X2.5 Preamp —nNo problems other than Sound

So I bought this piece of gear thinking I might like to hear the contemporary preamp to my X150.5. Well, at first thought it was fun and had incredible dynamic range. After a few days of listening, I felt I lost something. Something pleasing.
Then plugged the Susy iron pre back in and, well never had the urge to put the X2.5 back. Instead I've opened the hood. I first and foremost would like to understand the topology..fets on the input? I don't need to be spoon fed. A reference to an article would be preferred. But some topology I noticed Is this similar to zenpre? And what function? And DC blockers in and out? Possibly early tech of a resistive ladder volume control? What smd resistors were used? My ear is keen on resistors in the signal path . My iron pre has Vishay thin foil input voltage divider and that change was was night and day in my rig with new alps exercised with deoxit to get them to read even close. I want to like this pre but I'm struggling.. any suggestions? The money guy in me wants to pass it on. But. I have a hard time with that. A schematic would be really helpful. Awe. Nobody wants to be benched for a better player...
Official Court Jester
Joined 2003
Paid Member
A schematic would be really helpful.

For understanding the concept, you don't need schematic

it's in general - relay based input selector followed by Wayne's superduper resistive matrix attenuator made of bunch of smd bjt transistors (instead of relays we are used to see in similar ones), followed by UGS stage

generation of UGS stage is written on small pcbs (2 of them)

attenuator each channel having one big pcb, those with plenty smd bugz; (so two of them)

everything fed with decent linear regulators

if UGS doesn't ring a bell - just search here on forum and you'll know; simplest explained - very similar to BA3 Bal FE; take in account that UGS is gain cell ( as name is saying - Universal Gain Stage) and how it's nested in any specific gadget can be slightly different

what difference you're hearing between X2.5 and Iron Pre (Bal) is no result of parts used per se; of course that sort or quality or "quality" of resistors and caps iused in circuit will bring some difference in performance ( one sort vs. another) but these differences are subtle and certainly not in category of game changers

differences in sound are due to differences in construction, in short - Iron Pre is having lesser amount of signal conditioning and - not to dismiss - some amount of xformer bloom, which you evidently like

lemme see

both preamps are having relay based selector

both preamps are having resistive attenuator ......... and only if you use MUSE module in your Iron Pre, it's somewhat on par with Wayne's in X2.5

X2.5 is using series regs, while Iron Pre is having shunt regs; ZM biased heavily, matter of choice and caprice, not brain

X2.5 is having "active" gain and existing NFB, while Iron Pre gain is solely of autoformer, plain physical device

so, whichever way you made yor Iron Pre, be sure that PL gadget is of higher quality, technically

but, liking - phenomena easiest explained as - one likes more apparatus which demand lesser brain activity to process ....... and that's highly subjective thing , about which one only need to rise level of awareness, nothing else

  • Like
Reactions: 4 users
ZM. Thanks for your thoughtful response.. helpful in many ways. How does the fet after the input relay function? I only counted one TI chip reg per rail unless dual die? Yes a wonderfully well built piece of gear. The modular aspect of the ugs has me thinking there could be some non invasive fun there. Roll UGS. Iron ? Power supply could be low hanging fruit. Pictured is a PS Audio HCPS. Needs quasi snubbers but could work. 40 volts give or take. Or, if I understand you, shunt provides low impedance PS by design so no need for many caps etc? Where can I get shunt reg boards? Fun to imagine, but I'll likely just reassemble and listen some more while I complete 32 step 4 level takman metal film r volume switch thingy for iron pre. I might just short the DC blocker caps though.... Peace
Official Court Jester
Joined 2003
Paid Member
How does the fet after the input relay function?


I only counted one TI chip reg per rail unless dual die?

resuming it's for X2.5, then yes, and no double die

no, if you don't exactly what you're doing; big No

Power supply could be low hanging fruit.

resuming it's comment for X2.5......... I already wrote that Shunt regs are my Fetish, but no - fact that X2.5 is not having Shunt regs is not a reason why you like it less than Iron Pre
it could be several explanations and reasons, but be sure that nothing wiuld change even if one make exactly same circuit (as X2.5) but using proper shunt regs
THD Spectra will stay the same ( not dictated by sort of PSU but dictated by active signal circuitry itself) ........ and - as I wrote above - reason why one is liking Gadget A more than Gadget B is simply that one's brain is working less (or working in more pleasant way) while using Gadget A

Rest of your post I don't get - why is PS Audio PSU spoken of, in this context?
Joined 2002
Paid Member
The X2.5 is about 20 years old and a different creature than the fine Iron pre.
It is cap coupled on the input and output and probably used Panasonic thin film resistors in the volume control.
The volume is a shunt with one input resistor and then switches to ground. What board revision on the main board?
Wow Wayne. Thanks for your response. Deep respect. I'm an idiot savant who questions everything and builds and rebuilds. The coupling caps are blue and seem really small for 1uf. Polyester? My first thought about this was to short all those blue polyester things and if it worked, put them in a bin. I've ordered a
Bunch of parts from parts connexion and mouser. I know it can sound better. I'm in the mountains listening on battery but will get back on board number. Why are there IRF 610 off the relays?
I have noticed that i prefer the sound of my X2.5 on it's Low Gain setting. It runs a pair
of Aleph 2 clones - and i've always considered the sound very enjoyable.
I set it to low gain. I was at first struck by speed and dynamic range but quickly tired. Fatigue.. The iron just brought back all the Jfet love and a depth and warmth and detail..oh just so pretty.. especially with the X150.5 at 50deg C with fans and that is an iterative process to get right but bloody cool. What speakers are you hearing ?
ZM 610 off relays. I probed for continuity. Surely you know. I saw a schema once that seems like it had your name on it..too lazy to try to understand on my own...And my PS audio picture was just a poke about how the Boulder boys were doing preamp PS about the same time as the X2.5. The PCA-2 . A preamp that's been on the bench for a while..I have some schema for but they opted the interesting parts. I'll attach the schema I have. Probably need my work station so it might be a minute


  • Gain Stage Rev E.pdf
    80.2 KB · Views: 55
  • PCA2 Display Schematic Rev. E.pdf
    220.2 KB · Views: 37
My question unanswered is about Pre power supply impedance and the transient response of the shunt vs a big bank of caps. And what meter is there which measures transient response of power supply? But bloody hell man your shunt is Just of so pretty.. At the end of the day
Official Court Jester
Joined 2003
Paid Member
power supply impedance and the transient response of the shunt vs a big bank of caps

what you have of replies being in range of broad generalizations ?

And you can get only that sort of replies if question isn't proper

anyhow - proper answer, whatever your question is - if speaking of "Pre power supply" - there is no comparison between "big cap bank" and "shunt"

cap bank, whatever size of it , must be part of rectifier and filtering block

regulator of any sort goes (and must go!) after cap bank, and - by my opinion - is equally important as main signal circuitry is