The Tillman FET preamp.

[Blueteeth]

Hi,

Well, seeing as I have a few guitars, and some peizo-equipped acoustics, I have finally got around to making the tillman premap cable for them.

FET Preamp Cable

One question, does it 'really' matter how long the cable is between the FET part (inside the Jack housing) and the power supply/output cap ?

I realise the output impedence is around 6.8K (using his values) so I'm guessing the cable can be up to 5m long.

I'm planning on prototyping it to get the biasing right, before putting it into a Jack housing. I'm not using a microphone cable, just a standard unbalanced cable, with a mono 3.5mm plug at the other end, going into to power supply box. (which will be a small 1.1V to 9v high frequency boost converter, as I hate 9v batteries).

Does anyone see anythign wrong with this setup? (ignoring noise from the boost converter). I could use a 2-core cable, as per his original design, but thats just extra cost

Blueteeth

 

[schmitt trigger]

The capacitor and the equivalent parallel resistance of R2 and your amp's input resistance will create a low pass filter. But a quick calculation shows that the equivalent resistance would have to drop below about 1.7k to hit a corner frequency of 20 Hz...so you are OK there.

 

[Blueteeth]

Hi there,

Yeah, I've been stung filter characteristics by changing capacitances and resistances before. I'm *trying* to keep this circuit stock, and resist the temptation to start modding it, but, as always, I have fallen at the first hurdle. It seems very difficult to get the drain voltage below 8V using the values shown, and so....its input range is far too low....clips with my guitars signal, even with mild playing. I measured a maximum of
3.06V peak-to-peak straight out of the guitar when playing 'hard'. Thats 1.1Vrms maximum input signal.

Ultimately, I would like this circuit to have large input impedance, and relatively low output impedance. This should be a good buffer for piezo pickups and high impedance guitar pickups. - as a bonus, its simple, low current ( < 3mA), and will fit inside a 1/4" jack plug, to be as close to the guitar pickups as possible, without actually having to stick it inside the instrument.

I simulated it with LTspice, trying to account for battery source impedance, guitar signal impedance, output impedance etc... of course its only as good as the models, but heres the weird thing.... my prototype (on a breadboard) is damn close to the simulation results. (which is a bad thing).

I have heard that JFET's have terrible manufacturering consistancy, perhaps I got a 'good' batch but they seem pretty consistant to me, with the drain voltage being between 7.5 and 8.1V. I have replaced the source resistor with a 10K trimpot, this can drop the drain voltage down to <7V, but the value of the pot ends up being around 220ohms. Thats 10x smaller than the stock value (2.2K). Plus I suspect it would reduce the voltage input range even further.

So, any audio/analogue gurus out there (yes, I meant you audioguru) can you recommend any similar FET buffer? I'm not after gain, unity would be perfect, but I'm sure 'some' gain will be inevitable. I have looked into source follower circuits, but in simulation I cannot seem to get close to unity gain, whilst maintaining a good input voltage range.

here's the LTspice circuit. Any help would be appreciated.

 

[audioguru]

Yes, Jfets have a very wide range of voltage gain so they might be saturated or cutoff in this simple circuit.
Yours is almost cutoff all the time.

You could make a variable negative reference voltage for the gate and adjust it to your FET for symmetrical clipping.

Why not use an inexpensive Fet-input opamp (TL071) instead?

 

[Blueteeth]

Hmm, its going to be tricky to get a negative voltage to the circuit, as its inside the jack plug. Although I do have 2-core cable to hand.

I have considered an opamp, and, although Tillman doesn't seem tolike them much, for convenience and accuracy, they can't be beaten. I have some SOT23-5 opamps somewhere...unfortunately, not FET input, but I guess with a VCC/2 bias using 2.2M resistors, a 1.1M input impedance would be ok.

I did simulate the same circuit with 2N3819, and by trial and error for the source and drain resistors, got it to pretty much unity, with a 3v p-psine wave. Idles at 1mA, which I can live with. Of course, thats in simulation, but its an ecouraging start. Also, lowers the output impedance a bit.

So, my options:

Single SMT opamp circuit: Lower input impedance, but with a wide input common mode voltage range (close to 1.0v to the rails), and as a simple voltage follower, it could work from 5v - 12v happily. Bigger circuit size, but still around 11x7mm...at least thats what I managed in eagle. Using a TS461 SOT23-5. I have some analog true 'rail to rail' opamps somewhere though for even more headroom.

Different FET: Easier, cheaper, but requires tweaking of values. Looks like I'm going to have to sit down with a prototype and a scope to see its true gain and any distortion.

One thing though, I am puzzled as to why the tillman circuit seems so popular. Sure, its a wonderful idea, the J201 is a low noise device, and runs at very low current, but surely many non-savy electronics peopel have built this 'stock', even though it clips (albeit, very gently) without serious changes to the values. I wasjust worried that I was missing something, assuming that, given the correct 'tweaks' of values, it would do what the design suggests. Would a source follower circuit be any better? I haven't managed to simulate a 'decent one' - by that I mean, with a gain close to 1 and no clipping/distortion up to an input of 1v RMS,9v powered.

 

[audioguru]

FETs have a very wide range of gain. One will be saturated and the next one will be cutoff.

There are thousands of Fet-input opamps available. use one.

 

[mneary]

If you have a 3Vp-p input and a 9V supply, any gain takes you too close to the rails for a simple circuit. If unity gain is OK and all you need is lower impedance, I agree that a source follower is the way to go.

 

[Blueteeth]

Hello again,

Well to wrap up this thread (sorry if it seemed to be a bit of a basic and vague question) I've got it behaving. Found a 2N5457 in my bits box, plugged it in...and with stock resistor values does the job almost perfectly...which frankly is luck. 3dB gain and remains unclipped until around 3V p-p, then its soft clips very nicely asymetrically. I'm pretty sure that's thats maximum input my guitars can output.

As an added bonus, increasing the source resistor slightly drops the gain to almost nothing, adding even more headroom for the input. Current draw was measured at 0.39mA with a 1kHz 1V RMS sine wave going through it. A PP3 9v should last months and a single (or double) AA boost converter (0.9-1.5 -> 9v) will last more than long enough. Next up will be an op-amp version, for 'decent' gain and lower output impedance.

As with all my projects I've probably over engineered it, but at least I learned something

Blueteeth.

Btw, thankyou all for your help/advice. Especially you audioguru for seeing my hint.