Discrete Opamp Open Design

[grhughes]

Scott: Please forgive my non familiarity with electronics and audio design but if TO126 or TO220 (mje171/181 or mje243/253 output devices are used then how much heat sinking is required? How close to Class A can it be biased? Can this signal dependent bias be pushed into class A? What do you think the quiescent or collector current in the output devices would amount to? Thanks! Ray

[scott wurcer]

Originally Posted by janneman

Scott I assume you meant the 2nd definition re: LTspice?

tr.v. en·joined, en·join·ing, en·joins
1. To direct or impose with authority and emphasis.
2. To prohibit or forbid.

jan

Yes, I usually use that definition.

[bcarso]

Originally Posted by grhughes

Scott: Please forgive my non familiarity with electronics and audio design but if TO126 or TO220 (mje171/181 or mje243/253 output devices are used then how much heat sinking is required? How close to Class A can it be biased? Can this signal dependent bias be pushed into class A? What do you think the quiescent or collector current in the output devices would amount to? Thanks! Ray

Most device datasheets will have a ambient temp and a case temp spec on power dissipation. Or, at least a thermal resistance spec for chip to case and chip to ambient. With the maximum operating chip temp also given, one can then calculate the permissible dissipation.

The circuits published (for example 1568) only have a weak signal-dependent bias, as Scott commented. The bias for the output stage current sources is just convenient, not essential to the circuit operation.

[scott wurcer]

Originally Posted by grhughes

Scott: Please forgive my non familiarity with electronics and audio design but if TO126 or TO220 (mje171/181 or mje243/253 output devices are used then how much heat sinking is required? How close to Class A can it be biased? Can this signal dependent bias be pushed into class A? What do you think the quiescent or collector current in the output devices would amount to? Thanks! Ray

I left the bias values at the ones in the JE990, there are several descriptions floating around describing the thermal issues for the module. If you want very low impedance drive at higher power I would put the bias diodes and output devices on a piece of metal either case or heat-sink.

I just noticed that I left off the output bias trim and instead matched the Is of the diodes to the outputs.

[grhughes]

I run a small recording studio and do location recording of bands. I need 16 stereo headphone amplifiers (32 channels) for performer reinforcement. NO BATTERIES! They have to drive mostly Sennheiser HD380Pro ear buds (54 ohms). If the output stage can't handle that I'm not interested. Thanks, Ray

[scott wurcer]

Originally Posted by grhughes

I run a small recording studio and do location recording of bands. I need 16 stereo headphone amplifiers (32 channels) for performer reinforcement. NO BATTERIES! They have to drive mostly Sennheiser HD380Pro ear buds (54 ohms). If the output stage can't handle that I'm not interested. Thanks, Ray

Headphone driver no problem I assume you are not putting 10V rms on earbuds. I suspect this circuit is also fine in Dick's 1V rms on 30 requirement. With real musical crest factors the average power dissipated while well blasting your ears is fairly small.

[stinius]

Soon ready to do the PCB layout.
Attached: Schematic for PCB, simulated THD and PSRR (pos and neg).
The target is:
8 pin DIL. Pin compatible with commersial opamps.

S

[scott wurcer]

Originally Posted by stinius

Soon ready to do the PCB layout.
Attached: Schematic for PCB, simulated THD and PSRR (pos and neg).
The target is:
8 pin DIL. Pin compatible with commersial opamps.

S

Looks good, all SMT right? How small can can you get it?

EDIT - Those complimentary pairs are a great idea, and I love those prices. How low do you think the BOM could get?

[bcarso]

Originally Posted by stinius

Soon ready to do the PCB layout.
Attached: Schematic for PCB, simulated THD and PSRR (pos and neg).
The target is:
8 pin DIL. Pin compatible with commersial opamps.

S

Very nice!

[Waly]

Originally Posted by stinius

Soon ready to do the PCB layout.
Attached: Schematic for PCB, simulated THD and PSRR (pos and neg).
The target is:
8 pin DIL. Pin compatible with commersial opamps.

S

Careful. BF545 has a minimum pinch voltage of -0.4V. If by chance you run into the worst case combination of BF862 and BF545, the BF862 may not saturate (but stay in the linear region).

Better find a n jfet with higher pinch voltage, like this. Only 25V Vds but dual, and -2V minimum pinch voltage.