Standalone preamp: output impedance/buffer question

[jaywalker512]

So I'm building a standalone tube guitar preamp and I've got yet another question, this time about output impedances and power amp input impedances and such.

The output impedance of my tube preamp is pretty high, so I plan to use a jfet or bipolar transistor to buffer the output (some potential examples blow), thus lowering the output impedance and eliminating my problem. The power amp I'm using is a solid state power amp with a 10k input impedance. All sound/look good?

http://beavisaudio.com/techpages/Buf...Transistor.gif
http://beavisaudio.com/techpages/Buf...Transistor.gif

Related situational question: The power amp is actually a Fender Frontman 212r and I'm using the effects loop in. The manual says its sensitivity is "930mV for 100W into 4 ohms at 1Khz". However, my preamp puts out some 15-40v AC and the Fender power amp still doesnt put out the full 100 watts that the Fender alone (as a combo amp with its own preamp) does. I've measured its preamp out, and it maxes out at 0.9v AC, and that makes it blast out its full 100 watts. It might seem like a stupid question, but is this only because of the largely different output impedances (my tube preamp being at least 30K ohms, and the Fender preamp being an unknown much lower impedance because it is solid state)?

For me to use one of these transistor buffers, is the output impedance the only thing I need to worry about for driving low impedance power amps, or should I make it have some current gain? IE: does a low output impedance imply higher current output than a same-voltage high impedance output?

[GainFreak]

If your preamp's output is really in the 15-40v range the "regular" buffers you linked to are going to distort like hell. With this output you should either think about tube buffer or a high voltage MOSFET buffer.

[guitician]

You could try taking your output from the cathode of the last stage of your pre-amp. That should help drive the SS power amp.

[Steve Conner]

For me to use one of these transistor buffers, is the output impedance the only thing I need to worry about for driving low impedance power amps, or should I make it have some current gain? IE: does a low output impedance imply higher current output than a same-voltage high impedance output?

Yes. A thing that converts a high-impedance signal into a low-impedance signal does so by having current gain.

More mathematical explanation: Impedance is voltage divided by current, so for the same-voltage condition you stated, a reduction of impedance requires an increase in current.

RG Keen has some information about MOSFET buffers on his site.

[jaywalker512]

If your preamp's output is really in the 15-40v range the "regular" buffers you linked to are going to distort like hell. With this output you should either think about tube buffer or a high voltage MOSFET buffer.

You could try taking your output from the cathode of the last stage of your pre-amp. That should help drive the SS power amp.

Right, forgive me for not specifying that I have a circuit worked out to cut the voltage down to acceptable levels for the transistors. Also, I will run them from a 16v power source so they have a bit more headroom. Id rather avoid using a cathode follower, mainly because I dont think the tubes I'm using could take it without making some major changes to my circuit. Or were you meaning something else?

Yes. A thing that converts a high-impedance signal into a low-impedance signal does so by having current gain.

More mathematical explanation: Impedance is voltage divided by current, so for the same-voltage condition you stated, a reduction of impedance requires an increase in current.RG Keen has some information about MOSFET buffers on his site.

Alright, so given that I lower the preamps output voltage as to not overdrive these transistor buffers, they would still do the job (I'm still a noob to transistors)? The simulations I've done with them dont seem to have much current gain, or would a circuit like this be better suited (even though it provides voltage gain as well, so I suppose the other examples would still be lower output impedance than this one?):

[Gregg]

Alright, so given that I lower the preamps output voltage as to not overdrive these transistor buffers, they would still do the job (I'm still a noob to transistors)? The simulations I've done with them dont seem to have much current gain, or would a circuit like this be better suited (even though it provides voltage gain as well, so I suppose the other examples would still be lower output impedance than this one?):

You need a buffer, not a gain stage.

[Albert Kreuzer]

RG Keen has some information about MOSFET buffers on his site.

Listen to Steve:

http://www.geofex.com/Article_Folder...osfetfolly.htm

HTH
Albert

[jaywalker512]

Hey guys, sorry to bring up a dead thread but I'm having trouble with the buffer I've been trying to use. I chose to go with the first buffer I listed in my first post based on the MPF102 because it was the cheapest and simplest. Now the problem I'm having isnt getting it to work, because it works fine when I power it from a 9v battery. However, I want to power it by rectifying the 6.3vAC heater supply of my preamp for ~9v so I dont need a separate power source. The problem I'm having is incessant buzzing/humming but only when I power it from the heater supply. I've replaced the filter cap, all the diodes in the bridge rectifier, to no avail. Here's what I'm working with:

A schematic for how the buffer is wired (as seen immediately after the tone stack) (note, the schem here shows a 2N3819 in the circuit, but only because I dont have an MPF102 model for LTspice. I'm using an MPF102 in reality):


And a schematic of how its wired up for power:

[Steve Conner]

That just won't work. And if your center tap really is grounded, you'll burn out the diodes and/or your heater winding. Think about it.

I and others have published circuits here in the past to get a 12V DC supply from a heater winding.

[jaywalker512]

Alright, well could you point me in the direction of one of these circuits (I couldnt find one searching your posts)? If possible Id like to be able to power this buffer and maybe a 5w power amp chip without having to use a separate PT. Though I did already try powering the buffer alone from a separate 12v transformer and had the exact same problem as I am now.

[jaywalker512]

Sorry to bump this again but could someone please give me an example of what Steve was talking about (12v supply from heater winding)? Or if it's completely impossible to power the heaters and the buffer from the same winding, could someone confirm?