Basic Common Emitter Amp Help

[Mooly]

Hi Obe1,
It's one opamp and as many inputs as you want.
It's a "virtual earth" mixer configuration.
You have each input going to it's own "R1" and then to the switch. Each input goes to a different pole on the switch. The wiper of the switch goes to the opamp... easy.

You select each "R1" to give the same volume for each source. It the classic inverting amp, gain being RF/R1.

[Mooly]

Originally Posted by Obe1

Mooly,

Your amplifier is just exquisite!

Thank you. It's all the casework and metal work that I don't like doing.

[Mooly]

OK that's me done for today...

[Obe1]

Originally Posted by Mooly

Hi Obe1,
It's one opamp and as many inputs as you want.
It's a "virtual earth" mixer configuration.
You have each input going to it's own "R1" and then to the switch. Each input goes to a different pole on the switch. The wiper of the switch goes to the opamp... easy.

You select each "R1" to give the same volume for each source. It the classic inverting amp, gain being RF/R1.

Ahhhh! Now I get it. Very clever.

[AndrewT]

Originally Posted by Obe1

Source 1 ---------------------> selector switch --|
Source 2 ---------------------> selector switch --|----> Vol. pot----> Power Amp
Source 3 ---> Preamp ----> selector switch --|

Source 1 ---------------------->selector switch --|
Source 2 ---------------------->selector switch --|-->Vol. pot-->inverting unity gain buffer>--------> Power Amp
Source 3 -->inverting Preamp -->selector switch --|

Adding the extra inverting opamp after the volume pot allows your pre-amp to drive interconnect cables and any sensible power amp input impedance.

[AndrewT]

Hi,
need to add that the last opamp can be made switchable phase, inverting/non-inverting with just a single pole single way and two extra resistors.

[Mooly]

Originally Posted by AndrewT

Hi,
need to add that the last opamp can be made switchable phase, inverting/non-inverting with just a single pole single way and two extra resistors.

Indeed it can.
I wonder, have you ever done any serious listening regarding absolute phase Andrew ? Must admit I never have.

[Obe1]

Originally Posted by AndrewT

Source 1 ---------------------->selector switch --|
Source 2 ---------------------->selector switch --|-->Vol. pot-->inverting unity gain buffer>--------> Power Amp
Source 3 -->inverting Preamp -->selector switch --|

Adding the extra inverting opamp after the volume pot allows your pre-amp to drive interconnect cables and any sensible power amp input impedance.

Hello Andrew,

Thanks! I'm game for trying this. For the sake of simplicity, should I use a coupling capacitor between the stages? And if so, would the cap go before or after the volume pot?

Best regards,

Obe1

[AndrewT]

the inputs to the selector switch can be labeled with the voltage sensitivity to drive the Power amp to clipping.

If the power amp has 20Vac maximum output and a gain of 30 then both input 1 and input 2 are labeled, 660mVac.

If the gain of the input 3 pre-amp is 2times (+6dB) then label 3 is 330mVac.

This third input could go to a second selector switch with different input resistors (or a switchable parallel resistor) to give alternative sensitivities.

I would have a DC coupled output to the RCA socket and a second parallel route through a 10uF polypropylene audio cap to the AC coupled output RCA. Add a 1M5 or 2M2 after the cap to discharge any static build up on the AC output pin.

I have not tried evaluating absolute phase.

[Mooly]

Obe1,
Volume controls should always be AC coupled as a general rule. That means a cap feeding the signal to the "top end" of the pot and another connected to the wiper (output of pot).
The reason why is any DC at all (even a millivolt or two) makes the pot "noisy" as you turn it.
Caps get a lot of bad press, usually undeserved.