JR. wrote:... something other than the old drive the speaker as a current source, but that works too.
That was investigated by Mills, B&W and does reduce distortion but so does ACE-bass
Synthesis of Loudspeaker Mechanical Parameters by Electrical Means - Karl Erik Stahl
A method for extending bass response and lowering distortion in louspeaker systems, which differs from both equalized and servo or feedback systems, is described. It can be seen as an extension of the technique to increase loudspeaker damping by giving the driving amplifier a negative output resistance. However, this method also controls the moving mass and compliance. Very good results, such as flat frequency response down to 20 Hz and excellent distortion data, have been achieved with moderately sized bass-reflex enclosures.
AES Convention:61 (November 1978) Paper Number:1381
Current drive is MUCH more complex to get right.
.... actually measuring the VC temp ...
This is what David's bridge did. I think he never had a preprint (Inst. of Acoustics, Windermere 198?).
ACE bass is the best method I've tried and I had some success with incorporating David's VC temp compensation in it.
Low distortion transformers seem counter productive for all the folks who use them solely to add coloration.
Golden Pinnae Brigade aside, I think there are still good solid reasons for transformer in/out. With all respect to the excellent THAT stuff ..
- better, easier real-life RFI bombproofing.
- easy P48V
The best I've managed with transformer mike i/p is about 1.6dB NF but this is only important with a low output ribbon like Coles 4038. For that, a proper Cohen (or even THAT 1510) without the P48V bits but full RFI, would be slightly better.
For output, you rarely NEED more than 22dBu (comfortable NE5532 on +/-15V) I don't think the fancy balanced outs are worth the PCB space. I'd rather use "Balanced Impedance". When you DO need more than 22dBu into a low load, it is hard to beat a transformer as in the earlier Lundahl note.
If only Swedish virgins and Unobtainium cores weren't so rare, large & expensive.