LM3875 Newbie Question re: sensitivity - diyAudio
Go Back   Home > Forums > Amplifiers > Chip Amps

Chip Amps Amplifiers based on integrated circuits

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 11th January 2004, 07:58 PM   #1
diyAudio Member
 
Join Date: Jan 2004
Location: Northumberland
Default LM3875 Newbie Question re: sensitivity

HI Everyone

I know how to set the gain of an IGC amp. But how do you measure or calculate the sensitivity ie. the minimum RMS input signal required to give full output without clipping? I can measure this easily on my tube amps using a sine wave generator & scope, but feeding a 300 - 800 mV sine wave into these chips makes them red hot very quickly even with a large heatsink?

Does any know the sensitivity of the IGC circuit with 10K input and 220K feedback?

I wan't to try and match the sensitivity of some of my tube amps, so I can use the IGC's to power additional speakers for a 5.1 surround system.
  Reply With Quote
Old 11th January 2004, 09:07 PM   #2
GregGC is offline GregGC  Canada
diyAudio Member
 
Join Date: Oct 2003
Location: Toronto
Default Re: LM3875 Newbie Question re: sensitivity

Quote:
Originally posted by Gringrath
HI Everyone

I know how to set the gain of an IGC amp. But how do you measure or calculate the sensitivity ie. the minimum RMS input signal required to give full output without clipping? I can measure this easily on my tube amps using a sine wave generator & scope, but feeding a 300 - 800 mV sine wave into these chips makes them red hot very quickly even with a large heatsink?

Does any know the sensitivity of the IGC circuit with 10K input and 220K feedback?

I wan't to try and match the sensitivity of some of my tube amps, so I can use the IGC's to power additional speakers for a 5.1 surround system.
Pretty close guess would be + V-5V. If you have a 30 v rails it'll start cliping at 25 IF the other power considerations are met.
If you want to measure it remove the load and see when it'll start clipping by increasing the V source. So, for the prev. example if you have a gain of 22 and the V ps is 30V I expect to start clipping at around 1.2V @ the input.
  Reply With Quote
Old 11th January 2004, 09:09 PM   #3
Bozzz is offline Bozzz  Netherlands
diyAudio Member
 
Join Date: Jan 2004
Location: Amsterdam
Send a message via ICQ to Bozzz
Hi Grin,

Read the part "generalized audio power amplifier design" (p 13/14) in the LM3875 datasheet. Equations also apply for Inverted configuration. (right?)

Hope this answer your question.
  Reply With Quote
Old 12th January 2004, 01:05 PM   #4
macboy is offline macboy  Canada
diyAudio Member
 
Join Date: Oct 2003
Location: Ottawa, Canada
Quote:
...feeding a 300 - 800 mV sine wave into these chips makes them red hot very quickly even with a large heatsink?
Then there is something very wrong with your amplifier. It is probably not stable.

Figuring the sensitivity is simple. If you simply assume that the clipping voltage is equal to the power supply voltage (it will be a few volts less actually), then the peak input voltage required is equal to the rail voltage divided by the voltage gain of the amp (which you determine by design). To find RMS, divide that number by the square root of two (1.414).

So if you have an amp with a gain of 20 and voltage rails of +-30 V, then the peak input voltage to clipping is 30/20 = +-1.5 V peak (or 3 Vp-p if you wish). The RMS voltage is then 1.5/1.414 = 1.061 Vrms.
  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
newbie question : driver/output relationship, input sensitivity mahleriana Tubes / Valves 1 17th August 2008 08:10 PM
Newbie question regarding LM3875 chipamp Rev 3 PCB colinB Chip Amps 6 27th September 2006 08:41 AM
Newbie with a question about speaker sensitivity Dylan Multi-Way 5 10th September 2004 04:59 AM
Newbie question: why use non-isolated LM3875? tcpip Chip Amps 41 27th September 2003 04:35 PM


New To Site? Need Help?

All times are GMT. The time now is 08:06 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