# Any Class A Car Amps?



## jtholley03 (Feb 22, 2006)

Looking for something for the tweeters. XM2000R is a bit of an overkill.

5 or 10 watts would be plenty.


----------



## Guest (Apr 22, 2006)

Hello ... for what it's worth, I have discussed, argued and maintained that Class A amps can be a smart choice for high efficiency drivers, like tweets and horns. Two reasons :

1. High efficiency means you don't need much electrical power, so 50~100 watts of Class AB is typically overkill. Said another way, the 25% max theoretical efficiency of Class A hardly matters when you're only supplying a few watts to the drivers.

2. High efficiency drivers are more likely to expose "residual" crossover distortion in Class AB output stages ... meaning poorly biased or drifting (age, temp) bias in the output stage. High effiency drivers expose this, because the distortion artifacts from Class AB mis-bias tend NOT to reduce with signal level, anywhere near to the extent that other classic analog distortion mechanisms do. In other words, the "notch" distortion from Class AB tends to remain somewhat constant as level drops, meaning Signal-to-Distortion Ratio can degrade at low output powers, where high efficiency drivers will operate.

From an engineering perspective, there's simply no need to split the signal and drive positive/negative swings through different circuit paths if your power needs are very small.

Also from an engineering perspective, there's no fundamental reason why a low-power Class A amp should _cost_ more than a mid-high power Class AB amp. But this is where the market is failing us  There are a few "Class A" designs on the market ... but typically only operate CLass A for a few watts, then transition to Class AB, which strictly speaking makes them hot-bias Class AB ... and these amps are EXPENSIVE. Examples include : Sinfoni and Tru Technology (bigger, more Class A power) ... actually, I don't think the Tru A-Class is still available, but it's way overkill for tweeters anyway.

Monolithic used to offer true Class A amps for the car ... based on idle current and heat, I think they really were Class A.

Another true Class A amp for cars, overkill for tweets to be sure, is the HSS Fidelity HT 230. Single ended, Class A tube power, Delivers 2x30 watts, idle current more than 20 amps (!) ... consistent with 60 total watts at 25% efficiency.


----------



## JasonH (Oct 27, 2005)

Soundstream used to have some, I see them on Ebay. Pioneer also used to have some, ODR series I think, I haven't seen those for along time though.

http://search.ebay.com/search/searc...&fpos=N6J+2X9&ftrt=1&ftrv=1&saprclo=&saprchi=


----------



## Guest (Apr 22, 2006)

Genesis makes one or two ... Dual Mono Class A, something like that. May be a hot-bias Class AB, or hot-bias Class G (Class G = Class AB with variable supply rails, as I understand it, allowing for higher efficiency under certain conditions).


----------



## MarkZ (Dec 5, 2005)

werewolf said:


> 2. High efficiency drivers are more likely to expose "residual" crossover distortion in Class AB output stages ... meaning poorly biased or drifting (age, temp) bias in the output stage. High effiency drivers expose this, because the distortion artifacts from Class AB mis-bias tend NOT to reduce with signal level, anywhere near to the extent that other classic analog distortion mechanisms do. In other words, the "notch" distortion from Class AB tends to remain somewhat constant as level drops, meaning Signal-to-Distortion Ratio can degrade at low output powers, where high efficiency drivers will operate.


Experimentally, I don't think there's any evidence of this, werewolf. Doug Self went into this issue in his last book and concluded that, in the properly designed output stage, distortion actually decreases with decreasing signal level -- and in fact, this decrease is enough to cause a decrease in THD *percentage* as well, not just absolute level of distortion. The percentage is important, because it does a little better job of taking into account masking.

Basically, the only reason I think class A is useful for tweeters is because of the rapid decrease of the influence of the negative feedback signal at high frequencies. At low (or even midrange) frequencies, it's been shown fairly recently that a class B output stage can actually be designed to BEAT a class A in terms of distortion performance. This is a major shift in what had once thought to be a slam-dunk argument. The problem is: just how good are the circuit topologies of today's high end car amplifiers? Class A may still be "better" if the industry isn't putting forth the best design strategies in their class B or AB designs.

To the original poster: I think you should look for more than a 5 or 10 watt amp (do they even go that low in power?). Although tweeters normally dissipate *on average* watts somewhere in the single digits, transients can easily exceed 10dB, often requiring as much as 30 watts or more. If you're looking to spend the exorbitant prices on a class A amp for the sake of purity, then it would defeat the entire purpose if you were to introduce distortion by clipping the transients! Personally, I'm fine with a solid class B design with low noise for my tweeters.


----------



## MarkZ (Dec 5, 2005)

werewolf said:


> Genesis makes one or two ... Dual Mono Class A, something like that. May be a hot-bias Class AB, or hot-bias Class G (Class G = Class AB with variable supply rails, as I understand it, allowing for higher efficiency under certain conditions).


Class G can also incorporate a class A output stage. Maybe Genesis is doing that? IMO, considering the efficiency issues involved, it seems like a very reasonable way to approach class A.


----------



## Guest (Apr 22, 2006)

Mark, couple things. Most analysis of Class AB output stages does not take into account possible age and drift mechanisms. Even when properly biased from the factory, Class AB output stage quiescent current can tend to drift ... this is because the output stage bias current is established not only by local negative feedback (typically through an industry-standard Vbe-multiplier, together with emitter degeneration resistors in the output legs), but also by matching of the Vbe-multiplier bipolar to the output stage bipolars. Now the multipier transistor is usually tightly coupled to the output stage heatsink, to help track the temp drift of the output devices. But mismatch over age, and temperature, is still very real.

I recently debated the issue of THD, resulting from crossover distortion, versus signal level over on carsound. Several good references were cited, including one or two to refute my position ... only to find that as signal level dropped by 2X, SDR didn't in fact increase by 2X ... just 1.6X 

Here's the 4-page debate, for anyone interested :

http://www.carsound.com/UBB/ultimatebb.php?ubb=get_topic;f=18;t=011169#000001

And many may take issue with this point ... but Class AB amplifier topologies haven't changed very much at all ... over _several decades_. I've seen several car amp schematics to support my position  So modern or old, Class AB output stages are not without their issues 

I'll stand by my position : If I were designing a car audio amp from scratch, even with cost-optimization as a goal, to deliver just a few watts to a high-efficiency driver ... horn or tweet, significantly less than 10 watts ... I wouldn't incorporate a Class AB output stage (or a switching DC/DC power supply, for that matter )


----------



## Thoraudio (Aug 9, 2005)

yeah... but then where would that pesky old headroom go? 

even a 93 db efficient speaker is going to run outta juice pretty quick with program material with a 20 db crest factor with only 10 watts...

That being said... I'm also on the lookout for a killer tweeter amp to mate with by BG Neo3s.... seriously considering the Sonic Impact units...


----------



## MarkZ (Dec 5, 2005)

werewolf said:


> Mark, couple things. Most analysis of Class AB output stages does not take into account possible age and drift mechanisms. Even when properly biased from the factory, Class AB output stage quiescent current can tend to drift ... this is because the output stage bias current is established not only by local negative feedback (typically through an industry-standard Vbe-multiplier, together with emitter degeneration resistors in the output legs), but also by matching of the Vbe-multiplier bipolar to the output stage bipolars. Now the multipier transistor is usually tightly coupled to the output stage heatsink, to help track the temp drift of the output devices. But mismatch over age, and temperature, is still very real.


I agree, and I probably should have been clearer in my post that what the industry DOES and what they SHOULD do are entirely different things. Bias issues are handled poorly in most of the designs I've seen. First of all, they use the common heatsink as the temperature reference, which has been shown to be vastly inferior to using the surface of the TO-220 or TO-3 can (or whatever they're using) as the reference. Secondly, most of the designs I see incorporate an emitter follower output stage, which I believe can be more susceptible to the problems you're describing than other forms, like CFP designs (due in part to temperature reference and component matching). So yeah, I should make it clear that the way the industry has approached the two classes is probably dramatically different. I haven't seen the class A designs that you and others have listed, but I'd suspect that some of them incorporate better strategies in places beside the output stage.



> I recently debated the issue of THD, resulting from crossover distortion, versus signal level over on carsound. Several good references were cited, including one or two to refute my position ... only to find that as signal level dropped by 2X, SDR didn't in fact increase by 2X ... just 1.6X


I haven't reading the entire thread yet, but it appears that the focus was on a lot of amps that were tested. There's no telling how these amps were achieving bias tracking. I don't think you can extend the argument to something inherent in class B or class A operation as much as you can simply say that "these amps exhibited these characteristics."



> And many may take issue with this point ... but Class AB amplifier topologies haven't changed very much at all ... over _several decades_. I've seen several car amp schematics to support my position  So modern or old, Class AB output stages are not without their issues
> 
> I'll stand by my position : If I were designing a car audio amp from scratch, even with cost-optimization as a goal, to deliver just a few watts to a high-efficiency driver ... horn or tweet, significantly less than 10 watts ... I wouldn't incorporate a Class AB output stage (or a switching DC/DC power supply, for that matter )


If it was ONLY for tweeters (that is, not just two channels of a four or six channel amplifier), I'd probably also consider class A, but for a different reason -- global NFB. If I can get my scanner to work tonight, I can show you a class B design out of Self's book that beats the standard class A in distortion performance, if you're interested.


----------



## Guest (Apr 22, 2006)

Of course I'm interested! Sometimes old guys like me fall prey to the "there's nothing new under the sun" type thinking 

Yeah, the link I posted tends to wander off-topic quite a bit (my fault, I fear) ... but a couple good references within that thread are worth a look.


----------



## MarkZ (Dec 5, 2005)

werewolf said:


> Of course I'm interested! Sometimes old guys like me fall prey to the "there's nothing new under the sun" type thinking
> 
> Yeah, the link I posted tends to wander off-topic quite a bit (my fault, I fear) ... but a couple good references within that thread are worth a look.


Finished the thread. Looks like RobM already offered a Self reference and you addressed it...problem is that the website he referenced is down and I can't remember exactly what tests and figures you were addressing exactly. I'll dig up the book later and try to figure it out, and I'll link you to scans later tonight.


----------



## Guest (Apr 22, 2006)

thanks dude 

As I see it, the wonderful thing about Class A amps is that, even in the absence of any overall negative feedback, the distortion levels must drop quite rapidly as signal level decreases ... simply because there's not even a hint or suggestion of discontinuity around zero. The square-law-induced second harmonics will drop 4X (in absolute value), for example, as signal level decreases by 2X, _improving_ signal-to-second-harmonic by 2X.

The price you pay for this inherent (even open-loop) small-signal linearity, of course, is piss-poor efficiency.


----------



## Guest (Apr 22, 2006)

Oh .... it's my belief that _large-signal_ linearity is quite over-rated. Aural masking, and other psycho-acoustic "shut downs", tend to turn-on for hot signals.

However, I suspect that _small-signal_ linearity is perhaps under-rated  At the risk of a gross over-exagerration, most things in nature tend to get more linear as signals get smaller, and I suspect that over the countless millenia, the ear/brain has come to "expect" such behavior  

Just an opnion, fwiw ...


----------



## MarkZ (Dec 5, 2005)

I'm not sure exactly what you mean by small signal and large signal nonlinearity in this context.


----------



## Thoraudio (Aug 9, 2005)

high and low wattage. 

Potentially audible differences are more likely at 1 watt in a 93 db/oct speaker than at 100 watts in a 79 db/oct speaker.


----------



## MarkZ (Dec 5, 2005)

I'm still not following. Sounds like two different issues.  

If the acoustic output is the same in both cases, potentially audible differences shouldn't be any more prone to being noticable in either condition.


----------



## tyroneshoes (Mar 21, 2006)

nice class a for tweets. good price too.

http://cgi.ebay.com/Used-Phoenix-Go...716743254QQcategoryZ79820QQrdZ1QQcmdZViewItem


----------



## raamaudio (May 4, 2005)

Since the Class A amp I found interesting is a bit out of my price range I am having a digital amp built for my ID neo horns. I have another one already completed for my soon to arrive Edgarhorn slimlines (upgraded to oil caps and Fostex tweeters) that is being tested by Doc right now. I do not have the funds for the tube amp I want (car project is eating all my extra money) so going to try these digital amps. 

Maybe I will let one or both out to be tested by a member here

Rick


----------



## MarkZ (Dec 5, 2005)

Alas, I can't find the book to scan it in. It's undoubtedly in one of my many unmarked boxes. But I did find G. Randy Slone's book, where he offered his best schematics for a class A and class B design and noted that the distortion levels were virtually identical for both of them. I can scan those in if you'd like, and remain on the lookout for the Self book...


----------

