Buffalo II & transformers

[Joe Rasmussen]

Originally Posted by Terry Demol

Just put a 6 ohm resistor across the Sabres OP.
I don't think you need the ground reference.

T

Hi Terry

Yes, see above, the point I've made as well. But both should be tried if the opportunity presents itself.

Cheers, Joe R.

[nullspace]

Originally Posted by Joe Rasmussen

The context was a DAC with voltage offset, usually near 2.5V but also in some cases 1.65V - you can't just bring it to ground without potential damage (know somebody who did and poof!!).

DACs like PCM1792/1794/1796 (also DSD prefix) all have zero offset voltage (they do have offset current, but that hardly matters) and hence no danger to bring it to ground or even short it.

Thanks Joe. I've been mulling this over, and now get the difference between DC current offset & DC voltage offset DACs in this context. Can hardly believe I'm this dense/ took so long to get it...

I have a COD module in transit. Once I get a chance to mess with it, I'll start a new thread.

Thanks again for your help.

Regards,
John

[Joe Rasmussen]

Originally Posted by nullspace

...now get the difference between DC current offset & DC voltage offset DACs in this context.

I now avoid current DACs with offset voltage - so that I can do direct-coupled (no caps in signal path) zero feedback I/V post DAC circuit. The other point is that, in this case, offset current is not a problem, but oh boy, offset voltage makes offset current a real problem. But once you get it, as you say, it helps a lot.

My thanks to Terry who I used as a sounding board on this, some time back.

Cheers, Joe R.

[hkminn]

Originally Posted by Joe Rasmussen

I still think somebody should try this with a Sabre DAC using Tubes.



Normally Tubes are only good for voltage DACs, but use a transformer like this and you will have a Tube I/V stage - and it is the Sabre DAC that makes it possible - offset current = 8mA (I wouldn't do this with any other DAC I could think of). It can be pulled all the way to ground on the Primary and the Secondary grounded - and just input to the grid of your favourite Tube - and away you go.

Somebody should try it - requires gain about 50.

Cheers, Joe R.

Hello Joe,

I have tried this with a Buffalo24 board, using Sowter 8347(1:17) followed by SRPP stage using 6111 subminiature tubes.

First I tried 3R registers. The result was amazingl!! Lots of air and more details without losing too much dynamics comparing to a Buffalo32 with opamp I/V stage.

One problem was too much gain and I could not use more than a quarter of volume control. So, I replaced 3R registers with 1R, and the sound seems to be as good as with 3R (supposed to be better?) and I now can use the volume control in comfortable range.

One concern though. Isn't 1R register too low for the DAC? I asume it is safe since I don't see smoke, but I would like to have your confirmation.

Thank youl.

[Jeffrey Davison]

How much gain does the stage have?

Thanks

JD

Originally Posted by hkminn

Hello Joe,

I have tried this with a Buffalo24 board, using Sowter 8347(1:17) followed by SRPP stage using 6111 subminiature tubes.

First I tried 3R registers. The result was amazingl!! Lots of air and more details without losing too much dynamics comparing to a Buffalo32 with opamp I/V stage.

One problem was too much gain and I could not use more than a quarter of volume control. So, I replaced 3R registers with 1R, and the sound seems to be as good as with 3R (supposed to be better?) and I now can use the volume control in comfortable range.

One concern though. Isn't 1R register too low for the DAC? I asume it is safe since I don't see smoke, but I would like to have your confirmation.

Thank youl.

[blackeye*_0]

Originally Posted by hkminn

Hello Joe,

I have tried this with a Buffalo24 board, using Sowter 8347(1:17) followed by SRPP stage using 6111 subminiature tubes.

One problem was too much gain and I could not use more than a quarter of volume control.

Isn't 1R register too low for the DAC? I asume it is safe since I don't see smoke, but I would like to have your confirmation.

Thank youl.

Not surprising. The figure shows a 1:1 transformer, and since you're using a 1:17, I think you must be getting more than 20db gain before going into your SRPP stage. The 6111 is medium mu, so depending on how you have it set up- you may have a lot of unnecessary gain (overall).

[Jeffrey Davison]

Just ran my numbers..... as I mentioned in an earlier post I'm using a dual mono setup.
With a 1:1 traffo, using 10R on each primary leg to ground (assuming 97.5 R on the B2 DAC in mono) for each channel, that should net around .2v at the output of each transfomer. With a single gain stage of a 6N6P with a gain of about 20db, output voltage should be close to 2v. per channel. Right at my desired target.

JD

[hkminn]

In the datasheet, the gain of 6111 tube is 20. But I do not sure when it is used in SRPP.

[staccatiss]

1R is no problems as the ESS9018 chip can be used down to 0 Ohms into virtual ground. That is what the IVY uses.

Originally Posted by hkminn

Hello Joe,


One concern though. Isn't 1R register too low for the DAC? I asume it is safe since I don't see smoke, but I would like to have your confirmation.

Thank youl.

[pietjers]

Hi hkminn,

Interesting approach!
The Sowter datasheet suggests that the IV resistor may also be used across the secondary winding of the transformer (taking into account that the value of the resistor is reflected on the primaries by the ratio of the transformer (18.7) squared: 18.7 x 18.7= 350). Did you try that also?

What are the drawbacks of the high transform ratio? Or is there something like a free lunch?
Datasheet for Sowter 8347 and 9055:
Sowter Type 8347 DAC INTERFACE TRANSFORMER

Peter