25W Class A amp with Lateral MOSFETs

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#301
Another query regarding the power transformer. Post #180 of Juma says 40vdc so backward calculating makes it 35vac secondary winding trafo but what amperes do you guys suggest. If using a single transformer to power both the channels should I get something like 0-35v @6A *2 dual secondary rated around 400VA or is it more if using with 8ohm speakers?

Thanks
 
#302
manniraj said:
Another query regarding the power transformer. Post #180 of Juma says 40vdc so backward calculating makes it 35vac secondary winding trafo but what amperes do you guys suggest. If using a single transformer to power both the channels should I get something like 0-35v @6A *2 dual secondary rated around 400VA or is it more if using with 8ohm speakers?

Thanks
Click to expand...
I got the calculation wrong here and it should be like 0-28vac 5A dual secondary to make it around 300VA for both the channels. I hope this is more than sufficient for optimal bias. By the way how is the bias done as I do not see any biasing resistor, so can I use like 1R 3-5w resistor in series with the supply voltage and check the current flow in mV across this resistor? How much should I be aiming for?

Thanks
 
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#304
manniraj said:
I got the calculation wrong here and it should be like 0-28vac 5A dual secondary to make it around 300VA for both the channels. I hope this is more than sufficient for optimal bias. By the way how is the bias done as I do not see any biasing resistor, so can I use like 1R 3-5w resistor in series with the supply voltage and check the current flow in mV across this resistor? How much should I be aiming for?

Thanks
Click to expand...
Amperes you can back out from the impedance of your speakers. I'm OK with 4.5A per channel @ 40V.
The voltage over D1 controls the bias. In my build I wanted something I could regulate and used a TL431 instead of a blue LED.
I use 700mA bias mostly b/c this gives the maximum temperature I am happy with on the heat sink.
You can use different bias current values and see how you like it. I found that at somewhat valiant listening volumes more is better maybe up to 500mA. Beyond that I didn't feel more makes a difference. I gave it 700mA because I can.
 
#308
manniraj said:
Another query regarding the power transformer. Post #180 of Juma says 40vdc so backward calculating makes it 35vac secondary winding trafo but what amperes do you guys suggest. If using a single transformer to power both the channels should I get something like 0-35v @6A *2 dual secondary rated around 400VA or is it more if using with 8ohm speakers?

Thanks
Click to expand...
My thumbnail calculation is the DC will be 1.3 times the AC voltage and you want the power rating of the transformer to be something like twice the idle draw of the circuit.
 
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#311
Nelson Pass said:
My thumbnail calculation is the DC will be 1.3 times the AC voltage and you want the power rating of the transformer to be something like twice the idle draw of the circuit.
Click to expand...
if we are using a SMPS power supply like laptop brick power supply, what rating do we need to use when compared with idle draw of the circuit ? how much head room do we need to have for the SMPS power supply.
 
#312
Any issue if I replace the BC560c in this class A 25w circuit with BC557c or 556c? I have few pieces of 557/556 NOS rather than 560 sometimes coming as rejected/fake ones when I tested them in the Salas Ultra BiB circuit. Hence I hope using the replacements should not change anything.

Thanks
 
#313
manniraj said:
Any issue if I replace the BC560c in this class A 25w circuit with BC557c or 556c? I have few pieces of 557/556 NOS rather than 560 sometimes coming as rejected/fake ones when I tested them in the Salas Ultra BiB circuit. Hence I hope using the replacements should not change anything.

Thanks
Click to expand...
AFAIK the only difference in spec is the noise figure (and max Vcb)
 
#316
With my -30vdc rails on the Juma board and I used pair of red leds after checking/matching to get close to 3.53v using my LCR meter I powered up one channel. Bulb tester passed as no current is being passed. Now without bulb tester none of the red leds glow and I see the voltage of 30v across the source (S) of Q2 (J310) against the ground and almost the same voltage across the source (S) of Q3 (BC560c). Offset is also around ~29.8v with the trimmer not helping to reduce or increase any of these voltages. Any idea what I am missing as the schematic says to set -20v across the Sources of Q2/Q3 using the trimmer (I presume that is half the supply voltage and in my case its 30vdc so I need to set to -15v).

Thanks
 
#317
Some pics of my build so far:
021196E5-2DDE-411D-A2B0-F087473E0D7C.JPG


1646457944330.jpeg


1646457976647.jpeg
 
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#318
I am going to replace the red leds with the Green ones having forward voltage of 3.5v and the led2 with jumper. Hope that is the issue and also suspecting the BC560c from a local source. Plan to source few pieces from genuine lets see if I am successful.

Thanks
 
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