High impedance mic preamp design

[Tashi1]

Hi!

In my experience with buiding a low noise pre-amp for a ribbon mic, as we decrease input impedance, high frequencies indeed are killed first.
But, above 10kOhm input impedance, it makes not much difference. Below 4.7kOhms we clearly can see the difference showing up.

See attached some measurements I've done some years ago comparing different input impedance values versus the baseline of 10Mohm input impedance.
10k, 4,7k, 2.2k, 1k and 560Ohms. Charts zooming 10 to 10kHz, 100 to 10kHz, 1k to 10kHz and 4kHz to 10kHz so as to better observe the frequency response in each range.

These measurements must be observed from the relative perspective: same mic, same pre-amp circuit and same loudspeaker. The only variable changed was the insertion of a resistor at the input so as to vary the input impedance.

I ended up building this pre-amp with 2 pairs of JFET LSK389 (parallel to reduce noise) with 100kOhm input impedance to gnd.

Thanks for sharing those, that's really interesting to see! I wonder if older mics with a lower output would start to get loaded even sooner.
How did you solve the current sinks when you built it?

 

[Tashi1]

Hi Tashi1, Hope you are not offended but I would prefer not to share the final schematic of my design.. as I now started to build these and offer them as a "commercial" product.
But the preamp is performing in Class A , push-pull and drives a small signal transformer at its output. The LSK series is well suited because of the matching of the pair of FETs, and they are indeed very quiet. I went for about 24 dB of gain (variable feedback) and at full gain the noise floor is super low, I think better than 110 dB signal to noise ratio. For instrument levels I felt that 0 to 24dB is more than enough range for the gain. The transformer is not a super high end, though good enough for guitar and bass etc for the frequency response.
I have also tested it with higher end output transformer with very good results. For your mic /instrument preamp, you'd probably want to use something like the Jensen, Cine-Mag or Hammond studio series transfos. They are pricey but you'll get full audio bandwidth , balanced and isolated signal for DI applications.
If I were doing a preamp that would work for both mic and instrument, I would probably try to optimize the mic preamp front end separately from the instrument front end and merge the two at the middle or output section. You definitely don't "need" 60 dB of gain for the instrument level.
Cheers!

No worries, thanks for the suggestions!

 

[Tashi1]

The original post showing Bob Cordell's proposed design seems pretty reasonable to go with. It's not exactly a complete circuit, as it is showing the LSK 389/489 as an open input +/- differential pair. It is also leaving the current sinking parts as a black box..so you need to design that aspect.
A similar mic preamp design by Elliot Sound Products (Project 66) for low noise balanced microphone preamp, but uses bipolar transistors in the front end differential amplifier. I'd suggest having a look at that one to get some direction on tying in the differential pair +/- inputs to the microphone...for input impedance and grounding. Also the power supply to the first and second amplification stage should be very well regulated.
ESP Project 66 low noise mic preamp

Cheers

Yes figuring out the sinks to operate perfectly is probably beyond me at this stage, I'll start with the easier on-chip route for now

 

[Tashi1]

Drew this up, INA163 with 1646 as a line driver. Ability to use jumpers to select a different input impedance & also bypass the phantom caps (that's a particular use-case for me - sometimes I need to split a mic output that is carrying phantom).
INA163 should give about 7-75dB, then another 6dB from the 1646. Sorry its a bit messy. Any glaring issues?

PSU circuit. U3 is a BUF634 as a rail splitter, J7 is a DC input jack. Ability to use jumpers to input a proper dual rail supply

 

[ron68]

How did you solve the current sinks when you built it?

Hi, I used the popular current source as shown below to sink current from the JFET differential amp.
You build a voltage divider (10K and 4.7k) so you get a certain fixed voltage at BC549 base (in this case 4.8V above the -15VDC).
This voltage, minus 0.6V, will be applied to the 1k resistor (4.2V) which forces 4.2mA on the emitter.
The emiiter current will be reflected to the collector of BC549, thus you get a fixed current source in the BC549 collector.

 

[MarcelvdG]

Any glaring issues?

94 second charging time constant for C11 and C12 when the preamplifier is set in high-impedance mode. You will have to be patient after turning on the phantom supply.

Working voltage of C1 and C2 just too low to survive a 48 V +/- 4 V phantom supply when there is no microphone connected and the voltage happens to be at the higher side of the tolerance interval

0.8 pA/√Hz input noise current is fine for microphones, but may be a bit high if you still want to connect electric guitars. Then again, maybe it's no big deal, as I have been told that electric guitars usually pick up enough mains hum to mask the noise.

 

[shanx]

Yesterday, I simulated the design in LTspice and I wasn't able to remove an oscilation that seems to have its origin in the input stage.

On the Project 66 circuit? Was it open at the input, and what frequencies of oscillation?