ESS Amt Monitors restoration

[Juha]

I have started restoring ESS Amt Monitor speakers from early 1980's. They are big: height 100 cm, width 40 cm, weight 55 kg. Featuring a 12 inch woofer plus 12 inch passive radiator and the Heil Amt driver.

First impression was totally overblown top end. It was so harsh that metal fillings in my teeth vibrated. I started with recapping the crossover. Mundorf M-Caps replaced the totally inadequate originals.

The pot in the middle is a brightness control. On the left we have bi-amping terminals.
http://hoone.com/ess1.jpg

The sound is now elegant. Smooth, but still too much top end.
Maybe this was the norm in California. Some music sounds very good like this, but it is tiring in the long run.

Today, I put a parallel 2.2. ohm resistor on the Amt's. This helped to bring down tweeter levels.
http://hoone.com/ess2.jpg

ESS woofers need new surrounds. Current surrounds are too stiff and partly broken.
Stiff surrounds could cause woofers to operate under desired levels at least on low volumes.
http://hoone.com/ess3.jpg

Testing with Squeezebox Touch, Velleman 8020 valve pre-amp, Quad 522 amp. The small speaker on the left is Amphion Helium Plus.

[DSJR]

Hardened surrounds can lose several db in output so no wonder these are currently screaming your head off - very old Spendor Bc1's with white surrounds on the cone are the same, more "modern" balance because they don't honk any more, but the power handling becomes shot to pieces.

Do you know what the tweeter impedance is? I only ask because adding a parallel resistor might take it down too far????? Adding it in series with the return might be a better short term option until the bass units are sorted methinks?

[Juha]

On DiyAudio forum the Amt tweeter impedance is quoted as ~3,2-3,7 depending upon frequency. Amt Monitor speaker impedance is 4.2 ohm.

Thanks for the tip. I will try serial resistor.

[DSJR]

Good luck, easily changed if it's worse :)

[sq225917]

I'd check that the stated values of all the components in the XO actually match what they say they are. The gap in the surround on the bass driver will also result in changed output level.

Nice speakers, a classic.

[walpurgis]

Nice speakers. I have heard these and the AMT1 Tower (the big transmission line job) back when they were in production, most impressive.

As has been implied, the polyurethane foam surrounds have no doubt age hardened and are causing the frequency imbalance. I'm sure things will improve once you fit the necessary new surrounds. If you need to further trim the treble output, a quick Google will find loads of suitable circuit mods and designs online.

[edna]

I would remove the resistor as it will change the crossover point. The only way to do a resistor is by series and matching in parallel. Check the crossover values as the ESS crossovers tended to start a little early for the bass driver creating a slight gap...also measure all values (Impedance, capacitance and mH)using a meter and optimise values. commercial tastes usually dictate that a suck out at the crossover point is easier to sell then a flat response plus too hard for production to be that accurate. I have also made a frame and suspended my Heil using a bungy cord to reduce Microphony and yes its the best thing I have EVER done to a speaker. A square frame with two holes at the top - bungy cord going thru Heil then out the bottom and off to the sides to another set of holes. Careful elimination of damping minimising the stuff adds a lot of life and dynamics but don't go too far...probably a half of whats in them from factory will do and experiment with different textures of damping - mixing them inside the cabinet....common sense. Good luck - rewire as well of course...

[DSJR]

Phillip, PLEASE can we have some piccies of your mods in a separate thread. We LOVE this kind of thing here, especially with products that are rare on these shores :)

[Juha]

I measured all resistors on the crossovers. Changed two that were too far from nominal values. Both speakers had identical wiring and cap values. Original caps were replaced with M-Caps.

I tried to find crossover schematics. ESS has had so many variations and models that I was unable to find anything useful. So I don't have mH values.

I have now found a local shop that will refoam the woofers.

My AMT Monitors were in storage for 20 years. Had these since early 80's.

[edna]

Crossover on AMT is usually just 12 db and pretty easy to work out...you will need to buy a multi meter which can measure mH values and desolder an end of the coil to measure correctly. Biggest issue I feel with the standard AMT speakers is the suck out in the Mid. I use some Kevlar 10 in (I have AMT 1) with the dust cone doped with PVA glue. This makes the midrange a little clearer...you might want to experiment with the ESS 12 inch drivers which are a good driver and can do midrange well for a 12 inch.

[The Grand Wazoo]

Philip,
I've left a message on your profile page (http://theartofsound.net/forum/member.php?u=7552) - please read and act on it, if you don't mind.
Thanks a lot.

[edna]

P.S. you need to get rid of the original wire if you are still using it...its BAD...something like QED 72 would be fine and a big improvement without costing the earth. I used vdHul skyline which is not expensive.

[Marco]

Hi Philip,

Welcome to AoS :)

Could you please do me a favour and before going any further, go to the Welcome area and intoduce yourself to our community, telling us what system you use and what music you like, as this procedure is required of all new members - cheers! :cool:

Marco.

[Juha]

ESS woofers are back from speaker repair shop. They are 11.5 inch, not 12 inch.
This made it more difficult to find new surrounds. Surrounds are now rubber. Originals were foam.
The repair tech said foam always fails in the long run.
I was lucky to find this place. Even in Tallinn they repair woofers on a daily basis.

New surrounds improved sound a lot. Bass was a little too boomy.
There was no damping material behind the passive radiator. I filled the bottom of the speaker with 30 mm polyester damping. This cured it.

Now ESS Amt magic is back.
Next step will be making the cabinets more beautiful.

Repair cost so far 8 Mundorf caps + new woofer surrounds totals 195 eur.

[Juha]

ESS Amt Monitor restoration is now ready.
The sound is sweet.

Cabinet veneer was lightly sanded and finished with two coats of Liberon Finishing Oil. Speaker fabric was removed and washed with mild detergent.
The chip boards holding the fabric needed some touch up with black paint.
I fastened the fabric with 3M Spray Mount glue.

Lessons learned:

1. I was too quick to dismiss these speakers. They needed restoration to sound good.

2. Mundorf caps really made a difference.

Here is a picture of my current setup. Quad 303 / Quad 522 are fantastic.
The Harman Kardon HK 3470 amp looked promising with a large transformer and discrete transistor amp. But is it no match to the Quads.

Speakers are Magnepan MG 2.5/RT, Amphion Helium plus, ESS Amt.

http://hoone.com/essready.jpg

[DSJR]

Quick suggestion. If the Quad 33 is stock, it'll be a sonic bottleneck these days, much as I love the old thing. Loads of things you can do to it, both in upgrading the standard boards and especially the power supply, and in replacement boards from the likes of Net-Audio, which all but change it into a different preamp in Quad clothes...

[Juha]

My Quad 33 has been upgraded with the Dada Electronics kit. The kit and volume pot rebuild made it usable.

I have tried other pre-amps after the upgrade, but always end up with this one.

It sounds so good with electrostatic Magnepans. I often wonder if Quad did some tricks on Quad 33 to make their electrostatic panels sound better.
Quad 33 is not neutral. It does something.

[DSJR]

Mine was dada updated and I also changed C400 on the output boards from .068uF to 1uF film caps which opened out the line stage no end in terms of 3-D and bass quality in general. Using the tape monitor input and adjusting the preamp gain there, the Croft, AVI and old updated Crown to my ears do give closer matching of input to output than via the 33 in updated form. The power amp you have has been "discovered" recently and this series aren't as anonymous as they once were, sadly...