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Old 14th January 2014, 04:35 PM   #11
ishiru is offline ishiru  Indonesia
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mine was using 35VDC both driver and finals w/ 2A bias over 0.15Ohm emittor resistor.
make sure you didnt get dc offset at the output.
mine using 1 pair MJL3281/1302 and have no problem on the bias.both pnp and npn biased the same .from my studies, this bias difference also caused by dc offset

also try to use lower tolerance emitter resistors.they would makes the measurement more accurate
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Old 14th January 2014, 06:00 PM   #12
esgigt is offline esgigt  Netherlands
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Quote:
Originally Posted by richie00boy View Post
The idle power dissipated is 40 watts.
I think it's a bit more.

The supply is +/- 40V @ 1 A idle current. So idle power is 2x40W = 80W

At a Class A power of just 4 Watts: a lot of heat..

@dihmels : have you already listened to the amp? How does it sound?
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Old 14th January 2014, 06:26 PM   #13
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Would two sets of transistors not double the current and therefore the class A watts to 8?
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Old 14th January 2014, 06:30 PM   #14
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It sounds nice. I am comparing it to an Arcam A75 and P75 into Spendor S5Es. It sounds much more effortless at low volumes than the Arcam output stage. But the gain is not great and I see that the board suppliers are now suggesting 15v more for the input stage from the original 10v differential (+45 and +60)
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Old 14th January 2014, 07:16 PM   #15
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Quote:
Originally Posted by esgigt View Post
The supply is +/- 40V @ 1 A idle current. So idle power is 2x40W = 80W

At a Class A power of just 4 Watts: a lot of heat..
You are correct. Both upper and lower halves dissipate 40W each.

And I agree on the waste

Quote:
Originally Posted by dihmels View Post
Would two sets of transistors not double the current and therefore the class A watts to 8?
No, I had accounted for two pairs. The 0.5 amp flows in each pair so 1 amp total.

Having said that, I have seen some claims that a push-pull output stage can deliver double power still class-a. But I'm not sure about that. I only see it as being able to reach a higher max power for a given set of rails.
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Last edited by richie00boy; 14th January 2014 at 07:20 PM.
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Old 15th January 2014, 12:26 PM   #16
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Well, I just want to built amp that can deliver more clean power than what I have now. The Arcam / Spendor setup is alright but it seems to be missing some life. Any suggestions. I'm not opposed to building or buying new amp boards.
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Old 15th January 2014, 12:46 PM   #17
esgigt is offline esgigt  Netherlands
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Originally Posted by esgigt View Post
At a Class A power of just 4 Watts: a lot of heat..
I see I've omitted the Idle-current calculation for the KSA-50 (50W class A in 8 Ohms) specification: 3.54 Amps !

So each channel requires some 285 Watts of energy

Indeed, I would settle for a lesser setting: 1.5 a 2 Amps max. (only if the heatsinks can handle this )

Each channel would require a 500VA tranny (and a soft-start)
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Old 15th January 2014, 01:11 PM   #18
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I can't imagine achieving that much idle current even if my power supply could handle it. I'm not sure of the va of the toroidal but it's quite large, about 5 by 5 inches, more of a big marshmallow shape than a doughnut. So if I'm fussing this much for 4watts I would rather move from class A to the best class AB possible. Any suggestions?
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Old 15th January 2014, 01:27 PM   #19
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Just build it and set the bias so that you measure 25mV across each emitter resistor. Listen and see, it will most probably be totally fine. That will give you more "clean power" than amps of bias.
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Old 15th January 2014, 01:39 PM   #20
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0.5A bias pr device and 2 pairs in push-pull configuration is 16Wrms at 8ohm.
peak class A current is twice the bias. so 2A peak.
2*0.707=1.414A rms
1.414*1.414*8ohm = 15.99Wrms at 8ohm.
so 80W dissipation is nothing to whine about. besides. the high dissipation is do to the higher power class A/B. you can drop the rails to +/-20V or so. and have 40W dissipation and still 16W class A at 8ohm.

Last edited by AudioSan; 15th January 2014 at 01:52 PM.
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