FET Preamp Cable

[bancika]

Hi,
I recently installed piezo in my acoustic and wanted to build some sort of preamp. This ( http://www.till.com/articles/PreampCable/ ) came up and seems interesting. It says in the article that it can be plugged straight into 48V powered mixer without having to build the pedal part of it. Anyone tried it and can confirm that it would work. I wouldn't want to fry my interface or something
Thanks,
Bane

[drolo]

Hi Bane,
I'm not too experienced with phantom power but looking at the till info it seems ok. I don't think it could do any harm to your interface (as long as the phantom power is not shorted I guess)

But I hope someone with better understanding will chime in ;-)

[bancika]

Thanks! I'm a bit puzzled because two pins 1 and 3 of the xlr are connected together...so it's lile regular mono cable on both sides. I know nothing about phantom power, maybe it's ok.

[Seljer]

Phantom power or the mixer side is 48V supplied through two 6.8kiloohm resistors. So even if you short it out to ground it's not going to draw more than 7mA.


In theory if you shorted 1 and 3 together at the preamp end instead of at the XLR there may be a slight benefit to the noise rejection. (pin 1 goes to the shield)

[bancika]

So you think that these 4 components will be enough? That's awesome, I have to give it a try.

[Seljer]

It'll work just fine for how simple it is, not much headroom though so no use with super high output humbuckers


Heres something that'll give you a somewhat better balanced output if you're going to be exclusive using it with phantom powered mic inputs but at the the cost of many extra components. It probably won't fit in a jack with all the large capacitors required.

[bancika]

I need it as a piezo preamp for acoustic. I guess it's not as hot as humbuckers on electrics, but haven't measured

[bancika]

btw, how does j201 bias with 48V and 2.2k source resistor? I've been looking at output curves but couldn't figure it out  Haven't built anything in a year or two and I already forgot everything I knew

[PRR]

> how does j201 bias with 48V and 2.2k source resistor?

See the 12V Zener?

Forget the Phantom junk, replace the Zener with a 12V battery, I think you'll find it biases fine. 2 or 3 Volts at Source, Drain 2 or 3 V down from 12V.

This also happens to be, for a nominal 2N5485, very nearly the optimum split for a cathodyne (whatever you call it in JFETs). Divide 12V by four, you want S and D to be at 3V and 9V. But for smallish guitar signals you don't have to be spot-on optimum.

Put the Phantom junk back, works the same.

The 22K Phantom resistors are barely small enough. For some 2N5485 you won't get quite 12V at the Zener. This too will work as long as you have the big capacitor. My taste would suggest using 10K instead of 22K. However 22K gets real close to the old-old Phantom spec of 2mA per jack. Nearly any modern Phantom will allow near-short (over 10mA; 14.1mA in dead-short) so higher current should not be a problem.

[bancika]

Thanks. I understand how the more complicated ome works. I was wondering how the simple one biases with 48v and 2.2k.
Tnx

[Seljer]

Thanks. I understand how the more complicated ome works. I was wondering how the simple one biases with 48v and 2.2k.
Tnx

Theres 2.2k on the source going to ground, and the drain is connected to 48V through a 6.8k resistor thats installed in the mixer/micpreamp/whatever, same setup as the phantom power box with the 9V that's shown but with 48V instead.
It's probably not the optimal biasing condition but the fact is that it'll work well enough for how simple it is (and that it fits inside the plug)

[diydave]

I've build the pre-amp cable because I've got tired of screwing and unscewing the backpanel of my guitar to change the battery.
It's a normal guitarcable, no XLR. Fet, gate and source resistor are tucked in the jack on one side of the cable. It's a bit tricky to solder and isolate everything, and you need a large housing, but it's doable.
In a 1590B box there's the drain resistor, condensators and 100k outputresistor to ground. The thing is powered with a 9-volt battery in the box.

The preampgets powered as soon as I connect the "non-fet"-jack in the box. The signal-wire acts like a long wire between drain-resistor (in the box) and the fet (in the jack). So the signalwire has about 5 volts DC + carries the guitarsignals AC to the box.  

Can't play without it anymore.

In a next multifx project, I'm planning to build in and have 2 inputs: one for the preamp-cable and one without the preamp. Thereby I can ommit an extra box which is no put before my pedalboard.

[bancika]

It's probably not the optimal biasing condition but the fact is that it'll work well enough for how simple it is (and that it fits inside the plug)

yes, that was my concern. But I cannot figure out how to choose the value of bias resistor to have optimal bias for 48V.

[Seljer]

Well the 6.8k is fixed at the XLR plug on the mixer so the only thing you can mess around with is the source resistor. In theory, lowering it will give more gain.
After some quick simulations it seems that with the J201 the amount of headroom doesn't really change much (that low Vgs really kills it). If you're only going to ever use the 48V phantom voltage you can probably use different FET and get better results.

[bancika]

Hmm, interesting, any suggestions on the replacement? Maybe BS170?

[Seljer]

In SPICE I have it behaving well with a 2N5485 and anything between 470ohms and 1kiloohm between the source and ground. It biases to around 30V on the drain and has a whopping 15dB of gain, might even be too much for a mic input on a mixer :shrug:

I say breadboard it for good measure

[bancika]

hmm, it's rated for 25V Drain-Gate Voltage. Do you think it will be fine in that configuration?

[Seljer]

Hmm, good catch. Though the J201 is only rated at 40V as well. I'm clueless from this point onwards.

You can't get a mosfet like the BS170 to bias right with this topology. You'd need to add another couple of megaohms from the gate to the drain so the gate voltage isn't 0V, which means you'd also need to add a couple of nanofarads to block DC on the input. And a zener diode to protect the gate from static and such also probably wouldn't hurt . You might be able to pull all this off within a plug with SMD resistors and caps.

[bancika]

thanks for the help. I'll try j201 and we'll see how it goes.

[bool]

If you're scared, just add a 33V-36V zener accross the fet's s-d (that should be small enough to build and it will brute-force protect from overvoltage) and try to bias the drain to circa 25V...