Interface Circuit

The interface contributes massively to the sonics of an Acoustat ESL - obvious I suppose. As the elements and transformers are excellent quality, we have concentrated our efforts on improving the interface components and its "interface" to the speaker elements, amplifier and wall socket. Although our mods haven't changed the electrical characteristics of the interface, they now seem to be easier to drive i.e. they seem to go louder for a given input. However even though they are easier to drive, they still represent a difficult load - see graph below.

Audio Section

Replacement Wiring

An early mod was to upgrade the internal wiring to Monster Cable. However this cable suffers from internal corrosion (green discoloration) so should be replaced. I use Kimber Cable but any cable deemed to be 'better' can be used. We also bypass the phenolic tagstrip and push clip used for bass boost (transformer tapping dependant on the number of panels) by joining the two wires together. The tagstrip holding the 1 ohm R4 was moved to a better position to shorten wiring to the bass transformer - see pic below. The hole and bolt for mounting the tagstrip was previously used for p-clips holding the monster cable.

The input terminals of the interface have come in many guises. We prefer any heavy duty binding post as the originals were very poor.

There is also an input fuse. We took this out of circuit many years ago - do so at your own risk noting failure modes of the power amp and bearing in mind the transformers are no longer available! If the amplifier is solid state, we would not do this.

High Frequency Capacitor - LT side

The HF capacitor (C1, C3, C3) may have already been upgraded as part of an earlier mod. The value of this capacitor will depend on whether the interface has the medallion modification.

This capacitor will determine the voice of the speaker and is really system dependant. I have tried a number of capacitors in this position and found they all give a different presentation. Solen, North and Ansar all make metallised film capacitors that are suitable. To my ears, they all exhibit the "metallised" sound which is not my preference - the Norths the least of the group by far. I believe paralleled capacitors in some systems will provide the "zing" to get the system to sound alive but ultimately I find this to be tiring long term and compromises tonal purity. Some of us have used Black Gate types in this position. I did use them for a bit but lately haven't tried them again in revisiting all the recommendations on this site. So the North Zen that doesn't fit very well is in there at the moment. Check out the size difference below between the Black Gate option and metallised PP film. Some time soon (early 2003) I hope to get back to the BG and see how he goes.

High Frequency Capacitors - HT side

These looked like paper caps in the original (they're covered in paper but may be mylar type) but as I don't have them to hand I can't recall accurately. Later these were yellow MKP types. We replaced these (C4 and C5) with 0.01uF WIMA FKP1s rated at 2000V DC. We measured the AC voltage when playing the speaker loud and although these are of lower rating than the originals or medallion replacements, they seem to survive. WIMA do have a 6000V rated 0.01uF FKP1 and they are probably a better bet. Very powerful solid state amplifiers can cause the 2000V capacitors to fail but a series parallel combo seems to fix this. I now use 0.01uF 5kV rated film/foil PP capactors from SEC.

Low Frequency Resistors

The original resistors (R2 and R3) were ceramic cored wirewounds. We originally replaced these with series parallel combination of Resista MK4s but they couldn't handle the power and died. We then tried Dale 47K 50W resistors (the ones that look like a gold heatsink) and they are what we used until recently. Further experiments have shown that Caddock power film resistors to be quite grainy in comparison. We are now using 2 off 25K 10W Ohmite resistors of the same type as the Norths. Check here for help in sourcing parts.

We have experimented with removing the 1 ohm resistor in series with the bass transformer (R4). In the past we have thought removing it to be of benefit but lately we have realised this is a mistake. It seems to roll off the bass transformer. Without R4 the treble is splashy and unrefined and the midrange congested. This can make the speaker sound more exciting but not accurate. The quality of this resistor in very important and the winner in this position at the moment are from North Creek.

Removing R4 does not improve bass performance. From Andy Szabo previously of Acoustat .... "The resistor (R4, 1 ohm) is there for a very good reason, and it is not for reducing the output of the low frequency transformer. A transformer has very low DC resistance, in this case only a fraction of an ohm. Some amplifiers are unstable into such a load, so the 1-ohm resistor was added to guarantee that the DC resistance of the speaker was never less than 1 ohm. Additionally, if your amplifier has any DC offset on it's output, then this very low resistance can cause significant current flow, possibly causing instability and/or overheating."

High Frequency Resistor

This was originally a rheostat (R1) used to adjust the HF balance. The size of these resistors will depend on whether the interface has the Medallion modification. For non-Medallion interfaces I used 2 ohm and 4 ohm (2 ohm to speaker terminal) but have used 1.5 and 4.5 ohms in the past. For Medallion interfaces we used 1 and 15. We favour the North Creek resistors in this position too. We have used Caddock MP type power film resistors but they are grainy and closed in compared to the Norths. The down side is losing the flexibility of the HF control which can be a problem if the system or room changes. I'm revisiting these recommendations so living with the rheostat for a while and then swapping back again - this being early 2003.

Please note: We have chosen the 10W North/Ohmites for their sonics first. It is quite possible with very high horsepower solid state amplifiers that the resistors we've chosen may get hot. Although we listen loud at times, others may listen louder in bigger rooms.

Power Supply Section


Teflon Board

We used Teflon 2mm thick the same size as the original PCB. The capacitors are mounted on top and the circuit connected together using the diodes on the bottom. This was one of the first modifications we carried out so used all the original components. Highly recommended!


I have been using the original 500M resistor (R5) feeding the diaphragm. Now experimenting with the HV thick film resistors from RS, 500M 7.5kV, type 296-0645. The Caddock MX450 (used for some time by others) seems to be as most Caddocks, grainy and closed in compared to the original. If there isn't a problem, then the original is hard to beat.

Mains lead

We have experimented with mains lead feeding the interface and find there are benefits. I have used reconstituted mains lead (solid core twin and earth) wrapped around "XLO style" a PTFE tube former. Richard and a few others use RG214 RF cable, Andy uses XLO mains cable on his Quad ESL63s.


Most of the above modifications refine the sound and generally increase the speakers resolution, bandwidth, dynamic range and ultimate loudness - in other words the voice of the speaker changes dramatically, we believe in the right direction. Some of the modifications on their own may reveal an unpleasant result but that usually means their has been another problem area exposed OR you are just hearing more of the truth upstream.

Sourcing Parts

The Ohmite resistors can be hard to get outside of the values North Creek supply. Use the Ohmite Website Worldwide Inventory Search Tool to locate stock. Note: Check Ohmite values for resistors as they use non-standard values.

Further Tweaks

Interface Housing

The interface is very susceptable to vibration which is exacerbated by the fact the Model series use it as a major frame support component. In our early frames, we used the interface to weigh down the speaker. When we isolated the interface from the vibration, we experienced a less confused sound. Moving the interface out of the rear sound wave improved things further. Mounting it on damping material and a single hard point such as a cone improves things again.

EHT Modification

In messing about with mains regeneration, it occured to us that the mains and/or the multiplier was messing with the sound. The multiplier has no effective smoothing so we added capacitance to the output to smooth things out a bit. The effect was far greater than we could have imagined. First up we used the spare treble HT coupling caps of 0.01uF as we had them lying around having replaced them. Later we tried various sizes of PP film/foil SEC capacitors; the same as we used for the audio part. Currently I'm still using 0.033uF rated 5kV having tried 0.01uF of the same type. Not sure if there is any difference; see pic below for placement. It changes the speaker dramatically removing a greyness to the sound, opening it up dynamically seeming to allow them to play louder and more extended at both ends. Quite remarkable!

Note that this added capacitor will store energy after the mains has been switched off and could prove to be very dangerous. Only do this mod if you are absolutely sure you know what to do as a mistake could be fatal!


Some Pics

Revised R4 position, interface box, Black Gate size comparison


Teflon cct board and carpet eye view of EHT mod capacitor.