Buffer, Gain??

[Shakey 1047]

I have just built this buffer and it works great, now the only thing I'm wondering is, what would I have to change to get a few dB of gain. I did try the Tillman buffer which offers about 3dB but could never get a clean sound(even after building 6 or 7 of them. I would like to be able to adjust the gain also. Can anyone suggest anything. Cheers Dave.

[teemuk]

I have just built this buffer and it works great, now the only thing I'm wondering is, what would I have to change to get a few dB of gain. I did try the Dillman buffer which offers about 3dB but could never get a clean sound(even after building 6 or 7 of them. I would like to be able to adjust the gain also. Can anyone suggest anything. Cheers Dave.[ATTACH=CONFIG]17069[/ATTACH]

Google search: common emitter amplifier

You will never get any voltage gain from an emitter follower.

I'd jump straight to opamp based circuits, though.

[Jazz P Bass]

If you want gain, then you need another stage after the emitter follower (which will have slightly less gain than unity)
Multi-stage Amplification

[Shakey 1047]

Please forgive my ignorance but you are talking about emitters. This is a Jfet circuit, I'm afraid you will have to explain it to me as I don't know where the emitter is in this circuit.?????????

[Jazz P Bass]

We are into semantics here.
BJT transistors as a buffer would be refered to as a common emitter configuration.
Substitute a JFet and it is now a common source.
It still is true that a buffer, as such, will have an output that is less than the input.
It will have a high impedance input so it will not pull down on the input signal.
And it will have a low impedance output that can "drive" another stage (which you seem to want, if you want more gain).
So you need another stage.

[mozwell]

In between the drain & +9V, put a 22k resistor. Move the + end of the capacitor from source to drain.
This will give you some voltage gain.
Try different values of resistor between 10k & 22k, or use a 10k resistor with a 10k pot in series (wired as a variable resistor).

Whatever current there is through the source 10k resistor, will also be through the drain resistor, so if the drain resistor is twice the ohms of the source resistor, you will have a 2x gain.....
try different resistor values till you get something you like.

[Shakey 1047]

Thanks Mozwell

[Jazz P Bass]

Yeah but,...
Now it is not a true buffer.
Read this web site.
The whole jfet buffer, gain thing is explained very well.
Link: JFET Buffer and buffer cable

[Arthur Dent]

The schematic in the first post is not a common source circuit, it is a common drain, sometimes known as a source follower. Mozwell has turned it into a common source circuit. Common drain circuits have basically no voltage gain, but they have a very low impedance output. Common source circuits generally have the most gain of any of the three typical JFET configurations, but relatively high output impedance. Most JFETs have a much lower output impedance than a pickup anyway, so this doesn't really matter much in my opinion. Then there's common gate circuits, which are a bit more complicated. I use them all the time, but you shouldn't .

Jazz P Bass, what on earth makes a common source circuit not a "true" buffer? It would also be a gain stage then, but I see no reason why that would disqualify it from fitting the definition of a buffer. A common source stage can be used as a buffer. It accomplishes the same thing, in any case.

Shakey, that circuit as is will not produce any gain. Modify it as Mozwell said and it should have some. If you want more, put a large (~5uF or more) capacitor from source to ground, in parallel with the 10k resistor. To adjust the gain then, put a variable resistor in series with that capacitor.

[Shakey 1047]

Thank you Arthur, your reply is exactly what I wanted, no disrespect to anyone else that replied but this I understand.
Cheers Dave.

[J M Fahey]

Agree that putting a resistor at the drain will give you some gain.
Fine.
But you said the Tillman circuit didn't work for you .... and you are building exactly that.
*Maybe* you have hot pickups and the 4 o5 5Vpp available with the "Tillman" or the mod proposed , combined with a mild 3 to 6 dB gain are not enough.
Tough call.
If that's the case , maybe you should follow the Teemuk path (Op Amps) which offer the best of both worlds.
I'd only add, spend a little more and get some rail to rail Op Amp; a standard TL072 or similar has trouble getting much closer than 2V from any rail (including ground), which is bad for headroom and sheer signal swing.
If anything, the humble TL061/62 family is better adapted to such low voltages.