Transformer for Gainclone

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Painkiller82

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Joined 2007
#1
#6
Painkiller82 said:
What about bridge rectifiers. How much current do they have to handle?

Are any of these ok?:

http://www.banzaieffects.com/Bridge-Rectifiers-c-522-p-3.html
Click to expand...

Hi,
the peak current and the average current through the rectifier is determined by the design of the capacitor input filter hung on the end.
8ohm speakers need +-20mF/channel for deep bass but, almost all chipampers believe this to be balderdash. I stick with this recommendation.
Reduce the smoothing to +-4.7mF/rectifier and your 10A 280V version will survive without any added resistance to reduce peak current.
If you go for +-10mF then, 10A may run too hot.
 
#8
AndrewT said:


Hi,
the peak current and the average current through the rectifier is determined by the design of the capacitor input filter hung on the end.
8ohm speakers need +-20mF/channel for deep bass but, almost all chipampers believe this to be balderdash. I stick with this recommendation.
Reduce the smoothing to +-4.7mF/rectifier and your 10A 280V version will survive without any added resistance to reduce peak current.
If you go for +-10mF then, 10A may run too hot.
Click to expand...

Is that assuming a single 10A rectifier per channel?

For what it's worth I am using 20,000uF and two 8A 1000v SIL rectifiers per channel, one rectifier doing the positive side and the other one doing the negative side. With a pair of 8 ohm speakers at near full volume, playing music, the rectifiers on both channels are not even warm to touch, without any heatsinking. Same thing with a pair of "4 - 8 ohm" speakers I have.
 
#10
As above... If my experience is anything to go by then I think you should be totally fine with that power supply schematic and those 10A rectifiers.

I use the 8A version of those in the same configuration, though I would have used 10A if I could find them at the time, and they don't even get warm. I do have a separate supply per channel though (i.e a total of 4 rectifiers). I think even with a single supply for both channels (2 rectifiers in total) would work fine.

I wouldn't worry about it.

Be sure to use a 100W-ish light bulb in series with the mains live before the transformer when testing the amplifier. It can help protect against some faults faults and, I believe, possibly prevent damage. The light bulb should light up brightly for a second or two while the supply capacitors initially charge and then go dim / not lit up at all. If it stays bright then something is wrong.

Also, don't have your face too near the thing incase you have accidentally wired a capacitor reverse polarity etc and things explode! Might even be worth wearing safety goggles. I've got the supply reversed before on a tiny little opamp (NE5532) and you'd be surprised how violently they can explode. The chip's top flew up to the ceiling with a very, very loud bang!
 
#11
Thanks!

I'm a new member so it takes some time before my messages show up. So I actually posted that last question before you posted the perfect answer to it 🙂

I will try the parts I ordered first, and if anything burns up I just have to try something else. The lm3886 shuts down if it gets too hot right?
 
#12
Hi,
I don't think there is any significant difference in dual/single rectifier when considering power dissipation for a single channel dual polarity PSU. The main difference is the isolation of power ground from the transformer, but that is irrelevant to amperage in this context.
It's nice to know that 8A rectifiers can cope with +-10mF without resort to added resistance and not overheating in a ClassAB amplifier.
 
#15
I'm using +/-30V , 19,000uF per rail, (2) single secondary trafos, each with a 12A/1000V rectifier and they don't exceed room temperature driving 94+ db/8ohm speakers to volume. My setup doesn't even come close to taxing this amp/psu combo.


Have fun with your project!

7/10
 
#17
calculating volt amps?

Can someone please clarify calculating voltamps with these power transformers?

For instance, if you use V * A on the secondaries, you see 21.5V * 2.7A = 58.06 VA per secondary. For both secondaries, you then have 116.1VA total.

However, people are talking about 160 VA min for these amps. Do you actually calculate VA on the primary (giving 297VA for a single primary), even though amps are specified per the secondary?

Thanks much
 
#20
The LM3886 does 68 Watts RMS into a 4 Ohm load MAX. Add on a few extra watts of pwr for housekeeping and losses and 80VA will cover ONE LM3886 chip full range at any level the chip can tolerate.

The 120VA torid is pretty much ideal for a SINGLE LM3886 run full range.

It will easily handle 2 chips high-passed or moderate to low volume listening.

I have used that exact trafo on both LM3886 and LM3875 chip-amps full range on B&W 805 Matrix speakers.

My co-listener commented that he could feel the port blowing his pants. (speakers were nearfield, ~ 8-10 ft., we were evaluating speaker stand heights from 1 foot to 4 feet.)

I would suggest a good bypassed filter supply >5KuF per rail.
 
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