Can anybody explain how McIntosh's "Unity coupled" circuit works?

[goldear]

I have a rough idea looking at the schematic of various McIntosh amps that they just use some tirtiary windindings, etc on the output transformers to couple some serious negative feedback around the output transformers which should theoretically couple the speakers better than would happen with just primaries and secondaries alone. Ok, I understand this as a very high-level concept. But I'm as clear as mud on a couple of the details.

For example: How does making the screens on the output tubes swing in the opposite direction of the plates even work? I would have thought that they would have largely cut-off much of the tubes operation by doing that.

Secondly: How are these amps biased? I understand both fixed bias, and self biasing. But macs seem to have fixed biased, but without any ability to adjust their bias. Normally only self-biasing lacks adjustment. So how was Macintosh able to successfully bias their output tubes with this arrangement?

 

[battradio]

I have a rough idea looking at the schematic of various McIntosh amps that they just use some tirtiary windindings, etc on the output transformers to couple some serious negative feedback around the output transformers which should theoretically couple the speakers better than would happen with just primaries and secondaries alone. Ok, I understand this as a very high-level concept. But I'm as clear as mud on a couple of the details.

For example: How does making the screens on the output tubes swing in the opposite direction of the plates even work? I would have thought that they would have largely cut-off much of the tubes operation by doing that.

Secondly: How are these amps biased? I understand both fixed bias, and self biasing. But macs seem to have fixed biased, but without any ability to adjust their bias. Normally only self-biasing lacks adjustment. So how was Macintosh able to successfully bias their output tubes with this arrangement?

There is no voltage gain , only current gain in the output stage of the Mac's is the simplest way to explain it all the voltage gain is accomplished in the previous tube stages . It is like a preamp with a cathode follower tacked on the back to provide a low impedance drive on the back of it .

The plates don't do any work they are just along for the ride and is cathode biased .

 

[dcgillespie]

Gold -- The Unity Coupled circuit is as you say, a tertiary feedback arrangement. But to be more specific, it is exactly a 50-50 arrangement. That is, whereas in a conventional arrangement, the load for either tube is presented entirely at the plate, in this case, 50% of it is at the plate connection, while 50% is at the cathode connection. These two sections together then make up the total impedance that would normally be seen at just the plate in a conventional arrangement.

Since the peak AC voltage seen at the plate under full power conditions would normally be about 350 volts in a conventional arrangement (assuming the same tubes and operating/loading conditions), that means that 175 peak volts is appearing at the cathode with the 50/50 split arrangement that the Unity Coupled design uses -- which produces some serious challenges for the driver stage, requiring the boot strapped cathode followers etc., that appear before the output stage.

But this 175 peak volts that appears at the cathode would also cause serious reductions in power output if the screen voltage were held constant. If this were done, the application of increasing power output causes the cathode voltage to increase, which against an otherwise steady screen voltage, would produce a net reduction in screen/cathode voltage as power increased. With an effective ever increasing reduction in screen voltage with increasing power output then, maximum power output becomes significantly limited.

What to do? Attach the screen to the opposite tube's plate! Why? We know that when one tube is conducting, its cathode voltage is rising, and therefore, its plate voltage is falling. But the exact opposite is happening in the opposite tube. It's plate voltage is INCREASING (while its cathode voltage is falling). Since the cathode voltage is rising in the tube that is conducting, and with the screen of this tube connected to a source that is also rising by the same exact amount (it must since the windings are all 50/50, remember?), then it means that now, the effective screen voltage does not change, in spite of increasing power output. Quite a neat, but very necessary little trick.

As for the bias, because there is so much feedback employed in the output stage (100% if you base it on just the signal developed at the plate), then a critical setting of bias became unnecessary for Mac to achieve its stated performance specifications. They could even relax quiescent current draw, and still meet the specs because of it. With such relaxed current levels, virtually any tube could be installed without any real concern for excessive quiescent dissipation levels, since such a relaxed setting would accommodate virtually all tubes in this setting. No doubt, the bias could be tweaked for even better performance, but to achieve the published specs, it wasn't necessary.

The whole affair requires very tight coupling in the OPT between the various windings, which in part, generated the "Unity Coupled" name given to it because the windings are so closely coupled. Special bifilar and trifilar winding techniques were used to attain such tight coupling, with the results being more expensive to produce, but producing performance which speaks for itself.

I hope this helps!

Dave

 

[gadget73]

Thanks for the explanation. That was an output stage I've never really understood either. I knew it worked, but just looking at the schematic I'd say someone screwed up their wiring.

 

[peterh]

Thanks dave for an excellent presentation. I have myself tried to understand without much success. Now i see the light !

 

[sony6060]

Thanks Dave, never fully understood it until you explained it. :thmbsp:

 

[goldear]

Thanks so much Dave! Now I finally understand! This is something which has puzzled me for years. Not sure why I didn't post that question years ago.

 

[goldear]

Dave, another question for you:

Why does the MC225 lack the cathode followers that the other Mac amps seem to have? I'm assuming that the 7591 must be easier to drive than are the larger output tubes. But I'm not entirely clear on why that is.

 

[dcgillespie]

For two reasons. First, the 7591 is more sensitive (it's a higher Gm tube), so it takes less to drive it, and secondly, it operates at lower voltages, meaning that there is simply less voltage being developed at the cathode for the driver to over come. Between these two criteria, Mac was able to use a more conventional driver for their 7591 version, rather than having to resort to the more exotic driver design of their higher powered versions.

I haven't looked at the 225 schematic in a long time, but I too recall the use of a more conventional driver, and the use of the 7591 is entirely the reason.

Dave

 

[Pio1980]

Dave:
Would you consider re-posting this explanation in the McIntosh forum for a "Sticky"?

Gold -- The Unity Coupled circuit is as you say, a tertiary feedback arrangement. But to be more specific, it is exactly a 50-50 arrangement. That is, whereas in a conventional arrangement, the load for either tube is presented entirely at the plate, in this case, 50% of it is at the plate connection, while 50% is at the cathode connection. These two sections together then make up the total impedance that would normally be seen at just the plate in a conventional arrangement.

Since the peak AC voltage seen at the plate under full power conditions would normally be about 350 volts in a conventional arrangement (assuming the same tubes and operating/loading conditions), that means that 175 peak volts is appearing at the cathode with the 50/50 split arrangement that the Unity Coupled design uses -- which produces some serious challenges for the driver stage, requiring the boot strapped cathode followers etc., that appear before the output stage.

But this 175 peak volts that appears at the cathode would also cause serious reductions in power output if the screen voltage were held constant. If this were done, the application of increasing power output causes the cathode voltage to increase, which against an otherwise steady screen voltage, would produce a net reduction in screen/cathode voltage as power increased. With an effective ever increasing reduction in screen voltage with increasing power output then, maximum power output becomes significantly limited.

What to do? Attach the screen to the opposite tube's plate! Why? We know that when one tube is conducting, its cathode voltage is rising, and therefore, its plate voltage is falling. But the exact opposite is happening in the opposite tube. It's plate voltage is INCREASING (while its cathode voltage is falling). Since the cathode voltage is rising in the tube that is conducting, and with the screen of this tube connected to a source that is also rising by the same exact amount (it must since the windings are all 50/50, remember?), then it means that now, the effective screen voltage does not change, in spite of increasing power output. Quite a neat, but very necessary little trick.

As for the bias, because there is so much feedback employed in the output stage (100% if you base it on just the signal developed at the plate), then a critical setting of bias became unnecessary for Mac to achieve its stated performance specifications. They could even relax quiescent current draw, and still meet the specs because of it. With such relaxed current levels, virtually any tube could be installed without any real concern for excessive quiescent dissipation levels, since such a relaxed setting would accommodate virtually all tubes in this setting. No doubt, the bias could be tweaked for even better performance, but to achieve the published specs, it wasn't necessary.

The whole affair requires very tight coupling in the OPT between the various windings, which in part, generated the "Unity Coupled" name given to it because the windings are so closely coupled. Special bifilar and trifilar winding techniques were used to attain such tight coupling, with the results being more expensive to produce, but producing performance which speaks for itself.

I hope this helps!

Dave

 

[dcgillespie]

Pio -- It's fine with me, but that would be a job for the Mods if they want to, as I believe that power rests with them!

Dave

 

[Tubologic]

The best explanation I've ever read about the unity-coupled Mac output stage, and without any schematics or maths. Congrats !

 

[Pio1980]

Pio -- It's fine with me, but that would be a job for the Mods if they want to, as I believe that power rests with them!

Dave

http://www.audiokarma.org/forums/showpost.php?p=7589200&postcount=2

 

[Pio1980]

Pio -- It's fine with me, but that would be a job for the Mods if they want to, as I believe that power rests with them!

Dave

I think they need a repost in the forum by you to do it. Edit it for changes, additions, ect, if you desire.

 

[dcgillespie]

Done!

Dave

 

[nj pheonix]

Must be a hell of an explanation. I think I actually followed (understood) it.:scratch2:

 

[shelly_d]

Dave:

Another Thank you for that clear explination.

Shelly_D