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Old 26th August 2005, 10:04 PM   #1
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Default Increasing caps on existing receivers

Hi guys, I'm pretty new here, I'm a regular poster/lurker on the class-D forum but have a small question.

A lot of surround receivers out there boast 5 or even 7 channels of audio - yet if you open them, there's a fairly decent transformer inside (judging from size, never is VA rating mentioned) but almost always the caps seem small - probably because it's a very pricey part of any amplifier.

For example my Yamaha DSP-A2 (anyone have experience with these?) has 2 caps totalling 22.000uF (If I remember correctly) - it's rated at 230V/880W (which says nothing I guess)

Newer 7 channel receivers are reported to have only 15.000uF, in the same price range my yahama was - and this for a bipolar class AB design.

Would adding caps to a receiver of this type help for sound quality/available power, or is the transformer also a very limiting factor? I'm also worried about added inrush, or is this not a big factor in amps with non-toroidal transformers?
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Old 26th August 2005, 10:36 PM   #2
MikeB is offline MikeB  Germany
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Hi,
I also own a Yamaha-dsp a2, and it has a BIG transformer, yes...
About the other receivers, i don't think that you can that easily
increase quality, depending on the circuit used the undersized
psu only limits power when all channels are playing simultaneously.
You risk loosing reliability of these receivers by replacing parts.
The guys constructing these are not dumb, but they have to keep
prices low...

Mike
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Old 26th August 2005, 11:41 PM   #3
anatech is offline anatech  Canada
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The problem is ... how do you get rid of all the heat? Stereo heatsinks were getting smaller each year, then they started bolting this other stuff on. Air flow is generally restricted inside the case. Bigger caps could just push these over the edge.

What I would recommend (in a perfect world) is to build/buy large multichannel amps and have them autosense to turn on. Hide them and plug the pre-outs into the real amps.

Now you have good sound, reliable amps and the wife acceptance factor goes up 'cause she can't see them and there aren't any more switches.

Listening to surround receivers is like racing minivans. They almost make it.

-Chris
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Old 27th August 2005, 12:34 AM   #4
Arius is offline Arius  United States
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I wholeheartedly agree with Anatech.

It's kinda pointless to improve the PSU caps when the amp circuit is so basic. Case in point - compare a high priced (yes) receiver to a low priced hi-fi amp (say something cheap from Parasound, etc). Find something with similar power, e.g. 100W per ch. Look at the amp sections - you'll see what I mean.

It's also interesting to compare the input AC power. I have yet to see a receiver that draws in more power than what it can give out.
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Old 27th August 2005, 09:21 AM   #5
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Thanks for the help.

I've went (partly) the seperate way a while ago - Adding a stereo class-d amp to my receiver.

It sounds nice as it is, the DSP-A2 was no underpowered machine, well not like the new receivers out there anyway... But i'm going to upgrade my receiver at the end of the year, and I'm afraid I'll have to upgrade ever further, adding at least a seperate amp for my center speaker.

then the receiver will just have to do the surround speakers, and even then I think 15.000uF is not a lot.

Even my vintage akai stereo 100W machine from the Eighties has more caps than those receivers...

My new stereo amp has more uF for one channel even!
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Old 27th August 2005, 02:45 PM   #6
anatech is offline anatech  Canada
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That's basically been my point. Use the surround receiver as a front end and supply your own amp.

-Chris
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Old 31st August 2005, 05:46 PM   #7
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It's bad. It's done to keep things cheap........As long as it filters the recified DC enough so it doesn't hum much, it's enough for cheap commercial stuff. The caps aren't really holding the voltage up much. Some amps hum, while others may remove AC hum through PSRR.

I'd recommend get as big of caps you can get, make sure the filter diodes can handle the extra inrush current.

Here's an example of commercial cheapness: An amp I'm replacing for a subwoofer, old Sharp amp with IC amp, advertised 120W/ch x 2. Unit has a large heavy transformer. Input on back says AC 230W LOL

IC Chip specs say only 100W/ch (STK4231-2)
Trans measures 44-0-44 V AC, 60-0-60 DC no load
Filter caps are only 5600 uf each (2 of them)

Amp sounds strong at low power levels, but at high power, the rails sag, so the amp never really outputs 120W/ch. Also, the amp hums with no music and your ear is about 1ft away from speaker!

My replacement amp still in the works has 23,500uf per rail 10 caps altogether (4,700 x 5 per rail) and has no hum. Yay for DIY!!!
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Old 31st August 2005, 06:53 PM   #8
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Yeah...

Unfortunatly, receivers are the most bang-for-the-buck processors out there - I'm even looking to mod the pre-outs. The A2 is certainly not bad, if you know it's limits.

I only use internal amps for back channel duty now (and center but it will change...)

I was just thinking, giving the fact I have good amplification in front now, can I spice up the receiver a little so that it has more reserves? Of course there's no way telling what the VA rating of the transformer is...

For front end I've went with a class-D solution, UcD's, with 44.000uF of caps just to run 2 channels, fed from an old 350VA for the moment, but this will be 800VA soon. I'll add a third channel in there, that can be switched off. BUT of course this isn't the class-D forum

I'm just afraid if I add bigger caps in receivers I might make the inrush current worse? I'm not worried about the diodes, I've got large 35A bridges here, one €/piece that'll handle just about anything...

Thanks for the tips you've given guys, very much appreciated!
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Old 31st August 2005, 10:29 PM   #9
anatech is offline anatech  Canada
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Hi Yves,
Don't bother. You have reduced the load on the supply and there is not that much going on in the surrounds compared to the fronts and centers. In effect, you have increased the available capacitance to the rears.
Don't mess with it.

EWorkshop1708,
What you say is generally true, but you are picking on one of the cheaper units out there. You may make an incremental improvement but that's all. By the same token, the outputs are rated on the edge and so are the heatsinks. Then there are the amplifier circuits - cheap. Then the low voltage supplies, preamps and signal processors. Then muting and selector switching. Where are you going to stop?

It is what it is. Build real amps if you need to but don't waste your time on the receiver unless it's an experiment. A cheap receiver will always be the limiting factor - unless you got the speakers as a package.

-Chris
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