newbie attempts 8 channel mic interface box (phantom/pad)

[ombudsman]

I think I'm going to try to build something, which is a bold move for a guy like myself with a history of frustration about audio gear, not to mention my specialized needs.

But it would be a relatively simple project, and I wanted to ask the tribe about a couple of basic points to make sure I am thinking about it correctly before I start to spend money.

The general idea is to have one or two front end mic interface boxes that would give me a -20 db U pad and a phantom switch on each channel, so that I can more easily use my old fixed gain preamps.

I'm thinking I might be able to accomplish this by ordering rack chassis pre punched from Par Metal, and using the JLM Go Between kits, and an Acopian or similar power supply. But JLM also has a 48 v supply.


The questions I had at first are:

1) for vintage tube preamps like Cinema Engineering 102AX and WSW 811 301, do I need to make sure that these preamps aren't seeing phantom power at the inputs ?

2) if yes, does the JLM Go Between kit have blocking resistors that would prevent the phantom power from going back to the preamps ? (Or if not, could they be added in a simple way, like hanging from the XLR connector on the back of the box ? I can solder, but that's about it.)

3) the JLM power supply that I am looking at says it "Runs on mains voltage of 85vac to 265vac without switching." Does that mean it is linear when run off of a 120 vac outlet ? Or do I need to go with something like this ?

http://www.acopian.com/store/productdetail.aspx?q=i481

If this works out, I'd be happy to share the details, might at least save someone else some time in ordering the same chassis for the same type of project.

thanks, Mark

 

[PRR]

> "Runs on mains voltage of 85vac to 265vac without switching." Does that mean it is linear

Means the *user* does not have to flip a switch when touring Japan USA and France.

It is surely a switcher. If JLM is selling it, I am *sure* it is good for me or you.

 

[ombudsman]

thanks PRR. I had been under the impression that linear was important for phantom power.

If anyone has thoughts on 1 and 2 I would appreciate it. I have read that V76 and similar units can have their input transformers destroyed by phantom at the input, and the tube preamps I have are from around the same time, so I am guessing I should find a way to get blocking resistors in place. But I don't know if they might be already built in to that JLM, or if it makes sense to add them elsewhere.

 

[Whoops]

I should find a way to get blocking resistors in place.

Blocking Capacitors maybe...

Capacitors Block DC , Resistors Don't

 

[ombudsman]

Thanks Whoops.

 

[emrr]

The general idea is to have one or two front end mic interface boxes that would give me a -20 db U pad and a phantom switch on each channel, so that I can more easily use my old fixed gain preamps.

The questions I had at first are:

1) for vintage tube preamps like Cinema Engineering 102AX and WSW 811 301, do I need to make sure that these preamps aren't seeing phantom power at the inputs ?

2) if yes, does the JLM Go Between kit have blocking resistors that would prevent the phantom power from going back to the preamps ? (Or if not, could they be added in a simple way, like hanging from the XLR connector on the back of the box ? I can solder, but that's about it.)

3) the JLM power supply that I am looking at says it "Runs on mains voltage of 85vac to 265vac without switching." Does that mean it is linear when run off of a 120 vac outlet ? Or do I need to go with something like this ?

1) Phantom will not hurt those preamps so long as they do not have a primary center tap tie to ground.  If they do, you should disconnect it from ground.  I have yet to see a preamp that suffered from center tap being lifted. 

2) not a resistor, a cap.  Most phantom supplies are cap isolated.  I have gone the other way with some phantom supplies and removed the blocking cap for use with transformer coupled pre's, which don't generally need them (unless they are that lame-o Telefunken stuff with dodgy construction.  If you love Telefunken, do what you need to do), and marked them accordingly. 

3)  The JLM needs a power transformer, and they at least used to have an info section telling you roughly how to figure what you need. 


You will vastly overspend and overbuild if you go JLM for this.  I would buy a multi-channel phantom box, and either drill and wire in some pad switches, or buy XLR barrels and make inline pads.  It's hard to find a modern multi-channel phantom box, but there are older units from Crown, AKG, etc, which have the necessary space to add other features.  I picked up (5) four channel Crown off Ebay last summer for $50/shipped (admittedly a deal).  Price a box and XLR's plus some JLM kits and a power trans, you'll see what I'm getting at, at even double that. 

 

[Whoops]

I would buy a multi-channel phantom box, and either drill and wire in some pad switches,

That seems quite a good idea actually,
if you can get one of this boxes for a good price second hand , then you can just add a switch and 3 resistors for an U PAD, and even a switch for Polarity reverse for each channel. You dont need a PCB for that.

Of course if you want to do it yourself with JLM Go betweens, it will be fine also, Go Betweens are nicely made. The JLM switches are also quite good.
Just be aware that it can become costly depending on where you live, since JLM is in Australia you might have to pay Customs Fees, and those can really increase the final price in 30/35%, besides the shipping

 

[cyrano]

It doesn't even have to be a phantom box, but that would fit your use case better, obviously...

It's just that these multi-channel phantom boxes are rare second hand and quite expensive new.  Strangely enough, the single and dual channel boxes are cheap.

I just repurpose every 19" case that has a lot of XLR's. Cheaper than buying decent XLR chassis parts. I've just gutted an intercom 4-wire interface. It already had four female XLR's, so I've just added four males to build a four channel mic transformer isolation box.

And that's one dangerous box less. The XLR's on this box carry 30V DC on pin 3...

 

[ombudsman]

Wow,  thanks Doug, I think you just saved me not only considerable expense but also time and trouble.

This Rolls 8 channel phantom unit is $180. Unless they've got electronics jammed toward the front, that panel looks great for adding the switches to disable phantom per channel and for the pads. And I can do two of these the same way and have 16 channels of this mic interface/common patching point in front of all my fixed gain amps.

Unless anyone knows of a downside with this particular unit in which case I could look for something else. I already have a 4 channel Audio Technica but it isn't a rack unit and I would like to have uniformity, as if this was the top row of controls for the channels of a console, with output attenuators and faders below (have those mostly covered already, except for 4 Cinema Engineering 102AX which are in the process of being set up with a power supply) . This would make for a much better organized system compared to what I have now.

https://www.bhphotovideo.com/c/product/1316286-REG/rolls_rpb488_phantom_eight_8_channel.html

If anyone else wants to take on a similar project, I'd be glad to share pics and notes along the way, might be a couple of minor problem solves along the way that others could use.

 

[emrr]

I can't speak to that Rolls unit.  No idea how much space.  You'd likely have to cut traces on a PCB to add features.  No XLR's either. 

 

[ombudsman]

Ah, OK, they also make a 6 channel version with built in XLRs, so 3 of those might be a better way to go.

https://www.bhphotovideo.com/c/product/492050-REG/Rolls_RPB486_RPB486_Phantom_HEX.html

Cutting traces sounds scary to me. (Maybe it shouldn't.) I was hoping to unsolder the connectors, hang some new small parts there, and run lines to front panel switches. Maybe I'll start with a single one of the 6 channel version and show the guts to a friend locally who may be able to help.