I have learned the hard way that speaker protection is necessary. Very much so!
I was away for the weekend, and forgot to turn off my First Watt F5. The system somehow developed an issue where the amp would output full positive DC rail voltage (around 24 V) to the speakers. Needless to say the woofers were toast. Worse thing was the terrible smell of burnt electronics in the house, and I am still glad there was no fire.
I added a speaker protection to this (and every other) amp since then. Burning down the house is just not an option.
@codyt,
I just ordered the gerber files you shared. ^^
BTW, I have a few questions.
In this work, you used IRFP240, which is different from before. Is there a special reason why you changed it from 150 to 240?
And is the optimal value of the 2K trim pot, called 'Aleph Gain', just the point that sounds the most pleasant to the ear? Or is there another standard?
This is the first time I've seen a trimpot that can be turned by hand. Could you please share the part name?
Thank you
I believe I used the IRFP240 because NP had recently (at that time) used that part in the top section of his F8, so I felt comfortable using it too. That said, I've used both parts and don't have a preference. The 'Aleph Gain' pot value of 2K is meant to cover the original Aleph J value of 1K2. You can run into stability issues if you run that value too low (say below 800R). If I were doing this project over again I'd probably put 750R in series and make the pot 750R. You can probably figure out how to modify the board to make that happen, or just be attentive when adjusting that pot. I would set it at 1K2 initially, then experiment with a turn or two in each direction once everything else is dialed in. I always made that adjustment realtime while measuring the distortion, and I always preferred it very close to the stock 1K2. Of course, that's the other option – just use a resistor instead of the pot.
P1 is most likely Bourns 3362.
P1 is most likely Bourns 3362.
@codyt
I have a few questions.
Is the source resistor value you have set last 0.1 at the top and 0.0 at the bottom?
I read somewhere that the results are good when the current contribution of the top and bottom are 50% each. The current of the top can be obtained by measuring the voltage value between source resistor, but in the case of the bottom, how can the current value be obtained when the source resistor is removed and shorted?
When setting the initial bias, I should start with a small bias value and gradually increase it.
If I want to set the initial bias value to a small value, should I set the resistance value of P3 as small as possible? (Set the 2nd pin of the trimpot as close to the 3rd pin as possible)
Finally, if I apply ground lift, should I mount the related components on the back, cut the lines marked with cut, and connect the jumper?
If I reverse the ground lift application, should I just remove the mounted components and short R19 (HBR)?
If I apply ground lift after the components are finally mounted, it seems like PCB cutting will not be easy, so I plan to start by doing this first. (cut marking cutting, jumper connection, R19 short)
Thanks You!
I have a few questions.
Is the source resistor value you have set last 0.1 at the top and 0.0 at the bottom?
I read somewhere that the results are good when the current contribution of the top and bottom are 50% each. The current of the top can be obtained by measuring the voltage value between source resistor, but in the case of the bottom, how can the current value be obtained when the source resistor is removed and shorted?
When setting the initial bias, I should start with a small bias value and gradually increase it.
If I want to set the initial bias value to a small value, should I set the resistance value of P3 as small as possible? (Set the 2nd pin of the trimpot as close to the 3rd pin as possible)
Finally, if I apply ground lift, should I mount the related components on the back, cut the lines marked with cut, and connect the jumper?
If I reverse the ground lift application, should I just remove the mounted components and short R19 (HBR)?
If I apply ground lift after the components are finally mounted, it seems like PCB cutting will not be easy, so I plan to start by doing this first. (cut marking cutting, jumper connection, R19 short)
Thanks You!
Start wherever you want regarding source resistors. I would always consider Nelson's choice for the Aleph J an ideal starting point. Somewhere in this post I uploaded a folder of 20-30 distortion measurements at different source resistor settings. The Iq is the Iq, so if you can measure it on the top across that 0.1 resistor, you're good.
Yes, start bias low and work higher. I believe decreasing resistance increases bias, but I'm not confident. Biasing is all stock Aleph J, so you may want to reference other Aleph J posts and check against them. I know the I usually ended up around 65K or so.
Yes, your understanding of the ground lift is correct. I've built a couple of these and never needed to use the ground lift.
Hope that helps!
Yes, start bias low and work higher. I believe decreasing resistance increases bias, but I'm not confident. Biasing is all stock Aleph J, so you may want to reference other Aleph J posts and check against them. I know the I usually ended up around 65K or so.
Yes, your understanding of the ground lift is correct. I've built a couple of these and never needed to use the ground lift.
Hope that helps!