Transistor Research Labs ST-225 Integrated amp


There’s not a lot I can say about Paul Weitzel and his contributions to high end audio, (Tube Research Labs, FIM Cables, Cable Research Labs, Fidelis Records and Diversity Records) that I have not already said over the years. I make no secrets of a heavy personal bias towards Paul's products and their reproduction of music which is, and has been, a revelation for me over the last 13 years of listening to music and gear.

Transistor Research Labs represents his latest contribution, along with his brother, Brian Weitzel of Record Research Labs. Together, they have made a believer out of a "tubes only" guy for the past 33 years.

I've owned various pieces of Tube Research Labs amps and preamps since meeting Paul at 92 Winter CES, where he showed with Avalon, First Sound, and Cardas with his huge GTRP-800's, which are 96 tube, 4 chassis, 1240 pound shipping weight amps that now retail at $140K. At the time, I owned Jadis amps (200's) and preamps, all heavily modified and Avalon Ascents. Since I couldn't afford the big amps that Paul took to CES, I bought a Stereo 100 Watt, which replaced my Jadis.

I've been working my way up the product line as finances allowed, and planned on settling on the GT-400's which retail for $85K. (Although I've seen them used here at Audiogon for much less).

My latest Tube Research amps were the GT-200's along with the GTP-4 line stage, which I replaced with the new Transistor Research Labs ST-225 (retail $5500).

Simply put, I no longer need to dream of the day that I can own (and have room for) the Tube Research Labs GT-400's. Paul has designed an solid state amplifier that doesn't sound like solid state, (and not like mosfets either). Typically, in the past, I have found that transistors are edgy, have glare, have a lack of high frequency dynamics, are not as holographic of imaging, and to be fatiguing. Solid state has always left me wanting to leave the room, rather than stay up late listening. Not the case with the ST-225 ... I don't sleep much anymore.

The ST-225 is dead quiet. I usually dislike power LED's or indicators, but am happy that they included a blue LED in the face plate. When the music starts, the ST-225 throws a deep, wide stage, with incredible focus and dark area around each instrument/voice. The dynamics really surprised me the most. In my opinion, micro-dynamics is where the heart and soul of music reproduction lies. The ST-225 excells here. Minute details are finally revealed. Musical information that has always been there is now evident. Macro-Dynamics jump right out at you, as they do in a live event, powerful and strong. Transient response is very quick and decay is very natural. Sonically, there is no fault that I can find. Compared to my Tube Research, I have found that I have been listening to, or have accepted tube hysterious effects or response for years. It's hard to describe, but once you experience it, it becomes really bothersome.

Weaknesses: I'm used to the 1 inch thick billet aluminum faceplates used on nearly every piece of Tube Research Labs gear that I've ever owned. I understand that the guys wanted to keep tthis new line of gear affordable, but I miss the nice, thick engraved faceplate. (Probably not enough to offset the price, but I've become accustomed to it). And, the amp is quite heavy (around 80 pounds) for a simple integrated.

Strengths: Seperate input selector switches for each channel (3 inputs each), custom made to weitzel's spec with silver contacts. Seperate 45 step attenuators custom made to Weitzel's spec with silver contacts. Cardas premium jacks (look like GRFA's) and Cardas posts (look like CCGR's). And, they use a Heineman circuit breaker switch for the power, no fuses.

I'm very happy. My new ST-225 is superior to the tube gear that it replaced and at a fraction of the cost. I no loger need the heat that 10 KT-88 outputs tubes/channel produce, along with the other 10 power supply tubes/input tubes. (on the GT-400's). Nor do I have to save for the 400's.

Sources used: TRL modified Marantz SA-14 and a TRL modified Scully 270 open reel, playing back 15 and 30 IPS 2 track production masters.

Speakers: Magico 3's

Cables/interconnects: FIM/CRL Gold throughout.

Room size: 23 X 12 X 8

Room treatments: Shakti Hallographs in the corners behind the speakers, along with custom made room treatment panels on the back wall, the side walls, and the ceiling.

Length of Audition: Around 700 hours of use on the amplifier.

Type of audition/review: Product owner.

Thank you for your time,

Jack Seaton
Jack, can you post a picture or two of the amp somewhere?
I'm an integrated amp kind of guy so I really appreciate reviews like this...thanks for the review Jack...although I think I hear the tube camp close on your heels. ;o)

Deb...there is a picture of one for sale right definitely looks like it means business!!...however...I have to agree with Jack that it needs a different looks like it belongs in a laboratory...not necessarily some diamond encrusted, highly polished audio jewelry kind of front...but something a "little" less industrial.

keep the knobs though...they are sweet!

just my 2c

Jes45, with dual 45 step attenuators lacking indicator markings by which to judge relative volume level, how does one match left and right volume...just kinda by listening and "feel"?
Thanks Ellery for the info on the pics.

Tvad, yeah I had a passive pre at one time that had 23 steps and no indicators, you just count the number of steps. Sometimes there's the wierd record that doesn't have the vocal in the exact center, then you have to use your ears to adjust for proper balance. Comes with a little listening experience. Tvad, why are you so angry and negative and ...
Ellery911: It is a little "industrial", but I like it. The huge knobs really set the thing off. (You should have seen the Wife's friends with the big volume knobs, turning them back and forth ...). I do miss the 1" thick face panel, but at the price offset, I'll learn to manage. :) Especially considering the sound.

TVAD: I'm actually happy that they don't have numbers around the knob as an indicator. I would find that a little ugly, (think 70's el-cheapo reciever) in my humble opinion. I'm used to having step attenuators and have always used my hearing as Deb describes to set the proper level for each channel. It doesn't require much acuity, IMHO. And, if I do change the volume setting from one recording to another, it is easier to count off 2 or 3 clicks than it is to lower my eyes to look at indicator marks anyway. :)


Deb, why does my question make me angry and negative? I think it's reasonable.

I own a preamp with dual stepped attenuators, and each has volume markings that make it easy to match left and right volume. I don't have any experience with trying to match left and right volume with unmarked step attenuators, therefore my inquiry about the ease of matching left and right volume on the ST-225 is a valid one.

Thanks for your response.
OK, Jack. Thanks for your take on the subject.
Tvad, actually the first really good preamp I had was a First Sound passive preamp that I picked up used. It didn't have any numbers to indicate what the volume was. Got along just fine with it. Never gave it any thought that it should have had numbers on it.
I had the pleasure of spending an afternoon with the ST 225 with a friend who was trying it out with his system a few weeks ago.
I liked the ST 225 very much and agree that it had a very low noise floor which allowed the musical information to be fully revealed. Almost devoid of glare or harshness, yet produced detail and resolution nicely.

As for the volume attentuators-they were a little tricky, to be honest, for me and my buddy. I am sure one would get used to them.
We both are hitting the age where bifocals are being recommended so can't see for shit sometimes :)

I have an old Parsound power amp in my pro audio rig with L/R level controls with tiny numbers and no steps you can feel. Actually with my middle aged eyesight the numbers don't help at all anyway!

Update: With countless hours, the ST-225 is definitely fully broken in. As most TRL owners can attest, it takes a while to fully break in one of their pieces of gear or even just their digital mods.

The amp has opened up dramatically, most noted between 300 to 550 or so hours and the stage got wider and deeper as well. The low noise floor of the amp, along with the low noise floor of the TRL/SA-14, is unbelieveable. I can't believe how much noise we are accustomed to and just tune out. The amp can be at 95 dB levels (and higher) and be dead quiet between songs. When the music starts, it makes you jump the first couple of times, which I completely forget when guests come over. (Sorry).

Anyway, I couldn't be happier. And I am not upset with my decision to sell the Tube Research GT-200's ... a statement that I thought that I'd NEVER make.

Best wishes,

Jack Seaton
I am somewhat of a latecomer to the TRL parade. Would it be possible to post a picture of the amp? The old link has expired. Audioezra is supposed to be getting a separate transister amp from TRL. I am very curious to hear it. I asked Paul whether these amps could drive the load presented by my Apogee Scintillas. He is looking at circuits that could push low ohm loads. I am anxiously awaiting the return of my TRL SA-14 so I can hear in my system what all of the buzz is about. Bob
I know this has been asked above, but are there any pictures of the amp out there?
I'm with Tvad on the unmarked attenuators issue. The numbers on the face plate of First Sound pre-amps (also dual stepped attenuators) do not detract from its looks IMHO, and they DO help orient one's volume settings---especially for returning to the same volume in an evaluation of some thing in the system or to use the same setting for a particular cut of music. It could get confusing if you are using multiple cuts of music with specific volume settings that you want to return to every time.
Hi Jack

Wondering if you are still satisfied with your ST-225?



He's probably too busy listening to respond:) I actually received the latest model, the D-225 which is a non-integrated version. If you're interested you can email me and I'll send you a photo. I have about 250 hours on it and it has opened up quite a bit since I got it. Most notable of the changes is the smoothing out of the high end. The bass and low end was excellent right out of the box.

Thanks :-)
What did you have before the D-225? How does it compare so far? Would the TRL guys take back a unit if it didn't make you happy?



I was using a Cary V12i stereo amp (I have it listed here now). I had it outfitted with 12 NOS Siemens EL-34s, 2 NOS Aperex 6DJ8s and 2 early Russian 7139/EL-84s. Combined with my Joule Electra LA-100 MkIII this was a very rich and warm set-up. Very lush and sweet mid-range. The bass was acceptable, made slightly better by the Siemens tubes.

The D-225 is much more neutral. Initial observations was that it was a bit thin in the high end. The low end was very solid out of the box and even tighter today after 250 hours. As it has been breaking in the high end has smoothed out and while I know it will never be as warm and fat as the Cary with those sweet mids, the D-225 does so many things right in other areas where the Cary was lacking that I'm not missing that lushness at alll right now. Not to mention the D-225 drives my Spendor 1/2e speakers (88db efficiency) like no other amp before drove them. Much more dynamics from these speakers now. I also now appreciate the Joule even nore and know exactly what it contributes to the equation where before its signature was mixed in with that of the Cary.

I got mine on a trial basis. Give Paul Weitzel a call and talk to him. These guys stand behind their work and treat me more as a friend than a customer. They understood I was hesitant to make the move to solid state and they truly wanted me to be happy with my purchase.
Kalen: To each his own. Since most of my listening is done in a very dim lit room, the whole numbers issue is void. Besides, I'd be embarrassed if I couldn't set the volumes by ear anyway ... it would be like owning a Porsche 911 and requiring a shift pattern on the gear selector knob. :) And, since we are on the topic of First Sound/Presense Audio, did you know that Paul Weitzel (of Tube Research Labs and Transistor Research Labs) designed the circuit for the Presense Audio active preamp? You have likely (like TVAD) been enjoying his efforts for years. :)

Oldpet: Sorry, Paul. I haven't dropped in here for a long time. And yes, I'm still enjoying the amplifier every day. Yesterday (Christmas) we had most of the Wife's immediate Family over. The Sister and Brother in Law stayed in our guest room which is adjacent to the listening room. The BIL knows that I leave gear on 24/7, so he pushed play on the digital player and listened to music for a while the other night. Being a former professional himself, (studio musician, acoustic guitar) he was amazed at the sound of the music. I found out later that he stayed up until around 2:30 AM listening to various discs. (Looks like we have another converted audiophile ...) Anyway, he asked prices, etc. and plans on setting up a system sometime this year.

All I can do is grin ...

Best wishes,


P.S. Clio09 is right ... I've been enjoying the music and haven't been checking in on this review, which is an easy thing to do with the ST-225. :)

Same here with the D-225. I can't seem to pull myself away. Interesting point though on the First Sound (Paul sure gets around, heard he's designed for George Cardas, as well as FIM and others). My Joule Electra is not marked as well, nor does it have steps. I set it by ear, just like I shift my '72 911 by ear.

Tony: Betcha didn't know that VTL's Signature series transformers were Paul's design that he did not want, so he released the design to Greg Scott, who later offered them to VTL.

And the Shallco 45 step attenuators used in my ST-225 were designed by Paul a few years back and built to his spec. Some of his ideas have trickled down to their other models.

And that the van den Hul "Copper Hopper" and Black Beauty were Paul's design, too.

There are a few others, but I'm not sure if they should be made public or not ...

BTW, I used to shift my 911S without looking at the knob, too. :)


I spent an afternoon listening to Tony's TRL 225. It seems considerably more transparent than his Cary, with a more defined soundstage. I was impressed.

I had a '70 911T from '71 to '94 and put nearly 100 K miles on it. I shifted by the tach. I much prefer the gated shifters in Ferraris to the vague gate of the 911.

Donbellphd: Cool. I have never owned a Ferrari, so I'll have to take your word on it. Which model do you have?

As for the TRL ST-225 integrated, at $5500, it should fit many budgets.

I've heard Tony's D-225 and I'm not shocked at your impressions. It is a very musical, accurate, detailed and dynamic amplifier. Stage layering and depth are remarkable.

Best wishes,

Well Jes45, I guess I should be embarrassed now; I do like the numbers on my volume controls. I also actually look at the shifter knob the first time a drive a car (to orient myself to the gears) before I drive it. (Of course, once underway, no need....)

I can set the volume by ear on a given cut on its own. No problem. I think I mentioned that the numbers are helpful for returning to given settings over several different cuts of music (at different volumes) for repeated evaluations. They just help to make quick “return to” settings.

If that should make me embarrassed, I guess I have no audiophile credentials. Maybe I should hang out with you so you can show me how this stuff works.
And, since we are on the topic of First Sound/Presense Audio, did you know that Paul Weitzel (of Tube Research Labs and Transistor Research Labs) designed the circuit for the Presense Audio active preamp? You have likely (like TVAD) been enjoying his efforts for years. :)

Sold my First Sound Presence Deluxe II.

I couldn't live any longer with the embarrassment of having to look at the numbers on the volume control. Now, I hope I can be a full fledged member of the club.

What'da ya say?
Tvad: I figured that you had to sell your Presence Deluxe II because someone told you not to own anything designed by Paul Weitzel ... :)

Kalen: I suppose this is a topic where we will have to agree to disagree. I'm the type of guy that doesn't require numbers to set the volume.

And yes, you are invited to hang out any time.


Jack Edward Seaton