ECP Audio T4 Headphone Amplifier

One of the first things to notice about the T4 design by dsavitsk are some unique features with the PCB:


The cutout for the Power Transformer is obvious, but this was done to avoid any cross-contamination of the 60Hz AC with the PCB. On other thing to notice about the top of the PCB (also on the bottom) are the heat sinking areas directly on the PCB. This is because the power transistors in the solid-state, fully differential buffer are SMD chips soldered directly to the PCB. However, despite the fact that they are small, SMD chips, they build up as much heat as many TO-220 transistors. You'll see later on how the heat transfer goes together, but there is a very thick, aluminum "heat bar" that is bolted to the bottom of the PCB and to the bottom of the casework. It attaches directly to these rectangular pads with ceramic insulators in-between. The ceramic insulators are needed, because there's close to 300V that pass through these pads. The numerous vias are used to fully integrate the sinking planes from the top of the PCB to the bottom of the PCB. Fairly unique, but it allows complete heat sinking through the entire casework, instead of bulky and somewhat inefficient (by comparison) PCB-mounted aluminum heat sinks.

Here's the PCB bottom:


Here you can see those heat-sinking planes duplicated on the bottom of the PCB, with the vias connecting from top to bottom. The other notable item is the complete absence of a ground plane. Usually, with most headphone amplifier designs that use a PCB, one surface of the PCB is used for the circuit traces and the other side is used for a ground plane that covers most of the area of the PCB. With the fully differential circuit beginning at the tubes and continuing to the output transformers, a common ground plane is impossible. However, there are many supporting parts of the circuit - power supply, tube heater supply, CCS circuits, and connectors - that utilize a common ground. That's the large trace pointed out above and the branches that follow.

 

The first step in populating the PCB begins with the lowest height part and ends with the tallest part. On the T4, that begins with the small diodes, followed by the tube plate resistors:


The small diodes are straightforward, you just need to make certain that the polarity is correct. As for the plate resistors, shown here as the large, reddish-tan, 75K resistors, there's a story behind those.

During the T4 predecessor amplifier design by Doug Savitsky (dsavitsk), the Torpedo III (or T3, for short), there was quite a discussion in the T3 thread here on SBAF about the plate resistors. Of course, back then the T3 was DIY and everyone was trying their own flavors of parts. Some worked, some didn't. Anyway, this particular builder noted that the plate resistors in a tube amplifier have an effect on the sound quality. He was absolutely right about that, but just not in his particular suggestion.

Doug and I had originally used standard, higher-wattage metal film resistors in the T3 as the plate resistors, but this builder wanted to try the boutique Amtrans carbon-film resistors. They come with a unique construction and reputation that can be found on PartsConnexion here: https://www.partsconnexion.com/media/pdfs/Amtrans_amrg.pdf.

After quite a bit of hoopla over them, I tried them … but didn't like them. They actually dulled the response in the highs, making the details of cymbal hits disappear in a fog. So, I dismissed the idea of changing the plate resistors from the metal films.

Until … many months later a customer sent me a T3 for repair and asked me to install a different style of resistor at the plate resistor positions: the Texas Components Corporation TX2575 resistors. These were "naked" bulk metal foil resistors, now owned by Vishay (what electronic part isn't owned by them anymore?). Again PartsConnexion carries them under the new brand of Vishay here: https://www.partsconnexion.com/VISHAY-80844.html.

When I installed them in the customer's T3 … Wow! They really made a difference: highs were cleaner and even more detailed than with the standard metal films. Unfortunately, as you can see in that PartsConnexion link, they are quite unorthodox and expensive, at almost $14 each. That was $56 for a single amp!

That said, when Doug designed the T4, I mentioned the TCC/Vishay resistors to him. His answer was the type of resistor shown in the pic above. It has very similar qualities, but in a more orthodox construction, availability, and price. Apparently Doug did a lot of his own research and tried out different resistors before finding these. They are still expensive at $3-$4 each, but worth every penny on this amp. Please understand that differences such as these often go unnoticed in a typical amplifier. With the T4 and even the T3 before it, every little increase in part quality yields positive results, because the amplifier performance is at that level.

The actual resistors used in the T4 can be found here: https://www.mouser.com/datasheet/2/427/cpf-1762878.pdf. They make a difference and are very little different in performance from the TX2575 resistors described above. Here's a pic in mid assembly, with the plate resistors' leads bent and ready to insert in the PCB:



By the way, that little red triangle lead-bending tool on the left is worth its weight in gold, especially when assembling several amplifiers such as this, with varying lead lengths from part to part.

(I'll try to be less verbose and have more pics on following posts.)

 

Next up are the SMD transistors, voltage regulators and MOSFETs. No big deal here, except that the assembly is typical SMD: liquid flux in a tube, small solder, small soldering iron tip, and tweezers to hold the parts:

1. Apply solder to a single pad at each part position.
2. Re-melt the solder with the soldering iron in one hand, while at the same time, holding the part with tweezers in your left.
3. Use the tweezer to move the SMD part into position, into the molten solder.
4. Remove the soldering iron while still holding the part with the tweezers.
5. Let everything cool, remove the tweezers and solder the other leads/etc. of the SMD part.

The tabs are particularly important here. One needs to be certain that the transistors are flat on the heat sink pad and that the transistor tab is soldered completely flush with a clean joint. Otherwise, you lose heat sinking capability.


Closer (tabs on right have not been soldered yet):

 

Lovingly hand crafted ECP audio T4 amplifiers. No wonder this hybrid headamp is at the top end of sound quality in my lab.

 

Ahh … this is the very, very boring part of populating a PCB - the rest of the resistors:


You can see the PCB flipped over here, so that I can solder all of the leads:


It's a lot of work and you don't really get a sense of having accomplished a lot, once completed. It's absolutely necessary, though, and probably because of my complaint - is often where focus is lost and the resistors get mixed up, causing untold grief in trying to troubleshoot once the amp is built. I keep all the resistors in their labeled bags and insert only one value in their proper positions at a time, to remove the chances of those errors.

Once again, Doug surprised me. I actually replaced his basic resistors in the prototype with the familiar Vishay-Dale brown sausage-like resistors. The V-D resistors have been used by DIY-ers for at least 20 years, now. They are absolutely consistent in quality and manufacture and as metal films, are light years ahead of the old carbon comp resistors. They're hard to beat.

I had a small flirtation with PRP resistors, sold at PartsConnexion and many other places as a colorful (they're red) V-D replacement. Some people were excited about them around 10 years ago. I even had a line on a local distributor that was going to supply me in bulk with these. Unfortunately, when trying them I found that they were inconsistent: sometimes they were "blobs" instead of "sausages," and the red epoxy shell seemed to break and fragment very easily. You have to almost use a pair of pliers on a V-D resistor to break one. So, I was back to the dependable V-D resistors quickly.

So, I asked Doug about the T4 - "Why do you keep using these cheap, colorful foreign resistors?" "Because they're better," he tells me. "Whaaaat?" I said. Sure enough, he got me again:
https://www.mouser.com/datasheet/2/427/mbxsma-1286643.pdf
Turns out they are German and started in Berlin as Beyschlag. Their power rating is better than the V-D RN55 resistors, so is their tolerance, and they're smaller. So, I switched, too:

Another shot in the middle of resistor work:


Long view of the Beezar Audio "production line:"


I'm in the boonies, now, so every once in a while, I get to see deer crossing my front yard out of that window. That helps with the boring resistor work.

 

Thankfully, things start going faster (or at least the feeling that you're accomplishing something) after the TO-92 transistors. The first of the large parts, the tube sockets are next. With a tube amp, it's important to have the standoffs very, very close to the tube sockets. Or, better yet - directly under each tube socket. This is because the act of plugging a tube into a socket puts a lot of stress on the socket and the PCB. If it's not properly supported, bad things can happen.

Once again, Doug spec'd an improvement in tube sockets:
https://s3.amazonaws.com/tubedepot-...ached_files/00004280-VT9-ST-C1.pdf?1383248764
The Belton socket has a couple of advantages as a PCB-pin tube socket:

1. Fiberglass construction
2. Low profile

With #1, you get high-temperature structural stability, along with vibration resistance. You don't get any vibration resistance with a typical ceramic socket. Old NOS sockets made out of bakelite are not exactly vibration-resistant.

With vibration resistance, you minimize microphonics. Every tube is probably microphonic to some degree, simply from the mechanical construction involved. So even if it's not audible, it might be measurable and have an otherwise non-localized affect on the sound quality that could be audible.

With #2, every instance I've measured and tested where the lead lengths were increased on tubes, either from socket protectors, flying leads, or what-have-you, the longer signal path causes measurable increases in noise. So, low profile is good from that perspective.

From the perspective of using standoffs, low profile is bad. It means the standoff mounting screws below are captured by the tube sockets. IOW, if you don't put the screws in first - before soldering the tube sockets in place - you're in a world of hurt. I won't tell you what I went through with the first few T4s when I forgot this.



So with those screws highlighted above, here's a shot of the standoffs underneath the PCB:


And with the soldering done:


Top view of the sockets, along with the Molex plug-ins for the power transformer, the power inductor, and film filter caps also installed:


As you can see, there is a center hole in the sockets, so one is always able to tighten, loosen and remove the standoff. However, the screw stays secure under the socket, trapped by the socket pins.

Because the sockets undergo so much stress, I am careful to ensure that solder flows up the pin legs from the pads below. This ensures maximum strength with the socket's attachment to the PCB.

 

At this point, I'm able to stack the PCBs to save space while working on them:


As you can see, I've installed the Z-switch. At 4-pole, double-throw, it means 12 pins to solder for each switch.

Next up are the rectifiers, 4 for the tubes' B+ supply, and 4 for the tube heaters power supply. These are irritating to me. In a TO-220 form with only two leads each, they will shift not only backwards and forwards, but side-to-side as well. So, I solder one pin each with as best alignment as I can then flip over the PCB to check the alignment. Some creative bending is performed as necessary, then the PCB is flipped over and the remaining leads soldered:



I do four at a time, to ensure that I don't mix up the high-voltage rectifiers (tube B+) with the low-voltage ones (heaters). It helps that one set are completely rectangular, while the others have chamfered corners on top.

 

The wonderful Lundahl output transformers are next. From AtomicBob's extensive tests, the Lundahls result in the following output impedances for the T4:
HiZ: 26 ohms
LoZ: 6.3 ohms
I believe these are unheard-of values for a tube output-transformer amplifier, especially that one at LoZ: 6.3 ohms.

This is what allows the great versatility with the T4: world-class performance with 300 ohm HD800 headphones and also great performance with 50-80 ohm Focals, or even 32-ohm Grados.



They have 20 pins each to solder! However, they're fairly low profile and flat. Plus, with that many pins, shifting is held to a minimum. Nevertheless, I solder two opposing ends pins first, on each transformer. Then I flip the PCB over to make certain they are straight and flush. Then flip the PCB back over again to solder the rest. I alternate soldering a pin on one transformer, then solder a pin on the other transformer. They're pretty tough, but the winding wire is awfully small and it's probably best to solder them this way so that not too much heat is built up. Alternating soldering from one OT to the other helps one cool while the other is heating up.

Mandatory use of the exhaust fan is needed, because the heat from the soldering iron definitely causes the lacquer to smoke. The lacquer around the windings stays untouched, thankfully, but I believe they probably immerse the entire transformer in the lacquer, which means it's on the pins and all around. So soldering the pins adds lacquer smoke to the solder flux smoke. It also causes the joints to dull. So, when you look at the bottom of a finished PCB, it may seem that the pins on the output transformers are cold joints - but they're not! That is the melted lacquer preventing the solder from having a shiny appearance.



Above are the output transformers installed, with a Lundahl-custom, ECP Audio part #:

ECP07
Made for
ECP Audio
by
LUNDAHL
TRANSFORMERS
SWEDEN​

The Lundahls are a different size and shape than either Edcor or Cinemag PCB-mounted transformers, which we used in the past on both the Torpedo I and Torpedo III. The Edcors and Cinemags were square steel, with plastic runners underneath that held the PCB pins. Here's a pic of one on the Edcor website:


The Cinemag output transformers used on the Torpedo III are similar, although taller. The metal is also very shiny, being nickel-core instead of mild steel as with the Edcor above. So, the Lundahl is almost completely different from Edcor/Cinemag by comparison:


It starts with a yellow PCB material on the bottom. The metal stack and bobbin winding are rectangular shaped, smothered in lacquer, then a clear plastic wrap is used to cover the windings. Note that the plastic wrap is trivial and may be slightly frayed on the edges, but this is cosmetic, only. The assembly is fixed to the PCB material with a light-tan epoxy/resin material. Similarly, you may notice some cracking in the resin base. Again, this is cosmetic, only, and has no effect on the transformer performance. So on the surface, the construction might look a little less robust than the Edcor/Cinemag design. However, I can tell you from experience that the single plane, rectangular PCB - covering the entire bottom of the transformer - is much more robust in use and installation than the plastic rails used in Edcors and Cinemags.

In addition to dsavitsk's implementation of a tube-parafeed design, and his proprietary DSHA-fully differential circuit, these Lundahls are a large part of the heart of the T4.

Here are pics in a steady progression of installation to the six PCBs under construction. You'll note the stock of Lundahls at the left in the last pic:




The two tiniest Lundahls on the white foam are used as line-level outputs on the ECP Audio Walnut DAC.

Note that I can no longer stack the PCBs while working on them. The pins from the Lundahls on top will pierce the plastic wrap on the Lundahls underneath.

 

A finished T4 in use by a delighted owner. An excellent way to enjoy riding out isolation.

 

Which dac do you find more synergic with T-4, Spring 2 or Matrix XSP?

 

Things start going fairly fast, now. At least the parts are bigger, so it looks like more is being accomplished. Here you see the boards where I've installed the RCA jacks, heater power supply caps, and the Alps "Blue Velvet" RK27 volume pot. The RCA jack assembly is simple, with two plastic prongs that hook into holes on the PCB. They don't move after that. You only need to solder the signal and ground leads. The two electrolytic caps for the tube heater power supply are similar - just two leads, each. However, I take great care that they are soldered flush to the board. You would not believe how common it is with commercial electronics, including supposedly high-fidelity equipment, where electrolytic capacitors are soldered in all sorts of alignments, lead lengths, bent leads, etc. - everything but simply straight and flush to the PCB. I think a lot of high-fidelity equipment might all test about 10% better if greater care was taken with how electrolytic caps are soldered.

Anyway, with higher quality Panasonic FM capacitors such as these, it's actually the rubber plug that ends up being flush with the board. So, that's also a built-in vibration isolator. As with some of the other parts, I solder one lead each on the caps, flip the PCB over to check alignment, then solder the remaining lead.

I buy the Alps Blue Velvet stereo volume pots directly from Alps in Japan. The 50K version is best for headphone amplifiers, but it's not a standard stocked production model. So, any request for a 50K Alps must be a custom manufacturing run from the factory in Japan. I last purchased 400 of them from Alps soon after the horrible tsunami years ago. The Alps factory was hit by the tsunami, so they wanted 400 as a minimum order, instead of the previous 200 minimum. With that many pots, I still have a few dozen left today. Maybe in a sense, Beezar Audio contributed to Japan's reconstruction after the tsunami.

Anyway … the pots are difficult to solder, not because of anything having to do with the six pins, but because alignment is absolutely critical. They're also somewhat top-heavy. All of the pins are concentrated at the front and the weight of the shaft wants to make it topple off of the PCB. So, it's not so straightforward to get soldered in straight. Once you marry up the front plate and the giant machined-aluminum knob we use, it's too late to correct any tiny misalignment. I guess I should use a protractor mounted on the shaft or something like that, but there's no guarantee the PCB itself is aligned during soldering. So, I eyeball it - but very, very carefully. There is a scale square drawn on the PCB for the pot and it provides a great guide for alignment in the azimuth. However, alignment in the vertical is just as important. I solder the opposite back and front pins, then heat and cool the solder joints repeatedly while adjusting both the horizontal and vertical alignment. Once I'm happy it's flush and straight, then I solder the remaining pins. It takes a while for each pot, but is very important.


To the left, you can see the XLR jacks are next to be soldered. Here they are soldered in place:

 

The choke (comes last) and the IEC inlet require both a mechanical connection and a soldered connection. The IEC inlet is up next:


The IEC inlet is designed to match a single, three-pronged IEC design to serve as the same basis for differing wall outlet plugs from all over the world. So, except for changing the voltage inside the amp (detailed later), you only need to pick and purchase the correct cord for your country and it will work.

Also part of the IEC inlet assembly is the power switch (lighted in this case) and a fuse box. The fuse holder is shown at right. The fuse only makes contact with the metal tab and the single socket at top inside the middle of the IEC inlet. The rest of the space is intended as storage for a spare fuse.

Back in the day with the original Torpedo I and Torpedo III, I once thought it was a neat idea to serve my customers well by supplying a spare fuse in that storage pocket. Unfortunately, not actually making an electrical connection with a tab and a spring, it would rattle if you moved the amp around. So, I had customers pulling out a brand new amplifier from the shipping box, hearing it rattle, and immediately thinking the amp was broken in shipment. No good deed goes unpunished. This caused me so much grief from customers that I quickly stopped including a spare fuse.

In the T4, I have a little tool kit with screwdriver and spudger tool that I include, so it's simple to include a spare fuse there, too.

So, the IEC inlet is bolted in first. There is some play in the soldering connections and dsavitsk actually designs the case bottom with slotted holes for the IEC inlet. However, I've found it good enough to simply ensure that the IEC inlet is moved back away from the edge of the PCB as much as possible. The screws and standoffs are tightened down, then the leads are soldered into place. Here's a view from the bottom with the IEC bolted into place (with standoffs), and its three leads (main, neutral, and ground) soldered in place:

 

currently I prefer the Spring2 as I progress through a NOS DAC phase. But please read my profile information section on how to interpret my comments. Don't assume anything I say will be directly applicable to your auditory nirvana without considering convergence or divergence of your personal preferences with mine.

 

Don’t you have the May DAC?

 

The KTE May was here on loan for measurements and evaluation. Then the KTE Spring2. Subsequently I'd commissioned a KTE Spring2 which arrived a few weeks back but is in quarantine. Measurements of both the loaner Spring2 and my personal Spring2 will be forthcoming. Spring2 gets me 98% of the way towards KTE May in a single chassis. Form factor was important for my lab use. Spring2 with T4, DSHA-3F, SW51+ and Magni3+ have yielded many enjoyable hours of listening for me during isolation.

 

Can you elaborate on what kind of quarantine is recommended for a DAC, vs wiping its surface carefully with 70% isopropanol?

 

Spring2 shipped through China so the exterior was wiped with disinfectant. But to error extremely on the side of caution the box wasn't opened while giving three to four weeks for any virus to die inside with the desiccant. I'm probably overdoing it but I am in the target population that doesn't have good prognosis if infected.

 

Yeah, I sympathize being no spring chicken. I have ordered a few gadgets since starting WFH, but ended up just doing a careful gloved unpacking dance followed by thorough isopropanol-based wiping. So far so good, but no DAC, however good, is worth catching covid-19 for BTW, this is the most detailed peer-reviewed paper I know on the topic https://www.nejm.org/doi/10.1056/NEJMc2004973

 

Just to be clear, Beezar Audio practices safety practices constantly. I have limited my trips to the Post Office to twice weekly and they are practicing social distancing with shields in front of the clerks and masks for everyone. UPS and FedEx is a much different matter, with very little interaction with the clerks involved … and no lines to speak of in the FedEx/UPS stores.

All of my parts come directly from Mouser or DigiKey,* with the following exceptions:

1. The Alps pots, as mentioned previously, date back to the Japanese tsunami.
2. The Z-switch is of Japanese origin, from the very high-quality, NKK company.
3. The XLR connector is Neutrik (German), but it may be manufactured in East Europe, I'm not sure.
4. Tube Sockets are Belton. Korean, I believe, and ordered from Tube Depot before the first production T4 was ever built.
   
5. The casework is all locally contracted, fabricated and anodized in the Atlanta suburbs. Wood is provided directly by Doug Savitsky from local sourcing.
6. The Lundahls are Swedish and come by way of K&K Audio in North Carolina.
7. The power transformers are manufactured by SumR in Canada.
8. The Audience Auricap XO parafeed capacitors (up next) come from California, but were purchased back in November.
9. The PCBs come from Taiwan, I believe (through Doug), but they predated the COVID-19 outbreak by about six months. Plus, they will all undergo a multiple 91% alcohol rinse to remove solder flux.
10. All hardware was purchased through McMaster-Carr, mostly in 2018.
11. Solid state chips and rectifiers are all reputable mfrs - TI, IXYS, Vishay, and ON. Some may have been outsourced to China, but the act of soldering surely exposes them to more than enough heat.
12. The Shurter IEC inlet is made in Czechoslovakia.
13. Resistors, as previously noted, are Vishay - mostly from Germany.
14. The LED for lighting the ECP Audio logo is definitely Chinese mainland, but date back to 2017 and earlier (many back to 2009).

* The RCA jacks are Mouser's Kobiconn house brand and are probably of Asian mfr, but "J.T." is all I can find, certainly no "China" marking.

I think that covers it, but if anyone has any other questions about the part sourcing, please ask and I'll try to find out.

My local Post Office yesterday. Everyone practicing social distancing and masks, except for the old guy on the right. My fingers are in the bottom right - not used to taking pics with packages in one hand and while wearing a mask.

 

Up next are the parafeed capacitors, almost the last thing in populating the PCB before the choke and a couple of other capacitors.

Doug has written a great tutorial on headphone parafeed amplifiers here:
http://diy.ecpaudio.com/p/parafeed-tutorial.html
My level of understanding doesn't approach his, but I'll give it a try. If I state something incorrectly, hopefully Doug will correct it.

The output transformers in a parafeed arrangement are basically AC-coupled. There is a giant reason for this: safety. Doug has tried in the past to DC-couple the output transformers through the use of a servo. Unlike other servos you've perhaps seen in the SOHA II (Cavalli design) or Cavalli-Kan-Kumisa (another Cavalli mod), the servo that Doug has tried to make work runs at the high voltage of the amp. In the SOHA II, the tube was powered separately from the solid-state buffer circuit. The buffer circuit simply used a typical solid-state voltage supply at +or- 15VDC. Doug has been attempting to use a servo with the full plate voltage of a 12AT7, around 250VDC. This is the voltage passing through the DSHA-differential buffer of the T4 and the T3 before it.

His experiments resulted in the T5 a few years ago, but he could never lick the audible oscillation that would develop in the servo. This is all to zero out the DC offset, of course. Without that, the output transformer cores will saturate and bad, bad things can result if that happens. From what little I understand, it would mean the output transformer would basically change into nothing more than a resistance on the end of the 250VDC, exposing you and your headphone to that voltage, besides arcing and perhaps burning up the output transformers.

So … the parafeed arrangement with the output transformers use couple capacitors to block all DC offset ahead of the transformers. This is largely steady-state circuit offset. In normal turn-on and turn-off modes, the output transformers completely insulate the output from any voltage spikes.

So, the parafeed capacitors are extremely important to the circuit and even though the transformers are the real output, the capacitors are directly in the signal path. Good quality capacitors are thus very important.

We have used many, many different kinds of parafeed capacitors in the history of the Torpedo. The Torpedo I initially used Clarity SA capacitors shown here:


Since the original Torpedo was DIY, builders used a variety of caps.

Epcos:


Cornell-Dubliers (CDEs):

The one above was actually built by Dr. Kevin Gilmore (more on the CDEs later).

When the Torpedo III came along, it was the first Torpedo or "T" amp to use the DSHA-differential buffer circuit. With the nickel-core Cinemags, it was more important than usual to protect the transformers from DC saturation, so the differential circuit needed four capacitors. We eventually - based on builder reports - used the Mundorf Silver-Oil EVO caps. These were very difficult to shoehorn into the existing design planform. They required a detailed lead-bending plan and custom insulating spacers that Doug designed. They also required a Dow-Corning adhesive (the white goo in the pic) to keep them from shifting in shipment and use:


Nevertheless, the Mundorfs had a "dry" sound signature that caused complaints from some. For some reason, 12AZ7 tubes seemed to counteract that the best, but they develop a lot more heat, which brought additional problems. CCS boards were added for the DHSA-differential buffer and for the tubes that complicated things even more. In addition, some 60Hz leakage from the power transformer was still showing up in testing. It was never enough to be audible, but was probably contributing to some false "warmth" that was never really there in the tube and Mundorf capacitors. Anyway, the problems with the tweaks and planform resulted in the T3 becoming way, way too difficult to build by the typical DIY-er (especially the inexperienced looking to save a quick buck). So, we quit selling them as kits and Doug focused on a future design.

The T4 was the result. Based on the previous experience above, Doug's first prototype used the Mundorfs. When I got it, the amp sounded thin and lean. We suspected the Mundorfs and replaced them. Since Dr. Gilmore seemed to like the CDEs, that's what I put in first. The amp sounded great!



EDIT: It's kind of interesting to see that the T4 amplifier has quite a heritage and can trace its lineage back many years through many versions, with improvements all along the way.