The Acoustat "C" mod (as found in the MK 121-C interfaces) is always strongly recommended by Acoustat gurus such as Andy Szabo (who has answered questions about Acoustat for over 10 years at DIYAudio) and Roy Esposito (who rebuilds transformer interfaces). They are both former Acoustat employees (the Acoustat company does not exist anymore). "C" mod can easily be applied to any previous Acoustat transformer interface. Though the factory "C" version included the Medallion transformers also included with "B" version, the Medallion transformers are not necessary to perform the mod.
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| Acoustat A and B vs C mod |
"C" mod was the last iteration of interface designs before the significantly different Spectra series of Acoustat speakers were introduced. So "C" mod was the last factory design for Acoustat models such as 1+1 and 2+2.
One of the reasons it is recommended is that it attenuates the low frequencies being applied to the HF transformer, reducing distortion.
At first glance, it seems very strange that in the original MK-121 interface there is no actual "crossover" on the primary side of the two Acoustat transformers. With both the "A" and "B" mod interface designs, the HF transformer is DC coupled to the input signal, with only a capacitor bypassed series resister in line (whose purpose is tuning the high frequency peak around 10kHz, though it seems weird such a large capacitor and small resistor would have that effect, when their own RC constant suggests they work in the midbass, but their effect is just enough to shift the ultimate HF pole).
The actual "crossover" for the HF transformer is only on the secondary side, a 0.01uF capacitor coupling the HF transformer output to the panel drive signal. The LF transformer has no "crossover" at all except it's own inductance rolling off the high frequency response, which was the reason why the Acoustat dual transformer interface was invented in the first place.
Is there a difference between crossing over the transformer on the primary or secondary side? I'm not sure, I would have never thought about crossing over a coupling transformer on the secondary side, that it even works OK wouldn't have been obvious to me. It would have seemed to me that even with the secondary wide open, the transformer primary would operate like a choke, and would still be affected by potential bass saturation. (However, chokes do less and less at low frequencies anyway.)
Comparing that capacitor with the 50k resistor coupling the LF transformer, a first approximation of the highpass cutoff of the HF circuit is (2pi/RC) 318 Hz.
I can see a one pole at 318 Hz might be a bit inadequate for fully isolating the HF transformer from low frequencies. If the C mod HF control were turned all the way up, it would introduce a second cutoff caused by the 57uF capacitor being loaded by 16 ohms, around 174 Hz. Additional loading from the transformer itself and turning the control to midpoint, push this cutoff somewhat lower than that.
Further protecting the HF transformer from low frequencies seems like a good idea (though it's not clear how much of a problem it was).
But at what cost?
People smarter than me, or at least more familiar with SPICE modeling, have analyzed the B and C mod circuits. And the incredibly curious thing is despite how they look entirely different, and wouldn't work the same at all, in fact they work very much the same. There is relatively small difference between the interface outputs with B or C mod in place. Which is as it should be, presuming B mod was reasonably well designed in the first place.
But there is some difference, the two very different circuits obviously do not work exactly the same. I would have expected more rolloff in the deepest bass, but in fact the C mod might have slightly higher output, like 0.1dB or so, at 20-40 Hz. But it's in the midbass through the midrange, 100-1000 Hz, that the C mod has up to 2dB less output. The absolute advertised gain for the C mod in the extreme highs around 10kHz and above is very very slight, it's only relative to the 2dB attenuated output at 1kHz that it's "relatively" significant.
If you played these two versions in a simple test without adjusting the levels to compensate for the greater loudness of the B mod, I'd bet that almost all of the time audiophiles would prefer the louder seeming "B" mod. It might still win in a properly level matched (best to use C-weighted pink noise) comparison because 100-1000 Hz is basically the heart of the music. The relative and tiny absolute gain in the very extreme highs above 10kHz is debateable, I've had an ear tuned notches tamping 2200, 5200 and 9100 for C mod actually. It certainly has more 'punch' if you don't adjust the level downwards (when switching from C mod to B mod) to compensate (I've found a 2.5dB downward adjustment in gain for B mod (vs previous C mod) for the Acoustats to work best by ear, eliminating the 'listening fatigue' I get when it's set higher, relative to the subs, but after you compensate for the difference in output with a 2.5dB downward adjustment...I don't yet if the 'punch' is different, I think it is slightly but not as much as I'd hoped when I first heard it, the increase in punch was shocking, but I hadn't realized I was playing 2.5dB louder 100-1000 Hz either. (Plus, I'd always set the HF level down before by accident, not realizing "flat" was 3 o'clock instead of 12 o'clock. So it's louder in the highs not, despite not being a difference because of the change to B mod in the my most recent 2+2's but instead because so far I've left the newer speaker at it's factory "0dB" setting, which is tricker to change then with my first 2+2's where I could just turn a knob, but now I'm thinking I like more highs anyway, so I'm not planning to adjust the new but rather to measure how the HF level control on the older one's works, and how much and whether I still need the EQ's with the newer speaker for it to sound best. So, added highs is an additional factor possibly adding punch.)
In fact, a number of Acoustat users and even some self-appointed Acoustat gurus do prefer the "B" mod, a fact I only discovered yesterday (note: the figures in this post come from that thread).
I'd generally found the C mod midbass through midrange to be somewhat weak, but I've never broadly EQ'd that upwards, I've only notched out a few peculiar resonances to make it more smooth, and curiously added two tiny 1/3 octave 3dB boosts at 850 and 1kHz, where there was a curious suckout, I determined when I had the chairside EQ (in the repair queue for about a year now). But I had been disturbed by the general 'weakness' of that region. Well, going back to the B mod fixes that general 'weakness' from 100-1000 Hz, or at least the new speakers do.
The significant loudness increase with my newest 2+2's (which have B mod) might be entirely due to B mod! Which I had previously assumed was a step backwards. But there might be other reasons too.
With more midrange, midbass, and deep bass, the B mod might have "punchier" sound as well.
The only noticeable lack in the B mod from the measurements is a very slight extension in ultimate high frequency response, which looks pretty small and fairly unimportant to me.
I'm not going to assume C mod is necessarily better any more. I now have the ability to swap B and C mod interfaces into my newest 2+2's and I will measure them and listen to them and see.
Note that while the B mod will play as much as 1.5dB louder than C mod with the same input signal, the ultimate loudness limit (provided sufficient amplifier power is available, < 200 watts into 4 ohms) will likely be about the same. The transformer and panel limitations will remain the same.
DIYAudio poster Bolserst posted some more illuminating simulations and measurements to the aforementioned thread.
First, he shows B mod mixer drives the HF transformer with the full bass signal, but the C mod mixer rolls off the bass to the HF transformer (this doesn't seem to show the effect of the capacitor in the secondary circuit however).
In later graphs, he shows these spice models do fit the measurements almost exactly. Including the curious lump in the LF transformer response, which goes away in the C mod. It's actually that "lump" which causes the boost in B mod vs C mod output in the midrange. The HF midrange response is identical!
There is no change to the LF circuit at all, so what explains this lump and how it goes away with C mod? There are no changes in the LF circuit at all! That brings us to Bolserst's theory that the C mod reduces the low frequency backward coupling of the LF transformer into the HF circuit, which somehow pulls the LF response higher.
Greater output in the midrange might make it sound better, on the other hand, if there's more distortion added, that could make it sound worse. In my previous measurements the Acoustat (with C mod, all I had ever used before) is very low distortion, but perhaps distortion is slightly higher in B mod.
Since I've readjusted the level to -2.5dB from before, I haven't had any more episodes of listening fatigue, but I wouldn't be surprised if C mod was slightly cleaner. So perhaps it will win in the end? I'm going with my newer speakers anyway because I can switch from the current B mod to my original 1+1 speakers hot rodded (external Solen cap) interfaces, because my latest 2+2's have the compact interfaces which are quickly swappable.
That feature alone is important, but for now I'm thinking they sound at least as good with B mod as my originals do with C mod, or possibly better.
*****
Update December 23
No question anymore, the 'hardness' of the new-to-me 2+2 with a B mod interface is intolerable. I don't know yet what the problem is, perhaps I just need to turn down the HF level comparable to my other 2+2's. Perhaps the C mod does reduce audible distortion, and once you've gotten used to it nothing else will do. I can make all these changes with equipment on hand, I would love to swap out the interfacees in the new-to-me 2+2's with my hot rodded 1+1 interfaces from back in the day, with huge external polypropylene cap and everything. But to be fair, I'm going to take the first step and changed the new-to-me 2+2's to their lowest HF level and see what that does.
It occurs to me that the way the HF level control "works" is that it actually creates HF ringing. The "flat" position is with a little ringing, and the +2 is with more ringing. -2 is probably what you need for no ringing.
And the ringing probably is more audible with B mod having in effect higher Q.
Still, I imagine many would prefer the 'louder' sounding sound.
Update December 28
I should not have conflated the differences between my two sets of Acoustats with the difference between B and C mod.
It now appears the fatiguing quality may have been mostly that the arcing in the right speaker, though impossible to hear except with ear at speaker, was making it more distorted. The electric charge was non-uniform.
Also the difference in HF level. But now, very indirectly so far, it does seem like B mod may sound somewhat louder irrespective of HF differences. (And also that in my room and to my taste, the HF level needs to be turned down at least 2dB.)
I will shortly be able to swap in my C mod interface, which will be the closest I've come to doing a fair test. I still have the B mod speakers in front of the C mod, giving the B mod an artificial advantage one would think.
I've disconnected the arcing speaker, because it might have damaged my amplifier, and I can't risk my next amplifier, the amplifier I'm using now, which I'm liking more than anything ever right now, the Aragon 8008 BB, with Mapleshade carpet feet and my superlative bias adjustment.
Until I get that arcing fixed, I'm only going to play it with "expendable" amplifiers. For now that speaker going back to the repair pool.
I might be first repairing my original right channel with C mod. It also seems to have some distortion playing You Think Too Much About Flying Saucers, but, playing the same right channel signal on the new B mod 2+2, there is no distortion.
So it's looking very much like when and if I get them matched, I'm going to be mixing my new and old 2+2 pairs until I get at least one of the defective ones fixed.
And to really do that correctly, I'm going to pull my old modified C mod interface out, so both will have C mod, at least for starters.
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