I am working on an incarnation of the preamp in my old, beloved Chilton QM3 console, and Ive hit a little snag.
(schematic attached)
If I load TP4 with an output transformer, I have quite a bit of current on the transformer, and correspondingly, quite a bit of signal (voltage) loss.
but thats not all...
I also have an active stage I'd like to be able to switch into the after circuit after TP4 with a high impedance, opamp input. When I switch it in, current through TP4 is restricted, but even though I have no signal (voltage) loss, it introduces hum (I am guessing that the lower drive current and relatively higher voltage messes with the transistor biasing - I'd like to know if that explanation makes any sense, or I have taken a wrong turn thinking).
So for these reasons, I'm thinking the solution would be to put "something" after TP4 of this circuit that does not put a load on source current and puts out a low impedance signal that retains the signal voltage.
A low impedance input, low impedance output driver/buffer!? I only have examples of low impedance output drivers, but none with the low impedance input I would like to try.
...Also...Vbe on Q3 is 0.3V no matter how the circuit is loaded...thats odd, right? So maybe I "just" have some problem that should be solved, which would result in a lower source current on TP4 and cancel the need for the interstage.
Gustav
(schematic attached)
If I load TP4 with an output transformer, I have quite a bit of current on the transformer, and correspondingly, quite a bit of signal (voltage) loss.
but thats not all...
I also have an active stage I'd like to be able to switch into the after circuit after TP4 with a high impedance, opamp input. When I switch it in, current through TP4 is restricted, but even though I have no signal (voltage) loss, it introduces hum (I am guessing that the lower drive current and relatively higher voltage messes with the transistor biasing - I'd like to know if that explanation makes any sense, or I have taken a wrong turn thinking).
So for these reasons, I'm thinking the solution would be to put "something" after TP4 of this circuit that does not put a load on source current and puts out a low impedance signal that retains the signal voltage.
A low impedance input, low impedance output driver/buffer!? I only have examples of low impedance output drivers, but none with the low impedance input I would like to try.
...Also...Vbe on Q3 is 0.3V no matter how the circuit is loaded...thats odd, right? So maybe I "just" have some problem that should be solved, which would result in a lower source current on TP4 and cancel the need for the interstage.
Gustav