valve mic noise

[wthrelfall]

I've got a Red5 audio valve mic which is pretty noisy.  I've tried swapping out the valve for known working ones but to no avail.  I ran an analyser on the input and interestingly there are spikes at 100hz, 200, 300, 400, 500.. etc, every 100hz up to 1k.  what can be causing this issue?  PSU?

 

[Matt Nolan]

Yes, those are the harmonic frequencies from a full-wave rectified power supply on the UK 50Hz mains.

Surprised they are that strong. Did you have the mic or PSU case open?

 

[wthrelfall]

No, that's with case on under normal conditions.

 

[synthiaks]

PSU filter Caps?

 

[Matt Nolan]

PSU filter Caps?

Sounds like it.

 

[ruffrecords]

Sometimes it can be due to using semiconductor rectifiers. These can cause sharps spikes when they turn off and the spectrum of these spikes is lots of harmonics of  once or twice the mains frequency. They will benefit from 100nF snubber capacitors across them or, if they already have them, perhaps they need replacing.

Cheers

Ian

 

[PRR]

> pretty noisy

50/60Hz and overtone spikes may stand-up above the hiss floor yet have cause very little offense to the ear.

What does it *sound* like? HISS or BUZZ ??

Totally different causes and cures.

 

[Monte McGuire]

If the microphone power supply is grounded through its AC connection, there could be a ground problem between the mike and the preamp. If this is the case, the "mike will produce noise" when the mike's connected to the preamp but the mike's power supply is turned off, since the noise is entirely from the ground loop between the preamp/console and the mike PSU.

If the noise only happens when the mike's tube is powered up, then there is something wrong with the PSU - maybe defective filter caps?

 

[Matt Nolan]

Sometimes it can be due to using semiconductor rectifiers. These can cause sharps spikes when they turn off and the spectrum of these spikes is lots of harmonics of  once or twice the mains frequency. They will benefit from 100nF snubber capacitors across them or, if they already have them, perhaps they need replacing.

Cheers

Ian

If this is the issue and there isn't room for snubber caps, you can use fast recovery diodes like Schottky diodes instead of bog-standard rectifiers.

 

[Gus]

I googled "Red5 audio valve mic" http://www.red5audio.com/acatalog/Valve_Microphones.html

 

[wthrelfall]

Thanks to everyone for the useful advice.  Looks like the PSU is the culprit, probably why they have an "updated psu" advertised with the mic on their website.  This one is a few years old and i'd imagine was sold with the original psu. 

 

[CJ]

those FRED's are expensive, and they pop like popcorn,

alternate?  vac tube rectifier, just a small one for this app, maybe a 6X4, no switching noise here,

smooth as butta, 

buttachunk? have not seen him in a while,

 

[wthrelfall]

here's the inside:

 

[trancedental]

Got one of these old Red5Audio RV15 valve mics,  seems that it likes a change of valve, tried a few old Mullards & Brimars instead of the stock 12AX7, a  Mullard 12AT7 / ECC81 sounds much better in there! The PSU is quiet on this model.

Does anyone know what type of design or circuit it uses, looks like a AKG Solidtube or similar! Any advice on improvements would be more than welcome!

 

[radardoug]

You cant just put a 12AT7 in place of a 12AX7! They are totally different tubes, even though the pinout is the same!

 

[abbey road d enfer]

You cant just put a 12AT7 in place of a 12AX7! They are totally different tubes, even though the pinout is the same!

People do it everyday, with different results. Typically, a 12AT7 will be less noisy than a 12AX7 and have less gain. Its operating point may not be optimum, but it will make sound.

 

[radardoug]

Duh! Of course its less noisy, its got less gain! Just because the tube has the same pinout, doesn't mean you should put it in there!

 

[abbey road d enfer]

Duh! Of course its less noisy, its got less gain!

No. It has less EIN (Equivalent Input Noise) because Gm is higher. Noise  and Gm are related. The higher Gm, the lower the noise.  Indeed, the fact that gain decreases also decreases the output noise. But if the tube was included in a NFB loop that maintained constant gain, the noise figure would be better.

Just because the tube has the same pinout, doesn't mean you should put it in there!

As I said, people do it everyday, and they are not all stupid... It's often a means to clean up the sound of a guitar amp...