If same as PA5 : 10KI couldn't find a spec but I'll bet it's somewhere around 10k to 22k ohms. I would plan on it being 10k ohms, as that seems pretty standard these days. What will you be driving it with? The output impedance of your sources should be no higher than 1k ohms. Almost all audio sources have low output impedance these days, but there are some analog preamps that have higher output impedance, especially vintage vacuum tube phono preamps.
Thanks Rongon and danibound for the info. Maybe it's not safe for my case: I do have a lovely Sansui Tu-517 tuner. It's only output is variable, and I guess its impedance must be quite high. I am not certain, but I do own an Accuphase T-101 which, according to the manual, has around 2k in its variable output (and only around 200 in the fixed output).If same as PA5 : 10K
Well your Sansui is 600 ohm output impedance, so that is fine. And a higher impedance on a variable output is less of a problem as you'll adjust the ouput level to compensate the voltage dropThanks Rongon and danibound for the info. Maybe it's not safe for my case: I do have a lovely Sansui Tu-517 tuner. It's only output is variable, and I guess its impedance must be quite high. I am not certain, but I do own an Accuphase T-101 which, according to the manual, has around 2k in its variable output (and only around 200 in the fixed output).
As long as both load and source are purely resistive (they should be), there should be no impact on THD or frequency response. Only signal level should be impacted by a relatively high output to input ratio.The 'rule of thumb' concerning matching components' impedances is that ideally the load (target) impedance should be 5X to 10X the source impedance.
The larger the ratio of Zout:Zin the better the signal voltage transfer, least THD, and widest bandwidth. But 1:5 (2k:10k) should be okay.
Every day is a school day. You are correct, I'd not considered the impact of any DC blocking capacitor.The thing is, the signal source will usually be AC coupled (have an output capacitor to block DC offset), which will form a high pass filter with the load impedance. This will be especially likely with a source having a vacuum tube output stage, as these will usually have low value output DC blocking capacitors with a value of 2.2uF or possibly less. (They need to be rated for at least 250V DC working voltage, and they're usually film capacitors as opposed to electrolytic types.) It is true that a DC-coupled output stage with near-zero DC offset shouldn't exhibit any reduction in low frequency response relative to mid-frequency. But how many vintage analog sources have a DC-coupled output stage?
Also, many line level amplifier stages will exhibit increased THD driving a heavy load as compared to how they'll perform lightly loaded. This is likely to be worse in a signal source that has high Zout, such as a vintage analog signal source like a tuner or phono preamp, and especially if those have a vacuum tube or discrete transistor output stage (as opposed to a modern op-amp). It is true that not all signal source output stages will exhibit markedly higher THD when heavily loaded. But many will.
Lets put to one side tube based designs. Niether of the tuners mentioned are tube based, and if someone is selecting tube based pre-amplification, we can probably assume they are not that worried about frequency response and distortion compliance.
Then FR flatness will depend on the ratio of total resistance (Output + input) to reactance of the blocking cap. In this case either of the resistors increasing in value will tend to flatten the FR. So Ouput R = Input R would have a flatter FR (Although at a lower level) than Ouput R< 1/10 Input R.
Similarly a high output R will actually reduce the load on the line level amp. I'd also hope that 20K would not be considered a significant load even for a vintage line stage. Though I have little knowledge of what such a vintage output may look like, so that hope might be misplaced.
you'd have to have a bloody long cable to get that at a typical 50 to 60pF / m (yes you made me look it up)and a long cable with let's say 2.5nF (2500pF)
Nor do I, but I have the full circuit in the service manual. I think TR 5 6 9 and 10 are part of a mute circuit (called pop protection). Basically switch them on and nothing comes out.I have to admit that I don't have the chops to just look at that circuit and see exactly how its output stage is working,