Solid State Amplifiers > Amplifier Discussion

Power amps - and power supplies

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R.G.:
I'll rephrase myself here, as the forum is new.

Power amps are best thought of as a power *supply* that lets some of the power out under special, carefully controlled circumstances. The power amp itself is really just a highly specialized wart on the power supply. In most cases, the power amp circuits proper are a trivial part of the cost of the unit.

The power supply needs to be the FIRST order of business in designing a power amp. Going to get 50W into 8 ohms? You need to know how to tell what power supply you need before you dig into what power amp or chip you'll use.

After the power supply conversion stages, voltages and currents, etc. you need to consider thermal matters. Again, this is substantially independent of the power amp circuit itself. If you're putting out 50W into 8 ohms with a class AB solid state amp, the power needed from the power supply is 50/.74 = 67.6W worth of DC coming out of the power supply at peak power. But that's not where the peak dissipation on the output devices happens. That peak comes at  forty (something) percent of the max power out. The heat has to be dissipated by heatsinks so the output devices are kept below the boiling point of doped silicon. Otherwise, you have just bought a new set of output transistors - or a new chip amp. The heat generated in the power supply itself also gets released inside the box. Where does that go?

After you have a good power supply design and a good thermal design, run wild with whatever power amp circuit you like.

junkman:
hello r.g. im new to this forum. i have been foolin around with this stuff for a while tryin to make amps out of junk i find. and i have realized recently that the power supply is the starting point for building and amp. its the part ive been overlooking and recently ive beeen trying to figure out transformers and diodes and caps  and i think i have the basics down, i am currently experimenting with a lot of tda 2030 chips i got. i think my power supply is in order and im working on the main circuit now. if i can get this thing working the only part i have paid any money for is the tda2030, the rest is from dumpster finds.

junkman:
i have a question about filtering caps in a power supply.   if more capacitance is better, is there a point at witch you gotoo far? can you have a cap thats too large?   

teemuk:
Sure.

When a capacitor charges its impedance is very low (its almost like a short circuit). A high filtering capacitance will therefore draw big current surges that stress the power supply components. The current surge during power up can be enormous, the surges during charging the capacitors when the amp is under loading will also be much higher than in the case when you use a smaller amount of filter capacitance.

I attached an image clarifying this: V(n002) and V(n005) are rail voltages of two circuits under similar loading. Basically the circuit consists of a power supply (having an internal resistance of 1 ohm), bridge rectifier and a 4-ohm resistor from the (rectified) supply rail to ground. There is also filter cap in parallel with the resistor (naturally). The only difference in the two circuits under comparison is that the filtering capacitance in the other one is ten times higher. You can see the effect of that in the smaller amplitude of the ripple.

I admit that this is not a circuit you would encounter in a practical amp as the loading of the supply rail drops this low only momentarily when amplifier is delivering its full power. Anyway, it nevertheless provides a fine example. For some perspective of realistic behaviour, the graphs at the bottom show the power up and “idling” with a 10-kilo-ohm rail load. This type of circuit you could actually encounter in a real amp.

I(D1) and I(D6) are currents flowing through one of the rectifier diodes in each circuit. The current flow through the other three diodes would be identical. I(D1) is the current in the circuit that has the higher filter capacitance. You can see that the initial current surge drawn to charge the filter capacitor is enormous. It’s quite high in the other circuit as well but the still the difference is drastic! Think how you would have to rate the transformer, diodes and fusing to tolerate this characteristic.

Then there are the surges that occur periodically when the capacitors are charged. The circuit with higher capacitance requires shorter charge time, thus the charging peak is shorter but also much higher in amplitude. Again, it is more stressing for the power supply. The characteristic is quite negligible in most circuits though. You can see the difference most prominently when the rail load is quite high (graph at bottom right).

Anyway, in essence you are dealing with making a compromise between high, transient-like current draw that will stress the power supply components - and a higher amount of ripple that will creep to the signal. You should calculate how much ripple you are willing to tolerate and choose the filtering capacitance accordingly. The reason why commercial designs often use less than 10 000uF per rail is actually quite understandable when you begin to consider the drawbacks of using too much capacitance. Often there isn’t even much of point in going “overkill”: A typical power amp is quite immune to ripple (which begins to occur in notable amount only during severe loading anyway) and the rails of a typical preamp are often regulated so the much more delicate preamp does not suffer from the ripple either. Having a very high filtering capacitance is basically more of an issue in single-ended circuits that also have single-ended preamps powered from rather high voltage rails that lack regulation.

junkman:
let me make sure i have this straight,
   the caps are not just dumping out enough juice to even out the pulsing dc, they dump all of their charge and have to be completly recharged, so too much is wastefull and taxing on the transformer or would require a larger transformer with no improved filtering.   

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