Power amp class AB- Class D, class H..!?

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webstersp

Inspired
I know that cliff said that class AB was the best option for the axe fx but I would like to know why?
What are the differences between class AB-D and H beside the fact that AB is a bit harder to find!?:p
 
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I'd like to know if anyone has direct experience with the Hypex UcD180ST Class D modules. I've read great things about them. But how do they sound?
 
Overly simplified explanation: Most guitar amps are class AB. Many low wattage (<10W) guitar tube amps are single ended class A. Lots of high wattage poweramps are class D because this design allows for smaller, lighter, very high efficiency designs.

However, to the end user the operating class is pretty much irrelevant. I wouldn't worry about it one bit, there are more important specs.
 
I remember reading in the old forum that cliff recomended class AB power amp and that he did not recomend class D and H for the AxeFx.
Just want to be sure and want to know why before i buy!
 
Depends what you using it for. In basic terms, Class A is the best for audio, but is very inefficient - which means hot due to the ammount of heat generated, and heavey because of the fans/heat simks etc, and low power (too much power and the heat/weight is too much). It amplifies the input waveform continuously so you get an exact replicat out as you put in.

Class B is a good way round this - though its still fairly inefficient its manageable. it splits the waveform in half, and uses a pair of devices each apmping one half. The two halves are then put back together. As each device is only opporatin half the time its cooler as much as anything. There are issues though. Unless you put the two halves back together perfectly, and the switchover is instant you can get distortion (not like were used to but in that the output waveform isnt quite what was put in).

Clas A/B is similar to B, but the corssover point is blended rather tha switched instantly which solves most of the distortion issues. Its also still pretty close to class A in its audio quality.

Class D uses on/offs and moves further away from perfect audio. it is however much much more efficient - so cooler and lighter. For high power application is much more suited tha Class A/B because of this. Also at the volumes your dealing with the extyra distortion you get isnt really an issue.

However for critical listening, like HiFi in a relatively low volume environment the differences can be heard.

So - Class A gives you the most faithfull replication - class A/B is very close and a good compromise on heat weight for low power (under 300 to 400W that is really). Class D is less faithfull but much lighter and cooler.

Read this for a much better discription:

http://www.bcae1.com/ampclass.htm
 
I also see class H like on the QSC PLX2 series (2-Tier Class H).
Would be nice to try all of them to hear the difference!
 
There is no inherent reason why class D moves further away from perfect audio.

There are bad/cheap implementations of class D amplifiers and then audio quality tends to suffer.
There are also very high quality class D designs that rival the very best class A and AB amplifiers when it comes to sound quality.

Just because an amplifier is class D doesn't mean it is any worse then a class AB. In some cases it might be better.

Some people on this forum worry way too much about amplifier classes instead of looking at the performance of individual amplifiers but of course it is difficult to get a bunch of them together and do a good comparison.

There is absolutely nothing that makes an amplifier either good or bad just because it is class A, AB, G, H or D.
All off them can be very good or very bad depending on the specific design, requirements and implementation.

S.R.
 
srooijens said:
There is no inherent reason why class D moves further away from perfect audio.

There are bad/cheap implementations of class D amplifiers and then audio quality tends to suffer.
There are also very high quality class D designs that rival the very best class A and AB amplifiers when it comes to sound quality.

Just because an amplifier is class D doesn't mean it is any worse then a class AB. In some cases it might be better.

Some people on this forum worry way too much about amplifier classes instead of looking at the performance of individual amplifiers but of course it is difficult to get a bunch of them together and do a good comparison.

There is absolutely nothing that makes an amplifier either good or bad just because it is class A, AB, G, H or D.
All off them can be very good or very bad depending on the specific design, requirements and implementation.

S.R.
Click to expand...
This agrees with everything I've read. As for worrying about amplifier classes, since Class D is so efficient, lightweight, inexpensive, and unfamiliar to so many people, I understand the curiosity. If what I'm reading is true, as Class D designs improve, this type of amp may become a real alternative even for audiophile applications. It also appears that designing a good one is a non-trivial task.
 
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The problem with Class D, as I understand it - and I don't really understand it, is that, historically speaking, they have a hard time with accurate reproduction of treble frequencies compared to Class A/B designs.

The reasons for this are kind of murky in my own mind but have something to do with the quality of the digital processors they use to control the output transformer.
Class D amps are not "digital amps" in the sense that there is a AD and a DA converter involved.
The digital component of Class D amplification involves some sort of digital control of the push-pull processes of the output transformer, I think.
Supposedly, they need a very high speed digital processor in order to accurately reproduce the treble frequencies. Processors like that have not been available in the past.
Or something like that.

So, Class D has a rep for being a good choice for bass amplification, jazz guitar amplification, and for powering sub-woofers in a multi-speaker system.
They have a bad rep for audiophile FRFR audio, thus far.
But year after year the Class D designs coming to market seem to be getting better and better.

The big advantage is that they are lightweight small-sized and powerful, and that's why they are taking over the market at the moment.
My own experiences with Class D amps has lead me to see them as being analogous to mp3 vs 44.1K audio.
I.e. They sound good, but something is missing.
 
webstersp said:
I have read about it, jsut want to know if someoe tried it it an Axe or versus an AB power amp.
Click to expand...

Anything I can tell you has already been said.
Class AB is the standard for high powered audiophile amplification.
But we're really getting into cork-sniffing territory here.
Most people here will not be able to hear any difference at all between well-designed amps using any of the topologies we've discussed here, especially when driving a guitar cabinet.
The only way you'll know if the QSC Class H amp works for you is to try one.

What I did, was to buy the best power amp that I could afford, that meets my current power requirements, and that I was also comfortable lugging around with me, and that turned out to be a used Bryston 2B-LP-Pro. I use it in mono bridged mode for 200 watts into my 8 ohm guitar cabinet.
It cost me $400 on eBay, and I couldn't be happier.

My philosophy is that Cliff has designed a box that fairly faithfully duplicates the audio signal of the tube amps that it simulates.
The goal of my monitoring system is to faithfully reproduce *that* signal as closely as current technology will allow for.
Most hi-powered amps are not built for audio-phile purposes.
The Bryston hi-powered amps are an exception to that rule.
Bryston's rep is pretty much unrivalled as far as I can tell.
While the 2B-LP is not really a high powered amp, in bridged mono mode, it suits my particular needs.
Before the Bryston I used an ART SLA1, also bridged.
The SLA sounded fine, but the Bryston sounds noticeably more musical, and with it the illusion of playing through an actual tube amp is even stronger.
 
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Of course - as has been said - a well designed Class D amp will be more "musical" that a poorly designed Class A/B amp. The issue there is cost.

A class A/B amp in the same price group as a Class D amp will normally be a much better solution for audio - but will also be heavier in general. The weight is down to the cooling heat sinks, and mains transformer. Most A/B amps use Torroidals which are big and heavey (relatively). It is possible however to make a Class A/B amp using switch mode PSUs which cuts the weight down considerably. Contrary to belief, the PSU makes next to no difference in audio quality (provided its designed and filtered correctly). You amplification device cant tell teh difference how its power is generated - as long as its the correct power.

Class G and H are actually better than D. That said - try them because what sounds good its good in general.
 
This is how I understand it as well, though I should point out that elec guitars don't really generate hi freqs, so for use with the Axe and guitar I'd "guess" that a quality Class D amp would be fine for elec guitar. Yes, distortion generates harmonics but most guitar cabs/speakers roll-off by 5.5K - 6K which are hi-mids etc. Acoustic guitars picked up with hi quality mics have harmonics that maybe go up to 10K, still not a real hi freq?

That said, I've done no listening tests so Class D amps might still suck LOL?

joegold said:
The problem with Class D, as I understand it - and I don't really understand it, is that, historically speaking, they have a hard time with accurate reproduction of treble frequencies compared to Class A/B designs.

The reasons for this are kind of murky in my own mind but have something to do with the quality of the digital processors they use to control the output transformer.
Class D amps are not "digital amps" in the sense that there is a AD and a DA converter involved.
The digital component of Class D amplification involves some sort of digital control of the push-pull processes of the output transformer, I think.
Supposedly, they need a very high speed digital processor in order to accurately reproduce the treble frequencies. Processors like that have not been available in the past.
Or something like that.

So, Class D has a rep for being a good choice for bass amplification, jazz guitar amplification, and for powering sub-woofers in a multi-speaker system.
They have a bad rep for audiophile FRFR audio, thus far.
But year after year the Class D designs coming to market seem to be getting better and better.

The big advantage is that they are lightweight small-sized and powerful, and that's why they are taking over the market at the moment.
My own experiences with Class D amps has lead me to see them as being analogous to mp3 vs 44.1K audio.
I.e. They sound good, but something is missing.
Click to expand...
 
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I'm not very technical with amps, but my Stewart World 1.2 sounds very good. The bass comes through very smoothly without distortion
 
DJD100 said:
This is how I understand it as well, though I should point out that elec guitars don't really generate hi freqs, so for use with the Axe and guitar I'd "guess" that a quality Cass D amp would be fine for elec guitar.
Click to expand...

You raise an interesting point and it makes me wonder, is it only the highs that suffer when one runs into issues specific to an amplifier class, or can other problems occur?

Terry.
 
What about the low dumping factors on D class amplifiers? If you're looking for a tight bass on your guitar tone, will this low dumping factor (~100) deliver what you need?
 
The problem with Class D is two fold.

Firstly, as you probably know, it uses an "on - off" output stage, that is constantly switching full on, or full off many thousands of times a second, and it varies the pulse length and then filters the output .. the idea is basically to re-create the original shape of the waveform from all the pulses ... a bit like the 1 bit A-D convertors used in some odler CD players.

At low frequencies (say 100 Hz) if the switching frequency of the class D amp is say, 50 KHz you have 500 pulses to re-create the original subtle shape of the waveform ... no problem, with 500 pulses you can very accurately re-create the shape of the 100Hz bass signal .. but what about 10Khz? at 10KHz you will only have 5 pulses to re-create the shape of the waveform ... it will not look much liek a sine wave!

But this is not the worst part ... the output filter which is reposnsible for shaping all these digital pulses back into a nice sine-wave is really only designed to work at one impedance .. so if it is designed to work well at 2 ohms, it wont work well at 16 ohms ... most class D amps have plenty of current and not much voltage, so you will find they are specced to 2 ohms, or even 1 ohm .. but at 16 ohms, the signal can look terrible ... and don't forget, your 8 ohm speaker is not really 8 ohms, the impedance can vary across the frequncy range quite a lot .. all this leads to a non-optimal impedance being presented to the output filter ... and then the signal begins to become very strange at the output, not like the nice sine wave you hoped for ...

Then there is the problem of clipping ... when a class D amp clips it can go quite crazy ... the problem is they often extend the individual pulses so they actually become one big pulse instead fo 2 or 3 individual pulses, and this is effectively a lower frequency, 1/3rd or 1/2 of the intended frequency ... and this escapes through the output filter and you see a horrible signal at the output.

Class AB and similar (Class H etc) usually clip smoothly, class D does not.

Every class D amp I have heard sounds OK or even good on bass, but the top end has been harsh, brittle and not very musical. Even manufacturers like MC2 seem to havve accepted this and are now producing a class AB mid top amp to work with their class D amps .. class AB for mid tops and monitoring, class D for the bass and sub bass in PA systems. There is no place for Class D in quality audio reproduction .. it is best suited to things it does well ... high power sub bass and high efficency, low quality (like the amplifiers for the speakers in mobile phones)!

If you really want to see it, try putting a triangle wave through a class AB and a Class D amp .. the class AB will produce something looking exactly like a tringle waveform .. the class D will look nothing similar .. maybe they will improve, but so far, the only improvment seems to be to lower the cost and improve the profits of the retailers.

So .. in my opinion .. if you are going to spend several thousand pounds on just the right guitar, made with the best quality maple and ash, and the right pickups .. and then put it through an AxeFX with really good quality DSP and high resolution A->D convertors and D-A convertors where the idea is to produce a sound that is exactly what you need and carefully tweaked to have the subtle nuances of tone .. the last thing you want to do is put it through a device that makes it all sound crap ... your sound is a result of each component in the chain ... the overall quality will be no better than the weakest part of that chain and in my opinion, sticking a Clas D amp on the end of a high qulity device like an Axe FX is plain crazy
 
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