The I-pod produces a stereo signal, R & L. The two 100 Ω resistors are low enough to cause some cancellation of the combined signal if the phase of the R and L signals from the I-pod are not equal in phase and amplitude - which most of the time, they won't be. Hence, a reduced input to the volume control and a reduced O/P from the valved amplifier occurs. So I suggest that you increase the values of those resistors for a start: this increases the inter-channel isolation from the I-pod. Then there is the capacity of the capacitor that sits between those two resistors and the top of the volume control to consider. Since the amp. is a valved unit, I think it is safe to assume the volume control is in the range 500 kΩ → 1 MΩ, so a capacitor in the range 0.1 µF → 0.01 µF should be suitable.
All of that is summarized in the drawing below. The jack socket you're using is not shown, but you should be able to incorporate it into that drawing. It needs to sit between C1 and the 'top' end of the volume control.
[attachment=6513]
Al. / Nov. 27 //
All of that is summarized in the drawing below. The jack socket you're using is not shown, but you should be able to incorporate it into that drawing. It needs to sit between C1 and the 'top' end of the volume control.
[attachment=6513]
Al. / Nov. 27 //
