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Old 7th April 2012, 10:57 PM   #11
DaveG is offline DaveG  United States
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so this was interesting-
for some reason I expected 3-4 ohms also, but I guess I never measured. I have several models available, so here is a list. Also, in the original Acoustat owners manual, they are all listed as "Nominal Impedance:4ohms". After I had the first unexpectedly low number I decided to get a bit more serious.

So these numbers are straight resistance numbers at the speaker input banana jacks hot and ground, and on 2 of the interface units I was able to get directly to the low frequency primary taps. I checked both channels of each Model and left and right channels were all basically the same.
All these numbers are pretty much at my measurment limits using an old 4 1/2 digit meter. I did check it against a 3.92 ohm 0.1 percent resistor and my meter reads 3.90 so hopefully I am fairly close, but don't bet any money on these numbers.

so here goes:

Model 3 with MK-121-2A (no Medallion labels) (each tap setting measured at input jack hot to ground) (brown paper transformers)(interface appears unmodified with a blue 220uf EL cap and yellow 0.01 uf and 10uf film cap and, and a monster cable label, and corroding internal monster cable)

At input banana jack. Red tap = 1.22 ohm
At input banana jack. Orange tap = 1.3 ohm
At input banana jack. Yellow tap = 1.33ohm
yellow to brown = .38 ohm (Directly across the low frequency transformer primary at tap terminal strip)

HF transformer alone = 0.07 ohms (red lead lifted to common)

I resoldered the red lead and again measured at the output jack and unplugged the low frequency tap lead and measured 5.99 ohms, so that is the resistance of the HI FREQ balance control in series with the HF transformer and without the low frequency transformer in parallel.


Model 1+1 with MK-121-B with Blue and Red Medallion labels. (Set at yellow tap)(black paper transformers)
At input banana jack. Yellow tap = 1.62 ohm
yellow to brown = .55 ohm (Directly across the low frequency primary at tap terminal strip)


Model 2+2 MK-121-2A no Medallion labels (I recall these having brown paper but did not confirm at this time)
At input banana jack. = 1.3 ohm (did not open box to see what tap)



Maybe you could try a 1 or 2 ohm resistor in series, at least to see if your amps then function properly. I'm going to try just that with my PP tube amp to see if anything changes for the better. It never sounded good on the 8 ohm tap and the bass is fairly fat even on the 4 ohm tap.
hope this helps-
Dave
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Old 7th April 2012, 11:09 PM   #12
john65b is offline john65b  United States
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Yup, so I am not crazy, and my speakers appear OK.

Putting th 10 ohm resistor across the primary side of the treble tranny (C- Mod) probably does some good, but a series resistor on the speaker inputs would probably help make more amps usable, albeit lower spl...
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Old 8th April 2012, 04:50 AM   #13
john65b is offline john65b  United States
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I put a .649 Dale resistor in series with speaker input and now my Aleph 5 with a 600VA toroid plays just fine. Without the series resistor it pulsed...

The TNT-200 still pulses and the series resistors get pretty hot.

It is strange how the TNT plays fine on other speakers including my CLS (which also has a 1 ohm resistor and two very low DCR Step up transformers.
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Old 9th April 2012, 01:11 AM   #14
DaveG is offline DaveG  United States
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so the small series resistor brought the DCR up just enough for one amp- good to know. It seems your now onto TNT repair. I don't have any experience with the Acoustat amps, but keep us posted on the outcome.
Dave
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Old 9th April 2012, 12:36 PM   #15
john65b is offline john65b  United States
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I have a feeling those large PS caps in the TNT may be a little tired, and need replacing. Its been 30+ years...
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Old 9th April 2012, 01:04 PM   #16
sreten is offline sreten  United Kingdom
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Quote:
Originally Posted by bear View Post

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.
Hi,

There is no reason that should be the case. Same as if you measure
the DCR of a valve amplifiers output windings, they are much lower.

DCR of transformer windings are much lower than designed impedance,
otherwise they would be at best 25% efficient, clearly not the case.

rgds, sreten.
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Old 9th April 2012, 01:33 PM   #17
john65b is offline john65b  United States
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I put an inline amp meter that reads the amps and watts off the mains plug and with normal speakers or no speakers the TNT pulls just short of 300 watts from the wall, but when I have the Model 3 plugged in I read it pulsing from 300 watts up to 900+ watts, back to 300 watts, and all the while the room lights dim as it peaks to 900+ watts.

My Krell pulls the normal 750 watts from the wall with the Model 3, as always.

Clearly something not right with the amp, or the combo.
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Old 9th April 2012, 08:22 PM   #18
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I would suggest the Acoustat amplifier has some sort of marginal stability issue. Normally, the Acoustat amp should be able to drive the speakers with ease.

The DCR of the speaker will be on the order of only a few ohms. If your ohmmeter does not have good resolution or accuracy in that range, your measurement could easily be in error by an ohm or three.

The speakers require only a few moments to reach playing charge, but improvements in sound and dynamic capacity will be noticed during the first 24 hours of being energized. I suggest the speakers be left energized at all times unless you are expecting a long period of non-use.

The choice of 3-amp or 5-amp slo-blow fuses is personal one. For moderate sized power amps and most listening conditions, 3-amps should be fine. If you have a large amp, and like playing loud music, a 5-amp fuse may be a better choice. The idea is to use as small a fuse value as possible without 'nuisance' blowing, while still affording some protection to both speakers and amplfier.
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Old 12th April 2012, 02:01 PM   #19
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I suggested that to measure the DCR place say a 10 ohm resistor in series with the meter, measure the 10ohm + transformer. Now measure the 10 ohm, subtract the two, that is much closer to the actual DCR. The meter is more accurate at 10 ohms than at 1ohm.

Seems that something is up with the amp.
Do both channels do the same thing?
Just one channel, right?

I'm suspicious of the cap that is in the feedback network - which I think is there, it is an unusual feedback network - intended to pull the DC gain to unity... might be unhappy. Could be something else, but I'd have to look and see if I have the schematic on this laptop or not, don't think I do.

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Old 12th April 2012, 02:22 PM   #20
john65b is offline john65b  United States
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Thanks Bear, I will re-check the DCR tonight, but suspect it to be around 1.5ohms.

As far as the amp is concerned, it is both channels, and I think it is likely a spent electrolytic that goes a bit haywire once a low DCR speaker is connected, as I have no issues (noise, etc) driving other speakers. Since it is not my amp and owner is not using it on his accoustat speakers, I will let him decide what he wants to do.
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