I've heard of the 10 X rule, so my current preamp is way off then; 600 ohms X 10 = 6K ohms. Not even close to 50K. So as the previous poster mentioned I need min 2K ohms for a balanced pre and 5K ohms for single ended(?) I don't think I've come across a preamp with an output rating this high...
bluepearl, you may be a bit confused. The 10:1 rule of thumb is a rough
minimum to determine if your impedance match is greater than 10:1. In your case of 50K:600, that's a ratio of 83:1 and well above the minimum of 10:1. You're good.
Let me explain a bit further. The typical interface between a preamp (the source) and power amp (the load) is a line level, high impedance connection, where the output impedance of the source is usually in the order of about 50 ohms to 1000 ohms and the input impedance of the load is in the order of 10K ohms to 50K ohms.
This is known as a voltage bridge connection where the source is acting as a voltage source and almost no current is drawn. In fact, a perfect voltage source would have an output impedance of zero ohms at all frequencies. This would result in all the output voltage from the source being dropped across its load with no voltage being lost to the output impedance of the source.
Output impedance is basically the internal resistance of an amplifier seen at its output. The value can be slightly frequency dependent because of reactance caused by inductance and capacitance, but either way, a low output impedance is desirable.
In its simplest terms you can view the internal resistance of a source and its associated load as a voltage divider. The higher the output impedance of the source, the more voltage will drop across 'itself' instead of its load. This in effect means less voltage will be received at the load.
If the input impedance of a load device is not significantly higher than the sources impedance, the signal will be reduced or 'loaded down' and its signal to noise ratio and frequency response will suffer. Certainly the load can become too much for the source to supply adequately.
Generally, a high output impedance requires closer attention to cable lengths. The concern is that the high reactance (frequency dependent resistance caused by capacitance) of the longer cable, combined with a high output impedance of the source creates a low pass filter which adversely affects bandwidth. This distortion of the higher frequencies increases with higher output impedance's. To put it simply, your highs will suffer.
So, unlike speaker cables (where we don't give a darn about capacitance), with an interconnect it becomes important no matter what the output impedance, although the situation is exacerbated with a higher output impedance. The interconnects capacitance results in a parallel reactance (frequency dependent resistance) that will roll off the higher frequencies. It becomes more and more a factor with a higher impedance mismatch between the source and load.
Anyway, the old rule of thumb that says the input vs output impedance of a voltage bridge interface should be a minimum of 10:1 is specifying the minimum to ensure high frequency preservation. So, if I fed a load that has a 10K ohm input impedance, then I would just be OK with a 1000 ohm output impedance source, but I certainly wouldn't want to run long interconnect cables.
This reasoning is why they insist when you use a 'passive' preamp you use super short, super low capacitance cables, because they have fairly high output impedance's. This impedance usually varies too, because the output is connected directly to a potentiometer that alters the output impedance as the dial is turned - pretty undesirable. People weigh this off against the benefit of low noise of the passive preamp.
Tubes preamps also have fairly high output frequency dependent impedance. This is what gives them that 'warm' sound you hear about, (actually it's distortion). Cables are important to valve lovers for a reason.......Now you know why. In the case of a valve output of 600 ohms, you have no worries feeding an amplifier with a 50K ohm input impedance. I would start to be concerned about cables if the amplifier had an input impedance of 10K ohms though.
brucek
