Once boards cleaned, will turn to chassis. Remove heatsinks and start sizing them up to drill for the mosfets. Also drilling hole for transformer, and get chassis feet figured out.
Then power supply, before fire up. I will have plenty of questions then about this grounding and all, re reading a bunch of this and staring at pics here, see what I can figure out.
Then power supply, before fire up. I will have plenty of questions then about this grounding and all, re reading a bunch of this and staring at pics here, see what I can figure out.
Received the 'cut-down' PCBs from ZM -> all SMD devices included except the LTP FETs. With many thanks
Used a matched sets of LSK170s from the diyAudio store - 4FETs per board wrt transconductance as advised by ZM - he was worried I might try and "go cheap"!
BD139/140 -> OnSemi devices with "all plastic" cases, so direct mount with thermal paste only, but lower gain c.f. other BDs I have on the bench.
My pucks are IXFN140N30P / ITXN40P50P.
Needed to drop R28 to 1k to adequately adjust bias - using 1.5k maxed at 130mV. At 180mV per side after 90min idle -> HS temp 46C.
In my previous SMD-FET version, dV across BFR under bias setting conditions -> 70-90mV. Using the matched LS set -> 1 and 2mV respectively. I know there is no way on Earth anyone could hear this ... but half the joy in DIY is knowing what is 'under the hood'.
That was all > 48 hours ago ... so this is a clearly viable approach if like me you bought some matched FETs from the store "just in case".
Now ... the listening bit. In short ... the XA25 is sitting on my study floor and the 252 in my listening room
After several hours of moderate-level listening (not loud-loud but enough to annoy the wife) the HS temp -> 56C, so the Chinese case is "just adequate" for this format.
The 252 is certainly 'brighter' than the genuine XA25, with greater detail in the mid/upper range ... which works well with my speakers / room / musical taste.
My son is a jazz guitarist, so I have spent many hours listening to live jazz, and am accustomed to the sound of live brass, piano, top-hat etc ... to my ear, there is no question the 252 does this better.
I'll now sit in the background awaiting the XA252AX ... super-symmetry monoblocks might be the next adventure when ZM gets bored/inspired ... but very happy to rest for a bit and enjoy the music !!
voltage diff across BFR is rather going to be result of (Vbe) difference between BC transistors acting as left & right CCS-es for LTP
smd JFets, what I'm putting in LTP - all 4 matched to 0.1mA
now, that diff. is irrelevant in dynamic modem that's why I'm not bothering to match anything else except JFets
anyhow, I'm glad that you're happy with both original XA25 and its clone-not clone
lemme see - what's the same ........
- OS parts
- rail sense resistors for Iq biasing (value not same)
- limiting protecting circ - blatant xerox copy of Papa's
everything else is different
Blame Pa
mefinnis Very cool! I ordered some BD139/140's and noticed the same thing where they were all plastic. It is nice not to have to use insulators. So your amps are an insulator-free project at this point? I am working on an amp at the moment and was using mica insulators. I kept getting 2-3 ohms from V+ to chassis GND... Scratching my head for a few minutes, I loosened one of the output MOSFETs and sure enough, a bad insulator... So I just ordered some fancy ceramic ones...
I have some 2sk170s. It may be a fun experiment to swap them out and see what happens.
So did the boards with the 170 jfets put the XA25 on the study floor? Or was there not much of a difference between the 170's and the SMDs? That is impressive that the XA252 earned it's spot!
With you having those B&W speakers, I can see the bass output on the XA252 really complimenting the B&W's sound. I heard a set of B&W 802 D4's through both a set of McIntosh monos and a set of Jeff Rowland monos. They can really articulate bass with the right amp.
At 56c, are you running the amp at 180mv bias?
I know of a few speakers that would appreciate a SUSY xa252...
I am working on an amp at the moment and was using mica insulators. I kept getting 2-3 ohms from V+ to chassis GND... Scratching my head for a few minutes, I loosened one of the output MOSFETs and sure enough, a bad insulator... So I just ordered some fancy ceramic ones...I have some 2sk170s. It may be a fun experiment to swap them out and see what happens.
So did the boards with the 170 jfets put the XA25 on the study floor? Or was there not much of a difference between the 170's and the SMDs? That is impressive that the XA252 earned it's spot!
With you having those B&W speakers, I can see the bass output on the XA252 really complimenting the B&W's sound. I heard a set of B&W 802 D4's through both a set of McIntosh monos and a set of Jeff Rowland monos. They can really articulate bass with the right amp.
At 56c, are you running the amp at 180mv bias?
I know of a few speakers that would appreciate a SUSY xa252...
IXYS pucks?
if yes (and it could be normal having those for this amp), all you need to have is thermal paste applied - no isolation needed
they're electrically isolated from case plate/base, so if there is conductivity from drain to mosfet case, mosfet is Dodo
I've told ya all that it is good
No, a different amp. I personally like the IXYS pucks in part because you don't have to insulate them. Not to mention one IXYS pucks = how many IRFP240/9240's?. 🏍️
Yep, it is definitely a great amp!! So about the SUSY version.... ⏳
Bias = 180mV / 1.8A (approx).
As ZM states - no insulation for pucks, just well smoothed-out thermal paste - the milling on the Chinese HS is "relatively coarse", so for this case essential.
Likewise, for the "all plastic" BD's -> just paste.
Being honest, I would never be able to tell the difference listening between the SMD FETs per ZM or the LS variants ... I was just playing around as I had the parts.
Ergo, the difference between the 2 amps is fundamentally the design topology - as ZM hints.
Were in the land of "it's all good" - so there is a large personal preference component involved, however, I guarantee I could tell the two amps apart blinded, there is a clearly discernible difference in the mid/upper range with the 252 winning in my case.
As ZM states - no insulation for pucks, just well smoothed-out thermal paste - the milling on the Chinese HS is "relatively coarse", so for this case essential.
Likewise, for the "all plastic" BD's -> just paste.
Being honest, I would never be able to tell the difference listening between the SMD FETs per ZM or the LS variants ... I was just playing around as I had the parts.
Ergo, the difference between the 2 amps is fundamentally the design topology - as ZM hints.
Were in the land of "it's all good" - so there is a large personal preference component involved, however, I guarantee I could tell the two amps apart blinded, there is a clearly discernible difference in the mid/upper range with the 252 winning in my case.
XA25 FE is sorta BA3-like
"sorta" - few clever tricks making difference
B. XA252 - deliberately made to use classic LTP instead of (BA3) complementary JFet input stage
(reasons being - wanted to avoid scarce JFets and also wanted to avoid simple copying)
after that, XA25 is having complementary mosfet gain stage, feeding OS, while my concoction is having eenyweeny TO92 common emiter-gain stage feeding potent complementary bjt buffer feeding OS
sound wise - both of these topologies can be set to sound pretty much the same ..... there are several more criteria/topics asking to be covered here, but have no time/brain/eloquence for that
all in all - Papa's is having 2 (gain) stages followed with OS, mine is having 2.5 (gain) stages followed with OS
existing sound differences are practically meaning any or all of these things:
- Papa is (logically and factually) smarter than Mighty ZM
- I'm unable to recreate performance of his circuit
- There is a slight
difference between Papa's state of mind and Mighty ZM's state of mind - never been to Africa, but Elephant's fart is effective even in dreams
Even if one wanted the fancy k170 Toshiba JFETs for the XA252, the k170's seem to be more common than the j74's so I agree that the LTP is pretty handy. Mr Pass probably has a pillow stuffed with rare parts that he sleeps on at night. I bet it sounds good too.... Lucky guy.