How come signal input capacitors are not on every amp? - diyAudio
Go Back   Home > Forums > Amplifiers > Tubes / Valves

Tubes / Valves All about our sweet vacuum tubes :) Threads about Musical Instrument Amps of all kinds should be in the Instruments & Amps forum

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
Reply
 
Thread Tools Search this Thread
Old 9th July 2012, 03:09 AM   #1
mbeards is offline mbeards  United States
diyAudio Member
 
Join Date: Mar 2012
Default How come signal input capacitors are not on every amp?

I have seen amp schematics with and without a capacitor right after the signal input. Usually around .1 uf

I assume that this is to stop any unwanted DC from elevating the grid of the first preamp tube and throwing it off bias. If this is the case, how come they can't be found on every amp? Are there adverse effects like filtering out low frequencies?

Thanks
  Reply With Quote
Old 9th July 2012, 04:24 AM   #2
kevinkr is offline kevinkr  United States
diyAudio Moderator
 
kevinkr's Avatar
 
Join Date: Sep 2004
Location: Boston, Massachusetts
Blog Entries: 6
The best cap is no cap.. Yes, DC on the input will affect the operating point of the input stage, but IMHO this problem is best fixed at the source.

FWIW I haven't been using input caps for about 25yrs now, and have yet to hear of an instance where this has caused a problem.
__________________
"Extraordinary claims require extraordinary evidence." - Carl Sagan
  Reply With Quote
Old 9th July 2012, 10:11 AM   #3
DF96 is offline DF96  England
diyAudio Member
 
Join Date: May 2007
Filtering out low frequencies does not necessarily have an adverse effect, if it stops signals you can't hear from interfering with signals you can hear. Something will remove LF, and it may be better for a coupling cap to do it than the OPT.

If you know that all the sources you will ever use are incapable of emitting DC or subsonics then the input cap can be omitted.
  Reply With Quote
Old 24th July 2012, 09:31 PM   #4
diyAudio Member
 
Join Date: Jan 2011
Location: Columbia, MO
I just was wondering about this as I was considering making a pan-able input to a two-channel amplifier.

and it reminded me of a protective measure recommended by a guitar shielding tutorial where they put a large value 600v capacitor in series with the guitar signal to prevent DC from coming from the amp to the guitar strings in case of a short or other catastrophy.
  Reply With Quote
Old 24th July 2012, 09:52 PM   #5
DF96 is offline DF96  England
diyAudio Member
 
Join Date: May 2007
An amp input is unlikely to be able to emit a harmful amount of DC, unless something really catastrophic has happened. If you rely on a cap for safety then it must be a proper mains-rated cap, X or Y (I forget which), which is guaranteed never to fail short-circuit.
  Reply With Quote
Old 24th July 2012, 10:20 PM   #6
diyAudio Member
 
Join Date: Jan 2011
Location: Columbia, MO
yeah, the guy wasn't claiming that it would thoroughly stop the dc, but rather, buy the guitarist a moment of time until the cap failed, to get the guitar off. anyway, could be unfounded. here's a quote from the page:


An 0.33uf, 400V metal film capacitor. Note, this is a change (10/06/1998) from the 1uf capacitor previously recommended. This capacitor was originally placed to prevent DC shocks from defective tube amplifiers. I hadn't given much thought to blocking AC because I always check wiring recepticals and so on. A recent discussion on the REC.MUSIC.MAKERS.GUITAR newsgroup brought it to my attention that many others don't and that some gigging musicians simply can't check/control wiring. This new value will provide some protection from the dreaded PA-and-Amp-on-different-polarities problem while providing much better noise performance than the parallel combination of a tiny capacitor and resistor recommended by some. This will limit current on a 120V, 60hz system to a maximum of about 30ma – certainly enough to get your attention but less likely to be lethal for most people. Even so, if you gig a lot and don't have control over the wiring – play it safe by either deleting the string (bridge) ground connection entirely or by using the tiny capacitor and parallel resistor method to isolate the strings. See the very important article on electrical shock

Last edited by brilliantblue; 24th July 2012 at 10:22 PM.
  Reply With Quote
Old 24th July 2012, 10:25 PM   #7
DF96 is offline DF96  England
diyAudio Member
 
Join Date: May 2007
In olden days some guitar amps had no mains transformer so an input cap might be the only thing stopping the guitarist from appearing at the great gig in the sky.
  Reply With Quote
Old 25th July 2012, 02:41 AM   #8
mbeards is offline mbeards  United States
diyAudio Member
 
Join Date: Mar 2012
Perhaps it is more of a protective measure for the user rather than to block DC from the incoming component.

At least in guitar amps that makes sense. Last time I checked guitars aren't known for spontaneously producing DC

But.... you never know with active pickups these days.
  Reply With Quote
Old 25th July 2012, 09:33 AM   #9
diyAudio Member
 
Gorgomat's Avatar
 
Join Date: Aug 2009
Audiophools like to risk their gear by not using any input caps. Just in case your valve gets a anode-grid short, you risk of getting your audio source burned out..

One cap more or less doesn´t matter.

Last edited by Gorgomat; 25th July 2012 at 09:38 AM.
  Reply With Quote
Old 25th July 2012, 12:33 PM   #10
TheGimp is online now TheGimp  United States
diyAudio Member
 
TheGimp's Avatar
 
Join Date: Aug 2009
Location: Johnson City, TN
Could this have been a band-aid for electronics with two wire power cord problems?
  Reply With Quote

Reply


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
Briged amp from Balanced Input Signal reins Everything Else 15 29th December 2011 01:56 PM
Input signal and amp bias vdi_nenna Pass Labs 14 10th September 2011 11:56 AM
A 1.5W mono amp with no capacitors in the signal path? peskywinnets Chip Amps 7 27th September 2010 08:26 PM
CDP output signal too strong for AMP input?? Florian Digital Source 12 3rd August 2005 11:41 PM
amp with single input can't amplify pulse signal,please help me thanh Solid State 2 26th October 2004 01:56 PM


New To Site? Need Help?

All times are GMT. The time now is 05:04 PM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright ©1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2