your opinion: Will this power transformer handle it?

[brad347]

I finished a Dave Royer Mod MXL tube mic and am very happy with the results. I carefully laid out the power supply to leave room for another power supply circuit board and pair of jacks in case I wanted to make another one.

Well, I'm so happy with it that I want to do just that. My concern is the power transformer being able to handle it.

In the schematic presented in this article, Royer specifies a transformer with a 24VAC secondary and a 1A rating.

The transformer I used is a very beefy-looking Stancor 24V transformer but it's rated only 40VA, or 1.67A at 24V. To power two microphones, this would be about 1/3 amp less than specified.

I also have a lamp hooked up on the secondary side of the PT. Would probably have to move that to the primary side and use a 120V lamp. Or maybe not. How much does a little 24V bayonet-base lamp draw?

Does anyone here know more about the precise current draw demands of the Royer mod? Is the 1A transformer on the schematic over spec'd as some designers do? Does one of these mics really draw 1A of current?

Thanks in advance for any info you can provide.

[Matt Syson]

Hi
The rating will be fine.
a valve heater would take the most power in this application and is a little under 2 watts (VA).
The circuit is pretty wasteful getting 6.3 volts from a 24 volt transformer but even so it is not a massive deal (maybe 8watts, I didn't read the whole article). Yes I know Watts are strictly speaking not the same a VA but it is close enough for this job.
Matt S

[brad347]

that is great news. In your opinion, do I need to move the lamp to the primary side and use a 120v lamp? I estimate the lamp draws about .3 amps.

[Matt Syson]

Hi
No you don't need to.
I would suggest using a 28 Volt bulb taken from the secondary of the transformer, with perhaps a small resistor as well so that it 'glows' rather than 'shines'. On the AC secondary you are not 'wasting' the smoothing which would be the case with using it at a DC point and the wiring will be at a 'safe' 24 volts rather than having more wires at 120 Volts.
If I were doing this project I would be tempted to put a regulator on the heater supply as they are so cheap and it would buy some 'insurance' against high mains input and give a more consistant performance. A LM317 variable regulator would be fine, as would a 6 volt version of the LM7806 or similar family. The extra parts would run to another Dollar or thereabouts.
Matt S

[brad347]

thanks a lot for the info. It all makes my life easier.

Anyone considering this project should try it. I'll be at less than $400 all in for the pair of mics and I used NOS Mullard tubes, Jensen transformers etc (quality stuff)

I am getting really good results.

Here's a test recording I did of various sources in an untreated room.

http://www.soundsdifferent.com/music/MXL_Test.mp3