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Old 8th November 2003, 01:08 AM   #1
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Default Are these parts needed for the LM1875?

I am now starting to build a dual LM1875 amplifier, and want to keep it simple so the circuitry fits on a small board. Is the resistor and cap connecting from pin 2 to gnd neccesary? I can't see what purpose it serves, but I am sure somebody can point out its job. Also, is the 22k resistor on the input nessecary? I assume this has something to do with volume, and since I will be adding a volume control, do I need it?
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Old 8th November 2003, 01:13 AM   #2
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you could try the inverted way to reduce your parts count...
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Old 8th November 2003, 01:16 AM   #3
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What parts in the inverted mode would be removed? and can I just use the same schematic but change the + and - on the chip around? And what if I were to remove these parts witout going into inverted mode? Will the sound not be as good? Will it even work?

Thanks, Mike
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Old 8th November 2003, 01:28 AM   #4
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Default Re: Are these parts needed for the LM1875?

Quote:
Originally posted by soundNERD
I am now starting to build a dual LM1875 amplifier, and want to keep it simple so the circuitry fits on a small board. Is the resistor and cap connecting from pin 2 to gnd neccesary? I can't see what purpose it serves, but I am sure somebody can point out its job. Also, is the 22k resistor on the input nessecary? I assume this has something to do with volume, and since I will be adding a volume control, do I need it?
http://www.diyaudio.com/forums/showt...496#post259496

This is the simplest you can do for NIGC. Just use the apropriate pins for LM1875. Check the sch above for even one resistor less, but I'd put the resistor to sleep well at nigt.

Greg
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Old 8th November 2003, 01:29 AM   #5
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look at some of the inverted designs you see around the forum...would work for you as well...though the pinout might be different...and they say inverted mode sounds better...
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Old 8th November 2003, 01:35 AM   #6
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Maybe I will just use the extra components, since I already have most of it built. Thanks, though.
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Old 8th November 2003, 05:26 AM   #7
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No problem...tell us how it sounds after you have completed...
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Old 8th November 2003, 08:12 AM   #8
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Quote:
the resistor and cap connecting from pin 2 to gnd neccesary? I can't see what purpose it serves, but I am sure
You should make the effort to learn some basic opamp theory. The resistor is there to set the gain of x21; without it you get unity AC gain. The cap sets the gain to unity for DC. If you replace it with a short you'll get slightly worse offset and considerably better sound.
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Old 8th November 2003, 12:35 PM   #9
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Well I built one channel last night and am going to build the other today. I used an old split power supply that I built using a 26.6VCT transformer @ max of 1A, so through the rectifier it puts out about +-22. The rectifier also reduces current, so I am probably getting about 3/4A, right? So, I think that could and most likely is the cause for distortion. I am running an 8-ohm speaker with a portable CD player as the input. Of course the CD player could be the problem, but I think it is the low power input. It does go consiterably loud before distortion for how little power it is getting. I also noticed a large problem I am having. When there is nothing connected to the input, the speaker starts to buzz and sort of vibrate since the speaker in it is moving in and out rapidly. I really need to make it so there is no noise even with the input connected.

Also, analog_sa, what do you mean by worst offset? And by considerably better sound, do you mean crisper, less distortion, or something else?

Is it OK that I am using a 33k resistor for the feedback? That wouldn't be setting the gain too high, right? If so, I would have to use a 15k.

What is the purpose of the 1ohm resistor and the capacitor on the output? I didn't have the cap so at first I just used the resistor. The sound was not too good. So I decided to put a 0.5uf NP capacitor in place of the 0.22uf, and the sound became much better. What do these parts do?

Thanks for all of your help, Mike
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Old 8th November 2003, 01:05 PM   #10
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Quote:
Originally posted by soundNERD
Well I built one channel last night and am going to build the other today. I used an old split power supply that I built using a 26.6VCT transformer @ max of 1A, so through the rectifier it puts out about +-22. The rectifier also reduces current, so I am probably getting about 3/4A, right? So, I think that could and most likely is the cause for distortion. I am running an 8-ohm speaker with a portable CD player as the input. Of course the CD player could be the problem, but I think it is the low power input. It does go consiterably loud before distortion for how little power it is getting. I also noticed a large problem I am having. When there is nothing connected to the input, the speaker starts to buzz and sort of vibrate since the speaker in it is moving in and out rapidly. I really need to make it so there is no noise even with the input connected.

Also, analog_sa, what do you mean by worst offset? And by considerably better sound, do you mean crisper, less distortion, or something else?

Is it OK that I am using a 33k resistor for the feedback? That wouldn't be setting the gain too high, right? If so, I would have to use a 15k.

What is the purpose of the 1ohm resistor and the capacitor on the output? I didn't have the cap so at first I just used the resistor. The sound was not too good. So I decided to put a 0.5uf NP capacitor in place of the 0.22uf, and the sound became much better. What do these parts do?

Thanks for all of your help, Mike
follow analog_sa' s advice and read read a couple paragraphs about how op-amps work. a 33K resistor sets the gain too high.

the RC network rolls off the highs -- it depends on the cables you are using, length of cables, type of crossover etc. whether it is necessary -- the chip amps easily go into oscillation.

i think i have posted the link to the ChipAmp calculator on Nat Semi's website 3X so I am not going to post it again.
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