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Hi Thorsten,
I would like to build a DAC similar to the ADAGIO, but I don't know where to buy the Amplimo transformer you specified (I haven't found anything similar on the Amplimo website). I checked the specifications of many transformers from Audio Note, Sowter, Lundahl or Jensen and I have't found any suitable because of the offset current. Please correct me if I'm wrong, but I think if I use 20 Ohm for the I/V resistor and the DC resistance of the primary is about the same, half of the offset current will flow through the primary. If I use higher I/V resistor it will be even worse. The max. quiescent current is specified only for Sowter 8347 (-0.6 mA, which is too low). Maybe this problem is not so critical and I can simply ignore the offset current and use any microphone stepup transformer?
Any suggestions from anyone?
Thank you in advance.
Ciao,
Csaba
Follow Ups:
This is a wonderful chip and of course there is more than one way to get good sound out of it. Link to previous post of my preferred implementation below.Watching the power meter spin backwards,
WMS
Hi Folks,Just one or two notes. The TM-3 in the Adagio is part of a complete tuned filter which offsets the SIN(X)/(X) rolloff induced due to the non oversampling and staircase output waveform of the DAC. All this is tuned reasonably critical and hence is non negoiable.
Of course, you can do whatever is pleasing, but in the end certian specific issues need (IMHO) adressing.
Later T
Heh, that DOES say TM-3. I couldn't really tell before.Do you have a (bigger/clearer/otherwise easier to read) version of the Adagio schematic than the one on your website? I was going to price it out, because I was thinking of building one, but I can't read most of the part numbers.
Charlie G
:-P
That's what I noticed from running the simulation of that part of the output stage.
Nice bump at around 15kHz, 0dB at 20kHz and then going down very quickly.
The only thing that worried me is the phase behaviour, it's -100 degrees at 20kHz.
I understood a constant phase throughout the audio band is prefered ?
Hi,The SIN(X)/(X) rolloff also leads to leading phase, the filter compensates.
Later T
DCR of the primary of the Amplimo TM3 is about 160 ohms.
Dunno about the other ones, forgot to measure when I had the Sowthers.
I've compared the Sowther (18.7*) to the Amplimo (15*); The Amplimo is really small, the Sowther is much bigger and you'll get a higher (higher than ratios would make you believe) output level when used with I/V resistor and bipolar output DAC like PCM1702 and loaded with some 20k.
The Sowther IMHO is the better one (when used as direct output), but > 3 times more expensive.Depending how well your system downstream handles the aliasing images, I would go without transformer, use a I/V resistor like 68ohms with film cap parallel to it for 1st order filtering and couple to SRPP stage. If that stage has a gain of say 18, you'll end up with an output voltage of 4*68*18/1000=4.9Vpp = 1.7Vrms which is just slightly below spec.
You'll find posts about this higher I/V resistor, somebody measured with spectrum analyzer and his ears and came up with a higher than "normal" resistor which sounded best to him.
Greetings, Rudolf.
Hi Rudolf,
thank you for the info. Did you try the Sowter with the TDA 1541? I'm not sure, but I think I read a recommendation from you about the TDA1541 + 20 Ohm I/V resistor + Sowter 8347 earlier. I would go for this combination, because I want to drive a 6C45 directly without analogue filters.
Regards,
Csaba
TDA1541 + 20 Ohm I/V + Sowter 8347 + 6C45pi will give you around 20Vrms output (myself I use 40 Ohm per chip I/V directly coupled to 6C45pi buffer and get standard 2Vrms and Sowter 8347 has 1:19 voltage ratio). Is this what you're going to get?Bye,
Lesha
...yes, that's what I would like to get. The 6C45pi will drive the power triode. Only three transformers two tubes one resistor. Maybe it's too simple and I need some more gain somewhere.
Regards,
Csaba
Depends on the power triode. That's enough to drive triode connected EL34, you probably need more gain for bigger triode. Though you can try to increase the I/V resistor (the values up to around 100 Ohm give somewhat distorted but still rather pleasant sound) while you don't exceed the transformer's DC and peak signal current ratings.Bye,
Lesha
I tried the Sowther (on loan) in a Sony CDP-950 player that has Sony digital filter and TDA1541. I/V resistor was 22 ohms, load 20k. Output was quite below 2Vrms, but still enjoyable when volume was cranked up. This was so convincing, since then I don't want slopamps anymore (note there was a LM6172 in place already).Later I tried the amplimo in same player with same configuration. Output signal was unacceptably low, but somehow I was not suprised looking at the small size of the tranny.
But with higher loads and a seperate active amplification stage they most probably will have the same results, altough if one could cope with the price I would opt for Sowther, or even better, have 1:10 handwound (and bigger) transformers.
It sounds very promising. I know the best would be to try before buying them, but these transformers are pretty expensive.
Thanks,
Csaba
If you exceed the DC current rating the core will saturate and this will cause distortions. If you can't find the right transformer (though AFAIR at least Sowter makes custom ones at no additional charge) you can either try to compensate the DC current (though it is not easy to do unless the transformer has two equal primaries or a center tap) or recall the rule "the best transformer is no transformer, the best cap is no cap" and directly couple the I/V resistor to output buffer. In this case you'll have to use high transconductance tubes to get enough gain while still having low output impedance and have to meet very strict requirements for plate supply. Also your amp should be capable of handling a higher amount of RF noise without loss of sound quality.Bye,
Lesha
Hi Lesha,
thank you for your reply. I hope I'll find a suitable transformer. I would't try to compensate the offset current, I think the high subjective quality of TDA 1541 and PCM63 somehow related to the unipolar current output of these DAC-s.
Regards,
Csaba
Hi,> I don't know where to buy the Amplimo transformer you specified
Amplimo.nl - e-mail them.
Later T
Thank you very much for your answer!
Ciao,
Csaba
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