Valve DAC from Linear Audio volume 13

[MarcelvdG]

Originally Posted by acg

I did some searching...here is that scope shot. Not sure if it is worthwhile in this instance.

Must be a funny current waveform, roughly 1 kHz square wave with on average some 22 kHz on top of it, but frequency modulated by the square wave? It's not the megahertz signals I was thinking about.

[acg]

Originally Posted by MarcelvdG

You could indeed remove L6 and use a separate 5 V supply for U17. Any jitter caused by ripple on the 5 V for U17 is supposed to be suppressed by the current steering gate made of E88CCs, but that suppression is imperfect, so maybe a separate supply or separate regulator could improve performance to some extent. I don't know, as I haven't tried it...

Excellent. Where on the pcb would you ground the extra 5V supply if the +ve went to the correct pad on L6 (or somewhere closer to the load if physically possible)? Certainly not going to do this upfront, but perhaps at a later date as a trial.

Originally Posted by MarcelvdG

...it certainly makes more sense than using an ultra low noise regulator for supplying an FPGA board.

For suppressing really rapid current variations, physically small capacitors work best. Hence the use of 10 uF X5R SMD capacitors at various places.

Sweet.

Originally Posted by MarcelvdG

Regarding noise, as reported in the Linear Audio article, I measured noise floors of -85.76 dB(A) left and -91.29 dB(A) right with respect to a full-scale sine wave in the PWM8 mode. That's low enough not to bother me at all, but not nearly as low as some solid-state DACs.

Yes, some SS dacs have excellent numbers but not necessarily excellent sound. I am firmly in the "good enough" camp in terms of measurements and the decent noise performance, relatively simple circuit and your approachability is what has drawn me to this project. My playback system is enormous with 110dB/w/m sensitive horns and I have zero tolerance for hearing any idle noise with my head in the horns so noise is quite important in this context.

Currently have 40 vacuum tubes in the system including DHTs with both AC and DC in the filaments and have managed to get things perfectly quiet almost straight away with well planned layout, generous space where required and use of good components, particularly the iron, and there is a lot of iron...just don't want this dac to sound so great I want to keep it but have my horns hiss at me with little potential to lower noise. Not sure if that will happen or not. Had to go to some extreme lengths recently to chase noise out of a SS phonostage but got there in the end with particular attention to the power supply. The ValveDac is not so simple as a phono and there is a lot more to dropping noise than improving a bipolar power supply, hence my questions here somewhat in advance of actually needing to try to lower noise...they are more aimed at trying to figure out viable options I may need to lower noise and how much extra room to leave in the chassis just in case (I make all my chassis' from scratch). Hence my questions about reconstruction filters, DC blocking caps, 5V supplies. I've even considered asking about dual mono power supplies, but looking at the KiCAD files that would be difficult to implement with the pcb...would also require a dedicated -300V clock supply.

[acg]

On the subject of iron, Dave Slagle has posted the custom ValveDac output transformers today, wound on 82% nickel cores. It should not matter in terms of performance, but in my experience those nickel cores just sound better than anything else I have tried. There is Slagle iron in my valve LCR phonostage, preamplifier and the midrange channel of my power amplifiers, and soon the ValveDac.

Price is good too.

[acg]

Originally Posted by MarcelvdG

Must be a funny current waveform, roughly 1 kHz square wave with on average some 22 kHz on top of it, but frequency modulated by the square wave? It's not the megahertz signals I was thinking about.

I don't think that particular supply is aimed at the MHz of an FPGA, but is supposed to show the transient improvement when ultracaps are used on a generic, presumably lowish quality LPS. 325F is about 325 million times more capacitance than a 10uF X5R, with vanishing ESR, but they are quite large and would take some time to charge off the power supply before ready for first use.

EDIT: Just looked at the scope shots more closely...seems the power supply is the USB charger for a Rasberry Pi...not even a decent quality LPS to start with.

[agent.5]

Originally Posted by acg

On the subject of iron, Dave Slagle has posted the custom ValveDac output transformers today, wound on 82% nickel cores. It should not matter in terms of performance, but in my experience those nickel cores just sound better than anything else I have tried. There is Slagle iron in my valve LCR phonostage, preamplifier and the midrange channel of my power amplifiers, and soon the ValveDac.

Price is good too.

Do you have a link? I checked and could not find it in the intact forum.

[acg]

I just did what I normally do, sent Dave an email with the schematics, back-and-forthed a bit about about what I am trying to do and the specs it needs to hit and he talks me back to reality and then custom winds exactly what is required. If you send him an email I am sure he will be aware of the project and the transformers...the ones made for Anthony and the ValveDac.

[SonnyMarrow]

How's the price compared to Jensen? Jensen is $137/each I think.

[acg]

Price is comparable to the Jensens. I think Dave might want to see how mine work out before winding too many more, not sure.

I'll be building as soon as the pcb's arrive which should be any day. Power transformers are the biggest wait, but I do have some here that I can make do with as an interim measure.

[SonnyMarrow]

Are you going to omit L6 and use a separate 5V supply? I've got that extra ultra noise regulator coming, just need to figure out the best place for the ground.

[acg]

Probably not straight away, but I will likely give it a try once everything is working and verified. Across C18 looks the spot to do it, and it is easy to desolder a couple of SMD's to facilitate the change. Like Marcel says, maybe it will improve something, maybe not.

What I have done is order two complete sets of inductor bits and a load of polystyrene caps so that I can test various reconstruction filters just by switching them in and out. First decide between 6th order 0.05deg Linear Phase and Gaussian-to-6dB, then maybe try some lower order filters and/or lower cutoff frequencies. My hunch is that some experimentation there may yield some nice performance or sound quality benefits.