old trick of shorting unused inputs on preamp...

crooner

Tube Marantzed

• Nov 12, 2004

• #1

Didn't remember I had some shorting RCA plugs salvaged from an old Kenwood amp in my junk box.

Tried them on my PAS 3 tonight. Shorted the "special" and "tape head" inputs. Lo and behold! much lower noise floor and overall better performance with phono and high-level sources. The sound opened up with much better soundstaging...

The difference is not subtle. An excellent way to improve performance without modifying your preamp. I recommend it.

 

Last edited: Nov 12, 2004

spkrman57

wrongsideofthetracks

• Nov 12, 2004

• #2

It does work!

By shunting the unused inputs to ground will keep most noise/hum from being amplified along with the music.

Sometimes with high sensitivity inputs(especially tubed gear), it is not unusual to pick up adjacent inputs signals at a low level, even with just crossing between contacts in thin air.

Even the best units are sometimes suspect to this!

Ron

 

dshoaf

That high voltage buzz

• Nov 12, 2004

• #3

Contrary to current postings....

this isn't a new idea in the least. My Mac MX-110 has factory-supplied input caps for the tape head and second phono inputs - this one manufacturered in 1963.

Cheers,

David

 

piece-it pete

What, me worry?

• Nov 12, 2004

• #4

You mean, actually shorting them, as in more or less twisting the wires together?

Pete

 

Andyman

Scroungus Stereophilus

Subscriber

• Nov 12, 2004

• #5

crooner said:

Shorted the "special" and "tape head" inputs.

Sounds cool!

For us electronically challenged, what exactly did you do?
Connect the R Special to R Tape head, ditto left, or did you connect R special to L special, ditto tape head???

Click to expand...

Click to expand...

 

dshoaf

That high voltage buzz

• Nov 12, 2004

• #6

Simple, actually....

The input is shorted to ground. So, on an RCA jack, you would connect the center conductor inside the hole to the outside metal. To do this, you take a regular RCA plug and solder a small wire from the two terminals inside the hood and insert it into the unused input jack(s).

**Big Note**: You do *not* do this with Tape Output jacks. Only inputs!

Cheers,

David

 

Thatch_Ear

Addicted Member

• Nov 12, 2004

• #7

Thanks for that. I have found my PAS to be pretty high gain and to have a bit of background to it. I had completely forgot about doing this.

 

LBPete

Rolling Along

Subscriber

• Nov 12, 2004

• #8

RCA Shorting Plug

Is this what you are talking about?

http://mcm.newark.com/NewarkWebCommerce/mcm/en_US/endecaSearch/partDetail.jsp?SKU=27-5335&N=4

- Pete

 

Last edited: Nov 12, 2004

Bill Ferris

Lunatic Member

• Aug 27, 2016

• #9

 

L

Lesterbest

Super Member

• Aug 27, 2016

• #10

shorting unused inputs on preamp...

Only necessary on unused phono inputs. Doesn't do a damn thing with line level inputs. It may make you feel better, but............................................................

 

A

Audiovet

Well-Known Member

• Aug 28, 2016

• #11

Old thread - but aren't inputs mostly switched one at a time? In which case normal practice is for the relevant switch to do the shorting when a different input is selected. When not, I had the experience of e.g. a tuner input leaking through when using the phono input - depending of course on the physical layout.

 

Bill Ferris

Lunatic Member

• Aug 28, 2016

• #12

Audiovet said:

Old thread - but aren't inputs mostly switched one at a time? In which case normal practice is for the relevant switch to do the shorting when a different input is selected. When not, I had the experience of e.g. a tuner input leaking through when using the phono input - depending of course on the physical layout.

Click to expand...

Yes good designs usually short unused inputs, but some older preamp layouts had the wafer switch(which can have leakages as well) close to the front panel and in certain situations the longer wires from the unused open high gain input jacks(mic, tape head, and say phono 2 can bring up the background noise and hum on their way to the selector switch.. And worse, a nearby strong RF source(CB, Ham, CFL. ect.) have been known to leak in to ungrounded preamp inputs..

 

ferninando

Lunatic Member

• Aug 28, 2016

• #13

Certainly not a trik. Back in the day [50yrs ago] many mfrs recommended unused phono inputs be shorted in their user manual instructions.

 

ferninando

Lunatic Member

• Aug 28, 2016

• #14

so are you gonna beat up on him buglegirl?

 

SiliconTi

Super Member

• Aug 28, 2016

• #15

My Mac 4100 came with shorting plugs for the phono inputs out of the box.

 

WillVT

Super Member

• Aug 28, 2016

• #16

I'm so glad someone reopened this thread! I have a pair of shorting plugs that I got with my Onkyo TX-4500, and I had no idea what they were for. A while ago when I was not using this receiver, they somehow ended up, I think, on the dolby loop outputs, resulting in no signal. Took me a while to figure that out.
Anyway here is a pic of them, if anyone is wondering what they look like:

 

Last edited: Aug 28, 2016

ETLS

metacarpophalangealcranium

Subscriber

• Aug 28, 2016

• #17

WillVT said:

I'm so glad someone reopened this thread! I have a pair of shorting plugs that I got with my Onkyo TX-4500, and I had no idea what they were for. A while ago when I was not using this receiver, they somehow ended up, I think, on the dolby loop outputs, resulting in no signal. Took me a while to figure that out.
Anyway here is a pic of them, if anyone is wondering what they look like:

Click to expand...

Pic doesn't show.

 

WillVT

Super Member

• Aug 28, 2016

• #18

ETLS said:

Pic doesn't show.

Click to expand...

Fixed it. I think...

 

Lavane

Lunatic Member

• Aug 28, 2016

• #19

My Marantz 33 pre came with two sets. It has two phono inputs and a tape head input. I was suprised both sets were still there after all these years when I bought it a few years ago.

 

sKiZo

Hates received: 92644 43.20°N 85.50°W

• Aug 28, 2016

• #20

Yup ... that's an old thread ... er ... old skool kinda thang. My Eico's have them standard for the tape head. Those were higher signal levels for decks that didn't have their own preamps and tended to bleed some otherwise. A lot of those are still handy for juicing up a low output phono cartridge into an external stage that has RIAA capabilities.

And ya ... from personal experience, shorting line level inputs doesn't do much. If anything, use non-shorting covers to block any stray RF, and even then, that's minimal effect even with a full metal chassis w/cover.

Thanx for the reminder though - I've got an HF12a that I was planning to convert the tape head to standard line level. Winter project I guess. I'll check back in another 10 years or so and give yall a progress report. <G>