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Old 14th April 2016, 10:45 AM   #7101
xrk971 is offline xrk971  United States
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There's some good info here on advantages and disadvantages of various transistors for VAS.

http://www.diyaudio.com/forums/solid...sistors-6.html
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Old 14th April 2016, 03:49 PM   #7102
bimo is offline bimo  Indonesia
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Originally Posted by erik777 View Post
Bimo, can you clarify. Are you saying the 2SC3503/KSC3503 is a drop in replacement for the BC639/BC640, that will also perform better?

The power ratings are much different. Especially considering the FX8 max V is +/-40V.
In simulation it will perform better, but of course you should try yourself. "Perform better" is not necessary "sound better". Because people have different taste. Some people prefer high distortion
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Old 14th April 2016, 05:10 PM   #7103
AndrewT is offline AndrewT  Scotland
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Thick film SMD are metal oxide. These generally have a 300ppm/C tempco.
Metal film which are generally used for through hole have an equivalent in smd, Thin Film. These generally have a 25 to 100ppm/C
If you know your resistor has a very narrow temperature operating range from cold to fully warmed up then 25ppm/C rarely offers any benefit over 100ppm/C in audio. There are exceptions.
But 300ppm/C can introduce small errors that are audible and measureable.
An example is the Feedback resistor in the upper leg of the Amplifier NFB loop.
The feedback loop can have the peak transient voltage across it, a 100W into 8ohms amp gives peak output approaching 41Vpk to 42Vpk.
Apply this across a 20K:680r feedback loop and the instantaneous current is 2.031mApk
That gives an instantaneous power dissipation in the two resistors of 82.5mW and 2.8mW
The two resistors do not maintain the same ratio as they warm up and cool down. That is distortion of the output and it gets worse the higher the tempco.
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Old 14th April 2016, 06:07 PM   #7104
ivanlukic is offline ivanlukic  Serbia
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Originally Posted by erik777 View Post
Id be curious if you could figure out what Toshiba cob parts would perform better then the BC639/640 in there perspective positions and be a drop in replacement.
Small signal transistors in TO92 casings usually have very high ft and very low Cob. If current demand is not too high, there is very good reason to use TO92 transistors for VAS duty.
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Old 14th April 2016, 07:16 PM   #7105
bimo is offline bimo  Indonesia
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A gift from me. Not tested!
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File Type: zip FX8mod.zip (132.2 KB, 509 views)
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Old 14th April 2016, 08:04 PM   #7106
erik777 is offline erik777  United States
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Originally Posted by bimo View Post
A gift from me. Not tested!
So you picked 2SA1381 / 2SC3503. Nice PCB layout too. Don't you think Q7,Q8 should be on the edge of the PCB for a sink?

Last edited by erik777; 14th April 2016 at 08:29 PM.
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Old 14th April 2016, 10:04 PM   #7107
benz2 is offline benz2  United States
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Originally Posted by erik777 View Post
So you picked 2SA1381 / 2SC3503. Nice PCB layout too. Don't you think Q7,Q8 should be on the edge of the PCB for a sink?

There are quite a few varieties of physical layouts that will accommodate attachment of heatsink. This one appears to be the mounting of Q7 and Q8 to the bottom of the board allowing the screw to be tightened where the device peeks out of the left and right sides. This gives the devices a larger physical separation, and to my mind, better heat dissipation, at the cost of perhaps a less-convenient overall chassis design? I like this arrangement. Can't articulate it better. Might just be superstition.

I have only given this a quick glance, however... Should the heavy, flat line representing the back of the device be on the other side - i.e. are the devices rotated 180 degrees? Sorry, if I am mistaken. Alternatively, I have seen a few designs that actually run wires from the MOSFETs to the heatsink, apparently as much as 6-8 inches - not recommending it, but it's been done.

Last edited by benz2; 14th April 2016 at 10:15 PM.
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Old 15th April 2016, 12:12 AM   #7108
bimo is offline bimo  Indonesia
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Originally Posted by erik777 View Post
So you picked 2SA1381 / 2SC3503. Nice PCB layout too. Don't you think Q7,Q8 should be on the edge of the PCB for a sink?
Q7 dan Q8 mounted below PCB to heatsink
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Old 15th April 2016, 12:32 AM   #7109
bimo is offline bimo  Indonesia
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A gift from me. Not tested!
C1 and C8 = MKP.
C2, C5, C10, and C11 = Ceramic NP0 or Silver Mica.
R11 and R12 should be high quality metal film.
Q1 and Q3 should be match and tie (thermally coupled) for lower DC offset and stability from temperature.
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Old 15th April 2016, 05:52 AM   #7110
ivanlukic is offline ivanlukic  Serbia
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Q7 dan Q8 mounted below PCB to heatsink
I think that it is the most practical method for mounting of output transistors.
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