Controlling base current for Darlington

I posted on this Hitachi HA-4700 prev. on AAC:
https://forum.allaboutcircuits.com/...linear-ckt-design-please-help-explain.210716/
Serv. manual is here:
https://archive.org/details/manual_HA4700_HITACHI/page/n1/mode/2up

To cut to the chase, the LEFT CHANNEL seems to be acting (biasing) fine -- slowly ramping up to 7.7mV per serv manual, and holding there when warmed up-- despite , me having substituted in power transistors [Sanken 2SB1570/2SD2401 Darlingtons] that are NOT ideal for purpose. (That's all I have in my parts bin. Original replacements parts are difficult and $$ to acquire ).

Orig. Fujitsu 2SA1075/2SC1075 (both are BJT, non-Darlington )
My replacements for both L/R channels (currently installed): Sanken 2SB1570/2SD2401

I am not sure why the LEFT CHANNEL works (with decent sine and square waves), topping power at almost 80W!!

The RIGHT CHANNEL is not okay. Can't achieve bias above 0.0mV. Not sure why.
One reason for RIGHT CHANNEL issues might be the non-ideal choice of output trans. devices.

Note that the Sanken Darlingtons have DC current gain (hFE) much higher than the 2SA1075/2SC1075. I seem to have been able to control that with the orig. circuit, using traditional bias trim pots.
But I'm not sure why L channel is working.


============

ChatGPT reply to comparison query, compare "2SB1570 and 2SA1075":

The 2SB1570 and 2SA1075 are both PNP power transistors, rated for high current, but they have a crucial difference: the 2SB1570 is a Darlington transistor, while the 2SA1075 is a conventional single power transistor. That difference affects drive current, voltage drop, switching behavior, and whether they can be substituted for each other.



1. Specifications compared
Sources: Sanken 2SB1570 specifications and 2SA1075 datasheets. Specifications may vary by manufacturer and test conditions.


2. The key electrical differences


2SB1570 — high gain, low drive current

• Extremely high gain means the driver circuit needs very little base current.
• Its Darlington structure produces a higher base-emitter voltage drop, typically around two junction drops.
• Can be useful in power stages designed specifically for Darlington output transistors.

The Sanken version is listed as obsolete.




2SA1075 — conventional transistor, faster drive response

• Requires more base current than the Darlington.
• Has a lower base-emitter voltage drop, typically around one junction drop.
• Designed for high-speed power switching and amplifier applications.

The Fujitsu version is described as a high-speed ring-emitter power transistor.
fujitsu semiconductor


3. Can you replace one with the other?
Generally, no—not as a direct replacement.
Even though both are PNP devices rated at 12 A, their gain and base-drive requirements are substantially different. Replacing a 2SB1570 with a 2SA1075 could leave the transistor under-driven; replacing a 2SA1075 with a 2SB1570 could change bias, voltage headroom, and switching behavior. The pinout and safe operating area must also be checked.
For an audio amplifier, the difference in base-emitter voltage is particularly important because it can affect bias and crossover distortion.
4. Which one is better?

• For a circuit designed around a Darlington: 2SB1570.
• For a circuit requiring a conventional, fast power transistor: 2SA1075.
• For a repair or substitution: Choose based on the original circuit design, not just the maximum current and power ratings.

What are you using the transistor for? An audio amplifier, power supply, or another circuit? If you tell me the equipment model or the original transistor you are replacing, I can assess whether either part is a suitable substitute.
The 2SB1570 and 2SA1075 are both PNP power transistors, rated for high current, but they have a crucial difference: the 2SB1570 is a Darlington transistor, while the 2SA1075 is a conventional single power transistor. That difference affects drive current, voltage drop, switching behavior, and whether they can be substituted for each other.
Sanken Electric



1. Specifications compared

Sources: Sanken 2SB1570 specifications and 2SA1075 datasheets. Specifications may vary by manufacturer and test conditions.
Sanken Electric
+2

2. The key electrical differences



2SB1570 — high gain, low drive current

• Extremely high gain means the driver circuit needs very little base current.
• Its Darlington structure produces a higher base-emitter voltage drop, typically around two junction drops.
• Can be useful in power stages designed specifically for Darlington output transistors.

The Sanken version is listed as obsolete.
Sanken Electric
+1



2SA1075 — conventional transistor, faster drive response

• Requires more base current than the Darlington.
• Has a lower base-emitter voltage drop, typically around one junction drop.
• Designed for high-speed power switching and amplifier applications.

The Fujitsu version is described as a high-speed ring-emitter power transistor.
fujitsu semiconductor
+1

3. Can you replace one with the other?

Generally, no—not as a direct replacement.
Even though both are PNP devices rated at 12 A, their gain and base-drive requirements are substantially different. Replacing a 2SB1570 with a 2SA1075 could leave the transistor under-driven; replacing a 2SA1075 with a 2SB1570 could change bias, voltage headroom, and switching behavior. The pinout and safe operating area must also be checked.
For an audio amplifier, the difference in base-emitter voltage is particularly important because it can affect bias and crossover distortion.
4. Which one is better?

• For a circuit designed around a Darlington: 2SB1570.
• For a circuit requiring a conventional, fast power transistor: 2SA1075.
• For a repair or substitution: Choose based on the original circuit design, not just the maximum current and power ratings.

What are you using the transistor for? An audio amplifier, power supply, or another circuit? If you tell me the equipment model or the original transistor you are replacing, I can assess whether either part is a suitable substitute.