Weird question re: balanced connection

Ralph, that's excellent and useful information, of course. But I'm not sure whether or not you noticed that the question was not about monostrapping or paralleling. It was about the possibility of configuring two pairs of tube monoblock amps having unbalanced signal paths into the functional equivalent of a pair of *fully balanced* monoblock amps.

Kijanki & Zd, thanks very much for the kind words.

Best regards,
-- Al

If you are looking to monostrap a tube amplifier, the procedure is different from that of a transistor amp.

This procedure is outlined in the Dynaco Stereo 70 manual FWIW.

The amps to be monostrapped must be absolutely identical! The same signal must be fed to their inputs. A Y adaptor can be used. Essentially, the outputs of the amps are to be paralleled. This will cut the output impedance in half, so if you have an 8 ohm tap on the output transformers, they will be driving a 4-ohm load.

Some amps have a grounded 4-ohm tap so the accommodation is slightly different, but in general you are safe if you have a jumper between the minus terminals of the two amps and the highest tap available, as well as the taps that are being employed.

Obviously this hookup can really mess with things (destroy valuable parts) if not done correctly!

In our case our amps are OTLs and have no output transformer, but the same technique is used. Note that in some cases it may be more difficult to obtain correct bias readings on the power tubes unless the amps are separated.

The output power will be doubled and the amplifier will be able to drive impedances that are half of what they were before.

If you try to do this with transistor amps you can expect to damage them- this only applies to tube amps.

Very convincing, ok I won't do it, thanks again.

Zd542, you got it right!

If I ever join NASA, I'm not going up unless Al is in mission control making sure they get it all right.

Al, I did not realize it is a tube amp - thank you for clarifying it. I absolutely agree that it is very dangerous to play any games.

Cptaz, Follow Al's advise and don't do it!

I second Kijanki’s good comments, and I would add that in addition to the possibility of over-stressing the amps due to the halved impedance each of them would see, I’m not even sure that you could turn the amps on and off without creating at least a small risk of damage.

As you are probably aware, running a tube amp that has an output transformer unloaded is not a healthy idea, especially if a signal is being put into it, but conceivably also during the startup and shutdown processes even if no signal is present. Transients that may occur in the amplifier circuitry at those times might in effect create a brief “signal.” And in the configuration you are proposing whichever amplifier in each channel pair is turned on first or off last would in effect be running unloaded for at least a brief amount of time.

Also, although this amp is not bridged (it uses two 845 tubes in a parallel SET configuration), I’ll mention that combining two bridged amps into a balanced pair configuration would be a sure invitation to damage. Combining two amps into a balanced pair configuration requires connecting one output terminal of one amp to one output terminal of the other amp (with those terminals not being connected to the speaker). With amps having single-ended outputs it would be the common or ground terminals that are connected together. But with bridged amps the result would be that output signals which are inverted relative to each other would be shorted together. Definitely not healthy!

Regards,
-- Al

Thank you all for your responses.
Kijanki, that's exatly what I needed to know, thank you.

You have to be careful since you will get bridged outputs if amplifiers have single ended (ground referenced) output. Bridged outputs will get you 4x power (while two power supplies are designed only for 2x more power total) plus doubled output impedance (affecting speaker damping), not to mention being not able to drive your speaker's properly if half of their minimum impedance (for the same current since voltage is 2x) is below amplifier's ability. If amp has already bridged outputs then the only improvement will be keeping system balanced. Fully differential output will cancel even order harmonics but I wouldn't bother since your beef is with increased odd order harmonics. Both amps have to have exactly same gain and phase shift or common mode reduction will be reduced. Doing thing like that is always risky proposition. Most likely you'll get better results selling all four monos and buying two better amps.

Possibly if you also reversed the polarities appropriately on the speaker outputs.

Of course, you can just use a Y-connector.