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Class A vs Class D amplifiers

Posted: 22 Aug 2019 17:52
by Wrayster
Hi all,
Wondered if anyone has any views on which sounds better. I know the D is a lot more efficient than the A and won't get as hot. However I'm interested in views as to the general feeling about the sound quality.

I currently have a old Realistic amp (I've replaced all the caps) which sounds a little closed in at the top end. I'm running this with a modified Thorens table and an Airport express, same issue with both sources.

I'm considering building a Class D power amp (which I could run from the pre outs on my Realistic) based on the Texas Instruments TPA3255. There are a lot of positive comments on the electronics websites about this module however I haven't been able to find anything about it on HiFi forums so wondered what people thought?

As an alternative I may consider purchasing a secondhand power amp was thinking about the Rotel RB-850 or the Quad 303.

Comments appreciated, thanks.

Re: Class A vs Class D amplifiers

Posted: 22 Aug 2019 21:13
by jdjohn
Probably not a shock to hear that there are good and "less good" amps out there for both classes A and D. I don't think we can make a blanket statement about which sounds better. What you're describing about your vintage Realistic is a 'warm' sound, and I would suspect you'd get something very similar with either that Rotel or Quad, since they are a bit dated as well. You might search reviews for both, though, and see what folks have said. The Rotel may actually have a little brighter sound than the Quad.

My main rig is class D from PS Audio. I like it a lot, and know several others that have the same and like it as well. If you want to build a class D with the TI TPA3255, you might try going to the diyAudio forum. I did a quick search, and got all this: https://www.diyaudio.com/forums/search. ... d=21122576 And in particular, there is this quite extensive thread: https://www.diyaudio.com/forums/class-d ... 55evm.html 88 pages of stuff to read :D I think a really good power supply will be a key component if you go that direction.

Re: Class A vs Class D amplifiers

Posted: 23 Aug 2019 01:00
by analogaudio
It seems neither the Rotel nor the Quad are class A which makes me wonder why the thread is A vs D rather than AB vs D, AB being a common well established topology whereas A is very low efficiency and a fringe product.
I would chose class D from a reputable well established brand, the unit will be smaller for the same power, run cool, and probably cost less than the equivalent AB. No contest.

Re: Class A vs Class D amplifiers

Posted: 24 Aug 2019 22:40
by Wrayster
Thanks for the replies. Sorry for my misunderstanding over class A and class AB amps.

Re: Class A vs Class D amplifiers

Posted: 25 Aug 2019 00:04
by aardvarkash10
forget the technology, listen to the sound.

Having said that, a true Class A amp will burn SIGNIFICANT power for every watt of output. Because of this, it will tend to be a low power output device. Because of this, speaker selection is a vital part of your process.

Class AB moves toward higher efficiency, higher power output, and more relaxed speaker requirements. Most power amps are AB with varying amounts of time spent in "A"

Class D historically was used for high efficiency not high reproduction quality. Those times have changed. There will be plenty of people who wave their arms around and deride Class D as "sterile, cold, harsh" etc. Perhaps thats how it sounds from under their 2nd-harmonic rock, but that would be an invalid comparison.

If you are thinking about a kit already and want to consider Class A, check out the Amp Camp kits designed by "Papa" Nelson.

Re: Class A vs Class D amplifiers

Posted: 30 Aug 2019 23:30
by macster
I went from a CJPV5 and MF2500A to a Wyred4Sound PH1 and STI1000 and couldn't be happier. It took a while for things to "burn in" as I made a wholesale change of phono/intergrated/interconnects and powercord. But again I couldn't be happier no more darn tube fickleness, weird noises and associated maintenance.

M~

Re: Class A vs Class D amplifiers

Posted: 31 Aug 2019 00:16
by Jim Leach
I’m not saying this is the case here, but it seems quite common to cross confuse “class” and “quality”.

Class, is used to describe a mode of operation of an amplifier.

Historically, class A was a superior design with regards to Sonics because every component is at “full blast” all the time. So any transient, no matter how fast, would be amplified. The down side is the amp consumes 100% of its potential power consumption all the time. If it’s not amplifying, it is wasting that energy as heat.

Class AB operates as class A up to a low portion of the possible output. Usually up to 15-20 watts on a 100 watt rated amp. Doesn’t sound like much but it’s plenty for normal listening levels with decent efficiency speakers. Now, when it crosses mode of operation from A to B some are better than others.

Class D, “digital” or “demand” amps are sort of like switching power supplies. But only in description. Power for amplification is produced on demand. So the power supply is really critical. When those big transients come along and demand power, it has to come from somewhere.

In all cases, as already stated, how a type of amplifier is executed is more important than type. A cheesy class A can sound bad and a stellar class D can sound, well, stellar.

All that out of the way, how is the signal handled in your preamp? You may be fighting a loosing battle if the signal is already “muddied” before it enters the power amp. I don’t know your receiver so I’m just putting it out there. Try borrowing a few amp options and see if they all have the same character, it could be the preamp.

Re: Class A vs Class D amplifiers

Posted: 03 Sep 2019 06:20
by 1200y3
High quality Class A power amplifiers are very costly to build, almost prohibitively expensive. And a good high quality class AB amplifier can also be created with a lot of dedication (never mind the power supplies).

A few companies tried making Class A amps more usable, such as the JVC Super Class A circuit, or Quad's "current dumping" systems.

But as for Class D, it should be near perfection of Class B, which could be absolutely "dynamic", if one could exist. But some companies dabble in "Class T", which is Class D upgraded for better sound. I am hooked on Class T, although decent power output doesn't exist.

Re: Class A vs Class D amplifiers

Posted: 12 Sep 2019 13:15
by Wrayster
Thanks for all the replies.

Jim, my amp does have a pre out and a power in so if I can get hold of a pre (or power) I can have a play and see if either the power or pre is effecting the sound.

Re: Class A vs Class D amplifiers

Posted: 14 Sep 2019 00:55
by Jim Leach
That might be a valuable learning tool.

Nothing worse than dropping two months salary on something you didn’t need!

Re: Class A vs Class D amplifiers

Posted: 22 Sep 2019 19:53
by ChrisfromRI
Many Class D (aka: switching or PWM) amplifiers use the same basic output modules, either from IcePower or from Hypex. What primarily differentiates them is their input/driver circuits. For example, PS Audio uses IcePower output modules with Class A input drivers in their Stellar amplifiers. Jeff Rowland did this as well. I have built several Class D amplifiers around the IcePower modules, and they can sound very good and can be very linear. It's not uncommon for them to operate at over 90% efficiency.

If you're preamp has very low output impedance then the amplifier's input circuit is less critical - but if you're using a passive or tube preamp the amplifier's input circuit is typically quite critical.

I have also built Class A amplifiers, both solid state and tube. They can sound very good too, but generate lots of heat which can eventually cause their self destruction. The easy way to tell an amplifier is really operating in Class A is to put an input wattmeter on the AC line feeding the amplifier. With Class A the AC watts used don't vary between zero output to the speakers and full output to the speakers. Generally these are up to about 20% efficient - meaning that at full output volume 20% of the power being used is converted to power driving the speakers.

Re: Class A vs Class D amplifiers

Posted: 23 Sep 2019 02:21
by ChrisfromRI
Here is a picture of one of the Class D amplifiers I built. This one uses the IcePower 125ASX2 power output module, which is a stereo amplifier module putting out up to 125 Watts per channel. Mine is technically "Hybrid Class D" because I also used a two tube (one tube per channel) Class A Input Stage to drive the Class D IcePower module. It's small, efficient, and sounds very good on the system it is on driving fairly inefficient 4 ohm speakers.

https://live.staticflickr.com/65535/482 ... ecd7_b.jpgBO 125 acrylic lid by c f, on Flickr

Re: Class A vs Class D amplifiers

Posted: 29 Sep 2019 06:09
by smee4
1200y3 wrote:
03 Sep 2019 06:20

But as for Class D, it should be near perfection of Class B, which could be absolutely "dynamic", if one could exist. But some companies dabble in "Class T", which is Class D upgraded for better sound. I am hooked on Class T, although decent power output doesn't exist.


Class T was a term used for Tripath. They had some very innovative ways to control the class D amps switching and feedback loops. The company went under quite a while back.

Various other companies have been improving their class D control circuitry, to the point where they have become just as good, and even better than Tripath. The top of the heap at the moment seems to be Hypex. But even less specialised companies like Texas Instruments are doing good things. You'll find great amps out there with plenty of power.

Re: Class A vs Class D amplifiers

Posted: 29 Sep 2019 09:50
by Pauw
Wrayster wrote:
22 Aug 2019 17:52
Hi all,
Wondered if anyone has any views on which sounds better. I know the D is a lot more efficient than the A and won't get as hot. However I'm interested in views as to the general feeling about the sound quality.

I currently have a old Realistic amp (I've replaced all the caps) which sounds a little closed in at the top end. I'm running this with a modified Thorens table and an Airport express, same issue with both sources.

I'm considering building a Class D power amp (which I could run from the pre outs on my Realistic) based on the Texas Instruments TPA3255. There are a lot of positive comments on the electronics websites about this module however I haven't been able to find anything about it on HiFi forums so wondered what people thought?

As an alternative I may consider purchasing a secondhand power amp was thinking about the Rotel RB-850 or the Quad 303.

Comments appreciated, thanks.
Can I comment on the Rotel RB 850?

I would suggest finding two and running them as two mono blocks in bridged form? I think you will be more than satisfied. :D