Multi-channel amplifier for Theos

[edwinr]

I have decided to venture into multi-channel. I was concerned about implementing multi-channel without the high cost and complexity of separate discrete amplifiers. I was also concerned about the ability of any H/T receiver I purchased being able to drive the Martin Logan Theos speakers as well as my Bryston amplifier.

Last week, I took the proverbial by the horns and I purchased a NAD T757 multi-channel amplifier. I'm not a big audio visual receiver fan, but the NAD appealed to me because it lacks the usual bells and whistles associated with home theatre components.

Inserting the NAD into my system was easy. I have no center channel speaker, but I had previously purchased a pair of Martin Logan FX2 dedicated surround speakers. These speakers feature an aluminium cone woofer and 2 folded ribbon tweeters. They come in any color you want as long as it's black. I had placed them on rear shelves and the connecting cables were already in place, so it was a case of simply connecting the wires to the amplifier. I ran a tape measure to check the distance from the listening position and I used a Radio Shack spl meter to calibrate the levels and I was ready to listen.

The amplifier is rated conservatively at 60 watts per channel with 7 channels driven, and around 110 watts per channel with only two channels driven. So power is not an issue. The sound quality is surprising when driving the Theos speakers. I expected the normal shrill reproduction of the average H/T receiver but instead I was pleasantly surprised by the smooth, punchy sound similar to my reference Bryston B-60R. I think my Bryston sounds a little more spacious and 3 dimensional, but there's not a lot in it. The NAD drives the Theos to very high levels and doesn't even break into a sweat. I measured spls at my listening position of around 107 to 110 dB 'C' weighted and there was no sign of break-up or distortion. The amplifier was barely warm to the touch (Man, the Theos can go loud!).

Oh, did I mention? I bi-amped the Theos, utilizing the two back channels of the NAD. There's a simple option which you can select from the NAD amplifier setup which allows bi-amping. Frankly, I haven't tried the NAD without bi-amping the Theos. It sounds so good, I am happy to leave it the way it is.

I have a Sony BDP-S580 universal disc player which outputs SACD via HDMI cable. The player converts the dsd signal internally to 192/24 bit PCM. The sound quality is very good, although not quite to the standard of a dedicated SACD player with analogue outputs. I have several multi-channel SACD discs and these have not left the player in several days. I will be purchasing more as finances permit.

Listening to vinyl is also very rewarding. As I mentioned earlier, the sound is not quite to my preferred standard for vinyl and CDs, but it's a lot better than I expected. If I'm being really honest, there also might be a little biais on my part against the sound of the NAD. I naturally expect a dedicated audiophile amplifier to sound better than a H/T orientated component - so it does (whether or not it actually does is another matter - if you get my drift)

I am happy with my purchase. The cost was relatively low, and I have a backup 2 channel amplifier if required.

[MrBT]

.........measured spls at my listening position of around 107 to 110 dB 'C' weighted and there was no sign of break-up or distortion. The amplifier was barely warm to the touch (Man, the Theos can go loud!)........


........lhave several multi-channel SACD discs and these have not left the player in several days. I will be purchasing more .......

110db at peaks? Wow! That would require a clouple of hundred watts to reproduce without clipping and distortion. This "little" NAD must be really great or your Radio Shack spl meter foulty(?)

SACD these days can get really expensive. Lucky me! Most of my hundred sacd's I purchased years ago when prices were reasonable. Today I wouldn't be able to afford for example Propaganda Sacd for $200 or lets say Kraftwerk Sacd for $300 (both examples of great m-chanel mix) and many other today unique/expensive titles in my collection.

[edwinr]

The NAD is not so little. It has a massive power supply, and the specifications I quoted related to minimum continuous power output. Tests indicate a power output of around 178 watts into 4 ohms with 2 channels driven. The spl readings were recorded using 'peak hold'. I also checked the spl readings with my neighbor's digital spl meter. My meter recorded 107 dB peak and my neighbor's recorded the 110 dB. The distance from the Theos at the time was 3 meters.

[Northy]

I used to have a NAD receiver. It was a great piece. It sounded twice as powerful as its ratings indicated. I am curious. Why do you not power the woofers with your Bryston and the pannel with the NAD? Bi-amping feature of the Theos is great for this purpose. I am willing to bet that you will get more taut bass using this configuration. Moreover, the NAD will be free to only power up the panel which should result in better sound.

The Theos will be my next upgrade. I can not wait to experiment with amplification.

[edwinr]

Northy, I'm going to plug the Bryston back into the system tomorrow. It should be interesting. The Theos speakers are stunning. It's hard to imagine anything sounding better, but I'm sure moving up the Martin Logan range would reap further sound benefits. As it stands, I reckon the Theos occupies the sweet spot in the ML lineup.

I should mention the FX2. What a great sounding surround speaker. ML have really done their homework. The FX2 is so close tonally to the Theos, that I can't really pick where they finish and the electrostatic panels start. I thought initially that ML were asking too much for them, but after seeing the build quality and listening to them, they're worth every penny.

[MrBT]

Is NAD T757 a receiver and Bryston B60r an integrated amp? If so, is it actually reasonable to use this setup for bi amping? Or am I missing something?

[MrBT]

"It has been designed with pre-out/main in connectors in order to allow independent use of the preamplifier or stereo amplifier section within an audio or video system."

Just found the answer. Pretty neat function I must say. Quite often I thought why manufacturers won't implement such a useful addition to integrated amps and here we go.......

[edwinr]

MrBT, I have tried many amplifiers over the years, and I have learnt that mere specifications cannot foretell how well any particular amplifier will drive any particular speaker. I was concerned that the NAD may not be able to drive the Theos to high levels. I remembered how the Bryston changed my view on lower powered amplifiers being able to drive electrostatics, so I had some hope. Thankfully from switch on, the NAD impressed.

I should clarify my observations, though. The surround sound performance of the NAD actually did meet my expectations. And this is a fair expectation on my part. There are a number of very high quality H/T receiver options on the market, NAD have been doing this for a long time and if they didn't meet the standard, Onkyo, Yamaha and others would swallow them up. But, the other brands that I auditioned did not seem to be as successful with 2 channel (CD, phono etc). The NAD's imaging in 2 channel is first class, and I can place each image quite clearly in the soundstage. There's one thing though (this never made sense to me before), I think the Bryston has more depth or offers a more 3 dimensional view of things. There's not much in it. On lessor speakers you probably wouldn't notice.

(learned vs learnt: The *prescriptive* answer is:
"learned" should used in phrases such as "a learned professor", in which case it is pronounced with two syllables.
"learnt" should be used in phrases like "I learnt a valuable lesson today".

[edwinr]

Okay, I found the limits of the NAD when driving the Theos. I played 'Inception' last night on blu-ray. About one third through the movie I turned the sound up a little to hear the dialogue better, then there was a huge explosion and the NAD switched itself off. It turned back on after I disconnected the power and I finished the movie at slightly lower levels. I am thinking that the maximum spls I recorded earlier was when listening to music, so the peaks may have been relatively high, but at what frequencies?

I reckon the loud explosion was broad-band and really taxed the amplifier. I plugged in the Bryston and this little amplifier didn't even break into a sweat reproducing the explosion. I will have to play around with the amplifier preference options on the NAD - maybe I'll use the Bryston for the front channels and the NAD just driving the surrounds...

[jmschnur]

Good idea


J

[edwinr]

I spoke with NAD technical support. They said the T757 should be capable of driving 4 ohm speakers like the Theos to loud levels.

They recommended not bi-amping. So I tried it without bi-amping but the NAD still switched off when playing loud movies. NAD tech support will call me back, as they are curious why this happens.

Despite the switching off problem, the sound quality still impresses.

My adventure with multi-channel continues...

[Northy]

With all due respect edwinr, I am not sure why you are so obssessed to equal the performace of the Bryston to that of the Nad. They are two different beasts. As I said before, NAD puts out very fine products but they are not in the same league as the Bryston's amps. You should realize how impressive the power reserve section of the Bryston is with its huge torodial tansformer with lots of microfarad capacitance. The fact that the Bryston doubles its wattage at 4 ohms (ie. doubles its current) is a testimont of outstanding power delivery.

In closing, as you said it yourself, you should put the Bryston in charge of tightly controlling the Thoes and let the NAD be responsible for the centre and surrond speakers. You will have a much better sounding system.

[Rich]

I spoke with NAD technical support. They said the T757 should be capable of driving 4 ohm speakers like the Theos to loud levels.

The Theos is not just a 4 ohm speaker. It is an electrostatic speaker, and as such, the impedance varies with the frequency. It dips to less than 1 ohm at the higher frequencies. This can be death to lesser amplifiers. If your amp doesn't double its current output with every halving of impedance, then it may very well not be able to handle electrostatic loads, especially with material that has a lot of dynamic peaks played at higher volumes. Did you inform the folks at NAD that this was an electrostatic speaker?

[MrBT]

Edwinr,
Just a word of caution: keep in mind next time when you venture into world of 110 db that you have 8" mid-bass drivers. Panels can take a lot of abuse but cone drivers can not. Know it from my personal experience because I had to change my Vista drivers three times in the past. Probably Theos and Vista share similar components. Also like Rich said stable amplifier at lower impedances is a must with electrostatics and I'm very sceptical with any kind of receivers(they have to pack a lot of electronics into one small package, which means compromise). After a years of experimenting with amplifiers and ML electrostatics I finally settled down with my small behemots 750w/8ohm.

[edwinr]

Thanks everyone for your input. I've never had multi-channel before, so this is all new to me. After doing more listening and research, I think the 'problem' lies with the fantastic dynamics offered by the new HD sound tracks on blu-ray. I've noted that the NAD defaults to a higher cross-over frequency than I would normally select for the main speakers. In the setup menu I originally choose to run the Theos full range with my REL sub taking their high level signal from the same speaker terminals - this is my preferred option when listening to music. When asked to reproduce explosions and such like with blu-ray movies, I think my decision in running the Theos full range is taxing the NAD receiver beyond it's capabilities.

I also take into consideration your warnings about over-driving the 8 inch woofer. So what I tried was setting the Theos to 'small' in the speaker (I think on this setting the NAD starts rolling off the Theos from 40 Hz and down). The sub now reproduces all frequencies below 40 Hz. This has made a huge difference. All of a sudden the NAD is reproducing the blu-ray soundtracks without switching off, and probably more importantly the Theos woofers are protected from very loud non-musical signals.

I'm not saying I will not upgrade the NAD to a higher powered unit, nor have I discounted the idea of a more powerful sub, but it's important I get this right so that any upgrade I make is done for the right reasons. I've been toying with the idea of buying the Montis - but this would be a last resort. The Theos is the first speaker I have ever owned that I actually like.

NAD were informed about matching the Theos and FX2 speakers with the T757 - in fact a technical support person said the match should be 'workable'. NAD have been very helpful and are assisting me with my technical queries.