Seeking Advice: Best Preamp Design for Low-Noise, Balanced Microphones

[Voyager10]

Myself and several others don't see any value in keeping the NJM chip, it doesn't provide any useful control over gain.

 

[bigben83]

Myself and several others don't see any value in keeping the NJM chip, it doesn't provide any useful control over gain.

Yes I get that impression, but the availablity of the THAT CORP IC's is pretty scarce currently.

 

[ricardo]

Either the THAT or TI solutions would give you what you want with suitable Arduino control juju.

PGA2500 and the THAT solutions are SOTA for mike preamp performance (if you exclude some Earthworks designs under special circumstances)

PGA2505 isn't quite as good as PGA2500 & the THATs but the difference is moot for most people.

I wouldn't let the low voltage output of the TI devices put you off. VERY good soundcards have been made with PGA2500. If you want +20dBu output, use the THATs or have a 10dB gain OPA on +/- 15V rails after the PGA2500

The NJM chip is a Volume Control. Not suitable for Preamp GAIN which is what you want for SOTA performance.

If you use PGA2500, there are a few tricks from the THAT datasheets that improve performance and, more importantly, long term reliability.

 

[DaveDC]

Myself and several others don't see any value in keeping the NJM chip, it doesn't provide any useful control over gain.

+1 110%

Although they may seem similar in some respects, a variable-gain preamplifier is not at all the same thing as a fixed-gain preamp followed by a volume control. The former is used to match levels from a mic of unknown sensitivity exposed to an acoustic source of unknown SPL to a device of known sensitivity (console, recorder, etc.). The latter, which is what you've got with the NJM part, is just not suitable as an input signal conditioner.

Consider the case where the mic and source conspire to make a signal that just barely puts the SSM2019 into clipping. Ideally, you'd like to back off the gain slightly to avoid the clipping. But short of turning on the pad, there is no way for your topology to do that. The NJM will attenuate the clipped signal, but the signal will remain clipped at your console/recorder. If you have a source that clips the 2019 when the pad is on, there is no way for your design to correctly handle it.

This is why it is absolutely necessary to vary the 2019's gain to match the source. You can use a THAT5171, a motorized pot, or a bank of relay-switched resistors. Whatever method you choose, the gain set resistance needs to be absolutely voltage-linear, have low self-noise, and be isolated from any interfering signals.

Besides, the NJM has other liabilities, like limited voltage headroom and high sensitivity to noise on its control pin. It is a "lo-fi" part that will just degrade a your SSM2019's performance.

 

[DaveDC]

Yes I get that impression, but the

of the THAT CORP IC's is pretty scarce currently.

Mouser is showing considerable stock of the 5171

 

[bigben83]

where in Australia bigben83?

Tasmania

 

[ultrafonic]

Tasmania

Brisbane here. Elec eng as well, if you ever want to chat

 

[Estomagado]

Yes I get that impression, but the availablity of the THAT CORP IC's is pretty scarce currently.

The title of the thread is

Seeking Advice: Best Preamp Design for Low-Noise, Balanced Microphones​

Using the NJM would mean it's not the "best" and not "low noise" (as it is a crappy cheap part), and insisting in using it would negate the "seeking advice" part, as many people advised against it and you will use it anyway. And would not provide adjustable gain, as it is a mere fader. The only thing that's left in your design is that it's balanced...


EDIT: I have been bitten by the DIY bug, as everyone else in here. But sometimes buying commercial products is cheaper and would perform way better. You are not in a recording studio, it is a Public Address situation (in a church, no less), you don't need the lowest possible noise achievable to mankind. Any average app-controllable chinese-made small mixer would suffice, and there are a lot of them being offered at AliExpress.

Or something like this:
https://www.behringer.com/product.html?modelCode=0605-AAB