Handling inactive inputs in source selector - diyAudio
Go Back   Home > Forums > Source & Line > Analog Line Level
Home Forums Rules Articles diyAudio Store Gallery Wiki Blogs Register Donations FAQ Calendar Search Today's Posts Mark Forums Read

Analog Line Level Preamplifiers , Passive Pre-amps, Crossovers, etc.

Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Thread Tools Search this Thread
Old 8th September 2013, 09:04 PM   #1
vputz is offline vputz  United States
diyAudio Member
Join Date: Apr 2012
Default Handling inactive inputs in source selector

Like so many others, I'm building a source selector/attenuator; in this case the source selector will be a bunch of DPDT relays run by a microcontroller, shift register, and Darlington array.

What's totally unclear to me is what to do with the signals when they're not connected, and I see lots of methods that I'm not sure I understand.

- Some say disconnect signals and ground completely. However the diyaudio grounding page (here) says "...now that we have cleaned up the grounds, it may not necessary to switch the grounds." Sounds good to me, as I'd rather not buy lots of extra relays, and I see many designs that leave the grounds connected.

- One of those is the Twisted Pear darwin, but in that schematic all the signals are always tied to ground through a resistor, whether connected through the relay or not. The diyaudio grounding page does recommend tying signal to ground through a resistor, but only when unselected: "If there is still a problem you can connect (with a relay) a 50 Ohm resistor between the signal and signal reference of all inputs except that which is selected."

- I've seen other places where they recommend connecting signal to ground with a capacitor on unused inputs.

With the idea that the diyaudio grounding page probably was pretty well researched, I'm inclined to tie grounds together and connect unused signals to ground via a 50-ohm cap, but is there something I'm missing before I go through with it?
  Reply With Quote
Old 8th September 2013, 09:09 PM   #2
diyAudio Member
JonSnell Electronic's Avatar
Join Date: Dec 2012
Location: The Jurassic Coast, England. GB
Don't switch the grounds. The grounds should be connected permanently. Just switch the signal lines and leaving them Open, when not in use, is fine.
Support for Fender, Marshall and all Valve Equipment; Audio Innovations, Audiorama FU29, Quad and Leak. www.jonsnell.co.uk
  Reply With Quote
Old 8th September 2013, 11:06 PM   #3
diyAudio Member
indianajo's Avatar
Join Date: Jan 2010
Location: Jeffersonville, Indiana USA
both my selectors, the PAS2 tube preamp, and the RA88a op amp disco mixer, tie all the grounds together and leave them tied permanently. The disco mixer had a hum improvement when I changed the ground tie point to the board near the op amp inputs, rather than to the case at the RCA jack panel.
The PAS2 shorts all the inputs not used with a rotary selector switch. this was fine in 1960 with tube equipment, which runs fine powered up into a short. In 2010, with CD and MP3 players having potentially overcurrent sensitive output devices, shorting the unused inputs is not so smart. Isolating the low level (magnetic phono cartridge and dynamic mike) inputs with a grounded bulkhead from the high level inputs (FM, CD, MP3) may eliminate crosstalk if using the turntable with mag phono cartridge, and a high level input is left turned on.
commercial mixers do not have bulkheads, but have the low level dynamic mike inputs located away from the high level tape monitor and effect loop inputs.
Dynakit ST70, ST120, PAS2,Hammond H182(2 ea),H112,A100,10-82TC,Peavey CS800S,1.3K, SP2-XT's, T-300 HF Proj's, Steinway console, Herald RA88a mixer, Wurlitzer 4500, 4300

Last edited by indianajo; 8th September 2013 at 11:10 PM.
  Reply With Quote
Old 13th September 2013, 02:01 PM   #4
AndrewT is offline AndrewT  Scotland
diyAudio Member
Join Date: Jul 2004
Location: Scottish Borders
Does adding muting to the unused input/s help cut down on crosstalk?

If this does help, can we use a muting resistor instead of a dead short, so easing the load on the unused source?

How big can that muting resistor be before the crosstalk approaches what it was before adding the muting.

Am I just chasing my tail?
regards Andrew T.
Sent from my desktop computer using a keyboard
  Reply With Quote
Old 13th September 2013, 02:09 PM   #5
Mooly is offline Mooly  United Kingdom
diyAudio Moderator
Mooly's Avatar
Join Date: Sep 2007
You should here absolutely nothing from an unused input fed with its maximum allowable signal level, even with the volume turned fully up.

Open circuit switching elements (relays, switches etc) still couple the signal via stray capacitance. Its always audible in my experience. The method I use is series switching in combination with shunt to ground for unused inputs. These all feed a virtual earth input stage so there is no "actual measurable voltage" present to couple capacitively. Use a non virtual earth input stage and you can still get traces of breakthrough... yes its many times better than conventional switching... but the virtual earth stage kills it all.
  Reply With Quote
Old 15th September 2013, 12:05 AM   #6
diyAudio Member
Join Date: Nov 2004
Location: Germany
Originally Posted by JonSnell Electronic View Post
Don't switch the grounds. The grounds should be connected permanently.
The reasoning behind this would be avoiding potential potential problems with sources, right? A device floating somewhere could cause nasty signal spikes upon switching if the ground contact lagged behind the signal contact for some reason.

Disconnecting grounds would have the advantage of potentially breaking a bunch of ground loops though, which are quite common in practice. Maybe one could leave things connected by a 1k-10k resistor, just enough to keep sources from floating away into outer space?

Obviously presenting a high-impedance input to each ground connection seems even smarter.

Last edited by sgrossklass; 15th September 2013 at 12:12 AM.
  Reply With Quote
Old 15th September 2013, 07:57 PM   #7
vputz is offline vputz  United States
diyAudio Member
Join Date: Apr 2012

Sigh, more things to learn. Link to a schematic? I did some reading about virtual earths but I confess I'm not completely clear what you have in mind here.
  Reply With Quote
Old 16th September 2013, 06:17 AM   #8
Mooly is offline Mooly  United Kingdom
diyAudio Moderator
Mooly's Avatar
Join Date: Sep 2007
Post #2 here,
My MOSFET amplifier designed for music.

You can add as many inputs as you want by duplicating the input section (one resistor and two FET's per input)

Post #201 explains the FET operation,
My MOSFET amplifier designed for music.

The FET's could also be replaced by CMOS switches such as 4016/4066 etc.
  Reply With Quote


Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off

Similar Threads
Thread Thread Starter Forum Replies Last Post
Volume / Source selector - open source project ? AuroraB Analog Line Level 22 22nd September 2012 02:21 PM
source selector mnr102 Multi-Way 0 24th February 2008 11:11 PM
Adding Wordclock inputs to source mlihl Digital Source 2 25th July 2006 02:36 AM
Here's a source selector DragonMaster Everything Else 0 7th August 2005 03:52 AM

New To Site? Need Help?

All times are GMT. The time now is 02:05 AM.

vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2017 DragonByte Technologies Ltd.
Copyright 1999-2017 diyAudio

Content Relevant URLs by vBSEO 3.3.2