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pauldune 26th March 2012 09:10 PM

Another Tube Preamp for balanced DACs with SE output:build thread
4 Attachment(s)
A while ago I decided to purchase a DIY DAC, with the intention to build and/or design my own tube I/V converter for it. After some reading, the Buffalo III from twisted pear audio came in sight, so I bought that.
Luckily it came standard without integrated output stage, so that was ideal.
The DAC delivers 16mA/channel. (it's not per sť a current output DAC, but that is another story)
I wanted to keep it as simple as possible. So my first idea (see schematic #1) was to just take the balanced + and - out, put two 1 ohm I/V resistors between that and GND, connect the - output to the GND and the + output to the input of a voltage amp stage with reasonably low output impedance and a gain of ~70x. Ready.
I forgot that then both the (differential) -outputs would be connected to the same ground, so I had to come up with some other idea.

This is what I came up with. (schematic #2)

Again, as simple as possible. Its a standard LTP with CCS in the tail, based around D3A in triode mode. (low noise, high gain, low distortion, low plate resistance)
I calculated its gain as ~28x. With a 4.7 ohms I/V resistor, that should be enough for 2v output. It doesn't need input capacitors, in fact the 1.68V DC offset from the dac helps to put a little more voltage over the CCS.
The CCS is a 10M45S, and the lower end of the tail is -6.3V. (the 6.3v heater voltage) So the total voltage over the CCS is ~9.23V (6.3+1.25+1.68)
It's current is set to 40mA.
The D3A is set to 140v Vanode, and 20mA Ia. The output is taken (using a good quality capacitor) from the tube with the + phase, the other tube is loaded with the same load as my power amplifier.

What do you think?
Any ideas are welcome!

greetings, Paul

pauldune 27th March 2012 06:32 PM

Woops, today I read somewhere that the 10m45s only works good with Vak larger than 20v.
Kan someone confirm this?


PET240 20th April 2012 01:24 AM

CurIosity, is it two tubes per channel?

Have you looked at John BroskieS unbalancer from tube cad/glassware?

Just a thought, he has a CDs in the cathode of a dual triode as well.



pauldune 20th April 2012 02:35 PM

2 tubes per channel; 4 tubes for stereo. This schematic has evolved into this:
because I wasnt satisfied with this version.
Yes I looked at the unbalancer; part of my inspiration came from it, but I wanted a simpler schematic, and less coupling capacitors.
So I needed more gain and lower output impedance; thats why I used d3a, wich has a mu of 70 in triode, combined with under 2k output impedance. Plus it is a very low noise tube. But for optimal results it needs ccs's in the anode line, very low distortion is then possible: 0.02 % without feedback. Sounds good too...


pinnocchio 31st May 2013 02:50 PM

Hi Paul,

Where are you in your Bal to SE progress?


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