Power double ac amplifier from 30V DC

tigox

Member
2012-07-10 7:24 pm
Hi i looked up a lot of amplifiers on ebay but the only good ones i can find are powered by double AC, so i tought ... is it possible to boost 12v to 30V with a boost converter, power the amplifier with that 30V and GND after the bridge rectifier ( so it i would bypass the rectification block) and then reference the audio input to a virtual ground i create with or without a buffer?
Thank you :D
 
So many questions generate so many questions.

What "good ones on e-bay" are you referring to?

Do they take a centre tap transformer?

If not then you don't need a virtual ground from your 30Vdc.

If they do then it would be just as easy to take +12 and make a +/-12 and use that.

(or even +/- 15V)

As for the +30V with a "virtual ground", It would have to be more than just for the input circuitry.

Hope that helps

:)
 

tigox

Member
2012-07-10 7:24 pm
So many questions generate so many questions.

What "good ones on e-bay" are you referring to?

Do they take a centre tap transformer?

If not then you don't need a virtual ground from your 30Vdc.

If they do then it would be just as easy to take +12 and make a +/-12 and use that.

(or even +/- 15V)

As for the +30V with a "virtual ground", It would have to be more than just for the input circuitry.

Hope that helps

:)

when i say good amplifiers is 2.1 amplifiers with more than 20-25wrms that don´t need center tap...
but most need 20-0-20 ac to work
what sort of external circuitry are you talking about (refering to the virtual ground response you gave)
 

tigox

Member
2012-07-10 7:24 pm
when i say good amplifiers is 2.1 amplifiers with more than 20-25wrms that don´t need center tap...
but most need 20-0-20 ac to work
what sort of external circuitry are you talking about (refering to the virtual ground response you gave)

... reference the audio input to a virtual ground i create ...

Just following your lead.

:)
 
Why create an artificial one when you already have the real one?

Now for a more general (and useful) answer: artificial (or more properly "active") center taps or virtual grounds, have one problem: they must source or sink *all* the current pasing through the load, same as a "real" ground would.

It's practical in a preamp , or in a lot of 9V powered Guitar pedals, small battery powered mic mixers and such, because since they basically process *signal* (meaning voltage into high impedance loads), current is very low.
So, say, a humble Op Amp which can provide 5 to 10 mA on demand, is adequate.

But in a Power amp, we are talking big Amperes.
A 100W into 4 ohms amp supplies 7A peak current into the load (and into the speaker ground return wire) all day long, and such return should be able to flawlessly pass 10A, to have a slight margin.
Very easy task for 8 inches of #18 (1 mm) wire which cost a few cents and at most will drop a few millivolts.

But if you want to create an active ground, it will need to pass the same current, will drop much more voltage (at least 1 or 2 diode drops at the junctions) and in general will need semiconductors as beefy as the power output ones.

So it's not that it's technically or physically impossible to do it, it's just that it's inefficient and expensive compared to the alternative.

While in, say, a preamp or pedal, 1/2 TL072 (or even a 741) , 2 resistors and 2 100uF caps will add to less than 1 U$.
 

tigox

Member
2012-07-10 7:24 pm
Why create an artificial one when you already have the real one?

Now for a more general (and useful) answer: artificial (or more properly "active") center taps or virtual grounds, have one problem: they must source or sink *all* the current pasing through the load, same as a "real" ground would.

It's practical in a preamp , or in a lot of 9V powered Guitar pedals, small battery powered mic mixers and such, because since they basically process *signal* (meaning voltage into high impedance loads), current is very low.
So, say, a humble Op Amp which can provide 5 to 10 mA on demand, is adequate.

But in a Power amp, we are talking big Amperes.
A 100W into 4 ohms amp supplies 7A peak current into the load (and into the speaker ground return wire) all day long, and such return should be able to flawlessly pass 10A, to have a slight margin.
Very easy task for 8 inches of #18 (1 mm) wire which cost a few cents and at most will drop a few millivolts.

But if you want to create an active ground, it will need to pass the same current, will drop much more voltage (at least 1 or 2 diode drops at the junctions) and in general will need semiconductors as beefy as the power output ones.

So it's not that it's technically or physically impossible to do it, it's just that it's inefficient and expensive compared to the alternative.

While in, say, a preamp or pedal, 1/2 TL072 (or even a 741) , 2 resistors and 2 100uF caps will add to less than 1 U$.

if i am willing to have a voltage drop what can i do? because a voltage drop is actually good for me or has no efect at all in my application
 
How about using two batteries in series?
The top +12v goes to the positive of the amp, where the ground of the top bat and the +12v of the bottom battery are joint together is your real ground, and the ground of the bottom bat is your -12v, problem solved about virtual grounds and you also now have 24v across the poles of the bats.
 

tigox

Member
2012-07-10 7:24 pm
How about using two batteries in series?
The top +12v goes to the positive of the amp, where the ground of the top bat and the +12v of the bottom battery are joint together is your real ground, and the ground of the bottom bat is your -12v, problem solved about virtual grounds and you also now have 24v across the poles of the bats.

yeah i know but i have 2x5500mah batteries that give me 11.1v each, to have those 24v (+-12v) i needed 4 percebes?