Rebuild Packages - Acoustat TNT-200 Amplifier

Introduction:

There's not a lot to add to an introduction about the TNT200 that fans of this amp don't already know, but I would like to say there's more to this amp than most people would realize. The TNT200 has a lot of capability and even more potential, but it needs to be tapped properly for a really nice amp to emerge. Most of the classic vintage amplifiers fit this profile and will benefit from a rebuild, providing the rebuilder knows what they're doing. My observation throughout the years has been that rebuilding amplifiers is a lot like rebuilding engines. It just depends on who's doing the work and the care they take on detail. It's rather common to find factory personel that still service gear doing what they've been taught, and others whose services include mods of a limited nature, but to find someone who can do a top to bottom overhaul with an understanding of the subject is less common. The most effective rebuilds would include not only a parts upgrade (proper parts that is), but circuit changes where necessary as well. I have yet to see an amp that didn't need some sort of boost in both departments, and the 200 is not an exception.

This rebuild involves changes and parts upgrades that will take this amp to where it needs to go. The most prominent features I can point out are the rebuild depth and resultant sound that will most likely exceed any of your expectations. I'm saying this based only on results coming from customer approval. What I come up with in my lab known only to me doesn't do anyone much good, so I leave it to customers to be the judge. I also get a lot of inquiries asking if I know where one might be for sale, and it bums me out that I can't help. I'm just sorry there aren't more TNT200's out there for everyone to have. About the only thing I can do is tell people to keep an eye out, or perhaps I may come across one that will be listed in the "Items for Sale" section of this website.

Following are the engineered changes and parts upgrades currently available.

Front End Power Supply:

There is a power supply change for the front end I have developed that will take the stock approach out to the woodshed. I don't usually like being dramatic saying things of this nature since it's an overdone part of audio that most everyone has turned a blind eye to, but this is actually the Achilles heal of this amp and must be changed, or the amp is going nowhere fast. True, better parts will improve this section, but it's on such a level as to not be worth considering. I'm afraid installing capacitors that belong to the flavor of the month club, calling it quits, and making outrageous claims isn't my style either. Besides, that's not even a challenge. I know there are individuals out there craving the real deal, and it's my directive to give it to them. This change does that.

Incircuit Tweak:

A circuit tweak has also been included that enables a slightly better performance in circuit balance and symmetry to be achieved. Without specifically getting into it, a certain component value is changed to a more suitable level and the circuit seems to respond to this change yielding a somewhat more solid sound.

Resistor Swap:

Additional work done is a resistor swap. This is not a trivial aspect of amp rebuilding. Even though there is some published information on the subject including manufacturer's data sheets, it's a trick to interpret this data properly in a way best suited for audio. I have found particular types of resistors work well in particular circuit locations where each type can be utilized accordingly. I have not found this approach published anywhere and know of no one else using it. Locations for the TNT200 include the front end source and drain locations, the main voltage amplification stage, gate stoppers, and power supplies. Each one has their own unique requirement and I choose the right resistor for the job. The sum total of these changes do make a difference and yield a more natural overall soundfield. It is on par with what cap swaps can do for a piece of gear.

Mosfet Matching:

If the front end mosfets are not matched as well as what they can be, I can install ones that might yield slightly better performance. I work a lot with mosfets and have the experience. Some amps are not as balanced electrically as what they could be and upgrading the front makes it possible to restore the balance, although there are certainly other factors involved, one of which includes the incircuit tweak as described above. Mosfets, like jfets, are famous for being lop-sided in balance unless time is taken to properly make a match. With the mosfet mods I perform on other equipment rebuilds, I do take the time for this process.

Main Power Supply Caps:

All 4 main power supply caps are upgraded, but not just replaced with any old cap. I communicate frequently with factory engineers to get unpublished information on products, which can be quite enlightening at times. How do I get this information? I just ask! These guys will sometimes bend over backwards to break up their dull schedule. This is yet another big factor in the sound improvement. The big can caps made yesteryear were ok for their day (what other choice was there?), but quantum leaps in performance have been made since then. Many individuals can also be turned off by the costs, and much of that view stems from a lack of understanding of how caps work and their importance in power supplies. Despite all that has been written on this subject, it still remains a rather gray area for most. To further complicate the issue, there's an art and science to choosing the best one for this particular amp. The TNT200 has a unique topology that makes these caps do double duty, both as power supply caps in the traditional sense as well as being coupling caps to the speaker load. It's actually a very fascinating setup with it's own set of requirements. The caps I'm currently using are of a higher voltage rating too, anywhere from 100Vdc on up (inventory dependent), so the operating point isn't so close to the edge anymore.

Diode Bridge & RC Snubbing:

Another power supply upgrade item is a step up to a new, modern low-leakage diode bridge. Some people go to the trouble of installing a FRED-based diode bridge, but these can carry a rather hefty pricetag that just adds to the expense which would have to be passed along to you. Instead, I opt to install RC snubbers across the tranny secondaries that squelch the EMI/RFI quite nicely. This eliminates the need for an expensive "audiophile" part and will actually make the circuit properly engineered. If such parts are needed, that's one thing. But if they can be circumvented, it's more affordable to everyone involved and the expense can be used elsewhere. It's the reason I can keep my prices low, yet still deliver exceptional sound. It's also a perfect example of not giving into hype and being smart about getting the job done, which is a thumbs up in my book.

Gold Plated RCA Connectors:

Last but not least, you get new gold-plated RCA jacks. These jacks looks nice, are nonmagnetic, have a splined contact piece, and probably will be a welcome change from the stock units. They should provide a more reliable, if not better connection than what you have been getting.

An option package:

This is an upgrade that took a little while to come up with, but it finally dawned on me one day thinking about the output section and how it really works. I don't stop thinking about ways to improve any amp I do, no matter how well established it is. What this option involves is improving the accuracy, driving force and stability of the output section even better than what it is already.

1) I install a 16ga power cord and get rid of the stock 18ga unit. What this does is create less voltage drop by almost doubling the copper content for current draw. Most other amps with their more conventional topology wouldn't benefit as much from this change, although there are certainly a variety of factors at play here. It's a simple yet effective change in this case.

2)The other part of this package is to swap out the 4uf polypropylene caps that are in shunt with the main p.s. caps for some 10uf ones. This more than doubles the poly cap shunt effect and is also an effective change. There happens to be a limited space in which to accomplish this, but it can be done. While trying my best to keep prices low for people to actually afford having a rebuild done, I'm going to have to charge $50 for this option. There's a total of 4 caps and a power cord involved and prices add up fast, not to mention the added time and effort. The cap install in not minor, but it is doable, worth it, and available to those that want it. I also don't mind doing a little extra work for my customers as I actually do it all the time, but the parts will need to be paid for. The additional cost aside, I do recommend this package. It's a solid and real, non-snakeoil type of upgrade that is beneficial.

What To Expect:

These upgrades address everything I can think of for the best possible performance. All weak points are re-engineered and most all passive parts are upgraded. I don't know what else can be done other than to comment on the result: It's not the same amp at all. The stock and rebuilt versions couldn't be anymore different than night and day. You can take any aspect and it will be totally shifted. Everything that the stock version isn't, the rebuild is, and vice versa. I'm talking about resolution, soundstage, bass, mids, treble, .....and I think I'll run out of page if it were all listed. This is where I've got to say, "You'll just have to hear it." As stated in the introduction, it will in all probability exceed any expectation and reference sound you have heard. If I rattle on much more, it will sound too much like a magazine review. But, I can safely say that it is both an electrically and very sonically accurate amp. I'd also like to make clear that I do not change the basic design. The original intent is kept intact, I just do some fudging here and there to shake things up a bit.

Pricing:

Price on this rebuild is $500 for a working amp, and $550 including the option. For those with non-working amps due to fried outputs, I can offer a little help in the way of free installation of the mosfets. The price for these is as listed in the "items for sale" link at $45/trio. It takes 2 trios to complete one channel, which includes both N and P polarities.

Things seldom mentioned about the TNT200 you might like knowing:

This amp was made specifically for driving Acoustat speakers, but you might have luck driving others too. I've had them hooked up to some Martin Logan CLSs and there was no problem. It's been reported by some that it can sound sort of funny. Those instances were probably based on listening to a stock unit, and based on that alone I can't take them too seriously. Any electrostatic speaker would most likely be suitable, and would be the speaker type that I would recommend using them with. There's an understated aspect to owning one of these amps, and it might be one reason that they are primarily used with stats. First, the amps are indeed designed into such a load, but some units do possess an elongated turn-on thump that can hit the speaker. Acoustats speakers have an input fuse on most models (I think) and can blow protecting the panels. The common way around this is to unplug the wal-wart power supply to the diaphragms, power up the amp, then plug the speaker power in. It works, no problem. If any other speaker type is used, then the DC offset may cause harm in the extreme case. I have known it to blow fuses and make certain amp-speaker matching impossible. It will be up to the user to properly use these amps.

The reason there can be a pronounced turnon thump lies with the assymetrical circuit and how it powers up. The main p.s. caps are connected to the output which also make a contribution. I have looked into this and haven't found an easy way around it. If something was done, it would probably put the fidelity at risk, so I think the best bet is to know about the issue and work around it. Use it within it's intended scope and you should be ok.

Another comment I'd like making is that I too wish it had more heatsink, but it does seem to work ok as is. The trick going on here is the types of mosfets being used. At the proper bias, they are supposed to have thermal stability with a zero temperature coefficient, so runaway isn't a problem. They are also very tough and can take the heat. About the only way I am aware these can blow is if they are severely strained. This can come about due to mismatched trios, or being improperly hooked up. I have seen where an improper bridging attempt was made, and one side of the outputs did not survive. So, with that being said, you pretty much have to abuse these to break them, but it can be done. Be careful!

A word about bridging:

I receive inquiries on rare occasion about bridging these amps. I know with a few it's a popular thing to do and they swear by it. As for me, knowing what I know, I tend to be a bit more conservative on developments such as this unless they can be done "correctly".

Done properly, one would need a balanced source. That is, a signal feed with both a + and - polarity. One polarity would be fed into one input (right) of an amp, and the other polarity into the other channel (left) of the same amp. The same approach would be needed for a second amp to serve the second channel.

A less desirable, but most popular way includes distortion I'd rather avoid, technically speaking. A regular signal is fed into one amp input, followed by the output being fed back into the other amp's input in a manner that inverts the signal, thus obtaining equal and opposite signals at both outputs. This second channel acts as a unity gain inverter, and has to be configured as such when the bridging switch is thrown.

This less desirable approach has 2 aspects I don't care for, but it's certainly not the end of the world if it happens. One, the inverting channel must be configured as a unity-gain inverter. This turns the channel into a different amplification circuit, inverting the signal along with much increased feedback. It would be preferable for both channels to operate identically. Two, all the distortions from the 1st channel are fed into the 2nd channel. This ain't good no matter the argument. However, if one must do it, working with an accurate amp is certainly preferable to an inaccurate one. To be honest, one amp with the parts I install will drive a speaker quite loudly.

There is yet one more downside to bridging, and that's making the amp stout enough to handle the job. The current requirement doubles in bridge mode, which places twice the burden on each output stage. So, the need to be careful about load impedance increases. I've already expressed some wishful thinking about the TNT200 having more heatsink, and the bridging option stresses it even more. I'm not sure where the limit is with such a setup, but if I was starting over and building the TNT200 from scratch, I'd give it more heatsink for such an application. The one thing I can think of that may work in the user's favor driving electrostatics is the impedance curve. It's common to find impedance levels higher than the 4 ohm nominal value at the lower to mid frequencies, which is where the bulk of music energy lies. There's not so much content at the higher frequencies where the impedance tends to dip. This may well ease the requirements somewhat and be a more gentle load than what it initially appears. I'll not comment any further but you can be the judge. This is just strictly commentary on my part and is meant to be a helpful if not informative guide to interested parties.

In conclusion, I am not offering a bridging option at this time. Eventually I may, but under conditions, guarantees, warranty, and possibly other criteria yet to be determined.



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