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Old 15th February 2007, 01:21 AM   #1
Jed is offline Jed  United States
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Default Forte Model 4 ( Power Supply upgrade)

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.
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Old 15th February 2007, 01:51 AM   #2
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What's the rail voltage?

Grey
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Old 15th February 2007, 03:25 AM   #3
Jed is offline Jed  United States
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From the positive terminal of the first cap to the ground plane I get 39V.
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Old 15th February 2007, 06:35 AM   #4
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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.

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Old 15th February 2007, 12:17 PM   #5
Jed is offline Jed  United States
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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.
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Old 15th February 2007, 12:39 PM   #6
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My experience tells me that min. 50V-rating is safe size for the 39V rail. Brand? I think that the quality gap in electrolytic caps of today is paper thin. I just choose nice color and good looking caps!

Other members might give you better comments.

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Old 15th February 2007, 04:59 PM   #7
steenoe is offline steenoe  Denmark
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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
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Old 15th February 2007, 05:11 PM   #8
steenoe is offline steenoe  Denmark
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Found the Goldtunes for you: Nichicon KG Goldtune, Muse and misc capacitors for sale
I beleive Ruach has some 63V's too.
Steen

BTW, that is cheap!
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Old 15th February 2007, 06:03 PM   #9
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Just curious, is the Forté Audio Model 4 an NP design ?

I've never seen the IGBT model, so no idea how it sounds.
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Old 15th February 2007, 06:06 PM   #10
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Quote:
Originally posted by jacco vermeulen
Just curious, is the Forté Audio Model 4 an NP design ?

I've never seen the IGBT model, so no idea how it sounds.
No, it is not. I don't recommend IGBT's for audio, and I left
Threshold with a paper entitled "IGBT's - Threat or Menace?"

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