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Old 9th December 2006, 06:36 PM   #11
adason is offline adason  United States
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Quote:
What would be a good gain in your mind?
the unswer to your question is....
yes, no and maybe

definitely yes, even with unefficient speakers, if you are going to listen at normal listening levels

definitely no, if you are party animal and you want to shake the house /I can make 150 watts amp constantly into clipping after some significant alcohol consumption/

go for it, you will see later

ed
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Old 9th December 2006, 06:44 PM   #12
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any small diffeences in sound caused by these considerations will surely be swamped by the mp3 decoding and outoput stage of the mp3 player, i would just go with something int he middle range
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Old 9th December 2006, 11:46 PM   #13
CarlosT is offline CarlosT  United States
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Nobody liked my idea of a DPDT switch with two sets of R1/R3 resistors...High Gain/Low Gain?
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Old 9th December 2006, 11:50 PM   #14
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Quote:
Originally posted by CarlosT
Nobody liked my idea of a DPDT switch with two sets of R1/R3 resistors...High Gain/Low Gain?
No
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Old 10th December 2006, 12:31 AM   #15
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Quote:
Originally posted by CarlosT
Nobody liked my idea of a DPDT switch with two sets of R1/R3 resistors...High Gain/Low Gain?
I'd probably try it [but I am a newbie]
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Old 10th December 2006, 07:52 PM   #16
scottw is offline scottw  United States
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I can appreciate that you are trying to design for a player with a low output, but I'm wondering how many here believe the subject line: "Less Negative Feedback Is Better?" as it pertains to power opamps?

This from the LM3875 datasheet:

"Although higher gain amplifiers provide
greater output power and dynamic headroom capabilities,
there are certain shortcomings that go along with the so
called “gain”. The input referred noise floor is increased and
hence the SNR is worse. With the increase in gain, there is
also a reduction of the power bandwidth which results in a
decrease in feedback thus not allowing the amplifier to respond
as quickly to nonlinearities. This decreased ability to
respond to nonlinearities increases the THD + N specification."

I'm wondering because my chip amp for the last several years has had a gain of ~32 and for my next build I was planning for a gain ~18, to see if there was much difference.


Scott
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Old 11th December 2006, 12:18 AM   #17
CarlosT is offline CarlosT  United States
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Just about every LMXXXX and GC/IGC setup the default gain seems to be 20.
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Old 12th December 2006, 07:51 PM   #18
CarlosT is offline CarlosT  United States
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So any last pearls of wisdom on what is a good feedback resistor combination for my application?
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Old 12th December 2006, 08:21 PM   #19
adason is offline adason  United States
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Hi CarlosT,

why don't you make it a pot
you can change it quickly and listen to it at various values
that's what diy is about!
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Old 12th December 2006, 08:54 PM   #20
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Quote:
Originally posted by adason
Hi CarlosT,

why don't you make it a pot
you can change it quickly and listen to it at various values
that's what diy is about!

You should be carefull here. Many chip amps have a MINIMUM gain below which they will oscillate. The reason is that with the increased feedback, the amp will have a wider bandwidth. That in turn means that the high frequencies are still amplified, even when they approach 180 degrees phase shift. If you feedback a 180 degrees phase shifted signal it turns into POS feedback rather than neg feedback -> oscillator instead of amp.
The datasheet should detail this.

Even getting close to the min gain should be done with care, because external circuitry, pcb layout etc can add phase shift to that of the amp itself.

Bottom line, stay away from min gain plus a healthy safety margin.

Jan Didden

PS For minimum DC offset (assuming your feedback network is DC coupled, no cap to ground) make the parallel value of the two feedback resistors equal to the DC resistance seen by the + input.
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