Acoustat Electrostatic Speakers

[Peter W.]

My brother has a very vintage pair of Acoustat speakers driven from what he describes to me as an Acoustech IV pre-amp, distinguished by having phono-jack outputs directly to the speakers. Which have large, heavy, powered boxes attached. After many years of sitting unused, they went silent on him. I suspect the pre-amp, but at-a-distance, I cannot be sure. No, I do not have the model number of the speakers, but they are self-powered, very nearly 2 meters tall, just over 6 feet, There are three fuses on the back, a removable power-cord, an on-off switch, the 1/4" phono jack input and a 5-position treble boost/cut knob. Which should narrow it down.

Question: is that specialized pre-amp required, or would any normal audio pre-amp do?

Thanks in advance!

Peter Wieck
Melrose Park, PA

[legg]

The preamp isn't special. You'll need one that can do a
reasonable output amplitude, even though the speaker
input impedance is high, as the accustats need good
drive levels.

Having both speakers go silent at the same time does
suggest the preamp.

RL

[Peter W.]

Thank you! I have in mind the Dynaco PAT-5biFET as it puts out ~15 volts, and is more-or-less contemporary with the speakers.

Thanks again!

[Trevor Wilson]

**I have some familiarity with a few Acoustat models. I am unaware of
any that came with inbuilt power amps, though I guess it is possible
that one or two were. I can tell you that the load presented by most
Acoustats is brutal. Only amplifiers capable of substantial current
ability and unconditional stability should be used. Here is an impedance
graph I ran on an old Acoustat amny years ago:

https://www.rageaudio.com.au/index.php?p=1_12

Check out the dip at 15kHz.


--
Trevor Wilson
www.rageaudio.com.au

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[Phil Allison]

Trevor Wilson Bullshitted:
======================

> >
> **I have some familiarity with a few Acoustat models. I am unaware of
> any that came with inbuilt power amps, though I guess it is possible
> that one or two were.

** The modlel " X " IIRC.

Direct drive form the tubes runnint at 5kV.

I can tell you that the load presented by most
> Acoustats is brutal. Only amplifiers capable of substantial current
> ability and unconditional stability should be used. Here is an impedance
> graph I ran on an old Acoustat amny years ago:
>
> https://www.rageaudio.com.au/index.php?p=1_12
>
> Check out the dip at 15kHz.
>

** Very easy load for any sanely built amp.

Trouble for your nutty MEs - were they ??


..... Phil

[Trevor Wilson]

On 8/12/2020 12:39 pm, Phil Allison wrote:
> Trevor Wilson Bullshitted:
> ======================
>
>>>
>> **I have some familiarity with a few Acoustat models. I am unaware of
>> any that came with inbuilt power amps, though I guess it is possible
>> that one or two were.
>
> ** The modlel " X " IIRC.
>
> Direct drive form the tubes runnint at 5kV.

**Fuck me. I think you're correct. Direct drive ESLs - the Holy Grail.
Very rare. Never seen one. I've worked on 2+2, 1+1 and Spectra models
(the impedance curve was a Spectra).

>
>
> I can tell you that the load presented by most
>> Acoustats is brutal. Only amplifiers capable of substantial current
>> ability and unconditional stability should be used. Here is an impedance
>> graph I ran on an old Acoustat amny years ago:
>>
>> https://www.rageaudio.com.au/index.php?p=1_12
>>
>> Check out the dip at 15kHz.
>>
>
> ** Very easy load for any sanely built amp.
>
> Trouble for your nutty MEs - were they ??
>

**The ME850, with zero global NFB, a 2.5kVA power transformer, 245,000uF
and 70 Amps of continuous (till the mains fuse fails) current capacity?
None at all. Perfectly stable, regardless of load. A perfect match for
any ESL (or any other speaker, in fact). The ME1400 would do even
better, without raising a sweat (5kVA power transformer, 310,000uF and
100 Amps).

Nutty? Maybe, in this era of pretty impressive Class D amps. Still,
people like 'em. I know I do.

[Phil Allison]

Trevor Wilson wrote:
=================

>>
> >> https://www.rageaudio.com.au/index.php?p=1_12
> >>
> >> Check out the dip at 15kHz.
>
> > ** Very easy load for any sanely built amp.
> >
> > Trouble for your nutty MEs - were they ??
> >
> **The ME850, with zero global NFB, a 2.5kVA power transformer, 245,000uF
> and 70 Amps of continuous (till the mains fuse fails) current capacity?
> None at all. Perfectly stable, regardless of load. A perfect match for
> any ESL (or any other speaker, in fact). The ME1400 would do even
> better, without raising a sweat (5kVA power transformer, 310,000uF and
> 100 Amps).
>
>

** You omitted the *very real* risk of damaging the input power limiter of a Quad ESL63 and any of its relatives.

But that speaker is not a problem load just because it dips at 15kHz.


...... Phil

[Trevor Wilson]

**Both amps would certainly fry the TRIAC(?), if pushed. If I recall
correctly, the ME850 (maximum output Voltage = 31 VRMS) would not exceed
the maximum Voltage limits of the ESL63? The ME1400 certainly would (42
VRMS). Overkill for the ESL63.

Let me know, I'll send one over for you to listen to. I'll cover freight
both ways.

FWIW: Musos love the way ME amps deal with transients. Musos seem to be
acutely attuned to the way a system delivers transients.

[Phil Allison]

Trevor Wilson wrote:
-------------------------------

>>
> > ** You omitted the *very real* risk of damaging the input power limiter of a Quad ESL63 and any of its relatives.
> >
>

> **Both amps would certainly fry the TRIAC(?), if pushed. If I recall
> correctly, the ME850 (maximum output Voltage = 31 VRMS) would not exceed
> the maximum Voltage limits of the ESL63? The ME1400 certainly would (42
> VRMS). Overkill for the ESL63.
>

** The ESL63 has two input limiting triggers.

1. A voltage peak value of about 35V using a power transistor or zener array.

2. A simple ionisation detector ( uses a 555 ) trips a triac for a short time.

It's often a 1.5ohm, 5W resistor in series with the speaker that can pop.
Later marks have a fuse to save it.
In any case Quad, warns not to use amps have no VI limiting with their ESLs.


..... Phil

[Trevor Wilson]

**An ME550-II then. MUCH more polite. 27 VRMS, 500VA transformer,
120,000uF and 40 Amps of PEAK current. Easier to freight too. Only 16kg.
All ME amps are current limited.

Peter Stein learned to his cost, way back in the late 1970s, that
pathological speakers loads, like the Infinity RS4.5 would kill a 38
VRMS amp, with around 70 Amps of current ability, but without current
limiting. The design genius' at Infinity, decided that it was a good
idea to place 2 X 4 Ohm bass drivers in parallel was a good idea.
Trouble was, the bass drivers also had a 2 Ohm voice coil, which was
paralleled with the 4 Ohm voice coils at LF. The importer smoked several
Phase Linear amps, a bunch of Yamahas and several others, before trying
the ME. It last 6 weeks, then died horribly. Output stage SOA was
exceeded. Peter began fitting current limiters to all subsequent amps.

[Phil Allison]

Trevor Wilson wrote:
=================

>
> > In any case Quad, warns not to use amps have no VI limiting with their ESLs.
> **An ME550-II then. MUCH more polite. 27 VRMS, 500VA transformer,
> 120,000uF and 40 Amps of PEAK current. Easier to freight too. Only 16kg.
>
> All ME amps are current limited.
>

** Really ?

VI limiting is another matter - intended to prevent output stage failure under
adverse loads including short circuits.
Means the short circuit I max is no more than half the I max into a few ohm
resistor - often more like one quarter.



..... Phil

[bruce bowser]

On Sunday, December 6, 2020 at 1:51:14 PM UTC-5, Peter W. wrote:
> My brother has a very vintage pair of Acoustat speakers driven from what he describes to me as an Acoustech IV pre-amp, distinguished by having phono-jack outputs directly to the speakers. Which have large, heavy, powered boxes attached. After many years of sitting unused, they went silent on him. I suspect the pre-amp, but at-a-distance, I cannot be sure. No, I do not have the model number of the speakers, but they are self-powered, very nearly 2 meters tall, just over 6 feet, There are three fuses on the back, a removable power-cord, an on-off switch, the 1/4" phono jack input and a 5-position treble boost/cut knob. Which should narrow it down.
>
> Question: is that specialized pre-amp required, or would any normal audio pre-amp do?

No idea, but that vintage stuff out there in Kutztown, PA is something compared to the microscopic breadboards 3D printed out in Silicon Valley. Those late-model "echo device" speakers of the new generation are really becoming loud, too.