DC coupling capacitor calculation - diyAudio
Go Back   Home > Forums > Design & Build > Parts

Parts Where to get, and how to make the best bits. PCB's, caps, transformers, 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
Reply
 
Thread Tools Search this Thread
Old 17th December 2007, 04:32 PM   #1
diyAudio Member
 
luvdunhill's Avatar
 
Join Date: Jul 2006
Default DC coupling capacitor calculation

I am looking to mod a preamp that uses coupling capacitors and wanted to know if there was a way I could measure the offset and compute the lowest value I could get away with, assuming the original manufacturer allowed for headroom in the design, or just threw in whatever was handy.

Is this possible, and how does one go about doing this?
  Reply With Quote
Old 17th December 2007, 04:39 PM   #2
diyAudio Member
 
PigletsDad's Avatar
 
Join Date: Jul 2007
Location: South Worcestershire
To have a reasonable safety margin, you want the voltage rating on coupling caps to exceed the power supply voltage in the pre-amp, so that the cap won't fail if big transient signal comes along. If the pre-amp has a dual rail supply (positive and negative), you want the difference between rails, as this is the largest voltage likely to come along under normal operation.

In practice, I would usually choose high rated parts, as they cost almost nothing more, and often have better performance. I would also try not use electrolytic caps as signal coupling devices if film types were practical (any value up to say 10uF or so). My experience is that pretty much any film cap is so much better than any form of electrolytic as to be a very cost effective upgrade. They are physically larger, and it can be hard to fit them into some designs.
  Reply With Quote
Old 17th December 2007, 04:52 PM   #3
AndrewT is offline AndrewT  Scotland
diyAudio Member
 
Join Date: Jul 2004
Location: Scottish Borders
Hi,
the size/value of the signal coupling capacitor does not affect the offset.

The coupling cap forms half of a high pass filter, the input impedance of the next stage forms the other half.

The F-3db of a power amp is usually set in the range 1Hz to 15Hz.
Preamps are usually set lower than this. eg Jensen suggest 220uF for the electrolytic coupling capacitors in each half of a balanced output.

I suggest an RC time constant >=150mS for your preamp.
This can be formed with 1uF+150k or 10uF+15k or 220uF+600r (well almost, what Jensen intended?)
__________________
regards Andrew T.
  Reply With Quote
Old 17th December 2007, 06:02 PM   #4
diyAudio Member
 
luvdunhill's Avatar
 
Join Date: Jul 2006
Quote:
Originally posted by PigletsDad
To have a reasonable safety margin, you want the voltage rating on coupling caps to exceed the power supply voltage in the pre-amp, so that the cap won't fail if big transient signal comes along. If the pre-amp has a dual rail supply (positive and negative), you want the difference between rails, as this is the largest voltage likely to come along under normal operation.

sorry, I probably wasn't clear. I am referring to the value of the capacitor, not the voltage rating.

Thanks though!
  Reply With Quote
Old 18th December 2007, 06:54 AM   #5
diyAudio Member
 
Join Date: Aug 2007
A long time ago (1973) I modded out a Citation 11 that had a low freq roll off around 4-5 Hz. I founnd the coupling caps on the board, read the numbers from the board parts, and then multiplied that number by 10 and added another cap in parrell. I left the 5-gang tone controls out of the mod. For what it is worth, and being ignorant of high quality caps, I used polyproplene caps, and got good service for many years via this mod.

A switch of the tone controls (with tone controls at 0db) on/off would allow one to hear the difference between a 4-5 Hz roll off and a 0.4-0.5 Hz roll off. It was audible, but not very (Bose 501 driven by Dynaco 150 power amp).
  Reply With Quote
Old 18th December 2007, 10:43 AM   #6
Bobken is offline Bobken  United Kingdom
diyAudio Member
 
Join Date: Dec 2002
Location: UK
Hi,

The reason why you could hear this difference is probably due to the fact that with a -3dB roll-off at say 5Hz, the roll-off actually *commences* at a decade higher than that. i.e. at 50Hz, and this is well within the normal hearing threshold, of course, although until at a point somewhere between these 2 frequencies when the reduction reaches maybe 0.5 dB or so, it is unlikely to be readily noticed.

Also, the phase change commences higher still at around 500Hz, which is also likely to be audible in a direct comparison, too, when still well within the normal hearing-range it reaches a significant level.

Regards,
__________________
Bob
  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
coupling capacitor value reading ccschua Parts 3 31st May 2009 01:51 PM
increasing coupling capacitor value in we 91 tmhajw Tubes / Valves 13 13th May 2009 12:25 PM
Pearl: Coupling capacitor h_a Pass Labs 5 10th January 2007 05:51 PM
Need 33uF coupling capacitor eehobby Parts 2 3rd September 2003 02:53 AM


New To Site? Need Help?

All times are GMT. The time now is 11:53 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