M, I'm sure some will say that these are the most-important caps in the amp. I don't agree totally with that, but conrad-johnson seems to believe it--some of their preamps use only film caps instead of electrolytics (els) in the PS. I can remember decades ago when the M75 (?) was upgraded to the M75a; they simply removed els from a decoupling position. I suppose that because the output transistors in a solidstate poweramp are connected directly to the last PS cap, the quality of the caps is MOST important.
I'd look at Michael Percy's el caps, here http://www.percyaudio.com/Catalog.pdf. See what you can afford, and don't forget that you can distribute the PS caps in different places around the amp and that it helps to use smaller values the closer to the output device the power gets. Also, I'd use as much polypropelene capacitance (the Solens are good choices here, as they're good quality, relatively 'small', and relatively inexpensive) as bypasses, with smaller, higher-quality bypasses (TRTs? MultiCap RTXs?), some as close to the transistors as I could get them.
It stands to reason that even though they are not in the signal path that upgrading caps with something better would still help the sound. Unless you don't think upgrading PCs, outlets, and wiring make a difference. If you don't, then it's a waste of time and money.
If you're talking about the large can caps, I renewed the ones in a Threshold S/300 Series II a few years ago. I searched the web for electrical equivalents. Then narrowed it down further to which ones were identical in size. Since my plan was preventative maintenance, finally I chose what was in there to begin with - computer grade Mallorys - only slightly higher in voltage and capacitance; exact replacements were no longer being produced. The Mallory website showed how many of each type were on hand and pointed me to an electrical supplier who could get them.
The switch out was simplicity itself, requiring only a screw driver. Nonethless, keep notes on the postions of the fasteners, the wiring and the orientation of the plus and minus signs marked on the caps.
I hear the rectifier bridge benefits from replacement at the same time but I didn't bother. When it was all done the amp was a maybe a little smoother and more liquid but it could have been my imagination. The main improvement was to my peace of mind.
The caps that you have are probably still within spec on this amp given its' age and the caps used. As such, i would add more to what you already have. Given the size constraints of the amp, you might be better off going with a bank of smaller caps. In turn, you could bypass these with an array of even smaller caps. Using a wider array of values increases the filtering action that is achieved, reducing in a blacker background and less grain. I would also replace the bridge rectifier while doing this. It is your choice as to go with a standard replacement or to go with some other type of "high grade" rectififiers. It it were mine, i'd be using high current Schottky's rather than stock rectifiers. This may lean the sound out a tad due to less ringing, but the increase in clarity and liquidity should easily out-weigh that. The use of snubber's across the rectifiers may / may not be beneficial depending on the quality of your local AC. Some say that these are not necessary with Schottky's, etc... but RF bypasses are always beneficial as far as i'm concerned. Then again, i live in a highly populated area with filthy AC. If your way out in the woods with pretty clean AC, it may not do anything for you. Sean
Jeff, "Rock", and Sean, you each offered something I would not have considered otherwise. Know that is appreciated.
While I agree with Sean that multiple filter caps result in a lower ESR. I do not share his affinity for Schottky bridge rectifiers. They will work but the expense/benefit ratio is very low. The jury is out on the benefits of Shottky diodes. Some say the diodes need to be fast to respond the charging requirements of the electrolytics but lets face it, with modern amps having 50,000uF + of capacity per rail, the discharge time constants are so long that regular diodes have no trouble keeping those caps charged.
Liguy: It's not only a matter of response time, it's a matter of ringing. Faster diodes produce less ringing and this can be seen as a reduction in harmonic content in the power supply. In case you haven't noticed, JA has recently measured quite a few amps that suffer from a measurably high harmonic content coming out of the rails. The use of higher quality rectifiers that are faster and produce less ringing would minimize / negate that problem. This is the reason that some folks think that Schottky's and other "fast" diodes "lean out" the device that they are installed in. That is, they get rid of the artificial "warmth" that the slow, ringing diodes contribute to the sonics of the device they were installed in.
As a case in point, Nelson Pass has never been a fan of "fast" rectifiers. Then again, most of his amps have never been known as being "bass monsters" either. As such, installing "fast" rectifiers into his older designs resulted in a presentation that he thought was too lean sounding, hence he didn't recommend that mod. He has recently taken a liking to these devices and his latest amps are making use of them. My guess is that he's found a way of taking advantage of them while retaining the sonic signature that he prefers.
This is something that everyone should think about. That is, many components are voiced based on specific system conditions. If the component was voiced with "dirty" AC coming into it, removing all of the grunge may leave you with a different sound ( for better or worse ). On the other hand, running a device on dirty AC that was voiced using highly filtered AC may not deliver the sonics that the unit is capable of providing. While "clean" AC should always work best, there are so many ways to "clean" AC and so many different component designs, it's quite possible that some combo's may not work that well together. If in doubt, contact the manufacturer and see what they say. Good quality gear already has quite a bit of filtering built into it and would be voiced accordingly. Sean
Mprime, come to think of it here are a few other observations. The Forte 4A was perhaps the most well received amp in Forte's brief history. While the compulsion to get more of a good thing is very persistant in audio, consider preserving the balance the 4A already possesses.
Upping the storage capacity of the large can caps significantly, say more than 25%, may necessitate a new bridge rectifier. Go too far and you'll need a bigger transformer. How much sleep do you want to lose :^)
In short, I'd look for 7A monoblocks before I went off the deep end modding an already terrific product like the 4A. Of course no condition is permanent so whatever you decide, have fun with it!
Well, you're right, Rock - as I listen to this amp I am quite happy, and if I'm inclined to upgrade, it's now the preamp.
I guess I'm just surprised how much amp I got for my $500.
BTW, I popped the hood of the Forte and wow: what a beautifully simple design! I don't know why I was expecting something much more complicated, but I can actually understand the circuit and it is layed out so cleanly. Really have respect for Mr. Pass as a designer.