Anybody advise me on Revox A78 MkII amp?

[Andrei]

Hi there!
An acquaintance of mine recentlly offered to sell to me his A78 Mk II amp, which (if it fits) will replace a Sansui AU-D907 amp that performs rather poorly with my speakers (planar speakers Magnepan MMG): the sound is quite unnatural and hushed and quite poor sonically. The technical problem, apparently, is that my current amp has difficulties working with the speakers that are initially of quite low impedance (4 Ohm) and sensitivity (84dB), while during playback the impedance of these speakers drops as low as 0.25Ohm...

My question is: what years were the A78's manufactured and what are their general performance with heavy low impedance loads, such as my speakers? It would be a shame if it performs as poorly as the Sansui amp (which is generally very good with other speakers). Or should I look for some other Revox model that is known to perform well wit low-impedance peakers?

Thank you very much in advance,
Andrei

[klingklang]

Hello Andrei,

i don`t believe that this older Device could drive such a low Loudspeaker Impedance. And even if it could, the question would be how stable?

I only know, from a Rvox Technican, that each B250 Output-Stage, can handle a Current of about 50 Ampere, for a short time. I dont know what he means, with a short time. But that seems to me that each Potput Stage can handle that current electrically, but not thermically. So after a few seconds driving 50 Ampere the output transistors could be damaged due termal overload.

But my recommendation would be: forget the magnepan speakers. Not why they are from magnepan, but they are Dipol Speakers. Those Speakers create a fake sound. It`s because these speakers offers Sound from the Front and the rear. The Sound Information from the Rear appear a little bit later what creates that live Atmosphere. But it`snothing else as a siple Echo due to the distance from the Backsode of the Loudspeaker and the Wall. So the Soundinformation gets to the Ear at many times, like in a club. The Walls offers two things time delay and diffusion of the sound-source. Thats the reason why you have to posotion dipol speakers with a minimum of 1 Meter from the wall behind. The Echo would appear after a too short time and that would cause a diffuse sound instead of that well sounding, fake, echo sound.

[Andrei]

KlingKlang,
Thanks for your comments. I do not think that this load is excessive for any SS amp if it is rated as being able to operate at 4Ohms, as speaker impedance is not stable in any speaker during playback, therefore the engineers that designed the amp would have thought of this. However, the key issue is HOW WELL the amp plays when driving the speaker.

Talking about the planar ribbon speakers: maybe I will follow your advice, but for a different reason: I am just tired of looking for the right amp. I has been about 6 months that I procrastinate in this venture, and haven't found the right amp. Tube amp seem to be working better, but quality tube amps are darn expensive, and, on top of that, only few of them can drive these speakers due to ow sensitivity. I sold my beloved AudioNote P1SE single-ended amp sfter having bought the planars as 10W output was too subtle an output to drive these speakers... I do like the sound, however. It sounds natural and spacious, 'cause if you look at any regular musician, all except tube players emit soundwaves in all directions, therefore they are identical as sound sources to dipole speakers.

Thanks for your comments,
Andrei

[Gerhard2]

Hi Andrei,

this has to be kept as a secret information because it´s not a revox amp.......;-))

Some years ago I fired my Infinity Kappa 9s (the initial version with very low impedance) with a simple Accuphase E205 without any problems. I used this comination for a long period with daily intensive use.

Maybe a Studer A68 can do it as well. Does anybody here around have some experience with it?

Kind regards
Gerhard

[Andrei]

Gerhard,
My use, although daily, can not be called intensive, as I listen mostly to soft and tranquil jazz with some modern incrustations like some Andreas Vollenweider, Gavin Briars and Aarvo Part. Tranquil indeed.

WHAT IS as Studer A68? Oh, by the way, what is considered to be the best Revox amp sonically? And how much should it cost in Germany? I think I can make it travel to my home :-)

Thanks,
Andrei

[Veteran]

Andrei,

I have a quite informative Studer A68 leaflet before me now and I think the data are quite similar to the A78, its successor. So briefly here are the main electrical data:-

Rated pwr output, 30Hz-15kHz, simultaneously modulated channels: 100W @ 8 Ohms, 175W @ 4 Ohms, bridged ie. mono, 350 W @ 8 Ohms.

Max. power output: 150W @ 8, 200W @4 400W @ 8 Ohms bridged (they called it the "Power Output At Treshold of Limiting, at 1% THD; cute, I would say it's a power capacity when the amp trips even when you look at it for a moment).

Freq. Response: 30 Ht to 15 hHz, +0 -0,5 dB, 20-20: +0 -1 dB. Excellent.

Inputs: Balanced and Floating (! this is important, match this) Input impedance: 5 kOhms stereo or thereabouts. Input sensitivity 0 dBu to 17 dBu whereas 0 dBu means 0.775 V.

THD: less than 0,1% up to rated power output; typ. 0,05 pct at 1 kHz). A good value for that time.

S/N Ratio: rms unweighted rating, better than 100 dB up to rated power, very good for the time when manufactured.

Crosstalk atteunation, abt 60 dB.

Damping Factor: better than 75, abt. 250 @ 1 kHz.

Power consumption varies from 100 to 800(hmmm, well, peak) Watts.

The values were taken at + 6 dBu input voltage.

A brief comment as regards the written data, a quality professional amp to be used in a studio setup ie. balanced, floating (twisted pair cable leads are live + live -, ground, the cable shield for the enclosures only) at rated studio voltage level (100% modulation = 0 dBu = 0.775 V). An amplifier without any of it's own "sound" and extremely linear.

I would trust it winken

All the best, Edi Zubovic, Crikvenica

[Andrei]

Edi,
Thanks a lot. One more question, though: what impedance do studio speakers normally have? Is it common for them to have 4Ohm or even less impedance?

The problem with my existing Sansui AU-D907 is that it sounds rather unnaturally hushed with the maggies: very little treble and generally metallic and harsh sound. I use a thoroughly modified CEC2100 as source (having a completely new DAC inside), should sound ok...

Thanks,
Andrei

[Veteran]

Andrei,

Passive studio monitors have 4 to 8 ohms impedance. I doubt they would have less than 4 Ohm. Older ears like one-point sources like vintage Tannoys. Other studio build they own monitors out from choosen drivers. The point is, that these are separately powered (bi - or triamped or even more units).
But the most common and widespread monitor loudspeakers are active ones, consisting of electronic crossover with quite steep cuts and separate amplifiers for each drivers -- but I bet you know this already. Europeans like 4 Ohm units. America and Japan prefer 8 Ohms; it has always been so. Some say 8 Ohm sounds a tad better; I don't know because I prefer an European sound lächeln . An amplifier should match the impedance of the loudspeaker, hence "4 to 8 Ohms".

As to Magnepans; I haven't heard much planar loudspeakers. Once I was surprised by the dynamics of Mangeplanar Tympani. I liked the sound.

As to monitor loudspeakers, generally they should not make any "own" sound so some say that they are boring sometimes. It may well be so but this depends on how good a recording was. I am used to this sound and although I remaster with the active monitors, I use a typical combination of a Technics 909 amplifier and Telefunken mini three-ways (from an old but fine "Telefunken Studio 1 M portable {? = abt 15 kg}) set just for checkout. This combination says somethimes what the big monitors wouldn't especially in terms of distorsion so I can control it better.

I would first try to position the Maggies immediatelly near the wall and inspect the sound with as low reflections as possible. Should this be OK, I'd experiment with the positioning (Klingklang is right as to the dipole loudspeakers). If the sound is harsh, try valves lächeln -- A Studer A78 would serve you very good but it is totally neutral and would require a good preamp with a balanced output. The sound would definitely be different then.

All the best,
Edi Zubovic, Crikvenica, Croatia

[Gerhard2]

Andrei,

you can see an Studer A68 here on the pictures taken during the Revox-Fan-Treffen:

http://www.revox.net/treffen/revox2003/Seite0003.html

Kind regards
Gerhard

[Gerhard2]

Andrei,

it is the third and fourth picture.

[Andrei]

First audition results:
I like the sound of the A78 mkII. It is open and natural, with nice open treble and real-size bass ;-)

Surprisingly how this little Revox outplayed my Sansui amp being two times smaller and at leas three time lighter...

However, if I like the Revox sound in general, maybe I should be looking for a better Revox amp? What are the Revox amps that are considered to be the best among all produced by this company? For instance, there is quite an variety of B750's offered, is it a better amp? Also, what is considered a fair price for the A78 MkII?

Thank in advance for your help,
Andrei