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Class-T amplifiers

The SI T-AMP finds some friends...

[Italian version]

Products: Sonic Impact T-AMP, Autocostruire 2020, DiyParadise "Charlize", 41Hz AMP-3.
Reviewer: Nick Whetstone - TNT UK
Reviewed: October, 2005


My first review for TNT! I had dreamed of something exotic to drop in my system, sitting back and enjoying the music. But no, my first assignment turned out to be a review of some class-T amplifiers. And not the sort that you just 'drop in' the system!

To be honest, if I had realised the work involved, I may well have passed on this one, but then again, how many people get the chance to compare a number of similar products in their own system at home?

After the Sonic Impact T-amp, and the accompanying rave reviews, along came some more class-T contenders. Class-T I am told denotes that the amplifier uses a Tripath chip! Rather late in the day, I had purchased one of the SI amps to see (or hear) one for myself. And at first, I couldn't understand what all the fuss was about! I could hardly listen for more than a few minutes without wanting to go back to my Gainclones! But the advice to stick with it and give the T-amp plenty of time to 'burn in' proved quite correct. It's surprising how mine went from 'so-so' to 'WOW'. But I have heard from several other T-amp owners who had a very similar experience.

By now the other candidates for this review were starting to appear. 41Hz supplied their AMP-3 in kit form and Autocostruire sent their 2020 based amplifier module. Then DiyParadise brought out their class-T amp based around the 2020 chip and kindly agreed to supply one for this review.

Now if all this sounds like loads of fun, it is! But the other side to it is the work involved! The SI amp comes 'ready to play', cased and only requiring a suitable power supply. The 41 HZ AMP-3 however, comes as a kit, using mostly very small SMD components. I have to admit that I failed to complete one and was helped out by fellow DIYer, Leo Kirkbride, who kindly came to my rescue. The Autocostruire and DiyParadise amps came assembled but still required putting into cases with all the necessary connectors.

I needed some cases, well four of them actually (as I decided to re-house the SI amp), and I couldn't afford to go out and buy something ready-made. But DIY comes as second nature to me these days so I had a look around the house to see what the possibilities were. (Ah! Now I can see why TNT recruited me!) I have previously made cases for my Gainclones from pieces of plastic pipe, the sort used as soil pipe. It's 110 mm in diameter and can be cut to the desired length. The truth is that any of the class-T amplifiers would fit in a much smaller diameter pipe but the problem is then finding enough space to mount the loudspeaker terminals, phono sockets, power connector and switch. This is a review rather than a DIY article so rather than go into how the cases were constructed, I'll leave you to read about that here.

I have briefly mentioned power supplies so I will elaborate on what I tried with these class-T amplifiers. All the examples featured in this article require a single rail supply or around 12-13 volts. The usual 'get-you-going' solution is a wall wart type regulated supply and that's what I tried first off. I have one of these units with selectable output voltages and it is normally used to power my digital camera.

Next I knocked up my own regulated supply using the LM338 regulator. Nothing too elaborate here, a 120VA transformer, a 10K cap before the regulator and another on the output. The output cap had a snubber but only because that's how it came out of one my Gainclones!

Next to be tried was a small SLA battery, with and without a 10K capacitor in parallel with the battery.

My favourite supply for the Gainclone had been an SMPS module (the Skynet 8080) and as it has a 12 volt output, it seemed natural to try it with the class-T amps too. My findings were that with all the class-T amplifiers, the SMPS (with a 10K cap on the output) was clearly the best supply, and fairly soon, I was using nothing else. Many people condemn the use of switching supplies in audio but I know from reading the various forums that I'm not alone in my preference for using an SMPS with a Class-T amp.

Here is an introduction to the four class-T amplifiers that I tested:

The SI T-AMP (model 5066)

Price varies (a lot) but expect to pay around 20 USD or 35 Euros (+ shipping)

Sonic Impact T-AMP

This is the class-T amp so widely reviewed, both here on TNT, and many other audio sites. If you have been living on another planet for the last eighteen months, I should tell you that this little box of tricks has sold by the thousand to people all over the world! It is built around the Tripath 2024 chip and sells for - well peanuts really!

The SI amplifier comes complete and ready to play. There are connectors on the back of the small plastic case, of the somewhat cheap and cheerful variety, and a pot on the input so you can control the volume. To get you going, there is a battery compartment and all you need to do is insert some AA batteries, connect it up to a source and some speakers, and listen!

Many users have reported a perceived lack of bass caused by the tiny DC blocking capacitors on the input of the T-AMP. I found when auditioning the amp in my 'second' system that is in a smaller room (about 3 metres by 3 metres), and playing through my IPL A2 reflex speakers, the bass was adequate. In my larger listening room though, I did notice the bass dropping away. So, when rehousing the T-AMP, I modified the input section and replaced the tiny stock capacitors with some 2.2 uF polypropylenes (ironically almost as big as the whole T-AMP internals) and this made a small but noticeable improvement to the bass quality.

I had reservations about modifying the T-AMP for this review. Shouldn't a product be reviewed as it comes from the manufacturer? Normally, I would say yes! But considering that the other three amplifiers in this review require some degree of DIY work, I argued that the type of person interested in the review would probably want to know how these four examples compare 'on a level playing field'. So I went ahead and modified the T-AMP. Apart from the input capacitors, I added a couple of 1600 uF caps to decouple the power supply rails where they connect to the chip. And the amplifier was then rehoused in a new case with some decent connectors.

The Autocostruire 2020-m

Price - 120 USD or 95 Euros (+ shipping)

Autocostruire 2020 module.Picture courtesy of Autocostruire.

This class-T amplifier is available either as a kit or ready-built. Normally I prefer to build this sort of item myself but in view of the amount of work building cases etc, I took up Autocostruire's very kind offer to supply a finished module.

This amplifier uses through-hole components as opposed to the SMD type found in the other modules in this review. The PCB is of very good quality, especially when compared to the one in the SI T-AMP (which in fairness, is a lot cheaper). A good quality Alps pot is included in the price which means that the module only requires a case and some connectors, and it's ready to play.

I should also mention that the 2020-m can be fine tuned to your speakers by playing around with two capacitors in the output section. The 2020-m uses air-cored output inductors which Autocostruire claim sound better than the ferrite core types. This seems to be a valid claim as some people have ordered these inductors and fitted them to other class-T amplifiers with good results.

For the purpose of this review, I first removed the pot. I also replaced the feedback resistors to lower the gain to the same levels as the other amplifiers. The Autocostruire module comes with quite a high gain, almost double the gain of the other examples in this review. Obviously it made sense to have a similar level of gain for each amplifier.

The 41HZ AMP-3

Price - 30 USD or 25 Euros (+ shipping)

41Hz AMP-3 module.Picture courtesy of 41Hz.

41 Hz were kind enough to supply a few of their AMP-3 kits for review. This turned out to be quite fortunate as my attempts to build one of them turned to disaster! I have to say that despite a lot of experience in building items like the Gainclones, I found the soldering of such small SMD components to be very difficult and would take issue with 41Hz over the claim on their web site:

AMP3 based on the Tripath TA2021B chip this is a tiny and simple to build 2x25W into 4 ohms amp with a big sound that will surprise everyone. Surface mount components.

True, I have heard of people building these kits and even one guy who claimed that it was his first project and had successfully built one, but it is certainly not simple! Anyway, as stated above, a friend managed to build up the second kit for me. When he sent it back, I put it into a case, connected it all up and tested the DC offset prior to trying it out. One channel had a very high 110 mV while,the other was a more acceptable 20 mV. After asking about this problem on both the 41Hz forum and the DiyAudio class D forum, it appeared that one or two other people had similar results. Somebody had a channel with 130 mV and had tried their AMP-3 without mishap so I decided to try my example. Fortunately, it seems to cause no problem in either of my two systems!

Despite my concerns about the difficulty of building an AMP-3, it must be pointed out that this module costs far less than the Charlize or the 2002-m. And credit to 41Hz, they have now brought out a version of the AMP-3 (called the AMP-6) that uses through-hole components and should be a lot easier to assemble for those of us not used to using SMD parts.

The DiyParadise Charlize

Price - 80 USD or 65 Euros (+ shipping)

DiyParadise Charlize module.Picture courtesy of DiyParadise.

The Charlize is another class-T amplifier module based on the Tripath T A2020 chip. This arrived all the way from Malaysia ready assembled and required only putting in a case. Unlike the Autocostruire amplifier, this one uses a mix of through-hole and SMD components.

DiyParadise considered offering a kit but decided in the end that it was better to offer a ready-built module due to the extra complexity of assembling the SMD components. Like the Italian 2020 version, the PCB is of very high quality and there are some 'audiophile' parts as standard.

For those of you who have never tried DIY hi-fi, one of these assembled modules would make an excellent introduction to the hobby and provide you with a quality of amplification that would cost you a great deal more to buy in a commercial amplifier. Fitting one into a case and wiring up the connectors etc is really not that difficult!

How do they sound?

All the class-T amplifiers in this review sound very good. When I first heard a Gainclone, I was amazed at the level of detail and with the class-T amplifiers, this is taken a stage further. They also have the ability to tightly control complex pieces of music in a way that only very expensive, high-powered amplification has done in the past!

This ability to control and reveal detail so clearly does mean that poor quality recordings are ruthlessly exposed. On some albums I noticed (for the first time) how poorly the recording had been mixed. Each part of the mix was clearly identifiable, rather than being blended seamlessly into the final mix. Somebody on one of the forums suggested that recording engineers would really have to get their act together in the future and I would strongly agree with that sentiment. And, as with any good piece of hi-fi, flaws in the performance are revealed more clearly.

But you have probably read enough positive comments about the class-T amplifiers and want to know how the different examples compare with one another so here are my findings. The listening tests were done using both of my current systems:

Cambridge Audio CD5 slightly modified Philips CD723 modified
 Scott Nixon DacKit DAC
Passive pre-amp using a stepped attenuator Passive pre-amp using a stepped attenuator
IPL A2 bass reflex speakers Goodmans 201 open baffle speakers (with subs)

The four class-T amplifiers in their 'pods'
The four class-T amplifiers podded up and ready to test.

As previously stated, the amplifiers were powered from a Skynet 8080 SMPS, and the comments below refer to using the amplifiers with that PSU.

The Sonic Impact SI T-AMP

I'm not so sure that my impressions of the T-AMP completely tallied with the reviews that I had read. It is a very good piece of hi-fi, even more so given it's cost. In both my systems it produced a massive sound stage. Tonally, it was very accurate. Voices sounded natural with hardly any sibilance. Clarity was very good and bass well controlled. Bass depth was good but note that I had replaced the stock input capacitors with 2.2 uF polypropylene types.

On the negative side, I sometimes got the impression that notes were truncated. There didn't seem to be the usual (natural?) amount of delay. This could make guitar strings (for instance) sound less 'fluid', or slightly staccato. This was more noticeable with the IPL speakers with their crossovers but not so on the open baffles with their (crossoverless) full-range drivers.

I also found that the T-AMP was tiring to listen to during longer sessions. I don't want to sound too critical of this amplifier however. For its price, it's very good, better with a few simple modifications, and lots of fun. It's just not, in my opinion, a long-term solution for pleasurable listening!

As an experiment, I tried the T-AMP with an active pre-amp (unity gain class A discrete transistor). I found that this made the T-AMP less fatiguing but at a cost of adding back one of the veils that it had removed. In other words, I was trading transparency for a 'gentler' presentation. I'm not sure which way I would go with the T-AMP if I was using it permanently but fortunately, there are alternatives that don't require the active stage. But if you have a T-AMP, and find it fatiguing, it may be worth trying an active pre-amp.

The 41Hz AMP-3

Considering the stress that it had caused me trying to build it, I had to remind myself not to have any preconceptions prior to listening to the AMP-3. But I needn't have worried and was pleasantly surprised as soon as I connected it up and heard the first bars of music emerging from the speakers. The AMP-3 is certainly impressive from the outset! It was clearly an improvement over the SI T-AMP (even with the mods)and I was happy to go through a few CD's before I left it to burn in for a few days prior to serious assessment.

The AMP-3 has many good qualities. Detail, timing, clarity and control are all very good. Like the SI T-AMP, it is quite neutral, has a large sound stage, and separates instruments and singers, putting plenty of 'air' between them. And this class-T amp has another trick up its sleeve. It has the ability to re-create the ambience of the recording venue, particularly 'live' venues, probably better than anything I have heard before. On some recordings it was possible to close my eyes and believe that I was in a concert hall watching the performance from somewhere in the stalls.

Like I say, the AMP-3 is impressive from the outset but for me, this impressiveness comes at a cost. It is not able to maintain my listening pleasure over an extended listening session. At times I found the AMP-3 a little strident and after a while, it gave me the same listening fatigue that I got with the SI T-AMP. Again, I feel it only fair to point out that this amplifier costs next to nothing in terms of the price of hi-fi components. But this is a review and I have to speak as I find!

The Autocostruire 2020

There was a 'rightness' about this amplifier from the first time that I powered it up. I did leave it to burn in for a few days but in truth, I found myself wanting to listen to it at the same time.

Like the other class-T amplifiers, detail and clarity are very good but with the Autocostruire model, this detail and clarity is allied to a presentation that isn't 'too much', too 'over-the-top'. It's an overall gentler presentation! I don't mean that it isn't up front and exciting at the same time but I have no trouble listening to this amplifier all evening and consequently prefer it to the T-AMP and AMP-3.

Once again, the sound stage is large: wide and deep. When I first listened to these class-T amplifiers in my second system, I found that the sound stage was shallower than with the Gainclones. But played though my open baffles, the depth is clearly there (with all four class-T amplifiers). In fact it is quite holographic! Different elements of the music are also clearly positioned and kept apart with space between them.

Bass on the Autocostruire is good, well controlled but not as deep as the Charlize. When I have time, I intend to try some alternative input capacitors as I suspect that may improve things.

The DiyParadise Charlize

Perhaps not unsurprisingly, as they both use the Tripath 2020 chip, I found the sound of the Charlize very similar to that of the Autocostruire. It does almost everything very well and I find it very difficult to choose between these two. Over time however, I found myself listening more and more to the Charlize. Most obviously, it has a tad more bass than the Autocostruire amp but I found the sound of cymbals more metallic on the latter (if perhaps slightly less well controlled). But apart from that I can't honestly say that it is any particular aspect of the Charlize that makes it 'better', just that overall, it's the one that I prefer.

The Charlize, like the other class-T amps does a splendid job of portraying instruments so that they sound if not real, then pretty close. The sound of the piano has always been difficult to get right in a hi-fi. I have long used a recording of a piano when auditioning new equipment to assess how good it is. With the class-T amps, particularly the Charlize and Autocostruire models, a piano sounds as close to 'real' as I have heard. There may be amplifiers out there that do a better job, but not at a price that the average hi-fi enthusiast can afford!

In Conclusion

I've very much enjoyed this review although it has been both time consuming and hard work. Class D amplification has been around for a while but now it is accessible to just about any hi-fi enthusiast. The four samples that I have auditioned are all at the 'entry' end of the market and acquit themselves very well. I have been using Gainclones for amplification for the last couple of years and they are very good. I felt that all four class-T amplifiers were in some ways, an improvement over the Gainclones. So, I will almost certainly keep a class-T amplifier in my main system but I have one slight reservation! During the five or six weeks of auditioning these amplifiers, I quite often said 'WOW' but I never quite had the hairs on my neck standing up. You see for me, the class-T's don't quite convey the same amount of emotion that the Gainclones do. And that's quite an important factor in enjoying any music!

What this review has done is to leave me wanting to hear the more 'up-market' class D amplifiers, and believing that this is the way forward for hi-fi amplification in the future. While what I have heard so far is very good, there is always room for improvement and I for one will be looking to see where we go from here!

In conclusion, I would strongly recommend an audition of any of the class-T amplifiers, particularly the Autocostruire and DiyParadise modules. Many audiophiles spend years improving their systems, making small improvements. Unless you already own very expensive amplification, you will probably find the class-T's a significant upgrade! And at the prices of even the most expensive example here, it really is an affordable upgrade. And with the cheapest costing not much more than a CD, there's no excuse not to!

© Copyright 2005 Nick Whetstone - www.tnt-audio.com

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