Spectral amp and VTL preamp


Hi guys, after some days spent with the Soulution 721, the Spectral DMC30SS Series 2 has returned to my system. The CD player is a Soulution 540, the power amp is a Spectral DMA260 (MIT Magnum MA speaker cables and Cardas Golden Reference interconnects). I’ve also tried a VTL 7.5 Series III and it was the sound I liked the most between these three preamps. Each of them has its strengths but I think that the VTL has a little more musical elegance. The (maybe silly) question is: are there some risks in using the VTL with the DMA260? I read dozens of times that Spectral power amps shouldn’t be used with other than Spectral preamps but I also read dozens of times that someone had very good results using a Spectral power amp with a non-Spectral preamp. Apart from the output impedance of the VTL (25 Ohms, max 150 Ohm at 10Hz) and the input impedance of the Spectral (10 KOhms), are there other things that should be considered? I don’t want to take any chances. Is there something else to consider? Some risks about potential DC-offset? Spectral amps should be DC coupled... and the VTL? I think that both of them should have some kind of protection against DC but I don’t know too much about this topic so I’m asking to you...
Thanks a lot in advance.
katfnc
Where are you Al? Are there electrical compatibility issues?
I’m here, John (Roxy54), thanks :-)

Katfnc, after looking at the specs and descriptions of your Spectral amp and the Spectral and VTL preamps I don’t see any technical issues that would arise with the VTL preamp/Spectral amp combination.

Regarding DC offset, I see that the amp incorporates a "DC protection servo" having a "0.5 volt range." Assuming (as I do) that the 0.5 volt figure pertains to the output of the amp, that seems to me to be adequate protection against DC that may originate anywhere in the system.

The gain of both preamps, assuming you are using balanced interconnections at their inputs and outputs, is somewhat higher than I would generally prefer to see when a digital source is being used. Those numbers being 19.2 db for the VTL and an option of either 15 or 21 db for the Spectral. But if that were an issue in your application (which depends on speaker efficiency among other factors) you would already have found that you have to use volume control settings that are undesirably low.

Regarding impedance compatibility, I don’t see any problems.

Good luck. Regards,
-- Al

Al, thanks for your full and comprehensive answer! This is exactly what I was hoping for ‘cause I like a lot how the VTL sounds with the Spectral. 
Thanks also to John (roxy54) for inviting Al to answer my questions! :-))
@Katfnc

Thanks for the nice response.

Thinking about it a little further, though, I’m not completely certain as to the correct interpretation of Spectral’s reference to a "0.5 volt range" of the "DC protection servo." And a good deal of further research I did failed to shed any light on its intended meaning.

One interpretation, which was the basis of my previous response, is that the design incorporates a protection mechanism that will limit any DC that may appear at the output of the amplifier to a maximum of 0.5 volts. That would be fine.

Another possible interpretation, though, is that the "protection" will only be effective for DC voltages at the amp’s output that would be no greater than 0.5 volts in the absence of the so-called protection. Under that interpretation the "protection" would essentially amount to a performance enhancer, as I see it, as opposed to providing effective protection against secondary damage that might result from major failures.

It might be worth contacting Spectral and asking them to clarify exactly what that spec means.

BTW, that same spec and wording appears in the description of many of their amplifiers, not just your DMA-260.

Best regards,
-- Al
Thanks Al, as always, and good luck katfnc.
@almarg Thanks for your further clarification. Does Spectral has an email address? I live in Italy and I’d prefer to contact them by email ‘cause it’s much more easy for me to write than to have an intercontinental phone call... also because I’m sure that they could better understand me (and I could better understand them!)
:-)) 
@almarg Another thing Al... I think that the crux of the matter is: has the Spectral DMC-30SS Series 2 some kind of protection that the VTL hasn’t? This because the power amp would be the same (DMA-260) and my doubt is if the use of the VTL could increase the risks of malfunction... So if with the VTL the risks are the same (or even less for some reason), keep the Spectral preamp won’t give me a safier situation...
Katfnc, you make a very good point in your post just above. And in looking further at the descriptions of the two preamps it appears to me that you would be at least as safe with the VTL as you are with the Spectral, regardless of how effective or ineffective the DC protection mechanism in the amplifier may be.

The VTL design includes coupling capacitors at its outputs, even though its output stage is apparently solid state, as well as coupling capacitors in its internal signal path for the tube stages. Those will block any DC that may originate in the preamp or the CD player (or in any other source component).

In addition to providing protection against DC, the Spectral preamp is described as providing protection against oscillation. But I would feel certain that the VTL requires no such protection, due to its much lower and more conventional bandwidth, and the fact that it uses zero global feedback and very minimal (less than 2 db) local feedback. The Spectral’s description does mention the use of feedback (although not quantitatively), and of course as is typical of Spectral designs it has ultra-wide bandwidth. So the reason it provides protection against the possibility of oscillation is that it needs it.

Regarding Spectral’s email address, it’s strange that their website provides neither that or a means of contacting them directly via the site. But I found elsewhere that their email address is, or at least has been, spectral@prodigy.net.

Best regards,
-- Al
Al, I couldn’t have hoped for a better and comprehensive response! You have been very clear and kind, thanks a lot!!
And thanks also for the Spectral email address. I’ll write to them to ask about the “0.5 volt range of the DC protection servo” in the power amp. 
If one day you’ll make a trip to Italy, you’ll find a cold beer waiting for you! :-))
In my system I’ve Audio Research Reference 3 pre-amp with Spectral Dma 360 s2 monoblocks power amplifiers  ( Matrix HD 38 signal cable, Mit  Oracle Matrix  HD 90 power cable  (the rest of the system includes CD Player Spectral Sdr 4000 Pro, turntable   Linn Lp12 Klimax and JM Lab Utopia speakers first
 I haven't encountered any incompatibility issues (and it sounds much better than the Spectral preamp!)

I spent about two years with the VTL 7.5 Series III along with the Spectral DMA 260 but things substantially improved when I decided to replace the Spectral with a Boulder 1060.

The Soulution 540 has been replaced by a dCS Rossini player and some months ago I also changed the VTL preamp with a Boulder 2010.

I must say that the least noticeable difference was that due to the change of preamp but I’m very happy with my current system.