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PostPosted: 12 Jan 2014, 12:38 
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Joined: 04 Jan 2014, 17:08
Posts: 3
Location: Ceveland Ohio
Hello to all from a new member,

I tried to increase the onboard PS capacitance in my Yamaha MX600U 2 channel power amp. There is a 18000uf 73 volt cap provided for each channel present on the amp main board. I purchased 2 10000uf 100 volt caps and installed these in parallel with the existing caps, one on each channel. After adding the caps, and upon powering up, the amps 10amp main circuit protection fuse blew. And continued to blow until I found that the bridge rectifier on the main board was cooked. I removed the new caps I had installed, replaced the rectifier with one I happened to have, and powered up the amp again and it has been working fine since. I have not made any further attempt to add any new caps.

My question is where I erred in my effort to add additional capaciitence? Also would adding the 2 new 10000uf caps actually improve sound quality? I was hoping to clean and tighten up the bass response a tad.

Also along this same line, instead of adding 2 additional caps as I have described, would purchasing or making a power supply board with increased capacitance be a better way to go? I have room in the amp chassis to do so. Does installing additional power supply rectification, in the form of an additional separate board present any conflict with the caps and rectification built in on the Yamaha main board?
Thanks and I hope I have posted this according to forum rules,
Dennis


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PostPosted: 13 Jan 2014, 21:03 
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Joined: 28 May 2008, 21:53
Posts: 4592
Location: Winnipeg, CANADA
Hi Dennis,

Adding more capacitance increases the inrush current on power up. Sounds like the increased inrush current exceeded the rating of the bridge rectifier. One way around this would be to use a "soft start" circuit or a current limiting device like a thermistor.

Welcome to the forum.

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PostPosted: 13 Jan 2014, 22:27 
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Joined: 04 Jan 2014, 17:08
Posts: 3
Location: Ceveland Ohio
Thanks for the welcome!
Looking back and being aware of my haste to ASSUME something way to often, I'm thinking I installed the 2 new additional caps in reverse polarity. It was very difficult to see the polarity of the 2 existing caps with any certainty without removing them so I installed the new caps +to+ -to- with my best guess. There is a delay soft start feature on the MX600. After installing the 2 new caps (probably backwards) upon powering, up the main fuse blew instantly and cooked the rectifier. No soft start delay taking place. Would the reverse polarity installation short the rectifier and blow the fuse instantaneously?

I would be tempted to reinstall the 2 new caps making sure the polarity was CORRECT this time, in parallel with the existing ones. The rectifier I replaced the faulty one with is substantially larger and heavier. It is a metal cased component rated at 25amps. Should I try it?
Dennis


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PostPosted: 15 Jan 2014, 08:58 
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Joined: 04 Jun 2008, 20:59
Posts: 4326
Location: Arizona, USA
Hi, Yes. They would be likely to kill the rectifier and cause the fuse to blow. In answer to another part of this....I doubt that they would would make much difference in the quality of the amp. The fuzzy bass is likely a consequence of the amplifier design. In theory a quality solid state amp ought to be great at bass as most solid state components handle high currents quite well. But in manufacturing them the companies are cost conscious and do things that reduce that capability. You didn't mention whether it was a class "T" aka "D" switching based amp or a linear one. In class T there are few things that can be done at the diyer level to get better bass in a linear some possibilities based on how the output stage is designed. If it has no coupling capacitor (output signal is symmetrical with reference to the ground you can't change much. If it is capacitor isolated then perhaps a larger coupling capacitor can help.

Good listening
Bruce

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PostPosted: 17 Jan 2014, 14:51 
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Joined: 04 Jan 2014, 17:08
Posts: 3
Location: Ceveland Ohio
I think my MX600U Is capacitor isolated. I do have a schematic for the unit and the circuit does appear to be so. Maybe??? What is the old saying? " A little knowledge can be a dangerous thing". I can not find any written advertised amplifier design type specified in any of the paper work I have in regard to this amp. Attached is a short blurb I copied out of the manual which might indicate a hybrid design of some type. AB ?. By the way the Yamaha is my backup beater and tinkering toy so not that much to risk.

I did reinstall the 2 10000uh 100volt electrolytic capacitors . Making certain of polarity. The amp is functioning with the mod this time around. I do sense some subtle improvement in the low end response. I am using the amp to drive a couple of subwoofers with low end signal fed from my Oppo 105 BDP multi channel SW output. No crossovers in the speakers. Bass is slightly cleaner sounding with more detail. Prior to adding the extra caps, low frequency from the Yamaha and the subs leaned toward a degree of murky puffiness. Especially when sub volume level was increased. With the additional caps in place, I can raise the sub volume level substantially higher now maintaing tighter detail. I do not listen to or want a lot of low end added to my system so I don't crank the sub volume much anyway. I do not think the subtle improvement is just a wishful perception.

I will get back with questions about coupling caps.

Hey, it is all good! I have enjoyed and continue to enjoy Hi-Fi audio tinkering for the past 40+ years. Though my ears sure ain't what they used to be! Thanks for your help.
Dennis


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PostPosted: 23 Jan 2014, 08:31 
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Joined: 26 Oct 2012, 21:00
Posts: 203
Location: ontario canada
from the way you described your mod/cap replacement i think you popped your mains fuse and cooked your rectifier simply because you didn't "form" the caps before you pressed them into service!
the in rush current would have been tremendous!
long live "dim bulb testers"!

implementing b+ regulation vs brute capacitance is the way to go for better bass imho.


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