The 400A is one of my all time favorite amps and a joy to bring up to modern day standards. All circuitry is addressed which includes the entire output section, main p.s. caps, circuit board, LED board, power supply rectifier, both power and ground wiring, new connectors, and even some options thrown in for free.

Output section:

The output section is upgraded to Onsemi's MJxxx series devices, which handle more current and offer a bit more reliability in service. You can push the kitchen sink with this setup. The 400A was originally designed into a very low impedance load, and the upgraded devices do it even better. Some who rebuild the 400A will also install these same Onsemi devices in the driver section, but I have found this is not the best approach. I use a modern day version of the original series which does perform better.

Output section assembly:

I only use mica insulators and synthetic thermal compound to mount power devices. You can't beat this setup in reliability, longevity, and performance. Long after conventionally mounted devices have had their compound creep all over the place, this will still be intact doing it's job. One thing I do not like about the stock 400A is the grease used and quality of assembly. There are no oxide fillers in the strictly silicone compound to aid in thermal transfer performance and the heatsink assembly fit isn't always the best. I have seen gaps where contact has failed between the sections and even worked on one unit that had grit and other contamination trapped inside. Yech! With an amp that runs as hot as the 400A is designed to do, this is not a good thing. While apart, I reassemble with synthetic compound and make sure that assembly is correct. When properly done, the 400A is now ready to pump some current.

Main Power Supply Caps:

Main power caps are updated to some that are truly outstanding, and yes, these make a difference. This is one issue I tend to press and talk about everywhere I can. The tendency here for most people is to avoid the issue because of the expense, but I won't do that. The caps you get will be much better than what the average DIYer tends to use. As a matter of fact, I try to keep in stock what I consider to be among the best on the planet. Availability is supplier dependent and happens to be a cap that fits the 400A, and very much plays a major part in the sound. If these are not available however, the caps I do use are not far behind performance wise and are still way better than your average cap. Depending on your system resolution, you may not be able to tell the difference.

Power Supply Bypassing:

I use 10uf polypropylene capacitors for bypassing the main large can caps. These poly caps are normally used for switching purposes and are fast with ratings up to 100khz. They are installed as an integral part of the power wiring harness and give that added "oompf" to the power supply.

Circuit Board:

The circuit board is rebuilt on several different levels, depending on options chosen and the maintenance needed. The level common to everyone involves: (more is described below in the Options section)

- A resistor upgrade. I use several different types according to their location and function in the circuit. The net result is a more natural sound and is on par with what cap changes can be. Some early versions don't even come with emitter resistors, but an upgrade is possible here if used in conjunction with the mosfet option (as described below).

- Cap changes. There's not a lot of caps on board, but the few that are can be changed, or alternatively removed altogether. An example would be an input coupling cap. Preamps don't have any DC to speak of anymore, so the input cap may be removed. Most are tantalums and need removing anyway.

- Transistor check. Some of the metal canned units are actually pretty good and can remain. Others don't measure so well and some may even have rotting leads and underside that can't be seen unless the unit is removed from the board. I've even seen leads so tarnished that it goes up into the case and eats at the actual die connection. These types can be updated to modern versions, and more often than not, there are updates. If you have the cascoded version, then you'll have TO-92 plastic units whose quality can vary. These are checked and upgraded when necessary.

LED Board:

The interface circuitry to the LED section and it's power supply are upgraded. I feel this upgrade is important because it contains some capacitors that I'd sooner remove in an amp if it were mine, so I include it in a rebuild. Mostly just caps are upgraded in the circuit from tants to film, which is a huge improvement. If any LEDs are non-functional, then these are fixed too.

Rectifier (Diode bridge) and RC Snubbing:

Another power supply maintenance 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 indeed give the equivalent or better performance. If these 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.

Wiring:

The wiring in the 400A is not the best, and even contributes to muddying the sound. There's actually more rewiring needed with this amp than in most others I do. I improve upon the routing, replace connectors, install slightly larger gauge teflon wiring in the power rails, and clean up the grounding. This latter upgrade is actually very important and will improve the detail like a cap change will, removing ground loops and unecessary voltage drops that rob the sound. I've heard detail coming from this change that borders on freaky.

RCA connector jacks:

I use Canare gold plated jacks to replace those old ones. These are the perfect connector and one cannot ask for a better one, period. They are made for the broadcast industry and fit into the oversized back panel hole perfectly. You cannot help but be happy with these.

Power Switch:

Sometimes the power switch will need replacing, even though these are stout devices. If so, then replacement will be at cost and installation is a free service to rebuild customers. It's not a normally replaced item.

Options:

I have done quite a bit of development with this amp involving original ideas, and am making some available to those that would like the improvements. These include both board and output section upgrades.

- Board: I can change the input stage to mosfets, which will be matched. This change allows an improvement to the feedback network, improves the DC offset characteristics, and allows for front end emitter (source) resistors to be installed where none were previously on earlier models. These are positive changes and ones I recommend. There is no charge for this option. The amp will sound better if these are done.

- Output Section: I can streamline the output section and give you back the high frequencies you've been missing. I do this by removing the sliding bias circuit and adjusting the affected circuitry accordingly. This is actually a good thing in my book and also highly recommended. I liken this mod to a badly needed open heart surgery. There is no charge for this option either. This mod will result in the open sound you'd like, but can't seem to get with the stock version no matter how high the bias has been set. There's no need to worry about the class A hype either. With the new design, you'll most likely never even reach the crossover distortion point, even with the toughest load. I cannot recommend this upgrade enough, but will certainly leave it stock for those that wish it left alone. I understand the desire to leave certain functions alone and know this may be one of them.

What to expect:

The amp will have a new sound. If all options are chosen, then the sound will be even better. Soundstaging, tone, detail, dynamics are all improved. You'll probably hear detail in some of your recordings that you never knew existed, but that will depend on some extent to the rest of your system. I know when I got all my gear upgraded, I could hear improvement at each step. The reliability factor will be improved as well. The 400A is known to come with a bug or two that cause failure, and these are corrected in the rebuild.

Price:

The price for all these upgrades is $500 and takes several days to complete and be thorough. Not only are there quite a few parts used, but a bit more labor is involved as well. Too many versions of this amp exist to make this an assembly line effort, but don't be mislead about the earlier non-cascode units versus later cascoded ones. I can get them all sounding super sharp because they all have the basic goods to work with.

Some noteable customer feedback who chose all options:

"When I first hooked my system up, I spent maybe 3 hours listening to some of my favorite CD's. I have all types of music. In the end, I have to say that I was kinda so-so about the sound. I did this every other day, spending up to 4 hours just listening and messing with the settings on the Pre amp. It was not until this last Wednesday that I got into my music again and WOOOWWW!!! I'm not sure what I did or maybe the amp just finally decided to wake up and do some unreal things. I had been listening to lots of jazz and blues type music. I popped in one of my favorite CD's that I have already listened to maybe 10 to 15 times since I hooked the system. I was shocked at what I heard. I could swear that this was not the same system that I had 2 weeks ago. All I can say is that this was WEIRD. It was as if someone came in and swapped out my system and replaced it with one identical to one I had, only this one is much better. I have always heard about systems that make your speakers disappear. I have also read about set ups that can actually place the musician and his position on a stage. I can honestly say that now I know what they are talking about. I'm now going back and listen to all the music that I have listened to earlier.

I've been in at least 7, maybe 8, sound rooms between San Francisco, Palo Alto and Marin County. These rooms have thousands and thousands of dollars worth of components. I never really came upon a system that if you closed your eyes, you could not point to where the speakers were standing. I was always wondering if I was ever going to have one of those systems that some people talk about. I'm sure as I upgrade here and there, things are only going to get better. Way to go Ken. I could not be happier." - Vic M.