wanna shed any additional light on response from outlaw on 7500 about amp class?

jrlouie

I recently submitted a request online to Outlaw audio to give me the specs indicating for how many watts their 7500 and M200 amps run in class A mode before switching over to B (since the 7500 is listed as AB and the M200 is G/AB). Here is their response. I'm a bit confused. I've never heard of an "A/AB" design and apparently their amps never run in class A mode. Really I don't know hardly anything about all of this stuff except for the basic amplifier classifications at a high level and a few pro's and con's.

I wanted to talk to the techs that build the 7500 and 7700 before answering your question.

Technically the 7500 and 7700 are what's called Hybrid Class AB operation. The amp does not run in Class A at lower volumes. What's more important than low power Class A operation is the fact that the input stage is Class A. This contributes more to the overall performance than having Class A operation up to 10 watts or so.

The 200 and 2200 operate in AB/G and also do not run in Class A.

The class that you describe is often known a Class A/AB and to my knowledge is not widely used in home theater amps due to the heat and inefficiency that is the byproduct of Class A output.

What makes the 7500 special is it's differential mode of output. Both the Red (Hot) and Black (Differential) speaker terminals are live, the Red is positive voltage and the Black is negative voltage. The speaker motion is controlled by the difference between Hot and Differential which yields a faster rise time, also known as "slew rate". This results in an amp that is more dynamic than a conventional common ground amp. Please let me know if you have any additional questions.


Thanks and Regards,

Porter

It's a class AB amp. That's different from class A.

Here's a breakdown of the different amplifier classes:

The techy version: http://en.wikipedia.org/wiki/Electronic_amplifier#Amplifier_classes

The simple version: http://www.audioholics.com/techtips/audioprinciples/amplifiers/audioamplifierclasses.php


Hope this helps.

The really interesting thing about the Outlaw is that it is a differential mode amp, not a common ground amp. That's one of the reasons they sound so musical even though they are Class AB.

Porter

Oh, and here's a diagram. It may help, it may not.



From http://tpub.com/neets/book7/25e.htm

jrlouie

Porter wrote:

It's a class AB amp. That's different from class A.

Thanks for the info. I just always had thought an AB amp operates in class A for its lower wattages and then later operates in class B for the higher wattages. It doesn't sound like this is the case for the Outlaw.

jrlouie

Just thought I'd post a link I found regarding class A/B amps versus A/AB. Some might be interested.
http://www.perreaux.com/class_ab.php

I still haven't found a clear description as to what class A/B amps actually do to make them A/B. All descriptions I have found actually sound like A/AB (i.e. - operating in A for X number of watts and then flipping to A/B for the rest).
If anyone is able to explain truly what happens in an A/B amp, I'd appreciate that info.



This confuses me that they don't acknowledge A/AB classes. So are they combining those with A/B?

Porter wrote:

The techy version: http://en.wikipedia.org/wiki/Electronic_amplifier#Amplifier_classes

Porter

You're misunderstanding the description.

A Class AB doesn't operate in A for the low wattages and then LATER in B for the high wattages, in terms of low or high volume on the knob...

A Class AB operates in both A and B at the same time, A for quiet parts of the waveform that don't require large current transfer, and B for sections of the waveform with greater magnitude that require the push. That's oversimplified, but in the ballpark.

It's not the most precise way to approach waveform reproduction.



Oh, and A/AB is a bit of a gimmick. It's still a class AB amp, just run at a slightly higher current. Note that they're a bit vague about it, for good reason. There's nothing wrong with Class AB amps, some of them sound fantastic... but the purpose of designing an amp around that architecture is for efficiency, not for sound quality.

heiney9

Most Outlaws are G or H class which mean they use switching power supply's. To say they are A/B, but never actually run in class A is misleading and confusing. The only possible way to get the kind of effeciency needed in such a small package is to use class G or H predominately. All classes have there pluses and minuses. IMO, G & H aren't near as musical as a good A/B design. Full class A getting the edge. Outlaws however are really for HT and for their price point and ergonomics they are hard to beat. For music I'd choose something else everytime.

mulveling

I'm not hugely knowledgable on amps design, but IIRC the "input stage" they are referring to is where the signal's voltage is multiplied. That's easy to make class A since there's little current (and hence little power & heat) involved. I think it's rather similar in practice to a preamp with fixed gain. The output stage that follows is what delivers current to the speakers, and that's where all the Watts come from. Speakers have much lower impedances that any other amplification stage input, so the output stage of a power amp really has its work cut out for it. For instance, the input impedance of a typical power amp ranges from 10,000 ohms to 100,000, and that's all the preamp has to drive. Current will be very low into such impedances, and as a result so will power (Watts of power equals current multiplied by voltage). Compare that to the 4 to 8 ohms of a typical speaker. That's why it's so hard to make a pure class A power amp with a decent amount of audio power. Pure class A headphone amps are much easier to implement since the impedances range from 32 to 300 ohms and the typical headphone's sensitivity is so much higher.

The input stage is important, but it seems far too convenient for them to claim it's more important than the output stage. A true class A amp will be pure class A all the way, no switching to AB, etc.

My PS Audio HCA-2 uses a class A input stage with a class D (pulse width modulated) output - hence the name "Hybrid-Class-A".