DIY Class A Amp

[LTig]

But the question is whether the sound quality of class a is worth the power cost? Are they considerably better or is it just hype?

IMV it's just hype from 50 years ago. In a proper controlled double blind listening test people fail to hear differences between decent power amps as long as they are not driven into clipping. Hence the best one can do is to make sure that the power amp never clips which means lots of power.

 

[MRC01]

So here's a related question: what is a good power amp for someone who needs 400 clean continuous WPC of power, to drive inefficient yet low distortion revealing speakers? Balanced XLR inputs with 24-30 dB of gain.
All options are on the table: class A, AB, D or whatever. But it must be built to last.
I'm happy with my current amp, but it's nearly 30 years old and one of these days it will die.
We have the usual suspects: Adcom, Krell, Bryston, etc. But that's old school.
What else or new?

 

[levimax]

So here's a related question: what is a good power amp for someone who needs 400 clean continuous WPC of power, to drive inefficient yet low distortion revealing speakers? Balanced XLR inputs with 24-30 dB of gain.
All options are on the table: class A, AB, D or whatever. But it must be built to last.
I'm happy with my current amp, but it's nearly 30 years old and one of these days it will die.
We have the usual suspects: Adcom, Krell, Bryston, etc. But that's old school.
What else or new?

I would look at the Apollon or Buckeye Purif based amps. Pretty hard to beat performance and durability is going to be based on the modules which should have good support at the very least.

 

[dlaloum]

So here's a related question: what is a good power amp for someone who needs 400 clean continuous WPC of power, to drive inefficient yet low distortion revealing speakers? Balanced XLR inputs with 24-30 dB of gain.
All options are on the table: class A, AB, D or whatever. But it must be built to last.
I'm happy with my current amp, but it's nearly 30 years old and one of these days it will die.
We have the usual suspects: Adcom, Krell, Bryston, etc. But that's old school.
What else or new?

I would suggest looking around for a used Crown XLS2500 (or the still current XLS2502) - I picked mine up used for $250.

It is designed / marketed for pro audio use (bands and PA) - but it is the same circuits and chips as used in the JVC Synthesis and Lexicon amps.

The way they are configured in the XLS series you get only 2 channels per amp, but you get heaps of current (ie: the full current of the power supply, which in some other models, powers multiple channles) - spec is 440W@8ohm, 775W@4ohm, 1200W@2ohm.

A number of people who own ML electrostatics with impedance going down around 1 ohm, swear by these

My personal experience, using my Gallo Ref 3.2's (3 ohm at woofer crossover, 1.6 ohm at tweeter) - is that if you have a "difficult" speaker, these can make it shine.

Having said all that - I believe that some of the audiophile alternatives (Hypex, Purifi, Benchmark) would probably drive my speakers fine... (at between 5x and 10x the price) - with my speakers, I have compared various lower powered "audiophile" alternatives - but these ended up sounding better... unexpectedly, as I purchased them as an experiment... one that has stayed on as my main amps.

P.S.
The XLS series have balanced XLR, unbalanced RCA, and unbalanced 1/4" jack inputs
The also have an onboard DSP which allows for crossovers, limiting frequency range etc... - but does imply an onboard conversion, and bandwidth limitation - they pretty much do nothing above 20kHz (but are very flat up to 20kHz), there is no way of avoiding the DSP in this series (Crown/Harman has other relatives of this amp - driven by Drivecore chips - that do not have onboard DSP... but I have no experience with them)

PPS:
Built to last... they are not Krell style over-engineered tanks - they weigh almost nothin (courtesy of Class D and switching power supply) - but are designed to be thrown around as Band / PA amps on the road.... - they are very robust! - Nothing "Bling" about these. Purely functional in a very 21st century way.

 

[MAB]

So here's a related question: what is a good power amp for someone who needs 400 clean continuous WPC of power, to drive inefficient yet low distortion revealing speakers? Balanced XLR inputs with 24-30 dB of gain.
All options are on the table: class A, AB, D or whatever. But it must be built to last.
I'm happy with my current amp, but it's nearly 30 years old and one of these days it will die.
We have the usual suspects: Adcom, Krell, Bryston, etc. But that's old school.
What else or new?

Your must be built to last would lead me to Bryston.
I don't hear any difference between the PuriFi amp and the Bryston 4B ST that I have.
I am not sure if the PuriFi will last as long as the pile of Bryston gear I have.

 

[dlaloum]

Your must be built to last would lead me to Bryston.
I don't hear any difference between the PuriFi amp and the Bryston 4B ST that I have.
I am not sure if the PuriFi will last as long as the pile of Bryston gear I have.

Honestly I am not sure if ANY Class D amps will last the way some of my 40 year old classics have... + The classic A and AB amps can relatively easily/economically? be maintained, caps replaced as they get past their use-by... and keep powering on.

With the Class D amps (or even modern AB's like the Benchmark) - I don't know how maintainable they are over the long term (measured in decades) -I believe most of them are part of the "disposable" age - if they are well built and designed, they may well last a couple of decades - but once it comes time to recap, I am not sure that these complex, crowded, surface mounted circuit boards, will lend themselves to individual component level replacement/update.

 

[captainbeefheart]

Class D is great if you need to carry around your amp like musicians or if you are one of those climate change yahoo's that think your amplifier is going to put us into global warming.

As others bring up they aren't the most reliable amplifiers and don't expect any Class D amplifier to last nearly as long as a well built linear amplifier. Now if you like to purchase new amps every so often then that's not an issue obviously.

Questions need to be answered before you get too deep into looking into Class A amplifiers and the major question is how much power do you actually need. The variables needed to be answered are: How large is your room? What are the sensitivity of your speakers? And finally what is your preferred listening volume?

With efficient speakers inside an average sized room with average listening you will be surprised how little power you actually need. BUT, with say a larger room with power hungry speakers and you like your house to shake and blow your eardrums out then you may require substantial power and Class A just isn't going to be a good option.

For example my system could get away with a flea watt amp (1 watt or possibly less) if I didn't like some volume once in a while. Now I don't want too much volume, say a hair under concert levels. I'm not talking heavy metal concert levels, I'm talking symphony hall levels and for that I still only require 20 watts. I have some friends with large rooms and very power hungry speakers where my 20 watt amp will not come close to doing the job of around live concert levels and we calculated over 200 watts was necessary. Remember you need to double your amplifier power in order to gain +3db so you can see how power can jump up very quickly because at 100 watts to gain +3db you need another 100 watts.

Do some research and figure out how much power you require and go from there. You may find you can get away with less power which will greatly help with heat and power consumption if going with Class A because a 100 watt Class A amplifier will be large, hot and use quite a bit of juice so if you can get away with say 25 watts then your not wasting as much.

 

[dlaloum]

Class D is great if you need to carry around your amp like musicians or if you are one of those climate change yahoo's that think your amplifier is going to put us into global warming.

As others bring up they aren't the most reliable amplifiers and don't expect any Class D amplifier to last nearly as long as a well built linear amplifier. Now if you like to purchase new amps every so often then that's not an issue obviously.

Questions need to be answered before you get too deep into looking into Class A amplifiers and the major question is how much power do you actually need. The variables needed to be answered are: How large is your room? What are the sensitivity of your speakers? And finally what is your preferred listening volume?

With efficient speakers inside an average sized room with average listening you will be surprised how little power you actually need. BUT, with say a larger room with power hungry speakers and you like your house to shake and blow your eardrums out then you may require substantial power and Class A just isn't going to be a good option.

For example my system could get away with a flea watt amp (1 watt or possibly less) if I didn't like some volume once in a while. Now I don't want too much volume, say a hair under concert levels. I'm not talking heavy metal concert levels, I'm talking symphony hall levels and for that I still only require 20 watts. I have some friends with large rooms and very power hungry speakers where my 20 watt amp will not come close to doing the job of around live concert levels and we calculated over 200 watts was necessary. Remember you need to double your amplifier power in order to gain +3db so you can see how power can jump up very quickly because at 100 watts to gain +3db you need another 100 watts.

Do some research and figure out how much power you require and go from there. You may find you can get away with less power which will greatly help with heat and power consumption if going with Class A because a 100 watt Class A amplifier will be large, hot and use quite a bit of juice so if you can get away with say 25 watts then your not wasting as much.

You are disregarding the case of a user with a 1 ohm speaker (in my case 1.6ohm) - and the recalculation of power requirements at desired SPL, at 1 ohm brings about quite a different picture.

The power requirements are then measured in figures that are an order of magnitude larger (strictly speaking 8x for 2 ohm, and 16x for 1 ohm)

So if for the average speaker you can get by with a fleapower 1W - for the 1 ohm speaker case, you need 16W.... if your base 8ohm requirement is for 20W - then your 1 ohm requirement is for 320W (!!)

Based on my measurements, and based on the power LED's on my amps - at my normal listening levels of circa 75db at the MLP - my amps are barely getting to just over -20db - which is circa 3.4W @ 8 ohm - and around 9W @ 2 ohm (probably circa 12W @ 1 ohm) .... (yes they aren't perfect... they don't double down as the impedance halves)

Which means the sizing of the amps is well gauged.... I have a bit under 20db headroom (my target was 20db headroom)

My 20db headroom takes my amps to their spec power of 440W@8ohm, 775W@4ohm, 1200W@20hm... (1 ohm not specified... but known stable)

(Speaker efficiency is 86db SPL/wm )

Yes - you can calculate out power requirements, room, etc.... but with many speakers, once you allow for 20db headroom (for music.... 15db for movies) - you end up needing a HEAP of power

If you are running Klipschorns with 114db SPL/wm it is a very very different picture.

What your amp requirements are is totally dependent on the speakers & room - speakers / amp / room need to be handled as a single system.

Klipschorn with 20W pure class A makes perfect sense.... Gallo Ref 3.2 with a 500W Class D makes similarly perfect sense (and was the way the respective speaker designers demonstrated their speakers!)

 

[MRC01]

...
Questions need to be answered before you get too deep into looking into Class A amplifiers and the major question is how much power do you actually need. The variables needed to be answered are: How large is your room? What are the sensitivity of your speakers? And finally what is your preferred listening volume?
...

I believe the OP mentioned he has Magnepans. If they're like my 3.6/R, they slurp down all the power you can feed them and ask for more. Not a good application for class A power. Yet they aren't a difficult load, 4.7 ohms in bass smoothly dropping to 3.3 in treble.

 

[dlaloum]

I believe the OP mentioned he has Magnepans. If they're like my 3.6/R, they slurp down all the power you can feed them and ask for more. Not a good application for class A power. Yet they aren't a difficult load, 4.7 ohms in bass smoothly dropping to 3.3 in treble.

My Gallo's are down to 3 ohm at the woofer crossover, and 1.6 ohm at the tweeter... don't seem to be using all that power current - but feed them with an amp with LOADS of power/current capacity into 2 ohm loads - and they definitely sound better.

The 100W AVR driving them - simply made them sound like a mess - lost all imaging/soundstaging, got a hint of harshness about it too - simply didn't sound great - using the same AVR pre-outs into Quad 606 (145W@8ohm, 90W@2ohm) - sounded a lot smoother, resolved the harshness and imaging/staging issues ... but giving it an amp capable of 1200W@2ohm - gives the whole system an "ease" it doesn't have otherwise.

You can run some figures, on what it takes to raise the voltage output of the power amp, to the required V to provide the SPL needed at the MLP (at peak for desired headroom)... with speakers that are "difficult" (low impedance) loads - the power/current requirements are often quite surprisingly high.

I theorise that my system could get by, and would probably sound identical with an amp putting out circa 500W@2ohm.... but my current 1200W'ers are managing fine....

 

[captainbeefheart]

You are disregarding the case of a user with a 1 ohm speaker (in my case 1.6ohm) - and the recalculation of power requirements at desired SPL, at 1 ohm brings about quite a different picture.

I'm not at all disregarding that, one of the criteria I mentioned where not to go with lower powered Class A amplifiers is if you have power hungry speakers. I would define a 1 ohm load as power hungry wouldn't you?

 

[DVDdoug]

This is probably somewhat redundant... I just scanned the responses...

I’ve never had a serious class A amp so wanting to hear the difference.

A good amp is often described as "a straight wire with gain". It's not supposed to have any sound of it's own.

And if there is a difference you can't assume that your class-A amp will sound like another class-A amp.

...I don't think I've ever heard a class-A "hi-fi" amp! But I remember some class-A small radios & car radios from the 1960's (some with tubes).

I understand the the benefit of class as that they eliminate transistor switching noise by keeping them on

As you probably know, Class-A/B has some bias so one doesn't turn-off until the other has "taken-over".

As above, the distortion isn't necessarily audible and overall the distortion may not be any worse than Class-A/B or Class-D. But, it's called "crossover distortion". It's somewhat "noise like" in-that it's more-or-less constant so proportionally worse at low levels and possibly more audible at low levels and inaudible at higher levels. But unlike noise it's not present with silence.

...But in our favor, imagine that we have "bad" 10% distortion with a barely audible signal. If we can barely hear the signal then the even-quieter distortion is completely below audibility. Then since the crossover distortion is constant, as the signal gets louder, the percentage of distortion is lower and even more drowned-out by the signal.

As a self taught technical person I’m worried about adjusting the bias Of class a amps. Is that difficult?

It shouldn't be too bad as long as you have a "procedure" starting with zero bias. If you start-out with too much you can burn-up the transistor or MOSFET! That's less likely if you build a class-A/B amp because there should be a reasonable to maximum to the adjustment... Class-A has to run hot so it's not as easy to put a safe-limit on the adjustment.

But... If you're building an amp "from scratch", you probably should have some extra parts on hand. anyway I build some hobby electronics and I always like to have extra parts. It's even more important with high current or high voltage, or just about any time you expect to need a heatsink because that's when you are pushing parts to near their limits..

 

[captainbeefheart]

You can run some figures, on what it takes to raise the voltage output of the power amp, to the required V to provide the SPL needed at the MLP (at peak for desired headroom)... with speakers that are "difficult" (low impedance) loads - the power/current requirements are often quite surprisingly high.

It's not really a voltage issue with a really tough load like 1.6 ohms, it's how much current can the output devices handle. So long as the amp says 2 ohm stable you should be fine just don't push it to full tilt for very long. Or some will also say 1 ohm stable and you know you could push the amp to full output without letting the magic smoke out. For a 1200 watt amplifier that could handle the 1.6 ohm load the output devices need to be able to deliver 27.3 amps of current. The rail voltage would need to be around +/-65 volts because you would need 43.8v rms across the 1.6 ohm load to make the rated 1200 watts.

Tube amps are a little different, a 1.6 ohm load is not an issue so long as the output transformer has the correct impedance ratio for that specific load. Basically as long as the tube is seeing a load it's happy with that's all that matters, so if it wants a 2.5k load and you wanted a 2 ohm tap the impedance ratio would be 1250:1 and the turns ratio would be 35:1. You would most likely not find one off the shelf and would need to have someone wind you one. A mismatch of 100% isn't a big deal so as long as you ran the amp on the 4 ohm tap I'm sure it would be okay especially if it's only one spot where it dips that low, the average impedance of your speaker is probably 4 ohms but dips to 1.6 somewhere in the bass region. In this case an off the shelf output transformer with a 4 ohm tap would suffice.

 

[captainbeefheart]

The problem with some Class AB amps is when trying to push the efficiency too high (low bias current) and you end up with nasty sounding distortion (higher harmonics) at lower power outputs which often many people do most of their listening in this lower power region. Of course this isn't an issue if care is taken to bias the amp properly and avoid all crossover distortion.

Another issue with BJT AB amps is getting thermal tracking correct. As you push more signal into the amp asking for more power from the output devices they heat up. BJT's increase Collector current along with increased base junction temperature. With Class A amps you don't have as much of a temperature change with the output transistors. Mosfet's are what I prefer, they require less current from the supply to run due to them not needing gate current like the base of a transistor and when using lateral Mosfets that don't have negative voltage coefficients you don't really need any thermal compensation like with BJT's. Of course Nelson Pass likes his Class A amplifiers and made vertical Mosfets popular due to less temperature change from the Class A bias making thermal compensation not really needed.

 

[captainbeefheart]

I don't think I've ever heard a class-A "hi-fi" amp! But I remember some class-A small radios & car radios from the 1960's (some with tubes).

Probably the most popular radios back in the day was the All American Five amps, they were in everything and everywhere because they perfomed well with only 5 tubes that ran directly from the mains (no power transformer) with the 5 tube filaments in series adding up to the line voltage. Plugging it in gave you a 50% chance of the chassis being hot!!! This is dangerous electrical practice nowadays. They were Class A output stages due to them only having 1 power tube so it needed to conduct the complete 360° signal waveform.

 

[roderickvd]

You could build an AMB b24 in class A configuration and have it output 40W in 8 ohm. Personally I would build it in the default A/B configuration and never worry about having enough power without running into clipping. Also the class A will draw require crazy amounts of power and produce lots of heat. The cost-benefit ratio is not there.

 

[dlaloum]

It's not really a voltage issue with a really tough load like 1.6 ohms, it's how much current can the output devices handle. So long as the amp says 2 ohm stable you should be fine just don't push it to full tilt for very long. Or some will also say 1 ohm stable and you know you could push the amp to full output without letting the magic smoke out. For a 1200 watt amplifier that could handle the 1.6 ohm load the output devices need to be able to deliver 27.3 amps of current. The rail voltage would need to be around +/-65 volts because you would need 43.8v rms across the 1.6 ohm load to make the rated 1200 watts.

Tube amps are a little different, a 1.6 ohm load is not an issue so long as the output transformer has the correct impedance ratio for that specific load. Basically as long as the tube is seeing a load it's happy with that's all that matters, so if it wants a 2.5k load and you wanted a 2 ohm tap the impedance ratio would be 1250:1 and the turns ratio would be 35:1. You would most likely not find one off the shelf and would need to have someone wind you one. A mismatch of 100% isn't a big deal so as long as you ran the amp on the 4 ohm tap I'm sure it would be okay especially if it's only one spot where it dips that low, the average impedance of your speaker is probably 4 ohms but dips to 1.6 somewhere in the bass region. In this case an off the shelf output transformer with a 4 ohm tap would suffice.

Which is why some low powered Valve designs manage remarkably well with speakers that some high powered transistor amps cry "uncle" with....

 

[Nobleman]

I would suggest looking around for a used Crown XLS2500 (or the still current XLS2502) - I picked mine up used for $250.

It is designed / marketed for pro audio use (bands and PA) - but it is the same circuits and chips as used in the JVC Synthesis and Lexicon amps.

The way they are configured in the XLS series you get only 2 channels per amp, but you get heaps of current (ie: the full current of the power supply, which in some other models, powers multiple channles) - spec is 440W@8ohm, 775W@4ohm, 1200W@2ohm.

A number of people who own ML electrostatics with impedance going down around 1 ohm, swear by these

My personal experience, using my Gallo Ref 3.2's (3 ohm at woofer crossover, 1.6 ohm at tweeter) - is that if you have a "difficult" speaker, these can make it shine.

Having said all that - I believe that some of the audiophile alternatives (Hypex, Purifi, Benchmark) would probably drive my speakers fine... (at between 5x and 10x the price) - with my speakers, I have compared various lower powered "audiophile" alternatives - but these ended up sounding better... unexpectedly, as I purchased them as an experiment... one that has stayed on as my main amps.

P.S.
The XLS series have balanced XLR, unbalanced RCA, and unbalanced 1/4" jack inputs
The also have an onboard DSP which allows for crossovers, limiting frequency range etc... - but does imply an onboard conversion, and bandwidth limitation - they pretty much do nothing above 20kHz (but are very flat up to 20kHz), there is no way of avoiding the DSP in this series (Crown/Harman has other relatives of this amp - driven by Drivecore chips - that do not have onboard DSP... but I have no experience with them)

PPS:
Built to last... they are not Krell style over-engineered tanks - they weigh almost nothin (courtesy of Class D and switching power supply) - but are designed to be thrown around as Band / PA amps on the road.... - they are very robust! - Nothing "Bling" about these. Purely functional in a very 21st century way.

Don’t mean to rock your boat but I considered the Crown XLS Drivecore amps but the Noise floor is is in the -70db range so worse than most mid tier class ab amps. This was the only turn off. They got power to drive a small moon for sure…for PA they should be fine but def not for home listening IMHO.

 

[captainbeefheart]

My personal experience, using my Gallo Ref 3.2's (3 ohm at woofer crossover, 1.6 ohm at tweeter) - is that if you have a "difficult" speaker, these can make it shine.

Just seeing this now which relates to my other response to you.

The 3 ohm load on the woofer is the tougher load even through your tweeter has a 1.6 ohm impedance. The reason why is there isn't much content in music up in the tweeter range. Look at the power rating differences between woofers and tweeters, the woofer in loudspeakers are designed to handle far more current than the tweeter due to this fact. So going back to my original statement so long as your amp can handle the 3 ohm load in the bass region you should be fine which is 20 amps of current. The rails will need to be higher at around +/-85v due to the 60v rms across the 3 ohm load to make 1200 watts. That's where your amp is going to need it's power, not up in the tweeter range even though it's a low impedance.

 

[MRC01]

... So going back to my original statement so long as your amp can handle the 3 ohm load in the bass region you should be fine which is 20 amps of current. The rails will need to be higher at around +/-85v due to the 60v rms across the 3 ohm load to make 1200 watts. That's where your amp is going to need it's power, not up in the tweeter range even though it's a low impedance.

How is level of power possible, given the limitations of a typical home outlet at 115 V and 20 amps, or max VA of 1840 VA (115 * 20 * 0.8) ? 1200 watts delivered to the speaker would require 1200/1840 = 65% efficiency.
Does this amp take a 230 V outlet or a 30-40 amp breaker?