More Acoustat questions

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Hello everyone!

Now that I have my new amplifier working (see: http://www.diyaudio.com/forums/showthread.php?s=&threadid=31224&highlight=), 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? :rolleyes:

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
 
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
 
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.
 
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.
 
My amplifier has absolutely no protection circuitry whatsoever, other than line fuses. The latest schematic is here: http://www.electronicpeasant.com/projects/amplify/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
 
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.
 
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.


Retro
 
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
 
I think your amp is the issue.

I never used 2's but lived with 3's for many years. Tried high power solid state and finally big mega tube amps. Manley 350's worked great, but settled on some Altec 1570's that were reworked completely.
The speakers were sent to Hollywood Fl and converted to the latest Medallion series during this period.
I found that some solid state amps could not deal with the low impedance at high frequencies. They would get very hard and it would quash the dynamics. They even blew up a couple when played at high volumes.
One test is is play at fairly high volume and see if you get a headache. The headache is a sure sign of ultrasonics. The top end of the 2's and original transformers may not do it, but the 3's could.
They should be smooth as butter on the top. The low feedback tube amps handled this without a whimper.
My moniker comes from living with a big ugly pair for so many years. Finally moved on, but they could be great in certain situations.

George
 
This could be part of the problem!

Look what I found when I removed the cover from the amplifier!:eek:
 

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Needless to say, the failed cap and it's conductive ejaculate all over the regulator and amplifier pcbs was having an adverse effect on the sound.

That's one failure out of twelve for those capacitors, so far, if another one fails I will replace them all. :whazzat:

Anyways, I replaced the capacitor and cleaned up the inside of the amplifier, and then I ran some tests. Using the speakers themselves as a load, I was able to check the output of the amplifier with both sine and square waves (unfortunately I don't have the equipment for tone burst testing). There was no significant distortion, ringing, or noise that I could see at any frequency from 10Hz to about 40kHz, where the corners of the square waves began to round off. This was the same anywhere from no signal to close to clipping. The amplifier seems to be quite stable from what I can tell.

As for the sound, some recordings are just fine, but others still seem to have an unpleasant hard edge to them that really comes out at higher levels. For the most part the speakers are sounding better than they ever have, so maybe part of the problem is that I'm hearing just how bad some recordings really are. Still, I certainly would like the overall sound to be a bit smoother, so I'll continue to work on this. I'll post again when I've had more time to investigate what is happening here. Thanks everyone for all of your help!

Take care,
Doug
 
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