First time poster.
I bought two Australian Monitor AMISPre-1 Mic amplifiers today.
I opened them up and discovered that they use one JRC4558 Op - Amp and two A733 PNP Amplifier Transistors, which are common parts, and because the circuit is quite simple, I believe I should be able to upgrade some components in the signal path to lower-noise components.
I have a couple of OPA2134 and 2N3906, and I'm thinking about soldering in some sockets and experimenting with different opamp/transistor combinations to see if I can lower the noise floor a little.
Do you think I should focus on the active components or the capacitors in the signal path to help reduce noise?
Any advice is appreciated!
I bought two Australian Monitor AMISPre-1 Mic amplifiers today.
I opened them up and discovered that they use one JRC4558 Op - Amp and two A733 PNP Amplifier Transistors, which are common parts, and because the circuit is quite simple, I believe I should be able to upgrade some components in the signal path to lower-noise components.
I have a couple of OPA2134 and 2N3906, and I'm thinking about soldering in some sockets and experimenting with different opamp/transistor combinations to see if I can lower the noise floor a little.
Do you think I should focus on the active components or the capacitors in the signal path to help reduce noise?
Any advice is appreciated!
Noise is about voltage noise and current noise of input devices, and Johnson noise of associated resistors - without the circuit its not clear which will be most important to reduce - there's no point changing active devices if the limiting factor is a high value series resistor, for instance, and there's also no way to assess noise performance without knowing the source impedance going into the mic amplifier - though often mics are 200 or 600 ohm.
The OPA2134 is a modern fast opamp and may need better decoupling than is already in the circuit (you really don't want oscillation in your circuit), so this needs to be looked at too.
I presume you mean 2SA733 transistors? The 2S is sometimes omitted on the package for space reasons.
The OPA2134 is a modern fast opamp and may need better decoupling than is already in the circuit (you really don't want oscillation in your circuit), so this needs to be looked at too.
I presume you mean 2SA733 transistors? The 2S is sometimes omitted on the package for space reasons.
2SA733 is a very good low-hiss part. The 2N3906 "can" be good, or not; you'd have to sort many parts to find a pair similar to a pair of 2SC733s.
The JRC4558 is not a real-low hiss part and here it does not have to be because the transistors do most of the gain. The spec look bad too. Yet the '4558 is notoriously "nice sounding", many people like their sound.
At anything but full-up gain, the hiss of a mike-amp is about the resistor values more than any reasonable devices.
Is your studio really-really quiet? The quietest room I knew, an unheated stone church, still rumbled significantly.
The JRC4558 is not a real-low hiss part and here it does not have to be because the transistors do most of the gain. The spec look bad too. Yet the '4558 is notoriously "nice sounding", many people like their sound.
At anything but full-up gain, the hiss of a mike-amp is about the resistor values more than any reasonable devices.
Is your studio really-really quiet? The quietest room I knew, an unheated stone church, still rumbled significantly.
Thanks for all your advice! I tried OPA2134 with decoupling capacitors yesterday and I'm pleased with the outcome. 😊
Here is the best DIY microphone preamp---pretty inexpensive and will perform as well or better than any commercial device:
https://sound-au.com/project66.htm
https://sound-au.com/project66.htm
Well you can change it to use ZTX951 / ZXTP2012's as input devices if you want even lower noise. Might want to increase the standing current to 10mA as well. Linearity can be increased too by using balanced feedback topology. Then it might be up there with the decent professional preamps. The circuit is lacking protection diodes across the EB junctions of the input devices - essential for low-noise circuits.
BTW the ZXTP2012 is the SMT version of the ZTX951 (it even is marked "951" on the package).
Ditto ZXTP2018 for the ZTX851 NPN. Both are 0.25nV/√Hz or so.