Buffalo DAC output stage

[Dave the bass]

I liked my answer better.

Pffftttt!

DTB

[Ali Tait]

[Mike H]

All very good, but then try and stop the joyriding tealeaves from nicking it

[papaiannis]

Hello Nick,
did you connect the outputs of the buffalo II directly to the grid of the 6N6P? Or did you use a series capacitor?
Did you tune up the circuit?
Regards.

Alekos

Leo, asked me about what I was doing with a output stage for a Buffalo DAC, so here is the simple as they come circuit I am using. The output is a little over red book 2v, but not enough to worry the non anal

[Nick]

Hi, No I directly connected the output to the grid's. The 1.3v or so offset on the outputs of the DAC were just offset by extra cathode bias.

[papaiannis]

Thank you for your answer.
Did you try to force the ES9018 into current source mode? And in the case, did you use a transformer?
Regards.

Alekos

[Nick]

No, I just used it in voltage source mode, into the valve grids, with a transformer output. To my ears at least is was vastly better than either of the ivy output stages. The downside is you get more than 2v output, but if that was a real problem, you could reprogram the avr controller to knock 3db of the gain without loosing much. Using it in voltage output mode in theory is higher distortion, but I think the simplicity and lack of caps in the valve output stage makes up for any paper advantage a output based on op amps should have.

[papaiannis]

Which is the overall gain of your output stage?
I also think that the gain can be controlled by inserting a resistor in parallel at the secondary of the output transformer. This way we obtain a lower output impedance as a plus...

I'd also like to try the Buffalo dac in current source mode. I was thinking of using a Sowter 8347 or 9055 and a low value resistor at dac output. Do you think this is a viable solution?
Regards.

Alekos

[Nick]

It was about 4.4dB. Yes, you can trim the voltage back with a load on the secondary, But don;t take it too far as you will be increasing the load on the anods and bringing up distortion, and mainly 3rd harmonic so to be avoided. Yes, I did think about using a transformer on the input as well, but that was more than the person I built if for wanted to spend. I have previously used a sowter 1:10 on the end of a pair of TDA1541 DAC's driven differentially, and that worked out very well, so I think a transfomer I/V with maybe a 1:0.7 ratio or just 1:1 would work out very well.

[papaiannis]

I was thinking to put a transformer at the input of the 6N6P-based stage, putting a low value resistor at the dac output, thus forcing it to work in current mode. Which transformer, among the Sowter or Lundhal models, does fit best for this kind of application?
Regards.

Alekos

[Nick]

I would think a Sowter 3603 would be fine, but maybe give Brian a call on Monday or a email and see what he suggests.

http://www.sowter.co.uk/dacs.php

[sondale]

Hi,

I currently have both a Buffalo 32s and a Buffalo II dac. The 32s is sitting on the I2S output of a QA-550 SD card player and the B II on the I2S output from a juli@ sound-card in cMP2 computer.

The 32s is using the built in IV stage with a Balsie Lite converting Balanced to Unbalanced; the B II had been using the Twisted Pear IVY III but I changed it to using a pair of O'Netics transformers.

I would like to change both of these to using the 6N6P circuit discussed here as my first ever valve project - the circuit certainly looks simple enough - my first question relates to the power supply for the 170volts - what is the simplest / best way (especially for a first-timer) to produce this voltage?

Secondly if I wanted to maintain the balanced output is it simply a matter of using the +/- outputs plus the input earth/ground?

[Nick]

First question:

Simpilest is probaly what I used, a custom TX with a heater and HT winding, simple cap choke cap filter. If I remember it was a Canterbury winding torroid with a 150v and 6.3v winding. For the choke, a hammond 158M would be fine, just add a solid state bridge from UF4007 and a couple of 220uf 250v caps and its done.

A more complex way would be to use a higher voltage transformer and a solid stage 170v regulator.

Another more complex way would be to use a TX with a 200v or so secondary and miss out the first cap to run in choke input.

Second: yes, just do as you say.

[jmanalo]

Hi,

The 170V to center tap of 8650, is that measured value after you connected the 8650 or before?

Just want to make sure, cuz I set the supply to 170V, after I connected the 8650 the voltage drop to about 135V.

Thanks,

Joe

[Nick]

Sounds like you are pulling more current than intended. What voltage is on the 6n6p cathodes?