What's the input impedance at the Collector?

[Bill Mountain]

I have been thinking about ways to make HI and Lo gain inputs for a dirt box I'm working on.  I have it figured out using an op amp with switchable gain settings but this is one of the ideas that I tossed around that intrigued me.

The only thing I wasn't sure of was whether or not there would be any tone suck without any isolation between the LO Gain input and the output of the transistor in the HI Gain input section.



You can ignore the opamp values.  I just placed it there to give you an idea of what I was trying to do with the seperate inputs.

Thoughts?

[Derringer]

how about just using switching jacks?

[Bill Mountain]

how about just using switching jacks?

Like a jack with a switch that breaks the connection?  That should work but I don't have any so I didn't think about it.  I'll look into it.  

I think I'm going to go with an op amp anyway because I wanted to have a high input impedance for both gain settings.  I have an opamp set up as a non-inverting gain stage with the resistor to ground attached to the ring lug on a stereo jack at the high gain input.  When the low gain jack is used the resistor is not grounded so the opamp becomes a unity gain buffer.

I haven't tried this yet but I'm pretty sure it'll work just fine.  If not I'll look into getting some switching jacks.

I think I'm going to breadboard the transitor version this weekend to see if there is any noticable tone suck.

[Gurner]

It'll suck.

A lot of folks only worry about impedance to ground, but in that schem -  parking what impedance the tranny presents (dunno, but it'll be lowish) - even removing it from the equation you've only 15k of resistance to your 9V rail  there ...a low impedance is a low impedance ...be it ground (the lowest) or a supply rail (still pretty low...even if a battery)

[Bill Mountain]

It'll suck.

A lot of folks only worry about impedance to ground, but in that schem -  parking what impedance the tranny presents (dunno, but it'll be lowish) - even removing it from the equation you've only 15k of resistance to your 9V rail  there ...a low impedance is a low impedance ...be it ground (the lowest) or a supply rail (still pretty low...even if a battery)

That's a good point. I could always try raising the collector resistor.

[ashcat_lt]

Isn't this normally accomplished via resistive divider?  Build the circuit with all the gain you want for the "high gain" input and then attenuate the "low gain" input.  I guess this usually expects a low-z active input to the "low gain" channel.  You'd need pretty big resistors to avoid too much treble loss via loading.  The bottom R in the divider will limit the input-Z of both inputs.

I think your trick with the opamp is a pretty cool idea, though.

[Bill Mountain]

Yeah the fender style input resistor divider would muck up the input impedance. The circuit was actually built for my main bass (which is passive with an ultra high output) but my guitars and my other bass sounded weak through it so I wanted to add a boost for when I don't have my main bass.

[Ronan]

A possibility is you could switch the transistor block in or out with a DPDT toggle switch, as a boost on/off switch.

[PRR]

Nevermind the collector impedance.... that transistor's base-input impedance is 470K/150 or ~~3K.

[Bill Mountain]

Nevermind the collector impedance.... that transistor's base-input impedance is 470K/150 or ~~3K.

I believe you but I figured it was closer to 100k.  I've tried to figure it out before but the math is over my head.  I read somewhere that a BMP has a 100k input impedance so I just go with that.

[gritz]

The 470k feedback resistor makes the circuit behave in a similar way to an inverting op amp (with the caveat that the tranny's open loop voltage gain is much smaller) so that the base of the tranny is *almost* a virtual earth. Similarly, that feedback resistor makes the impedance seen at the collector much smaller: pulling the collector more +ve increases base current, so collector current increases, resisting the +ve pull, etc.