If your bias multiplier is mounted on a daughter-board, then it was most likely modified sometime after its original manufacture. The original used an 1800(?) volt transformer and a 3-stage multiplier. That transformer proved to be unreliable, so it was changed to a 750-volt transformer and a 5-stage multiplier. It was Acoustat's policy that any interfaces submitted for the Medallion upgrade, or for any other repairs, were to have the bias supply upgraded at no extra charge.
I suppose it could be possible that such a 'modified' unit might have been built that way at the factory, if it was manufactured during the transition period between the two styles of bias supply. That was way before my time.
The 0.01-uF capacitors with brown paper and black epoxy ends would have been replaced with all-yellow polypropylenes in the Medallion program. Quite a few different capacitors were used prior to the Medallion upgrade, so it was standard procedure to replace all of the old-style capacitors with the new polypropylenes.
The transformer part numbers should be located on the side of the wrapper, or perhaps on the top or sides of the laminations. I wouldn't expect the numbers to be on the bottom (or mounting) side.
You now have me wondering if the Medallion program might have been started (on a very small scale) just before the bankruptcy that led to the acquisition by Hafler. That's when I joined the company. I do know for sure that the program started in earnest soon after the operation was restarted, as I was responsible for putting the program together, which included designing and purchasing the new Medallion labels. It was such a popular program that we had to hire several people just to handle the modifications!
Well, my bass transformer says 2008733, 012181 and STI, and I found the original invoice for an "Acoustat Mod" from my dealer that is from December 1981 and is $110 for parts and $40 for labour. What would the Medallion parts have cost at the beginning of the program? I can't believe I paid $150 for just upgrading my bias circuits because that was a lot of money back then. I bought a running motorcycle for about three times that back then.
Based on the transformer part numbers, the date (1981), and the price ($150) of your 'modification', I am positive this was NOT the Medallion upgrade. I am not sure what work might have been done. Perhaps Acoustat was charging for the bias upgrade at that time (it was later done at no extra charge when combined with the Medallion upgrade). The price you paid sounds about right for the bias mod: two bias transformers and two 'daughter' boards. It's possible your dealer marked up the price beyond Acoustat's fee, and this may have also included shipping charges.
The Medallion upgrade was much more expensive, as it involved replacing the two very expensive audio transformers in each interface. I don't remember the exact price, but I recall that it was in the range of $800 per pair.
Acoustatman- I think in a past post, you mentioned that the you use a Hafler 9500 in your system and that you thought the 9500 was better than than the tnt 200.
My question is- how is the 9500 better? Also, can a tnt200 be modded (I see a few places that claim to make them better) to work as well or better than a Hafler 9500?
My question is- how is the 9500 better? Also, can a tnt200 be modded (I see a few places that claim to make them better) to work as well or better than a Hafler 9500?
Since we're on the topic of amps for Acoustats, I can highly recommend a YBA Integre DT. You can pick them up for around $1000 now and it has no problem driving Model 3s to much louder than I want to listen. I used to use PS Audio Model 2 "Blocks" in mono and they would literally melt down trying to drive the Model 3s. The YBA stays luke warm at full volume and delivers solid bass and a steady image.
Based on power specifications alone, the 9500 is even better suited to driving ESL's than the TNT200 (which itself is no slouch in that regard). The TNT200 is rated at 200 wpc @ 8-ohms, (which is does meet, but without much margin), whereas the 9500 is very conservatively rated at 250 wpc. The 4-ohm rating of the 9500 is better, too, indicating a lower output impedance (or higher damping factor).
The 9500 was also designed by Jim Strickland of Acoustat fame, and it represents his 'next increment of improvement' over the original TNT series. And finally, all 9500's are newer than any TNT's, and since they are all getting pretty old now, that could make a difference in your long-term investment. There is also a 9505, which is the same amp with balanced inputs.
I cannot comment on the advantages of any 'modifications' offered for the TNT. In any case, no modification is likely to increase the power level to match the 9500.
Sorry for interweaving my posts in this thread, but nowhere else seems more appropriate. I just did the C-mod on my Model 3s (non-medallion) using Dayton polypropylene 47uF 250V caps and so far I like it. The thing that jumped out for me immediately was guitars sounded more natural. Imaging may be tightened up a bit as well. Still breaking them in, so I expect improvement over the next few dozens of hours. One of my one ohm LF resistors disintegrated and a chunk of the ceramic was sitting on the bottom of the interface box. There's a two week wait for parts at my local electronics store, but at least it still works. I'm sort of wondering if the proximity of the Dayton caps (these things are big) to the high voltage board could be a problem, because they're tucked right next to the standoff under the diodes and caps.
One of the frist things i did was to make a out-board bias setup on my Acoustats 3s............one i can set.. from 4k-7k.....an yes i dont think jim would have put the bias 5k less than 1" from the panel driver mixer.... on top of all the parts.......... if the cost an the looks did not rule the day.........an most say well thay still work after over 20years of stress....on all the time... right.......with the bias up 5-6k thay sound better than the SoundLab A1s..an i can drive them with 50 tube watts......out of 4-5 pr of 121 i seen all were low bias........like 3.5k
seeing how the bias can make all the parts an tranfourmers run cool if the bias is right....or cook the parts if it to low...........like the one ohm on your bass trany.....thats been over heated.....more than onces...... i would think...
i would ge two 2ohm 30watt....an run them in paralle........to get the one ohm needed
have fun...........L0ng Live Acoustat
seeing how the bias can make all the parts an tranfourmers run cool if the bias is right....or cook the parts if it to low...........like the one ohm on your bass trany.....thats been over heated.....more than onces...... i would think...
i would ge two 2ohm 30watt....an run them in paralle........to get the one ohm needed
have fun...........L0ng Live Acoustat
I have been playing around with my latest local acquisition - a pair of Spectra 11... really just wanted the pair of panels to make a a pair of Spectra 33... sounded kinda drab when I first hooked them up. I was kinda miffed spending all that money (which really wasn't much, but I am cheap). Anyway, took some bias readings with my multimeter - supposed to b around 75v, well I had more like 28v. The other was a little better, but not much. After fiddling with the pots, I couldn't get over 60v on one and over 48v on the other. I then found AAM posted Bias Fix. Did all the resistor swaps/pulls, the cap change, and coiled the antenna around the bias wire. I was then able to dial 75v on the money on one, and could not get any higher than 72v on the other. Still much better than before. Vacuumed out the panels, (did not wash them)...Sounded much much better.
I then started looking at the crossover circuit - previous owner really did a bang up job on the crossover. Real crappy soldering... the 10 ohm restor across the tranny was replaced with a 4 ohm, the series 1 ohm was changed to an 8 ohm, a series PP caps added to the 150uf and 100uf,...all sorts of junk. Not sure he knew what he was doing. The interfaces did not even match. Anyway, I returned everything back to original per schematic. Another step up in SQ.
Then I got kinda bold - something I have always wanted to do on these spectras. The Spectra 11 have white and blue direct off tranny - no series resistors- for full range (including HIGHs), while the yellow and black had the 330k resistor for MIDs and lower.
So I jumpered the two 330k. Now the entire panel (all stators) are running full range (all producing HIGHs). Still no beaming and sounds a little more brighter - cleaner. I think I will keep them this way. May even do the Spectra 22 I have.
According to the schematic, all the stators on the Spectra 22/33 have a series resistor (none are run straight thru from tranny, like the spectra 11) . I know these resistors are doing two things - trimming HF (why would you need that?) and adding a delay to reduce beaming effect. I removed the series resistors and appears to have no less/no more beaming than with the resistors in, but with more HIGHs.
Comments?
I then started looking at the crossover circuit - previous owner really did a bang up job on the crossover. Real crappy soldering... the 10 ohm restor across the tranny was replaced with a 4 ohm, the series 1 ohm was changed to an 8 ohm, a series PP caps added to the 150uf and 100uf,...all sorts of junk. Not sure he knew what he was doing. The interfaces did not even match. Anyway, I returned everything back to original per schematic. Another step up in SQ.
Then I got kinda bold - something I have always wanted to do on these spectras. The Spectra 11 have white and blue direct off tranny - no series resistors- for full range (including HIGHs), while the yellow and black had the 330k resistor for MIDs and lower.
So I jumpered the two 330k. Now the entire panel (all stators) are running full range (all producing HIGHs). Still no beaming and sounds a little more brighter - cleaner. I think I will keep them this way. May even do the Spectra 22 I have.
According to the schematic, all the stators on the Spectra 22/33 have a series resistor (none are run straight thru from tranny, like the spectra 11) . I know these resistors are doing two things - trimming HF (why would you need that?) and adding a delay to reduce beaming effect. I removed the series resistors and appears to have no less/no more beaming than with the resistors in, but with more HIGHs.
Comments?
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Anyone use a NAD 208 to drive their acoustats?
Does this look up to the task? Is it better, equal or worse than a 9500?
Product Review -
Does this look up to the task? Is it better, equal or worse than a 9500?
Product Review -
I use a Carver 4.0t on my 2+2's in 34x90 foot room, it's rated 375w at 8ohms, I can't get it off the bottom led at 100+db , nice sounding amp to, 
I've never found acoustats require lot's of power in huge rooms, they NEED room to BREATHE, or it's a waste a time, you'll only get half of what their capable of and work the hell out off the panels to get 80-90db in smaller rooms.Acoustats are just not meant to be couped up. JMHO Al
I've never found acoustats require lot's of power in huge rooms, they NEED room to BREATHE, or it's a waste a time, you'll only get half of what their capable of and work the hell out off the panels to get 80-90db in smaller rooms.Acoustats are just not meant to be couped up. JMHO Al
Regarding the bias system - I would set both interfaces at the same measured voltage to ensure equal efficiency in both speakers. So if one of them max's out at 72 volts, then set both for 72 volts. The exact voltage measurement will vary due to meter loading, so the most important thing is that both speakers are the same.
Bypassing the 330K sector resistors on the Spectra 11 should increase the high frequency output. Since the interface equalization is set up for only half the panel area to play highs, this only makes sense when you configure all panel area to play highs. In my opinion, this will negatively effect the imaging, but if you like it, I'm happy.
Moving on to the full range Spectras. Those transformers have both low and high frequency windings, and it is the R-C network that combines the output of those two windings. I would expect bypassing those components would really mess up the equalization, and possibly even cause damage to the transformers and/or cause impedance problems. I don't recommend doing that. When it comes to bypassing the sector resistors, see my comments above. But in this case, I would expect the negative effects on equalization and imaging to be even greater. But, no damage would occur due to bypassing the sector resistors. You can try it, but I doubt you'll like it!
I think damping factor is important with any speaker, as it can have a dramatic effect on the amount and quality of bass. This is why some amps seem to 'have more bass' when in reality they have a lower damping factor, which leads to less controlled bass.
For ESL's, which are generally low in impedance, damping factor is even more important. Since damping factor is calculated as load impedance divided by amplifier output impedance, a high damping factor is that much more important when the load impedance is low. In other words, the damping factor at 4-ohms is only half the value that it is at 8-ohms. So it is desireable to start with as a high damping factor as possible.
That being said, there are some highly-regarded amps out there with rather low damping factor: most tube amps for example. So the significance of damping factor will depend on your personal tastes and the speakers you intend to drive. Note also that damping factor usually varies with frequency, so if the speaker has a widely-varying impedance curve, the overall frequency response will be affected more by an amplifier with a low damping factor.
Yes, the full range Spectra models have two identical transformers: one for each phase (one 'push', and one 'pull'). Each transformer has three windings: a primary, and two secondaries. One of the secondary windings covers most of the audio band, and the other secondary is a 'bass boost' winding that fills in the bottom end. The bass boost winding has multiple taps for adjusting the amount of bass equalization appropriate for the number of panels in the system. Unlike the earlier MK121 series, these bass-taps are not really 'user-adjustable' since they require considerable disassembly to access the connections. The only time one would need to alter the bass tap is when using an interface that was factory-wired for a different model.
The output of those two secondary windings are combined through the large resistor and the 0.01-uF capacitor. That signal is fed directly to the full range sector(s) of the speaker. That signal is also fed through smaller resistors to drive the 'mids and lows' and 'lows only' sector(s) of the speaker. This accomplishes two things: it rolls off the high frequencies to those sectors, and it also introduces a time delay. This time delay effectively curves the panel array, thereby improving dispersion and imaging. In case you didn't know, the name 'Spectra' is an acronym for Symmetric Pair Electrically Curved TRAnsducer.
The Spectra 11 and 1100 use a single transformer in each interface. However, they are internally dual transformers, in that each contains two identical sets of primary and secondary windings, one for each phase. Because they are limited frequency-range transformers, there is no seperate bass-boost winding, and hence no RC network to combine the outputs. The output is fed directly to the full range sector, and then further through a resistor to feed the 'mids and lows' sector.
AAM - Yup. Makes sense now... Thanks for teh explaination!!
Maybe the Spectra 11 has some benefit of bypassing the series resistor (I think they sound better), while the 22 / 33 etc. may not...
Another test I always wanted do is to separate the two MK-121 trannies to their own separate panels (on a 2+2) run thru an active crossover - biamped...and bypassing the mixer circuit altogether - like Soundlabs...anyone do this yet?
Maybe the Spectra 11 has some benefit of bypassing the series resistor (I think they sound better), while the 22 / 33 etc. may not...
Another test I always wanted do is to separate the two MK-121 trannies to their own separate panels (on a 2+2) run thru an active crossover - biamped...and bypassing the mixer circuit altogether - like Soundlabs...anyone do this yet?