To monoblock or not to monoblock McIntosh MC275's

@g_nakamoto, for a given load impedance the ratio of two power levels is expressed in db by the following formula:

db = 10 x log(P1/P2)

where "log" is the base-10 logarithm.

From that it can be calculated that a 3 db increase corresponds to a doubling (not tripling) of power. Or more precisely, a doubling of power corresponds to an increase of 3.0103 db, which is usually rounded off to 3 db.

And a 3 db increase in the amount of power delivered to a speaker will result in a 3 db increase in the volume that is produced by that speaker, at a given distance, assuming that the speaker is not being driven so hard that "thermal compression" or other factors cause its behavior to become significantly non-linear.

Mr. Hirsch might also have referred at times to a commonly stated rule of thumb guideline that a **subjective** perception of "twice as loud" requires ten times as much power, which is an increase of 10 db.

I have no idea why or if he might have referred to a tripling of power.

Regards,
-- Al

Effischer, thanks very much for the nice words, informative references, and good comments.  Interestingly, I took the "Weber's Law" test four times, using four different settings of the volume control on the computer.  On the low volume trials within each test my answers were MUCH better (averaging 85% correct) than on the high volume trials (averaging 30% correct).  So between the two conflicting references that are cited at the second link you provided, my results would indicate that the Weber's Law reference is the correct one.

Thanks again.  Best regards,
-- Al
 

Thanks very much for the nice words, Cleeds. Regarding ...

It does sound like the two channels of a stereo amplifier must be very closely matched for parallel operation to work well. Because we’re talking about tube amps, I wonder how practical that really is.

... I suspect that in the particular case of the MC275 a factor that helps it avoid or minimize any sonic degradation when operated in parallel mono mode is that like many McIntosh amps it apparently uses a substantial amount of feedback. One indication of its liberal use of feedback being its specified damping factor (">22" for the Mk VI version), which is quite high for a tube amp.

In general, feedback will reduce the degree to which the behavior of an amplification channel is affected by variations in tube parameters, or variations in other circuit elements for that matter.

Best regards,
-- Al

Cleeds, the vintage Dynaco ST-70 is another amplifier that I recall has provisions for operation with the two channels paralleled. I’d imagine there are others, among tube amps that have output transformers.

It is of course physically possible to parallel the channels of any tube amp having an output transformer, by jumpering the appropriate output taps together and applying the same input to both channels via a y-adapter. I would not want to do that without a specific indication from the manufacturer that it is ok, however. One reason being that if a tube or something else were to cause a failure in one channel while the amp is operating the other channel would be attempting to put out a full power signal while that channel would be attempting to force their paralleled outputs to zero volts. Which of course may be unhealthy unless that possibility is specifically addressed in the design. And for that reason among others paralleling the channels of a solid state amp would be out of the question in nearly all cases, since the near zero output impedance of nearly all solid state amps would not provide any limiting of the resulting current flow. On the other hand, though, solid state amps can of course be designed such that they can be bridged.

As Tomcy6 noted earlier in the thread, paralleling the channels results in the load impedance that is nominally optimal for each output tap being cut in half, relative to stereo operation. The MC275 and ST-70 provide 16 ohm taps, so using those taps in parallel would presumably result in an optimal match for an 8 ohm speaker. But if an amp only provides 4 and 8 ohm taps, parallel operation would in effect only provide 2 and 4 ohm taps.

As can be seen in the specs for the MC275 parallel operation will double an amp’s rated power capability. While if a bridged amp is designed with sufficient robustness, in terms of its current capability and thermal management, it can potentially/theoretically result in the rated power capability increasing by as much as a factor of 4, since the output voltage provided across a load by a bridged amp would be doubled (relative to stereo operation) if the correspondingly increased current can be provided and sustained. (For a given load resistance power is proportional to the square of voltage, as you no doubt realize). As a practical matter, though, the increase in the power capability of an amp operated in bridged mode often falls significantly short of that factor of 4.

With respect to the sound quality of an amp operated with the channels paralleled, I’d imagine that a significant factor would be how closely the characteristics of the two channels match. Although I have no particular feel for how critical differences between channels that are typically encountered would tend to be.

Best regards,
-- Al

As a point of information, when the MC275 is operated in mono mode it is not bridged. The two channels are paralleled instead, which is different than bridging. And while I have no knowledge of how its sonics may compare between stereo mode and mono mode, the sonic downside that is commonly associated with bridged operation of an amp is not applicable to it.

Also, I believe that the human ear can potentially discern volume differences on the order of 1 db. I have seen it said by various reviewers over the years that to assure an A/B comparison between two components is valid levels should be matched to a small fraction of a db. And stepped volume controls on well designed preamps typically provide resolutions of considerably less than 3 db over at least most of their range.

Regards,
-- Al