A quick question to those who are following and reading the thread. If you could choose, money and space is of no concern.
The XP12 approach: Everything in the same chassis. (1 chassis)
The XP22 approach: Main unit and separate PSU. (2 chassis)
The XP32 approach: Main unit & PSU with dual channel separation. (3 chassis).
Figured I would investigate while studying my options regarding the UGS stage - yes, there are more than one approach
and besides, listened to Wayne's walk through of the Burning Amp Linestage and he happen to mention that the one found in the PASS LABS products is better. Maybe not a surprise, anyways. So yes, UGS is better and I think there are ways to move forward with similar circuits and not offer the exact same, similar but different. The original V3 offered is the foundation ofc and others have made similar attempts here at diya. So the SuSy topology is well known and used in this community. Point being, I want to design one and only one gain stage and if a kit or group buy end up as an alternative, then everyone gets the same.
The XP12 approach: Everything in the same chassis. (1 chassis)
The XP22 approach: Main unit and separate PSU. (2 chassis)
The XP32 approach: Main unit & PSU with dual channel separation. (3 chassis).
Figured I would investigate while studying my options regarding the UGS stage - yes, there are more than one approach
and besides, listened to Wayne's walk through of the Burning Amp Linestage and he happen to mention that the one found in the PASS LABS products is better. Maybe not a surprise, anyways. So yes, UGS is better and I think there are ways to move forward with similar circuits and not offer the exact same, similar but different. The original V3 offered is the foundation ofc and others have made similar attempts here at diya. So the SuSy topology is well known and used in this community. Point being, I want to design one and only one gain stage and if a kit or group buy end up as an alternative, then everyone gets the same.
The extra chassies are nice if you are selling "by the pound", which Pass Labs do. However, making extra chassies for DIY is just a lot of extra work and additional money, and it makes it harder to stuff all those chassies into your HiFi rack. I'd go for the XP12 approach.
There are some advantages when using dual enclosures and you separate the power supply, not to mention that if you use a transformer for the gain stage this one really likes having the mains transformer in another enclosure. Beside this when you have the ps in another enclosure you have more room for filtering and voltage regulators.
Even if it comes a bit more expensive I would go for dual enclosures.
Since I am using such a big volume wheel - at least 115mm/4,52" - the chassis height will be somewhere around 150-160mm or ca 6". Meaning there is ample space for a PSU in the bottom, separated by a thick aluminum slab divider, and the main electronics in the top. Further, each channel can be divided into chambers and basically be treated like the XP32. With some trickery, we would end up with "3 chassis". What bothers people is the EM and RF emitted by the PSU (doesn't matter if its linear or switch-mode, both can transmit), so as long as there is proper shielding in place, the sectioned approach should be fine as long as chassis are referenced to earth and audio is referenced to ground (neg) only. Plus, there are MU metal which one can invoke if that makes one feel better and so on.
If that was a real problem, then I'd wonder how it's possible to make a big power amplifier in a single chassis. Just take a look at some to the bigger class-A amps, which have huge power supplies. Still, they can be made dead silent in a single chassis.
Most of the noise issues related to power supplies have to do with high-frequency switching noise from the rectifiers. The switching noise can be prevented by using suitable snubbers, "soft switching" rectifier diodes, or active rectifiers. Simple and easy.
Good news for both UGS Muses Scion pre-amplifier and XA25 Scion SE and XA25 Scion X power-amplifiers, there is a full lineup of active JFET, BJT & MOSFET which are equivalent performance to the gain stage for XP and XA*.5/8 series FET choices. Meaning, the components will not be the downfall of this endeavor. The reason I am sharing this news is that EOL Toshiba parts are either rare or expensive so, in order to maintain a price and cost sanity, replacement parts that are relatively easy to find and somewhat inexpensive is prudent. While the goal in itself is not a 100% PASS replica, it is the inspiration and the whole reason for being here.
In other words, the selected FETs will not introduce enough of an audible difference. Also, I will go forth with complimentary pairs, so no same sex or quasi-complimentary. The JFet will maintain the TO-92 (TH) package while other parts will be SOT-23-3 (SMT) packages. This depend a bit on the FET type and location. Generally speaking, as long as there is enough heat dissipation, surface mount component have better and lower noise figures than the TH equivalent which is beneficial since a SINAD greater than -80dB is our goal.
Just a little weekend update for you all. Enjoy.
In other words, the selected FETs will not introduce enough of an audible difference. Also, I will go forth with complimentary pairs, so no same sex or quasi-complimentary. The JFet will maintain the TO-92 (TH) package while other parts will be SOT-23-3 (SMT) packages. This depend a bit on the FET type and location. Generally speaking, as long as there is enough heat dissipation, surface mount component have better and lower noise figures than the TH equivalent which is beneficial since a SINAD greater than -80dB is our goal.
Just a little weekend update for you all. Enjoy.
somewhere from first page , Boyz gave you information that you can buy Linear System JFets
LSK170 and LSJ74 are 2SK170 and 2SJ74 replacement; for LS, B suffix being same as Toshiba BL suffix
building bespoke gadget, price of those is not a biggie
considering that your intention is to bring this to masses, best way to contact LS directly, to arrange acquisition in quantity
LSK170 and LSJ74 are 2SK170 and 2SJ74 replacement; for LS, B suffix being same as Toshiba BL suffix
building bespoke gadget, price of those is not a biggie
considering that your intention is to bring this to masses, best way to contact LS directly, to arrange acquisition in quantity
Yes, the Linear Systems FETs are part of the overall picture, but its been brought to my attention that diya store now offers genuine Toshiba and in the grand scheme of things, that is mighty tempting. Expensive but tempting. Either LS or Toshiba is big money considering small power devices. Which one will ofc be up to the individual person, either will work fine. But as I said, if one goes through all of the effort in building this pre-amplifier, genuine Tosh should be ones #1. And besides, as long as diya and DigiKey offers LS FETs, I'm not sure I can get a better deal - but we'll cross that bridge when we get there.
I am contemplating if this UGS should get the standard NFB or CFB. The reasoning for contemplating is the Pass/Wayne clearly prefer CFB in the XA25 and I am wondering if CFB will become the next gain stage upgrade for the rest of the product line. Either way, there should be room for either to be chosen, just like Schade feedback or full Harmonic Generator or non. Meaning we would end up with a direct mode, S-mode and H-mode, all in one package. That is pretty neat if you ask me.
I am contemplating if this UGS should get the standard NFB or CFB. The reasoning for contemplating is the Pass/Wayne clearly prefer CFB in the XA25 and I am wondering if CFB will become the next gain stage upgrade for the rest of the product line. Either way, there should be room for either to be chosen, just like Schade feedback or full Harmonic Generator or non. Meaning we would end up with a direct mode, S-mode and H-mode, all in one package. That is pretty neat if you ask me.
There is not much to contemplate about but by now it seems you got used to this 😎.
Once you know what each one does it’s quite easy to take a decision.
Just search on google one vs the other and your contemplation will end as soon as you finish reading the article.
I have a document from Bob Cordell and the thread [Current Feedback Amplifiers, not only a semantic problem?] which I will look at. I know NFB/VFB is output-to-input while CFB is output-to-source - but currently have no idea how it impacts the performance and audible differences, which is the part is need to wrap my head around. Just saying I want the least discriminating solution isn't good enough, I am sure there are issues with both that are good to know.
- A quick one -
When we look at the UGS V3a gain stage used in the original UGS Muses preamplifier, we can clearly see that both VFB (RFB resistor) and CFB (Rcm) was at some point used but later omitted for VFB only. Meaning this iteration or version used in the UGS Muses is a VFB.
I then went into my PASS folder, containing information of virtually all the UGS stages, dating back to 1999 and discover that I've been blind
. PASS always used CFB (that is current feedback to JFet source) all along and whenever we see a diy PASS imitation circuit using VFB, then this is a departure from PASS philosophy over the past 20 years. Wayne Colburn is the master mind behind these gain stages and the same group of JFets, BJTs and MOSFETs have been in used since v4 which was used from early 2000. What has changed is the degeneration scheme, similar but different.- XP25 gain stage -
There is at least one power amplifier found in this forum which uses common degeneration and there is no reason why paralleling this resistor will not work for the preamplifier gain stage, so this alone should introduce better dynamics like N. Pass mentioned. And yes, the BJT and MOSFET is different on the power amplifiers compared to the preamp gain stage.
I will spend some time, on the side line, reading up a bit on feedback and distortion, but overall, the only real change for the UGS Scion gain stage is the omittance of series degeneration.
As for advantages of CFB over VFB: wider bandwidth and better slew rate (speed) which helps in transient performance.
Source:
https://positive-feedback.com/reviews/hardware-reviews/pass-labs-xa25-class-a-stereo-amplifier/
I can take a look at it, but FYI... the UGS6 is ready and have been for a while, I simply didn't want to disclose the full information. Since so many UGS gain stages already exist in diya, the harm is significantly less, but I don't want to share the schematic since it is a current product. I can show a pic of the PCB render but will disclose my altered version. Someone could probably figure things out via the official service manual and various other official documents available online - as long as it doesn't come from me - This is the one I will alter.
I also discovered that spite being the same transistor, sometimes the Cob and Mhz vary. Same platform but worse spec than another producer, which is interesting.
OBS: Some of the function is not on this pcb version.
You have paralleled the gain resistor [common degeneration] and needtubes have done similar work on his [UGS-UP-Mini: A 25W SUSY Amp] amplifier.
All the help I can get from prior work is of great appreciation
- This is the one I will alter.I also discovered that spite being the same transistor, sometimes the Cob and Mhz vary. Same platform but worse spec than another producer, which is interesting.
OBS: Some of the function is not on this pcb version.
You have paralleled the gain resistor [common degeneration] and needtubes have done similar work on his [UGS-UP-Mini: A 25W SUSY Amp] amplifier.
All the help I can get from prior work is of great appreciation
Besides dual mono everything, what does the XS have/do which the XP- series doesn't ... I am asking because I genially don't know.dunno
that's old story
you should go for XS preamp route
(I plan on dual mono everything anyway
)It looks like it is using a full power amplifier gain stage - aka the larger BJT and MOSFET's - I can ofc adapt the UGS to this, not too complicated.
"new gain-stages with auto-bias and DC compensation all combine for optimal performance." and the ceramic PCB... Telecom stuff
Auto bias is a nice feature ofc, Elekit have that in their tube amps, and I am sure many others. I would need to do a wide search to see if XS stuff would pop up. Going over all diy PASS and FirstWatt amp threads would take a very long time. But I can take a look ...
Auto bias is a nice feature ofc, Elekit have that in their tube amps, and I am sure many others. I would need to do a wide search to see if XS stuff would pop up. Going over all diy PASS and FirstWatt amp threads would take a very long time. But I can take a look ...