Basic Measurements of Spectral DMA 260

[Olli]

Hi, Here are some basic measurements of a Spectral 260 power amplifier that I bought as a pre-owned unit approximately two years ago and had it checked by a local electronics expert who used an AP analyser from what I can see.

 

[Panelhead]

I do not understand the 1Khz graphs. Assume the fundamental notched out. What was output? Also, the AC line interference is very high. How old is the unit? The filter caps could have high impedance.
This is a graph of my amp. I do not have the test equipment to verify as I has evolved. I expect it is improved, but is so low that it is hard to verify.
The amp is low power -40 watts. It is played at very low power output, around 100 mW. The 5 watt test would be around 110 dB acoustic output.

 

[Olli]

The amp is s 2012 model. Unfortunately I don’t have annymore details on the measurements than what I‘ve posted.

 

[DonH56]

Looks like THD was measured at 10 W, though the "1" looks like a "7" to me. The table above (item 3) clearly says THD+N at 10 W. It is normal to notch the fundamental to keep from overloading the analyzer's front end when doing distortion measurements like this. I agree that is a lot of power supply ripple (50 Hz line frequency, then 100 Hz and all the harmonics from the rectifiers in the supply). Looks like SINAD is set by the power noise rather than harmonic distortion.

 

[Olli]

So you think it would make sense to try another power supply?

 

[DonH56]

So you think it would make sense to try another power supply?

Ask Spectral and/or the tech if the power supply ripple is normal. If not, then the tech should be able to replace old filter (decoupling) capacitors -- that is the most likely cause as @Panelhead said.

HTH - Don

 

[Panelhead]

Usually a six or seven year old amp does not have dried out filter caps. Also, Spectral is a measurement first company. Wide bandwidth, low noise, fast settling times, and high current output.
It would be interesting to see the FFT without the notch filter. It may be this was run at 200 watts or some other high power level.

 

[Blumlein 88]

http://www.spectralaudio.com/DMA300/300Spec.htm

Specs on a similar Spectral. They show THD of .009% and typical of .005% at 225 watts into 8 ohms DC to 100 khz. So the above results on the DMA 260 don't look unbelievable. SNR which is max power vs idle I believe is listed as 120 db.

I had a DMA 50 at one time. Lower power, and higher distortion than listed above. It was the best amp to use in a series amplifier listening test. You simply couldn't hear it in or out of the circuit vs wire. It was listed as 98 db unweighted SNR. I attributed the transparency to low enough noise and the supremely flat frequency response which was said to be -.1db at 100 khz. One of the reasons I think SINAD is being over-emphasized and doesn't help us much about which gear is transparent enough.

 

[Panelhead]

I do not speak German, but it appears the SNR is 86 dB maybe weighted and 75 unweighted referenced to one watt. The measured power output is listed as at 193 watts at 1%. The spec sheet shows 225 watts into 8 ohms at 0.015%.
Maybe this is normal. I have listened to Spectral in others setup and found the amps, preamps, disc players, and cabling to all be top notch. Never owned any, was into tubes at the time and the Spectral gear was too uncolored for my likes.
Today I would love to have a Spectral preamp and amplifier.

 

[Swtoby]

Spectral only warranties their amps provided you also use expensive MIT speaker cables which Spectral considers as part of a system. Spectral claims they are required because filtering in the cable prevents the amp from oscillating. That may effect measurements.

 

[Swtoby]

Also, Spectral is a measurement first company. Wide bandwidth, low noise, fast settling times, and high current output.

Maybe. For me, Spectral is a boutique, faith-based firm.

 

[Blumlein 88]

Maybe. For me, Spectral is a boutique, faith-based firm.

They are and they aren't.

Spectral's are supremely good amplifiers. They may be expensive, and there is quite a dollop of boutique BS odor about them. But they perform well and you'll not hear one sound awful unless the source or speakers are awful. Seriously good gear, and I've never known of any product they make not to be excellent. Ditto for any of their preamps, and the CD player they made.

BTW, I don't think warranty is void without MIT cabling. Spectral predates MIT actually. It is a good idea in a sense. Lots of modern digital gear has some idle tones or high frequency noise. Spectrals having a couple megahertz bandwidth will happily amplify all that. So a cable that gently rolls off the ultrasonics isn't a bad idea.

 

[Olli]

I also liked the sound of the Spectral Amp very much so far. But is highly senible to heat issues, at the moment it produces a high pitched sound on the left channel when getting warm. My local dealers can reproduce the issue but can‘t find the cause. Does anyone know where to get a detailed circuit diagram?

 

[Sir Sanders Zingmore]

Spectrals having a couple megahertz bandwidth will happily amplify all that. So a cable that gently rolls off the ultrasonics isn't a bad idea.

According to this review on their website the issue is about sending the amp into oscillation. That’s quite different to the amp “happily” amplifying

 

[FrantzM]

Hi

I also like Spectral or perhaps, used to ... I'd like to see how these measurements compared to those of a DeVialet...

 

[Panelhead]

The bandwidth is why they recommend the high capacitance MIT cables. This limits high frequency response. At least with input cables.
The high capacitance speaker cables may be needed for another reason. The capacitance of early MIT cables could cause Naim amplifiers of that vintage to go into destructive occillation. The result was output device failure.
Suspect it was phase margin related, the Naim amps were not ultra high bandwidth.
The high pitch noise indicates a problem. You could hook it up to a scope to see what is happening. If you can hear it this is not ultra sonic ringing. The ultra sonic occillation usually causes the amp to heat up, but is not audible.

 

[Blumlein 88]

According to this review on their website the issue is about sending the amp into oscillation. That’s quite different to the amp “happily” amplifying

Yes it is different. Oscillating is one situation, and amplifying HF junk is another.

From that review:
The expert witnesses we consulted (outside of Spectral) were in sharp disagreement over the likelihood of disaster with a set of speaker cables like the Nordost.

They then proceeded to listen with the low capacitance Nordost cable and report not a single problem of stability. Which mirrors my own experience. I can understand them not wanting to warranty odd events where oscillation destroys their amplifier. But the amplifiers aren't that picky. I was even using them with a highly capacitive load in using ESL speakers. They also will happily amplify DC so any offsets need to be attended to in some cases.

So one should exercise some caution, but that isn't the same as saying don't connect anything other than MIT cabling.

 

[Blumlein 88]

I also liked the sound of the Spectral Amp very much so far. But is highly senible to heat issues, at the moment it produces a high pitched sound on the left channel when getting warm. My local dealers can reproduce the issue but can‘t find the cause. Does anyone know where to get a detailed circuit diagram?

Something is remiss then. Spectrals aren't sensitive to heat like that.

I would check for DC on the output (they'll amplify DC offsets from the input). I'd also put a clamp on amp meter on the power cord. See if there is some oscillation causing it to draw way too much current.

Also just basic trouble shooting if you swap everything on the channels does the problem stay in the left channel?

Panelhead's suggestion to hook up an oscope is a very good suggestion to just to see what is going on there.

Another goofy idea. Since the sound is audible you might try one of those RTA apps on a phone and see what frequency it is. Then see if swapping input or output cabling to something longer or shorter alters that frequency.

 

[HammerSandwich]

It was the best amp to use in a series amplifier listening test. You simply couldn't hear it in or out of the circuit vs wire.

I understand how to do a bypass test with preamp, but what's the method for a power amplifier?

 

[Blumlein 88]

I understand how to do a bypass test with preamp, but what's the method for a power amplifier?

You load the output with power resistors. Connect with a couple of voltage divider resistors so the output thru these dividers is the same as input. In the case of the Spectral I think it was 20 db of gain. So the dividers reduce amplifier output by a factor of 10x. So one volt in would get one volt out. I at one time had a matrix of switched resistors so I could quickly change this precisely with a few switches.

I had a pair of interconnects made to connect to these and feed another power amp. So you could use your usual amplifier and put another one between source and power amp. You could compare the effect of two different amps or an amp vs wire. Though unknown to me when I first did this the Swedish AES uses the same kind of a test. They've gone further and load the amplifier with a simulated loudspeaker load instead of just resistance. They also do sighted and then blind comparisons this way. I don't know the latest results, but at one time not long ago they'd only had either two or three amps pass the sighted test, and one (Bryston) pass the blind test as being transparent. They never tested a Spectral to my knowledge.

This was the most revealing listening test method for amps I've done or know about. The Spectral had no sound I could detect (mine was sighted only). Most amps had a little coloration. Some had lots. I used Quad ESL 63s at the time I've done these tests. Its been awhile since doing any more recent ones.

One of the most interesting things to come out of it to me were the sound of triode amps. They seemed to be more 3D, more airy, more real than say the Spectral. And tube lovers of course believed the Spectral incapable of making this sound. I could use the Spectrals on the speakers, and place a triode tube amp between source and Spectral. The result sounded just like the triode amp. Indicating it was all a coloration. When I initially did this my expectations were that inserting the Spectral would kill those extra qualities of the triode. I was trying to determine whether an SS amp lost 50% of the resolution or 75% or maybe only 20%. Turns out nothing was lost and all that extra "resolution" was actually an additive coloration. With the added coloration at the input the Spectral could create exactly the same sound.

People who like SET's or other low powered tube amps could use this approach to get the sound they like and feed it into beefy amps so that they aren't restricted to efficient speakers. I've had a hard time convincing them the qualities they like are colorations that a good amp can faithfully reproduce.