Valve DAC from Linear Audio volume 13

[SonnyMarrow]

I was looking at C18, but wasn't sure if it would be best to leave it or remove it.

[acg]

I think leaving it would be best unless the new power supply can offer something "faster" and very close-by, preferably only an inch or two away (i.e. no long wires), which is why I brought up the ultracaps earlier.


10uF X5R cap ESR circa 4.5mOhm...I think...tucked right in next to the load which is ideal.

vs

2 x 325F Maxwell Ultracap ESR circa 1.5mOhm each...from datasheet...3mOhm in series...a few inches of extra wire adding to the ESR...lets call it equivalent ESR to the 10uF X5R if we keep the wires very short. So no advantage to the ultracaps even though they cost $50 to implement compared to cents for the X5R.



Where the two implementations may differ is 325million times more capacitance with the ultracapacitor smashing down ripple/noise and the output impedance of the 5V power supply in parallel to the Ultracap/X5R, along with different FPGA noise which may or may not actually bring an improvement in performance of the clock/dac. That is the big question.

However, if I had your voltage regulator and wanted to use it then I would get it as close to C18 as possible without removing that cap (and of course would remove L6).

Please don't take this for gospel! I could be wrong...I'm a farmer/audio enthusiast not an electronics whiz like Marcel.

[SonnyMarrow]

I planned on mounting the little PSU right under that spot (if I even do this) so the wire run would be very short.

[acg]

Yep. You may even be able to silicon it straight to the dac pcb depending on what else is in the vicinity.

[MarcelvdG]

The main reasons for avoiding long wires and large-area loops are inductance and inductive coupling rather than resistance, otherwise I agree with acg. Definitively leave in C18 and mount the new regulator somewhere close, with short wires and only a small loop area from the regulator output to C18 and back to the regulator ground pin.

[MarcelvdG]

By the way, I added a section about the digital precorrection for the analogue reconstruction filter to the opening post:

Digital correction for the reconstruction filter roll-off:
The digital interpolation filters in the original valve DAC have a partial correction built in for the droop of the analogue reconstruction filter. No such droop compensation is possible in the raw DSD version.

Hence, for the flattest possible amplitude response, the reconstruction filter of the raw DSD version has to be flat by itself while the reconstruction filter for the original valve DAC has to have a roll-off that's about the same as the roll-off of the filter that the correction was designed for. Therefore, Butterworth filters give the flattest response for the raw DSD valve DAC and 0.05 degrees linear phase or Gaussian-to-6 dB filters give the flattest response for the original valve DAC. While the phase response of Butterworth filters is not as good as that of Gaussian-to-6 dB or 0.05 degrees linear phase filters, it is still pretty close to linear phase an octave or more below cut-off.

The digital droop correction filter has a magnitude response:

1 + 13/256 - (19/512) cos(2 pi f T) - (7/512) cos(4 pi f T) with T = 5 us.

This is only a small correction: 1.009680428 or +0.083678759 dB at 15 kHz, 1.016534293 or +0.142440687 dB at 20 kHz, peak of about +0.644660534 dB at 73.5 kHz.

[SonnyMarrow]

Does anyone have any thoughts on putting transformers and power supplies in a separate enclosure and using an umbilical vs putting everything in one case? I've always done two separate cases, but not sure if it's worth the extra complexity and cost.

[MarcelvdG]

For what it's worth, I just placed the reconstruction filters on one side and two toroidal power supply transformers on the other side. The distance between the potcores and power supply transformers is over 55 cm.

[SonnyMarrow]

Did you have to use a ground loop breaker or something to prevent hum?

[acg]

In regards to the power transformers in the case, it depends. My experience is that they need lots of separation from low level circuits and most certainly from any inductors. Steel is the best shielding material by far (ferrous metals) as is distance but all transformers will couple to the steel in a chassis and cause vibrations which is not what you want with valves and clocks in this project. Toroids are supposed to have less stray flux than EI cores but this is not always the case, and they are very susceptible to DC on the mains which makes them vibrate and sing...I still prefer to keep all transformers far away and tucked away inside a heavy steel sarcophagus of sorts.

At the moment I am designing a chassis to finally put away my breadboarded DHT preamp. The preamp is currently sitting on a lump of MDF and the EMF meter needs at least one metre (three feet) separation for interference to fall to background levels. By contrast, the fully boxed up mono power supply for my 6 channel SET amplifiers is packed with power transformers and chokes, operates on much higher currents than the preamp project (and ValveDac project), weighs about 100k each with the chassis made from 4mm mild steel. That 4mm thick steel, along with some generous spacing for the EI transformers in the chassis (keeping them at least 40mm from anything ferrous) gives me background RF and EM readings just 30cm (1 foot) from the case, which is a much larger task than the fields for the ValveDac.

For the ValveDac, I will use the EMF meter to measure any issues before making decisions about single or dual cases. Toroids will be used although I usually prefer EI because of the DC gap and lower bandwidth but the local guy that normally winds them for me is snowed under.