Mic Fet booster with stepped HPF and line drive

[Tubetec]

I dont have any metal can LSK389's ,
But Id hate for it to go up in a puff of smoke instantly ,

Balancing the circuit would double standing current to 300ma , output transistors might need heatsinks too .

[AnalogJoe]

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 ,
P1070592.JPG

Which LSK389 will you be using? they have very different IDSS values. The datasheet doesn't give typical values, but it does give the min/max values:

2.6 - 6.5 mA for the 'A' version
6.0 - 12 mA for the 'B' version
10 - 20 mA for the 'C' version
17 - 30 mA for the 'D' version

Lets take the mean for each, that would give 4.55 mA for the A, 9 mA for B, 15 mA for C, and 23.5 mA for D.

If we consider a pinch-off voltage, Vp, of -0.95 V (mean of -0.3 and -1.6 V) then VGS for the bottom JFET would be around

-405 mV for the 'A' version
-513 mV for the 'B' version
-588 mV for the 'C' version
-647 mV for the 'D' version

The drain to source voltage must be equal or greater than VGS-Vp for the transistor to remain in the saturation region. Which means that you would need at least 950 mV between the drain and the bottom leg of the 270R resistor, which is larger than the typical base-emmiter drop of a BJT. Also remember we are using mean values, the actual VGS might be wildly different. In any case, you should make sure that you are not operating the JFET in the triode region or very close to the knee of the IV curve. Otherwise, the JFET won't operate properly as a current sink and you might as well replace it with a resistor.

All these are ballpark values, simulations should yield more realistic results. However, the SPICE models provided by Linear Systems seem to be as basic as it gets, and I wouldn't place all my trust in them. I believe Bob Cordell has developed better models for these transistors.

[Tubetec]

Thanks for your help ,

I checked the suggested reference ,

LSK489appnote.pdf

(574.27 KiB) Downloaded 3 times

Theres some nice audio ideas here ,

Maybe figure C is more appropriate .

[AnalogJoe]

Those do seem like good options.

There is another issue. The problem with strapping the BJT base-emitter junction directly across the JFET current source is that you are operating the JFETs at very different VDS. The low DC offset that the circuit in your image accomplishes is possible because you have a matched pair of transistors AND because that dual supply is dropping the same voltage across the top and bottom transistors. If you have very different voltages across the drain and source of the transistors you will get an unbalance in VGS due to the channel modulation effect (aka Early Effect) and the zero offset at the output will be lost. If symmetry is really important, you really want the same voltage drop across both transistors.

One idea is to disconnect the bottom JFET from the emmiter of the BJT and add its own resistor to ground, so the voltage dropped across the bottom JFET is not a VBE but closer to the voltage drop of the upmost JFET. This would of course loose the bootstrap effect, but you can then add a capacitor between the emitter of the BJT and the lower leg of the 270R resistor so the bootstrap is done with AC coupling instead. Not as ideal as a piece of wire, but a possible solution. Pick your poison.

[Tubetec]

My understanding of transistors , how they work and the terminology involved is very poor .

The bootstrap seems important to the opperation of the HPF in the Sennheiser MZA-14 circuit ,


Im looking at the Neve BA-283 and wondering could I use the 2N3055 driven by the LSK389 buffer , obviously the whole circuit is repeated to make a balanced topology ,
For simplicity I prefer a single rail supply , I also want to be able to supply phantom power at the input for transistorised condenser mics .

I'll have a re-draw of what Im thinking to try and make things a bit more clear .

[mediatechnology]

Maybe you should reconsider the original topology using an LSK170.

[Tubetec]

There something uncomplimentary about the so called complimentary pair ,
there two fundamentally different devices .

[Tubetec]

I was able to find one of the Canford phantom and AB(T) power units used so I went ahead ,
Has 4 attenuation settings 0,-10,-20,-30db , 4 HPF positions , 20,40,100 and 400hz ,
I have a couple of Sennheiser MKH series mics Id like to try with it , a 416P48 and a 416T , they both came to me for less than 50 euros but are without the shotgun tubes and inner windsheilds .

Its likely the Canford unit is through hole components for easier field serviceabillity ,
Theres no schematic around ,but it should be easy to see whats going on inside and
pinch a trick or two for my own design.