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

[bigben83]

Hi everyone,

I'm currently designing a compact, low-noise 2-channel preamp module and could use some advice on the best preamp topology and components for the job. The module needs to support both microphone and line-level inputs, have balanced XLR outputs, and feature remote volume control via a 10k linear potentiometer. Selectable phantom power (24V) is also required.

Key Design Requirements:​

• Ultra-low noise performance for professional audio applications
• Flexible power: 12-24V DC or potentially phantom power
• Balanced XLR outputs (preferably electronically balanced)
• Remote volume control using a 10k linear potentiometer with a DC supply (Similar to this https://rdlnet.com/product/st-vca3/)
• Input gain range: -60 dB to +4 dB

I’ve been considering preamp ICs like INA163, ADA4627-1, THAT1583, OPA1656, but I'm open to suggestions on what would work best for ultra-low noise performance. Additionally, I’m weighing whether to go for active balancing or a transformer-based output stage—would love some input on the trade-offs for each approach.

Has anyone here worked on something similar or have recommendations on circuit design, component choices, or best practices for achieving low-noise, high-quality audio performance? Any insights on PCB layout techniques to reduce noise and interference would also be much appreciated.

This is for a specialised system used in a church setting.

Looking forward to your thoughts!

 

[gyraf]

Why would it have to be "ultra low noise" if used in a church setting anyway?

 

[bigben83]

Why would it have to be "ultra low noise" if used in a church setting anyway?

To be used in acoustically dead rooms. so I'm aiming for as low noise as possible

 

[gyraf]

Now you have me confused: acoustically dead rooms, in a church setting..?!?

Trying to figure out exactly what you are aiming for here, it's not clear at all yet..

 

[bigben83]

Ha Ha sorry to confuse you. Think modern church with carpet floor, accoustic ceiling tiles. dense foam padded seating. acoustic slat-style wall treatment.

All I am after is a really good quality balanced mic preamp. which I can possibly modify the volume control so it can be remotely controlled. via a small seat-mounted control.

 

[k brown]

Ha Ha sorry to confuse you. Think modern church with carpet floor, accoustic ceiling tiles. dense foam padded seating. acoustic slat-style wall treatment.

All I am after is a really good quality balanced mic preamp. which I can possibly modify the volume control so it can be remotely controlled. via a small seat-mounted control.

Remote control of the actual mic gain, or of the line-level output volume?

 

[micolas]

Why would it have to be "ultra low noise" if used in a church setting anyway?

@bigben83
I agree with @gyraf
Will the church be empty, without an audience?
Regardless of the acoustic treatment, the mere presence of a crowd of people produces a lot of noise. Breathing, coughing, whispering, other noises produced by people, rubbing, or touching clothes, objects, chairs, etc., all these cumulative noises are substantial.
The specific "live" noise would overcome the noise of the preamps.

 

[Newmarket]

Ha Ha sorry to confuse you. Think modern church with carpet floor, accoustic ceiling tiles. dense foam padded seating. acoustic slat-style wall treatment.

All I am after is a really good quality balanced mic preamp. which I can possibly modify the volume control so it can be remotely controlled. via a small seat-mounted control.

Any decent off the shelf micpre / mixing desk would be fine for the job.
Whilst not wanting to discourage DIY activity - it seems you might be starting from 'scratch'. Italmost certain to cost you more in time and money compared to buying something. And the church would be waiting a long time for the kit.

 

[bigben83]

Remote control of the actual mic gain, or of the line-level output volume?

Yes it would be the line-level output volume that would be remote controlled.

 

[bigben83]

Thanks to all that have replied,
I get the feeling that there isn't a preferred IC to use for preamps. I've put this task out as a job on upworker seems to be plenty of talent to help out.

 

[micolas]

@bigben83
But what is the ultimate goal?
Indoor/outdoor local live PA or remote PA for a relatively close location
Audio recording
Streaming, radio/tv broadcasting, etc
For all these purposes I have used easily accessible common equipment over the years and SNR has never been a problem

 

[Newmarket]

Thanks to all that have replied,
I get the feeling that there isn't a preferred IC to use for preamps. I've put this task out as a job on upworker seems to be plenty of talent to help out.

If putting it out for development then you need to rethink this:

• Input gain range: -60 dB to +4 dB

But how many of these systems do you envisage making ?
I have a feeling that it may not be enough to cover the developer cost let alone materials / fabrication etc.

 

[MidnightArrakis]

To be used in acoustically dead rooms. so I'm aiming for as low noise as possible

[To be used in acoustically dead rooms] -- Do you mean an -- anechoic chamber -- ???..... PRAISE GOD!!!.....

/

 

[rh001]

Whether for voice or musical instrument, and since low noise is a requirement, use a transformer input (noiseless gain) followed by a Neve 1073 (Neve 1272) gain stage and output stage (runs on +24vdc) to drive an output transformer set for +6db noiseless gain. You'll pay less for Cinemag transformers and get same or nearly same performance as Jensen transformers or European trannies. As for remote gain, for totally continuously variable gain you can put a THAT (formerly dbx) vca in between pre-amp and output stage for output level control, but that makes almost no sense. It is the preamp gain that must be controlled. To do that remotely you'll need to control the gain with relays. To control the relays you'll need a roughly 8 wire control cable or there are schemes to use binary to decimal chips so you will only need 3 or 4 wires in the gain control cable. Generally speaking it seems you're way off target and all you'll get from upworker is chudd cuz competent people don't work for pittance and even if your hope is to gain off someone in a 2nd or 3rd world economy what you may not have considered is the best you can hope for via that strategy is something analogous to a Phillipines call center experience.

 

[ccaudle]

advice on the best preamp topology

Solid state microphone amplifiers designed for low noise and distortion (as opposed to copying a 1960's or 1970's design for a particular distortion color) have all converged to an instrumentation amplifier topology, either to single ended output with a separate balanced output buffer, or the dual-balanced configuration popularized by Cohen in the 1980's.
The ThatCorp mic-preamp devices give an instrumentation front end, and it is up to you to select gain switching and output topology. Conveniently That also make an electronic switch for gain selection, and have a selection of devices for the output buffer as well.

From your starting point I would recommend looking at the ThatCorp application notes and design seminars, especially the Designing Mic Preamplifiers seminar, and the design brief on protecting against phantom power faults.

Flexible power: 12-24V DC or potentially phantom power

Output power to the microphones, or input power? Because you cannot get enough power from phantom power to make a decent mic preamp. And 12-24V DC "or...phantom power" doesn't really make sense as an output choice, I don't think you will find anything except phantom power as a requirement for microphones.

Remote volume control using a 10k linear potentiometer with a DC supply

That is putting the answer before the question. Why do you care that it uses a 10k linear potentiometer with a DC supply? Why not a log taper pot? Why not 20k? Why not a continuous encoder with microcontroller communication? (hint: that last option is what all modern designs use)

Input gain range: -60 dB to +4 dB

That sounds more like an input signal range than a gain range (unless you really mean you want to attenuate the input signal by up to 60dB, or provide 4dB of gain). Assuming it is signal level, you need to explicitly state the reference since dB is a unitless ratio, e.g. accept -60 dBu to +4 dBu nominal signals. It could be dBV instead of dBu, but +4 dBu is a common reference level for pro equipment, while +4 dBV is not, so I am guessing from context that you meant dBu.
You need to be detail oriented in your specifications if you want to get accurate suggestions.

You should either give an actual gain range you would like to support, or give the input signal range and the output signal level you need to be able to provide from that input signal range.
If discussing signal ranges you also need to decide on headroom relative to nominal. Old style professional equipment could often provide 24 dBu maximum output, so 20dB headroom over nominal 4 dBu operating levels, but maximum levels of 20 dBu are more common these days in all but the highest priced equipment.

PCB layout techniques to reduce noise and interference

Keep your signal loop areas as small as possible, keep any power transformers as separated from the high gain stage as possible, don't commit the pin 1 problem.
If you don't recognize right away what those terms mean, then you probably are not yet at the skill level to design a low noise remotely controlled preamp from scratch, and should start with something simpler and work your way up.

Yes it would be the line-level output volume that would be remote controlled.

That is at odds with your requirement for low noise. Lowest noise requires that the gain is optimized in the first stage, so you need at least switchable gain (and probably a switchable attenuator) in the front end. It may or may not need switchable gain for the output stage depending on the gain range needed.

All I am after is a really good quality balanced mic preamp. which I can possibly modify the volume control so it can be remotely controlled.

Every modern stagebox has between 8 and 32 remotely controlled mic preamps (as well as line outputs, usually driven by D/A converters on modern designs).
The lowest cost and fastest approach to get what you need is to go buy a Behringer digital mixer with a stage box. They use the Midas pre-amp design which is high quality, are very quiet, have remote control of gain and phantom power and high pass filter, and connect back to the mixer with a single ethernet cable running AES50 protocol.
You can buy a setup like that for about what custom casework will cost for a one-off project.