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2 plus 2, Interfaces, improve interfaces?

Read more on the Acoustat 2 plus 2 in our home audio section

Archie Monro2004-02-23 22:21

Hi Andy, I have owned and enjoyed two pairs of Acoustat 2+2\'s consecutively, over the past 15 years. The first pair were modified by only changing the Monster wire and bypassing the audio circuit fuse, which helped to improve the sound quality somewhat. My current pair are unmodified Mk121-2A\'s and I would like to get them to sound the best that they possibly can. I have read and re-read the articles published on your website and I have attempted to disseminate the different models and amp discussions from those pertaining to the 2+2\'s. I am sorry to say, I am still a little confused; for example Sam Kurutz mentions changing the 50ohm 50W resistors with Ohmites. There are no 50ohm resistors that I can see - so I assumed he meant the 50kohm resistors. Ohmite tells me their Audio Gold wirewounds only go up to 20kohms.

My goal is to improve the interfaces with as little experimentation as possible, so if possible I would appreciate your assistance, as you appear to be one of the few people who are knowledgeable about the speakers, pragmatic about the modifications, and still enthusiastic.

It would appear to me the bias circuitry is just fine - is there any need to change R5?

I intend replacing the Monster cable and bypassing the audio fuse, and accepting the small risk for the improvement in audio quality. I use Audio Research monoblock amps to drive them, which have given no signs of instability over the years.

If we assume that medallian transformers are not available, what other meaningful modifications that I can make to the audio circuitry? Did the medallian transformers alone make an audible improvement?

I would change C1 and C2 for Infinicaps.

If I look at the differences between the 2A and 2C circuitry; C3 changes in value from 220 microfarads to 47, and R1 changes from 6 to 16ohms by the addition of a 10ohm resistor. Should I make these changes? What brand of components should I use regardless of whether I change the values or not? Of course, a 47microfarad high grade poly capacitor is easier to deal with than a 220!!

C4 and C5 will be changed, but what do I do about those resistors R2 and R3?

Should R4 be changed and what for? Are Mills wirewounds any better?

Aside from the obvious Monster replacement where else should I replace the audio cable? It appears to me that the only place possible is from the high frequency balance control to the capacitor block - but I would appreciate your comments.

I would really appreciate your time in helping me with this task. I realize that these are questions you\'ve been asked time and again - but to compile them in one orderly document would really help me as I have very little understanding of the electronics, but a great appreciation for the musicality… and I can just manage a soldering iron ;-)

Regards Archie Monro


Andy Szabo2004-02-23 22:21

I am not convinced that there is any advantage to replacing the 50-kohm (R2 & R3) and 1-ohm (R4) wire wound resistors. I think the primary advantage of the so-called ”premium” resistors is at very high frequencies - frequencies much higher than the audio range. However, there is no harm in replacing them (other than to your wallet), as long as you replace them with the same resistance, wattage and body style.

There is no advantage in replacing the 500-megohm bias out-feed resistor (R5). This is already a quality part. Most of the discussions on this website regarding this resistor have involved finding a suitable replacement when the original has failed. If your resistor is still okay (and they rarely fail), I would leave it alone.

There may be some advantage in replacing the 0.01-uF capacitors (C4 & C5), if they are some of the earlier types used by Acoustat. These early types were Mylar dielectric, which are not considered the best for audio. It may be difficult to determine which type you have, since several brand names were used over the years. If, however, your capacitors are of the type used in later years, they are a polypropylene dielectric. I wouldn\'t bother replacing them if they are already polypropylene. Note also that these capacitors must be rated at least 6000 volts, and they are very difficult to find.

If you can tell me the brand and part number of your capacitors, I may be able to tell you which type you have.

The change involved with the ”C” version is more than component values. Note that the schematic of the high-frequency balance control is different. Concise instructions are available on this site for this modification, listed under ”Modifications”, ”MK-121-2(A) Medallion ”C” Modification”. These instructions detail the entire Medallion modification (including transformers), so you\'ll need to extract the portion that corresponds to changing the high-frequency balance control circuitry.

The original ”C” version does call for a 47-uF electrolytic capacitor (C3), and changing those to a polypropylene type would be beneficial. The 0.01-uF (C1) and 10-uF (C2) were always audio-quality parts, so replacing these is optional.

The advantage of the ”C” modification is cleaner high frequencies, and less tendency for the high-frequency transformer to saturate - which can cause distortion. It is well worth doing, and should be a clearly audible improvement to most listeners. Although this mod was combined with the Medallion mod, it can be done independently from the transformer change.

Ytchin2014-05-25 08:19

Dear Andy,

My acoustat 2 speaker was with original medallion transformers and c mod. As recommended I replaced the hf caps to solen caps (47uf, 10uf & 0.1uf). At first I could turned on my transistor amp and listened to the music of great improvement. However, when turned off "the amp and turned on later, the power amp dc input fuse blow on both channels. The hf resistor setting is 1 ohm and 15 ohm to ground as recommended. I shall grateful if you could tell me what had went wrong so that such thing happen. Thanks.

Acoustatanswerman2014-05-28 19:44

I'm at a loss to explain what exactly might have happened. But the fact that you began having problems soon after replacing the capacitors might suggest a problem with the installation of the new parts. I suggest you recheck the wiring and soldering of the capacitors - make sure that the capacitors are not connected to one of the terminal strip lugs that connect to the chassis.

Of course, it could be that the blown fuses on your amplifier are purely a coincidence. Have you tried replacing the blown fuses, or perhaps a different amplifier? I suggest checking the installation of the parts BEFORE trying to reconnect the same or different amplifier.

Feel free to write again if you continue to have trouble.

Yt Chin2014-05-30 12:57

Dear Acoustatanswerman,

Thanks for your reply on this matter and suggestion. The blown fuses on the amplifier are not coincidence as I had tried a few times same things happened on the fuses of negative current supply.

However, when i connected the wires back to the original capacitors assembly, the speakers sounded as per normal. This shows that the wiring is ok. I also tried to connect a one Ohm resistor between binding post and new capacitors, in this case the fuse wouldn't blow but the sound from the speakers were poorer than the original sound. In view of this, I tried to connect the one Ohm resistor between the 10 Ohm resistor and the negative side of high frequency transformer , my amplifier could not turned on at all due to main circuit breaker tripped for two attempt.

One more question, if i follow the MK-121B network whether could avoid this problem.

Acoustatanswerman2014-06-04 18:50

I've never heard of a problem with an amplifier having difficulty driving the C-modded interface. If anything, they should be slightly easier to drive. So I wouldn't expect that going back to the B-version would be an improvement.

I still wonder if you might have made a mistake in the wiring for the C-mod, or perhaps you have a defective component. Do you have a friend that could check it for you? Sometimes a new set of eyes can find a fault that you missed.

Yt Chin2014-06-06 05:31

Dear Acoustatanswerman,

Thank you very much for your advices and caution. I had checked and found that it is true that one side of the cap was connected to chassis. This is because I only checked the terminal strip lugs for low frequency transformer. On which all the lugs are all not connected to chassis and assumed that all terminal strips were the same.

Once I corrected this mistake, all the problems solved.

Acoustatanswerman2014-06-06 18:28

Great news...I'm happy to hear you solved the problem!

Mohan 2018-02-05 12:30

I replaced the R4 resistor and had problems with the sound amp couldn't drive the speakers very low volume breaking sound the actual value that I changed was a ohmite 1ohms 20w 750v.is that the right component sir.im a little confused on the value of the volts its not specified anywhere.
Many thanks


Mohan 2018-02-05 12:40

The resistor that I need to changed was the 1ohms 20w the original but i don't have and idea of the voltage. So when the part arrived it looks the same and its 1ohms 20w 750v,now im not sure if the 750v got to do anything with the sound

Acoustatanswerman2018-02-06 18:18

The 20-watt, 1-ohm resistor is in the primary circuit and is not exposed to high voltage. Therefore an ordinary wirewound resistor is fine. Since it is mounted on a terminal strip, make sure it is soldered to the correct terminals - and not one of the terminals that connects the terminal strip to the chassis. Likewise, make sure that no drips of solder are making contact with the chassis or an adjacent terminal.

Also make sure the low frequency tap on the same terminal strip is connected properly for your model of speaker. RED for 2-panels systems, ORANGE for three-panel systems, and YELLOW for 4-panel systems.

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