Paul McGowan writes:
If a preamplifier or phono stage uses only one power supply voltage to feed the circuit, as is typical in many classic tube designs, a blocking capacitor smack dab in the middle of the musical signal is required to interface to other kit. The reason this is true is that 1/2 the entire power supply voltage is on the output of the preamp or phono stage. In the case of a tube, with power supply voltages in the 200 to 300 volt range, that’s a lot of DC sitting on the output.
Transistor preamps and phono stages have a much easier go of it because we have a bit more freedom to run everything with two power supplies – which enables us to have zero volts on our outputs and inputs – thus we don’t really need blocking capacitors in the music’s path. The lack of these components means cleaner sound.
But nothing is perfect and when I write “zero volts” it isn’t entirely accurate. To be perfectly accurate there’s just really “low DC” on the output. Let’s call it on the order of 1/100th of a volt – chump change to be sure – but this small voltage can cause problems and here lies the rub.
Imagine a chain of three products all tied together and DC coupled without coupling capacitors. The first in our chain is a phono preamplifier, followed by a preamplifier and finally a power amplifier. And for argument sake let’s say the phono preamp has a DC output of 1/10th of a volt (0.1 volts). The preamp has a gain of 20dB (10X) and the power amp has a gain of 30dB (30X). Now, multiply the 0.1 volts out of the phono preamp through the preamp and it is now putting out ten times more DC than you put into it – or 1 volt of DC. All of a sudden, that’s a lot of DC. Now put that 1 volt of DC into your power amplifier – with its 30 times gain – and guess what? Yup, you now have 30 volts of DC on the output of your power amp, enough to fry any decent woofer. Not good.
So the problem isn’t so much the DC the units produce but what the attached products do with that DC that can cause the problems. So what do we designers do about it? Well we have a few solutions, the most common of which is to stop our designs from amplifying DC and just passing it along. How do we do that? The simplest method is to use our old friend the capacitor – but not directly in the signal path – indirectly in the path instead.
