It's time I built a D3a -> 5687 phonostage

[Nick]

I used two of these

https://uk.farnell.com/vishay/f339mx263 ... dp/1902188

Use spice to decide what value you want to use, or follow Andrews path and send yourself mad trying to find a way to avoid the caps.

[Andrew]

I used two of these

https://uk.farnell.com/vishay/f339mx263 ... dp/1902188

Use spice to decide what value you want to use, or follow Andrews path and send yourself mad trying to find a way to avoid the caps.

If you drive it with matching Z you can terminate with a high Z, I think that approach is viable at 1k5, but 600R certainly makes things harder. To do this you need chokes that are happy at anode DC. A matching termination and those aforementioned huge caps avoid such issues and the sound quality of the solution speaks for itself.

My almost certain mistake was I terminated the LCR with 3Meg3 then used a really really tiny terminating cap and grid leak bias'd the second valve which was a D3A. The very high grid Z was a source of noise due to strays, it doesn't take much stray current to generate hum/noise across a 3Meg3 resistor which then gets multiplied up by the gain of the second stage.

On top of that, I should note that my experience with the D3A is mixed, they used to have good availability but they aren't very robust valves, they have a tendency to go noisy, loose vacuum or in my case go into thermal runaway, if run hard, which is what happens in grid leak bias.

[Nick]

I guess you could use a interstage, but again I think you are assuming the problem exists that needs a solution. I guess you could use a small cap into a high impedance buffer such as a op amp with voltage protection clamps on its inputs.

[IslandPink]

Re. D3a ,They are a lot more expensive than they used to be, as well.

[Andrew]

I guess you could use a interstage, but again I think you are assuming the problem exists that needs a solution. I guess you could use a small cap into a high impedance buffer such as a op amp with voltage protection clamps on its inputs.

A lot of the Japanese "DIY" designs of yesteryear used transformers, one advantage is the impedance "goes up" with the square of the ratio, making a "good" engineering solution, but the price goes up accordingly, everything has a cost!

[Andrew]

Re. D3a ,They are a lot more expensive than they used to be, as well.

Haven't looked recently, usually paid above a fiver for mine.

[IslandPink]

Ha !
How about £25 each nowadays ?

[Andrew]

Ha !
How about £25 each nowadays ?

I should have bought more

[Thermionic Idler]

I guess you could use a interstage, but again I think you are assuming the problem exists that needs a solution. I guess you could use a small cap into a high impedance buffer such as a op amp with voltage protection clamps on its inputs.

Now there's an idea, I think the OPA161x series that I used in the linestage has them built in...

[Nick]

I guess you could use a interstage, but again I think you are assuming the problem exists that needs a solution. I guess you could use a small cap into a high impedance buffer such as a op amp with voltage protection clamps on its inputs.

Now there's an idea, I think the OPA161x series that I used in the linestage has them built in...

I would not trust them with 150v on the input.

[simon]

Ha !
How about £25 each nowadays ?

I should have bought more

Mmmm yes. Just had a quick look and £25 might be a bit low. C3g look even worse. Wish I'd bought a good few more of a few valves now, it's beginning to become prohibitively expensive to use valves now. The popular ones anyway.

[IslandPink]

Damn about C3g's too. There are also C3o's out there that I think are like the C3g, but with a 20V heater, like the C3m.
I think they were pretty rare even years ago, though.

[Andrew]

In the days when we all started out, I bought 25, or so, D3A for a bargain price from a chap in Oz, it was a group buy, they are were a really good deal, sadly at least 20% of them were dead on arrival, white as a sheet inside We split the bad ones amongst the group buy, any survivors are probably worth 8x that now.

[Thermionic Idler]

I guess you could use a interstage, but again I think you are assuming the problem exists that needs a solution. I guess you could use a small cap into a high impedance buffer such as a op amp with voltage protection clamps on its inputs.

Now there's an idea, I think the OPA161x series that I used in the linestage has them built in...

I would not trust them with 150v on the input.

But the smaller sized cap you mentioned would be blocking that wouldn't it?

[Nick]

Eventually yes.