Cannibalize a Class AB Stereo to Class A Mono block.
I have been thinking too much about building a class A amplifier. Transformers and Heat Sinks and boxes are the biggest costs for all amplifiers, and suitable transformers and capacitors are nearly twice the cost for Class A amplifiers.
In general, do members of this group think that most 100 W class AB stereo amplifiers could be converted to Class A mono blocks with the same output power? This way we could avoid the costs of buying or building boxes and buying expensive transformers and capacitors.
I doubt this will be quiet possible, as has theoretical efficiency of 78.5% and class A a theoretical efficiency of 50%.
Having looked at my 3 Quad 303's I notice they have a Center tapped transformer. Even though the Quad 303 is a single rail powered amplifier.
They have approximately 70 V supply, and deliver officially 30 W RMS and 3 Amps into 16 Ohms hence the name.
If I put a single rail powered JLH class A amplifier inside, do you think I would over load (so the transformer starts to Buzz or some thing worse) the amplifier as a 15W into 16 Ohm mono block amplifier.
The idea would be to use the center tapped transformer and two diodes to halve the voltage, and half the time the current is used, and then since its a mono block I halve the power demand to make up for the approximate halving in efficiency over a class AB amplifier.
I don't think I am going to do this crime (at least permanently) as the Quad 303 is a lovely beast but if they remain so cheap broken on ebay in the UK I could be tempted, this said I will probably just spend the money some time a build a 25 W JLH Class A into 6 Ohms as I preferred the sound of no coupling capacitor when I played with the 1969 and 1996 JLH topologies, ( though I did feel that the need for some DC offset protection was a must with direct coupled amplifiers) but maybe some other less classic amplifier with higher power and and more current drive could suffer the fate of becoming a single railed JLH 1969 Class A amplifier using the same approach.
So please, if this could work, suggestions on good amplifiers to perform this butchery upon and suggestions of good value suppliers of transformers and suggestions for how much headroom I should need in transformers
Now compare that Krell size/weight to a typical two channel ClassAB 100+100W into 8r0 amplifier.
To build a low cost class A amp, Ebay.de is a good place for cheap heatsinks and transformers.
the biggest question is whether "100 W" is the real continuous output power rating for both channels simultaneously - marketing has invented several "dynamic power" numbers to make under rated supplies and inadequate heatsinks have much bigger output power numbers – another way the rating is puffed up is by using non-standard load impedance – one chosen to best match the I,V swing of the ps/output stage rather than a standard loudspeaker
the math isn't so hard although I'd use ~70% for AB
an ideal Class AB amp putting out 100W continuous power into a pure resistive load with 70% eff takes 100 W/.7 = 140 W from the ps
so 40 W is the pwr wasted in the output Q/heatsink
ideal push-pull Class A can reach 50% eff, wastes 1 W for each W of output
paralleling the 2 channels you have 80 W dissipation capability from the 2 channels of 100 W AB - so at best you could manage ~ 80 W output in push-pull Class A into ~1/2 the load resistance that the original amp was rated for
of course there is extra loss from not swinging to the ps rail that is more expensive in Class A so you would expect less than the 80 W - also JLH output relies on Hfe match for push-pull symmetry and can be even lower efficiency – I wouldn’t be surprised if ~60 W Class A was the practical result of paralleling 2 100 W Class AB channels
I wouldn’t want to change output, drive Q and wouldn’t use the JLH output topology
Think about half of a KSA50 which is a two channel 50+50W ClassA into 8r0 amplifier.
Half that Krell is still enormous compared to most ClassAB product.
The 50W ClassA consumes ~150W per channel for the 50W of ClassA output.
That continuous consumption determines the transformer and heatsink and capacitor specifications.
A 100+100W into 8r0 that dissipates a continuous <=40W of quiescent and a non continuous maximum of ~80W (when delivering transient peaks) cannot be a good candidate for a 50W ClassA.
you're right that the ps current budget looks more constraining than the output device power rating
Class AB ps has to supply 1/2 of the RMS current/chan from either rail
full output range Class A ps has to supply the peak current continuously
so 2x 100 W Class AB channels ps could only be expected to supply continuous Class A current of only 70.7% the peak current that gave the AB 100 W rating - resulting in 50 Wrms of Class A output if limited by the power supply ability
under these assumptions then the max power load resistance is 2x the original 100 W Class AB amp's rated load
maybe you should start with a 5.1 amp to get a Class A monoblock with nominal pwr equal to a single AB channel?
Yes this is as I feared, now its time to kill this thread unless any one else wants to keep it alive and start another on cheap heat sinks some time (if I cant find a good one), as its clear transformers have to be bought for the job, and 99% of the ones on ebay are not suitable current drive when I did the math for a class A and the 1% that are suitable sell at high prices similar to the prices new. This said as soon as I buy one suitable 10 perfect examples will appear on ebay.de irrespective of how long I wait, you know how it is :)
Also making boxes is a pain. If all I needed for a 25-50 Watt class A was the circuit I would build one in a week. Its a costly game building a class A amplifier and to my memory of the sound difference it was noticeable but maybe beyond this and next months budget, as I do like restaurants, dancing and nightclubs as much (if not more) than HiFi.
Back to thinking about power management to reduce the electricity bills before I restart doing a class A amplifier or looking for second hand Class A amplifiers on ebay.
Thanks to you all
heavy bias Class AB such that -5 to -10 dB below clipping is Class A may give much of the sonic benefits with less stress on supply
or pro 500-2KW PA could be another starting point for case, ps, heatsink and output Q - just down rate the Class A pwr by a big factor and it will cost less than you can buy the parts individually
In most cases is a heavy biased class A/b amplifier enough to give you the class A pleasure and joy in sound quality. Take for example the Parasound JC1 which is in class A the first 25 watt. This will cover 95% of your musical signal, even on loud concertlevels. In worst case only a few peaks will be in class B.
Personally I prefer good class A/B designs with at least the first 10 watt biased in class A. This will do it for most cases.
With kind regards,
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