Power amps - and power supplies

[R.G.]

... If it can only provide, for example, 20 watts of power then substituting a TDA2020 with a TDA2040 won't mysteriously convert the amp to a 40W amp. Even if the heatsinks were big enough to tolerate the increase, the power supply simply isn't.

That's an excellent example!

[TomCarlos]

Can I jump in and ask a question (and please forgive my ignorance)??

My new internet buds on the ACC forum helped me rebuild an Acoustic Bass Amp.  It's a simple Power Supply Design.  The A/C Voltage is around 53 volts. That goes through the Bridge Rectifier and then connects to a 3900uf cap.  The DC voltage is around 70 volts.

I'm trying to learn about power supplies, ripple, peak current loads, etc.  I've often wondered, when does it make sense to use Chokes in a power supply?  I came across an article that talked about an LC circuit and a Pi Circuit (Cap, Inductor, Cap) to smooth out the Ripple.  Why is it that Guitar & Bass Amps do not use these types of filters? 

If cost was NOT an issue, would it make sense to design power supplies using LC or Pi filters?

I'd like to study this further.  If anyone has a reference to a web site or book on the subject, please post that here.  And yes, I will read pages 167 - 193 of Teemu's book.

Thanks,  Tom

[J M Fahey]

Well, you have self-answered, sort of.
Besides Teemu's excellent Book, keep reading general purpose electronics books.
Today's massive capacitors are cheaper and more efficient than chokes, in the 40's and 50's it was the opposite.
Fender amplifiers often used 20uF filters !!!
Single ended ones from the 30's only 10uF !!!

[joecool85]

Well, you have self-answered, sort of.
Besides Teemu's excellent Book, keep reading general purpose electronics books.
Today's massive capacitors are cheaper and more efficient than chokes, in the 40's and 50's it was the opposite.
Fender amplifiers often used 20uF filters !!!
Single ended ones from the 30's only 10uF !!!

Wow have times changed!  I used two 10,000uF caps on my LM3886 project 

[R.G.]

... in the 40's and 50's it was the opposite.
Fender amplifiers often used 20uF filters !!!
Single ended ones from the 30's only 10uF !!!

Wow have times changed!  I used two 10,000uF caps on my LM3886 project 

Times have changed, and capacitors are much smaller now due to improved materials. However, this is kind of misleading. The important thing to look at in thinking about capacitors is energy stored per unit volume.

The energy stored in a capacitor is E = (1/2)*C*V². The power supplies in the 50s were commonly about 400-500V. So for a 20uF cap at 450V, E = 0.5*(20E-6)*(450^²) = 2.025 Joule.

For a 10,000uF cap at 30V, the energy E = 0.5*(10,000E-6)*(30^²) = 4.5 Joule. The energy in the two is only different by 2:1.

The 10,000/35V rated cap will be smaller than the 50s 20uF/500V capacitor, but only by about a factor of two. So in 50 years, we have made capacitors able to store about four times more energy per unit volume.

The measuring stick that the pros use for caps is the product of capacitance and voltage per unit volume. In the definition of capacitance, C = Q/V or the capacitance equals the total charge stored (in Coulombs) divided by the voltage it's stored at. Big capacitance stores a lot of charge at low voltages. Small capacitance has to be packed very tightly by voltage pressure on the electrons to get the same charge; literally, the voltage is cramming them more forcefully into a smaller "bucket". So the charge stored (Q) is the product of C and V, and the "CV product" is the measure of how much storage you get. CV product per volume is the measure of how much charge you can store in what number of cubic centimeters.

[tubeAMP]

for you engineers out there if the power supply stress is such a big concern why no choke between rectifier and filter capacitors ?
I figured that these diodes are rock solid handling much higher current than vacuum tube rectifiers

[J M Fahey]

for you engineers out there if the power supply stress is such a big concern why no choke between rectifier and filter capacitors ?

       

[phatt]

+1,, Well answered.
Phil.

[teemuk]

The size of a choke filter suitable for solid-state amp's power supply would be nearly equal to size of the PT itself. Price would likely reflect this trend.

Capacitance is much cheaper and compact and effectively achieves the same result.

[J M Fahey]

I'd like to add that in SMPS, chokes *are* used, but there they are small, relatively inexpensive, and need only a couple turns of wire.

[tubeAMP]

there you have it.  not an issue

[Roly]

I came across an article that talked about an LC circuit and a Pi Circuit (Cap, Inductor, Cap) to smooth out the Ripple.  Why is it that Guitar & Bass Amps do not use these types of filters? 

Actually the split is between valve amps, which often do have, and solid state amps which normally don't; e.g.;


...and at least some of the Fenders.

if the power supply stress is such a big concern why no choke between rectifier and filter capacitors ?

The much higher currents in S.S. amps make large filter inductors uneconomic (calculate the values you would need), and unergonomic if you have to lug them.

This looks like a bit of fun...
http://demonstrations.wolfram.com/PiFilter/

[Anomaly]

I'm wondering, are mosvalve power amps decent? I really love the immediate response of SS, so i'm wondering if a Mosvalve would pair up well with a Mesa studio pre-amp??

[J M Fahey]

Yes.
Thy are standard SS power amps.