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Old 6th April 2012, 02:10 PM   #1
john65b is offline john65b  United States
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Default Acoustat Model 3 Questions

Hey all,

Just picked up a pair of Model 3's locally. They have the 121-MK-2A interfaces and do not believe they have the Medallion trannies (brown paper wrapper on trannies).

Previous owner said he couldn't test the speakers, as he did not have a suitable amp to drive them. I took the gamble and purchased them. Took them home and noticed one of the speaker inputs was open. Replaced the blown 3A fuse and got around 1 ohm, just like the other. Hooked up my Icepower 1000ASP monoblocks and took a listen.

Sounded a bit different from my DIY CLS, but had some pretty deep base that I was not expecting from any ESL. The KSA-50 Clone sounded equally as good, if not better (clearer highs, etc). Pretty impressed.

I then tried a friends Acoustat TNT-200 I happened to have (he has Acoustats, loves them and told me about the local sale). He dropped off his TNT-200 last week as he could not get one channel to work (replaced a fuse and she fired right up).

Anyway, as soon as I turned on the TNT-200, I got a power "pulsing" and one of the audio fuses blew on the same speaker that had the bad fuse initially. I checked all the schematics for 121-MK-2A and noticed 5A instead of 3A audio line fuse. I changed both to 5A and tried again, but on the TNT-200, it still get the "pulses" for a few seconds with no sound from the speakers, and I shut off the amp. The amp continues to work perfectly fine with other cone speakers, and my CLS (although crossed over at 125hz and higher)

The Icepower play fine as does the Krell, but at certain high-ish volume, I get speaker cut out on both, where I just lower volume and all is OK (amp protection at work?) I tried the Aleph 5 I have and it also pulsed like the TNT-200....

I, later, did the C-Mod as shown on the Acoustat thread. I thought maybe the 10 ohm resistor across the speakers inputs / step-up trannys would give something the amp could work against, but still get the pulsing.

Don't know what the issue is...Am I saturating the LF cores? Whyis it just these certain amps? And the TNT was specifically designed to run these amps...

Or could it be that the panels are not yet fully charged when I connect the problem amps? I did give it a few minutes to charge the panels before turning on the TNT amp...do these require a longer time to charge?
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Last edited by john65b; 6th April 2012 at 02:20 PM.
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Old 6th April 2012, 03:23 PM   #2
john65b is offline john65b  United States
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Default Schematics

Overall schematics attached - I reused the 6 ohm variable pot and added the 10ohm resistor (50W) as shown in attached...
Attached Images
File Type: gif MK-121C_Xovr_changes[1].gif (26.2 KB, 270 views)
File Type: gif MK-121C_Xovr[1].gif (39.5 KB, 265 views)
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Last edited by john65b; 6th April 2012 at 03:29 PM.
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Old 6th April 2012, 07:00 PM   #3
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The DCR should be higher than 1 ohm.

It should be the DCR of the bass transformer windings plus the series resistor (1 ohm).
The impedance of the IIIs are about 6 ohms minimum.

I would suspect that the Acoustat TNT has a problem.
Easy to check.

Play the good channel on each of the speakers - if it works, then there is an issue with
the amp.

Could be some bad electrolytics, they are old. Or other problems.

The "medallion mod" moved the LF point that the HF xfmr gets signal higher in freq.
The main thing is to have polypropylene caps and NO electrolytics in there.

You can't tell which transformers are which by looking at the paper on the outside, afaik.
Or iirc.

So, check the good amp channel on both speakers... that will tell you where the issue is, amp or speakers. Check the DCR looking in, should be (guessing) around 3-4ohms minimum...

You can compare the DCR of the two channels, LOW DCR on ONE channel indicates a shorted primary on that speaker.

There was somebody in Florida selling replacement transformers... a search here might reveal that, or Google.

Also the IIIs have substantial bass - distance from the rear wall will effect that.
About 3-4 ft seems a good range... The Acoustat TNT has nicer lower mids than most SS amps on them, and the best is a non-pentode higher power tube amp. >100watts required for decent output power.

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Old 6th April 2012, 07:37 PM   #4
john65b is offline john65b  United States
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Thanks Bear,

There is only the single 1 ohm resistor in series with the LF transformer. But the DCR of the low and high transformers audio side windings are usually quite low - less than 1 ohm a piece, correct? So series 1 ohm with the LF transformer and paralleled resistance of the HF transformers should be close to 1 ohm (btw - my voltmeter resolution at low resistance is crap). What should the DCR be at the speaker inputs?

My DIY interfaces I built for the CLS has a single 1 ohm resistor in series with the two Toroids stepups and it only shows 1 ohm DCR too.

If the trannies were bad, why would they work for the two amps (Krell and Icepower)?


It is the impedance that is 6 ohm min, not DCR, correct?
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Old 6th April 2012, 08:17 PM   #5
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the bad channel - if it is bad - may not like the load.

You should do the things I suggested.

One ohm is way too low on the primary of that transformer, imo.
At minimum there is a one ohm in series...

You can add another resistor in series and measure the whole thing, measure the resistor alone, subtract. Pretty accurate.

Krell is a very high current amp, will drive almost to a dead short.
Icepower? Dunno.

impedance 6 ohm min, not DCR, but DCR should not be far behind.
I am not near my Acoustats, so I can't just do a quick measurement.

Regardless, if the "good channel" drives each of the two speakers the fault is in the amp.
CLS is a different animal.
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Old 6th April 2012, 08:52 PM   #6
DaveG is offline DaveG  United States
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"do these require a longer time to charge?"

just a comment here about the panel charging time. I have always used power strips to plug in any of my electrostats (I don't like the possible risk of transformer failure with older vintage equipment when I'm not around), and they all play just fine basically immediately. This is not to imply that they won't improve with a longer charge time.
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Old 6th April 2012, 09:01 PM   #7
john65b is offline john65b  United States
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Yes, I would like to know what your Acoustat DCR is...

I had issues with the TNT not just on one channel - they were both "pumping / pulsing"
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Old 6th April 2012, 10:11 PM   #8
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ok...

i will not be able to measure mine for maybe week...

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Old 6th April 2012, 11:08 PM   #9
DaveG is offline DaveG  United States
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I can post measurements tomorrow-same Model 3's.
Dave

Last edited by DaveG; 6th April 2012 at 11:10 PM. Reason: typo
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Old 6th April 2012, 11:11 PM   #10
john65b is offline john65b  United States
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Yes! That would be great.
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