What does an impedance-matching issue "sound" like?

So this also effectively doubles the power going to each speaker - how does it affect impedance and is it safer for the amp and speaker?

Regarding safety, earlier I described a scenario involving failure of a small signal tube that could conceivably result in damage to an amp that has been monostrapped. (Thanks, Ralph, for citing that term). However that scenario obviously has a relatively low likelihood of occurring, and so IMO it would be reasonable to consider both approaches as being safe.

Regarding impedances and sonics, IMO it's probably one of those situations where you have to try it both ways to decide which is preferable. Since paralleling/monostrapping would in effect provide the amp with an output tap optimized for a 3 ohm load, as I had indicated, that would presumably result in the best match in parts of the bass region where both the impedance of the speaker and the content of a lot of music tend to be especially challenging.   But it would result in a worse match at mid and high frequencies, compared to both stereo operation and vertical biamping.

Regards,
-- Al
 

@jsqt, no, what you’ve described in the second paragraph of your post above is a horizontal biamp configuration.

In a vertical biamp configuration you would dedicate one amp to powering the left speaker and the other amp to powering the right speaker. One channel of each amp would power the low frequencies of the corresponding speaker, and the other channel of that amp would power the mids and highs of the same speaker. When identical amps are used vertical biamping is considered to be preferable to horizontal biamping.

There are several potential advantages of a vertical biamp configuration, in addition to the overall increase in power capability compared to stereo operation of a single amp:

1) In contrast to a horizontal biamp configuration, in a vertical configuration both channels of the amp are processing the same signal, and therefore interchannel crosstalk that may occur within the amp is essentially eliminated.

2) The amps can be located close to the speakers, reducing the length and potentially the sonic effects of the speaker cables.

3) Quoting a statement made a while back by GeorgeHiFi in a thread about biamping:

... the biggest advantage of vertical bi-amping is that the whole power supply joule storage of one amp, is dedicated to just one bass driver and not shared, so the bass/upper bass should be better on big dynamic transients. (unless the amps have true dual mono power supplies) which are rare.

Regards,
-- Al

@jsqt, thanks for providing the additional info.

It appears that your preamp has very low output impedance, as the non-Extreme version is said to have a nominal output impedance of 75 ohms. So the somewhat low input impedance of two paralleled channels of your amp shouldn’t be any problem for it, even though the two RCA output connectors the preamp provides for each channel are almost certainly just jumpered together internally and are supplying the same signal. And in any event, as Ralph (Atmasphere) and I indicated if impedance issues were present at that interface you most likely wouldn’t be getting the good bass response you’ve described.

Also, as George and Ian (ieales) suggested trying a vertical biamp configuration could very well be worthwhile.

Enjoy! Regards,
-- Al

@georgehifi

George, I understand what you are saying about the speaker’s impedance being challenging for a tube amp in parts of the bass region. But note that the OP has indicated that the addition of the second amp, with the two channels in each amp paralleled, has improved the bass (which is to be expected given that paralleling the channels in effect creates a 3 ohm tap), and he has not indicated that there are any problems in that region.

Note also that he has said that "fuzzy sound in some of the midrange on some material is what brought all this up." And as I indicated in my first post in this thread the speaker’s impedance in parts of the upper mid-range and lower treble is in the vicinity of 20 ohms or more! Resulting in more of a mismatch to the tap on the amp than when a single amp had been operated in stereo mode. Which leads me back to the first of the two hypotheses I proposed in that first post, with the second hypothesis also being a possibility especially given that the amp has an output impedance that is very low for a tube amp (0.6 ohms in stereo mode, as I mentioned earlier, which would mean 0.3 ohms in paralleled mono mode):

With the amp presumably now optimized for a 3 ohm load perhaps it isn’t happy driving such high impedances in the mid-range and lower treble. Or perhaps the tubes or something else in the amp isn’t well matched between the two channels, and the two channels are trying to put out slightly different signals and are therefore fighting each other to some degree as a result of having their outputs connected together.

Best regards,
-- Al

@georgehifi 

As I've indicated multiple times, the amp is not being bridged!

Regards,
-- Al

@jsqt

I don’t envision any possibility of damage. But whenever the impedance of a speaker is significantly mismatched to the load impedance a tap is designed for there may be compromises to both maximum power capability and distortion performance.

As I indicated, paralleling two channels each of which has a tap that is optimized for a 6 ohm load in effect creates a 3 ohm tap. Applying a 3 ohm resistive load to such a tap would typically result in the amp being able to supply approximately twice as much power as it could supply into 6 ohms when running in stereo mode. With the impedance of your speakers being much higher than 3 ohms at most frequencies I’m pretty certain that the amp is falling significantly short of realizing that 2x increase. And although I’m just speculating perhaps the mismatch is contributing to the adverse sonic effects you described.

Regards,
-- Al

More than likely the impedance of the speaker is too low for the amps in this configuration.

That would probably be the case if the amp were in fact bridged, as was initially indicated, since as you probably realize bridging causes an amp to "see" the speaker’s impedance divided by two.

But note that as indicated in the subsequent posts the amp’s two channels are being paralleled, not bridged. Which has the effect of converting its single 6 ohm output tap into a 3 ohm output tap. 3 ohms being less than the speaker’s impedance at all frequencies, and much less at most frequencies. So in this configuration the amp is being lightly loaded, perhaps even too lightly to be optimal.

Regards,
-- Al

Erik, I've seen some cases in which paralleled operation of the two channels of a stereo tube amp has been recommended by the manufacturer.  Examples include the McIntosh MC275 and the old Dynaco ST-70, despite the fact that like the OP's amp those amps have output impedances that are particularly low for a tube amp.  (JA measured the OP's amp as having an output impedance of about 0.6 ohms; the ST-70 and the current version of the MC275 have specified damping factors of 15 and >22 respectively, which in both cases is suggestive of output impedances of well under 1 ohm).

A concern I've always had about doing that, however, is the possibility that if a small signal tube serving one channel were to fail, and music is then played, one channel would be attempting to output a full amplitude signal while the other would be attempting to force the output to zero volts.  Which doesn't seem likely to be healthy for the amp, especially if the listener isn't nearby or for whatever reason doesn't promptly realize there is a problem.

Best regards,
-- Al
 

First, to be precise it sounds like what you are doing is paralleling the two channels of each amp, which is not the same as bridging. (Although the term "bridging" is sometimes misused to refer to "paralleling"). Bridging involves operating the two channels with opposite polarity signals, and connecting the speaker between the two positive output terminals. You are not doing that since you "used a jumper between each of the "positive" taps," and I presume you are supplying both channels of the amp with the same signal. Also, I suspect that this amp is not bridgeable.

An amp which can be bridged will usually provide a greater increase in power capability into 8 ohms or thereabouts than paralleling the two channels, compared to the amp’s power capabilities in stereo mode, but paralleling will usually be better able to handle low impedance loads.

In any event, it appears that the amp has a single output tap for each channel that is optimized for a 6 ohm load. By paralleling the two channels the tap would be optimized for a 3 ohm load. Looking at the impedance curve for your speakers that is shown in John Atkinson’s measurements it appears that the impedance magnitude varies between a low of about 3.5 ohms around 100 Hz and very high impedances in much of the mid-range and lower treble. The impedance being more than 20 ohms between about 2 kHz and 4 kHz.

Given all of that, I’m not sure what might be causing the symptoms you have described. With the amp presumably now optimized for a 3 ohm load perhaps it isn’t happy driving such high impedances in the mid-range and lower treble. Or perhaps the tubes or something else in the amp isn’t well matched between the two channels, and the two channels are trying to put out slightly different signals and are therefore fighting each other to some degree as a result of having their outputs connected together.

Also, just as an FYI, JA measured the amp’s input impedances as 54K and 34K for the direct and variable inputs, respectively. So by applying the same signal to both channels you are presenting the preamp with a load of 27K or 17K, depending on which input you are using. Some tube-based preamps will experience a bit of deep bass rolloff driving such low impedances, but it sounds like that is not occurring in this case.

Those are all the thoughts that occur to me at this point. Perhaps the background I’ve provided will result in additional possibilities occurring to someone else.

Regards,
-- Al