Amplifier classes

[essexbasscat]

Hi folks

Just done a bit of research on amplifier classes A, B, C,D etc to try and gain some understanding of the similarities and differences between the classes. A google search dug up a whole load of circuits and heavy, heavy info that made my eyes glaze over almost instantly TBH.

I also did a search with the engine for this site, but dug up nowt.

Can anyone describe in plain language, the significance of the whole amplifier classification thing ?

thank you for reading and a big THANK YOU in advance for any postings

the answers from this one is likely to lead onto a posting about Bi-amping

cheers

T

[Lfalex v1.1]

[url="http://en.wikipedia.org/wiki/Class_A_amplifier#Class_A"]Wikipedia says....[/url]
This link is a bit technical, but covers it fairly well.

To generalise (very broadly)

Valve amps are often Class A. Certainly the Hi Fi ones. Transistor devices can be made to run in Class A (by design), but do get quite warm
Class D are them new-fangled dinky amps with BIG power outputs - used in both In-car applications and (often) small (1u) power amps.

Have a read. It explains better than I can!

[essexbasscat]

Thanks for looking that one up Lfalex


That's the one I came across in researching this thing. All very technical, but I'm still a bit lost in relating the whole classification thing to the amps that we use really.

As far as I can gather, class A amps were valve in pre amp and output sections and ran hot, typically around before the late 60's when transisters began to emerge.
An Ampeg SVT could be an example of a class A amp ?

I suspect, but I'm not sure, that class D are the most up - to - date type of amp that deliver the more 'holographic' type of sound, as characterised by markbass etc

Not too sure about the whole classification thing really. Is there really a sound difference between them ?

There sure is a difference between my Ampeg B2RE and my Sa450. The Ampeg sounds more 'vintage' and solid, while the Markbass sounds more holographic.

anyone else come up with anything on this one ? It may be more a question about amplifier geneology / development perhaps ?

Wondered about this one for a while now

thanks again

T

Edited April 1, 2010 by essexbasscat

[JackLondon]

Does this help ?

It's pretty simple really.

The device used whether tube or transistor is BIASED so it is in conduction 100% through 100% of the input signal. You need to know how an amplifier works.

Lets just discuss tube amplifier. If the tube was unbiased and allowed to just conduct the tube would heavily conduct and do nothing but make heat.

A bias voltage is put on a control grid to do exactly that control the current flowing through the tube.

The cathode is Negative plate voltage and the anode is the positive point. This bias voltage placed on a grid located between the cathode and plate slows down the conduction, the higher the NEGATIVE voltage in respect to the cathode the more the tube is throttled down.

With enough negative voltage on the cathode the tube can be completely cutoff.

The input signal is placed also on this control grid and as its voltage varies up and down at the frequency of the music, the signal pulses from zero volts to positive and then swings negative and back again at the frequency of the note.

This signal is at a much smaller value than the control grids high negative bias value but the combined bias and signal voltage cause a HUGE variation in the current flow through the tube.

This small voltage to huge current flow change is how the amplifer works. The current flow changes in the primary of the output transformer causes voltages to be induced in the output transformer secondary winding and makes the voice coil move on the speaker.

OK back to CLASS... A class A amplifier is biased to conduct current in the final amplifer tube 100% of the input signal's cycle. Preamps are usually run this way and makes for a very pure amplified signal. A tube has a plate dissapation limit for heat. When there is no signal present in a class A amp it is still conducting like there is signal present. Very clean amp and very inefficient power and heat wise.

This is also a single ended amplifier as there is just one amplifying device. If you used a large enough tube and used a big enough fan, you could build a super large class A amplifier and that is what the esoteric audio guys are doing using 833's and other broadcast triodes with 450 watts dissapation tubes as giant clean class A amps. They also get $80,000 for their trouble for a pair of stereo channels.

A Class B amplifer on the otherhand is biased to cut off the final amplifier tube through 50% of the input signal's cycle. When the tube cuts off it is allowed to cool for 180 degrees; half of the 360 degree signal cycle or 50% of the time. The problem with this class B is you only amplify half the signal and it would sound like crap without putting the other half of the signal back somehow.

In RF circuits the output circuit FLYSBACK and induces the other half back in the amplifiers tank ciruit (the equivellence to a power transformer) however at AUDIO the ability to flyback cleanly doesn't exist in such a huge heavily inductive device so that half must be regenerated instead.

SO we use another method instead of a single ended amplifier; in push-pull they split the input signal positive and negative and send each half to two different amplifiers. The positive cycle to one set of tubes and the negative part of the cycle to the other set of tubes.

Each set of final tubes conducts in exactly 180 degree increments cooling in the other 180 degrees and the output is fed to each end of the output transformer. The transformer puts the two halves of the signal back together....

SO even at audio a perfectly biased CLass B amp could make 100% undistorted beautifully reconstructed audio... Well not quite.

You'd need to be constantly tweaking this amp over time. Tubes aging and voltage changes... The manufacturers have lazily decided to run the P-P amps class AB.

All that is is a sliding of the bias towards conducting a little bit more current during the tubes rest period.

This allows for imperfections in componants and a crumby barely regulated POS bias supply to still make a complete more non-distorted sign wave at the output.

A guitar amp is really a truly POS audio wise. That's why a tube amp breaks up and does the distortion game. This would never be allowed to happen in the Audiophile World.

Class C is only good for RF amps making morse code, digital, or AM amplified carrier based amplifiers (cut off to less than even 180 degrees of the cycle)

And class D is completely different and uses other processes.

[Alien]

To summarise:

Class A - Hugely inefficient, run hot, normally weigh a ton, but can sound fantastic even when overdriven.

Class B - More efficient than class A, but still not great, run hot-ish, still fairly heavy, prone to crossover distortion.

Class AB - Falls between class A and class B. Less crossover distortion than class B, but still heavy and run hot.

Class C - Doesn't work for audio, so ignore it.

Class D - AKA digital/switch mode/switching amps. Very efficient, run cool, light, some can run out of puff if driven hard for long periods.

Class H - Uses multiple power supplies to only supply what's needed. Heavier than class D, but don't usually suffer the same power fade issues. Not all that common.

That good enough?

A

[Beedster]

[quote name='Alien' post='792744' date='Mar 31 2010, 11:48 PM']To summarise:

Class A - Hugely inefficient, run hot, normally weigh a ton, but can sound fantastic even when overdriven.

Class B - More efficient than class A, but still not great, run hot-ish, still fairly heavy, prone to crossover distortion.

Class AB - Falls between class A and class B. Less crossover distortion than class B, but still heavy and run hot.

Class C - Doesn't work for audio, so ignore it.

Class D - AKA digital/switch mode/switching amps. Very efficient, run cool, light, some can run out of puff if driven hard for long periods.

Class H - Uses multiple power supplies to only supply what's needed. Heavier than class D, but don't usually suffer the same power fade issues. Not all that common.

That good enough?

A[/quote]

At last, a reply I understand! Could you give examples of each class?

Cheers

C

[essexbasscat]

that's some great posts there, much appreciated, that really has helped clear up some of the mystery.

That said, it would be interesting to have examples of each class.

Also, did amp manufacture develop in the alphabetical pattern i.e class A came first, then developed onto AB etc ? what were the time periods / years ?

How did it evolve over the years ?

T

Edited March 31, 2010 by essexbasscat

[Alien]

[quote name='Beedster' post='792749' date='Mar 31 2010, 11:50 PM']At last, a reply I understand! Could you give examples of each class?

Cheers

C[/quote]


OK, here goes...

Class A - Can't think of many bass amps that run in class A, other than small amps with one-transistor power stages, such as the Peavey Microbass. Valve wise, it tends to be restricted to small guitar amps (under 10 watts or so). Think Fender Champ, Vox AC4, Epiphone Valve Jr.

Class B - Not commonly used for instrument amps because they need to be perfectly set up to minimise crossover distortion. Can't think of any off the top of my head.

Class AB - Here's the richest vein of amps. Most bass amps fall into this class. Hartke, Peavey, Trace Elliot, G-K etc. Class AB valve amps include the Ampeg SVT, Trace V8 and the Mesa Boogie 400+.

Class D - Most of the new breed of super lightweight amps are class D. Markbass, Ashdown Superfly, Genz Benz Shuttle and the like.

Class H - The only class H bass amp I can think of is the Marshall 7200/7400 Dynamic Bass range.

Hope this helps

A

[essexbasscat]

Yes, thank you to everyone. At last, this whole class thing is making some sense.

mucho appreciated. Now for a Bi-amp question....

T

Edited April 1, 2010 by essexbasscat

[umph]

[quote name='Alien' post='792810' date='Apr 1 2010, 12:55 AM']OK, here goes...

Class A - Can't think of many bass amps that run in class A, other than small amps with one-transistor power stages, such as the Peavey Microbass. Valve wise, it tends to be restricted to small guitar amps (under 10 watts or so). Think Fender Champ, Vox AC4, Epiphone Valve Jr.

Class B - Not commonly used for instrument amps because they need to be perfectly set up to minimise crossover distortion. Can't think of any off the top of my head.

Class AB - Here's the richest vein of amps. Most bass amps fall into this class. Hartke, Peavey, Trace Elliot, G-K etc. Class AB valve amps include the Ampeg SVT, Trace V8 and the Mesa Boogie 400+.

Class D - Most of the new breed of super lightweight amps are class D. Markbass, Ashdown Superfly, Genz Benz Shuttle and the like.

Class H - The only class H bass amp I can think of is the Marshall 7200/7400 Dynamic Bass range.

Hope this helps

A[/quote]

svt is ab2

[JackLondon]

In a nutshell bi-amping is splitting the signal by using a crossover and sending it to the corresponding speakers! Take for example my GK, if I connect a GK cab that has 1x15 plus a tweeter to my amp the low frequencies will be sent only to the woofer only and the high frequencies will be sent to the tweeter only! I have 2 separate controls so I can changel the volume of each speaker indeoendently! You can achieve it with using normal amp, crossover and a power amp as well!

[Alien]

[quote name='umph' post='793091' date='Apr 1 2010, 11:13 AM']svt is ab2 [/quote]

This is a distinction that only affects valve amps.

Class ab1 amps are biased so as not to allow grid current to flow. Class ab2 amps allow the flow of grid current. This current allows the anode voltage to fall close to zero, which improves the efficiency (and therefore the available power) of the amp somewhat. However, this increase comes at a price - class ab2 amps have a less linear response than class ab1. Swings and roundabouts, innit?

A

[thebassman]

[quote name='Alien' post='792810' date='Apr 1 2010, 12:55 AM']OK, here goes...

Class A - Can't think of many bass amps that run in class A, other than small amps with one-transistor power stages, such as the Peavey Microbass. Valve wise, it tends to be restricted to small guitar amps (under 10 watts or so). Think Fender Champ, Vox AC4, Epiphone Valve Jr.

Class B - Not commonly used for instrument amps because they need to be perfectly set up to minimise crossover distortion. Can't think of any off the top of my head.

Class AB - Here's the richest vein of amps. Most bass amps fall into this class. Hartke, Peavey, Trace Elliot, G-K etc. Class AB valve amps include the Ampeg SVT, Trace V8 and the Mesa Boogie 400+.

Class D - Most of the new breed of super lightweight amps are class D. Markbass, Ashdown Superfly, Genz Benz Shuttle and the like.

Class H - The only class H bass amp I can think of is the Marshall 7200/7400 Dynamic Bass range.

Hope this helps

A[/quote]

How about the valve state amps or trans class as they call them, like Ampeg svt3 pro, mesa boogie walkabout/ carbine etc??

[Alien]

[quote name='thebassman' post='793285' date='Apr 1 2010, 01:32 PM']How about the valve state amps or trans class as they call them, like Ampeg svt3 pro, mesa boogie walkabout/ carbine etc??[/quote]

Most of these only have valves in the preamp, which more or less by definition runs as class A. When amp manufacturers call their amps class A, class D etc. they're referring to the power amp stage.

A

[essexbasscat]

Anyone know which type the Ampeg B2RE is ?

this is one interesting topic. Really appreciate the informed info. put together like this, it's gold.

Cheers

T

[essexbasscat]

OK. Can I ask a few things ? I know there are a lot of questions here, please excuse a beginner trying to get a grip on this...



The post by jacklondon initially describes how the current flow is influenced by input signal signal voltage, the amplitude of that voltage is influenced by the frequency supplied by the electrical signal generator, in our case the pick up ( +/- active ccts).

The next part of jack's post goes on to describe the differences between amplifier classes as a function of how the valve(s) treat(s) the current and the steps taken to cope with the effects of current i.e. Class A = one valve (simply) and constant or 360 degree run time.

Class B cuts the current cycle at the valve, so it has to be regenerated another way. Still one valve (simply).

Class AB uses two valves to reclaim the 360 degree cycle (theoretically, with ? efficiency)



Then copied from alien's post:

"Most of these only have valves in the preamp, which more or less by definition runs as class A. When amp manufacturers call their amps class A, class D etc. they're referring to the power amp stage."

I'm fine with the first bit, as that concurs with jack's explaination or valves in the pre - amp stage, but the second sentance throws me, as it seems fundamentally at odds with jack's explaination of the classes being described in terms of how the pre- amp operates.

Q. Can anyone shed any light here please ?


Not sure what lazily running the p-p class AB bit means though jack.



Q. What's an H class ?



Copied agaIn from alien later:

"class ab2 amps have a less linear response than class ab1". I understand that Linear gain is uniform progressive while Logarithmic gain follows a curve when charted.

Q. Can anyone explain what this means in real terms to the ear ?

Q. If someone like trace etc develop pre-amps with greater numbers of valves, is this a way of promoting greater efficiency in the current modulation stages of the pre-amp?



Once again, my real thanks to everyone that has taken the time to post on this thread. I've really learned something today.

CHeers

T

Edited April 1, 2010 by essexbasscat

[escholl]

[quote name='essexbasscat' post='793796' date='Apr 1 2010, 10:18 PM']Class B cuts the current cycle at the valve, so it has to be regenerated another way. Still one valve (simply).[/quote]
Class B will always be an even number of valves or transistors, similar to AB. Never only one valve (exceptions may be, theoretically, where one valve might have two gain stages, such as dual-triodes).

[quote]Then copied from alien's post:

"Most of these only have valves in the preamp, which more or less by definition runs as class A. When amp manufacturers call their amps class A, class D etc. they're referring to the power amp stage."

I'm fine with the first bit, as that concurs with jack's explaination or valves in the pre - amp stage, but the second sentance throws me, as it seems fundamentally at odds with jack's explaination of the classes being described in terms of how the pre- amp operates.

Q. Can anyone shed any light here please ?[/quote]
The two stages of amplification are not related to one another. Preamps run exclusively in class A for audio (or at least I've not encountered any that don't), as they don't really dissipate any power and thus efficiency is not a concern, however they present simplicity, lower distortion, better linearity, and better stability than other classes -- hence they make good input stages.

The topology of the power amplifier stage, whether it is class, A, AB, etc is unrelated to the preamp, and as Alien says, when manufacturer's talk about the class of their amp, they are referring to the power amplifier stage.

[quote]Q. What's an H class ?[/quote]
The power rails of the amplifier supply are continuously variable. By keeping the rails just a bit higher than what is required by the output waveform at that moment, the output stage of a class AB can be run more efficiently and will waste less power, at the cost of higher THD and increased complexity of design.

Class G is similar in concept with the exception of having discrete power rails that the amplifier switches between, instead of a continuously variable supply.

[quote]"class ab2 amps have a less linear response than class ab1". I understand that Linear gain is uniform progressive while Logarithmic gain follows a curve when charted.

Q. Can anyone explain what this means in real terms to the ear ?

Q. If someone like trace etc develop pre-amps with greater numbers of valves, is this a way of promoting greater efficiency in the current modulation stages of the pre-amp?[/quote]
In answer to your first question, it really depends on what the non-linearity is caused by. It could be a distortion of the waveform, or it could be something like compression (or expansion) of the signal. Basically anything where the waveform going in is not linearly proportional to the waveform coming out, but there's not a specific "sound" as it can be due to a multitude of things.

I'm not really sure what you mean by your second question, but there are never really ever any concerns with preamp efficiency, at least not in bass amps. In large circuits with many, many small signal amplification stages, such as mixing consoles, the power consumption of each gain stage can be a concern, at least in terms of designing the power supply (but even then they are always class A, regardless). But this is largely irrelevant to small systems such as musical instrument amplifiers. If they add more preamp valves, it is likely just to obtain a better, different, or more flexible sound.

To summarise, the only topologies used for audio pre-amps is class A, at least as far as I'm aware.

The only topologies used for audio power amps are Class A (including single ended and push-pull), Class AB (including AB1 and AB2), Class D, and Classes G and H, which are basically Class AB.

Class B does have some limited uses in audio, such as in mobile phones, where audio quality can be traded for efficiency while keeping to a simplistic design. In general, you may safely ignore Classes C, E, and F entirely for audio. Class T, if anyone ever mentions is, is not a real class -- just the marketing term for a (incidentally very good, and now sadly defunct) type of Class D.

Just remember the general differences between A, AB, and D, and you'll be fine. :]