Simple amp with DC on the output...

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I made a simple amplifier today.. and for some reason when I have nothing connected to the input I get a positive voltage on the speaker output.. :hot: why ohh why??

heres the circuit.. something I missed?? something I don't need? thanx.. I am using +-24 volt rails BTW.. and 2 chips share a common transformer and 10,000uF per rail... no caps close to the chips, the closest are the 10,000uF which are only about 5cm of wire away anyway... any ideas about the voltage?? THANX!!!
 

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Yes, there is something you have missed. Input offset voltage
and current.

A resistor to ground on the positive input will lower the DC
level when no source is connected.
The value must be a trade-off, since it will also lower
the input impedance of the amp.

You may still get problems with output DC both with and
without a source. I recommend a capacitor in series with
820 Ohm resistor. About 47uF should do, unless I miscalculated,
but let's make it 100uF.
 
Put a cap of maybe 100uf from the bottom of the 820R resistor to ground instead of to ground directly. This will reduce the dc gain of the cct to 1. Also, the input should have some resistance, say 47k, from input terminal to earth to stop it drifting when open cct.
 
Netlist said:
What opamp did you use?

/Hugo :)

LM3886

Circlotron said:
Put a cap of maybe 100uf from the bottom of the 820R resistor to ground instead of to ground directly. This will reduce the dc gain of the cct to 1. Also, the input should have some resistance, say 47k, from input terminal to earth to stop it drifting when open cct.

Will try that sometime...

peranders said:
Is the input floating or have you tied it to ground? The IC must allways have DC-paths from the inputs to "somewhere" (ground). If you not have bias currents you will get a "undefinened condition".


huh? lol
 
Christer said:
Yes, there is something you have missed. Input offset voltage
and current.

A resistor to ground on the positive input will lower the DC
level when no source is connected.
The value must be a trade-off, since it will also lower
the input impedance of the amp.

You may still get problems with output DC both with and
without a source. I recommend a capacitor in series with
820 Ohm resistor. About 47uF should do, unless I miscalculated,
but let's make it 100uF.


when did you post??
anyway... I was planning on running this directly from a computer soundcard with no volume control.. is there a problem with this?

I believe just keeping this connected to my source should prevent me from getting the positive supply rail on the output.. :hot:
 
SkinnyBoy said:

when did you post??

I had a feeling you would start this thread, so I answered a
couple of minutes before you asked the question. ;)


anyway... I was planning on running this directly from a computer soundcard with no volume control.. is there a problem with this?

I believe just keeping this connected to my source should prevent me from getting the positive supply rail on the output.. :hot:


Considering computers crashing and programs running amok,
I would say a pot might be useful so you can turn down the
volume if necessary. Actually, a pot at the input will serve as
the grounding resistor as well, so go for that. A 47k log pot
would be fine.
 
Christer said:


I had a feeling you would start this thread, so I answered a
couple of minutes before you asked the question. ;)




Considering computers crashing and programs running amok,
I would say a pot might be useful so you can turn down the
volume if necessary. Actually, a pot at the input will serve as
the grounding resistor as well, so go for that. A 47k log pot
would be fine.


HAHA very funny.. :p lol

I CAN"T use a pot... this is my computer in a CD player case project.. the case has to still look like a cd player... nothing added to the front... :) its going to have an inbuilt 30watt per channel amp.. lol

http://diyaudio.com/forums/showthread.php?s=&threadid=13905&highlight=
 
li_gangyi said:
dude...did you put a resistor before testing for offset??


of course not.. lol

my concern is the fact that I want to use the computer to control the volume and if there is nothing connected I get the voltage rail on the output... the amp will not be able to be on without the computer... but I still don't know..
 
You should consider some sort of muting for power up/down of the computer and/or a blocking cap on the inputs.

I connected my computer directly to an amp and there are huge pops on startup, especially after it locks up. (sometimes an hourly occurrence). I have a Dell Inspiron 8200. Other computers/soundcards may behave better, but why risk your speakers?
 
You still ought to have that resistor to ground at the input.

You should also be aware that there is now nothing that
protects your speakers from DC in case of amp failure. If you
care a lot about you speakers you may wish to include some
form of DC protection circuitry or, in the worst case, a cap
at the output.
 
BobEllis said:
You should consider some sort of muting for power up/down of the computer and/or a blocking cap on the inputs.

I connected my computer directly to an amp and there are huge pops on startup, especially after it locks up. (sometimes an hourly occurrence). I have a Dell Inspiron 8200. Other computers/soundcards may behave better, but why risk your speakers?


yeah... true..... I considered a delay speaker connecting thingy.. lol

or... I am going to have a line level out.. that will also connect to the amp.. I could make an external box with a pot that connects the output to ground.. yes? :)
 
Christer said:
You still ought to have that resistor to ground at the input.

You should also be aware that there is now nothing that
protects your speakers from DC in case of amp failure. If you
care a lot about you speakers you may wish to include some
form of DC protection circuitry or, in the worst case, a cap
at the output.


AHHH!!! STOP REPLYING!!! lol

yeah... but caps are big.. lol a 50volt 2200uF cap just takes up too much valuable space.. lol :D hehe
 
I meant when the computer powers up it sends some DC to the amp. The amp I use here (Hafler 500) has a speaker protection delay that includes DC offset protection. It is not fast enough to prevent the pops, it mutes after the noise then reconnects after a few seconds.
 
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