Help Improve Parts Express Plate Amp

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I have a Parts Express#300-794 subwoofer "Plate" amplifier, rated at 250W/4ohms.
http://my.fit.edu/~kozaitis/Speakers/300-796DataSheet.pdf

The two main power supply capacitors are "Jun Fu" (??) 6,800/80v units. Would it improve the bass power/control if I were to replace these with larger capacitors(within reason)?

I could use either one pair each of 10,000uf/80v, or 15,000uf/80v units(I have these on-hand), but am not sure how much is "too much"...

I do want to improve performance with my 4 ohm sub, but I don't want to "kill" the bridge rectifier or damage the output transistors.
 
Picture attached

Here is the amp in question.
6,800uf isn't very much for a class AB designed for bass duty...
 

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Wow!

20,000uf is a lot more than I dare use, esp. due to limited space.

Additionally, I'm concerned about the bridge rectifier that was used(small black block between the big caps, marked "KBPC804").

I've heard "too much capacitance could kill the rectifier bridge"...

Is this true?

If so, how large can/should I (safely) use?

TIA

-Chas
 
Re: Wow!

clm811 said:

I've heard "too much capacitance could kill the rectifier bridge"...

Is this true?

-Chas

The problem only occurs at power-on, when the cap(s) are charging up and draw a lot of current. One strategy is to put a thermistor into the line (like a CL60 or similar) to limit the current and let the caps charge up more slowly.

Your bridge rectifier is rated at 8A continuous, 40A peak according to a datasheet I checked.

I'm not sure that you will hear a great deal of difference with the added capacitance, though.

Cheers
John
 
The problem only occurs at power-on, when the cap(s) are charging up and draw a lot of current...Your bridge rectifier is rated at 8A continuous, 40A peak...

So do you experts here think that increasing the cap size to, say, 15,000uf will help or hurt?

Again, this is a subwoofer amp with 3db of boost @30Hz being used to drive a 12" 4 ohm acoustic suspension subwoofer below 100hz in a multi-channel music and home theater system.

-Chas
 
diyAudio Member
Joined 2007
Warning ------ Non expert opinion!!!

I'd look at replacing the bridge rectifier with a biggish 30 amp unit, should only be a couple of dollars, and use as much capacitance as you can fit in the space.

I have the same problem here, except that the power supply is even smaller 42V toroid 320 VA and the caps are 4700uF 60V, but that is what I expected when the whole sub cost less than $100 for a 12 inch driver in a box.
It can't hurt, just may not help as much as a well designed amp in the first place
 
Thanks for the reply, Dog.

I wonder if I can find a 35A bridge with leads to fit the PCB spacing for the 8A unit...

... that is what I expected when the whole sub cost less than $100 for a 12 inch driver in a box.

I'm glad YOU got such a deal on your entire subwoofer; Unfortunately, I paid about $150 for just the amp!

My sub's an (older) DIY unit using the venerable NHT1259 woofer(See Pic).

So, I guess I'll go with the 15,000uf caps, then...

-Chas
 

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Sounds like the reason fore asking this is concerns about sub control and power

Yes. I think I've done everything I can over the last 5 years regarding placement(moving it, measuring with test tones and SPL meter, crawling around the floor, etc), plus many frequency/phase adjustments.

The woofer itself is a tried and proven design that has garnered many good reviews over many years. When powered by another, my main amplifier(Parasound), the bass is taughter and is more dynamic sounding; Therefore, I find the plate amplifier suspect(I've tried it with and without bass boost).

Everything I've read (esp. here on DIYaudio) about SS amplifier design indicates a need for adequate power supply capacitance for sufficient current delivery to control a woofer system- most bass amps utilizing 20mf, 40mf or more in PS capacitance.

Amp design guru John Curl wrote:
"We have found that 20,000 uF and more is necessary, per channel, to make the bass sound right."
.

I was just concerned about the practical limits of this particular circuit. I am assuming that the 6,800uf caps in this amp were chosen for economy, not performance.

Since I'm not that familiar with "plate" amps for subs, I'd hoped someone here on the "Subwoofer" forum might have all the answers.

I will turn to the SS amplifier forum to see what answers I can get.

Thanks to all here for your efforts!
 
HELP!- PS Caps get HOT! Fuse blows

I finally got around to testing the plate amp, and upon powering it up, the
4A line fuse blew. I touched the two "Jun Fu" 6800uf electrolytic caps and they felt unusually warm for such a short power-up.

To be sure it wasn't just a turn-on surge, I slowly powered it up through a variac. The fuse didn't blow this time, but the caps felt
very hot !

Looks like I'll have to replace them anyway, after all.

Anyone else have this experience with these caps?

Recommendations?

-Chas

p.s. I tried to contact Steve at Apex Jr., regarding the Jun Fu caps(he sells these), but he's away on vacation for another week.
 
I tend to find 10,000uF caps are a reasonable compromise between value and price.

WRT the rectifier - in a recent rebuild, I did what MoonDog mentioned, ie, mounting it on the heatsink, with a few bits of wire. It gives a much greater choice of rectifiers, too.

I'm sure I've just replied to that post, but here goes...
Hot caps are an indication of something wrong, so maybe check positive/negative rails and stuff. And the voltage of the output of the rectifier. If that's above the voltage rating of the caps, you need to keep it switched off. Over-voltage on caps can explode them.
 
Caps Rattle when shaken

I removed the suspect caps and upon shaking them they rattle as if there is something loose inside. Other electrolytics on hand do not exhibit this.

Shall I assume they have "dried-out" and that's why they are bad?

I'm now shopping for some good quality 80V snap-ins...

-Chas
 
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