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Old 8th July 2004, 12:07 AM   #1
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Default More Acoustat questions

Hello everyone!

Now that I have my new amplifier working (see: New Amplifier Project), I have had a chance to see what happens when I use more power to drive my Acoustat model II speakers. While it mostly sounds very good, I found that I can drive the treble interface transformer into saturation quite easily, with the result being that the top end gets VERY hard and brittle. I have tried out the mods suggested on various speaker sites that I found by searching this forum, and while increasing the resistor values in the treble circuit has helped, I'm still not satisfied.

Has anyone attempted to install a beefier transformer, maybe a toroidal type, in place of the stock unit? Is it even worth the trouble? Anybody know how who could make one of these for me and how I could obtain the specifications? Or should I just get better speakers instead?

As well, I currently have an option to purchase some 1 + 1 s with the medallion transformers. Would this be a significant upgrade from my original model IIs? What should I expect to pay for a used pair of 1 + 1 s in good condition, what is a fair price?

Sorry for all the questions!

Take care,
Doug
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Old 8th July 2004, 07:53 PM   #2
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Default Hello?????

Anybody?
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Old 8th July 2004, 08:03 PM   #3
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How did you determine that you were driving the transformer into saturation? There are a lot of other things that can cause the high end to go shreiky with high drive voltages.
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Old 8th July 2004, 08:31 PM   #4
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Hello Sy,

Well, for starters, the amplifier isn't even close to clipping when this happens, and I see no distortion of the output waveforms when testing the amplifier with sine waves into a load resistor, so I don't believe that the amplifier is the cause.

If I increase the value of the resistor in series with the treble transformer, the problem gets better, and will almost go away if the value is high enough. Unfortunately, then I loose too much top end for my tastes. All of the capacitors in the speaker interface have been replaced with high quality film types, with no effect on this problem (although the fidelity sure has improved as a result)

I suppose it could be some strange problem with the speaker loading the amplifier somehow.

Can you suggest other possibilites for causing this problem, and any ways you know of to determine if this is in fact what is happening?

Thank you for your reply!

Take care,
Doug
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Old 8th July 2004, 08:42 PM   #5
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Acoustats of that era have a very low impedence (~2ohms) at high frequencies causing many amplifiers to distort because their
output protection circuits or whatever aren't designed for that. If your amp has those types of circuits (I haven' t looked) you may want to temporarily remove them for a listening test (being extra careful not to short out your speaker leads until you can redesign them to function silently in the normal use sense).

The model 2 also has the lowest efficiency and bass wrap around cancellation effect so that complicates the issue of getting
loud enough volume, despite having lots of power. You might want to look into getting larger Acoustats (threes, fours, 2+2s, sixes, eights...) if you can find them and fit them into your listening room, or buy/build other more efficient loudspeakers -- using your model 2s as a reference point.
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Old 8th July 2004, 09:27 PM   #6
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Doug, radical's idea was the first thing that came to my mind. The diagnostic check you ran by increasing the resistors is very consistent with that idea. The Zin goes low and has a high drive angle, just to make things worse.

You might also check out whether or not your diaphragm coating is starting to give up the ghost or if something else in the interface box is starting to have some leakage. The SPL limitations, assuming a good, strong amp driving it, are in the bass. Just running mine from 400 Hz on up, I can get them loud enough to vaporize my eardrums with no apparent transformer distress.
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Old 8th July 2004, 09:46 PM   #7
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My amplifier has absolutely no protection circuitry whatsoever, other than line fuses. The latest schematic is here: http://www.electronicpeasant.com/pro...y/amplify.html

If not for this problem I would be completely satisfied with the output levels from these speakers. I have owned these speakers since 1980 or so, when I purchased them new, but I always used a lower power amplifier on them and so never encountered this problem before. Is there anything else I can check to confirm that this is caused by the amplifier?

Does anybody know where I could find a pair of 1 + 1s or 2 + 2s for sale and what a fair price would be?

Thank you for your help so far!

Take care,
Doug
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Old 8th July 2004, 10:02 PM   #8
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Audiogon seems to have them now and again.

Try doing some bench tests of your amp with high frequency tone bursts (like 10-15kHz) while the amp is loaded with a 4.7uF cap shunted by an 8 ohm resistor. See if there's any funny overload behavior, oscillation, or the like. You might be surprised at how fast the amp clips or by weird behavior like sticking to supply rails after a brief overload.

Even better (and less risky), see if you can borrow a monster amp that is well-known to do a good job driving difficult low-Z loads- a Levinson, a Krell, something like that. If the hardness goes away, you've got a clue.
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Old 9th July 2004, 12:46 AM   #9
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Peasant,

Well, here goes another shot. Just based on what you say about resistors in series with the transfo, it sounds like it might act as a resonance suppressor for the upper frequencies. It's nothing you'd measure on the pri. side, and you won't be able to measure on the secondary cause the volts be too high and anything you do will load it down. It would take an acoustical measurement to troubleshoot i'm afraid. The problem might be that the capacitance of the panels is combining with the stray inductance of the secondary winding to create an irregular freq. response, peaking in the highs. The series resistor is there for the purpose of controlling (damping) this peak, where an optimum value just has to be found. Probably the only thing you got to use is your ears, although verifying the response through acoustical/electrical means would be nice. You didn't mention what
resistor values you were using. Would they be on the order of an ohm or 2 or 3? Hey, I'm just throwing an idea out here based on what I've read. It is a valid problem, although maybe not yours. If it is the case, you'll just have to find that happy medium if you don't change parts. New parts would probably involve a rather comprehensive re-engineering effort..(just my guess). Hope this at least give you a heads up anyway. Trying another amp might validate this, so it still looks like a good idea to me. A better amp could pronounce this effect more.


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Old 9th July 2004, 01:33 AM   #10
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The original Acoustat wiring had a 6 ohm potentiomenter in series with the transformer (wired end-to-end, not connected to the wiper), with the wiper itself connected to a capacitor bank bypassing the potentiomenter, depending on the setting.

I found on another website where Acoustat upgraded this resistor on the newer models to 16 ohms with only 6 of those ohms being variable, to prevent saturating the transformer and causing a harsh top end. I have experimented with various resistor and capacitor values, and I find that 16 ohms takes away too much of the top end, but anything less is still too harsh.

I am going to do some more testing on the amplifier to see if I can find anything there. The problem seems to be made much worse by any kind of harsh program material, so I can well believe that it is a resonance of some sort in either the amplifier or the speaker electronics. As well, for my preamp I am using an Audio Research SP-9 MKII, and there may be an interaction between that and the amplifier. I have heard that this model can have difficulty matching well with some amplifiers.

I'll let everyone know how this progresses, thanks again for the help.

Take care,
Doug
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