Need help with PTT audio amp circuit

[Besto]

This is acircuit that i reverse engineered from a ptt mic preamp circuit. Can someone explain the circuit.
InA and InB is the input .

 

[barry]

Which part don't you understand?

It basically looks like a differential input is amplified by two transistors and converted to a single-ended signal by the first opamp and then further amplified by the second opamp. (The second opamp circuit looks a little weird to me.)

There appears to be information missing. For example, there's a signal U191_3 that only appears in one spot.

 

[Besto]

Which part don't you understand?

It basically looks like a differential input is amplified by two transistors and converted to a single-ended signal by the first opamp and then further amplified by the second opamp. (The second opamp circuit looks a little weird to me.)

There appears to be information missing. For example, there's a signal U191_3 that only appears in one spot.

Thank you for the information. U191_3 is connecting to a different circuit. Is there any advantage to using this differential amplifier rather than an opamp?

This is a better view of circuit.

 

[D.A.(Tony)Stewart]

There are a lot of RC components in the front end of this push-to-talk preamp to suppress common mode (line) hum, differential RF (AM), low DC gain with stabilized NFB, HPF/LPF. These features are not part of an Op Amp.

The transistor is unique to high Beta and low noise carefully selected for this application somewhat better than the gold-doped 2N5088.

 

[Besto]

LM 13700 is also connecting to the second opamp. I didn't understand that section.
Also, is this a good preamp circuit to follow?

 

[D.A.(Tony)Stewart]

The LM13700 OTA was a Harris improvement on the RCA CA3080 from the 60's using linearizing diodes in the front end and a better current mirror with extra transistors. RCA also release a version CA13700, then both became National as LM..., now TI.

With careful design this was known to give good low noise/distortion results.

It depends what specs you want to define best. The best simply meet your specs.

Grok suggests this... FWIW

Comparison with Other Designs

While the LM13700 is not as commonly used in modern low-noise preamps as discrete JFETs or specialized op-amps (e.g., OPA1656, as seen in OPA1656: Optimizing for low noise in a 4 stage guitar preamp), its historical use in audio circuits and the ability to implement variable gain make it viable. Other designs, such as those using AD797 or BF862 (e.g., Low Noise Design Schematics), achieve lower noise (e.g., 0.35nV/√Hz), but the LM13700 offers unique flexibility for gain control, which is valuable in mic preamps.

If you need a the much better design, consider this.

HPS 3.0 Schematic

Low Noise Designs

www.synaesthesia.ca

But HPS 4.1 is apparently even better.

But you ought to choose the best mic for your application too
and then decide on specs vs budget

Make or Buy with a budget.

from $10 to $100 to $1000

https://www.ti.com/lit/ds/symlink/ne5532a.pdf

$3 no specs or supply.

New or used or mystery mass production or DIY disaster

Wired or wireless