Several of you have asked me how you can tell if the output capacitor of your preamp (if it has one) is adequate for the input impedance of your amplifier. If you’ll recall, I suggested in this post that a mismatch, while rare these days, will result in a loss of low bass or unwanted phase shift.
One thing you can do is ask the designers of your preamp if it is direct or capacitor coupled. If capacitor coupled then ask what the value of the output capacitor is. Once you know you can easily calculate the bass roll-off using an online calculator like this one. You need only two variables for this to work. The output capacitor value of your preamp, and the input impedance of your amplifier.
For example, reader Bill Riley wrote me the following note:
“BAT uses a 3uf output capacitor per phase in their preamps. They advertise their preamps will work fine with amplifiers which have 10K input impedance and higher. This may be overkill, just to make absolutely sure there is no roll off. I don’t know.”
Using our online calculator link we can see that these values provide us with a 5.3Hz roll off. So, what does that mean? BAT would tell you it’s meaningless since 5.3Hz is way lower than we can hear. Technically that’s correct but… I would tell you it’s too high because at that frequency you’ll get audible phase shift of the low-frequencies resulting in less than stellar bass or simpler put, wimpy bass.
Here’s what you need to know if you’re going to play engineer. Using the calculator gives you what we refer to as the 3dB down point. This is where a filter starts to do its work. The slope of the filter determines how much work it’s doing. In the case of a single capacitor and resistor, as we have in the BAT example, it’s called a one slope. Meaning that after the 3dB down point we just calculated, the filter rolls off the bass at 6dB per octave (an octave is doubling or halfing the frequency depending on which way the filter is moving). A two slope network is 12dB/octave, a three slope 18dB/octave and so far. Each additional slope adds an extra 6dB to the filter.
All this aside, what modern companies like PS Audio do is called Direct Coupling. This is where there is no output coupling capacitor to concern ourselves with and is how many modern amplifiers and preamplifiers operate. Direct coupling gives an important advantage. No roll off.
And now, you can see why direct coupling matters, even in a tube preamp.