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In Reply to: RE: I'd use a set of Zeros posted by Simon Templar on May 04, 2021 at 14:34:44
A few years ago Roger corrected an error in the backplate of the Sound Labs, making them significantly easier to drive (and sounding better with any amplifier). The low impedance is at very high frequencies so there's not a lot of power there. Before this change the MA-2 was about the only game in town that worked right on that speaker (about 80% of our MA-2 production is running on Sound Labs) but after the change you could do the same thing with a set of MA-1s.
Follow Ups:
I have one of the earlier pairs. Purchased in '98 IIRC. LONG before the PX series as well.
Panel membranes and frame sock have been replaced, but the backplates (and the toroids) are all original
.....sT
Have two different backpanels. The earliest uses EI core transformers and works great with MA-1s; IIRC they went out in the late 1990s.
The toroid version which replaced the first one can be driven by MA-1s after a simple mod. The problem is that the impedance of the Sound Lab is a moving target with frequency so a resistor is used to help set the crossover point. Modelling was used to design the crossover and it was off by a country mile.
But it turns out that if you just remove that resistor the speaker becomes much easier to drive. In most of the backplates I've seen the resistor is composed of a number of smaller 20 or 25 watt resistors; the composite resistor is a 200watt device which explains where the efficiency of the speaker went. By simply removing the resistor the speaker sounds better in every way and is easily driven by MA-1s. Sound lab says that the resistor prevents saturation of the high frequency transformer but in practice with the MA-1 or MA-2 that doesn't happen.
The Toroid 2 backplate reflects the design of the crossover using the correct impedance curve of the speaker and so has the same easy drivability.
> > > > > > > > The toroid version which replaced the first one can be driven by MA-1s after a simple mod. The problem is that the impedance of the Sound Lab is a moving target with frequency so a resistor is used to help set the crossover point. Modelling was used to design the crossover and it was off by a country mile.
But it turns out that if you just remove that resistor the speaker becomes much easier to drive. In most of the backplates I've seen the resistor is composed of a number of smaller 20 or 25 watt resistors; the composite resistor is a 200watt device which explains where the efficiency of the speaker went. By simply removing the resistor the speaker sounds better in every way and is easily driven by MA-1s. Sound lab says that the resistor prevents saturation of the high frequency transformer but in practice with the MA-1 or MA-2 that doesn't happen. < < < < < < < <
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Mine had the toroids as an option. But they did not have the resistors, however. West offered a kit which installed the resistors. It required the removal of a cap (I recall it was a Solen....but can no longer remember what the value was. Been a few years). After removing the cap, one replaced it with the grouped ceramic resistors.
So....Are you saying to remove those resistors and not re-install the cap?
....sT
I bought a pair of M1s around the late 1990s. I drove them with a pair of Atmasphere MA240s. That model was built on MA2 monoblock chassis and used six 6C33C Russian triode output tubes in lieu of a bunch of 6AS7s. In power it is roughly equivalent to a 12- or 14-tube MA1. With the M1s, I never felt short of adequate power to drive the speakers full range. About 8-10 years later, I bought a pair of 845PXs, and to save money, the factory was willing to sell me the panels as a separate entity; they noted that the backplate for the 845PX was identical to that formerly used for my M1s. Note that my M1 backplates had the first or early toroidal transformer which was fed by a first-order high-pass filter consisting of an RC network, where to the best of my recollection, R = 6 or 8 ohms and C = 36uF. The resistance was made up of four of those rectangular "coffin"-shaped resistors wired in series-parallel, so as to increase power handling. So this is the "old" set-up that Ralph mentions that both eats amplifier power and dramatically reduced speaker efficiency, if you use tubes. Moreover, some other guys who posted on Sound Lab Owners Group (SLOG) noted that it resulted in a dramatic impedance dip to ~2 ohms at 1-2kHz. For an OTL, that is NOT where you want to see such a phenomenon. However, the scuttlebutt was that the factory used solid state amplifiers to voice the speaker, and the impedance dip was not seen as a problem by them. Around that time, I was growing more and more dissatisfied with my 845PXs, mainly because I had expected a big upgrade compared to the M1s, and I wasn't hearing it. There was a problem in the midrange that I now believe was due to that impedance dip. Around then, a guy from Australia was posting on SLOG about a cure for these ailments. His name was "Will", and sadly he is no longer with us. Nice guy. I started up a conversation with Will. This led me to determine an impedance v frequency curve for my own 845PXs. That confirmed the low Z at around 1-2kHz, but it also showed that the intrinsic impedance of the speaker without any RC network is much higher than 6-8 ohms, more like 20-25 ohms from about 200Hz to 5kHz. Thus one can understand if you put a 8 ohm resistor in parallel with a 25 ohms impedance most of the energy put into the system is going to flow through the resistor, where it does nothing but heat up the resistor. In addition, that R puts a ceiling on the impedance; it can never be higher than the value of R in parallel with speaker impedance, in the range where it operates on the treble tranformer. I have already made a short story long, but the upshot was that Will advocated dumping all the crossover components, both R AND C, and replacing the treble toroidal transformer with a massive EI type full-range audio step-up transformer that he had sourced in Australia. I immediately bought a pair and that is what I use to this day in lieu of the toroidal treble transformer. No RC network at all. You still need the bass transformer in parallel with the new full-range transformer, in order to derive enough energy to get good bass response. I kept the inductor that precedes the bass transformer, along with the OEM bass transformer, and I have been happy as a clam ever since. Most of this was published on SLOG. You can look it up in the archives I think. All of this conversation on SLOG led Roger West, who was never less than a fine gentleman throughout the process, to revise his design. He did not agree with the idea of no RC network, but he did agree that perhaps the crossover point was in the wrong part of the spectrum, and he introduced a heftier toroid that can accommodate lower frequencies, thereby permitting him to lower the crossover point. I do not know what values of R and C are now used. I absolutely love my 845PXs, ever since I went naked. My speakers present an impedance in the range of about 20-25 ohms from 200Hz to ~5kHz, falling to 2 ohms at 10kHz, where the low Z does no detectable harm, and that's unavoidable since an ESL is a giant capacitor. Below 200Hz, Z goes up. I can drive them to chest-thumping levels at way below full power from my amps. I am guessing 50W would suffice. But I am not advertising for anyone to follow that path; the Australian step-up transformer is now long out of production. I would go with what SL now recommends as their latest set-up.
Edits: 07/06/21 07/06/21 07/06/21
Hi Lew,
Good to see your post and thanks for giving the background on how the AU90 transformer came to be used in SoundLab ESLs.
Unfortunately the archives were not carried over to the new SLOG site. Site owner Mark Gilmore indicated when the the new site came into being that he would try and attempt to port the archives over but up to this point that has not happened. There recently has been renewed discussion about the AU90's and it would be great if you could post your response over on SLOG to give the newer members historical context.
BTW, I have lately been in contact with Rob Mackinlay of ER Audio about his 1:90 audio transformers compared to the AU90s previously supplied, his response was:
"The 1:90 xfmrs on our website are electrically much the same as the ones we supplied previously to the SL Owners group. Externally they look different as they don't have the connector block across the top and the plastic mounting flanges but essentially they are the same. We use two of these in our ESL IV full range ESL wired parallel series to give a turns ratio of 1:180. In this speaker they are mounted on a "drop through" board, much like the output transformer on a tube amp, so the original mounting hardware is not required.
If they are used in a Sound Lab speaker, mounting brackets would need to be added."
So effectively a suitable equivalent transformer is still available for those interested in modifying their SoundLabs.
This explains my failure to follow up on SLOG.
It's great to know that a similar 1:90 audio transformer is available. I had forgotten the vendor, so thank you for providing his identifiers. So far as I recall, he was located in Perth, while Will lived in either Melbourne or Sydney, the opposite coast to Perth. Anyone intrepid enough to try this modification to an SL speaker should know that some who tried it back then, using the new production 1:90 transformers, did not like their results. Those guys eventually adopted the revised toroid purchased direct from SL. I never was able to figure out why the one or two guys I am thinking of were not happy, but I did determine for myself that phasing of the two transformers (the OEM bass transformer with the AU90) is very critical. If the two driving transformers are wired out of phase, the impedance goes down the toilet, like 1-2 ohms across the board, and the sound most likely would be awful. With any amplifier. The two transformers MUST be wired in phase with each other.
Edits: 11/14/21
That's a variant of which I've not seen or heard. All the toroid backpanels I've seen used both a resistor and a capacitor. The cap was either 2 or 3 units in parallel for about 30uf IIRC.
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