I traced a 'thing' but i'm pretty sure it ain't right.

[patrick398]

A friend asked me to fix a a switch on something which for legal reasons we will refer to only as a variable impedance microphone box. You've no doubt seen and heard about them. I was always curious what was inside, thinking it would be complicated and involve transformers. Well lets just say there's a lot of empty space in there.

I traced the PCB as best i could but a few of the traces were hidden under the SOICs so i tried to do a bit of multimeter continuity testing but i think i'm getting false readings because of the nature of the ICs. They are both LSK389F, dual N chanel JFETs. I'm guessing my meter is being tricked into thinking there's continuity where there isn't because some of the connections i've ended up with look mad.

Anyone know anything about them? There's not a lot about them on the web and they seem basically impossible to buy.

Here's where i'm at...apologies for the hideous schematic, i had to draw it from the PCB trace i've made and don't want to try and make it neater in case i mess something up...more so.


There's 3 SMD caps i can't identify, no markings. The huge one i think is a 0.22uf 250V, it's labelled: u22 J250 H4
There's also a couple of resistors which seem to go nowhere. The connections to the pots and switches use 3 pin headers but the pots and switches themselves only use 2 pins, some resistors go to the third header but this doesn't actually connect to anything. I can only assume these boards serve multiple purposes in different products.

There's no way the connections of the ICs are right, anybody have a better idea how it might be working?

[iainpunk]

i'm like super curious about your variable impedance microphone box.

i think you have the readings wrong, and the Jfet channel reads a short. i guess the first IC is a long tailed pair and the other may be a gain stage, but what you traced out makes not much sense.

cheers, Iain

[patrick398]

i'm like super curious about your variable impedance microphone box.

i think you have the readings wrong, and the Jfet channel reads a short. i guess the first IC is a long tailed pair and the other may be a gain stage, but what you traced out makes not much sense.

cheers, Iain

Yeah made no sense to me either, i'll have another look at it in the morning. There is up to 25db gain according to the specs, and the HPF works in tandem with the variable impendance. I think the Z and HPF sort of makes sense to me, though i've never done anything with XLRs before so something could be off. I might try and draw it out with individual JFET symbols tomorrow so i'm not just randomly shorting shit together ha

Also, if i could find some to buy i could just whip these ones off the board to get a proper look at the traces and replace them after. Given that i can't find them to buy i don't want to risk frying the current ones

[PRR]

The "variable impedance" is just the 20k pot and 150r stopper.

The rest is like the "fog jacker" commercial product a few years back, which has been implemented in FET (sexier) and BJT (cheaper). But your pinouts can't be right. And these inline boosters are a "Oh, wow!" toy; if your board preamps do not suck, you can get similar S/N without add-ons.

[Rob Strand]

The rest is like the "fog jacker" commercial product a few years back, which has been implemented in FET (sexier) and BJT (cheaper). But your pinouts can't be right. And these inline boosters are a "Oh, wow!" toy; if your board preamps do not suck, you can get similar S/N without add-ons.

R3, R4, R5 on Patrick's schematic made me think it's diff-amp but those resistors are going to the drains, so something has gone wrong with the trace.   The second chip might be wired as output buffers.

[PRR]

My 2 cents is, indeed, common-source diff-amp, then maybe cascoding for improved high voltage (+48V) work. Except as you say D and S seem to be linked. Which makes it a very expensive diode. Which may even be a point (obfuscation).

I do think it odd that the same 22r resistors are used as in the BJT plan from 2005 (I really had to dig for it).

[Rob Strand]

My 2 cents is, indeed, common-source diff-amp, then maybe cascoding for improved high voltage (+48V) work. Except as you say D and S seem to be linked. Which makes it a very expensive diode. Which may even be a point (obfuscation).

I do think it odd that the same 22r resistors are used as in the BJT plan from 2005 (I really had to dig for it).

That makes a lot of sense.   Those devices are only 40V max.

It's a toss-up whether the 39R is 390R and the 1.5k is 150R.

Given the PCB is a bit tricky to trace another angle would be to measure the voltages around the devices and also the voltage across the resistors since they will imply certain currents - but only if the values are correct!   If you measured the voltage across the 22R that would give the drain current, summing those will give the current down R3, then measuring the voltage across R3 will allow R3's value to be verified R3 = VR3 / (2*I_22R).   A similar idea for the 1.5k's.   Once a speculative circuit is produced it could be put into Spice and the circuit voltages could be checked against the measured voltages.

[iainpunk]

its basically a LPF for a Ribbon mic, so i guess designing one yourself wouldn't be hard and making a better device isn't out of arms reach as well. but most mixers already have great, if not better, cleaner and less noisy pre-amps than most '''line boosters'''

cheers, Iain