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The Signal Transfer Company: Trimodal power amplifier

Class A or Class B? You choose!

[Trimodal amp]
[Italian version]

Product: Trimodal power amplifier
Manufacturer: The Signal Transfer Company - UK
Cost: Trimodal PCB & parts: £125 each, input card, PCB & parts: £38 each, protection unit: £77.40
Reviewer: Maarten van Casteren - TNT UK
Reviewed: October 2010


Several years ago I purchased a book about power amplifier design by Douglas Self. I'm not an electronics engineer, so this book wasn't aimed at me, but it looked fascinating so I bought it. A friend of mine actually does design power amps, as a hobby, and he was quite keen on borrowing it from me. I had it back within a week, as he ordered a copy for himself almost immediately. This is an interesting book, and not just about the theory behind power amp design.

I should start with a warning. I don't think that Doug Self has a very high opinion of audiophiles. He calls us 'subjectivists', as opposed to 'objectivists' like him. Engineers like to measure things and don't always agree with people who just assess a piece of equipment by listening to it. I think the purpose of an amp is to be listened to, and therefore think it is fair to judge it by doing just that. After all, what I'm doing is very much like what you would be doing if you would buy a Trimodal amp: put it in your system and start listening to it. If I like it, chances are you will like it too. But I also know that human judgement is extremely unreliable and can easily be influenced by expectation. For that reason I always try to listen to components for an extended time and compare them to known other components as much as possible. But I do appreciate Self's point: for engineers measurements are an extremely valuable resource and much more practical than human judgements. He has to design the amp, after all, I just have to like it!

You might not know this but many audio components are not just designed, but also 'voiced'. The design phase is important to make sure the component works as expected and produces sound, not smoke. You need to make sure an amp performs to certain specifications, and does so safely and reliably. But once this has been achieved, many manufacturers start the voicing process, where different values and types of capacitors and other components are tried until the unit sounds 'right' and is still within its intended price range. Most manufacturers actually have a 'house sound', which is often the result of voicing and not of the design process on its own.

We audiophiles sometime have an even more extreme vision of reality: some of us seem to think that good sound is almost completely depending on exotic components or very single-minded design approaches. Compared to that I do have to admit that Doug Self's opinions come as a very welcome breath of fresh air. He doesn't agree with burn-in, nor with keeping electronics powered up all the time. He doesn't believe in 'boutique capacitors' or oversized power supplies. And he certainly doesn't believe cables have any effect on the final sound quality of your system. OK, he might almost seem as extreme as us audiophiles but I think he makes a lot of sense too, and I also think it is about time for some more down-to-earth thinking in audio.

[Trimodal amp]

In his book Doug explores the standard design of the modern transistor power amp. Most have a differential stage at the input for the purpose of global feedback followed by a voltage gain stage and finally the output stage which adds current, and thus power to the signal. Self identifies 8 fundamental sources of distortion within this framework, and proposes solutions for all of them. This leads to the concept of the 'blameless' amplifier. Blameless, because it doesn't suffer from any of these problems anymore. Obviously, the Trimodal is such a blameless amp.

One surprising finding by Doug Self comes from his research on distortion in Class A, Class B and Class AB output stages. Many audiophile amps use a Class AB output stage, meaning that the amp will work in Class A up to a certain level, and then moves into Class B. Received wisdom says that the more an amps works in class A, the better it will sound. But, according to Doug, Class AB has higher distortion than pure Class B! He agrees that pure Class A will give you the best results, but then finds that the next best thing is pure Class B. As this will produce more power and less heat than the equivalent Class AB amp, it then logically follows that Class AB should be avoided and pure Class B is the best choice for most application. Only when you want the lowest possible distortion should Class A be used.

The Trimodal amp puts this into practice. You might expect it to have three modes, but it only has two settings: Class A and Class B. The third mode comes into play when you overdrive the amp while in Class A: instead of simply clipping, it then moves into Class AB. So, the two modes are actually Class A/AB and Class B. Switching is instantaneous, completely quiet and can be done while playing music. The amp runs very hot in Class A/AB mode, with even the front panel becoming almost too hot to touch.

The Trimodal as reviewed here produces about 30W into 8 ohm in Class A and only about 35W in Class B. This difference is actually very small, and in this particular amp the Class B setting should be seen as the standby mode, or the 'summer setting' as Doug calls it. The 'winter setting' should only be used when listening. More power from the Class B mode should be possible, upto about 100 watt actually, but only with a higher voltage power supply. A switchable power supply could do this, but this is difficult to do, and probably also expensive, as it would involve switching both the amp as well as the power supply at the same moment. So, this actually is a Class A amp with a very practical standby mode, as far as I'm concerned.

There's not much more to say about the Trimodal, except that it can be bought as a kit, in various stages of construction, from just a PCB to a fully constructed amp like my review sample. The company selling all this is 'Signal Transfer', and they also sell another power amp, the 'Load invariant power amp', as well as a preamp and a phono stage, all Douglas Self designs. My Trimodal was built very solidly and had XLR inputs and Speakon outputs, by the way, which I don't think is a practical choice for home audio, but I'm sure Signal Transfer would allow you to specify any type of connector for both input and output. All inputs and outputs are very well labelled, with explicit phase and ground indications, as one would expect from a pro-audio company. The amp also features a speaker protection circuit, so should be safe to use with any type of loudspeaker.

One final issue concerns the price. Signal Transfer were not able to give me a price for the fully build unit at the time of review, but most parts can be ordered on their website. The Trimodal amp units are £125 each, for the PCB and parts. My review unit also contained two input cards and an amp/speaker protection card. The input cards are £38 each and the two channel protection card is £77.40. This all adds up to just over £400, but you will have to solder everything together yourself, and add a power supply, wiring, connectors, switches and a case, of course. I assume someone with some experience should be able to build an amp like my review sample for £700-£900, which would make it excellent value. The input cards and protections units are also available fully build and tested, but sadly the same is not true for the Trimodal amp cards themselves and if you are capable of building those yourself, I assume putting together the input and protection cards will be easy enough too. I certainly hope that a fully build and tested Trimodal card and a fully build and functional amplifier will be available soon.

[Trimodal amp]

The sound

Initially, I was expecting profound differences between the Class A/AB and the Class B modes. Most companies make so much out of the advantage of Class A that you cannot help but think that this should be deeply superior to Class B or AB. But, whatever the differences in theory, or in other amplifiers, the reality is that this amp doesn't sound that much different in either mode. There are differences, but they are subtle and it takes a little while to find out where they are. Let's put it this way: if you would let me do a blind test to decide between the two modes, I wouldn't be completely sure if I could reliably distinguish between them. In the end this doesn't matter that much, actually, as both modes sound fine as it is. I did develop a preference for Class A, though, and once I found out that there's very little difference in output power for both modes, I only listened in this mode.

This amp actually surprised me a bit with its sound quality. It certainly is one of the best amps I've had in my system and comes very close in overall sound quality to my reference, the Usher R1.5, but sounds very different at the same time. The Usher is a typical, audiophile, silly, subjectivistís power amp. It has a huge oversized power supply, weighs more than 40 kilos and is full of impressively overdesigned components. It sounds massive, with great weight and a large soundstage, just as you would expect a big muscle amp to sound. I love it.

The Trimodal on the other hand sounds very precise, and articulated. It is 'fast', in the sense that it doesn't smear details in time and can reproduce even the tiniest details with utmost precision without getting things muddled up. But strangely it is also 'slow' in the sense that it sounds very relaxed and natural, without the hurried feeling of many transistor amps. The bass doesn't have the weight of the mighty Usher, but it certainly is tight and deep, and has excellent definition. The top end sometimes seems a little bit rolled off, but it is also very, very clean and never, ever becomes aggressive or bright. It does sometimes seem to lack a little bit of crispness and bite, and can almost be too polite. Having said that, to my surprise the Trimodal is actually capable of sounding quite bright if it needs to be: when the recording happens to be bright. It is certainly not the case that it lacks anything in the treble and, more importantly, it doesnít seem to add anything either. I guess we all expect a 'fast' amp to sound bright, and a warm amp to also be a bit 'slow' and soft. The Trimodal proves that it doesn't have to be this way. It certainly isn't bright, but can reproduce any sort of treble when asked for. Actually, it probably is the amp that is most like the ideal 'straight wire with gain' that I've ever had in my system.

When switching directly from the Usher to the Trimodal the latter initially seems a little bit muted and slow, compared to the more lively and powerful sound of the Usher. Both frequency extremes are a bit more controlled and feel less extended in the Trimodal and the overall sound seems more restrained, but it only takes a little while to start to appreciate its talents, though. The bass is deep en articulate, the midrange is very transparent and the top end is the cleanest I've ever heard in my system. This results in an overall impression of precision and openness. Where the Usher relies on its weight and impact to make an impression, the Trimodal brings out more inner detail, tension and rhythmic nuances. It is more balanced and subtle across the board, but if a burst of energy is asked for, it delivers it with such control and clarity that you cannot help being surprised, and impressed. This is not the sort of control that stifles the music, but more the kind that liberates it, albeit in an understated way.

After a while it was the Usher's way with music I was starting to doubt, instead of the Trimodal's, but switching back to my Usher immediately made clear that this amp has more than enough qualities to convince too, they are just completely different to the qualities of the Trimodal. Soon I started wishing for an amp that could combine the strong points of both these amps. Wouldn't that be great! In reality these amps are both excellent in their own right, and the fact that they can stand up against an excellent amp with a completely different sound proves how well they both do on their own territory, and how little they actually loose on their own weaknesses.

When I had some audiophile friends over I was somewhat surprised that they preferred the Usher and almost seemed to discard the Trimodal as irrelevant. We were playing quite loud, though, and the qualities of the Trimodal shine more at slightly lower levels when doing intense and careful listening. The deeper, louder and bigger sound of The Usher will win on direct comparisons, but after much more extended listening I'm not so sure which amp I'd prefer. If you like big, bold hifi you will most probably prefer the Usher too, and with reason as it is an excellent amp, but I can just as easily see other people strongly prefer the Trimodal. It all depends on what you want from your hifi, and what sort of music you listen to. On the other hand, I cannot see anybody being too unhappy with either amp.

All of the before was while using my Dynaudio Contour 1.8 mk2 loudspeakers. They like a bit of power, so the Usher had a natural advantage there, being rated at 5 times the output power. But then I had the opportunity to review a pair of loudspeakers based on the SEAS Exotic full range drivers. These speakers have a higher efficiency: about 91 dB compared to the 87 dB of my Dynaudios. They also are 8 ohm instead of 4 ohm, making a much easier load for the power amp. In addition, the SEAS drivers are extraordinarily revealing. With these speakers the Trimodal really was in its element! Its squeaky clean midrange and treble really played to the strengths of the SEAS exotics and it clearly beat the Usher in this respect. Of course, the Usher still produced stronger bass, but it did now sound a little bit grainy in the middle and higher frequencies, and had also lost its power advantage, with the Trimodal now easily extracting impressive sound levels from the speakers too. With these speakers, I would actually prefer the Trimodal to the Usher, which means the reference has been beaten! This might not be very relevant for people with Ďnormalí loudspeakers, obviously. In most cases the Usher will be able to drive home its advantage, but if you use speakers with better than average sensitivity and transparency, you might find yourself preferring the Trimodal too!

[Trimodal amp]


This was almost as much a review for Doug Self's book as for the Trimodal amp, and in the end Iím very impressed with both. This is a very good amp, and quite unique in the world of audio in that it only makes claims about being designed for minimal distortion and nothing else. I really liked it, and can easily see other audiophiles fall for it too. Its qualities especially shone through when used with the excellent SEAS Exotics speakers (review coming), where it showed uncanny levels of transparency. It is immensely precise and shows an almost complete lack of signature or colour, playing the music in an honest and natural way. There is no exaggeration of anything, it doesn't have a preference for any musical style and it will never make you feel tired of listening to your favourite tracks. You cannot ask more of an amp. Very highly recommended.

© Copyright 2010 Maarten van Casteren - maarten@tnt-audio.com - www.tnt-audio.com

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