INA134 instead of input transformer????

Anyone ever tried using the INA134 instead of mic input transformer on any pre amps. I want to build multiple channels of an API 312 like circiut. I am thinking about using two with transfos and two with the INA134. Will this work. I am short of cash right now and can't build 4 transformer circuits.

[quote author="jeroddumas"] Will this work.[/quote]
Not the way you desire, perhaps. At least use an opamp specifically for mic pres like an INA217 or an SSM2019. You could fix it at 20dB gain, add phantom blocking capacitors and slap it in front of the rest.

> INA134 instead of mic input transformer

Noise voltage is 7uV; LD condenser about 1uV, dynamic about 0.2uV, good transformer-input mike-amp about 0.1uV-0.4uV. INA134 will be obviously noisy compared to common mikes and mike-amps.

If they brought out the internal nodes you could do a bit better... but they don't.

There ARE decent mike-input chips. Of course they can replace your whole 312. Using them at very low gain, as tk suggests, will work, though it seems like a waste.

You may get usable results if you go through your pile of 240VAC:24VAC power transformers, used backward. Split-bobbin transformers will lack treble, and most power transformers will pick up hum very well, but I've done it.

2-transistor 1-buck mike transformer replacement:


With the 56Ω resistor, gain is 10 (+20dB). With good generic Switch transistors, noise will be usably low. Break one leg of the 56Ω resistor, gain is 1 (0dB), but noise will be higher than most dynamics (lower than many LD condensers, which is what you'd use the low-gain setting for anyway). THD is very low at 100mV output, close to 2% nearly-pure Third at 1V output, so keep some gain in the 312 stage. CMRR is abysmal, about 12dB: this won't work in high-EMI rooms (though I would expect it to be OK in any room where a Fender can be used). No values are critical, use anything close, though it would be nice to match same-value pairs to 5%. The 47u caps need to be 63V; the 10u can be 25V.

Thanks PRR, I may just build this up for the hell of it. Should work fine at +/-18v right? :twisted: Now I can get more channels of API like pres. :twisted: :twisted: :twisted: :twisted: