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 on: February 12, 2019, 11:46:45 am 
Started by mikezietsman - Last post by Jim Williams
One can fit a MOV device (metal oxide varister) across the AC pins on the internals of the gear. Those clamp surge voltages and are effective and cheap.

 on: February 11, 2019, 05:40:20 pm 
Started by mikezietsman - Last post by Kai
Will voltage dips in the mains supply pose any risk to the ac701?

We’re currently in a phase of scheduled blackouts and I suspect the supply is not great.
A power line situation like this is always connected to large overvoltage spikes which are likely to kill all kinds of electronic devices.
Even here in Germany, when I had to put an over-voltage protection into my central breaker box, failure of studio devices immediately was reduced from often to very few.

Here is an example of such a protection:

These protections are simple, cheap ($200 for a professional version), very effective, and pay off soon. The stationary breaker-box versions are much more effective than the ones built into multi-outlet-strips.
Installation must be done by a qualified electrician.

Over-voltage protections don't deal with smaller scale voltage variations, they cut surges of 1250+ Volts only.
For sensitive devices I suggest to ADDITIONALLY use a battery backup computer UPS (uninterrupted power supply - K.H.) with integrated regulation and sine-wave output.
The cheap ones don't regulate and put out waveforms with huge amounts of overtones (called power line voltage distortion in %) which creep into the audio, so get information before you buy.

A good UPS can bridge blackouts for several minutes, and give you the possibility to properly power down your computers without data loss, too.

And while we're at this: I strongly recommend to install a "ground fault circuit interrupter", other names see:

This thing saves lives, especially with historical music equipment not built by today's safety standards.
In my studio it has saved at least one person from beeing electrocuted, who had the great idea to solder a directly connected line level audio output to the internals of a TV set he used as monitor for his computer.

 on: February 11, 2019, 12:35:05 pm 
Started by mikezietsman - Last post by mikezietsman
So either head will work on either mic as long as they are in Cardiod?

Will voltage dips in the mains supply pose any risk to the ac701?

We’re currently in a phase of scheduled blackouts and I suspect the supply is not great.

The power supply is an nn48h

 on: February 10, 2019, 10:00:02 pm 
Started by mikezietsman - Last post by klaus
Keep the M269 head in cardioid when testing on the U67 body. That's the only pattern that will work in this combination.

The AC701 is a very fragile tube that does not like voltage spikes: one hot plug (full, unloaded voltage from the power supply jamming into tube without gradual ramp-up time) may be deadly; so the best you can do to preserve the tube is to always connect the mic to the supply before turning it on.A gentle start-up with a well designed power supply that's well calibrated is your friend.

Read also my primer on the AC701 tube: https://repforums.prosoundweb.com/index.php/topic,1150.0.html

 on: February 10, 2019, 06:32:57 pm 
Started by mikezietsman - Last post by mikezietsman
Through some very good luck I have recently acquired a neumann m269 to go along with my pair of u67s.

I got a good enough deal that I didn't mind particularly that the capsule had been swapped for what looks to be a reskin. I am also lucky that in a past life I happened on a large batch of spare k67 capsules.

I am however looking forward to finally hearing, in person, the difference between a reskinned capsule and a legitimate neumann capsule.

Would it be safe to use one of the heads of my u67s to test the difference between capsules without the delay that comes with desoldering etc etc etc?

I also wanted to ask about the longevity of the ac701k in this circuit...

I know that tubes can go at anytime but, assuming that the power supply is properly calibrated... what is the predicted lifespan of the tube in these mics?

And are there any major no-nos with regard to tube life?

(I am in South Africa, we suffer from power blackouts and I suspect the occasional voltage dip.)

 on: February 08, 2019, 12:26:19 am 
Started by Mickeyrouse - Last post by Piedpiper
Eric, what say you of your CK12? And anyone else want to share their experience with them?

 on: February 07, 2019, 12:42:09 am 
Started by klaus - Last post by klaus
By "this' you mean....?

 on: February 06, 2019, 09:35:47 pm 
Started by klaus - Last post by aremos
So, this is the one used in the 47Fet?

 on: February 06, 2019, 05:36:57 pm 
Started by Piedpiper - Last post by Piedpiper
He mentions in here a Mullard dual triode.


 on: February 06, 2019, 05:32:32 pm 
Started by Piedpiper - Last post by Piedpiper
He offers a Neumann K47/49 capsule for an upcharge. He also mentions including two tubes by Mullard. Must be around the back of that photo.

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