Opinions concerning monoblock amps vs stereo

As title states, for those who have heard set-ups with a stereo amplifier vs monoblock amplifiers, what were your opinions?  Were there any discernible differences in sound staging or instrument separation?  Was the sound quality any different?  I imagine some here have had the opportunity to hear an st-70 or st-120 and also Mark 2, 3, 4 or M-125 in the same system.  Possibly you have experience with other brand stereo vs monoblock amps?  Please share your experiences/impressions?  Are the differences subtle or very obvious?  Thanks!

Having had both a pair of M-125s and a VTA ST-120, I couldn't hear a difference. The ST-120 is essentially a dual-mono amp in a single box, the only shared component being the power supply, through which no audio signal passes. Speaker efficiency dictates the horsepower you need. Above 90 dB, the ST-120 should do just fine. Below that, consider the -125s.

Interesting question. I keep two HK amps, the citation 16, and the Citation 19, both more-or-less Brute-Force amps, with the 16 being considerably more brutal than the 19. Both of these are two mono-block amps on the same chassis, sharing only the on/off switch. What does not happen with these amps is any sort of sagging power-supply when driven to their edge. Given that I also keep Maggie speakers, the 16 is challenged at substantial volumes quite often and performs very well.

That would be the only reason to keep separate mono amps, if one regularly runs at high volumes whereupon a sagging power-supply might be an issue. With the VTA products, the power-transformers are at the very least adequate. As Deepee states - purchase based on horsepower, not on separate power-supplies.

Answering only from a tube Dynaco view point. I have all of the amps rebuilt in stock form except for a pair of MK VIs. They all sound slightly differently but I attribute that to the differences in transformers, tubes and slight variations in the actual audio circuit.

Where I have noticed a difference that you may be looking for is in channel to channel balance or tone. The mono amps will be a little off sometimes. It usually shows me I need to go track down a problem. Tracking down problems is easy with the nice layout and easy access. My monos have slightly different B+ voltages between them. Not enough to make an audible difference but it concerned me when I noticed it. I have kept an eye on it but I do not consider that a problem.

Newer VTA (or other manufacturers) may not have these differences. My old Dynaco amps do.

"That would be the only reason to keep separate mono amps, if one regularly runs at high volumes whereupon a sagging power-supply might be an issue."

Actually there may be other reasons:

-Co-location of amplifiers with speakers to facilitate shorter speaker cables
-Co-location of amplifiers with speakers to facilitate shorter IEC power cables
-Allows draw from two mains breakers
-(If) one exotic power cable carrying ICC good, two exotic power cables carrying ICC/2 better (with apologies to E. A. Blair)
-Thermal isolation
-Mechanical isolation
-Electrical isolation

I can think of some potential down sides to pairs of mono amps for stereo playback:

-L vs R non-linearity exacerbated by individual supplies
-Increased interconnect length
-Manufacturing cost driven compromises due to redundant chassis and power supplies
-Physical space requirements
-Reduced dynamic headroom when playing stereo program material

Please note the interpolations.

j beede wrote:"That would be the only reason to keep separate mono amps, if one regularly runs at high volumes whereupon a sagging power-supply might be an issue."

Actually there may be other reasons:

-Co-location of amplifiers with speakers to facilitate shorter speaker cables
-Co-location of amplifiers with speakers to facilitate shorter IEC power cables

Answering both as one - assuming well made cables of adequate gauge to carry the load, differences between lengths of say... 10 feet in speaker cables, and 20 feet in power-cables would be negligible. Keep in mind that in the case of power cables, there is (at least) a few miles (KM) and multiple transformations between the power source and the device using that power.

-Allows draw from two mains breakers

If the two mains breakers are on the same leg, there could be differences in potential between the now-two neutrals and/or the now-two grounds. It only takes a few MV before words are lost and humming begins. If they are on different legs, same as the above + the potential for crosstalk. The most effective rule-of-thumb is that _all_ components on a single system should come off one (1) breaker via one (1) receptacle. And that line should be dedicated.

-(If) one exotic power cable carrying ICC good, two exotic power cables carrying ICC/2 better (with apologies to E. A. Blair)

If exotic cables are necessary in the first place. For the most part they are not. And, for the most part when they are, it is for reasons other than actual sound.

-Thermal isolation

Yes. And Yes.

-Mechanical isolation

Yes. And Yes.

-Electrical isolation

Two-edged sword, see above.

I can think of some potential down sides to pairs of mono amps for stereo playback:

-L vs R non-linearity exacerbated by individual supplies

True, and why it is that God invented the balance-control, and why it is that the amps in question should be as close to identical as is practical. Several makers of pre-amps, back in the day, included careful explanations on how to set up systems feeding monoblock and early tube amps so as to overcome these issues - much more common in the past than in the present. And why is that balance-knobs on items of this vintage are on round shafts with set-screws or splines such that they may be adjusted. The very best such manufacturers got into matching resistors, cable capacitance, and other expedients to reduce such problems. 

-Increased interconnect length

Not relevant as noted above, unless  extreme.

-Manufacturing cost driven compromises due to redundant chassis and power supplies

Yes. However not in the same discussion as exotic interconnects. Going to interconnects of good manufacture and proper gauge/capacity would obviate such differences over the exotics.

-Physical space requirements

Yes. But if one is so trammeled, there are likely other issues.

-Reduced dynamic headroom when playing stereo program material

Please explain? It has been my experience that amps are pretty catholic when accepting input signal.

I have three sets of bi-amp capable speakers, and several sets of identical amps such that I can either run one stereo amp to one speaker, each section, or two stereo amps, or two stereo amps strapped. I have found in _EVERY_ case that headroom, not amplifier multiplicity has won the day. And the same has been the case when not bi-amping. The HK Citation series happen to be two complete systems on one chassis sharing nothing but a power-switch. But, again, headroom has been the effecting factor in all comparisons. Maggies are very difficult loads - as are AR3as. And, of course, both are power-pigs.

There is much happening at the margins - however, if one is actually living in those margins,  it will be _very_difficult to overcome marginal problems with interconnects, cables and power-cords. In the dim-and-distant past, I lived in a house wherein my room had one (1) receptacle, on a line shared by two other receptacles in two other also-occupied rooms. That is a marginal set-up. What most of us experience these days in our houses with dedicated circuits with over-sized wiring on a direct home-run to the panel from a hospital-grade receptacle is not marginal.

I have a bad back so monoblocks are my preference.

I have heard both, the ST120 stereo tube amplifier and the m125's. IMO there is NO discernible differences in sound. Soundstage for me depends on ones room size and the identifiable instruments are very clear and intentional with both amplifiers and recording nuances are apparent. My choice and b/c I have B&W 803diamond speakers I chose the m125's. A bit more power but certainly nothing crazy. I personally like mono-blocks so that was another reason. I cannot see anyone being disappointed with either amplifier. They are both good performers.
To mention the power factor I orginally had B&W804S speakers with 600 watts per channel into 8 ohms. I was seriously disappointed. MORE does not mean BETTER! After listening to a new found friends' m125's with 20 year old kit form dynaudio speakers I was blow away with the lifelike presence and the clarity. I quickly sold my SS..up for sale the next day after 3months of ownership.
IMO power is overrated. The ONLY thing significant about those 600 watt mono-blocks were that they were loud...nothing else. Certainty there was NO improvement in quality of sound, soundstage, clarity, or simple nuances such as the foot pedal of a piano or the breathing of Diana Krall on the microphone.

Of course this is MO.

[quote  Of course this is MO.[/quote]

And probably a good one!

As far as massive power-amps go (600-watt units being a case in point), all too many of them sacrificed pretty much everything else for that power, and those same ones sound pretty much like glass in a blender as a result.  Phase Linear, Crown and several others come immediately to mind along those lines.

Peter W. wrote: [cut]

I have found in _EVERY_ case that headroom, not amplifier multiplicity has won the day.

So headroom is good... unless it isn't.

j beede wrote:

Peter W. wrote: [cut]

I have found in _EVERY_ case that headroom, not amplifier multiplicity has won the day.

So headroom is good... unless it isn't.

Silly me, I kinda-sorta assume that well-designed equipment is used in the first place in the comparison. That someone may be comparing an HK Citation unit to a Phase Linear 700, or a Crown DC300 would never cross my mind. My mistake.

Peter W. wrote:[quote  Of course this is MO.

And probably a good one!

As far as massive power-amps go (600-watt units being a case in point), all too many of them sacrificed pretty much everything else for that power, and those same ones sound pretty much like glass in a blender as a result.  Phase Linear, Crown and several others come immediately to mind along those lines.[/quote]
The Phase 700 had great specs at full power, but at any rational listening levels distortion levels were dreadful.

Peter W. wrote:

j beede wrote:

Peter W. wrote: [cut]

I have found in _EVERY_ case that headroom, not amplifier multiplicity has won the day.

So headroom is good... unless it isn't.

Silly me, I kinda-sorta assume that well-designed equipment is used in the first place in the comparison. That someone may be comparing an HK Citation unit to a Phase Linear 700, or a Crown DC300 would never cross my mind. My mistake.

+1

I've never characterized the output of a Phase 700 or Crown DC300, I doubt either measures 600W/ch. Those sorts of power dissipation numbers infer Bryston, Krell, or Perreaux hardware.

I have never owned either amplifier, but a quick comparison of the topologies of the Citation 16A and 700B reveals surprising similarities. Both utilize AC coupled differential BJT input stages, Darlington driven parallel NPN pullups and pulldowns, and negative feedback providing some dynamic bias. Both appear to be conventional designs derived from what you might see in a Motorola, RCA, or just about any power transistor databook's reference design section.

The biggest differences I see is 60V versus 100V rails and eight versus ten output devices. If anyone happens to know the VA rating of the 700B PT and the VA ratings of the Citations pair of "compact" PTs that would be interesting to know. I suspect the Citation 16's approach (dual mono) was a cost cutting measure that was shrewdly pitched as a feature.

Maybe the 700B really does sound like glass in a blender. If it does I wouldn't attribute that to bad circuit topology. Maybe the Citation 16A really is an overlooked "gem". If it is I wouldn't attribute that to its bifurcated PTs.

If I owned a Citation 16A I would certainly experiment with coupling its isolated power supplies to see what it sounds like with a bit more headroom. My guess is that Stu Hegeman would approve.