I have an ongoing project to build an induction loop to link to the telecoil receivers in my hearing aids. As background I was curious to know how a loop amp can efficiently drive up to 10A peak round a loop with resistacce of only 0.2 - 2 ohms. So I tried tracing the circuit of our disused PDA500 loop amp. From a visual inspectio, it cotains a LM1875 power amp and 2 complementary BJTs BD911 and BD912, presumably to boost the output current drive capability. Tracing the cirtcuit, and after double-checking, this is what I got:

Clearly, this is going to instantly blow both BJTs then blow the (slo-blo) fuses! Very strange also that the LM1875 supply appears to be fed from the BJT bases. Continuity checks show that the collectors are indeed tied together and tied to the LM1875 output, that the emitters are connected to the positive and negative supplies and that the bases are connected to the LM1875 supply pins. The amplifier worked immediately before decommissioning, though with a bit of noise and distortion on the output, possibly due to an aging LM1875.

Something must be wrong. But in my experience, if double-checking doesn't reveal your mistake, triple checking rarely does either. But 3rd party checking often spots the mistake instantly. Who'll be my 3rd party?

 

I'd guess you are missing the resistors from base to emitter for each of the output transistors. The voltage drop over these resistors from the power supply current for the LM1875 drives the output transistors.

 

Yes, that would make a workable amp, but I think not with the 1.5Ω ones form the collector to base. Perhaps those resistors are more like 150 or 1.5K and connect to the two rails?

 

Thank you, yes, the 1.5Ω resistors are between the base and emitter, not base and collector. That makes much more sense. Even so, a curious way of using a LM1875, not envisaged by its designers, I'm sure!

 

I've seen it done with opamps. While the LM1875 is rated as 4A, when it gets to about 1/2 amp the output transistors will start to turn on. They are rated for 15A.

 

Thank you, yes, the 1.5Ω resistors are between the base and emitter, not base and collector. That makes much more sense. Even so, a curious way of using a LM1875, not envisaged by its designers, I'm sure!

Normally that configuration is a cascode connection. I'm pretty sure the designers were hip to it.
https://www.tubecad.com/2023/01/blog0575.htm

 

I had no idea that opamp could output 4A. Now the 1.5R makes sense.

 

The LM1875 is a power amplifier with an output of 20W into 4 ohms.
Here is a similar power amplifier (TDA2030A) using booster transistors:

 

I moved the B-E 1.5 resistors.
When the current is low enough that the OP-amp can drive it, current flows from V+ to the amp and out to the load. (transistors are off)
When the current is high enough that we get 0.6V across the 1.5 resistor the top transistor turns on and lifts the load.

 

So, this configuration works on the "pass transistor" principal seen in series pass 3 terminal regulators, rather than the process of driving the bases with the output of the amp?

 

I have seen a similar circuit in a copy of "The Radio Amateur's Handbook", which very seldom has errors. But their version had the the resistor values a bit larger, and it used several 1K resistors for establishing the output at a middle voltage.
Magnetic loop coupling to hearing aids is interesting, but it seems that not all of them include it. They can charge much more for Bluetooth access although it is cheaper to add.