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Old 11th December 2011, 04:30 PM   #11
kevinkr is offline kevinkr  United States
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Quote:
Originally Posted by gabdx View Post
Hi , thanks for pointing this to me. I thought using only the + signal would work, the - signal would not connected.
It will work, but substantially degrades the performance of the DAC. The benefit of balanced operation is that even harmonics and noise appear as common mode signals on the output cancel when combined.

This design seems needlessly complex, a high quality 1:1 transformer will do your balanced to unbalanced conversion, giving you great common mode rejection and a single amplifier stage after that ought to be all that is required.
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Old 23rd December 2011, 12:15 AM   #12
gabdx is offline gabdx  Canada
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I build it and of course it was quite a challenge... First, do not build the circuit as shown; ill remove it, because the headphone section has all the wrong values calculated by lampizator website. (srpp correct voltage 165V, Headphone section : Ra=7500 Rk1= 200, Rk2 = 6500 V=165)
There is some parasitic noise in the volume control metal casings and the signal wires from the dac to the headphone amplifier. One tube diode socket makes the ez81 noisy, I can’t figure why yet.

The biggest problem is the dac output going to the srpp tube. The dac signal seems over-amplified and the dac is clipping badly when music is louder. I will try to load the input with 10K instead of 240K or 75K (made no difference). I will also try to put a 1k resistor in serie to normalise the output impedance of the dac.
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Old 31st December 2011, 11:10 PM   #13
gabdx is offline gabdx  Canada
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Project : completed !
I used a wolfson dac with build-in opamp, similar to tda1543
The circuit is simple, volume control shunt 100k input.
- cathode follower 1/2 tube left and right , 150uf output capacitor
CLCRC filter, AC heaters, EZ81 rectifier.
Sound ? Awesome ! no hum, no noise, perfect amplification, just a bit loud , I recommend lot of volume control attenuation !
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Old 1st January 2012, 07:50 PM   #14
gootee is offline gootee  United States
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Maybe you should post a photo of the circuit and a final schematic.
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Old 2nd January 2012, 03:54 AM   #15
gabdx is offline gabdx  Canada
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Yes ! It is not very pretty after 3 rebuilds, just a prototype. Of course it will help others who would like to go beyond the opamp and the discrete transistors solutions without making a mess.
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Old 2nd January 2012, 04:15 PM   #16
gabdx is offline gabdx  Canada
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Click the image to open in full size.

Important Parts : Hammond 369EX, Choke Hammond 181R 20H, EZ81, 6n6p,
Specifications : tube bias 8.8 ma on 1/2 6n6p, grid current 0.0005ma 1/2 6n6p
Grid bias resistors : grid-HT : 355k, Grid-Ground : 145k.
Power rating : 0.1W , 300R Headphones recommended.
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