I wonder if someone on the forum has detailed information on the John Koval Quad ESL mod and would shed some light on how one could re-arrange the '57's crossover/audio transformer connections to arrive at the same or similar configuration. The Koval mod is, to my knowledge, no longer available.
Some background on my plan: Whilst continuing to work on my Acorn build (still ongoing; updates in another thread) I started to realize what an amazing feat it was for Walker to have designed and manufactured the Quad ESL some 70 years ago as a reliable, consistent commercial product. It also began to dawn on me how coherent and musical the ESL '57's midrange really is, and how lucky I was to have enjoyed my '57s for years. Used to take it for granted so when all my '57s finally required a restoration to continue providing services, I kept them in storage without a definite timeline in mind for the rebuild. With the newfound appreciation for the '57 and since I already have the ER Audio re-diaphragm kit, no more procrastination! As for the Koval mod--I first read about it in a reprint of Dick Olsher's 1987 Quad ESL review, and his mention of the mod allowing the '57 "to possess flat response on-axis (within 2dB) from 700Hz clear out to 17kHz" really interested me. Since I have two pairs for rebuild, I plan to convert a pair per the Koval mod if possible, and do a comparison.
The Koval mod is said to convert the '57 from 3-way to 2-way and use no capacitors for the crossover. I presume this means that it feeds the two mid (or tweeter) strips and the treble (or "super tweeter") strip in between with the same signal, and relies on the panel's own capacitance plus the audio transformer's parasitic capacitance for the high pass filtering. However this is just my guess and I have not been able to find more specific information on the Koval mod.
Any information would be much appreciated. Suggestions or ideas are also very welcome.
Some background on my plan: Whilst continuing to work on my Acorn build (still ongoing; updates in another thread) I started to realize what an amazing feat it was for Walker to have designed and manufactured the Quad ESL some 70 years ago as a reliable, consistent commercial product. It also began to dawn on me how coherent and musical the ESL '57's midrange really is, and how lucky I was to have enjoyed my '57s for years. Used to take it for granted so when all my '57s finally required a restoration to continue providing services, I kept them in storage without a definite timeline in mind for the rebuild. With the newfound appreciation for the '57 and since I already have the ER Audio re-diaphragm kit, no more procrastination! As for the Koval mod--I first read about it in a reprint of Dick Olsher's 1987 Quad ESL review, and his mention of the mod allowing the '57 "to possess flat response on-axis (within 2dB) from 700Hz clear out to 17kHz" really interested me. Since I have two pairs for rebuild, I plan to convert a pair per the Koval mod if possible, and do a comparison.
The Koval mod is said to convert the '57 from 3-way to 2-way and use no capacitors for the crossover. I presume this means that it feeds the two mid (or tweeter) strips and the treble (or "super tweeter") strip in between with the same signal, and relies on the panel's own capacitance plus the audio transformer's parasitic capacitance for the high pass filtering. However this is just my guess and I have not been able to find more specific information on the Koval mod.
Any information would be much appreciated. Suggestions or ideas are also very welcome.
Boy this brings back memories !
I remember going to his house in Orange County and looking for information on stacking quads which he provided
( had just bought 4 -57,s from a studio ) and his mod info on single units but never bought the grey potted modules which meant
even money I didn’t have at the time (x4)
He had a set of Apogees also and can’t remember the model, maybe the first Duetta series I believe
Will have to look for the paper he gave me on the mod for each unit ( in case I changed my mind later)
As I recall it detailed what each module did specifically but wasn’t a schematic
Will look for the info
I remember going to his house in Orange County and looking for information on stacking quads which he provided
( had just bought 4 -57,s from a studio ) and his mod info on single units but never bought the grey potted modules which meant
even money I didn’t have at the time (x4)
He had a set of Apogees also and can’t remember the model, maybe the first Duetta series I believe
Will have to look for the paper he gave me on the mod for each unit ( in case I changed my mind later)
As I recall it detailed what each module did specifically but wasn’t a schematic
Will look for the info
Curious to know how you stacked your '57s AVWERK. Did you build your own stands? How did stacking affect the perceived height of the sound images? I am thinking about stacking my 2 pairs after they're restored and your first-hand experience would be most helpful.
And look forward to any info re. the Koval mod you could find or recall. Many thanks in advance!
And look forward to any info re. the Koval mod you could find or recall. Many thanks in advance!
The ESL57 with their stock legs have an image which is way too low. So much so I once asked PJ Walker why he liked his singer lying on the floor. He laughed and said "raise them". That was half a century ago.
I then tried the HQD, placing the upper & lower panels along their "natural" curvature. The image was good, but my favorite sitting position was outside of the beams. I'd suggest anyone wanting to try stacked Quad to allow adjustments of the orientation of each individual panel AND of the whole stack.
My current living room is way smaller than before, so I reverted back to single panels sitting on stands (actually Ikea cubes) about 30cm high, as PJ suggested long ago. Btw, the ESL57's work great as near field monitors: I had my very 1st one in my student dorm, driven by my guitar amp, in mono!
I then tried the HQD, placing the upper & lower panels along their "natural" curvature. The image was good, but my favorite sitting position was outside of the beams. I'd suggest anyone wanting to try stacked Quad to allow adjustments of the orientation of each individual panel AND of the whole stack.
My current living room is way smaller than before, so I reverted back to single panels sitting on stands (actually Ikea cubes) about 30cm high, as PJ suggested long ago. Btw, the ESL57's work great as near field monitors: I had my very 1st one in my student dorm, driven by my guitar amp, in mono!
My 57's Stacked are on a Home Produced Frame made from Plywood
(20mm Thickness x 200mm for the Base Height and 200mm Vertical Post Width) .
The Frames Side Walls are upside down T shapes, with a 600mm Base Width and the Vertical Post centred to the Base, the internal spacing between the side walls are the same as the Speaker with side side cheeks on + 6mm (1/4").
There are 4 x Plywood Cross Braces, 1 x 400mm Width that connects the bases and 3 x at 200mm.
The 200mm Cross Braces are set with the lowest 500mm (20") above the floor, this is also the mounting base for the lowest speaker. The next brace is positioned leaving a 5mm(1/4") clearance above the lowest speaker, this is also mounting for the above speaker.
The Top Brace is not critical, but positioned 5mm above the Top Speaker works fine.
Plywood can be tricky when using the exposed plies to anchor a fastening into.
For very secure fastening, a different material such as a solid wood can be selected for the Cross Braces.
For producing a very rigid frame, a few diagonal braces could be added to connect the frames. Diagonal Braces can be concealed by placing them on the rear.
Additionally 8 x Pieces of a 75mm Length Angle @ 50mmx50mmx6mm with a Hole Bored and Tapped on one leg to receive a 8mm Thread Bolt are also going to be useful as footers attached to the Bases, to assist with fine tuning the coupling to the floor and also having a control of the tilt selected for the frame.
My own set up has the Lowest Speaker Head positioned outside of the Frame, to achieve this the speaker is resting on a strap that is anchored to the Frames Vertical Legs.
If the Vertical Post Width was 300mm in width, the strap would not have been needed.
All Original Quad Feet are removed.
The Speakers are orientated so that the bottom one is Upside Down and the Top is the usual method.
This allows for short Jump Leads to bridge the Speaker Cable Connectors.
My own are left as a robust finish as they were an investigation, they have been very effective in their function.
I have a diagram for a improved version (more adjustment options), I will look to see of I can find it and add it to the thread.
(20mm Thickness x 200mm for the Base Height and 200mm Vertical Post Width) .
The Frames Side Walls are upside down T shapes, with a 600mm Base Width and the Vertical Post centred to the Base, the internal spacing between the side walls are the same as the Speaker with side side cheeks on + 6mm (1/4").
There are 4 x Plywood Cross Braces, 1 x 400mm Width that connects the bases and 3 x at 200mm.
The 200mm Cross Braces are set with the lowest 500mm (20") above the floor, this is also the mounting base for the lowest speaker. The next brace is positioned leaving a 5mm(1/4") clearance above the lowest speaker, this is also mounting for the above speaker.
The Top Brace is not critical, but positioned 5mm above the Top Speaker works fine.
Plywood can be tricky when using the exposed plies to anchor a fastening into.
For very secure fastening, a different material such as a solid wood can be selected for the Cross Braces.
For producing a very rigid frame, a few diagonal braces could be added to connect the frames. Diagonal Braces can be concealed by placing them on the rear.
Additionally 8 x Pieces of a 75mm Length Angle @ 50mmx50mmx6mm with a Hole Bored and Tapped on one leg to receive a 8mm Thread Bolt are also going to be useful as footers attached to the Bases, to assist with fine tuning the coupling to the floor and also having a control of the tilt selected for the frame.
My own set up has the Lowest Speaker Head positioned outside of the Frame, to achieve this the speaker is resting on a strap that is anchored to the Frames Vertical Legs.
If the Vertical Post Width was 300mm in width, the strap would not have been needed.
All Original Quad Feet are removed.
The Speakers are orientated so that the bottom one is Upside Down and the Top is the usual method.
This allows for short Jump Leads to bridge the Speaker Cable Connectors.
My own are left as a robust finish as they were an investigation, they have been very effective in their function.
I have a diagram for a improved version (more adjustment options), I will look to see of I can find it and add it to the thread.
I've got a couple sets of those. Every time I clean out my bins of spare parts I threaten to throw them away (it's not like I'd ever actually use them), but so far I haven't.
Anyone who pots things like this and does not describe what he is doing doesn't deserve any of my time or any further effort beyond ripping them out and tossing them.
Here's a picture of a set with the finest cardboard and tape as part of the setup:
Sheldon
Anyone who pots things like this and does not describe what he is doing doesn't deserve any of my time or any further effort beyond ripping them out and tossing them.
Here's a picture of a set with the finest cardboard and tape as part of the setup:
Sheldon
Wow, this forum is indeed a wealth of experience, insight and creativity!
Zung--While you found the HQD beamed vertically, did the stacked configuration widen the listening sweet spot at all?
wout31 and Sheldon--This is the first time I see pictures of the Koval module! Did you ever get your modules installed?
JohnnoG--Look forward to your diagram of the improved version. I was thinking of following Quad's own plan in their technical bulletin below, but it doesn't really allow any angle adjustment.
Quad advised connecting the stacked ESLs in parallel, but if I ever get my '57s stacked I think I will still give series connection a try. I really would like to keep using my Merz OTL amps.
Then again, perhaps I will find stacking unnecessary after listening to the first restored pair since I never found the '57 lacking in volume before, even though they were most definitely already out of specs due to age. This review of the Electrostatic Solutions rebuilt '57s seems to indicate that a well restored pair of the '57 could very well be something I can happily live with for a long time...
Zung--While you found the HQD beamed vertically, did the stacked configuration widen the listening sweet spot at all?
wout31 and Sheldon--This is the first time I see pictures of the Koval module! Did you ever get your modules installed?
JohnnoG--Look forward to your diagram of the improved version. I was thinking of following Quad's own plan in their technical bulletin below, but it doesn't really allow any angle adjustment.
Quad advised connecting the stacked ESLs in parallel, but if I ever get my '57s stacked I think I will still give series connection a try. I really would like to keep using my Merz OTL amps.
Then again, perhaps I will find stacking unnecessary after listening to the first restored pair since I never found the '57 lacking in volume before, even though they were most definitely already out of specs due to age. This review of the Electrostatic Solutions rebuilt '57s seems to indicate that a well restored pair of the '57 could very well be something I can happily live with for a long time...
The Image of the stacked frame is quite robust and not a completed design, as a outline, it shows how improvements were addressed from the design for the original Side walls and speaker mounting.
What it does show is a shaping of the side walls as now having kink (dog leg) with a forward leaning for the above speaker.
The Vertical Leg of the side wall is now 'off set' toward the bases rear, to give a little extra balance, as there is a increased forward lean with this design.
The side walls are slotted at the point the Speaker is Anchored to the Frame, this as a method, is one that allows for a furthering of the forward leaning tilt, adding additional slots and making the slots longer will extend the options for final orientation and tweaking the centre of gravity.
The Speaker/Frame Anchor Points have a Bracket fastened to the side wall, a suspension isolator is also identified as being able to be incorporated between speaker and bracket.
My experiences of most speakers inclusive of ESL's is that suspension as a support, tidies the presentation up to become a noticeable improvement.
The spacing between the Speakers has also allowed for a additional Tweeter of my choice to be incorporated.
A Horizontal Set Bracing, connects the side walls between the Speakers, this bracing is used to attach the Tweeter on to.
The Tweeter and Bass Addition is a subject that has supporters and naysayers, I know of a Tweeter type I am very keen to hear in use with these speakers, as well as a Ripole Bass.
The Base of the Side walls is now designed to have Metal Angles back to back fastened along the length of the base.
The horizontal legs will be drilled and tapped to add a footer, in this case a tapered tip on the threaded rod or bolt.
A castor wheel is also shown as a part, to enable easy manoeuvring during initial setting up, the Thread can elevate the speaker to make the castor redundant when the ideal positioning is discovered.
There are now being shown Vertical Set Braces, 3 x Braces connecting the side wall at the base, ( I liked the idea of these being metal to add a little ballast), A single Vertical Brace is also positioned at the Kink and Head of the Side Wall.
The overall structure should have much improved rigidity with the Speakers anchored to the side walls, they are functioning like a stretch skin typically used in timber framing.
If further rigidity is wanted, the diagonal brace attached to the rear can be utilised as well.
What it does show is a shaping of the side walls as now having kink (dog leg) with a forward leaning for the above speaker.
The Vertical Leg of the side wall is now 'off set' toward the bases rear, to give a little extra balance, as there is a increased forward lean with this design.
The side walls are slotted at the point the Speaker is Anchored to the Frame, this as a method, is one that allows for a furthering of the forward leaning tilt, adding additional slots and making the slots longer will extend the options for final orientation and tweaking the centre of gravity.
The Speaker/Frame Anchor Points have a Bracket fastened to the side wall, a suspension isolator is also identified as being able to be incorporated between speaker and bracket.
My experiences of most speakers inclusive of ESL's is that suspension as a support, tidies the presentation up to become a noticeable improvement.
The spacing between the Speakers has also allowed for a additional Tweeter of my choice to be incorporated.
A Horizontal Set Bracing, connects the side walls between the Speakers, this bracing is used to attach the Tweeter on to.
The Tweeter and Bass Addition is a subject that has supporters and naysayers, I know of a Tweeter type I am very keen to hear in use with these speakers, as well as a Ripole Bass.
The Base of the Side walls is now designed to have Metal Angles back to back fastened along the length of the base.
The horizontal legs will be drilled and tapped to add a footer, in this case a tapered tip on the threaded rod or bolt.
A castor wheel is also shown as a part, to enable easy manoeuvring during initial setting up, the Thread can elevate the speaker to make the castor redundant when the ideal positioning is discovered.
There are now being shown Vertical Set Braces, 3 x Braces connecting the side wall at the base, ( I liked the idea of these being metal to add a little ballast), A single Vertical Brace is also positioned at the Kink and Head of the Side Wall.
The overall structure should have much improved rigidity with the Speakers anchored to the side walls, they are functioning like a stretch skin typically used in timber framing.
If further rigidity is wanted, the diagonal brace attached to the rear can be utilised as well.
Misunderstanding.
What I mean is the Quad by themselves beam vertically. Because I positioned them along their "natural" arc, the beams don't reach my sweet spot, so it was a fail.
If oriented properly, there should be no vertical coverage problem, simply because of the height of the stack. However, there could be other issues, like this strange effect with line source: when I stand up, I have the impression the singer also politely stands up; this I experienced with the Maggie's.
No, I think you misunderstood me. I rip them out, I don't EVER install them.
This is what the crossovers should look like
Sheldon
Yes, they are 220v 1W zeners just like the original circuit but in a modern package.
The zeners are:
https://www.digikey.com/en/products...uctor-diodes-division/BZG03C220-M3-08/7898737
The rectifiers are (also used in the EHT circuits):
https://www.digikey.com/en/products...nductor-diodes-division/GP02-30-E3-54/2144215
I've put the boards up for anybody to buy straight from osh park (they are sold in threes):
https://oshpark.com/shared_projects/NPKtacmX
I use nylon hardware to attach a little pair of right angle brackets to the mounting screws for the crossover board. If you want to be tricky, these make nice little brackets for those boards if you drill out a second hole in them:
https://www.digikey.com/en/products/detail/te-connectivity-amp-connectors/181953-1/2308016
Sheldon