Schematics for AU90 transformer modification

[Ezequiel64]

Yes, I believe them to be very similar. The inductor and capacitor you see on top of the brilliance control, was there for a flatter frequency response in the midrange. I wondered at one point, if perhaps this filter was causing the issue, and confirmed with Dr West that may be the case because I run OTL’s. Dr West advised that I remove the inductor because it was causing a dip in inpedance that my amps won’t like. Also, I consider playing with the value of that cap in series with it, but found it sounded better with no cap at all!

[Ezequiel64]

And here’s the high frequency section.

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[lotus123]

Looks the same as my Majestic except it appears to have two inductor cores. I did not see brilliance control?

[Ezequiel64]

John V,

I believe the back plates to be ulmost identical. Along with some pics, here’s a diagram I found and modified to show how these were wired.

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[John V]

Hi Ezequiel64,

Could you take a picture of the underside of your backplates?
I’m curious as to how it compares with the backplates for the full
range SL ESL’s.

[Ezequiel64]

I see, and thank you for the link. If I can get the specs,it seems getting a pair duplicated will be difficult. There is a transformer designed by Meno Vanderveen, and built by Plitron. It has a different turns ratio, and requires a resistor to tame some potential ringing. Has anyone tried with any success using it as a possible alternative?

[John V]

Aside from the turns ratio I’m not aware that any other specs for the AU90 were ever available.
I would contact Rob Mackinlay of ER Audio to see if can supply any
specs. Also, if enough people have an interest maybe Rob would consider doing another run of transformers.

http://www.eraudio.com.au/index.html

[Ezequiel64]

Hello, and first time poster in this forum. Early last year, I purchased a one of pair of soundlab speakers known as the Renaissance R3&R4. They are a hybrid design with a panel for the highs and mid frequencies, and a dynamic woofer for the lows. In the past year I’ve been restoring them, as well as made some changes to the back plates in order to help alleviate a problem mentioned in this thread pertaining to the midrange suckout, that seems to be typical of the era around 2004. The Au90 transformer mod is very interesting, and wondered if anyone has the spec’s for it, so maybe it could be duplicated here in the states, or perhaps matched to one already made?

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[John V]

Hi lotus123,

Check out this thread on Audio Asylum. It contains Lew’s comments
about the AU90 transformer and Will. Lew evidently did not remove the inductor connected to the bass Xformer while Will did. I think you would want to keep the inductor to block the highs from entering the bass transformer.

https://www.audioasylum.com/cgi/vt.mpl?f=otl&m=35023

[lotus123]

Hi John, it’s good to hear from you again. This is important to me as my hair turned grey and now Will had passed away a few year back. And we met again in full circle. I am retired so I finally get around to updating M854PX vintage 2015.

I want to clarify if the core inductor hooked up to the bass transformer should be removed also in the mod? Will told me to rip everything out except for the power supply, bass transformer and add small value extra high voltage caps between AU90 and bass transformer but did not mentioned core inductor.

[John V]

Hi lotus123,

You are the one who purchased the AU90’s from me. And I thought I procrastinated, your as bad as me.;))
What vintage M854’s do you have? I don’t recall that you owned them at the time you purchased the AU90.

[lotus123]

Ha ha John, I think you were the one who sold me your unused AU90 about 5 years ago? If not, thank you along with D10 and Hifibuff for all your help. I am going to get interestingly busy to tackle the long awaited project over the holidays.

I just responded to a SLOG member about the new Prestige 1 speakers. Don’t expect Earth Shattering improvement, although the AU90 mod was and is an earth shattering improvement over stock. I was bothered by the mid range suck out in the 100 hertz to 160 hertz region in my M845s. No reviewer ever highlighted this flaw in any of the reviews. Evidently, we listen with our wallets, and the pocket is deep enough to silence any upfront critics in this smoke and mirror cottage industry of hi end stuff. When I get to audition Lew’s system in Maryland with the mod, the speakers transformed. It became so efficient that the music came through with utterly clarity and punchiness. I guess all the signal was not being absorbed by the passive components in the stock back plates. Mostly importantly, the mid range suck out was remedied.

[D10]

Lotus 123 – I had all pics how I put acrylic plate but I can not find them .. Will try again .. If I take one of my backplate I will take a new ones for sure ..

[lotus123]

D10 and HifiBluff, thank you both. I first got over the mechanical hurdle by building a mounting plate for AU90. The backplate for my M845s is spacious enough to fit a stainless steel plate to block off electromagnetic field from the bass transformer. Remember our sonic signals ride on the transformers. D10 if you can, I would very much appreciate to see a photo of the inside of you backplate so I can see the wirings (only when you happen to do the periodical checkup). Again thank you both.

[D10]

Do not use too thick acrylic plates – transformer could hit your floor

[D10]

Ha! Exactly the same schematic I had! It is very easy to make it !

[John V]

Hi lotus123,

Here is a copy of am email I received in 2012 showing the brackets made to hold the AU transformers:

From: David [mailto:dmcclesk@swbell.net]
Sent: Sunday, August 28, 2011 10:04 AM
To: ‘acousticsupplies@gmail.com’
Subject: FW: brackets for tranformers

Hey Will – Here is the info I sent Lew on subject…Let me know if you have any questions…David

PS: Make sure when you do the cutouts in plate 3 that you leave enough room on the outside edge for the machine screws. They are right at the edge…

From: David [mailto:dmcclesk@swbell.net]
Sent: Thursday, August 18, 2011 6:51 PM
To: ‘markoffl@yahoo.com’
Subject: RE: brackets for tranformers

Hey Lew – Here is a quick sketch of how I made the mounts. All three pieces are ¼” acrylic. The cut outs should be large enough for you to get a nutdriver on the nuts that fasten the mount to the backplate. I can’t replicate them as I would have to take my backplates apart to measure spacing and sizes again, etc…The finished mount is removable. The 4 – 8/32 or 10/32 machine screws should be tightly threaded from the underside of plate 3 to avoid slippage when fastening the transformer nuts…Let me know if you have any questions…There may be a more elegant solution but I was in a hurry when I made them…After the picture (below) of the backplate was taken, I removed the brilliance pot from the circuit and hardwired the transformer connections with Valhalla..only because I had it already from the original backplates.…Also while the 1.5 sledgehammer is still in place I have it switched out. I’ll probably remove it at some point. Let me know if you have any questions…

David

—– Forwarded message —–
From: “Lewis M” <markoffl@yahoo.com>
Date: Thu, Aug 18, 2011 10:35 am
Subject: brackets for tranformers
To: “dmcclesk@swbell.net” <dmcclesk@swbell.net>

Dear David,

Is there any chance you can get me a pair of the brackets you made or had made for mounting the 90:1 transformers in the backplate? I would pay all costs, of course. If not, perhaps you can send me a drawing or plan for making them here.

Lew

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[D10]

The last pic is identical as mine . The problem is that AU-90 could not fit in my U-1. So you need to use 2 acrylic plates join together with cement – one should have holes to fit on backplate and another to fit transformer. I do not use inductor in my SL
D

[lotus123]

@hifibuff, thank you. Did you encounter issues with wiring the two transformers out of phase? Did you keep the core inductor or had them removed like Will did?

[hifibuff]

No problems. Since my A3 are from 1989 the backplates are small. I had to remove the hi frequency pot (it was bypassed with a resister but left if to fill the hole), mount the switch PS vertically and use spacers under the mixer to clear hardware on the backplate.

[D10]

Lotus 123, wow .. no way Will passed last year .. honestly I forgot where I read about it . It has to be in old SLOG forum .. too busy to look up for it
D

[hifibuff]

I found this from my dusty files. These were a tight fit in my A3 backplates.

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[lotus123]

Earlingarizona, the schematic is for stock not the AU90 mod. I recalled many years ago Lew on the forum told me to insert two small value but extremely high voltage caps as he did.

[lotus123]

D10, wow it’s been that long even Will passed away last year. I remembered Will even removed the inductor core. Did you remove the inductor core as well?

I wonder where is Lew now? He and Will were the foremost in directing Dr. Roger West to revamping the backplate with a more efficient tranny.

Earlinarizona, thank you so very much.

[D10]

Damn! I will take a hard look to find it this weekend!
Unfortunately Will passed few years ago …
D

[Earlinarizona]

I had this from long ago but don’t know if this is what you are looking for.

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[bk12]

I can’t find the schematics either. Unfortunately, I believe Will, the Australian that figured this out and sourced them has passed away a year or so ago.

[D10]

Why don’t you contact guy directly in Australia. I used to have schematic but I can not find it . Mod is very simple – you connect AU90 instead of toroidal transformer and take out entire crossover network – as I remember. So no caps and resistors in backplate. AU90 has natural frequency slope with bass transformer.
Will – who was real SL enthusiast and genius used his SL with wide open tone controls on backplate – I do the same .
D

[lotus123]

Anyone can help in getting the schematics for the backplate modification for the AU90 transformer? This mod was very popular back in the 2011 with members like Lew and Will from KIS audio in Australia.

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