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ARSCLIST  August 2011

ARSCLIST August 2011

Subject:

Re: Hum from the power supply

From:

"Richard L. Hess" <[log in to unmask]>

Reply-To:

Association for Recorded Sound Discussion List <[log in to unmask]>

Date:

Mon, 29 Aug 2011 20:19:04 -0400

Content-Type:

text/plain

Parts/Attachments:

Parts/Attachments

text/plain (83 lines)

Hello, Jan,

I am rapidly getting away from unbalanced interconnects as much as 
possible.

In your example, it is difficult, but I would treat the disc playback 
system as an island and then run balanced audio off that island. My old 
RTS405 RIAA preamp has transformer outputs and that is securely bonded 
to the Technics turntable.

Many A-D converters have good differential inputs. You can lift the 
shield at the input end and see if that improves things.

I have been doing a bit more PA and musical instrument work with my 
older son who is a far better musician than I ever could be--and he's 
still learning. We have all sorts of audio transformers floating around 
and they get the hum out. On a positive note, I was pleased that both 
the $100 Steinberg USB audio interface and the Kurzweil PC3LE8 keyboard 
we bought this summer both have balanced interconnections. None of the 
stomp boxes that he wants have balanced interfaces. The nice thing, 
however, is his Ibanez electro-acoustic guitar's preamp has an XLR 
mic-level output. I do build him extra phantom blocking capacitor 
modules in a Switchcraft S3FM barrel to be safe.

One thing we discovered was having a Mackie 1402 VLZ Pro mixer and a 
Yorkville C80P speaker plugged into the same power strip with 6' line 
cords (both grounded) and using a quality coaxial cable unbalanced 
interconnect between the two still caused annoying hum. Replacing the 
unbalanced cable with a balanced one (both ports were balanced) reduced 
the hum to below audibility from being annoying.

I don't have any transformers in my studio (other than that built into 
my Studer A80s and A810s and the RTS RIAA preamp) and perhaps a few 
other pieces of equipment. Most of the gear is electronically balanced. 
I use Aphex 124A balanced-unbalanced converters to generate a balanced 
signal close to an unbalanced source and then run balanced into the A-D 
converter.

There are good reference papers about hum and buzz (as well as one of 
the best source for audio transformers) at www.jensen-transformers.com.

The Jensen ISOMAX with RCA connectors on both sides is pretty effective 
at breaking the ground loop as well. I have a couple of those that come 
in very handy--again in the field. Having at least one of these is a 
great troubleshooting tool. Radio Shack in the US has one that is a lot 
less costly and doesn't sound half-bad, but I wouldn't use it in the 
final configuration, but it's cheaper than the Jensen for testing.
http://www.jensen-transformers.com/ci2rr.html
http://www.radioshack.com/product/index.jsp?productId=2062214

As an aside, this product is an effective device when you have mostly 
mic inputs (with phantom power) to connect a playback device at a 
distance from your mixer.
http://www.radioshack.com/product/index.jsp?productId=2062444
It is far better than the price would suggest as long as the input is 
the standard -10 dBV levels. It will also work with many guitar pickups. 
I think between me, my son, and the church, we have ten of these now.

Cheers,

Richard

On 2011-08-29 3:56 PM, Jan Myren wrote:
> HI!
>
> I have sometimes problems with "ground" hum from the power on my set-up.
> The problems are most often with my 78 rpm turnable playback system
> consisting of a Rek-O-Kut turnable, Pro-Ject pre-amp, Packburn 323A and
> Rek-O-Kut re equalizer and de-hisser. It is not quite permanent.
> BUT; when it sometimes appear it seems like it also make the music signals
> sound distorted, especially at the end of the records.
>
> May such hum also have something to say for the music signals, like
> distortion, maybe, or will it just cause that "famous" "ground" problems hum
> sound from the loudspeakers?
>

-- 
Richard L. Hess                   email: [log in to unmask]
Aurora, Ontario, Canada           (905) 713 6733     1-877-TAPE-FIX
http://www.richardhess.com/tape/contact.htm
Quality tape transfers -- even from hard-to-play tapes.

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