A simple newbie power supply Question??
« on: October 08, 2006, 03:43:12 PM »
Hi Guys, I have a pretty simple question here for you folks and I think I allready know the answer but just need someone to confirm my thoughts...

Here is the situation:

I am designing and Building a Guitar amp based arround the LM3886 Power amp IC....This design also has a preamp stage useing the OPA2134 which runs off of 15v Rails but the LM3886 needs at least 18v but preferably higher (voltages from 18v up past 35v) but it doesn"t need to be regulated...

The problem is I don"t want to build a Supply that uses 4 regulators (+/-15v & +/-18v) because of space concerns and because the LM317/337 regulators have a max amperage of 1.5amps and this supply will be delivering more than 1.5amps (Useing a 18v-0v-18v 100vA 2.4amp torroid)....

What I was thinking of doing was takeing the voltage off of the supply rails for the LM3886 after the filtering Caps but before the Regulators to give me an unregulated aprox 22v DC for the Power amp and take the 15v after the regs for the preamp.....

So my question is will this work properly?? are there some unforseen problems I might encounter??

I think this should work fine but I was thinking that maybe when the power amp is at max power it might cause a voltage drop which would cause the 15v regs to recieve less Voltage than needed to regulate to 15v??

I also though it should still work OK as the OPA2134 can run at lower voltages than 15v....

This amp should deliver 35w to 50w into 8 ohms and up to 68w into 4 ohms but I can"t seem to find any info that will tell me if my 100vA Torroid will be strong enough to power this amp...

Do you guys know if 100vA is enough for a 50w amp, If it isn"t enough will it just mean that the amp won"t go as loud when cranked to full or will it mean that it will only work up to a certain volume level and any higher will just produce distortions??

Also Heatsinking the IC is very important so I was thinking of simply attaching a Heatsink from a CPU and Power the 12v fan off of the 15v power rails through say a 150r resistor to lower the voltage a bit...

Will that work ok or will I need a bigger heatsink?? Do I ground the heatsink??

Thanx a Lot for any help you can Give.....



A simple newbie power supply Question??
« Reply #1 on: October 08, 2006, 04:04:50 PM »
So my question is will this work properly?? are there some unforseen problems I might encounter??

It will/should work. Its the way its usually done.

cause a voltage drop

How big are your filtercaps?

You will have about 25-0-25V unloaded DC coming off the filtercaps, it will drop under load but you should be allright.

if 100vA is enough

VA has little to do with output P.

The only load the secondary will see is the filter caps.

Im not helping this instead:
"Relaxing on the axis of the wheel of life."


A simple newbie power supply Question??
« Reply #2 on: October 08, 2006, 09:44:45 PM »
Thanx for the Link but that stuff is WAY Over my head.....I am absolutly terrorable at complex math.....

From what Info I do understand It seems that my 100vA might be too small to power a 50w amp....

My Filter caps are as big I guess as I make them...I have a lot of 1000uF 50v caps so I could put say 5 1000uF on each of the Power rails bypassed with 0.1uF metalized poly caps...Should that add edequate filtering???

If I build a very well Filtered power supply will that help with the amount of power I will need for this amp?? (It seems to suggest that in the Link you gave me)

The Chip is a Quasi Class A/B amp so I don"t know really what that is supposed to mean....

Here is the data sheet for the chip:

What in the data sheet do I look for when trying to calculate the size of transformer I should use??



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