A New Take on the Classic Pass Labs D1 with an ESS Dac

[qusp]

looking at using the semisouth jfets (R550 or R085) and PPS films on mine instead of wimas and IRF610. will try the IRF540 as well.

hows your SMD design going owen if you are still monitoring this? maybe too late for my own build, but i'm making another ackodac for a fellow compatriot and thought if you werent too far off would use your PCB for the lineout and another design for headout

[PierreQuiRoule]

Originally Posted by qusp

looking at using the semisouth jfets (R550 or R085) and PPS films on mine instead of wimas and IRF610. will try the IRF540 as well.

It would be interesting to compare the voltage at the output of the DAC (input of the I/V) for all these devices - under the same bias condition.

From what I gather from the posts, the current-output ESS DAC produces less distortion when driving a virtual ground. So the higher the transconductance of the device, the lower the impedance seen by the DAC, and the lower the distortion. But I am sure there are factors at play

If I may ask, is there a particular reason for not using the IRF610? If it is availability, I can share some.

[qusp]

Originally Posted by PierreQuiRoule

It would be interesting to compare the voltage at the output of the DAC (input of the I/V) for all these devices - under the same bias condition.

indeed it would, I may just do that at some point

From what I gather from the posts, the current-output ESS DAC produces less distortion when driving a virtual ground. So the higher the transconductance of the device, the lower the impedance seen by the DAC, and the lower the distortion. But I am sure there are factors at play

this is true, the lower the impedance of the input of the I/V stage, the more like a current source the sabre looks. and we are talking really low here, so yes these experiments are interesting indeed.

If I may ask, is there a particular reason for not using the IRF610? If it is availability, I can share some.

for the reasons above partially, but also just to put my mark on the build, the 610 isnt really capable of what I desire as far as high resolution output either. I will be trying the semisouth devices, the IRF540 and also the toshiba 2SK170 (a few in parallel) as recommended by Papa.

thanks for the offer though, nah they are easily available, but I think I will start with a different device. I plan on trying several devices in there to see which works best for my purposes. just need to think on how I might rig up some mini T092 heatsinks and pump Papa for more info on the specific tweaks needed to drop the R550 in will keep you in the loop. most other parts for the D1 will be here on friday, so will post some pics sometime next week

[lauret]

Originally Posted by Nelson Pass

The only advantage the Mosfet has over the JFET in this
application is a lower impedance at the Source pin, being that
the Mosfet has a much higher transconductance. This results
in lower voltage seen at the output of the DAC. If the DAC is
not sensitive to this, then there is no advantage.

This is from another thread, but why would you use JFETs in the light of this quote?

[opc]

Hi Guys,

I'm going to side with lauret on the jfet issue, and the more I muck around with the circuit, the more it appears that performance is closely linked to transconductance. One of the best measuring devices was still the original RJK mosfet I used, and it's also the highest transconductance fet I've tried.

On that note, I have a few IRF1324's here that I think are going to do well, or at least they'll help me verify my theory on high transconductance. I'll post results tomorrow when I get them soldered in and tested.

The main board layout is done, and it's designed to drop the new buffalo board right on top, and use some 0.1" jumpers to connect the two. I'm just sending the Gerbers out for quotes this evening.

Cheers,
Owen

[qusp]

well actually I was looking at JFETs due to nelsons advice,but I guess it was late and I missed the relevance. but yes indeed if a mosfet will present a lower impedance and higher transconductance to the sabre, this will help to maintain current mode, so thats the way to go. I knew the relevance of the low impedance already, but i'm fairly new to all manner of mosfet circuits, as I havent needed the power so much in the past (headphones) so stuck to low current JFETs, but its becoming clear the sabre is a strange beasty and new tactics must be used to attain that magical 140db.

arrggh, I think i'll grab both I actually already have some IRF540 on the way, so its a matter of suck it and see I guess. I guess some of you havent noticed my bowerbird tendencies yet. I never just choose one direction, but rather several and use what works best for me, regardless of the numbers. plus i've been looking for an excuse to use the sic parts, guess I can just use a vishay sic mosfet. one problem with some mosfets for me though , is the fact I use a lot of high resolution source, I do not want an upper limit presented by the I/V stage when the rest of my rig is capable of much more. thus playing with the D1 cap RC values too.

on that note Owen, where are you planning on setting the LPF? I need to maintain at least 192 and preferably 384khz due to my studio use and there is a balancing act there that must be respected.

and lauret, i'm using transformers on the output of my buff II at the moment and enjoy it very much, as far as the above its far from ideal, but its very pleasing all the same. good numbers please me a great deal, but they do not rule me

[qusp]

I know why I didnt see it; because it wasnt there I started my own thread, for those who didnt see it and Nelson said

I spent some time with the R550 last weekend. As an output
device it has more bandwidth than the other power parts such
as the R100's or IRFP240's, and this is due to lower capacitance.
With that comes greater distortion at lower frequencies. For
this it would be natural if your source impedance was high or
you were building a tweeter amp.

The IRF610 still has lower capacitance, but also much lower
transconductance, about 1/5 as much.

I don't know what a D1B1 is, but if it's the IV for a DAC, then
you are better off with JFETs like the 2SK170's, maybe a
few in parallel.

so yeah that was enough for me to want to grab some; along with some mosfets already ordered, TO TRY

[lauret]

Originally Posted by qusp

I know why I didnt see it; because it wasnt there I started my own thread, for those who didnt see it and Nelson said



so yeah that was enough for me to want to grab some; along with some mosfets already ordered, TO TRY

Please let us know what works best for you so we all can learn!

[lauret]

Originally Posted by qusp

on that note Owen, where are you planning on setting the LPF? I need to maintain at least 192 and preferably 384khz due to my studio use and there is a balancing act there that must be respected.

This interests me as well. Pierre changed a couple of capacitors to stay at the same cutoff frequencies. Should there also be changes to the circuit by Owen?

Originally Posted by PierreQuiRoule

To keep the same cutoff frequencies (RC) for the low pass filters, the following capacitors were adjusted accordingly:
- C40, C45 increased from .01 to .033 uF
- C15, C16 increased from .0027 to .0086 uF

[opc]

Hi Guys,

I certainly didn't mean to discourage you qusp, you should, by all means, try every possible device you can and see what works best. I was just generally trying to convey the trends I've seen in the 8 devices I've tried it with.

As I mentioned above, the lateral BUZ fets were some of the best I've tried, and there was no rhyme or reason to it so far as I could see. Keep in mind that things like gate charge and the associated capacitances don't seem to carry much importance here since the gate and source don't actually swing by any appreciable amount. For that same reason, I don't think that "faster" devices (lower gate charges and capacitances) would provide any significant benefit in bandwidth. I will check this though.

As for the capacitor values, I came upon mine by simulation, but I'll confirm with a few measurements tomorrow. As qusp mentioned, it's a tradeoff between how much HF garbage you're willing to let through, and how high you want your response to extend. I'll work out the values for -1dB at 50k, 100k, 200k, and 400k providing the circuit makes it that high.

Cheers,
Owen