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In Reply to: RE: Beveridge? posted by airheadair on February 17, 2016 at 09:37:46
I'm not sure if you are being serious LOL! .. if you look up at my first reply to the thread you'll see I posted a BIG pic of the amp and one up its skirt.
I've hand built a lot of amps and preamps but I just don't have the time any more nor the level of commitment to carry on a direct drive project to the end. So, Modjeski got the call after reading through his thoughts on Audio Circle some time ago. He makes a lot of sense to me as I have been trained in EE as well but just don't have the background experience he does in this speciality. I wasn't willing to spend the time to learn and experiment when he has done the work. I had been frustrated enough rebuilding quad esl panels in the past and while successful- it wasn't worth my time in the end.
I have tried Modjeski's crossover and subs for the Beveridge's but I did not like the imposition the crossover put on the transparency of the Beveridges. So , I went a different route with REL subs and like the results much more. The Beveridge's are incredibly revealing and it is crime to use anything but the best with them. I hear stories of some folks driving them with AV receivers after rescuing a pair... yikes.
If I go active again I have built my own crossover and it is , I believe, much more transparent.
Sorry if this goes off-topic once again, but you probably know the answer to this question regarding the built-in woofer amplifier and electronic crossover circuits built onto the Bev 2SW amplifier chassis'. RM was kind enough to supply me with the amplifier schematics for a nominal fee, but I have never been able to obtain a full schematic that includes the crossover. I do understand that the built-in low pass filter part of the crossover is 18db/octave at 100Hz or thereabouts. But what is the slope of the high pass side of the crossover? On the amplifier schematic, a small value capacitor is shown to be in series with the input to the direct drive amplifier. If memory serves, the value is .022uF. This alone could constitute a passive first-order high pass filter in conjuction with the parallel resistance at the amplifier input, but is there upstream (active or passive) circuitry that adds to the slope of the hi-pass filter? My ohmmeter tells me that the upstream side of the .022uF capacitor is not directly connected to the RCA input jack, but beyond that I have never figured it out. Thanks for any info.
In practice, I use a restored Dahlquist electronic crossover for a low pass filter before the solid state amplifier that drives my Transmission Line woofers. I use whatever is built in to the Bev amp chassis for high pass, and I have always had the feeling I could do better, if I knew what was going on in that circuit.
Have you asked RM your question about the slope of the high pass filter? He should know.
I believe that the high and low pass filters on the external crossover he uses (mine is labeled Beverage RM 3 Universal Crossover) is 18 DB/octave on both sides. Roger told me he thought it important to get a lot of the bass out of the ESL panels as well as a lot of the midrange out of the woofers.
No, I have not asked RM the question. When I contacted his business in order to acquire a copy of the direct-drive amplifier schematic, I never got to talk to him directly. Once or twice I emailed him but never got an email response, either. However, his company was very prompt in providing me with the schematic, once I had sent payment.
In the pamphlet that Harold Beveridge concocted to go to purchasers of the 2SW, it does say that the low pass filter slope is 18db/octave. And I have always assumed that the high pass filter would have the same complementary slope, but that is not directly stated in the pamphlet. But I also have always wondered why there is that series capacitance in front of the amplifier input, which by itself would add a passive high pass filter with first-order kinetics. On the other hand, it does not seem to me that 6db/octave would be sufficient to sufficiently attenuate the demands upon the panels in the 2SW to produce bass response. So it is most likely a net 18db/octave slope with the gain maintained by an active element in the crossover.
I have not looked at this schematic in a long time. I am by no means an expert.
It is part of the motional feedback system Beveridge used by injecting Rf and feeding back net diaphragm displacement rf signal to crowbar or limit excursion on the line source.
He was a pretty amazing engineer. Both he and Peter Walker could put a lot of the modern types to shame I would think!
Although I responded to your PM slightly differently, on second thought, the function you are describing may be happening within the input and driver stages of the direct-drive amplifier proper, after the high pass filter, in other words. There's a 7.5M resistor that feeds back from the output stage to the driver stage, with ~1600V on one end and barely 3V on the end that is attached near the driver section. Lots of filtering going on in the input section as well. I don't pretend to understand it, but I have been able to work on it a bit only because of the tutelage of another EE Bev owner I encountered on TubeDIY. Like me, you can get the original Bev amp schematic from RM.
I quite agree, HB was at the very least a major innovator in the history of audio, if not a genius, and he gets very little credit for it, mostly because his company was so short-lived and the products are rarities. I do also think RM had a lot to do with the original dd amplifier. Every single aspect of the Model 2 or 2SW system is novel, yet the system as a whole is a smashing success. HB is easily up there with Peter Walker in my book.
Sorry, I didn't realize that your picture was your amp!
My Stax speakers are pretty revealing too, but maybe my ears aren't any more. Anyway i found that the midrange seemed better to me with the crossover in place than without it, perhaps because the bass was taken out of the amps/ESL.s
To clarify, what crossover are you using with the REL's?
The RELs tie to the outputs of the amp driving the Bev woofers and the RELS have their own internal crossover. It is REL "trademark" of how they hook up and the sub "sees" what the amp sees. This way, I don't have the linesources going through any active crossover. This keeps things clean.
The linesources are passively crossed over with the internal first order Beveridge crossover. I use a 150lb 211 SET amp on the line sources and I use a solid state amp on the Bev internal woofers , again passively crossed over internally. The SET obviously has some limitations but this is no ordinary SET it is a beast.
So, the system is bi-amped with the internal passive crossovers in the Bev.
The internal crossover caps and coils have been ridcuously upgraded at great expense.
It all seems to work and blend exceedingly well after alot of trial and level matching. I think the 2 subs helps even out room modes. Quite a few of my audio group have been suitably impressed on this contraption!
I used to use exclusively rebuilt Quad ESLs with all their limitations they continue to amaze.. the Beveridges sound bigger ( the Quads are small sounding) but the Quads play plenty loud for me and are really amazing if they are rebuilt.
I have the USA monitors as well - as good as they are- they are not ESLs or Beveridge's.
The Quads play really well with the Futterman- amazingly well.
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