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Old 8th Jul 2020, 3:56 pm   #1
Join Date: Nov 2013
Location: Penrith, Cumbria, UK
Posts: 1,727
Default Electrostatic loudspeakers and suitable output circuits

I use a pair of Quad ESL989 in the main stereo system, very nice loudspeakers especially for my tastes.
Now as a rule I only use either one of my Valve power amplifiers which vary from a 35W kit KT88 design through a home brew 30W EL34 design down to my beloved old Leak stereo 20. All of which cope handsomely.
The solid state amps I am happy to use are either the Quad 405 (of course!) and an old Mitsubishi 50Watt power amp. The Mitsubishi can get flustered when pushed and I only feel safe using it because its output protection circuits function well.
So far so good.
So the question is this, what output circuits are OK to use, mainly on older gear as I reckon modern kit will have output protection built in if its been designed properly.
I am aware of certain NAIM amps having self destruct abilities into difficult and capacitive loads so we can dismiss those.
I was curious as I would be interested in listening to the refurbed Rogers Ravensbrook via the quads but it has no active speaker protection circuit, merely a Zobel network across the output and a 1A fuse on the power supply after the rectifiers/reservoir capacitor.
The output stage is transformer coupled to a pair of TO3 transistors in pushpull, capacitor coupled to the speakers. I suspect they are a version of the 2N3055, on this amp Rogers used "specially selected" Solitron SDT9210 but different production runs used different types from different manufacturers which is why I suspect they are just 2N3055 equivalents selected and matched.
I have tried to paste a copy of the output circuit into this post.
So will I do a nasty to the output stage connecting to a pair of ESL? They seem to need around 5watts on them to provide a reasonable level, they are very insensitive.

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Last edited by bikerhifinut; 8th Jul 2020 at 4:00 pm. Reason: extra info
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Old 10th Jul 2020, 9:47 pm   #2
Join Date: Nov 2013
Location: Penrith, Cumbria, UK
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Default Re: Electrostatic loudspeakers and suitable output circuits

In the absence of any responses I took the plunge, based on that the amp is already capacity coupled to the speakers and being transformer coupled, the worst that could happen was the output pairs could get stressed.
Also turning the slope filter to max would reduce the HF input as that's where quad esls dip to near dc loads.
Well its survived an hour. But kind of proved that early 70's transistor amps work best with conventional speakers of a reasonably high sensitivity....... Quelle surprise! as they say.
It's a tough old amp, I should know that, but in essence I reckon those ESLs do their lovely thing with a decent Valve amp, or something like a Quad 405, or even the 303, and obviously later iterations as they were always specifically designed to perform well with ESLs.

It's been fun, but I am so pleased I didn't flog my Kudos cardea1 standmounters. Lovely things, sensitive and a lovely fruity sound. And don't get on one about that, my view on "monitor" speakers is they should stay in the studio. I have noticed in my own homes how speakers sound so different according to their acoustic surroundings. And for me a reason why we should value decent, honest dealers with real world shops where you can borrow a pair of speakers and compare in the actual environment they will be used.
OT there but valid I feel. No point bragging about saving a few quid if the end result sounds worse than your mate's system with a pair of 40 Wharfedale diamonds.

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Old 10th Jul 2020, 10:02 pm   #3
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Default Re: Electrostatic loudspeakers and suitable output circuits

The ESL989s were designed to be OK with any amplifier from 40 to 100W, the loudspeakers contain a protection circuit and present an almost resistive load. The only way to decide what amplifier is best to drive these is to use your ears. I've owned a pair of these for few years and wouldn't change them and drive them from a EL34 ultra-linear 30W output amplifier.
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Old 10th Jul 2020, 10:30 pm   #4
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Default Re: Electrostatic loudspeakers and suitable output circuits

Well they work with a 10W transistor amp. Its working bl**dy hard though.

I Also prefer my EL34 UL 30W amp, it's not perfect but works well with ESLs. As does the integrated 35W KT88 Amp.

No surprise also that a quad 405 really does the job too.

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Old 10th Jul 2020, 11:01 pm   #5
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Default Re: Electrostatic loudspeakers and suitable output circuits

Originally Posted by bikerhifinut View Post
merely a Zobel network across the output
The bit that goes ACROSS the output is only half the proper zobel network. The full thing also has a bit that goes in series with the output.

The series bit disconnects the output from the amplifier at high frequencies, while the bit across the output (and upstream of the series bit) comes in at high frequencies and connects a little dummy load. The dummy load isn't needed per-se, but in the crossover region it helps damp a resonance.

It is the disconnection business which protects the amplifier from seeing excessively capacitive loads at high frequencies.

Capacitive loads are a classic problem for feedback-controlled voltage amplifiers.

So it's chiefly the section in series with the output which does the hard work.

You see some amplifiers with the series part missed out (oops, someone didn't understand what it did)

You see some amplifiers with the shunt element after the series (Double oops!)

And then there are some amplifiers where the designer wrote up how the Zobel diplexer was simply wrong and shouldn't be included, for the best sound. These amplifiers, as you mentioned, have a reputation for self-destruction if they see a load which looks capacitive at high frequencies. Certain boutique cables for example.

Relatively few degree level electronics designers are aware just what this little network is for. You also need to have stayed awake, alert, and following through several mind numbingly boring lectures on control theory and stability analysis... or you need to have taught yourself once you realised there was something you were missing.

Zobel was a very smart cookie into network analysis and filter design at Bell Labs he was one of the people who laid down some of the foundations of what Antol Zverez at Westinghouse later pulled together. There isn't one Zobel network, he did loads!

Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done
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