Aurycle DIY Tube mic kit.

Hey Everyone,

Next fortnight I thought that, since I've only got one decent mic, a Focusrite Scarlett CM25 condenser mic, I would finally get around to ordering an Aurycle DIY Tube mic, here's the link to the web page:

http://www.aurycle.com/aurycle-a5500...icrophone.html


There is one downside, it looks like the pre-built power supply has been designed to run on the US 120 V AC mains power system, so, I will need a 240V AC to 120V AC stepdown transformer to power it properly, I think I might be able to get one from my local Jaycar Electronics shop though.


It looks relatively easy enough to build.

If I was in the market for this kind of microphone then I'd spend the extra $30 and get the multi pattern version... But it is in US$ and there will be freight on top of that so when optioning up these things money wise can get out of hand pretty quick. And as it is from the US there potentially is that 120VAC only issue with the power supply.

It's a little disappointing that only 1/2 of the 12AX7 is being used, which makes me think that if I got the kit and it's already built to take a 12AX7 sized tube then I'd be keen on experimenting with single pentode tubes like the EF86 just to see what happens... or you could just play with the 2nd half of the 12AX7 with cathode follower circuits or active filtering...

Yeah I noticed that while checking out the circuit diagram, I reckon it would be quite easy to wire up the second half of the 12AX7 tube as a cathode-follower, you'd only need to connect anode 2 (pin 6) to the junction between R1 (10k) and R2 (100k) where C1 (1uF/400V) connects to, then connect grid 2 (pin 7) to anode 1 (pin 1), and then connect cathode 2 (pin 8) to ground via say a 100k resistor (a typical value found in most Marshall amps), R7 (3k) might need to be tweaked a bit, and then connect C8 (1uF/250V) to cathode 2, I think doing that will probably reduce the residual harmonic distortion a bit by lowering the output impedance, it might also change the tone a bit too, my understanding is that the output impedance of a typical common-cathode triode amplifier stage, such as the one formed by the first half of the 12AX7, is pretty high (something like 68k or so but I could be wrong), whereas the output impedance of a cathode-follower is lower, therefore it would be able to drive the output transformer better.

I may even look into doing that modification.

If you look on the main products page you'll notice that the PSU can be set for 150V or 230V operation. The CE marking should also have been a giveaway as they wouldn't have CE marked it for a product that couldn't be used in Europe.

Be careful when messing with valve mic circuits. They are never going to be as quiet as a FET driven mic, so you want to minimise the circuit noise. Also, one reason that people want a valve mic is for the harmonic distortion you get when they are driven hard (but you don't want too much distortion, just a slight colouring). It's not a valve guitar amp, there's no gain control on it and the higher the circuit gain, the more noise it's likely to pick up. You only need enough gain to get the output similar to a FET mic. You'll be better off noise-wise in using a bit more gain on your nice quiet mic preamp than adding a lot more from a valve at the mic end.

If you want a quiet valve mic, then go for a FET model.

Grounding everything can be an issue with these kit mics. The capsule enclosure needs to be well grounded otherwise the capsule can pick up a lot of noise. I've seen several kit mic builds on Sound On Sound where the makers have struggled to get a good ground for the grille assembly and get a huge amount of noise as a result.

Like Marcel, I'd be tempted to spend the extra money and go for the multi-pattern version. Far more versatile.

Two features missing from the mic that would make it even more useful are 1) a pad switch and 2) a high pass filter switch.

Quote:

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Originally Posted by Simon Barden

If you look on the main products page you'll notice that the PSU can be set for 150V or 230V operation. The CE marking should also have been a giveaway as they wouldn't have CE marked it for a product that couldn't be used in Europe.

Be careful when messing with valve mic circuits. They are never going to be as quiet as a FET driven mic, so you want to minimise the circuit noise. Also, one reason that people want a valve mic is for the harmonic distortion you get when they are driven hard (but you don't want too much distortion, just a slight colouring). It's not a valve guitar amp, there's no gain control on it and the higher the circuit gain, the more noise it's likely to pick up. You only need enough gain to get the output similar to a FET mic. You'll be better off noise-wise in using a bit more gain on your nice quiet mic preamp than adding a lot more from a valve at the mic end.

If you want a quiet valve mic, then go for a FET model.

Grounding everything can be an issue with these kit mics. The capsule enclosure needs to be well grounded otherwise the capsule can pick up a lot of noise. I've seen several kit mic builds on Sound On Sound where the makers have struggled to get a good ground for the grille assembly and get a huge amount of noise as a result.

Like Marcel, I'd be tempted to spend the extra money and go for the multi-pattern version. Far more versatile.

Two features missing from the mic that would make it even more useful are 1) a pad switch and 2) a high pass filter switch.

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Ah okay, I thought that the power supply was only 120V AC, so then if it can be set to 230V AC then all I'd need to do is replace the existing mains plug with an Australian 3-pin 240V AC type, I know where I can buy them from, I guess I could spend some extra money and get the multi-pattern version, I'll have to wait till I see what next fortnight's budget is, I have a feeling I might have a bill from my ISP soon, I recently paid my last power/water bill so I won't get another one for another two months or so, if everything goes well with regards to building the kit up into a working mic, I may order another one so I'll have two that I can use, my Focusrite Saffire Pro40 audio interface has eight mic/line inputs, two of them have pad switches included so no pad switch on the mic itself isn't really an issue for me, although I agree, a high-pass switch would have been a useful extra feature, I'm seriously considering buying a small multi-channel mixing desk for my studio room later on this year, most modern desks do include high-pass filters on the channel strips, at least the ones I've had a look at anyway.


I do understand that these tube mic kits aren't going to be anywhere as good, or high-endy as say a Neumann U47, which are hideously expensive, especially if it's a vintage one.


As a side-note, yesterday I did some work on a 60's Electrovoice EV-676 dynamic mic for a mate of mine, it's the one that Jim Morrison from The Doors used onstage, anyway, my mate wanted me to mod it so that he could plug it into an XLR socket on his PA mixer, the mod I did to it worked perfectly first time and my mate is really happy with it, here's what the mic looked like:

Attachment 24914

The pad on the mic would be between the capsule and the valve, to stop the valve going into mild overdrive if you're recording a loud source you want to keep clean, rather than let the valve drive a bit to add some valve 'character'.

The pad on the interface obviously reduces the signal level into the preamp stage to stop the A/D converter being overloaded, but can't prevent the mic from sending an overdriven signal.

These mics can be made to sound very good (they'll sound good anyway), but as they say, you'll need to swap out most of the provided components for more upmarket devices - especially the transformer and probably the capsule.

Capacitor C4 is the one that needs the most care in installing as it needs to be well insulated from ground. That's why it's provided with ceramic insulators for the leads, to keep the body away from the board.

Quote:

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Originally Posted by Simon Barden

The pad on the mic would be between the capsule and the valve, to stop the valve going into mild overdrive if you're recording a loud source you want to keep clean, rather than let the valve drive a bit to add some valve 'character'.

The pad on the interface obviously reduces the signal level into the preamp stage to stop the A/D converter being overloaded, but can't prevent the mic from sending an overdriven signal.

These mics can be made to sound very good (they'll sound good anyway), but as they say, you'll need to swap out most of the provided components for more upmarket devices - especially the transformer and probably the capsule.

Capacitor C4 is the one that needs the most care in installing as it needs to be well insulated from ground. That's why it's provided with ceramic insulators for the leads, to keep the body away from the board.

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Yep, C4 is the coupling capacitor between the capsule and pin 2 of the 12AX7 tube, it's job is to block the DC polarizing voltage but let through the AC signal, what's more is that C4 is located in a high-impedance circuit, so any leakage (either high or low impedance) path to ground will degrade the performance of the mic, hence the ceramic insulation.


And then there's R6, which is a 200M grid-leak resistor going to ground from pin 2 of the 12AX7.

The DC polarizing voltage is supplied through R10, which is a 1G, or one giga-ohm (that is if I've interpreted it correctly and I'm not mistaken) resistor, that means there's not much current being supplied to the capsule, so, any leakage path to ground is going to load the capsule's DC polarizing supply and cause it to drop quite a bit, a high-impedance leak will also cause noise too.


1 Giga Ohms equals 1,000 Mega Ohms.