If I bought an amp like this one, which has a robust enough power supply to do 2x 160W at 8-ohms (or 2x 230W at 4-ohms), how might I rebias it so it would run Class A? Is it totally impossible? Or too much trouble to be worth it? I am dying to have a Class A amp, but don't feel up to building my own these days. Thanks for any pointers you may have.
nearly every solid state amp. has a trimpot to adjust biasing. Often you can go from classB to somewhere at deep AB, but keep in mind, that sufficient cooling is the main borderline, not power supply. Probably you should change heatsink and/or run forced cooling. Not worth. If you want class A, buy class A or listen to class AB at very low levels.
regards
regards
You would need to change the transformer to lower the voltage, otherwise the heatsinks are too small for class A.
With the correct transformer the heatsinks would be able to handle about a 40W per channel class A amplifier.
You would also need the schematic as there will need to be other changes.
With the correct transformer the heatsinks would be able to handle about a 40W per channel class A amplifier.
You would also need the schematic as there will need to be other changes.
Not to discourage one but I would consider that a pretty whimpy power amp. If you are lucky and the outputs can take it(if there are enough to get you into a decent SOA you might get it to do 5 to 10 watts class A but I doubt that the circuitry would just allow you to crank up the bias to that level with out a major re-design. Look at the kind of heatsinking it takes just for a Mini Aleph and the Aleph design is some 25+ percent more efficient than a straight class A amp like the KSA-50 is. Those building KSA-50's should select what they think is enough heatsinking and then double that! Do check out both the Mini Aleph and KSA-50 threads and you'dd know in short order what it takes.... Now get yourself an old fan cooled BGW 750 to strip out and you'd have a pretty good basis for building a KSA-50 or something in that power range.
Mark
Mark
thats true but what if he is only looking for a few watts class a? I have a similar amp I use made by hafler. Even when it's pushed hard it doesn't get warm. I wouldn't mind being able to get five watts class A out of it if I could. My guess is that too really do this to an amp you would need to redesign the circuit and that would make it a waste. For the same money you could build a small class A amp and it would most likely sound better.
The problem in using such a relatively high power class AB amplifier is that the power supply voltages are rather high. This results in a lot of amplifier dissipation if you increase the bias current.
That Behringer amplifier is specified at 160W in 8 Ohm, which means that the power rails are about +/- 55V.
If you want to get, say, 4W output in class A into 8 Ohm, a modest amount of output power, you need 1A peak current out or 0.5A bias current. Dissipation is then 110V*0.5A=55W. This is probably the limit for this amplifier in terms of long term heat dissipation.
So, efficiency is very low. Using lower supply voltages would have helped a lot. On the other hand, high supply voltages allow for far higher output power if needed. It just shifts into class AB.
Steven
That Behringer amplifier is specified at 160W in 8 Ohm, which means that the power rails are about +/- 55V.
If you want to get, say, 4W output in class A into 8 Ohm, a modest amount of output power, you need 1A peak current out or 0.5A bias current. Dissipation is then 110V*0.5A=55W. This is probably the limit for this amplifier in terms of long term heat dissipation.
So, efficiency is very low. Using lower supply voltages would have helped a lot. On the other hand, high supply voltages allow for far higher output power if needed. It just shifts into class AB.
Steven
Hi needtubes,
A Class A amp can be simply a Class AB amplifier of around three times the power run with lower supply voltages but the similar VA
supply with more C, and much larger heatsinks, if it has been designed to.
For example, my new 150W into 8ohm Class AB module (see website) will do so from a +/-56V supply of >150VA transformer and about 15,000uF per rail. This is to contain ripple voltage to <1V. As it uses high performance current sources for biassing the whole amplifier, it can be operated on +/- 32V rails and the bias turned up to 1.75A to produce 50W into 8 ohms in Class A. As this is a much higher constant draw from the supply the C's are increased to 30,000uF to keep ripple within limits. Dissipation will be about 115W requiring a heatsink of around 0.3degC/W. Also there is a simple 'tweak' to enhance low order harmonic 'bloom' to sound more tubelike, if that's your preference. The board is also designed to back off bias in response to a possible heatsink mtd thermal switch and turn on a overheat LED.
Too easy.
Cheers,
Greg
A Class A amp can be simply a Class AB amplifier of around three times the power run with lower supply voltages but the similar VA
supply with more C, and much larger heatsinks, if it has been designed to.
For example, my new 150W into 8ohm Class AB module (see website) will do so from a +/-56V supply of >150VA transformer and about 15,000uF per rail. This is to contain ripple voltage to <1V. As it uses high performance current sources for biassing the whole amplifier, it can be operated on +/- 32V rails and the bias turned up to 1.75A to produce 50W into 8 ohms in Class A. As this is a much higher constant draw from the supply the C's are increased to 30,000uF to keep ripple within limits. Dissipation will be about 115W requiring a heatsink of around 0.3degC/W. Also there is a simple 'tweak' to enhance low order harmonic 'bloom' to sound more tubelike, if that's your preference. The board is also designed to back off bias in response to a possible heatsink mtd thermal switch and turn on a overheat LED.
Too easy.
Cheers,
Greg
Well, then sure it might do a few watts class A but you'd still need to do some re-design and the power supply would also have to be re-worked. Is it all worth it... certainly not from a monetary standpoint but if one is looking just for something to do then why not.... Would the amp sound better..... only modding it to do a few watts class A this would answer that.....
Mark
My mains are efficient (102dB@1w), so I could get away with a few watts, but I would really love to have something a bit more powerful. I just discovered the Marantz PM7200, which is normally a Class AB amp devilering 95WPC, but can be switched to 25WPC in Class A. Not a bad deal, I suppose, but more than I was hoping to spend. Money is a tad tight, but I've been dreaming of Class A for so long that it is eating me up.
I am intrigued by the concept of the amp presented above by amplifierguru. 50W of Class A power is quite a bit, though. Heatsinking and transformers would most likely run in the hundreds of dollars, so still out of my price range after I have to buy PCBs or kits. Are the amplifier kits for stereo or do you have to buy two kits to make stereo?
I had the feeling it would take more than simply changing a few components to rebias an AB amp to A. I suppose I was kidding myself thinking it could be easy...
I am intrigued by the concept of the amp presented above by amplifierguru. 50W of Class A power is quite a bit, though. Heatsinking and transformers would most likely run in the hundreds of dollars, so still out of my price range after I have to buy PCBs or kits. Are the amplifier kits for stereo or do you have to buy two kits to make stereo?
I had the feeling it would take more than simply changing a few components to rebias an AB amp to A. I suppose I was kidding myself thinking it could be easy...
With careful parts shopping at surplus centers one can build the Krell KSA-50 clone for about 300.00, much less if one can latch onto heat sinks on the cheap. In fact of you only need about 10 watts class A you could build it very inexpensively, its a very simple circuit, old design but super reliable and super stable, still used by many audio reviewers. I have about 200.00 tied up in my first one... 54 watts pure class A then goes to 80 watts class AB at clipping. Shopping for parts on E-Bay can pay off if done carefully.
The Mini Aleph is another good amp to build anf it sounds great... not at all like a 10 watt amp!.... then there are the chip amps... pass the dip please.....
Mark
The Mini Aleph is another good amp to build anf it sounds great... not at all like a 10 watt amp!.... then there are the chip amps... pass the dip please.....
Mark
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