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Electronics => Beginners => Topic started by: christos on October 12, 2015, 03:40:24 am

Title: Audio amp voltages
Post by: christos on October 12, 2015, 03:40:24 am

http://s21.postimg.org/s4oem13c7/400_W_Mosfet_Power_Amplifier.png (http://s21.postimg.org/s4oem13c7/400_W_Mosfet_Power_Amplifier.png)


Does this work with lower voltage..lets say 40v instead of 70-80v that its supose to work?will it work with lower watts? what's the downside if it works
Just a question..trying to figure how audio amps works

Title: Re: Audio amp voltages
Post by: kc8apf on October 12, 2015, 04:20:08 am

The overall design should work at lower supply voltages but many of the component values will need to change.  For example, the bias resistors will need to be changed to get the appropriate bias currents. 

This is a push-pull design.  Lowering the supply rail voltage is how you get lower power output.  Consider the case when the upper output FETs are fully on, the output will be driven to +70V.  Typical speakers are nominally 8 Ohm so the output current will be 70V/8R=8.75A.  P=IV so P=8.75A*70V = 612W.  The 400W rating presumes that you aren't driving it to the limits to keep the distortion low.

Title: Re: Audio amp voltages
Post by: SeanB on October 12, 2015, 07:14:57 pm

Also remember those Mosfets will have a threshold voltage, which limits the output voltage somewhat. Maximum voltage on the output will be around 10V lower than the positive rail, but the negative swing will go almost to the negative supply.

Google quasi complimentary amplifiers, and look for the stuff by John Lindsey Hood on them.

https://web.archive.org/web/20110720133138/http://www.tcaas.btinternet.co.uk/ (https://web.archive.org/web/20110720133138/http://www.tcaas.btinternet.co.uk/)

Title: Re: Audio amp voltages
Post by: Kleinstein on October 12, 2015, 07:50:39 pm

The plan has a small mistake: the SB546 is a PNP, and needs to be. The plan shows an NPN symbol instead.
Changing the bias (two 33 K resistors) is not that critical. It might still work at +-40 V.

Less voltage means less power. Distortion will not change very much. With MOSFETs, there will quite some voltage lost on the positive side - so the loss in power may be more than one might expect.

Title: Re: Audio amp voltages
Post by: Kappes Buur on October 12, 2015, 08:24:37 pm

Check this out as well http://raylectronics.nl/pdfs/Crescendo_Millennium_en.pdf (http://raylectronics.nl/pdfs/Crescendo_Millennium_en.pdf)

Title: Re: Audio amp voltages
Post by: christos on October 13, 2015, 03:55:40 am

Thx all of you..got what i needed..thank you!  the schematic i posted was only an example, so i dont mind if its wrong .