| SkinnyBoy |
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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| Christer |
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. |
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| Netlist |
What opamp did you use?
/Hugo :) |
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| Circlotron |
| 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. |
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| peranders |
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 "undefined condition".
He, he, three people were equally fast as I.
Opamp? Probably LM3886! |
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| SkinnyBoy |
| quote: | Originally posted by Netlist
What opamp did you use?
/Hugo :) |
LM3886
| quote: | Originally posted by Circlotron
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...
| quote: | Originally posted by peranders
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 |
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| peranders |
| Benrangelpojken, how much offset? It helps if you tell us that. |
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| SkinnyBoy |
| quote: | Originally posted by Christer
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: |
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| Christer |
| quote: | Originally posted by SkinnyBoy
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. ;)
| quote: |
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. |
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| SkinnyBoy |
+23 volts offset.. lol
which is why I wasn't calling it offset.... I think I just need something on the input... I don't want a pot... and I am not sure what the computer soundcard does while the computer is starting up.. I am also quite sure that 4inch woofers don't like 23volt applied to them.. lol |
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| li_gangyi |
| dude...did you put a resistor before testing for offset?? |
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| SkinnyBoy |
| quote: | Originally posted by Christer
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/showthre...3905&highlight= |
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| SkinnyBoy |
| quote: | Originally posted by li_gangyi
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.. |
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| BobEllis |
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? |
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| Christer |
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. |
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| millwood |
| if you are getting that big of a DC offset, I would suspect the wiring. |
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| SkinnyBoy |
| quote: | Originally posted by BobEllis
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? :) |
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| SkinnyBoy |
| quote: | Originally posted by Christer
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 |
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| BobEllis |
| 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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| SkinnyBoy |
| Perhaps I'll just put a pot on the back of the case.. lol there really isn't any room though... :rolleyes: |
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| li_gangyi |
| If you dun wanna use a pot...use a 50K resistor and tie it to ground...and remember to attach an 8ohm power resistor before trying to measure any offset... |
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| SkinnyBoy |
| quote: | Originally posted by li_gangyi
If you dun wanna use a pot...use a 50K resistor and tie it to ground...and remember to attach an 8ohm power resistor before trying to measure any offset... |
hmm... ok.... and perhaps a capacitor on the outputs.. :) just to be abit safer... and maybe a.... umm.. I forgot what I was saying.. :xeye: |
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| carlosfm |
Hey SkinnyBoy,
As Christer said, you NEED to put a resistor (say, 47k) on the RCA inputs (from + to ground).
If you don't have a pot, you need this.
DON'T CONNECT THE AMP TO ANY SPEAKER BEFORE DOING THIS!!!:eek:
23v DC offset may kill any speaker.
Oh, and you should put an input cap.
It's not big.
Nothing on the output.
Oh, BTW, don't put water on this one.:bawling: :clown: |
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| li_gangyi |
| Yeah...same thing as I said...though I suggested 50K...might not be available though... |
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| SkinnyBoy |
| quote: | Originally posted by carlosfm
Hey SkinnyBoy,
As Christer said, you NEED to put a resistor (say, 47k) on the RCA inputs (from + to ground).
If you don't have a pot, you need this.
DON'T CONNECT THE AMP TO ANY SPEAKER BEFORE DOING THIS!!!:eek:
23v DC offset may kill any speaker.
Oh, and you should put an input cap.
It's not big.
Nothing on the output.
Oh, BTW, don't put water on this one.:bawling: :clown: |
I already connected it to a speaker... an old 4inch driver.. and unpluged the input.. lol |
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| li_gangyi |
| Ok..check for a missing rail...one side might be gone...or shot..therefore the offset... |
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| carlosfm |
| quote: | Originally posted by SkinnyBoy
I already connected it to a speaker... an old 4inch driver.. and unpluged the input.. lol |
:D
No Bang?!:eek:
Good speaker, man, lol!!! |
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| peranders |
Skinnyman, have you made any troubleshooting?
1 Connect exactly as the schematic
2 Check the component values
3 Ground the input via a 10-47 k resistor
4 Connect nothing to the output
5 Measure all voltages with a DMM. Post the result to us.
6 Read carefully the datasheet and AN-1192
7 Take a photo of it and post it here
With the connection you have chosen you will get less than 300 mV offset I'll guess.
Skinnyboy, we talked about learning in an another thread, this LM3886 happens to have an excellent application note and I think you should read it carefully and try to understand it. DC-paths for the inputs ARE ESSENTIAL! If you don't understand why, just trust us. Meaning you must have a pull down resistor when you test this amp with no signal source. It's also wise to have all times just case the amp is switched on and the input is disconnected. |
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| SkinnyBoy |
| quote: | Originally posted by carlosfm
:D
No Bang?!:eek:
Good speaker, man, lol!!! |
no bang... the speaker cone went in about 3mm, and sat there hovering....
| quote: | Originally posted by peranders
Skinnyman, have you made any troubleshooting?
1 Connect exactly as the schematic
2 Check the component values
3 Ground the input via a 10-47 k resistor
4 Connect nothing to the output
5 Measure all voltages with a DMM. Post the result to us.
6 Read carefully the datasheet and AN-1192
7 Take a photo of it and post it here
With the connection you have chosen you will get less than 300 mV offset I'll guess.
Skinnyboy, we talked about learning in an another thread, this LM3886 happens to have an excellent application note and I think you should read it carefully and try to understand it. DC-paths for the inputs ARE ESSENTIAL! If you don't understand why, just trust us. Meaning you must have a pull down resistor when you test this amp with no signal source. It's also wise to have all times just case the amp is switched on and the input is disconnected. |
I haven't connected the resistor yet, ok? it was like midnight... :rolleyes:
1. The circuit is connected correctly
2. I chose those component values at randon (well, almost) as I didn't have the ones in the diagram...
3. I shorted the input to ground (with the CD players output) and measured about 45mV ofset
my digital camera is rubbish... I'll take pics anyway.. everything is once again mounted across the chip.. :)
Thanx for the help.. I'll connect a 47ohm (I probably don't have any) resistor across the input... |
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| Christer |
| quote: | Originally posted by SkinnyBoy
Thanx for the help.. I'll connect a 47ohm (I probably don't have any) resistor across the input... |
Probably just a typo, but in case it wasn't, the value shoould
rather be 47 kOhm. |
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| SkinnyBoy |
| quote: | Originally posted by Christer
Probably just a typo, but in case it wasn't, the value shoould
rather be 47 kOhm. |
It wasn't a typo.. 47k? thats pretty high.... I'll use something lower... closer to 10k :) |
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| soundNERD |
I didn't have time to read all of the responces, but in the ones I read, I saw nothing related to the simple fix.
You said that the output voltage increases when an input source is connected. Portable and cheap CD players tend to put out a voltage in the output. I had the problem where when the CD player is turned on, the speaker would suck in about 1/2 inch then come back out with a pop sound. To fix it, I just placed a small cap inline with the input. I myself used 2uf. It solved the DC voltage problems.
Good luck, Mike |
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| SkinnyBoy |
| quote: | Originally posted by soundNERD
I didn't have time to read all of the responces, but in the ones I read, I saw nothing related to the simple fix.
You said that the output voltage increases when an input source is connected. Portable and cheap CD players tend to put out a voltage in the output. I had the problem where when the CD player is turned on, the speaker would suck in about 1/2 inch then come back out with a pop sound. To fix it, I just placed a small cap inline with the input. I myself used 2uf. It solved the DC voltage problems.
Good luck, Mike |
nah.. theres nothing wrong with it.. just my lack of electronics knowledge... lol... |
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| Christer |
| quote: | Originally posted by SkinnyBoy
It wasn't a typo.. 47k? thats pretty high.... I'll use something lower... closer to 10k :) |
Probably works fine. My soundblaster card has no problem
driving headphones from the line output (not sure if that is
advisable, though).
However, you were very reluctant to put in the resistor at
all, and now you want a low value. A 47k is closer
to no resistor at all than 10k is. |
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| SkinnyBoy |
| 33k it is.. lol cos they are prettier.. :D |
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| Christer |
| quote: | Originally posted by SkinnyBoy
33k it is.. lol cos they are prettier.. :D |
Maybe, if the bottom colour is blue, otherwise I can think of many
other more estethical values, to my taste, that is. :) |
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| li_gangyi |
| Should be ok after you got the 47k there..or at least a pot... |
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| carlosfm |
Definitely.
The sound card is always to be kept at max volume.
That's because you loose 1 bit of resolution for every 6 db of digital attenuation.
SkinnyBoy, if you care about sound quality, put a pot on the amp and control the volume there.
Let the sound card at maximum volume.
:angel: |
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| SkinnyBoy |
| quote: | Originally posted by carlosfm
Definitely.
The sound card is always to be kept at max volume.
That's because you loose 1 bit of resolution for every 6 db of digital attenuation.
SkinnyBoy, if you care about sound quality, put a pot on the amp and control the volume there.
Let the sound card at maximum volume.
:angel: |
my soundcard clips at maximum volume!! HAHAHA!! and anything about half way gets distorted... lol
I could always put a pot INTERNALLY... so.. the max volume on the soundcard is the max level I would listen to it (or the "normal" level, or the level at which the amp clips.. lol) |
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| li_gangyi |
| Dude...what kinda sound card have you got there?? Seems that mine does not clip at max... |
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| peranders |
| quote: | Originally posted by SkinnyBoy
3. I shorted the input to ground (with the CD players output) and measured about 45mV ofset |
Does it work now? 45 mV is very good! |
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| SkinnyBoy |
The soundcard that clips badly is the one in this computer, not the one I am using for this new project... the clipping card is an old ISA card.. I believe it has an inbuilt amp that causes the problem..
| quote: | Originally posted by peranders
Does it work now? 45 mV is very good! |
I dunno if its 45mV it was before I connected a resistor to the input.. it wasn't a proper measurement.. (I don't think) but I can't test it again as I am working on installing it in teh case.. :) |
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| carlosfm |
lol crappy lol soundcard.:D
Not good for testing.:rolleyes:
lol.:cannotbe: |
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| SkinnyBoy |
Some pictures of stuff...
:D |
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| SkinnyBoy |
| Please sir, have another.. :p |
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| SkinnyBoy |
| JUST TAKE THE STUPID PICTURES!!! |
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| SkinnyBoy |
| I wasn't going to waste my ISPs bandwith hosting these pictures.. and slowing my internet connection down... :p instead I chose to host them by the only website I visit, to slow that down instead.. lol |
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| carlosfm |
Nice, lol.
It only lacks a picture of it's face.:clown:
I like to look gear in the eyes, lol.:D
Where's the front panel pic?:dodgy: |
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| SkinnyBoy |
well.. theres none in the original thread.. lol
http://diyaudio.com/forums/showthre...3905&highlight=
its just a standard CD player... nothing special at all.. :) well.. not from the front anyway.. and it will look like a standard CD player when finished.. (Except probably a few air holes in the top.. lol) |
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| li_gangyi |
| Looking good...maybe you might wanna consider another soundcard... |
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| SkinnyBoy |
| quote: | Originally posted by li_gangyi
Looking good...maybe you might wanna consider another soundcard... |
this soundcard is inbuilt.. I was talking about a different one, in a different computer.. :) sorry for the confusion..lol |
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| li_gangyi |
| try reducing the level of output from ur soundcard...some members here might be able to help...or maybe remove the "amp" section from the sound card... |
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| peranders |
| It's possible the reduce the gain of the LM3886. Much easier :nod: |
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| SkinnyBoy |
| quote: | Originally posted by li_gangyi
try reducing the level of output from ur soundcard...some members here might be able to help...or maybe remove the "amp" section from the sound card... |
would you just shutup about the sound card.. :rolleyes: if it makes you happy, I have swapped cards now.. happy?? |
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| sss |
| quote: | Originally posted by SkinnyBoy
JUST TAKE THE STUPID PICTURES!!! |
well , a cd player+amps+motherboard and processor in one box :)
it was easier to put the amps inside the pc case , dont u think?:)
it looks better this way though :) |
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| li_gangyi |
| Ok...so you swapped the soundcards...guess the amp now works properly now ?? And no DC on the outputs... |
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| SkinnyBoy |
| quote: | Originally posted by li_gangyi
Ok...so you swapped the soundcards...guess the amp now works properly now ?? And no DC on the outputs... |
I'm not even going to answer that, the question doesn't even make sence.... |
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| li_gangyi |
| I mean...does the amp work properly now?? And without DC on the outputs... |
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| SkinnyBoy |
| quote: | Originally posted by li_gangyi
I mean...does the amp work properly now?? And without DC on the outputs... |
I still haven't tested it with the changes... |
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| carlosfm |
| quote: | Originally posted by SkinnyBoy
I still haven't tested it with the changes... |
Just don't test it with your Grand Utopias.:D |
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| SkinnyBoy |
I tested it.....
less than 150mv with just the resistor on the input...
and
less than 50mv with the computer connected to it.. :) |
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| li_gangyi |
| great...so are you satisified with the final product?? |
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| SkinnyBoy |
the heatsink is too small... it gets too hot... and I still haven;t added a cap to the input for DC filtering... I haven't blown the cones out of the 3inch woofers its running YET, but if I'm not carfull I probably will...
there is no way I could use this to run 4ohm speakers.. the heatsink is far too small... I might try and fan cool it or add some fins to it.. but space is really a problem... |
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| li_gangyi |
| try water cooling...might also help the PC you are building this thing in... |
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| SkinnyBoy |
| umm.. there is no space to water cool it..... it gets too hot to leave your hand on it.. lol |
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| li_gangyi |
| hmmm...how about lifting the amp outta the PC?? or maybe free up a drive bay and use the whole space for a heatsink... |
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| SkinnyBoy |
the 3inch woofers cost $9 for the pair... they are sortof TLs.. they are sitting on my desk facing up with a 50cm piece of pipe resting ontop.... they have a usable response down to like 50Hz..... but not at too much power...
the amp cost me $2.70 to make (even cheaper than the speakers) all I had to buy was the speaker terminals...
EVERYTHING else was recycled, except the chips with are samples.. :) |
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| SkinnyBoy |
| quote: | Originally posted by li_gangyi
hmmm...how about lifting the amp outta the PC?? or maybe free up a drive bay and use the whole space for a heatsink... |
did you even read previous posts and look at the pictures? |
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| li_gangyi |
| I did...maybe a thicker aluminium extrusion would help...how about a thick aluminium block...drileed and with fans at one side...to help dissapate heat...or attach some fins... |
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| SkinnyBoy |
| I suppose I could at a PINCH bolt another piece of aluminium onto the back of the current heatsink, and bend it so it goes over the network card... |
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| li_gangyi |
| wonder if it's enough for the amp...relocate the amp to the sides...and have heatsinks extruding out of the case?? Izzit a possibility? |
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| SkinnyBoy |
the amp has to stay where it is...
there is nowwhere else for it to go, and where it is the mains wires, and secondary connection to the transformer are as short as possible... :) as is the DC from the filter caps and bridge... :) |
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| carlosfm |
Hey Skinnyboy,
Even staying where they are, you have some space to put the chips on cpu heatsinks (with fan).
It may be tight, but I think it's manageable.
Fans working at low speed (lower voltage) don't make much noise and are effective.;)
lol. |
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| SkinnyBoy |
| hmm.. I don't think I do have space.. lol the aluminium plate they are mounted on now rests on the motherboard, and the top of the case.... and, the surface that the chips are currently mounted on is only 4.5 cm high.... |
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| li_gangyi |
| that is really too small...will your mini PC have a cover?? Perhaps you can use a big heatsink as a cover.. |
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| SkinnyBoy |
| quote: | Originally posted by li_gangyi
that is really too small...will your mini PC have a cover?? Perhaps you can use a big heatsink as a cover.. |
umm.. yes.. it will have a cover.....
no.. the cover can't be a heatsink... |
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| li_gangyi |
| groans...how do we cool the amp then?? |
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| SkinnyBoy |
| we don't.. if it overheats.. it overheats... no big problem... |
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| carlosfm |
| quote: | Originally posted by SkinnyBoy
hmm.. I don't think I do have space.. lol the aluminium plate they are mounted on now rests on the motherboard, and the top of the case.... and, the surface that the chips are currently mounted on is only 4.5 cm high.... |
Naughty naughty, I see that you changed some things.:whazzat:
Your picture is from version 1.0, and now you're on version 1.1.:bawling:
Is the new version better?
Well... at least it's:hot: . |
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| SkinnyBoy |
| quote: | Originally posted by carlosfm
Naughty naughty, I see that you changed some things.:whazzat:
Your picture is from version 1.0, and now you're on version 1.1.:bawling:
Is the new version better?
Well... at least it's:hot: . |
umm.. no.. its the same... I'll remove the word "now" from the post you quoted.. (not literally) |
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| carlosfm |
Don't tell me you can't fit some heatsinks or cpu coolers in this space.
I see some space here.
:smash: |
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| SkinnyBoy |
| quote: | Originally posted by carlosfm
Don't tell me you can't fit some heatsinks or cpu coolers in this space.
I see some space here.
:smash: |
that gap is like 3.5cm X 4cm X 16cm perhaps a heatsink.. but NOT a CPU cooler... yes.. I can fit SOME heatsink in there.. and I will try... |
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| carlosfm |
One single PentiumII cooler (with fan) would fit there.
If you can find one.
And it would do fine for the two chips.;) |
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| li_gangyi |
| yeah was thinking of that as well... |
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| SkinnyBoy |
| umm.. no it won't.... |
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| carlosfm |
| quote: | Originally posted by SkinnyBoy
umm.. no it won't.... |
Yes it would.
You may have to cut a little of that l-shaped aluminium so that the fan fits, but it would do.
There are (were) some thin PII coolers, about 2cm thick (the heatsink), and about 1cm more for the fan.
Remember, hot chip means problems.
As you have it, you can't play your music loud, the chip will protect itself against destruction.:D
lol, and when the music is good, turn up the volume!:) |
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| SkinnyBoy |
| quote: | Originally posted by carlosfm
Yes it would.
You may have to cut a little of that l-shaped aluminium so that the fan fits, but it would do.
There are (were) some thin PII coolers, about 2cm thick (the heatsink), and about 1cm more for the fan.
Remember, hot chip means problems.
As you have it, you can't play your music loud, the chip will protect itself against destruction.:D
lol, and when the music is good, turn up the volume!:) |
it won't fit... |
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| li_gangyi |
| ...you just gotta get a heatsink there pal... |
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