Mic Fet booster with stepped HPF and line drive

[Tubetec]

The Sennheiser MZA-14 48v battery supply with discrete transistors, an attenuator and HPF , with 3 positions each , its no longer made ,

Id like to exapand the HPF and provide finer control over attenuation .
Abbeys LSK389 input stage looked like a good modern solution , instead of using standard series resistors to supply phantom , 48v supply fed through a center tapped line transformer ,housed in a DI box style enclosure .

A proximity filter to counter proximity effect .
designed to directly feed a mic to a DAC at line level .

[Tubetec]

[Tubetec]

Just to explain a bit better ,
In the case of a dynamic or ribbon mic a fet mic boost like Abbeys circuit could be placed inline towards the mic end of the cable , supplied with phantom power in the usual fashion .
MZA-14 is obviously designed to handle condenser mic inputs , so no need for the fet stage in that case , in practice it might take the form of a male/female xlr barrel adapter , placed in the circuit only when required .

The MZA uses parrallel switched resistors to determine the HPF settings , instead Id like to try switched series capcitance chains , probably SMD ,mounted on a 24 pos dual wafer rotary , additional gain control steps would also be an advantage .

I also had switchable transformer coupling at the output in mind, as much for the funk it adds to the sound at higher output levels but also to provide ungrounded connection if longer cable runs are needed and the ground paths are likely to interact .

The output would need to drive a DAC and line input for the monitor mix,

Ive tried the single ended switched coupling cap as HPF in high-z valve gear , similar to Baxandalls ribbon mic proximity effect filter from the 50's ,
12 steps , only 6db per octave , but to my ears it gives a very natural tone when you hit the sweet spot on dial with a given source .
Viscious cut-off slopes arent usually required and arent even desireable in my opinion unless your trying to prevent LF interference, noise or vibrations of some sort ,
Typically the bass bloom you get with close micing extends into the lower midrange so a cut starting in that vicinity is useful Ive found .

The reason this idea evolved in my head is I often find the standard audio interface mic input unsatisfactory , boomy at best and requiring digital eq after the fact , your into patch up jobs and headroom diminished because the system has to accomodate and overly bassy signal at the ADC input .

[mediatechnology]

Nice use of bootstrapping. (R21 and R22.)
Thanks for posting it.

[Tubetec]

Canford Audio stole a trick or two from Sennheisers MA-14 , but adds a few nice extra features ,
It works out around 400 euros once you add the tax , Id probably get caught with import duty on the way from the UK also .

20-750 - CANFORD PHANTOM POWER SUPPLY P48_AB(T) PP3 battery powered.pdf

(198.66 KiB) Downloaded 6 times

Looks like it wouldnt be that difficult to make up a protoype on verboard , its symetrical layout should just transplant across ,
Someone did show a circuit design software designed for making stompboxes on prototype board , I might give that a try if I can find it .

[Tubetec]

Could the above circuit replace the BFT46 in the Sennheiser circuit ,driving a BD139 at its output ?

[mediatechnology]

It could but I think it would need a negative supply.

[Tubetec]

Looks like the buffer circuit should work just the same with either +/- 15v or a single +30v , only difference would be the input would reside at +15 v with single rail , or close to zero with +/- supply .


Below is a rough sketch of what I had in mind ,

[mediatechnology]

OK, got it.
Easy enough to prototype one up.
The 100Ω emitter load looks like the Iq would be about 150 mA.

[Tubetec]

I can see this post belongs in the design section but I didnt know if it possible to put it there .