, maybe that's why he was reluctant to consider this. However I think this is a different problem, and any harmonic signature will be much reduced when we are talking such small signals. The other EE benefits for MM cart input are much clearer.
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izzy wizzy
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#135 Re: Born Again Vinylista Phono Stage

Post by izzy wizzy »

Obsessing and design prevarication is what forums are about. At least ideas can be hammered out before building happens. Saves oodles of time. We're all guilty of it from time to time.

This link may help focus all the design decisions
https://www.diyaudio.com/forums/diyaudi ... reamp.html

No one else may come to the same conclusions (I've tried everything in at some point) but it does deal with the stage by stage issues along the way and imo, in the right order.
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