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Yes, thanks microstrip … your explanation is same as I received from Roger and only thing I can add (If i didn’t mention before) is that actually the SL speakers will not saturate with the metal core if used within their power rating and if are not over-driven. Amplifiers generally have a lot more power than they did 20, 30 40 years ago and it is now pretty easy to happen to have an amp that is overkill for SL and therefore to over drive and cause saturation of the mids…I personally think Roger became a little tired of having the occasional complaint that the mids did not perform well at high levels and while he can (and did) say that the speakers were being over-driven, it is somewhat easier to not have any complaints and use of the air coil offers that …although the speakers would still be over-driven and damage may eventually occur just the same with or without any saturation … hope it makes sense. Chris
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Hi Chris,
A coil can only saturate if it has a magnetic core – saturation happens when the magnetization of the material reaches a maximum, even when we increase the applied magnetic field. An air core is saturation free – however due to the resistance of the copper wire there is a limit to the current that a coil can accept – heat released is proportional to the resistance of the coil and the square of the current. Although it depends on a few factors such as geometry and air convection, it is accepted that a 15AWG copper coil has a typical current rating around 20 amps.
Although if we accept large size and cost the rule does not apply usually there is compromise – air core has higher resistance and no saturation, magnetic core has lower resistance but saturation issues. Many people tell us that a properly dimensioned coil with an adequate core does not have listenable saturation effects before speaker damage.
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Hi Chris. Thanks for update, and in general all your recent informative posts. My guess is that the new air-coil is probably 18ga, similar to this:
Jantzen Audio 4.0mH 18 AWG Air Core Inductor Crossover Coil
15ga version will be massive and heavy, probably overkill, although it will have much less DCR.
Will be useful to confirm. Not that I will modify the backplate, but good to know.
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Good day Nirvana,
Roger responded – he is just on the road currently until Friday, – he confirms the previous backplates are 10amps but doesn’t offhand remember the new air-coil rating, but supposes it should be about the same. Note he mentions this is a rating for ‘musical’ amps which “have a duty cycle considerably less than DC amps”.
All good?
Cheers,
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Thanks, Chris, for asking Roger.
Thanks John V! I did not see the Toroid II schematic that you posted. That explains it well. I think my M2 hotrod/consummate back plate has that configuration circa 2013. If I traced it correctly, looks like the effective inductance values based on midrange selector switch positions are:
-6 dB (5mH + 15 mH = 20mH)
-3 dB (10 mH)
0 dB (5mH)
+3dB (1.5mH)Those seem to match the ideal values Roger mentioned based on his calculations.
Curious what current and voltage ratings those are, in case you know.
That also explains the new air core inductor with 4mH is similar to what we had before with 5mH (0dB) setting. I never felt the need to set other than 0dB for midrange, and also for Bass control. They interact with one another. 0 dB setting for both has worked for me with my setup.
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Regarding SL_nirvana’s question about the values and power handling of the muti-tap choke, below is an explanation I received from Roger West back in 2018 as to why he resorted to an outboard choke to obtain the values he needed at the time:
“Hi John,
I’ll try to clarify the coil issue a bit more. Initially, we used a tapped coil that had 5, 10 and 20 mHy. I needed 1.5 mHy to complete my design, so it had to be an outboard coil. The outboard coil was attached to the 5 mHy coil, thus all values were boosted by 1.5mHy: 1.5, 6.5 , 11.5 and 21.5 mHy. At the time I felt that this slight change wouldn’t make any audible difference, and it didn’t. The small difference couldn’t be detected by the best of ears around here, but my addiction to my math analysis couldn’t be shaken, so I decided to change the values to what I initially wanted them to be. I changed the values of the coil to 1.5, 5 and 10 mHy and added the 8.2 mHy unit (mounted on a steel plate which increases the value to 10 mHy). No difference was to be heard, but I felt better. I found a source that could provide all four taps in one unit, which will eliminate the outboard coil. I hope I haven’t made this more confusing.
Best, Roger ”Note that the schematic I posted the other day does show the values mentioned above.
On another topic, regarding the brilliance control. Roger has mentioned that periodically its not a bad idea the rapidly rotate the control back and forth to remove any oxidation that may accumulate om the wiper over time causing a change in possible value.
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… and adding to microchip’s accurate info regarding replacing the Brillance Pot with the equivalent of six 12 ohm R’s in parallel to achieve 2 ohms, that is equivalent of having the Brillance control knob set at a 2:30 0’clock position. This is also where I’ve always had my control set for the the most natural sounding flat response in my listening room.
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I would add that the brilliance control is a power variable resistor – in fact a resistive wire wound potentiometer.
This one is an 8 ohm unit and was taken from old A1’s – we can see that the operating zone of the wire had became very hot from the color change of the wire. In my speakers I get flat balance with 2 ohm – using six 12ohm Mundorf 10W resistors in parallel. -
Hi Chris. Could you check the mH and current ratings of mid-range inductors on the older back plates that have the +3/0/-3/-6dB selector? I assume the -3/-6dB settings use a multi-tap inductor in a black box… I see a separate inductor for +3dB outside of the inductor box.
Also, do you know the current rating of the 4mH inductor on the latest version back plate?
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Note that I am not at all qualified as an electrical engineer, so I apologize for lack of appropriate terminology.
Note 2: It is rare to harm the electronics, but if something does ‘burn out’, it is most often the brilliance control, which is actually fairly inexpensive and not difficult to replace. And is so intentionally because it acts as much like a ‘fuse’ or early warning sign that you would easily notice and it prevents the likelihood of severely damaging the more expensive components (torroid) in the backplate. If the brilliance control does become damaged, your hi frequency will disappear, but turning the control fully clockwise can be a useful diagnosis because it will by-pass the control and hi frequency will be fully restored (with no attenuation)… Hope this helps.
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Sure Massimo,
The continuously adjustable brilliance control is basically an attenuator for high frequencies.
The frequency response of the panel (not considering any room effects etc) is designed to be linear or flat with bass and mid set at 0db and the corresponding 0db or flat response is having the brilliance set at 3pm.
This is not really indicated anywhere.
If you turn the brilliance fully clockwise to its maximum position, the contact in the control are actually bypassing the attenuation mechanism and you are hearing the high frequency the same as if there was no brilliance control (attenuation) at all. Note that if there is no attenuation, the impedance does fall to less than 2 ohms at the highest frequencies. -
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The short reason for Roger’s not putting detail frequency info for the controls is his hope that people should decide more by listening rather than the ‘number’.
I am new to the group here though I have been representing Sound Lab in Japan for over 12 years and have decided to give my best support to SL basically around the world… my new website is coming. (I am originally from Canada)
Small note – perhaps you all know already – the brilliant control set at 3pm is actually equivalent to 0db (flat) and that turning the brilliant all the way up actually circumvents the control completely. -
Good day all,
I’ll try to help out as I can.
First – here’s the wiring diagram for air coil back plate. -
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Am running a pair of M1s purchased used two years ago. They are 20 years old but in great shape. Had them shipped to SL to check the panels and upgrade the backplates. The backplates are last generation with midrange control. I tried boosting midrange to see what happened. The sound was terrible in my room with my rig.
Re. the DSP remarks, I’m using it to offload frequencies below 120 Hz to my subwoofer. While my panels will get down below 30 Hz, at regular listening volumes (~85db) the panel membrane will slap the stators at lower frequencies, usually with close-mic’d drums. Not a pleasant sound.
Using Anthem Room Correction installed on my Anthem STR preamp, I’m correcting only to 300Hz. Since the subwoofer integration is in the preamp, it’s well controlled. The other advantage is that a lot of load is transferred off the panels, so they are more efficient. These things need lots of power.
I sent a note to Roger about the stator slap. He confirmed it and basically told me there’s nothing I can do except upgrade to new bass focus panels, which won’t happen.
DSP has worked well for this. I’ve tried extending the top DSP frequency to 5K – sounds terrible. I’m with WillB here. These great speakers don’t need any help with mids and high frequencies. Open. Pure.
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Re: DSP
I wouldn’t want to add an additional layer of – digital signal processing- to the playback chain, especially when sourcing analog. Call me a purist but weighing the pros and cons, I’ll opt out.Having experience in the digital audio (recording studio) workstation world I would not want DSP EQ or SRC (sample rate conversion) in my stereo system.
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I have never understood the reason why the ‘mid’ control has 3dB steps just like the ‘bass’ control. This is a gross error: the isophonic curves of Fletcher & Munson have shown that the human ear is much more sensitive in the midrange. Much more correct would be a control in 1dB steps.
To be honest, I have never used the ‘mid’, but the one time I checked its operation I got indeed the impression that the control does not have as much emphasis or de-emphasis as claimed.Another thing that has always puzzled me is the fact that the frequency range is not stated.
Finally, I have to add that, as much as the manufacturer’s effort to offer greater flexibility in the use of these loudspeakers that favours their placement in different listening environments is appreciable, the current offer of equalisation systems and dsp for room acoustic correction makes them superfluous. -
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Good catch WillB!
According to the schematic I have the zero DB position uses a 5 MH coil.
Also, the coil, or coils, are part of the Bass Transformer low pass circuit, assuming I’m reading it correctly. -
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It is WBF (What’s Best Forum)
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Today Chris wrote the following:
I checked with Roger and I’ll paraphrase his response. As usual he always gives a very complete and detailed explanation and I am happy to share that with you but this is probably not the place to do so. If you wish to have more information, please contact me thru my website ‘contact’.
In essence Roger does not recommend spending the time/money to change unless you normally play your speakers at very high levels (which is not recommended as you can imagine). The performance difference between metal core and air core is that at high current levels the metal core will saturate and lose inductance, mids frequencies will suffer and have distortion – but only at high levels. Air coil does not saturate. These days many new amplifiers are pushing out much more power and it is easy to push the speaker to limits and even beyond, the air coil prevents any distortion to the mid frequencies, but it is also not a good reason to continuously push the speaker past its intended power rating – the original metal core coil will not saturate if played within the speakers max power rating. You might say this is not so much of an ‘upgrade’ but a move ‘side-ways’. Please let me know if I can be any more help and/or explain further. -
Maybe this has been discussed here. I found on WTB Forum. I didn’t know about it.
I wrote:
Nice: Website: https://soundlabelectrostats.com … I noticed the photo of the backplate has no mid-frequency adjustment.
Chris replied:
Thank you, yes, that is correct. The latest generation backplate (from about 1 year ago) has been effectively set at ‘0’. There were a couple of reasons for this but mainly the decision was made to use an air-coil (which avoids saturation) rather than metal core inductor and is overall a better construction for the backplate. Thanks for noticing.
I asked: Do you know if there is a way for users to modify their Backplates (with adjustable mids) to this new version? I have never used the mid adjustments over 28 years. I have the latest version prior to this new version.
Chris replied:
Let me get back to you on that. Should be possible, but I have not been asked that before and I will give a confirmation shortly.
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