Class A solid state amplifiers.

[Wimbo]

Why is it so hard to come up or find an answer to this question. "What is the RMS power value of a Class A amplifier?"

Anybody? And no highly complicated algebra please. Simple algebra is fine

[Guest]

Ok, Class A, it is a little complex but

Current x volts = watts, into the output

So, the FET's (or other devices) that provide the output to the speakers are limited by the maximium current they can flow into a given speaker impedance at maximum voltage the power supply can dish up under full load or Amps (squared) x Resistance of load 

 Class A these devices are biased to be flowing current all the time right up to their maximum, almost the same as a valve, waiting for input signal, so a terribly inefficient device.

 

So to identify the rating of the amp I suggest first identify the output device, weather it be a FET or Valve or whatever.

Then google this part number and find out its current rating

This number can then be calculated out using the voltage rating of the power supply stage of the amp.

Limiting factors could include the fact that at high currents into low resistance loads, like 4 or less ohms the amp power supply can saturate and run out of steam.

 

Example, 4 x 10A FETs (40A) x 70v(power rails) = 280W 

This number can change when going lower in speaker impedance.

Any clearer ? These are very highly Mathematical and subjective in nature, Someone else will probably chime in with a better explanation.

 

 

 

 

Edited March 4, 2017 by Guest

[mwhouston]

It doesn't matter what sort of amp it is, RMS value of power out is all the same; its RMS. 

 

The way I test my tube and SS Class A amps is to drive a 1Khz sine wave into an 8ohm resistor until the sine was starts to flatten. That's max power before heavy distortion. 

 

Take the the amplitude of the sine wave peak to peak, halve it and multiply by 0.7071. That is the RMS power out before distortions. I use a digital CRO.

Edited March 4, 2017 by mwhouston

[Wimbo]

 A Class A/B amplifier can have its RMS value worked out easily by looking at its maximum Linear power rating and multiplying it by 0.707.

Is this the same for a Class A solid state amp?

Say amp is rated at 20 watts Class A. is its RMS value then .707 of 20 watts? See what I mean.

I have a solid state amp that is 15 watts Class A but its switchable to Class A/B and Rated at 60 watts RMS.

So then, is its Class A RMS value then 10.6 watts? I know about the log values when talking about power but I never can find anywhere to work out Class A RMS value.

Sorry bout the vagueness.

I also know that some Class A amplifiers are biased a certain amount in Class A and this is its rating, even though at higher outputs it slides into Class A/B. ARGHH.

Edited March 4, 2017 by Wimbo

[Zaphod Beeblebrox]

1 hour ago, Wimbo said:

Why is it so hard to come up or find an answer to this question. "What is the RMS power value of a Class A amplifier?"

 

First off, a pedantic point:

 

There is no such thing as RMS power. It is, despite thousands of references, advertising literature and millions of Google hits, wrong. The correct terms are:

 

Watts

Continuous Watts

 

Either is calculated from the RMS Volts or RMS amps. The answer cannot be RMS Watts. 

 

Now, to your question: 

 

The continuous power rating of an amplifier is whatever the designer/manufacturer wants it to be. Doesn't matter if it is Class A, B, A/B, C or D. So, a Class A amp can be 0.0001 Watts all the way to a million Watts (if you can find somewhere to plug it in). In real life, most audio Class A amps range in power from around 1 Watt to about 200 Watts. Much more and severe problems WRT cooling and places to plug it in become more critical. 

 

Perhaps you may care to refine your question, such that it can be answered with a little more clarity.  

 

1 hour ago, Wimbo said:

Anybody? And no highly complicated algebra please. Simple algebra is fine

 

Sure. Make your question a little clearer and I'll try to help.

[Wimbo]

4 minutes ago, Zaphod Beeblebrox said:

 

First off, a pedantic point:

 

There is no such thing as RMS power. It is, despite thousands of references, advertising literature and millions of Google hits, wrong. The correct terms are:

 

Watts

Continuous Watts

 

Either is calculated from the RMS Volts or RMS amps. The answer cannot be RMS Watts. 

 

Now, to your question: 

 

The continuous power rating of an amplifier is whatever the designer/manufacturer wants it to be. Doesn't matter if it is Class A, B, A/B, C or D. So, a Class A amp can be 0.0001 Watts all the way to a million Watts (if you can find somewhere to plug it in). In real life, most audio Class A amps range in power from around 1 Watt to about 200 Watts. Much more and severe problems WRT cooling and places to plug it in become more critical. 

 

Perhaps you may care to refine your question, such that it can be answered with a little more clarity.  

 

 

Sure. Make your question a little clearer and I'll try to help.

Continuous watts are rated at an RMS value? Right.

Edited March 4, 2017 by Wimbo

[Zaphod Beeblebrox]

Just now, Wimbo said:

Continuous watts are rated at an RMS value? Right.

 

Continuous Watts (or just plain ole Watts) are calculated from RMS values of Voltage and current. Yes. That does not make the answer 'RMS Watts'. Yes, I know everyone uses it. It's still wrong. 

[Wimbo]

4 minutes ago, Zaphod Beeblebrox said:

 

Continuous Watts (or just plain ole Watts) are calculated from RMS values of Voltage and current. Yes. That does not make the answer 'RMS Watts'. Yes, I know everyone uses it. It's still wrong. 

Ok Trevor. Simply put. What is the continuous power of a 20 Watt Class A amplifier? Thank you in advance.

[Zaphod Beeblebrox]

20 Watts.

 

[Wimbo]

1 minute ago, Zaphod Beeblebrox said:

20 Watts.

 

Simples. Thanks mate.

[Guest]

20 watts

More importantly, what is the current rating of the devices switching into the output and at what voltage in relation to the resistance of the load. Answer, 20w 

Edited March 4, 2017 by Guest

[Wimbo]

5 minutes ago, 125dBmonster said:

20 watts

More importantly, what is the current rating of the devices switching into the output and at what voltage in relation to the resistance of the load.

You mean,20watts class A into 8 ohm, 40w class A into 4 ohm etc?

As some class A rated amplifiers will die in the ass driving lower impedances.

Edited March 4, 2017 by Wimbo

[Zaphod Beeblebrox]

Just now, Wimbo said:

Simples. Thanks mate.

 

You're welcome, though I suspect I know where this is headed. I will try to save some time.

 

Most Class A amps sound more capable (louder)  than their equivalently powered Class A/B amps. There is a good reason why this SOMETIMES occurs. A Class A amp must have a power supply which is cable of delivering maximum Voltage and current 100% of the time, day in, day out. A Class A/B amp only requires a power supply that can deliver maximum Voltage and current for short periods of time. Thus, in the budget end of the market (and often in the not so budget end), manufacturers can cut significant costs in their amplifiers, by using much smaller power transformers and filter capacitor banks than a Class A amp. 

 

HOWEVER, some manufacturers who build really good Class A/B amps, pay great attention to power supplies and thus their products can easily rival the performance of a Class A amp. 

 

BTW, one last, very important thing to note: Despite what many manufacturers claim, very few amplifiers are really Class A. They may run quite warm, but they're not Class A. Some manufacturers tell fibs. Shocking, I know., 

[Zaphod Beeblebrox]

Just now, Wimbo said:

You mean,20watts class A into 8 ohm, 40w class A into 4 ohm etc?

As some class A rated amplifiers will die in the ass driving lower impedances.

 

Oh dear, can 'o worms time. Here's the dirty little secret about Class A amps:

 

A Class A amp rated at 20 Watts @ 8 Ohms, will deliver:

10 Watts Class A @ 4 Ohms

5 Watts Class A @ 2 Ohms

2.5 Watts Class A @ 1 Ohm

[Guest]

On 3/3/2017 at 8:21 PM, Zaphod Beeblebrox said:

 

Oh dear, can 'o worms time. Here's the dirty little secret about Class A amps:

 

A Class A amp rated at 20 Watts @ 8 Ohms, will deliver:

10 Watts Class A @ 4 Ohms

5 Watts Class A @ 2 Ohms

2.5 Watts Class A @ 1 Ohm

Time to open the can here Zaph.

I am guessing the missing part of the equation here is a constant, this being maximum current flow of the out put devices/PS

Or am I missing something ?

Edited March 4, 2017 by Guest

[Wimbo]

2 minutes ago, 125dBmonster said:

Time to open the can here Zaph.

I am guessing the missing part of the equation here is a constant, this being maximum current flow of the out put devices/PS

Or am I missing something ?

And I saw one of those Pioneer exclusive Power amplifiers rated at 300 watts Class A into 8 ohms and 600 into 4. Must be a Class A/B rating.

[Zaphod Beeblebrox]

Just now, 125dBmonster said:

Time to open the can here Zaph.

I am guessing the missing part of the equation here is a constant, this being maximum current flow of the out put devices/PS

Or am I missing something ?

 

Nope. You nailed it in one. That said, there is no reason why a push pull Class A amp cannot deliver more power into lower impedances, provided the output stage and power supplies are suitably rated, but with single ended Class A amps, you're pretty much screwed: Lower impedances = lower power outputs.

[Guest]

On 3/3/2017 at 8:31 PM, Zaphod Beeblebrox said:

 

Nope. You nailed it in one. That said, there is no reason why a push pull Class A amp cannot deliver more power into lower impedances, provided the output stage and power supplies are suitably rated, but with single ended Class A amps, you're pretty much screwed: Lower impedances = lower power outputs.

Yep, understand, enter the single ended Class A, thanks

Cheers

[Zaphod Beeblebrox]

Just now, Wimbo said:

And I saw one of those Pioneer exclusive Power amplifiers rated at 300 watts Class A into 8 ohms and 600 into 4. Must be a Class A/B rating.

 

A 300 Watt Class A amplifier will be running very hot indeed. In fact, I doubt that any amp has been built, short of something using industrial fan cooling or water cooling, to deliver that much Class A power. That said, if the amp was rated at 300 Watts Class A, then, provided it's power supply and output stage was sufficiently robustly rated, there is no reason why it could not deliver 600 Watts into 4 Ohm loads. It would, of course, be operating in Class B at that power level. 

[Wimbo]

1 minute ago, 125dBmonster said:

Yep, understand, enter the single ended Class A, thanks

Cheers

Single ended Solid State?

[Guest]

I think Zaph is suggesting that a real class A Amplifier is single ended only  

[Wimbo]

2 minutes ago, Zaphod Beeblebrox said:

 

A 300 Watt Class A amplifier will be running very hot indeed. In fact, I doubt that any amp has been built, short of something using industrial fan cooling or water cooling, to deliver that much Class A power. That said, if the amp was rated at 300 Watts Class A, then, provided it's power supply and output stage was sufficiently robustly rated, there is no reason why it could not deliver 600 Watts into 4 Ohm loads. It would, of course, be operating in Class B at that power level. 

So, all lower impedance values are generally Class B.

[Wimbo]

1 minute ago, 125dBmonster said:

I think Zaph is suggesting that a real class A Amplifier is single ended only  

Ahh ok, simple class A.

So a push/pull class A amplifier that is rated at 100 watts class A has a Continuous rating of 100 watts as well.

[Guest]

On 3/3/2017 at 8:39 PM, Wimbo said:

Ahh ok, simple class A.

So a push/pull class A amplifier that is rated at 100 watts class A has a Continuous rating of 100 watts as well.

Correct

 

[Zaphod Beeblebrox]

Just now, Wimbo said:

Single ended Solid State?

 

Sure. Way back in the dawn of time (the 1960s) pretty much all car radios (AM only, of course) were single ended, Class A solid state units. The go to output transistor was the mighty 2N301. I still have one in my collection. Rated at a phenomenal 40 Volts and 3 Amps, with 50 Watts of power dissipation, it was perfectly suited to high end car radio use. 

 

The point is, of course, that you can use transistors in single ended if you wish. Like valves, transistors like the 2N301 work better with output transformers, due to their lowish current ratings.