Hi,
I need some advice on upgrading the power supply on my Forte Model 4 amplifier. In case you are not farmiliar with this design it is 50W Class A using the IGBT devices. There are 4 main caps and 2 bridges. The stock caps are rated at 100V 15,000uF each. Is 100V rating really necessary?
Right now there is some hum in the amp and I'm wondering if a cap is going. This amp was built in the early 90s. I an hear the noise in the tweeter, mid and woofer.
Right now I'm thinking of replacing the 4 main power supply caps and rectifier with FREDs.
What parts do you recommend to make the best of this amp? Are CDE caps of good quality?
Anyone have a schematic?
Any other words of wisdom? I've done a search and see many with these noise problems have ground problems, but this noise is recent and I haven't done anything to the amp.
I need some advice on upgrading the power supply on my Forte Model 4 amplifier. In case you are not farmiliar with this design it is 50W Class A using the IGBT devices. There are 4 main caps and 2 bridges. The stock caps are rated at 100V 15,000uF each. Is 100V rating really necessary?
Right now there is some hum in the amp and I'm wondering if a cap is going. This amp was built in the early 90s. I an hear the noise in the tweeter, mid and woofer.
Right now I'm thinking of replacing the 4 main power supply caps and rectifier with FREDs.
What parts do you recommend to make the best of this amp? Are CDE caps of good quality?
Anyone have a schematic?
Any other words of wisdom? I've done a search and see many with these noise problems have ground problems, but this noise is recent and I haven't done anything to the amp.
I understand that the high cycle fatigue always comes earlier to the tubes, bulbs and electrolytic capacitors than to the other components – mainly due to the high operating temperatures. I would first replace the electrolytic capacitors with of min. 50V-rating, and will see whether any other replacement is also demanded.
Yeah, 50V caps seem about right... but I'm worried since the original amp uses 100V. +- voltages are each 39V. Was the manufacturer just doing it overkill or something because I can get a much higher amount of capacitance with 50V obviously- like over 4 times what is in there. I can replace the bridges to something more robust. I only want to do this once so any more feedback would be great. I've sourced some parts from mouser.com, and if you have any other recommendations for parts let me know. Brands etc. I'd like to keep the diameter the same which is in the stock amp, which is around 51mm.
Jed,
Babowana's advices are all sound and healthy. Using lower voltage rated caps only brings better performance like: better Ripple, lower ESR and so on. Its a bit hard to understand that the manufactorer used 100V's, unless he had a bunch of them at hand ofcourse. If you want to play it safe, use 63V caps. Regarding the brand; if you want the best use, Rifa PEH169 if you can get them. Otherwise Rifa PEH200's are second best. The Rifa's looks are not good, though. I beleive that Pass Labs uses Panasonic's , which are also very good caps.
Come to think of it, there was a guy selling Nichicon Goldtune caps in the marketplace, not so long ago. Might be just the thing for you. They are audiocaps with very good cosmetics, if that should be of importance. I beleive the user selling them was Ruach
I am not sure changing the rectifiers will bring much, but if you have to, you have to I like the MUR series from OnSemi a lot.
Steen
Babowana's advices are all sound and healthy. Using lower voltage rated caps only brings better performance like: better Ripple, lower ESR and so on. Its a bit hard to understand that the manufactorer used 100V's, unless he had a bunch of them at hand ofcourse. If you want to play it safe, use 63V caps. Regarding the brand; if you want the best use, Rifa PEH169 if you can get them. Otherwise Rifa PEH200's are second best. The Rifa's looks are not good, though. I beleive that Pass Labs uses Panasonic's , which are also very good caps.
Come to think of it, there was a guy selling Nichicon Goldtune caps in the marketplace, not so long ago. Might be just the thing for you. They are audiocaps with very good cosmetics, if that should be of importance. I beleive the user selling them was Ruach
I am not sure changing the rectifiers will bring much, but if you have to, you have to
I like the MUR series from OnSemi a lot. Steen
Found the Goldtunes for you: http://www.diyaudio.com/forums/showthread.php?s=&threadid=92191&highlight=
I beleive Ruach has some 63V's too.
Steen
BTW, that is cheap!
I beleive Ruach has some 63V's too.
Steen
BTW, that is cheap!
(Nelson can correct me on this) but like the Models 1 and 3 which were the same amp just wired differently (one a 50w class-A, the other a 200w class AB). Same chassis/sinks/circuits/caps etc
In the Model 3, NP had 45v caps running in series (as 90v's) on 78 volt rails. In the Model 1 (1a?) he wired the Trafo in series to get 50% voltage... with that you connect the caps in parallel and bias it high and Voila, you have a 50 watter class A with 39vdc rails.... using 45v caps. Again same caps as 200 watter just wired parallel vs. series.
I am guessing the 100v caps on the Model 4 were optimized for the 200 watt version. Where as NP had the 45v caps (24000uF each) optimized for the 50w class-A version in the BJT versions (1 and 3)
Hope this helps...
PS: I have spare 45v 24000uF caps that I pulled out for a couple of Forte 3's... if interested, email me.
In the Model 3, NP had 45v caps running in series (as 90v's) on 78 volt rails. In the Model 1 (1a?) he wired the Trafo in series to get 50% voltage... with that you connect the caps in parallel and bias it high and Voila, you have a 50 watter class A with 39vdc rails.... using 45v caps. Again same caps as 200 watter just wired parallel vs. series.
I am guessing the 100v caps on the Model 4 were optimized for the 200 watt version. Where as NP had the 45v caps (24000uF each) optimized for the 50w class-A version in the BJT versions (1 and 3)
Hope this helps...
PS: I have spare 45v 24000uF caps that I pulled out for a couple of Forte 3's... if interested, email me.
steenoe said:
BTW, the subject title did some misleading, I guess
I was misinformed that Nelson had designed the Model 4. Sorry for the confusion with the title. Now I'm curious why IGBTs are not recommended for audio. The Model 4 is a coveted amp on the used market (not that that means anything in reality
Just wanted to say thanks to everyone who helped me with my questions on the Forte. I replaced the old 15000uf caps with new 50V 39000uf caps and upgraded the bridge rectifiers to Vishay 35A 600V units. Not only did the noise go away, but the sound is so smooth and expansive, it sounds like a new amp. Thanks again-Jed
Jed said:
NP on IGBT
igbt principle
If you look at fig3 of that paper, you may probably call the IGBT a hybrid single package Sziklai.
If the output stage of your Forté A ever collapses i'm sure a couple overhere will advise you to change the type of output device, including step-by-step aid on how to do it.
afaiu, there's another problem associated with the IGBT which makes it a tiresome device for audio.
I'm too shy to ask Big Daddy Pass if he still has the Threat to the Menace paper and willing/able to place it on an /np timewindow for prosperity.
ps: don't mention IGBTs to high speed passenger train aficinados.
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