Microphone Preamp design

[CSlee]

Hi all,

So im putting the 1KW project to one side, and am going for something a little smaller.

Im planning on building a mic preamp, I think possibly a stereo pentode preamp, and also a stereo triode preamp.

Now ive had a look at the other designs on this forum, like the Cascode mic preamp on this thread...

Microphone Preamp

.. it has some interesting points on it, I plan on stealing the pad and invert switch idea, but it seems very large.

I have designed this preamp to be as simple as possible, trying to minimize how many components the signal will go through to keep it as clean as possible.

Im really asking you guys if you can comment on the circuit, any obvious faults or changes I should fix. In the diagram there are 2 level pots, I think when building it ill only use one at the input.

I was going to attach a phantom power circuit I made, but its very similar to the cascode design on the other thread.

I think thats it for now, other than this is the transformer I plan on using.

High perforamance microphone and line input transformers

Looking forward to your comments.

Cheers

Charlie

[CSlee]

OK, Brian Sowter has very kindly suggested that I should use this for the input for more gain and less noise..

High perforamance microphone and line input transformers

And this if I want a balanced floating output..

Isolating and Balancing Transformers

very helpful, thanks Brian

[DF96]

There is no decoupling on EF86 screen grid, which will make pentode partition noise even worse. The ECC83 does not make a very good CF, as the gm is too low - try ECC81? Keep DC away from pots, and don't trust them as DC paths, so you need a separate grid leak resistor and DC block capacitor at the EF86 grid. A diode or neon from grid to cathode on the CF would prevent any arcing during warm-up.

Your diagram seems to confuse g2 and g3. Why is there a capacitor from g1 to ground? Why use a pentode at all? A triode would be quieter.

[CSlee]

Thanks for those pointers, ill change my circuit around.

The idea is to have the option for pentode and triode preamplification.

I have a 12AU7 spare, would this act as a better cathode follower?

thanks

[CSlee]

My mistake, G2 and G3 on the EF86 are the wrong way round, thanks

[DF96]

ECC82 is not a good CF as it too has low gm. Better to use ECC81 or ECC88.

[CSlee]

Here is an updated design,

Its essentially the white cathode follower from Morgan Jones, it just seemed like the next logical design step. And ive altered the EF86 to triode mode to reduce noise.

All your comments are really helpful guys

thanks

Charlie

[Arnulf]

Since you seem to be planning to run your EF86 a bit hotter than suggested by the datasheet perhaps a 100R between anode and G2 wouldn't hurt ? This keeps screen at slightly lower potential than the anode, thereby reducing screen current (and dissipation).

[CSlee]

thanks for that, the real reason for using the EF86 is that I have a couple spare. But if theres a more suitable valve id certainly consider buying one.

thanks

[Arnulf]

I think EF86 is a fine tube for microphone amplifier. If I were you I'd copy one of existing schematics (there are quite some floating around, use Google to look them up). This will give you decent results and something you can learn from.