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 7 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.

[AnalogJoe]

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

P channel and N channel FETs tend to be quite different, even the so-called complementary versions. N channel devices tend to have better performance than their P channel counterpart almost in every aspect, with some exceptions (e.g., 1/f noise may be lower at some frequencies in P channel transistors). The main reason for this is the fact that the majority carriers in N channel devices are electrons, while the majority carriers in P channel devices are holes. And electrons have almost three times the mobility of holes. Higher mobility translates into higher conductivity (or lower resistivity), among other things.

Since electrons have higher mobility than holes, a N-channel JFET will usually have more channel conductivity (less resistance) than a P-channel JFET, all things being equal.

CMOS IC designers usually have to make the channel width to length ratio (referred to as W/L) twice (sometimes more) as big in PMOS devices compared to NMOS devices to try and equalize the resistances between devices (e.g., in a logic inverter or NOT gate), because increasing the width lowers the resistance of the channel. However, the downside is that it increases capacitance, since capacitance increases proportionally to area. This in turn detriments the performance of the device at high frequencies. So it becomes the same old game of chasing ones own tail. At even higher frequencies, for instance in the GHz region, even N-channel MOSFETs are too slow and you need to use completely different devices like MESFETs or HEMTs (High Electron Mobility Transistors).

But FETs are not the only ones that suffer from these differences, BJTs do as well. In NPN BJTs the majority carriers are electrons, whilst PNP BJTs have holes as their majority carriers. In the early days of transistor power amplifiers you really couldn't find many PNP power BJTs with good performance specs, which is why you would often see a Sziklai or Complementary Feedback Pair in place of a PNP transistor at the output of push-pull power amplifiers.

[Tubetec]

Thanks for the extra detail on that ,
I know of a few old style guitar amps that use a pair of 2N3055 for 50-100W , power output depending mainly on supply and speaker load .
They really are very robust in terms of overload , they also tend to distort in a relatively musical way , compared to the usual NPN/PNP .

Im a big fan of the Neve Ba-283 output stage , Im trying to imagine a push pull version , balanced Jfet input , center tapped transformer in the 3055 collector circuit , with an additional output available from the emitters , single rail supply .

The Canford audio unit ended costing a bit more than expected ,
What started out advertised at 40 sterling (48 euros) doubled to 90 euros by the time I added shipping and import duty from the UK .
Brexit and its tarrifs has killed all the good in ordering from the UK to Ireland ,
Were getting well and truly ripped off .