Pin 1 problem - build a "Hummer"......

[AudioJohn]

A couple of years ago I came across an article on the Web, based on an AES paper which detailed the construction of a very simple tool to investigate "Pin 1" problems (hum, or ground loops).

This problem has probably plagued most of us on the forum - but since building and using this unit - any problems have been easy to locate and cure.

It is so simple and most readers will have all the parts available to build one:

http://www.jensen-transformers.com/as/as032.pdf

I built mine into an aluminium die cast box, with 4mm terminals - to which can be connected a variety of test leads. I have a lead connected to pin 1 of a male XLR, a female XLR and one with a crocodile clip.

To use, simply plug into an XLR connector on the unit under test - and attach the crocodile clip to the chassis. There should be continuity - this will be indicated by the LED illuminating - and about 50mA of AC (or hum) will be passing through the chassis earth path.

Now monitor an unused audio output with a millivoltmeter (or listen!) - is any hum present?

If all is well absolutely no hum will be present or audible!

Move the connections from the hummer around to pass the current through different parts of the chassis and earth - again no hum should be audible.

Do let me know if you have also tried this successfully.

 

[peter purpose]

Cheers John, that looks v. handy... :thumb: :thumb:

 

[warpie]

It is so simple and most readers will have all the parts available to build one:

http://www.jensen-transformers.com/as/as032.pdf

I thought I'd bring this thread back to life instead of starting a new one...

I have plenty of AC/DC wall warts but no AC/AC and for some reason it's pretty hard to find any in Europe.

I only found this but it's 15VAC.
Is it safe to use 15VAC(670mA) instead of 12VAC(50mA) as suggested on the paper?

If not, do you know any source where I can get the proper wall wart AC/AC converter?

Thanks 

 

[JohnRoberts]

I am too lazy to look at all the links but yes, you just need to add or change the current limiting resistor (make it bigger) . You may need to use a power resistor depending on how much power you need to scrub off.

You can determine resistor value using Ohm's law.. Voltage/R=amps. Watts = amps X volts

JR

 

[warpie]

thanks John,

If my math is correct I need a 300R/9.3W resistor so I better find a different wall wart.

or maybe use ten 3K/1W resistors in parallel...

 

[bruce0]

You will be ok... The amperage of the wall wart is NOT the amperage you want to use.

The amperage (is that really a word?) is what will pass through your 300 ohm resistor 15/300=.050 or 50mA.

The LED will drop about 2 volts and the diode will drop about .7V and the wall wart unregulated will produce more than 15V usually measure it but I guess 18V unloaded so 17.3/300=0.058 or 58mA.

So you need a resistor to handle that 58mA so 0.058 X 17.3 = .997 Watts... So a 1 Watt resistor would work, but it might get pretty "warm" and might burn you so, and good practice would have you over-rate it by 20%, so use a 2 Watt resistor.

You can use any size (amperage) wall wart you want, it is just a matter of measuring the RMS AC voltage it produces.

 

[warpie]

OK, that makes more sense, thanks

 

[ruffrecords]

Try your local Ebay:

]http://www.ebay.co.uk/itm/12V-300mA-AC-240v-Mains-Adapter-Power-Supply-Charger-Transformer-UK-Plug-/150989287617[url][/url]

or Radio Spares:

http://uk.rs-online.com/web/p/plug-in-power-supply/0458923/

I searched for "12v ac plug in transformer"

Cheers

Ian

 

[warpie]

Hi Ian,

I already did but couldn't find anything. All of them would be shipped from US, China, Hong Kong, etc so I'll get one from Farnell as I'll be placing an order with them anyway. But thanks for the suggestion.

 

[JohnRoberts]

For extra fun and extra credit you could wire a triac light dimmer in front of the transformer, that could reduce the voltage and make the waveform even nastier. 

JR

 

[Speedskater]

In the new (2012) Bill Whitlock paper, see page 122 for more on the John Windt "Hummer".

"An Overview of Audio System Grounding and Interfacing"
Sept 4, 2012
by
Bill Whitlock, President
Jensen Transformers, Inc.
Life Fellow, Audio Engineering Society
Life Senior Member, Institute of Electrical & Electronic Engineers

http://centralindianaaes.files.wordpress.com/2012/09/indy-aes-2012-seminar-w-notes-v1-0.pdf

 

[bruce0]

Fantastic presentation.

I have read the Rane and Whitlock papers on grounding before, but this really lays it out in a step by step manner that was very helpful.  The whole paper is great.
Thanks Speedy!

In the new (2012) Bill Whitlock paper, see page 122 for more on the John Windt "Hummer".

"An Overview of Audio System Grounding and Interfacing"
Sept 4, 2012
by
Bill Whitlock, President
Jensen Transformers, Inc.
Life Fellow, Audio Engineering Society
Life Senior Member, Institute of Electrical & Electronic Engineers

http://centralindianaaes.files.wordpress.com/2012/09/indy-aes-2012-seminar-w-notes-v1-0.pdf