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Basic Measurements of Spectral DMA 260

Frank Dernie

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#21
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.
I used a Spectral preamp and power amp on Sonus Faber Extremas here in the UK and when I wanted to move my active Apogee Divas back here when I left France in 1995 I wanted to use 2 Spectrals instead of the massively electricity guzzling Krell KSA 200Bs I used there. I wrote to Spectral about driving ribbons but just got a rude reply saying they disapproved of active speakers so rather than buying another Spectral I sold the lot and bought a Goldmund preamp, monoblocks and speakers. Still got them.
 

Blumlein 88

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#22
I used a Spectral preamp and power amp on Sonus Faber Extremas here in the UK and when I wanted to move my active Apogee Divas back here when I left France in 1995 I wanted to use 2 Spectrals instead of the massively electricity guzzling Krell KSA 200Bs I used there. I wrote to Spectral about driving ribbons but just got a rude reply saying they disapproved of active speakers so rather than buying another Spectral I sold the lot and bought a Goldmund preamp, monoblocks and speakers. Still got them.
Don't know which Goldmund you have. I repaired one of the Mimesis models from around that time once. One of the middle inputs had become noisy and if left on a few hours quit altogether. Goldmund had each input using its own TO92 transistor. They were using a high bandwidth capable version. The noisy input had its TO92 just beneath a ribbon cable. They were running something like 400 milliwatts on this TO92 rated for 500 milliwatts. I thought that a poor design decision. I'd like to see 50% or less of rated power on such devices. With restricted ventilation that one had cooked. Fortunately the maker of that little transistor had come out with an exact replacement only with 1000 milliwatts dissipation rating. It was a slightly odd bird and I had to order it at the local electronics supply house.

I explained all this to the owner after I fixed it. Told him about having to special order and that the upgraded part I thought was the smart move, but this part cost a little more than twice the original part. I told him this in a serious tone, and he was looking like, "oh my, how much is this costing me". Then I told him the new part was nearly $2. He was much relieved.
 
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#23
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.
This is from one of Spectral's dealers:

"Note: Whereas other Spectral Audio components (i.e. their preamplifiers and digital source components) may be used with any other brand of equipment, Spectral mandates that their non-Universal amplifiers be driven by a Spectral preamplifier to ensure reliable operation. Spectral Universal amplifiers however may be driven directly by any brand of preamp, D-A, or surround sound processor. In addition, Spectral amplifiers need to be used in conjunction with the proper Spectral-approved MIT interconnects and speaker cables. These usage guidelines are now a condition of Spectral's warranty on their amplifiers. There are several reasons for these guidelines:

  1. Because of their wide bandwidth, Spectral amplifiers can amplify high frequency signals in the MHz range at massive power levels. Therefore radio frequency noise must be kept out of the amplifier. The MIT interconnects contain RF protection, and Spectral preamplifiers are stable at these high frequencies.
  2. Unlike most all solid state preamps, Spectral preamplifiers are true discrete designs, are DC coupled, and can output the high current that the (non-Universal) Spectral power amps are designed to be driven by. Other brands of preamplifiers often use integrated circuits or other devices which may not be able to drive the amplifier properly. It is interesting to know that IC's (integrated circuits) simply cannot provide for high current output in a preamp design which utilizes them. Whereas a fully discrete Spectral preamp—fully discrete meaning no IC's in the signal path—can provide on the order of 1 ampere of output current per channel!
  3. As part of their stability design, Spectral amplifiers are designed to be used with the type of output network found in the Spectral-approved MIT speaker cables. (You might be interested to know Spectral has found that by placing this network close to the load, i.e. the speaker, they can achieve superior results.)

What will happen if you don't follow these guidelines? Well, it's a probability thing. You may not encounter any problems—or you may encounter premature failure of some components in the amplifier. Or (worst case) your amplifier may suffer severe RF problems, and you will have to pay to have all the output devices replaced (which is expensive). Therefore, we highly recommend that you follow Spectral's operational and warranty guidelines for both peace of mind as well as highest performance and fidelity."

When I owned Spectral gear I remember the warning about using anything other than the approved MIT cable could void warranty in the manual. The dealer also made that point clear. My takeaway: use the MIT cables, if not, any damage that may be caused by not following Spectral's guidelines was on me.
 

Blumlein 88

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#24
This is from one of Spectral's dealers:

"Note: Whereas other Spectral Audio components (i.e. their preamplifiers and digital source components) may be used with any other brand of equipment, Spectral mandates that their non-Universal amplifiers be driven by a Spectral preamplifier to ensure reliable operation. Spectral Universal amplifiers however may be driven directly by any brand of preamp, D-A, or surround sound processor. In addition, Spectral amplifiers need to be used in conjunction with the proper Spectral-approved MIT interconnects and speaker cables. These usage guidelines are now a condition of Spectral's warranty on their amplifiers. There are several reasons for these guidelines:

  1. Because of their wide bandwidth, Spectral amplifiers can amplify high frequency signals in the MHz range at massive power levels. Therefore radio frequency noise must be kept out of the amplifier. The MIT interconnects contain RF protection, and Spectral preamplifiers are stable at these high frequencies.
  2. Unlike most all solid state preamps, Spectral preamplifiers are true discrete designs, are DC coupled, and can output the high current that the (non-Universal) Spectral power amps are designed to be driven by. Other brands of preamplifiers often use integrated circuits or other devices which may not be able to drive the amplifier properly. It is interesting to know that IC's (integrated circuits) simply cannot provide for high current output in a preamp design which utilizes them. Whereas a fully discrete Spectral preamp—fully discrete meaning no IC's in the signal path—can provide on the order of 1 ampere of output current per channel!
  3. As part of their stability design, Spectral amplifiers are designed to be used with the type of output network found in the Spectral-approved MIT speaker cables. (You might be interested to know Spectral has found that by placing this network close to the load, i.e. the speaker, they can achieve superior results.)

What will happen if you don't follow these guidelines? Well, it's a probability thing. You may not encounter any problems—or you may encounter premature failure of some components in the amplifier. Or (worst case) your amplifier may suffer severe RF problems, and you will have to pay to have all the output devices replaced (which is expensive). Therefore, we highly recommend that you follow Spectral's operational and warranty guidelines for both peace of mind as well as highest performance and fidelity."

When I owned Spectral gear I remember the warning about using anything other than the approved MIT cable could void warranty in the manual. The dealer also made that point clear. My takeaway: use the MIT cables, if not, any damage that may be caused by not following Spectral's guidelines was on me.
Well they weren't quite that adamant about it back some years. Seems a waste to have 2 mhz bandwidth and then use a low bandwidth cable all the way. I understand some of why they say they prefer this (beyond just warranty safety), but it is a dumb idea in my opinion.
 

Frank Dernie

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#25
Don't know which Goldmund you have. I repaired one of the Mimesis models from around that time once.
Goldmund called all their amplifier components Mimesis plus a number for a number of years. When I first came upon them the top stereo amp was a Mimesis 9 top mono amp the Mimesis 9.4 and top preamp a Mimesis 2.
The UK importer stopped selling them, rumour had it that Krell, a much bigger brand he also imported, didn't like the competition.
When I was living in France I bought the Golmund Europe distributors demo Goldmund Reference turntable when they discontinued it and got to know him a bit.
Quite some years later, having used Krell and Spectral electronics for years, I came across the Goldmund electronics and speakers at his place when I went to listen to the, much hyped, Wilson Audio WAMM which he was selling used for a client. I was slightly underwhelmed by them and took the opportunity to listen to his demo Goldmund speakers that day. I was impressed and eventually bought some, around that time Goldmund bought Job to obtain the intellectual property of their amp circuit. As I understand it they brought out a new range of amplifiers using this circuit and preamps by adding a 2 after the Mimesis name, so the new top preamp was the Mimesis 22 and the top stereo amp the 29. These were complete redesigns with a much more elaborate and bigger case as well as the new circuit. These products had several updates and I believe the latest version of the Mimesis 22 is still their top analogue preamp but they really went digital active a very long time ago and their analogue amps are a bit of an outlier.
They changed the amp circuit again a few years later and changed the name of the power amps from Mimesis to Telos. They also have a slightly less expensive range called Metis with simpler case and power supply and the mail order only Job range in a still simpler less expensive case and power supply.
Since they are not easy to sell I still have the Mimesis 22 and a pair of Mimesis 29.4 monos, and the Mimesis Ph2 phono stage. The amps weigh 75kg each so I can barely move them.
 

trl

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#27
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.
[...]
Extremely low M1 and M2 harmonics, to bad the hum noise is around -90dB. Does the 50/100/150Hz hum becomes audible, like here, with high volume (between tracks or no music on inputs)?


Picture from manufacturer's website. Notice the perfect layout arrangement, ground, star ground on the audio inputs, huge caps, clean and airy PCB layout. I totally love the way it looks inside.

Usually, mains hum is induced from the transformer or PSU.

However, worth reading one paragraph from http://www.spectralaudio.com/dma-260.htm, although I've no idea where the transformer is located, so I guess it's SMPS inside...or not? Here's a similar amp: http://www.spectralaudio.com/DMA300/DMA300 Top Open.gif.

"THE POWER VAULT

The Focused Power output section of the DMA-260 is only one of the many Spectral innovations designed to create the ultimate in amplifier control and resolution. A unique transformer architecture supports the critical requirements for lowest noise and high power capability of the powered output section array.
In traditional high-end power amplifiers, output transformers are located away from active amplifier circuitry for purposes of noise isolation and construction ease. This virtually universal method extracts a significant sonic price, however, because of the speed and regulation losses which are imposed by the long power supply wiring harness. In addition, the EMI and noise which radiate from the supply harness invariably contaminate sensitive low level circuitry by noise modulating the musical signal. Spectral engineers have long recognized the problems associated with power supply noise contamination and have proposed a unique solution in the DMA-260 High Resolution Amplifier. The Power Vault electromagnetic field containment system virtually eliminates EMI radiation to the amplifier signal path circuitry as well as power and regulation losses through the traditional power supply harness.
The architecture starts with Spectral's elaborate high performance custom power supply transformers. These unique designs are the result of years of research and development to maximize isolation and regulation. Each transformer features individually isolated windings to support each individual output transistor in the amplifier output section, a premium technique shared only with the Spectral DMA-360 Monaural Amplifier. The twin power transformers are mounted between massive machined mounting plates for damping and resonance control. The combined 38 lb. transformer structure is then mounted on a constrained rubber floating suspension system employing multiple compliances similar to a fine turntable suspension. The finished transformer assembly is then suspended inside an EMI, RFI tight welded aluminum containment box located directly beneath the DMA-260 output section. The resulting transformer Power Vault containment system results in an uncompromising solution to the universal power supply noise problem. The high-current power transformers are now intimately sited next to their own high-current output devices with extraordinary noise isolation. The high-speed driver circuitry of the DMA-260 operates in a pristine environment free of the noise radiation and phantom signals which plague other power amplifiers. Dynamic range limitations are removed with a sonic clarity and quietude which is palpable."
 
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