Another Tube I/V converter/preamp for balanced DACs with SE output:build thread - diyAudio
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Old 26th March 2012, 09:10 PM   #1
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Default 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!

greetings, Paul
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File Type: jpg P1020333small.jpg (286.1 KB, 604 views)

Last edited by pauldune; 26th March 2012 at 09:27 PM.
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Old 27th March 2012, 06:32 PM   #2
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Woops, today I read somewhere that the 10m45s only works good with Vak larger than 20v.
Kan someone confirm this?

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Old 20th April 2012, 01:24 AM   #3
PET240 is offline PET240  Australia
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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.


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Old 20th April 2012, 02:35 PM   #4
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2 tubes per channel; 4 tubes for stereo. This schematic has evolved into this:
I need help with ccs based LTP
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...

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Old 31st May 2013, 02:50 PM   #5
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Hi Paul,

Where are you in your Bal to SE progress?

PREAMPs : Bal AMB α10 + JC-80 | Bal DCB1 | Bal LDR3x | AMPs: AMB β24 | ExtremA | FO v1.4 | USSA5 | TSSA | Telos 390 Clone | NX Amp | FC-100
DACs : TPA Buffalo II + OPC's NTD1 I/V | JLsounds | Mirand AK4490 + Discrete Output Speakers : Troels Gravesen ATS-4 | Sagwee LT V3
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