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

[Barry Farmer]

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!

A couple of THAT1583 with their associated remote digital preamp controllers (THAT5171 etc), would be more than adequate - even were you to achieve an anechoic chamber, in your church, the noise of people breathing, etc., would far out-weigh any electrical considerations about "noise". I believe there are "development boards" available for these IC's

 

[Cqwet Dbdfte]

Remote controlled pots are very common on the Bay, I have some, and also a pre amp with passive 7-bit Digital Step Attenuator, using relays, (128 steps using thru hole metal film resistors). It is clickfree.
As has been pointed out, OP's requirements are not very unique except maybe for the volume remote.

 

[ccaudle]

this is what I am thinking of doing

That NJM device is a single supply 5V device, which means that signals are limited to approximately +/-2V peaks relative to mid supply. That is 5 dBu max level, barely consumer grade, not suitable at all for equipment expecting professional standard levels.

 

[RCAguy]

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!

1) "Remote volume control" may throw a monkey-wrench into possibilities. Would you be satisfied recording at 32bit? 2) By "preferably electronically balanced," you mean transformerless? Or would an impedance balanced (single-ended) output be OK, e.g. Schoeps' new CMC1?

 

[ccaudle]

"Remote volume control" may throw a monkey-wrench into possibilities.

The ThatCorp devices are made for that. They are not the cheapest, but That gives essentially a turnkey design you can use.
It is digital control though, that adds a slight additional learning curve. If you really wanted to use an analog voltage as a control you could probably whip up an Arduino sketch to read the ADC and write gain values to the That digital gain control.

 

[DaveDC]

That NJM device is a single supply 5V device, which means that signals are limited to approximately +/-2V peaks relative to mid supply. That is 5 dBu max level, barely consumer grade, not suitable at all for equipment expecting professional standard levels.

+1

It is curious that the OP is willing to put up with the ~1% THD of the NJM device. And that doesn't even include the intermodulation from the interference picked up on the control voltage cable.

This block diagram is how it would have been done 50 years ago. 25 years ago, we would have done a relay-controlled analog preamp + analog output driver with control via serial data (MIDI, RS-422, etc.). Today we would use an integrated preamp + ADC (QSC Q-SYS, etc.) sending networked audio straight into a digital console. And all at about the same price point.

 

[MidnightArrakis]

+1

This block diagram is how it would have been done 50 years ago. 25 years ago, we would have done a relay-controlled analog preamp + analog output driver with control via serial data (MIDI, RS-422, etc.). Today we would use an integrated preamp + ADC (QSC Q-SYS, etc.) sending networked audio straight into a digital console. And all at about the same price point.

[This block diagram is how it would have been done 50 years ago] -- Several years ago I worked as a "Design Consultant" with an aerospace/avionics company whose -- unofficial -- motto was....."We're the leader in 15-year old technologies"!!!

However, while such a motto does sound amusing, it is also the truth!!! When you are designing products for commercial aircraft that will be carrying hundreds of "human cargo" at a cost of -- tens-of-millions -- of dollars.....you just can't gamble on using any of the latest "whiz-bang" technologies!!! You need to use components, standards and techniques that have been -- proven -- to be completely reliable with a verified history.

A part of my job was to take PCB's that had been designed back during the 1980's and early 1990's and redesign them now using Surface-Mount Components instead of Thru-Hole Components. The amount of PCB real-estate reduction was such that we ended up being able to cram 2, 3 or 4 Thru-Hole PCBs onto a single Surface-Mount PCB!!! We even went from "double-sided" PCBs to using 4-layer and 6-layer PCBs because now, it not only "made sense", but was also more cost-effective. Multi-layer PCBs have already been proven to be reliable beyond a "shadow of a doubt", especially when fabricated by top-notch PCB-fabricators.

>> This is just a general "thrown out there" comment. That's all.....

/

 

[bigben83]

That NJM device is a single supply 5V device, which means that signals are limited to approximately +/-2V peaks relative to mid supply. That is 5 dBu max level, barely consumer grade, not suitable at all for equipment expecting professional standard levels.

Thanks @ccaudle I hadnt picked that up. back to the reasearch.

 

[bigben83]

A couple of THAT1583 with their associated remote digital preamp controllers (THAT5171 etc), would be more than adequate - even were you to achieve an anechoic chamber, in your church, the noise of people breathing, etc., would far out-weigh any electrical considerations about "noise". I believe there are "development boards" available for these IC's

Thankyou @Barry Farmer I will do some research into this.

 

[bigben83]

+1

It is curious that the OP is willing to put up with the ~1% THD of the NJM device. And that doesn't even include the intermodulation from the interference picked up on the control voltage cable.

This block diagram is how it would have been done 50 years ago. 25 years ago, we would have done a relay-controlled analog preamp + analog output driver with control via serial data (MIDI, RS-422, etc.). Today we would use an integrated preamp + ADC (QSC Q-SYS, etc.) sending networked audio straight into a digital console. And all at about the same price point.

Yes i looked at using something like the ADAU1467 to get full control. but I thought this was a bit of an overkill for what I was doing, even though I am considering a ADAU1467 board with XLR in and out for ease of use.

 

[bigben83]

I have seen there is a discussion on the THAT1512 on this forum
https://groupdiy.com/threads/that1512-pre-diy-reasonable-replacement-for-cheap-preamp-art.86210