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Ribbon interference solutions

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Ribbon interference solutions

Postby Cest Moi » Sat Dec 22, 2018 1:23 pm

I’ve got myself an old ribbon mic and an Art Tube preamp to up the gain. My problem currently is that (as expected really) my cable picks up local radio.

I’ve got a couple of ideas to resolve and it would be good to hear people’s opinions or other ideas for resolutions.

1. In an ideal world I’d sit inside a faraday cage and all my problems would go away. Is there a cable I can get with shielding that would have the same effect?

2. Run another XLR cable parallel to the mic cable and feed both the mic signal and ‘Aerial’ cable in to a dpdt switch so that I can reverse the polarity and remove the interference and fees the (hopefully clean) signal in to the preamp.

Any opinions on these options or other ideas appreciated.
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Re: Ribbon interference solutions

Postby Wonks » Sat Dec 22, 2018 1:29 pm

Do you know if the ribbon provides a balanced or unbalanced output? If you know the make + model then that may help others if you aren't sure yourself. Has it an XLR output or some other connector with an adapter?

An XLR cable with a balanced mic would normally provide more than enough RF rejection. It sounds rather like the output is unbalanced (though it could probably be made balanced with some simple mods to the mic wiring), and a modified cable could probably cure your problems.
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Re: Ribbon interference solutions

Postby Cest Moi » Sat Dec 22, 2018 1:52 pm

Apologies, should have been more specific and added pictures to start with.

Unbalanced I believe (with a 30 or 50 ohm option). Some pictures can be found here:

https://1drv.ms/f/s!ABufNhfQcixYgSs
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Re: Ribbon interference solutions

Postby Sam Spoons » Sat Dec 22, 2018 2:00 pm

Using a second XLR cable will not help but rewiring the mic for standard balanced connection should be fairly easy. Wired as it is it will be vulnerable to accidental connection to a phantom powered input which (unless there is, as I suspect there is a transformer in the barrel) would probably destroy the ribbon.
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Re: Ribbon interference solutions

Postby Wonks » Sat Dec 22, 2018 2:25 pm

Of the 3 pins in the connector, one should be a common signal ground and the other two will be 30 or 50 ohm connection options. From an old SOS post, I understand the 50 ohm option was designed for connection to a valve guitar amp, and needs a really high input impedance to get any top end on the signal, so you'll need the 30 ohm connection.

I'm sure Hugh, ef37a or Folderol can suggest the exact wiring to use as I don't know much more than that.
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Re: Ribbon interference solutions

Postby Sam Spoons » Sat Dec 22, 2018 2:30 pm

This might provide some useful info http://www.coutant.org/reslo/
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Re: Ribbon interference solutions

Postby James Perrett » Sat Dec 22, 2018 2:53 pm

If you are picking up radio you need to check your earthing first and check that the screening on the mic cable is properly connected to earth. If everything in your system is double insulated you may want to find a way to add an earth connection to the system.
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Re: Ribbon interference solutions

Postby Cest Moi » Sat Dec 22, 2018 3:48 pm

Far more detail than I was ever expecting. Lots for me to investigate. Much thanks to Sam spoons for the reslo page. I’m sure the guides will go a long way to stop me from destroying the mic.

What’s my best way to investigate earthing? The preamp has no earthing option itself so would I have to ground it via another piece of kit?

I’ll also dig through the cable I have and make sure I’m using something with decent shielding.
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Re: Ribbon interference solutions

Postby Sam Spoons » Sat Dec 22, 2018 4:05 pm

Shielding matters but common mode rejection is down to the twisted pair. If the shielding is good then rewiring for balanced should be well worthwhile.
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Re: Ribbon interference solutions

Postby blinddrew » Sat Dec 22, 2018 4:57 pm

If in doubt, Stew at xaudia.com is an expert on vintage ribbons, so if you decide it's getting outside your comfort zone give him a shout. :)
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Re: Ribbon interference solutions

Postby Hugh Robjohns » Sat Dec 22, 2018 7:10 pm

James Perrett wrote:If you are picking up radio you need to check your earthing first

^ THIS.

Always the first port of call with interference issues is to check the system grounding.

If the preamp is grounded, it serves as a faraday cage for the signal connections. That's the whole point... And if it's not grounded the whole system becomes an aerial. So I'd check that (and rectify if necessary) before thinking about Messing with the mic.

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Re: Ribbon interference solutions

Postby Mike Stranks » Sat Dec 22, 2018 10:49 pm

The experts have spoken and my practical experience of these low-impedance Reslos backs up their comments.

I inherited two of these that had been well cared for and were wired unbalanced to standard jacks. Never used them apart from a bit of testing, but there were no issues with them at all in terms of noise, RF breakthrough etc. Made a nice little sum when I sold them with no comebacks from the purchasers.
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Re: Ribbon interference solutions

Postby Hugh Robjohns » Sat Dec 22, 2018 11:44 pm

I'll second the rcommendation for Xaudia who offer a very worthwhile 'Beeb' upgrade.

http://xaudia.com/shop/the-beeb/

...which is based on this BBC R&D departmentbreport from 1961:

http://downloads.bbc.co.uk/rd/pubs/reports/1961-05.pdf

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Re: Ribbon interference solutions

Postby ef37a » Sun Dec 23, 2018 7:39 am

I have a Reslo RB 50 Ohms and never had an RFI problem with it ( not now nor donkeys ago in the Rubber Duck days and PMR taxis and we ran some VERY long lines!) . Mine suffers from hum, low level, 50Hz and due I think to it not being "buck" wired although the sensitivity might also be down after all these years?

Have we absolutely established that the mic is the ~50k unbalanced version? If so the shield must be intact otherwise it would not work at all and would hum like ****! Thousands of these HZ mics were used, bingo halls, auction rooms and plugged into guitar amps by Beatle wannabes (see old Cavern shots) rarely did RFI rear its head. If the mic is LZ and balanced and the shield continous, easily checked with a cheap meter, the problem resolves IMHO to two factors, 1) you are in a particularly strong RF field. Can you identify the station? Or/and 2) the Art pre amp has poor (or no!) RF protection.

Not a lot you can do about 1 but the pre amp can be modded by any competent tech' or indeed yourself if you are handy with a solder iron. You could try winding the cable around some ferrite absorbers or building an LC filter in a tin but external measures are usually not as effective as the rejection that SHOULD be in the amplifier to begin with. I despair of kit these days that boast a response into the IF region of AM radio. Those designers never went "on the road" with PA kit in MY back yard! Rugby and Daventry.

BTW, "balance" is a myth at RF. Op amps have long run out of gain and so can offer little to no common mode rejection. Good transformers with inter-winding shields can help a lot, pricey mind.

I shall try to find a schematic for that Art. Is there a model number?

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Re: Ribbon interference solutions

Postby ef37a » Sun Dec 23, 2018 8:09 am

http://www.sowter.co.uk/specs/8754.htm

One of those should help if a 50R mic. You are looking at the thick end of £100 by the time you have put it in a decent diecast box with XLRs but the IW shield should stop the RF.

I found a map quite easily and it ain't a "toob" input! Goes to a pair of transistors per "leg" and they will have a response to several MHz. There is a bit of RF stopping but the series R is a miserable 10 Ohms although the 330pf shunt capacitor is adequate. I would insert a small RF choke in series with each 10R or better, a small toroidal common mode choke.

There might be a cheaper, easier solution. Chokes and caps in a tin close to the input. All up the components should not cost more than £15, less if you don't use a posh tin. In addition you could include some 47mfd caps for permanent phantom power protection.

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Re: Ribbon interference solutions

Postby Mike Stranks » Sun Dec 23, 2018 10:17 am

Dave:

The pics supplied by the OP show that it is the 'black label' 30/50 ohms mics, not one of the 50K mics.

I have a 50K dynamic mic here which I use very occasionally. Works hum-free with a Eagle step-down transformer - 50K TS input: 600 XLR output.
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Re: Ribbon interference solutions

Postby blinddrew » Sun Dec 23, 2018 11:52 am

I have one of these Art tube pre-amps: https://www.andertons.co.uk/art-tube-st ... P1EALw_wcB which i use all over the place (including with a ribbon) and never had any rf problems. Not sure if it's the same as the OP's?
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Re: Ribbon interference solutions

Postby Xaudia » Sun Dec 23, 2018 1:06 pm

The Reslo mics are a bit of a minefield because although they use the same plugs, the mics are not wired consistently.

With the 30/50 ohm RB and CR mics, the third pin is not connected to ground. Ground is made through the barrel of the Reslo plug, which is a bit of a flakey connection to be honest. Grounding the third pin does no harm with these.

But for the microphones marked "30/50 and High', the high impedance output is on the third pin. This was Reslosound's way of getting alternative impedances out of a three pin plug. But grounding this skews the frequency response, so don't connect it.

With the SR, VR and RV models, the third pin IS grounded.

My current approach is to convert (nearly) all the Reslos to XLR output and wire balanced and grounded through pin 1. It gives much better grounding and less hum and other interference.

Some further advice and the Reslo documents are here at my blog....
http://xaudiaelektrik.blogspot.com/2012 ... guide.html

Hope that helps.
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Re: Ribbon interference solutions

Postby Hugh Robjohns » Sun Dec 23, 2018 1:22 pm

Notwithstanding all the excellent advice about rewiring the mic and impedance-converting transformers and all that... and the very real possibility that the mic needs some attention,,, the FIRST and EASIEST thing to check really is the grounding of the entire system.

That ART preamp is a great little thing and I've used it myself in projects for clients... but it has an external PSU and I can't remember whether it is grounded through its mains plug... I strongly suspect not... In which case, it will be relying on passing on a ground to the mic which it receives via it's line output connection.

Now if the ART preamp is plugged into a USB-powered interface which is plugged into a laptop the likelihood is that there is no effective ground at all, and the entire system is floating!

So that needs to checked first and foremost... If the OP has access to a multimeter it is a moment's work to check for continuity between the ground of the preamp and mains earth. If not, then using a guitar cable to ground the preamp to a known earthed device would prove the point (press the tip of one plug to something known to be earthed (eg the fixing screw on a wall socket) and something grounded on the preamp (like the barrel of the XLR. If the interference goes away you'll know the system was lacking a ground and needs one! If it doesn't change you're either not effectively grounding the system or it is already grounded and you have a problematic mic.

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Re: Ribbon interference solutions

Postby Cest Moi » Mon Dec 24, 2018 11:50 am

Thanks to a good few of few I’m most of the way there. @Sam spoons, thanks for the reslo links, I’ve made up a new cable which is now balanced off of pins a and b and the shielding connected to the barrel (doubled over and fed through the screw @xaudia).

Wired correctly and balanced I have no more RF interference.

@Blinddrew almost the same preamp it’s the Art Tube MP rather than the studio.

Hugh, you read my mind. Running a usb desk with no grounding, though monitoring the output directly rather than via my laptop (I have some Cubase 5 and Mac “developer compatibility” issues preventing me from launching the app, but that’s for me to do some thread searching). I do have some 50hz hum, so grounding the preamp may be a job for another day with some better kit (I have some known high pitched whine on my test desk, an Alesis multimix).

A quick connection with a jack lead to equipment I believe is earthed changes the sound but I need to do some thorough testing, perhaps after Christmas. But the mic works, it sounds nice and I have quite a few of you to thank for that.

Have a good Christmas all and perhaps I’ll see you round the forums when I get a chance to try and get cubase up and running and I’ll come back and share any news on my hum once I’ve had some time to look into it further.
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