Experience with this DIY DAC ?

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#3,122
Good to hear from you Bill, it's good to be back on DAC 😀

Bill Fuss said:
The filter's still good.
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Thanks for confirming, I can eliminate ringing as the cause of harshness heard...

Bill Fuss said:
... did you see the posts about the incorrect voltage feed on the upsampler board?
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I did, but I had forgotten, so thanks for the prod. This could make sense - in bygone posts EI was reckoned to sound better than toroidal, but maybe the 50VA EI is able to push the upsampler feed over the edge, whereas the weaker toroidal cannot.

I will whet my multimeter.
 
#3,123
Bill Fuss said:
Magnetized trafos have hysteresis distortion, low level low frequency odd order harmonics. You can degauss them with a beefy preamp and a signal generator. Feed a 60-100hz signal to them strong enough to saturate them and then slowly turn the signal down to zero.
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I think my Tams are probably ok then, thanks, as I do not hear any nasties at low frequencies.

For future reference, might a headphone output of a pc sound card serve as a beefy preamp? That way I could use a software signal generator and not spend any £$. If not, how about using an audio power amp with some delicate knob control - or is that very silly?

Thanks
 
#3,125
Ref: Post 3030
RichLund said:
...I received this interesting email from Bill Whitlock of Jensen Transformers regarding suitable transformers for current output DAC's:...
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I also received a similar email from Bill back in July. I was wondering if Jensen made a suitable transformer for the PCM1798. He seems to say that I could probably use my 10K JT-11P-1s. This would save me from ordering a pair of rather expensive LL1674s or the K&K passive output stage.
Of course I can't figure out the last part about capacitive load etc...

Hello Mike,

Thanks for writing. As with virtually all current-output DAC's, the
manufacturer of the PCM1798 does not specify how much voltage swing is
allowable on those pins ... because they're designed to work into the
virtual ground of an inverting op-amp (which keeps the voltage swing at much
less than 1 mV). Some DACs will respond very poorly if the output pins are
allowed to swing even a fraction of a volt. Others will allow substantial
voltage swings in one direction but not the other. Therefore, I must defer
to those "golden ear" types on the DIY and audiophile forums to determine
what "works" and what doesn't.

I appreciate that you're pleased with our JT-11P-1 in a similar application.
I see no reason that, once an appropriate value for the load resistors is
found, the same transformer wouldn't also work just as well with the
PCM1798. In either case, the JT-11P-1 secondary must have less than 100 pF
of capacitive load to maintain its specified high-frequency response. In
practice, this generally means no more than about 1 meter of cable (assuming
about 30 pF per foot of cable capacitance).

Cordially,
Bill Whitlock, president & chief engineer
Jensen Transformers, Inc.
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#3,126
Mono mode connections

Here are a couple threads showing mono mode connections. Interesting in the Ciuffoli tests the PCM1798 in mono mode didn't rate high until he switched to an active output. And, he immediatley ditched the shunt regs after listening to them. He mentions measurements of the PCM1794 in mono mode but doesn't seem to follow through with a listening test of that chip.
.
DAC Final
.
http://www.diyaudio.com/forums/digital-source/169097-comments-pcm1794a-dac-design.html
 
#3,127
McCrackers said:
Ref: Post 3030


I also received a similar email from Bill back in July. I was wondering if Jensen made a suitable transformer for the PCM1798. He seems to say that I could probably use my 10K JT-11P-1s. This would save me from ordering a pair of rather expensive LL1674s or the K&K passive output stage.
Of course I can't figure out the last part about capacitive load etc...
Click to expand...

You need a gain of around 20 so you would still have to feed it to some type of gain stage. The capacitance of the cable will act as a low pass filter, keep them very short.
 
#3,130
Hi guys, sorry for late answer about my problems with 50 Hz hum, just recently had time to fiddle with it.
First, I looked inside my amp and saw that it's ground is tied to the case, so, after some argument with myself, I've decided to tie DAC ground to the case also and connect both grouds through the wall socket (followed Protoplasma's advice).
After that the hum was gone, and I was happy, but after re-reading posts with answers to my problem, I think it is not the right way of getting rid of the hum - see this post by Bill. I think Bill is right, and amp's ground should not be tied to the case in the first place. To support this thought, when I connected grounded DAC to my reciever (remember, there was no hum, when DAC's ground was not tied to the case), the hum was back, I guess Yamaha's engineers were wiser than the builder of my amp (which is DIY). After all, I've decided to leave it as it is, i.e. both grounds tied together, although it is wrong, but there's no hum 🙂. Next step would be resolve ground problems in the amp.

So, to sum it up - one of possible reasons for hum could be your amp's ground tied to it's case.

Thanks to all you guys for valueable input, and especially to Bill, who is really the guru of this thread. 😉
 
#3,132
indeed no way should the grounds be tied directly to the case. for best results tie grounds to the ground of the first main filter/reservoir cap of the power supply or center tap of the transformer and then for safety ground this should be tied to IEC ground and the case through a ground loop breaker. this last step is only for safety, there is no reason to do it otherwise as it only hurts the sound.
 
#3,139
seams everybody is satisfied with this Chinese dac + endless modifications...but what's your though when comparing it to other breed of DAC like the EZ DAC that employs PCM1749 or the popular Quang Ho AD1865 with tube stage and those fancy shunt regulator.....???? Does anyone have been audition those fully loaded DAC before???
 
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