Using tube amp with electrostatic speakers.

Lewm,

For some reason, I did not ask the question of what resistor value is currently being used in the updated back plates with the Toroid ll when I discussed my upgrade.  Dr. West did confirm what you disclosed that the Toroid ll will not saturate as early as the Toroid l so you can increase the resistor value (I just don't know what they are using now, maybe someone else will chime in).

I have always wondered what the hot rod upgrade is for the Soundlab backplate.  Do you know what this upgrade is?

Atma-sphere - I agree that I am limited in power that can be delivered in the bass region of the speaker due to the high impedance of the speaker and the voltage that can be delivered.  Dr. West told me that the wattage claims for Soundlab speakers were to give people an idea of what kind of amplifier to buy (but he really mean't the wattage to infer what the power supply rails would be to adequately provide bass).

Regards,
Gary

I always loved the large Sound Labs speakers so when they announced their new 545 series I thought great! Got them in and after trying them with several different amps over 30 days (including my Atmasphere Novacrons) they just didn't have the magic that the larger Sound Labs  panels had. And yes I tried them with Zero Autoformers still without much success. To bad because they were made beautifully, got them in Birdseye maple. 

(Dealer disclaimer) 

Just to be clear-er, the Brilliance control is an "inexpensive" L-pad that in my opinion and that of many others colors the sound quite audibly.  Removing or bypassing it has absolutely no effect on the crossover point.  If one were to find that the treble is too bright with no Brilliance control in the circuit, one could add a high quality, high wattage, fixed value resistor in place of the L-pad.  The way to do that might be to set the speaker so you like the tonal balance with the L-pad in place.  Then measure the resistance across the L-pad.  Then replace the L-pad with that much resistance.  In my case, for my room, my ears, and my modifications, I am quite happy with no resistor at all.  If anything, I'd like to boost the treble a tiny bit, ideally.

For the example given above where 10 ohms and 36uF values of R and C give you a X-over point of ~440 Hz, removing the R entirely would give you a high pass -3db point at ~4400 Hz.  This would leave a big gaping hole in the midrange, because the bass transformer does not work much above 2kHz, based on my earlier measurements of my speaker.  I am sure you are aware of all this, which is why I think I might be misunderstanding the nature of your recommendation.  You could remove R and preserve the x-over point, if you add more C, in this example.

The issue here is that the old toroid was not set up right. As you know, the crossover is a bit of a moving target because the impedance of the transformer is on a slope with respect to frequency (increasing as frequency goes down, which means that the existing capacitor in the crossover is actually allowing it to go lower than 500Hz- the resistor was there to sort of 'enforce' that there was a set crossover frequency). In practice (rather than theory) removing the resistor on the older original toroidal backplates (which were replaced by the toroidal 2 about 4 years ago) works out quite well. The big concern Sound Lab had was low frequency saturation of the toroid, but in practice its actual impedance was so far off that this isn't a problem with most amps. We have a number of customers that have done this and been very happy. No-one has reported a gap in the midrange because its not there.

I agree the fact that you or your customer(s) have tried it, and it sounds good, would trump my purely mathematical analysis.  And you are correct in noting that the formula I used does not take into account the third variable, which is the intrinsic change in speaker impedance vs frequency (although my off the cuff thought on that is that the actual -3db point with no resistor would be boosted above the calculated 4400Hz by a greater degree than is the calculated vs the actual -3db point with the 10 ohm resistor, because speaker Z is going down as frequency goes up, and the -3db point is inversely related to total Z).  

Something in my memory is telling me that both Will and I did try removing the resistor from the RC network in front of the old toroid, before proceeding on to Will's ultimate solution.  And the sound did improve when we did that, although not to the degree that we heard/hear with the replacement of the treble-only transformer with a full-range one and no R OR C in the circuit.  Anyway, I am happy that everyone else is happier, no matter how they got there.