MAC Autoformers?

Speaker manufacturers are not so honest about their impedance range and a curve is the only way to know. If they didn’t do so many tricks in the crossover we would not have this problem. The drivers are not the problem, the crossover is.

In my experience most speaker designers do not know much about electronics or care about what the amplifier may have to do.

+1 on this, I’ve felt this for a long time.

I’ve heard this from ESL owners where the impedance rises in the bass to very high values. They believe that OTL amplifiers are better for their speakers because they can supply the extra voltage to drive the high impedance in the bass. They indeed get more bass if the damping factor is low, but not the bass the speaker was designed to give. They get more and perhaps like more but it is one note bass.

[snip]

This is where we get in trouble with the power paradigm which had led people to believe that the speaker wants constant power. It does not. I cannot think of or find a modern speaker that wants constant power, the varying impedance and flat response insure that the designer uses a constant voltage amplifier, ie one with high damping.

@ramtubes

The first paragraph is false. You don’t get a one note bass at all, even if the OTL has no feedback (and most do use feedback). Please keep in mind that one of my employees (who now works at Bel Canto) owned ESL57s, and don’t forget Bill Toberman (RIP) who had ESL63s. The one note bass phenomena only occurs when the panel speaker (it doesn’t have to be an ESL) is too close to the wall behind it, allowing the back wave to reinforce around one frequency. This happens a lot if the ESL user has a solid state amp because getting bass out of the speaker becomes a challenge- most solid state amps can’t make power into 30 ohms or more!

The last paragraph is a bit misleading. The Power Paradigm does not lead people to think speakers want constant power. If anything, most people think the idea is ridiculous. And as you state, most speakers are Voltage Paradigm. Its not until they hear what a properly set up Power Paradigm system can do that they might begin to think that its not bunk (ask clio09). And there are manufacturers that make speakers that are Power Paradigm and I’ve mentioned a few already- Audiokinesis, Classic Audio Loudspeakers, DeVore Fidelity, Lowther, PHY, Feasterex, Onken, Volti Audio, PureAudioProject, Evolution, Sonic Flare.... I can go on and on but I hope you get the point.
Most horns and panel speakers are Power Paradigm devices unless something unusual was done to change that (there’s a guy named Wayne Piquet who does just that with Quad ESL63s, by adding an extra panel to the speaker). For example, Magnaplanars are Power Paradigm but because of their resistive nature work fine with Voltage Paradigm (constant voltage) amplifiers.

Sometimes its the intent of the speaker designer that makes the difference, so the speaker might be acoustic suspension, like the AR-1, or it might be bass reflex and so on. Your comment at the top of this post has a lot to do with that- many speaker designers went to the school of ’by gosh and by golly’ and are simply trying to make the speaker work with a particular type of amplifier, like an SET (which is likely the most common application). If they get the latter right, they will have a Power Paradigm device.

In case you are thinking I made this stuff up, this is really all about history; take a look at this Google search, in this case one for a Fisher A-55:
https://www.google.com/search?client=ubuntu&channel=fs&q=fisher+A-55&ie=utf-8&oe=utf...

One of the first hits is an image from YouTube, showing the damping control on the amplifier, which is a dual-gang pot. If you look closely, it is labeled "constant voltage" with the control all the way down, "constant power" with the control at noon, and "constant current" with the control all the way up. In this case, voltage and current feedback are balanced against each other in the noon position.

Now current drive never really developed into a thing (meaning there are almost no speakers based on current drive), but power drive and voltage drive did. This amplifier was built about the time (mid-late 1950s; Fisher, ElectroVoice and a few others had such ’damping controls’ on their amps during this period) when the voltage rules were first being proposed and so amplifiers had to be adaptable to whatever was out there. Instead of balancing current and voltage feedback, you get similar results with no feedback at all, which is understandable because as you know, current feedback **raises** output impedance while voltage feedback decreases it.

@bifwynne
Roger has one of the best tube matching services in the country.

I suspect that one reason you have issues with the taps is that the ARC amp really needs a few more db of feedback; 15 db is slightly on the low side to get right.

BIF,

I can match your tubes, may have to sub or sell you a few. Will need to know the plate, screen voltages and cathode current which we do know. Write tubeaudiostore@gmail.com to set it up.

I would still like to see a schematic of the amp. Can you get one? 

What ARC did to balance things in design is what I do also. Use the 4 ohm tap, it should sound better.

In case you are thinking I made this stuff up, this is really all about history; take a look at this Google search, in this case one for a Fisher A-55:
https://www.google.com/search?client=ubuntu&channel=fs&q=fisher+A-55&ie=utf-8&oe=utf...

One of the first hits is an image from YouTube, showing the damping control on the amplifier, which is a dual-gang pot. If you look closely, it is labeled "constant voltage" with the control all the way down, "constant power" with the control at noon, and "constant current" with the control all the way up. In this case, voltage and current feedback are balanced against each other in the noon position.

I dont think you are making it up Ralph. Do look more closely at the video. The pot is not dual but single with a switch attached, thus the click into the high damping condition which cuts out all the Zmatic circuitry resistor losses. It is indeed a combination of voltage and current feedback as seen in this schematic

 http://audiophool.com/Schem_A/Fisher_100_schem.gif

One can clearly see the the switch that grounds out the current sense resistors and returns the amp to conventional voltage feedback. The damping spec on this amp is 26, quite good for an amplifier of this vintage. It is interesing to me that although the Z matic control was of interest they made the click to entirely remove it and its losses. 

@bifwynne
Roger has one of the best tube matching services in the country.

I suspect that one reason you have issues with the taps is that the ARC amp really needs a few more db of feedback; 15 db is slightly on the low side to get right.

Thanks for the compliment. I did start the whole computer matching program and feel the only correct way to do it is at fixed plate current which I can enter in the test program.

Whatever feedback they use is not of importance, its the resulting output impedance that matters. For instance, a triode output tube has inherently low output impedance and provides a damping factor typically of 4-5 without feedback. This is childs play to figure out. Its just the output transformer primary impedance divided by the plate resistance. For a 300B tube that is about 3500/700 ohms= 5.

For a pentode output tube its much higher for a 6550 about 3000/27,000 = 0.11 ouch, less than one. Now we need a lot of feedback to get that down. Even triode connected it is only 0.22. Thus not a good triode comparred to the 300B.

This is why SET ampifiers can be made without feedback. These tubes were created to do this job. The 45, 2A3, PX4, PX25 all have similar characteristics as does the 6EM7 which I use because it is very close to a 2A3.

There are three classes of output tubes. Low impedance triodes as mentioned above, Pentodes (EL34, EL84) and Beam Power tubes (6550, 6L6, and all KT series). The pentodes and beam tubes all have very high plate resistance, so high that we don’t even try to match it.

When I was a young lad the idea of measuring feedback was a total mystery until I found out how easy it is to measure. Heres how you do it.

Simply connect the proper load to the amplifier, set the output to a very low voltage, remove the feedback and see how many dB the output rises. ARC looks at another way, which I appreciate and use in my thinking. If you think of an amplifier as a power supply then you can easily measure its regulation, as power supply specs do. It can be noted either in percent or dB. This is measured by noting the output voltage with the load connected and then with no load. We want to see the smallest change, as does the speaker.

This is a great way to demonstrate what is going on with your speaker. If you pull the load on an amplifier with a damping factor of 1 the output will go up exactly 6dB. It will do about the same on your speaker when it goes above 20 ohms!

@ramtubes,  I was in contact with Anthony today.  I will ship off 2 quads of KT-150s next week that I bought from a 3rd party tube vendor.  Hopefully, Roger can get some good matched pairs out of these tubes.  Damn expensive!!  Otherwise, it's back to ARC.

Anthony advised that I should "light load" the amp off the 4 ohm tap, which Atkinson measured as having an output impedance of .55 ohms.  Presumably, FR should be flatter and DF higher. The impedance in the bass saddle is a smidge south of 4 ohms. I estimate the DF to be approximately 7 in the saddle.  Hopefully, that will be enough to get the bass kick I like.

Also, Anthony echoed that even if the 4 ohm taps presents an impedance to the output tubes off the primary coil of the output tranny that is higher than the rated 3000 ohms output impedance of the tubes, no harm will occur.  In fact, it will make for less strain on the tubes and lower distortion.  The cost, … about 3db of power.      

I can live with that.  My speakers have a sensitivity rating of 92 db.  They play plenty loud.

Thanks  Roger and Anthony