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Using tube amp with electrostatic speakers.

Moons ago I started similar discussions and thought I had been given enough good advice not to approach the subject again. Here goes anyway. I've used Martin Logan electrostats for well over 30 years with quite a few different amps but have recently switched to a tube amp and dynamic speakers with which I am very satisfied.  It consists of the Cary Rocket 88R amp and Serie Reference 3 speakers. 

My brother was visiting last week and was so impressed with the sound that he decided that he might want to try a tube amp also (probably the same one as mine).  However, he is using a pair of SL3's that I gave him years ago and I'm concerned primarily about the current requirements of the Martin Logans as well as other concerns that I'm not thinking of.  I don't want him spending money on something that may not bring him improved sound so would appreciate more advice to pass on to him.  He currently uses a Rogue Audio SS amp with his SL3 speakers and, to me, it sounds very good. 
jimbreit
lewm
12,983 posts
 
Gary, That's a pretty accurate summary.  The notion to remove or bypass the brilliance control is actually older than the ideas to modify the backplate that came from Will and me.  

I remain puzzled that Sound Lab would upgrade to a larger toroid with presumably a better low frequency response and then continue to use the same values of R and C, thereby maintaining the same old crossover point (at round 440Hz for the values 10 ohms and 36uF).  But I don't actually recall what values SL used as OEM standard with the "old" original toroid, so it could be that the old values were actually something less than 10 ohms with something less than 36uF capacitance, which together or separately would give a higher crossover point.  Therefore, the values of 10 and 36 are "new" to Sound Lab.  As I now recall, the old values might have been 6-8 ohms (depending upon year of manufacture) and maybe 32 to 34uF.
gwho
47 posts
 
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
sksos1
387 posts
 
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) 
lewm
12,983 posts
 
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.
atmasphere's avatar
atmasphere
11,778 posts
 
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.
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