Another Tube Preamp for balanced DACs with SE output:build thread
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!
Woops, today I read somewhere that the 10m45s only works good with Vak larger than 20v.
Kan someone confirm this?
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.
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...
Where are you in your Bal to SE progress?
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