vdi_nenna,
The way I see it, R15, just controls the voltage/current down the power resistors. You can change it. R8 and R15 form a voltage divider, and it is approximately the case that the voltage drop across the power resistors is:
V = 0.65 (R + 1) / R -- 0.65 is BJT vbe drop
where R in kOhm, V is the voltage from top of R1 to bottom of R3.
The output bias current down the power resistors is:
I = V / 0.575 -- 0.575ohm is combined power resistor value
Power dissipation = I * Vss, Vss is typically 19V supply voltage.
With no mod R15 = inf, V = 0.65, I = 1.13A, power dissipation = 21.5W.
With R15 = 2.21K, V = 0.94V, I = 1.64A, power dissipation = 31.2W.
With, R15 = 2.67K, V = 0.89V, I = 1.55A, power dissipation = 29.5W.
With, R15 = 2.67K, your amp will get hotter than stock, but not as hot as with R15 as suggested value. You get nearer to the bias sweet spot for good sound, but not quite on it. Please anyone, correct me if I'm wrong!
The way I see it, R15, just controls the voltage/current down the power resistors. You can change it. R8 and R15 form a voltage divider, and it is approximately the case that the voltage drop across the power resistors is:
V = 0.65 (R + 1) / R -- 0.65 is BJT vbe drop
where R in kOhm, V is the voltage from top of R1 to bottom of R3.
The output bias current down the power resistors is:
I = V / 0.575 -- 0.575ohm is combined power resistor value
Power dissipation = I * Vss, Vss is typically 19V supply voltage.
With no mod R15 = inf, V = 0.65, I = 1.13A, power dissipation = 21.5W.
With R15 = 2.21K, V = 0.94V, I = 1.64A, power dissipation = 31.2W.
With, R15 = 2.67K, V = 0.89V, I = 1.55A, power dissipation = 29.5W.
With, R15 = 2.67K, your amp will get hotter than stock, but not as hot as with R15 as suggested value. You get nearer to the bias sweet spot for good sound, but not quite on it. Please anyone, correct me if I'm wrong!
I've ordered a couple more of kit only ACAs. 6 channel ACA 
I'm now pondering cases and heatsinks.
I have a couple of recycled heat sinks - Mac I think. They are 135mm X 100mm. The fins are 70mm tall starting at 3mm at the base rising to 1mm. The base is mostly 7mm think but has a large section of 14mm thick. It's about 80mm x 90mm. I think where it contacted the processors.
This calculator (Thermal Resistance Calculator for Plate Fin Heat Sink) gives around 0.24 °C/W but I'm going to use fan cooling. This brings it down to about 0.11 °C/W which is good for 2 ACA boards.
My question is can I safely mount two ACA boards to the 90x80 14mm thick section? Are the output devices able to be in that close proximity even if the heatsink is adequate?
I'm now pondering cases and heatsinks.I have a couple of recycled heat sinks - Mac I think. They are 135mm X 100mm. The fins are 70mm tall starting at 3mm at the base rising to 1mm. The base is mostly 7mm think but has a large section of 14mm thick. It's about 80mm x 90mm. I think where it contacted the processors.
This calculator (Thermal Resistance Calculator for Plate Fin Heat Sink) gives around 0.24 °C/W but I'm going to use fan cooling. This brings it down to about 0.11 °C/W which is good for 2 ACA boards.
My question is can I safely mount two ACA boards to the 90x80 14mm thick section? Are the output devices able to be in that close proximity even if the heatsink is adequate?
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I got the 24v power supplies today all wired up bias at 12.5V stable and fired them up, it is different i like the sound even more, just more capable without changing the character
hardest part as ever is trying to move that pot....just a little bit just a little bit now playing music again
heat wise i think im ok just, hands on heatsinks 6 seconds.......might get some fans
hardest part as ever is trying to move that pot....just a little bit just a little bit now playing music again
heat wise i think im ok just, hands on heatsinks 6 seconds.......might get some fans
I use the largest PC fans I can get which are 20cm/8inch. They shift 105cfm at 900rpm and are near silent at that speed. With a variable voltage wallwart they'll spin up at 3-5v very slowly and gently move the air across the entire heatsink in TOTAL silence
Can't wait to be 6xACA @24v!!!!
Can't wait to be 6xACA @24v!!!!
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I use the largest PC fans I can get which are 20cm/8inch. They shift 105cfm at 900rpm and are near silent at that speed. With a variable voltage wallwart they'll spin up at 3-5v very slowly and gently move the air across the entire heatsink in TOTAL silence
Can't wait to be 6xACA @24v!!!!
where did you get them? amp on all night so far and it is hot!
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Yes, 1/4 W resistors will work fine there on a ACA. R1 thru R4 need to be 3 W, of course.
Thanks Bangla!
The size of MPC71/74 looks ideal for this. Just wonder what's the sonic differences between metal plate resistor (MPC71/74) and the original metal oxide film resistor (Panasonic/Mundorf's).
Best.
The size of MPC71/74 looks ideal for this. Just wonder what's the sonic differences between metal plate resistor (MPC71/74) and the original metal oxide film resistor (Panasonic/Mundorf's).
Best.
MPC74 has non magnetic material
Both have low inductive resistance because inner construction, better explained
on datasheet.
http://www.farnell.com/datasheets/72873.pdf
The MPC74 is a bit better than. I only could get 0,47R here.
Cheers
Both have low inductive resistance because inner construction, better explained
on datasheet.
http://www.farnell.com/datasheets/72873.pdf
The MPC74 is a bit better than. I only could get 0,47R here.
Cheers