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Old 14th July 2008, 10:47 AM   #1
JmanGTA is offline JmanGTA  Canada
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Default Amp suggestions for Acoustat Spectra 22's?

Hello all,

I bought the spectra 22's recently. So far I've tried just a few amps


Samson S1000-

500 x 2 @4ohms - cheap "pro" amp, very shrill around 3khz, sloppy bloated bass, vocals tend to recede, this amp does not like low impedance loads


Behringer ep2500-

1200x2 @2ohms - much better than the Samson, the shrillness is gone, better control in the bass range, surprisingly decent


Dynaco ST70-

35 x 2 @4ohms - only good at low volume, runs out of gas at anything above moderate volume and just sounds like hash/mud


Adcom GFA 555 mk2

325 x 2 @4ohms - Stereophile loved it in their review from the 80's, similar sound to the Behringer, just got it today from a friend so I'll need more time to compare, the behringer holds up well to it (so far)


Other Acoustat owners seem to like the Hafler Trans Nova amps and some use the crazy Acoustat amps with 5000 volt output terminals (no thank-you).

Has anyone compared other amps on these speakers?
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Old 16th July 2008, 03:32 AM   #2
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Default well have read that ....

the Forte amps are excellent. I know that Meitner/Museatex amps work very well as ED voiced his amps on 2+2's. The Jas SET amp works very well as do some of the digital switching amps and the Moscode is superb. ( have heard NuForce and ARC tripath) The Spectra series should be a much easier load than the original plus series with the MK121 interface.
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Old 18th July 2008, 02:15 AM   #3
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I dunno that the Spectra is an "easier load" than the 121 interface or not...

But the ST70 if modified will not turn into hash at all, but still will lack full SPL for these speakers. They were a bit SPL challenged anyhow... the first step is to haul out the crap stock caps in the signal path, check the tubes, and increase at least the second leg of the PS filter caps to >100ufd... that will get you about 50% of the way and render them in the listenable range, imho.

A pair of them modified and run in mono might be credible, unless you like really loud and/or have large or dead room...

You want an amp that can remain stable into capacitive loads, doesn't wildly overshoot in that case, and can swing the requisite volts (200watt class amp) AND doesn't have the typical "grain" that plagues an awful lot of solid state amps.

(Adcom is not on my list of candidate amps... fyi)

Of course you could build your own... there are few designs around here that might suffice...

_-_-bear
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Old 18th July 2008, 05:03 AM   #4
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Default Bear you should try this...

if you have a min of a two panel Acousta per channel. Run one panel directly off the secondary of the treble transformer (lift the CT from ground and insulate) and leave the input cap on the primary side. Run the other panel directly off the bass transformer. Input (for the bass transformer) via a 6 ohm 50 watt resistor to the red tap and via a 2uf cap to the yellow tap (disconnect the CT from ground and insulate it). Give it a try and tell me what you think. No secondary eq at all no 50 k resistors and no 0.01 uf caps in the signal path any more. Try it and comment. Make sure that you do not run the two secondary transformer sides in parallel to the same panel. Each panel gets its own transformer drive. I would also recommend you install two extra ladder sections to the HT suppply to bump the diaphragm voltage to closer to 5 Kv. That helps a lot. Be prepared to be surprised.
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Old 20th July 2008, 11:59 PM   #5
JmanGTA is offline JmanGTA  Canada
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I thought those caps were only in the signal path when using the bass re-routing toggle switch? I'm letting my Lexicon do bass management so I don't use that.
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Old 21st July 2008, 04:29 AM   #6
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Default are you running a MK121 interface?

if so the two 50 K resistors and the two 0.01 uf caps are always in the mix everything goes through them.
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Old 21st July 2008, 01:24 PM   #7
JmanGTA is offline JmanGTA  Canada
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Ya mine have the 121 interfaces. The Spectras I got were literally brand new. The owner bought them back in 91 and never even opened them.

So should I start with replacing those caps?
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Old 21st July 2008, 05:51 PM   #8
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Default if you want suer...

you can replace the caps and resistors but that will get expensive. google up the Izzy Wizzy site annd you can see what one dedicated Acoustat owner tried as that may give you direction as to what to try for replacment partl. Getting the parts out of the signal path is to me a better option.
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Old 23rd July 2008, 04:40 PM   #9
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Default spectra 22s

1st i had 33s, sold them.
missed them got 22s.
on both i use conrad johnson 150 watt monoblocks.
i will tell you there is nothing like high power beam tubes push pull on electrostatic peakers.
i did for a time use a SS amp, a Bedini 150 mk II and it did well, lots of juice but not the better relaxed brilliance of the tubes handling midrange.

i will never sell my 22s likely.
right now they are the rear channels in my theater augmented by JBL 8" woofers a pair each in the walls flush be hind them.
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Old 30th April 2012, 01:54 PM   #10
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Ih Moray,

Its Mathieu from France. I discovered your post which interests me a lot. Your philosophy here is completly different from mine (mine is similar to the izzi wizzy one...). Today I mostly completly rebuilt the K121 interfaces of my 1+1, the result is good to my ears.

Before i make the test can you give me aditionnal details ?
- Do you still think in year 2012 that this tweak is a good one ?
- Why do you add a 6ohm resistor and a 2uF cap on the primary of the Bass transformer ? Unless I'm wrong this is a 6db/oct Highpass filter...

Many thanks,
Mathieu
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