Pass X250 concept ?

[ANALOG GUY]

I used ACAUDIO & MY & DAVID BERNING design concept for a new PCB layout, it's quite good result with simulation and now I designing this schematic (I don't use 2SK389, 2SJ109, ZVN3310, ZVP3310, ZTX450, ZTX550) but I used 2N5458/5461, IRF610/9610, BC550/560, so I have to change parts value for best frequency response with 220uF coupling IN/OUT, UNBALANCED/BALANCED.

Maybe I have a wrong schematic but I don't care because it's only DIY project.

Best regard

analog guy

[CheffDeGaar]

Well, as you guys seem to be full of ideas, here's the version I use :

Pretty close to acaudio's one, but I made some cosmetic changes :

- Higher bias for the input jfets (3.8 mA)
- BJT cascode for the first stage
- Choose Zetex mosfets (lower capacitance)
- Added "McMillan magic resistors" to stabilize the differential offset, as done in the Aleph X

Straight from the box, it already shows a ridiculous absolute offset (+/- 5mV).

But with matched mosfets (M1 matched with M2 and M3 matched with M4) I can go under one mV. In free air, the offset still wanders a bit, but I lately added a little thermal coupling with aluminium bars for all the active devices, and the absolute offset stays under 1 to 2 mV for hours. The relative offset is always ridiculous and typically under 0.5mV.

Now for the sound... All I can say is : Wowwww
It easily blows away my BoSoZ. A perfect match to my AXs. It's a rocket : fast and stratospheric. Added depth and width, so quick on transients. Marvelous highs, no part of the spectrum is privilegied. Widens the scene, reveals every aspects of the sound. It was the first time I heard a noise gate shutting so clearly...

And dead silent... Even if I made PCBs for them, the whole thing is breadboarded with thin wires, low quality volume pots at the output... And despite of that, absolutely no noise...

Made some measurements for what it's worth. Absolutely flat response from 5Hz to 20 kHz, mainly H2 distorsion (-70/-75 dB), but I will remake the measurements with the final version.

I'll try to post some pics on Friday (I'm having hard times finding time ), showing the breadboarded horror

All I can say is : Thanks, Mr Pass

Cheff

[CheffDeGaar]

And the PCB I use.

This one is temporary. I'm currenly making a double sided one, mainly for aesthetic purposes More seriously, I want to add some compensation caps, since I detected very little oveshoot on square signals. Very layout dependent, since I didn't have this problem on the very first prototype...

Cheff

[jam]

Hi Cheff,

How is th offset? Do you need coupling caps?

Jam

[CheffDeGaar]

Hi Jam, nice to see you rambling around here

No coupling caps were necessary

With thermal coupling, absolute offset is under 1 to 1.5mV, with slow variations due to home wind . But the whole setup has no casing, so I thing it overestimates the offset variations. It's mainly a thermal problem, since I clearly saw the differences before and after the thermal coupling. Before, the 0.1mV digit on the DMM changed at each reading, and after, the same digit stays stable for 5 to 10 seconds, even longer in case of fair weather conditions . BTW, if people want to use it without coupling caps, I think it's better to leave it on all the time. At cold startup, the offset begins at 20-25 mV, and rapidly stabilize to its nominal and almost null value.

Anyway, even without thermal coupling, and with matched mosfets, the offset very seldom rises above 5mV...

Differential offset is always below the mV (0.2 to 0.5mV max typical)

Well it's only at prototyping stage for the moment, so specifications may change without notice

Cheff

[gl]

Hi Cheff,

A very nice piece of work indeed. Thank you for sharing it. I particularly appreciated your comment about the circuit sounding better than other simpler circuits like BOSOZ. It is a concern of mine when things start becoming too complex.

Cheers,
Graeme

[CheffDeGaar]

Hi Graeme,

Thanks for your kind comments. But also thank Nelson
Indeed it outperforms my heavily biased (65mA/Fet) BoSoZ, especially in the area of high frequencies. The BoSoZ seem to cast a veil on this part of the spectrum, while the UGS reveals it. But no harsh, just silk
And it also goes very low, without any boomy aspect. Everything is where it should be...
But the main thing is that it seem to widen the scene in a spectaculay way... I dunno where it comes from... And all of this without any special or audiophile component... Black magic, or am I just another proud father ?

And looking closer, it's not that complicated. Not more than my CCSed BoSoZ...

[gl]

Hi Cheff,

Yes you're right. I thank the man every chance I get. You're right too about the simplicity if you look at the overall component count. What I was referring to is the number of stages. There's something about the elegance of a single transistor line stage that is very compelling - unless it's a zero transistor line stage.

We could probably toss this back and forth all day.

Graeme

[zinsula]

Graeme

Some remarks:

- it seems to me that you have positive feedback. Your circuit is not inverting (like Cheff's), so you have to feed back the + output to the - input and vice versa

- turn Q7, drain must be attached to the negative rail, not to output

- with R38 and R29, do you want some "current feedback"? For offset reduction?

Success
Tino

[gl]

Hi Zinsula,

Yes you are right about the number of inversions. I realized this after re-reading may notes last night. The inside ends of R39 and R31 should be swapped. R38 and R29 return to a common point so they're fine. Without the followers this is actually a single gain stage amp. I have not used current mirrors in this way before and was intending to sort all this out experimentally before posting.

Correct on Q7. Thank you.

R38 and R29 are the "MacMillan" resistors used to help stabilize absolute offset. They are taken from the X2 preamp ver 2 schematic.

Thank you for the input.

Graeme