Thank you. Now a mosfet is cold!
I am thinking the batch was flawed. I ordered 1% 5w 1ohm resistors for the mosfet source. I hope this improves the matching!
The funny thing is that bias drops with time. it is supposed to be 3A and that value is reached only in the beginning when the amp is cold. then it drops to 2.56A. could it be a "cold" mosfet is causing this?
I am thinking the batch was flawed. I ordered 1% 5w 1ohm resistors for the mosfet source. I hope this improves the matching!
The funny thing is that bias drops with time. it is supposed to be 3A and that value is reached only in the beginning when the amp is cold. then it drops to 2.56A. could it be a "cold" mosfet is causing this?
no pictures , no measurements in important points , and you are expecting even additional confirmation of everything we replied to you ?
I understand lack of experience , but you really need to read what people are replying
temperature equilibrium is just that - equilibrium - it doesn't matter is it on 35 or 50C
crank up that bias and enjoy
I understand lack of experience , but you really need to read what people are replying
temperature equilibrium is just that - equilibrium - it doesn't matter is it on 35 or 50C
crank up that bias and enjoy
I am waiting for some components for second channel. Strange thing is that positive rail sees less current than negative rail. 3.5A vs 3.1A
I am waiting for matched irf9610 to reduce dc offset
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I am waiting for matched irf9610 to reduce dc offset
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Ah i forgot the heatsinks do still get quite warm but i plan large undervolted fans under it (20cm x2). the heatsinks weigh 15kg each 60x60x10cm
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Remove R19 (ref Aleph2 schem) and leave open. Sweet amps!! Are those wheels I see? I've used some 12V Noctua fans, seen in CPU heatsinks, undervolted by a few volts, they are silent.
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Thanks. Yes i removed r19 completely. I plan.on installing two ixys brick mosfets and.make thwm interchangeable. Yes there are wheels so i can move it around (80kg + )
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Second channel is still missing but i am also planning to add 250000uf of caps per channel to allow for higher bias
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more bias doesn't necessarily require more PS caps, however it will help lower PS ripple. Is that 6X60000uF per channel? If it is, I think you have enough.
I haven't measured with new configuration of rc. The amp is completely silent with my ear placed directly on high efficiency speakers. The caps are 50v 69,000uf
The amp is quite manageable. It will be encased in black plexy once finished
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The amp is quite manageable. It will be encased in black plexy once finished
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Out of curiosity, are you measuring amp draw with a current meter on the AC line? If you are, it should come out to 2.5A as stated in the service manual.
Why remove R19? What is the value and where is it in the circuit? What does it do?
R19 trims DC bias, it's found in the upper portion of the output stage current source. 56.2K is the value for Aleph2.
More discussion:
http://www.diyaudio.com/forums/pass-labs/5007-dc-offset-bias-setting-aleph-2-a.html
More discussion:
http://www.diyaudio.com/forums/pass-labs/5007-dc-offset-bias-setting-aleph-2-a.html
Thanks.
A friend of mine built me a pair of Aleph 2's about 6 years ago using KK boards. I had planned to have a look at their internals anyway and see what could be upgraded. This goes into the box of "must check ..."!
I measured 1.2v drop across 1ohm resistor. My transformer is boosted to 250v primary. I am using 300w of power. Strange for an aleph 2 with no 56.2k resistor!
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The 1 ohm you have in the PS? That's not how you measure the amplifier bias.
This one needs clarifcation.
I usually measure bias using the 1ohm 1% resistors in the rc network. 3.5A on the positive rail and 3.2 on the negative rail.
My transfor is deisgned for 230v but i increased mains to 250 via a step up transformer to increase voltage output. Simple.
My transfor is deisgned for 230v but i increased mains to 250 via a step up transformer to increase voltage output. Simple.
Ok, got it on the transformer front.
Bias for Aleph 2:
Looking at the lower portion of the output stage (the 6 drive outputs):
(voltage across Source resistor / Source resistor value = current in that device)
0.5V/1R=0.5A or 500mA multiply that by 6, which is 3A.(0.5*6)
I'm not quite sure what the voltage across the lower source resistors will be when R19 is removed, but it will no doubt be higher, my guess is prob 10mV or slightly higher. For ex. when I removed R19 in my Aleph5, it jumped from the standard 0.5V to around .64-.66
You don't measure amplifer bias by voltage across a PS resistor. And the positive half of a Class A amp bipolar power supply will generally have a slightly higher draw than the negative half.
Looks to me like you're right on target. Hope that helps! To the experts, correct me if I'm wrong. I just read stuff and regurgitate it.
-John
Bias for Aleph 2:
Looking at the lower portion of the output stage (the 6 drive outputs):
(voltage across Source resistor / Source resistor value = current in that device)
0.5V/1R=0.5A or 500mA multiply that by 6, which is 3A.(0.5*6)
I'm not quite sure what the voltage across the lower source resistors will be when R19 is removed, but it will no doubt be higher, my guess is prob 10mV or slightly higher. For ex. when I removed R19 in my Aleph5, it jumped from the standard 0.5V to around .64-.66
You don't measure amplifer bias by voltage across a PS resistor. And the positive half of a Class A amp bipolar power supply will generally have a slightly higher draw than the negative half.
Looks to me like you're right on target. Hope that helps! To the experts, correct me if I'm wrong. I just read stuff and regurgitate it.
-John
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