Quad Eight AM-10 DOA. Why no potting or thermal coupling?

[Helsing]

Can anyone explain?

Helsing

 

[Bo Hansen]

Hi Helsing,

It is no need to epoxy potting a discrete build op-amp to get acceptable thermal coupling.

There are some important thing to do insted for a potting:

Be sure that you place the two bias diodes CR6 and CR7 that set the quiescent current in the output transistors near the output transistor, or better, place one diode on each output transistor with a small dot of heat sink silicone compound, and if there are more than 10 mA quiescent current and you use TO-39 output transistors, it can be good to have a small heat sink on each transistor.

If you want better dc offset stability, use a matched transistor pair for the input transistors Q2 and Q4 and place they together.

--Bo

 

[Helsing]

What should the input pair be matched for? HFE?

Helsing

 

[Bo Hansen]

Yes, the hFE/current gain match.

You can also use a dual NPN transistor as Analog Devises MAT-01 and there are also some nice Japanese types as 2SC-3381 that is a lot inexpensive then the MAT-01 (or MAT-02/LM-394)

--Bo

 

[Helsing]

Bo,

Whats a good cross for the 2n4249, 2n2102 and 2n4036?


Helsing

 

[Bo Hansen]

For your experiment/prototype you can use BC550C for all NPN and BC560C for all PNP small signal TO-92 transistors and BD139 for the NPN and BD140 for the PNP output transistors or BC141-16 NPN and BC161-16 PNP if you want TO-39 case (maybee with a small heat sink)

--Bo

 

[Helsing]

Thanks Bo. I'll give those a try.

Helsing