power amps and speaker impedance

[Vitrolin]

hi
I have been wondering about what sets the limit for a power amps load.

example:
if a amp is rated minimal 8Ω and for some reason I want to load it with 4Ω.
lets say its a discrete push-pull with one complementary transistor set, could I then just add another complementary set, and related circuitry?
the above completely ignores that th power supply should be upgraded.

or is it the power supply current capability that limits the amps load impedance?

[Roly]

what sets the limit for a power amps load

In a word - current.  The ability of the output stage to pass it, and for the power supply to deliver it, all without damage.

In simple terms what you propose, double up the output stage devices and beef up the supply, is exactly what manufacturers do.

In s.s. amps the power supply is actually an intimate part of the output stage, they really have to be considered together, and together they are designed for a certain current capacity determined by the lowest intended load.  So you seem to be on the right track, however it's not a case of either/or but the totality of the power supply and output stage.  The entire current loop from supply, through the OP stage, loudspeaker, and back to the power supply has to be up to the task.

HTH

[Enzo]

^^^ what he said.


Look, if all you want to do is connect your 4 ohm speaker and run it at reduced power, you can do that.  it is like towing a trailer with your car.  Your car may be rated only to tow so much weight, but if all you want to do is hook up the car to move the trailer 20 feet down teh driveway, you could get away with doing a heavier trailer than the car is rated for.

All things else being equal, a 4 ohm load will draw twice the current an 8 ohm does.  So if you reduce the output voltage level by half, that would put the current draw back down to what an 8 ohms would want.

But to run full out at 4 ohms means you need to double up the output devices to handle the doubled current, and the power supply has to be able to deliver that as well.  And that most likely means you would need a new power transformer, because ultimately that is what supplies current to the circuit.  And it is highly unlikely the one in there now has twice the capacity it needs.

[J M Fahey]

Fully agree and add: you will also double heat dissipated ... you will need double heatsink surface (and the space inside the amp for it) and probably an added fan.

But the basic idea is right.

FWIW I started making SS amps ariund 1972 (had been cloning tube amps since 1969)  and was *heavily*  limited by rail/supply voltage which then current transistors could stand.

The Industry standard (for many years)  was 2N3055 and its variants (such as 2N6254) which could stand 70V (meaning +/-35V rails) ; careful selection yielded some 50% which could stand +/- 42V railks, some even +/-45V ... period.

Good for some 60/70W into 8 ohms, 100W into 4 .

I then made a groundbreaking (in its day)  200W RMS @ 2 ohms amp and then a 400W RMS @ 1 ohm beast (driving 2 x 2 ohms each 4x12"cabs) .

It was the standard Argentine Festival/Stadium Bass amp for almost 20 years, replacing coveted Ampeg SVT in most Rock/Metal/Pop bands backlines, simply because it was louder and meaner.

Today's unwritten standard in large MI amps (not large rack PA amps which are specialized and go even higher) is around +/-70V , good for some 200/250W RMS into 8 ohms; 300/350W RMS into 4 ohms, some 500W RMS into 2 ohms.

As you see, the old limitations and tricks around them still hold, only at somewhat higher voltage.

Even Class D amps still hoild same limitations.

Designers have to be well aware of what reasonable cost, available real world transistors can achieve or designs die quickly under grueling stage and touring conditions.

[Vitrolin]

thanks for the answers, and i am glad that i was on the right track.

my doubt started some time ago when i was reading about a power amp design and it was said to be stable down to 8Ω, and i started to think about it.

[Roly]

it was said to be stable down to 8 ohms

"Stable" in the sense that the magic smoke won't come out of the output transistors.   :lmao:

There are a couple of things that you need to keep a close eye on when designing an output stage.  Just for example the venerable 2N3055 will indeed pass a maximum of 15 amps, but the obscure gotcha is that the device current gain falls down a hole above about 5 amps, so in practical terms this generally means a load no lower than 8 ohms and a power limit of about 50 watts out per pair.

The second gotcha is under-estimating the amount of heatsink required to keep the transistor chip itself below its maximum temperature under all operating conditions.  An output stage that works fine on the bench at a 25ºC ambient might be in trouble on a 40ºC ambient summer night under stage lights.

The 2N3055 is rated at 115 watts and a maximum case temperature of 200ºC, however it can only handle 115 watts up to a case temperature of 25ºC and has to be derated above that, to about 60W at Tcase = 100ºC, and no watts at all at 200ºC.

The thermal resistance of the chip to case is 1.52ºC/W and with a really big heatsink giving 0.5ºC/W to ambient, at 25ºC ambient and 50 watts out you have another 50 watts of waste heat to get rid of.  With a total chip-to-ambient thermal resistance of 1.52+0.5 ~= 2ºC/W that's 25ºC ambient plus 2ºC/W * 50W or 100ºC + 25º = 125ºC chip, and the device has to be derated to around 50 watts rather than the 115 watts in the datasheet.  In an ambient of 40ºC it's down to only 35 watts.

Every now and then you come across a "100W into 4 ohm" amp circuit using a pair of 2N3055's that has been "designed" using only the optimum headline device specs, and yes, this can be made to work provided you don't mind bolting an air conditioner onto the back of your amp - otherwise it's a in-joke.   8|