04-03-2017, 10:18 AM
Don't forget Miller effect on the 5M6!
I've found the schematic on RM.org. I won't upload it, but Mike could as he put it on RM.org originally
The output of this stage feeds the tone control network and volume control. So the impedance they present - in parallel with RL (Ra3) - is highly dependant on frequency and the settings of the user controls. That makes it near-impossible to determine the open-loop gain of the first stage, and hence near-impossible to predict the Miller effect on Ra1.
The input impedance of the following stage is rather low as well.
My 14k does seem unreasonably low, and as I said, it won't be that low in practice because of the above. But it will vary according to the position of the user controls. Bad design, but presumably "good enough", especially if the gain structure is such that the volume control can never get near to "max" before gross clipping sets in...
Trevor makes a good point about the 1n input capacitor. But don't forget the source impedance of the cartridge, which is in series with the capacitor. If that's 1M, then the LF rolloff will be in the order of 100Hz. The DIN socket is worrying, because signals from that also go through the 1n caps, but the source impedance from that might well be 100s of k.
To get a 100Hz rolloff with 1n in circuit, the total impedance "seen" by that 1n cap would need to be 1.6M. As described, the "best-case" input impedance be ~500k, before it's been lowered by Miller effect.
I've found the schematic on RM.org. I won't upload it, but Mike could as he put it on RM.org originally
The output of this stage feeds the tone control network and volume control. So the impedance they present - in parallel with RL (Ra3) - is highly dependant on frequency and the settings of the user controls. That makes it near-impossible to determine the open-loop gain of the first stage, and hence near-impossible to predict the Miller effect on Ra1.
The input impedance of the following stage is rather low as well.
My 14k does seem unreasonably low, and as I said, it won't be that low in practice because of the above. But it will vary according to the position of the user controls. Bad design, but presumably "good enough", especially if the gain structure is such that the volume control can never get near to "max" before gross clipping sets in...
Trevor makes a good point about the 1n input capacitor. But don't forget the source impedance of the cartridge, which is in series with the capacitor. If that's 1M, then the LF rolloff will be in the order of 100Hz. The DIN socket is worrying, because signals from that also go through the 1n caps, but the source impedance from that might well be 100s of k.
To get a 100Hz rolloff with 1n in circuit, the total impedance "seen" by that 1n cap would need to be 1.6M. As described, the "best-case" input impedance be ~500k, before it's been lowered by Miller effect.
