LM3875 Newbie Question re: sensitivity
 User Name Stay logged in? Password
 Home Forums Rules Articles diyAudio Store Blogs Gallery Wiki Register Donations FAQ Calendar Search Today's Posts Mark Forums Read Search

 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
 11th January 2004, 07:58 PM #1 Gringrath   diyAudio Member   Join Date: Jan 2004 Location: Northumberland 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.
GregGC
diyAudio Member

Join Date: Oct 2003
Location: Toronto
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.

 11th January 2004, 09:09 PM #3 Bozzz   diyAudio Member   Join Date: Jan 2004 Location: Amsterdam 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.
macboy
diyAudio Member

Join Date: Oct 2003
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.

 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 Forum Rules
 Forum Jump User Control Panel Private Messages Subscriptions Who's Online Search Forums Forums Home Site     Site Announcements     Forum Problems Amplifiers     Solid State     Pass Labs     Tubes / Valves     Chip Amps     Class D     Power Supplies     Headphone Systems Source & Line     Analogue Source     Analog Line Level     Digital Source     Digital Line Level     PC Based Loudspeakers     Multi-Way     Full Range     Subwoofers     Planars & Exotics Live Sound     PA Systems     Instruments and Amps Design & Build     Parts     Equipment & Tools     Construction Tips     Software Tools General Interest     Car Audio     diyAudio.com Articles     Music     Everything Else Member Areas     Introductions     The Lounge     Clubs & Events     In Memoriam The Moving Image Commercial Sector     Swap Meet     Group Buys     The diyAudio Store     Vendor Forums         Vendor's Bazaar         Sonic Craft         Apex Jr         Audio Sector         Acoustic Fun         Chipamp         DIY HiFi Supply         Elekit         Elektor         Mains Cables R Us         Parts Connexion         Planet 10 hifi         Quanghao Audio Design         Siliconray Online Electronics Store         Tubelab     Manufacturers         AKSA         Audio Poutine         Musicaltech         Holton Precision Audio         CSS         Dx Classic Amplifiers         exaDevices         Feastrex         GedLee         Head 'n' HiFi - Walter         Heatsink USA         miniDSP         SITO Audio         Twin Audio         Twisted Pear         Wild Burro Audio

 Similar Threads Thread Thread Starter Forum Replies Last Post mahleriana Tubes / Valves 1 17th August 2008 08:10 PM colinB Chip Amps 6 27th September 2006 08:41 AM Dylan Multi-Way 5 10th September 2004 04:59 AM tcpip Chip Amps 41 27th September 2003 04:35 PM

 New To Site? Need Help?

All times are GMT. The time now is 02:55 PM.