Retro Tech : Electrostatic Loudspeakers

[Nightbrought]

Hey guys !

 

I recently acquired some Electrostatic Loudspeakers (Quad ESL-63), and I think the technology is not known enough and is so cool

 

I'm currently rebuilding them, as the glue and materials have deteriorated over time (I think mine are at least 35-40 years old)

 

If anyone here has any tips for the project I'm starting (getting them functional again) please don't hesitate to comment

 

Also, I'm curious as to if there are any plans for LTT to do a video on those, as I feel like the technology behind them is really cool

 

 

 

[da na]

That's awesome! I've heard of electrostatic headphones but never speakers.

How big of an amp do you need for that? I know electrostatic headphones consume a ridiculous amount of power, so I can't imagine a driver 20x the size...

[Nightbrought]

51 minutes ago, Mel0nMan said:

That's awesome! I've heard of electrostatic headphones but never speakers.

How big of an amp do you need for that? I know electrostatic headphones consume a ridiculous amount of power, so I can't imagine a driver 20x the size...

I'm not sure, as I'm still very new to Audio Tech

 

My father is the one helping me figure things out and he knows a little more.

 

He gave me an old Denon AVC-A1 amp which from what I understand can power this, and apparently the speakers need between 100 and 150W to function !

 

 

This is mostly a fun project for me to discover more about this tech, and hopefully have dope speakers for my living room ! (Just moved in)

[H713]

They're very cool speakers. I have not had a chance to hear a set of Quad ESLs, but a few of my friends will attest that they sound quite good for the era.

 

I've attatched some service data for these in the off chance that you don't already have it. Watch out for some pretty high voltages in these (manual says 5.3 kV). You'll need an HV probe if you want / need to measure this. A Fluke 80k-40 is what you'd need, but do be aware that the 1 G$\Omega$ impedance presented by this probe may still present significant loading effects here. As a warning, these voltages jump - you don't even have to touch anything to get a shock. Hopefully you don't have to do too much in this area of the speaker, since it's both tricky and a bit dangerous to work on.

 

Essentially these work by putting a very thin (I believe it's a metalized mylar) film between two perforated plates. Those perforated plates are held at + / - HV potentials. You can then move the film (which is the speaker diaphragm) by applying high-voltage audio signals to it. Those audio signals come from a step-up audio transformer.

 

Now the big warning: Electrostatic loudspeakers are a punishing amplifier load. They're almost purely capacitive. Hopefully the amplifier designer tested stability with this kind of load, and hopefully the amplifier has decent SOA protection on the output devices since the current will be 90\degree out of phase with the output voltage, which is the worst case scenario for a class B output stage. 

 

 

 

ESL63_service_manual2.pdf

[Spuriae]

5 hours ago, Mel0nMan said:

How big of an amp do you need for that? I know electrostatic headphones consume a ridiculous amount of power, so I can't imagine a driver 20x the size...

Electrostatic headphones aren't as inefficient as you might think. Sure, they need very high voltages to function properly, but because their impedance is primarily capacitive (very high at audio frequencies), the total power consumption isn't too crazy. The Stax SR-007Mk2, for instance, is a 94pF load with a sensitivity of 100dB/100V. Because this translates to 1.7MΩ at 1kHz, the 1kHz efficiency in standard units is 92.3 dB/mW, which is about the same as an above-average efficiency planar.

 

Electrostatic speakers typically contain a transformer to allow them to function with standard amplifiers, in which case they are appear electrically to the amplifier as having impedances in the normal speaker range. With electrostatic headphones the voltages involved are still reasonable enough to allow dedicated high voltage amplifiers to be commonplace, though even for those transformer adapters for standard amplifiers are common.

29 minutes ago, H713 said:

Now the big warning: Electrostatic loudspeakers are a punishing amplifier load. They're almost purely capacitive. Hopefully the amplifier designer tested stability with this kind of load, and hopefully the amplifier has decent SOA protection on the output devices since the current will be 90\degree out of phase with the output voltage, which is the worst case scenario for a class B output stage.

Quad does a good job of making the load more reasonable.

 

Here is the Stereophile measurement of the ESL-63's impedance:

Here is a later measurement of another quad, this time including phase. The similar magnitude graph suggests that the ESL-63 has the same phase behavior.

 

The phase is flat through most of the audible band, then increases at high frequencies. The schematic shows what is going on.

 

The speaker's panel is connected as a series of smaller capacitive panels, each separated by inductors. This causes the overall circuit to present itself as a transmission line to the amplifier for a more pseudo-resistive load. Near the end of the transmission line, R3, R4, and C13 create a Zobel network, increasing the phase margin at higher frequencies. The ~100Hz behavior is mechanical.

 

For more of an "amplifier killer" capacitive load see Sound Labs:

This design has (as far as I can tell) an optional series rheostat-connected potentiometer to tame the load a bit, but using it also severely reduces the treble response, so the chart shown here is the intended configuration.

 

Martin Logan also has some terrifying designs:

 

[H713]

Oh, that's way better than I was expecting. I think the Martin Logan impedance curve was the one I had in mind earlier. 

[Heats with Nvidia]

Wow, this is a really nice one! I have heard and worked on some nice and rare speakers, but there was never an ESL among them.

By the way if you intend to actually use it, replacing the electrolytic caps is never a bad idea when working on such old equipment.

[Nightbrought]

On 5/24/2022 at 3:57 PM, Heats with Nvidia said:

Wow, this is a really nice one! I have heard and worked on some nice and rare speakers, but there was never an ESL among them.

By the way if you intend to actually use it, replacing the electrolytic caps is never a bad idea when working on such old equipment.

Thanks ! I do intend on using them, however when digging deeper into the speaker itself I found some tiny holes in the diaphragm of some panels, so I'm trying to contact a specialist to know if I should replace it or no (the diaphragm seems in perfect shape otherwise)