Alpha Nirvana 39w 8ohm Class A Amp

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#783
danny_66 said:
Hi,
The input KSA992 should be matched for Hfe so that the output DC offset is minimal.
But is there a reason to go for lower or higher Hfe ?
The question came up because at Mouser you can buy the KSA992 in different classifications:
FA: hfe2 300 ~ 470
FB: hfe2 430 ~ 600

Grts,
Danny
Click to expand...

Hi X,

Whilst what danny says above is the "ideal case", I thought R113 was there as the adjustment you use, to get a <10mV offset result?

Matching the transistors is an expensive job - you probably have to buy 30 or 40, in order to end up with a matched pair (and by matched pair, I mean Hfe a max of 5 apart).

Andy
 
#785
xrk971, have you or any other builder have plans to test this class A amp using the FTC method of testing? 5 minutes of full power onset of clipping?

as you know, Mark Levinson has a 25 watt class A, much bigger and heavier than your amps, i wonder how yours compare...
 
#786
On an ammo belt I can usually get 2 to 3 pairs in a dozen. They are not expensive and I buy maybe 50 at a time. But just buy a dozen or so and you should be set. The resistance adjustment does not provide auto temperature tracking provided by a matched pair thermally bonded together.
 
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#787
TonyTecson said:
xrk971, have you or any other builder have plans to test this class A amp using the FTC method of testing? 5 minutes of full power onset of clipping?

as you know, Mark Levinson has a 25 watt class A, much bigger and heavier than your amps, i wonder how yours compare...
Click to expand...

Sure, Tony - I’ll get right on it. On a Class A like Alpha Nirvana, the current flowing through the transistor is the same regardless of time spent at clipping. It’s not going to thermally stress the amp any more than idle. In fact, the outputs might be cooler at full power as less dissipation through the MOSFETs but more out the load resistors. I have a fan cooled 300w EBG resistor. I’ll connect it for the 5 minute at clip onset test. What are you hoping to see? Should I take an OScope shot at 0min and 5min and compare?
 
#788
Being single-ended class-A, the load can't ever get more power than the bias, so the full power test should actually let the amp run a bit cooler as all the bias current is put into the load, instead of the heatsinks. 😀

EDIT - I see X answered it the same time I was typing. :yes:
 
#789
xrk971 said:
Sure, Tony - I’ll get right on it. On a Class A like Alpha Nirvana, the current flowing through the transistor is the same regardless of time spent at clipping. It’s not going to thermally stress the amp any more than idle. In fact, the outputs might be cooler at full power as less dissipation through the MOSFETs but more out the load resistors. I have a fan cooled 300w EBG resistor. I’ll connect it for the 5 minute at clip onset test. What are you hoping to see? Should I take an OScope shot at 0min and 5min and compare?
Click to expand...

thanks, people should understand that any amp is just as good as the power supply components you put into building it, so different sizes of power traffos may yield different results, no matter that it may be small..

a sine wave source, 1khz test frequency, 8 ohm dummy resistor loads and a scope....this testing confirms what you have calculated as the power output..
 
#790
6L6 said:
Being single-ended class-A, the load can't ever get more power than the bias, so the full power test should actually let the amp run a bit cooler as all the bias current is put into the load, instead of the heatsinks. 😀

EDIT - I see X answered it the same time I was typing. :yes:
Click to expand...

that is the theory, actual testing will reveal by how much, now is a good time to learn that much..

class A have efficiencies not higher than 50%, theory says...
 
#791
Hi,

The Alpha4R that I'm listening now will be upgraded to the Nirvana.
I'll be using the same case and PS, so the change will be easy.

Since my PS is 25V I opted for the 8R version.
Of course I had to play a little in LTspice to see the outcome of putting a 4R load on the 8R Nirvana.
With normal bias at 1.65A and 15vpp the output currents I(R141) I(R142) are out of balance and the negative is clipping.
But when I augment the bias to 1.93A (0.18r) it's nicely balanced, no clipping with 15vpp and 4R.

So 1.93A seems a nice alternative.
Now I only need the blessing of Hugh or X 🙂

Regards,
Danny
 

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#792
6L6 said:
Being single-ended class-A, the load can't ever get more power than the bias, so the full power test should actually let the amp run a bit cooler as all the bias current is put into the load, instead of the heatsinks. 😀

EDIT - I see X answered it the same time I was typing. :yes:
Click to expand...

Quoting your colleague Sangram: .... it's a SE amp, operated in PP.
Hugh Dean, the designer: it's SRPP (series regulated push-pull).
Kokoriantz: it's SRPP....
 
#798
Hi Danny,

I examined your schematic and you are almost there.......

If you reduce the two 0.18R source resistors to 0.15R each you will increase the quiescent to 2.3A. With 24V rails, you will dissipate 110.4W, but if you have enough cooling with your LARGE mosfets there will be no issue.

Add a 100p ceramic NPO of 47pF from base to collector of T4. This removes a glitch at MHz level when the transistor is coming off top clip.

You will then find that with 24V rails and 2.3A quiescent you can reach 36Vpp into a 4R load, which is no less than 40.5W and THD of 0.0413%!

I think you will be pleased with this........

Hugh
 
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