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Tube Microphone Pre-amp Inputs
#1
I've been thinking about building some microphone pre-amps for recording purposes. Since the microphones I'm using are Lo-Z I was looking at matching transformers and at the cathode input circuit from TUT-2.  I've built pre-amps with matching transformers but have not had any experience with a cathode input circuit on a mic pre-amp before.  Are there advantages or disadvantages to that circuit versus a transformer?  Do they sound different?  Thanks guys!
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#2
Hi makinrose

Most microphones are low-z and have very low noise as a result. To match this to a tube circuit without compromising the overall noise usually requires a step-up transformer. The Tx provides "free gain" and gets the signal up to where the tube's noise is less problematic and the overall circuit can actually compete with SS performance.

The cathode of the tube can be used as an input with the grid at AC ground. The tube exhibits the same gain this way but z-in is now much lower allowing a direct connection to a low-z source. The tube's noise can be lower this way but you now have to manage the microphone connection so there is no DC through it and low-frequency response is not impaired. The latter requires the use of a high-value cap, preferably polypropylene. Fortunately you can get some seriously high values on PP now and they are not too large.

An alternative to using the cap is to use a split rail supply for the tube and add a DC servo circuit to minimise the DC offset. This involves an opamp and maybe a level shift transistor, none of which is in the forward signal path.

With a grid-driven interface, there is the opportunity to have variable gain for the first stage, where this is difficult to do with the cathode input interface.

Of course, any circuit optimised for low-THD should sound like any other, but that is not always the case. Such optimisation with tubes means you are essentially trying to rid the signal of any tube character which leads to the question: why use a tube then? I would suggest building both circuit types and evaluate them for yourself.
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#3
Kevin,
Thanks for the through and informative response. I've currently have built a version with a transformer input so I think I'll give the cathode input a try. You make an excellent point about PP caps. After I posted this I did find one example of a 50's console that used a cathode input that used and electrolytic cap at the input. The author commented on this as being problematic.

You've raised an interesting question about why I'd want to use tubes as well. I have been thinking about as well especially since SS circuity could easily and cheaply do the job with great fidelity. In my case my goal is add character to the sound. While I'm concerned about adding noise I'm much less concerned about perfect fidelity. The digital recording interface I have has great fidelity but sounds cold to my ears. It's going to be an experiment!
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#4
Hi makinrose

The digital interface will sound cold mostly because its distortion harmonics are high-order and its IM is not great. It is IM that makes us feel that a sound is good, bad, harsh, cold, etc, much moreso than the THD present. Using tubes to add a veil of low-order THD harmonics and the attendant IM will warm the sound overall.

Electrolytics have leakage currents that are too high to let the cap be effective in this coupling application. The same applies to any cathode-tied cap for gain control. The pot will be scratchy in the latter case and there will be contact noise with the microphone. The DC offset will physically offset the mic diaphragm from its usual resting point, leading to asymmetric distortion of the captured sound.

The other problem with an electrolytic in most signal coupling apps is aging. The cap stays healthy with DC across it, so if there is no DC as in an opamp circuit with split rails, the cap dries out over time and its ESR, DA and DF go crazy contributing distortion to the circuit that will swamp that of the active devices. In the cathode-coupled mic pre, there will at least be a tiny DC bias voltage across the cap to keep it healthy to an extent.

A plastic coupling cap of any type eliminates all of these issues, but you have to use polypropylene to have zero THD from the cap itself.
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