Upper Limits of Biasing 2SC2922 2SA1216

[DreadPirate]

How high can I go? I've found best sound at 20mA. Is there an easy way to determine biasing limits from the spec sheets or is there alot more to it?

[AndrewT]

look at Sanken datasheet.
fT is max @ 2A to 5A
hFE is max @ 500mA to 1500mA
These are clues that this device operates at it's optimum when currents are high.

In a ClassAB output stage the output device bias current is normally set to minimised crossover distortion. This bias setting is referred as optimal ClassAB bias. The optimum bias is when the total output Re times the bias currents generates a voltage of 26mV @ Tj=25degC.
The internal Re is quite small and for larger values of external Re is often ignored.
setting bias to 25mVre is common, particularly if the external Re=0r47.
If external Re=0r1 then Vre might be around 18 to 20mVre.

Taking those recommendations and creating a table similar to what D.Self shows in his papers/book gives the following:
Re=0r56 Vre=25mVre Ib = 45mA
Re=0r47 Vre=25mVre Ib = 53mA
Re=0r33 Vre=23mVre Ib = 70mA
Re=0r22 Vre=22mVre Ib = 100mA
Re=0r15 Vre=20mVre Ib = 133mA
Re=0r1 . Vre=18mVre Ib = 180mA

None of these optimal bias currents gets as low as 20mA.

But, take care with device Dissipation (Pq) and Thermal Stability.
High Vcc does not go well with low Re (see Cordell)
High bias current does not go well with small heatsinks, nor with high Vcc.

[lineup]

I would go for 0R22
At for example 3 Ampere peak output, this would create a
voltage drop of 3 x 0.22 = 0.66 Volt across emitter resistor.

For a 0R47 resistor drop would be 1.41V at peak current 3A

*** 3 Ampere peak output corresponds to 36 Watt RMS into 8 Ohm

[DreadPirate]

Well the amp came with a 20mA biasing and I increased it to 30mA and found the results near unlistenable. I did not put a scope to it, but did not have to.

[lineup]

40-50 mA is what is used often for power amplifiers BJT

[AndrewT]

Originally Posted by DreadPirate

the amp came with a 20mA biasing and I increased it to 30mA and found the results near unlistenable.

what output stage is fitted?

[DreadPirate]

Darn, come to think of it, its 20mv at the test points, I'll have to check the emitter resistor values. Its a Class AB amp with 2 pairs of 2SC2922/2SA1216 per side.

[DreadPirate]

Ok, the emitter resistors are .33ohm and 20mv measured across them, so 60mA or so bias current. I'm getting a 170 F temperature of the heat sinks. I took the bias current to about 90mA with obvious sound degeneration. So, are my heatsinks possibly too small? They got significantly hotter. For the four devices per side, I have an 8" x 2.5" extruded aluminum heat sink w/ 3 fins/inch.

[AndrewT]

90mA requiring Vre (external) of 29.7mVre is far too high.

The 20mVre seems much more likely as the optimum ClassAB bias. This will take account of 6mV (total = 26mVre) dropped across the internal RE of the Sankens which do have a higher value than some due to their construction.