Why 4 Ohm Loads Stress Your Amplifier

[crooner]

I have noticed that Pioneer specify output powers for their SX-x3x, SX-xx50, and SX-xx80 receivers into both 4 and 8 ohms, except for the SX-1280 and SX-1980 for which output power is quoted into 8 ohms only. Similarly, the SPEC 4 is rated at 150 watts into 8 ohms and 180 watts into 4 ohms, whereas the output power for the larger SPEC 2 is only quoted into 8 ohms (250 watts).

My suspicion is that this is because the cooling on these higher power units is insufficient to allow them to develop the same (or greater) continuous power into 4 ohms as they can into 8 ohms (output power being measured by the old FTC regulations which required 1 hour pre-conditioning at 1/3 of rated output, i.e. maximum heating of the output transistors).

Inadequate cooling is not a reason for the lack of 4 ohm ratings. The real reason is that at 4 ohms current draw increases considerably and many of these models had a safety limiter to prevent damage to the output transistors.

The SOA of high power transistors in the 1970's was still limited at lower loads. There were "brave souls" such as Flame, I am sorry, Phase Linear that used them to their maximum capacity. However, the Japanese placed utmost importance in safeguarding equipment, hence the current limiters.

The units can actually be used safely with 4 ohm loads, only that doubling of power at such loads is not possible due to the limiter. THD probably creeps a little bit as well. Remember back then manufacturers were in a THD race. Nobody wanted to show higher THD levels at 4 ohms for about the same power available at 8 ohms.

 

[oldtubes]

Here is a good article on transistor power dissipation. Just don't ask me to explain it.
http://sound.westhost.com/patd.htm

 

[crooner]

I forgot to mention that the other units rated at 4 ohms also use current limiting since the power does not actually double at that load.

I don't view current limiting as a bad thing. Unless you are driving Apogee Scintillas it's not a problem. Thanks to these circuits many of the classics are alive and well today.

 

[Whitehall]

In any design, there will be some bottleneck, some weak link that sets the limits for the system.

Look at the sequence of power flows. First, the power cable - it would have to be pretty skinny to be the limit. Then, the power transformer - it could saturate. It could be the rectifier diodes. It could be the filter capacitor ripple current rating. Do this part right and you have a "stiff" power supply that holds a near-constant voltage with current draw - that's good.

It could be the power transistors SOA or the heat sinks watts/deg rating.

When we connect low impedance speaker loads, the whole system gets challenged to either deliver more amps or reject more waste heat.

So where did designers of the gear we use decide it was most economical to accept the bottle neck? Heat sinks aren't cheap and they eat a lot of real estate. Transformers weigh alot. Filter capacitors are bulky. An extra pair of power transistors per channel is also expensive.

I suspect that the reason 4 ohm loads were seldom rated is that the market for low impedance speakers was slim - designs were just progressing downward from the 16 ohm vacuum tube era to early solid state at 8 ohm. Current designs are tending lower due to cheaper power electronics being exploited by driver designers - let the owner pay more for electronics to keep the price/performance of new speakers higher.

 

[crooner]

OTOH, actual loudspeaker loads vary tremendously according to frequency. Static measurements with a fixed load don't always tell the whole story.

 

[rwortman]

You show that at half power into 4 ohms that the dissipation of the transistors is way higher than at full power into 8 ohms. This is true for every AB amplifier and many of them are safe with 4 ohm loads so while interesting in proves nothing about the amplifier in question. What you need to know is what is the heat removal capacity of the heat sink at the safe operating temperature of the transistor and at what power that heat load will occur into a 4 ohm load. The manufacturers may have either not tested into 4 ohm loads so were unwilling to predict the results or had designed close to the edge and really thought a 4 ohm load would blow something. If I had one of the old lunks and some 4 ohm speaks (such as my PSB's), I would either not use it or I would stick a thermal switch(set to the safe transistor temp) onto the heat sink and wire it in series with the coil of the protection relay.

 

[EchoWars]

...also consider that, although at half-power, that's only 3db down from full output, and still way beyond a 'normal' listening level for most.

Most amps don't have a problem with 4 ohms at reasonable listening levels, 'reasonable' meaning 10W and under. At the highest listening levels the culprit is current (with high dissipation being a byproduct of that). Continuous listening at high levels with a low impedance load should be done with an amplifier designed to do just that (and if you have to ask 'can my amp do this?', then the answer is probably 'no'), but for normal listening it likely isn't a problem.

You're on the right track and I see no reason to pick your argument apart. Nice work.

 

[thinker10]

Thats like some Yamaha, Onkyo receiver old and new that have an Impedance switch.

The only way they can do 4Ω .

Thank You

 

[crooner]

Any heat issues arising from operation at lower impedances would then be a funcion of the output device itself not the heatsinks attached to it. The device will be drawing much more current and if limiting is not employed it will self-destruct no matter how large the heatsinks may be...


The big receivers of the late 1970's pretty much reached the limit for safe usage of the devices available at the time. It's been said that the Dollar-Yen exchange rate killed the power wars. I would also add that any practical receivers beyond 330 wpc were simply not possible without applying new technologies.

 

[bully]

Gotta say that the European DIN figures are HIGHER OUTPUT than the USA FTC-mandated output. My Yamaha CR-1040 is 70 wpc at 8 and 4 ohms, and 80 or 100 watts DIN. The DIN does not require the serious preconditioning the USA FTC mandated standard for stereo amplifier claims. Big difference.

BTW, the Concept 16.5 receiver is rated at 165 wpc into 8 ohms, and a staggering 250 wpc into 4 ohms. The similar SX-1250 (a very fine receiver, I had the black SX-5590) is 160 and 200 wpc into 8/4 ohms. Another one of the reasons I got rid of my other TOTL receivers and kept the 16.5--that and the incredible sound the 16.5 provides. It really does rival separates.

 

[crooner]

Wow! That Concept is dynamite. I wonder what kind of devices they used.. :scratch2:

 

[andy]

What about amps that have a 4/8 ohm selector? I've never looked at what this switch does. I assume it either drops the supply voltage to reduce power dissipation, or it limits the maximum output level to keep the current under control.

 

[ToTo Man]

Just out of interest, do 'Class A' amps generally run a lot hotter than 'Class AB' amps?

 

[240 Volts]

Just out of interest, do 'Class A' amps generally run a lot hotter than 'Class AB' amps?

Yes!! :yes: (and maybe No!! )

The "Yes" bit : Class A amplifiers get hottest when they are idling with no signal. So when a class AB amp would be almost stone cold the class A amp is stinking hot.

The "maybe No" bit : At maximum output power I'm not sure if the class A amp will be any hotter than the class AB. :scratch2:

- Richard B.

 

[240 Volts]

Just added a bit more detail to the original post - regarding how the voltage / current / power curves were derived.

- Richard B.

 

[wineslob]

Yes!! :yes: (and maybe No!! )

The "Yes" bit : Class A amplifiers get hottest when they are idling with no signal. So when a class AB amp would be almost stone cold the class A amp is stinking hot.

The "maybe No" bit : At maximum output power I'm not sure if the class A amp will be any hotter than the class AB. :scratch2:

- Richard B.

I use a Bedini 150/150 amp in my system. It's a AB design that runs in B only while at idle. As soon as a signal is present it switches over to A. It runs hotter at idle than at half power or so, but gets close to the idle temp when pushed. This is on 5 ohm Apogee Duetta's that have a relatively flat impeadence curve.

 

[Retro Stereo]

I have noticed that Pioneer specify output powers for their SX-x3x, SX-xx50, and SX-xx80 receivers into both 4 and 8 ohms, except for the SX-1280 and SX-1980 for which output power is quoted into 8 ohms only. Similarly, the SPEC 4 is rated at 150 watts into 8 ohms and 180 watts into 4 ohms, whereas the output power for the larger SPEC 2 is only quoted into 8 ohms (250 watts).

- Richard B.

Hold the presses! There is an 4/8 ohm switch on the back of all my SPEC 2s. In the up position it says 8 ohms, in the down position it says 4 ohms. How could it NOT be rated for 4 ohms with a switch like that on the back?

Having rebuilt two of them recently, maybe EW will chime in here with some answers for that question.


Retro

 

[andy]

Even at full power, a class A amp is much less efficient than a class AB, or B. I forget the exact numbers, but a simple class A amp will get about 25% efficient at full power and it's much worse at low power. A class B is something like 80% efficient at full power (AB is slightly worse). These numbers are based on calculations, real amplifiers are less efficient.

 

[240 Volts]

Hold the presses! There is an 4/8 ohm switch on the back of all my SPEC 2s. In the up position it says 8 ohms, in the down position it says 4 ohms. How could it NOT be rated for 4 ohms with a switch like that on the back?
Retro

Similarly, the SPEC 4 is rated at 150 watts into 8 ohms and 180 watts into 4 ohms, whereas the output power for the larger SPEC 2 is only quoted into 8 ohms (250 watts).

Retro, I'm sure the SPEC 2 will happily drive 4 ohm loads, indeed the specifications I have for it state "Speaker : 4 to 8 ohms".

What the specifications I have seen do not do however is to quote an output power figure into 4 ohms, unlike the lesser models.

So the first person to post a scan of an official Pioneer brochure giving the 4 ohm continuous RMS output power of either the SX-1980 or SPEC 2 can have one of my SX-450s for free (the winner will, however, have to arrange collection from the UK - I'm generous but not crazy :scratch2: )

- Richard B.

 

[Retro Stereo]

Retro, I'm sure the SPEC 2 will happily drive 4 ohm loads, indeed the specifications I have for it state "Speaker : 4 to 8 ohms".

What the specifications I have seen do not do however is to quote an output power figure into 4 ohms, unlike the lesser models.

So the first person to post a scan of an official Pioneer brochure giving the 4 ohm continuous RMS output power of either the SX-1980 or SPEC 2 can have one of my SX-450s for free (the winner will, however, have to arrange collection from the UK - I'm generous but not crazy :scratch2: )

- Richard B.

Damn, and I wouldn't be able to get a scan off till this evening anyway!

But, if you take a look at this thread:

http://audiokarma.org/forums/showthread.php?t=50369&highlight=SPEC

You'll see where EW put his dummy loads on the first one of mine he rebuilt and at 8 ohms it didn't clip until 340 watts per channel!

He then when on to say that 400 wpc at 4 ohms would not surprise him.

So, is all this good enough to get that SX-450?????


Retro