Hi,
I have built this exact circuit with an LM3886, (well actually its got a 770 instead of a 680R resistor and a 1K input resistor but they will be changed/removed tomorrow..) but im not sure whether i have a safe dc offset or not..
I have built 2 of them and get 59mV on one and 72mV on the other side..
Im have some expensive speakers so i have only hooked the amp up to a cheap subwoofer, it works well but i cant comment on the tone quality as yet
So are there any 'tricks' to lower the dc offset to a few mV or so?
Help and advice appreciated!
I have built this exact circuit with an LM3886, (well actually its got a 770 instead of a 680R resistor and a 1K input resistor but they will be changed/removed tomorrow..) but im not sure whether i have a safe dc offset or not..
I have built 2 of them and get 59mV on one and 72mV on the other side..
Im have some expensive speakers so i have only hooked the amp up to a cheap subwoofer, it works well but i cant comment on the tone quality as yet
So are there any 'tricks' to lower the dc offset to a few mV or so?
Help and advice appreciated!
The offset values you have got aren't anything to worry about if you are concerend about the speakers. My rule is <50 mV is good, bad over 100 mV. This is just my opinion.
The speaker has a limit when you will get increased dist and a second one when it become damaged (which is several volts).
The speaker has a limit when you will get increased dist and a second one when it become damaged (which is several volts).
More parts -- degrade sound ??
It looks like the chip has no other terminals that you can use.
One method might be to connect your 680 ohms to ground through a large capacitor..... I guess a bad thing to do - polar or non polar! But that would drop the dc gain to 1 and hopefully reduce the error. There can still be inherent errors that you can only remove ( like if you want 1mV ?) by injecting additional voltage to one of the inputs. Something that I would VERY strongly advise against.
It would be better to leave things as they are. Anything else added on to drop the dc offset might just degrade the sound.
You might also open a Pandora's Box ! Your chips are alive now. Let them stay that way .
Can't you keep your hands away from an already good performance ? Just enjoy the music .
Cheers.
It looks like the chip has no other terminals that you can use.
One method might be to connect your 680 ohms to ground through a large capacitor..... I guess a bad thing to do - polar or non polar! But that would drop the dc gain to 1 and hopefully reduce the error. There can still be inherent errors that you can only remove ( like if you want 1mV ?) by injecting additional voltage to one of the inputs. Something that I would VERY strongly advise against.
It would be better to leave things as they are. Anything else added on to drop the dc offset might just degrade the sound.
You might also open a Pandora's Box ! Your chips are alive now. Let them stay that way .
Can't you keep your hands away from an already good performance ? Just enjoy the music .
Cheers.
Re: More parts -- degrade sound ??
It's no harm to trim away this offset. The only thing you have to think of is to inject a clean current so you won't get more noise and hum. Just create a voltage divider 12 V -> 10 mV (or so). Make sure this 12 V is well filtered and stabilized. This voltage divider should go via a trimpot, so you can adjust.ashok said:
Break the connection between R5 and ground, and insert a capacitor in the break. Since the resistor values are rather small you'll need a large capacitor to avoid rolloff in the bass, so you might consider bumping up the resistor values by a factor 10 or so.
Then again, I don't think the offset is much to worry about in practice.
Rune
Then again, I don't think the offset is much to worry about in practice.
Rune
Thanks for the speedy replies guys!
....but i have another problem......
Ok i thought everything was fine, but when i changed the input resistor (R2 = 22k) to a 22k pot my DC offset went from -30mV on mute to +200mV fully open....!!!
I havent tried putting any caps from the 680R resistor to ground as yet..
I also have an input cap placed before the variable resistor, is this the correct place to have it or is it better to have it after the pot from middle pin to the non inverting input of the chip?
....but i have another problem......
Ok i thought everything was fine, but when i changed the input resistor (R2 = 22k) to a 22k pot my DC offset went from -30mV on mute to +200mV fully open....!!!
I havent tried putting any caps from the 680R resistor to ground as yet..
I also have an input cap placed before the variable resistor, is this the correct place to have it or is it better to have it after the pot from middle pin to the non inverting input of the chip?
Optical, you can calculate the offset. For this you must know:
Input voltage offset
Input bias offset
Used resistor values in feedback and at the input.
If you don't know how to calculate, draw a schematics and I (or somebody else ) calculate it for you. Offset is natural but you can reduce it... or increase it. Your choice.
Input voltage offset
Input bias offset
Used resistor values in feedback and at the input.
If you don't know how to calculate, draw a schematics and I (or somebody else ) calculate it for you. Offset is natural but you can reduce it... or increase it. Your choice.
Optical said:Thanks for the speedy replies guys!
....but i have another problem......
Ok i thought everything was fine, but when i changed the input resistor (R2 = 22k) to a 22k pot my DC offset went from -30mV on mute to +200mV fully open....!!!
I havent tried putting any caps from the 680R resistor to ground as yet..
I also have an input cap placed before the variable resistor, is this the correct place to have it or is it better to have it after the pot from middle pin to the non inverting input of the chip?
The sch you use is good for OPA541 where Ibias are very small and will not be affected by changing value of the pot.
That's a bit better.
http://www.diyaudio.com/forums/showthread.php?postid=259500#post259500
But I would do this:
http://www.diyaudio.com/forums/showthread.php?postid=259525#post259525
This circuit will give you steady DC offset at the out, no matter the position on the pot.
If you want to experiment:
1. omit R1/C1 (HF filter, I'd ceep just in case)
2. Check different res values (keep the ratio the same) like 6.8k/220k and see what the output offset will be. It should be fine.
Mind you I did it and it sounds very good:
http://www.diyaudio.com/forums/showthread.php?s=&threadid=23353
I have made some calculations:
R3 22k
R5 680 R
R2 22 k
=> Pot at min, 33-333 mV offset
=> Pot at max, AC coupled signal source, 170 mV to over 1 V.
The big problem is that you have a DC-connected amp (no cap between R5 and ground.
I say that your amp can't be good unless:
Insert a cap in the feedback
DC-servo
Offset trim by a pot.
I suggest that you take a good look at page 1 and typcial application in the datasheet of LM3875 (or maybe LM3386 since you used it). Check also AN-1192 from National.
EDIT: I totally agree with ashok below that you should avoid current paths, and sensitive ones through the pot. Isolate the + input with a cap or have at least a pull down resistor. In case of a bad pot you can get bad behaviour of the LM IC. The IC MUST have current to it's inputs.
R3 22k
R5 680 R
R2 22 k
=> Pot at min, 33-333 mV offset
=> Pot at max, AC coupled signal source, 170 mV to over 1 V.
The big problem is that you have a DC-connected amp (no cap between R5 and ground.
I say that your amp can't be good unless:
Insert a cap in the feedback
DC-servo
Offset trim by a pot.
I suggest that you take a good look at page 1 and typcial application in the datasheet of LM3875 (or maybe LM3386 since you used it). Check also AN-1192 from National.
EDIT: I totally agree with ashok below that you should avoid current paths, and sensitive ones through the pot. Isolate the + input with a cap or have at least a pull down resistor. In case of a bad pot you can get bad behaviour of the LM IC. The IC MUST have current to it's inputs.
Leave it alone.
Hi Optical,
Anything you do now is not going to audibly improve the sound. In fact there is a chance you might degrade it . About 60 mV dc offset will dissipate less than 30mWatts in your speakers coil. That's nothing compared to the music signal that will be going through it.
You can experiment with some opamps to see how things work. Leave your gain clone alone!
If you put your input cap between the slider and the input of the gain clone , it will stop dc flow through the volume pot. That might be a good thing to do. I read somewhere that dc through the pot can eventually make it noisy ( while rotating it ). I am not sure if this is true. But I do know for sure that dc through the pot will cause lots of noise ( while turning ) if it gets dirty or the coating on the slider flakes off slightly. Better keep the dc out.
Larger resistor values will increase dc offset.
Cheers.
Edit : I just noticed that if you use a cap between the input of the gain clone and the volume slider , you will have no dc path to ground. So you will have to add a 22K resistor from the + input to ground.
Have a look at the other post " another gain clone ". It is done correctly there. The circuit is given there.
Hi Optical,
Anything you do now is not going to audibly improve the sound. In fact there is a chance you might degrade it . About 60 mV dc offset will dissipate less than 30mWatts in your speakers coil. That's nothing compared to the music signal that will be going through it.
You can experiment with some opamps to see how things work. Leave your gain clone alone!
If you put your input cap between the slider and the input of the gain clone , it will stop dc flow through the volume pot. That might be a good thing to do. I read somewhere that dc through the pot can eventually make it noisy ( while rotating it ). I am not sure if this is true. But I do know for sure that dc through the pot will cause lots of noise ( while turning ) if it gets dirty or the coating on the slider flakes off slightly. Better keep the dc out.
Larger resistor values will increase dc offset.
Cheers.
Edit : I just noticed that if you use a cap between the input of the gain clone and the volume slider , you will have no dc path to ground. So you will have to add a 22K resistor from the + input to ground.
Have a look at the other post " another gain clone ". It is done correctly there. The circuit is given there.
The dc decoupling cap in the 680ohm leg.
This has been brought up many times.
Electrolytic capacitors generally need a dc voltage across them to function properly. This has to be larger than the ac signal . In applications like these there is hardly any dc voltage across them. So you could expect some degradation in sound. However the capacitor ( polar ) is found in this position in most amps. Using a film type is better here but possibly hard to do because of the large values required.
If you can't avoid it , you should at least check out the polarity of the small dc voltage at this point and orient the cap accordingly.
I would prefer to avoid it and face the penalty of a small dc offset.
The input of the amp should be dc decoupled from the source to prevent it from amplifying that.
Try it out with and without the cap and see if the sound degrades . You might get poorer quality bass and transients apart from other effects. If you can't hear any difference , that's good for you.
Cheers.
This has been brought up many times.
Electrolytic capacitors generally need a dc voltage across them to function properly. This has to be larger than the ac signal . In applications like these there is hardly any dc voltage across them. So you could expect some degradation in sound. However the capacitor ( polar ) is found in this position in most amps. Using a film type is better here but possibly hard to do because of the large values required.
If you can't avoid it , you should at least check out the polarity of the small dc voltage at this point and orient the cap accordingly.
I would prefer to avoid it and face the penalty of a small dc offset.
The input of the amp should be dc decoupled from the source to prevent it from amplifying that.
Try it out with and without the cap and see if the sound degrades . You might get poorer quality bass and transients apart from other effects. If you can't hear any difference , that's good for you.
Cheers.
As Carlos & Rune suggest a cap between the 680R and ground will affect a cure. I had DC offset and switch off 'thump' problems with the LM3875 and following previous advice on this BB cured it by inserting, at first 10uF, a 'lytic cap between the 680R and Ground, this gives a 23HZ -3db Bass rolloff. I have since moved to a 100uF 50V Low ESR cap and have no issues with offset or 'thumps', sound is unaffected as far as I can tell. Bass response is now 2.3HZ -3db. The datasheet does list this component. Hope this helps.
Ok thanks for the tips guys
I am in the process of doing a A/B comparison of each method, so far i have completed this method, it works well! i have about -10mV on the output at all times!
But compared to a inverting GC i built a little while ago it seems to be lacking a bit in the mids and highs..
I am about to build the cct with the cap from the 680R to ground!
as a side note would these caps be any good to use?
they are described as 'crossover caps'..
http://www1.jaycar.co.nz/productVie...&pageNumber=&priceMin=&priceMax=&SUBCATID=305
I am in the process of doing a A/B comparison of each method, so far i have completed this method, it works well! i have about -10mV on the output at all times!
But compared to a inverting GC i built a little while ago it seems to be lacking a bit in the mids and highs..
I am about to build the cct with the cap from the 680R to ground!
as a side note would these caps be any good to use?
they are described as 'crossover caps'..
http://www1.jaycar.co.nz/productVie...&pageNumber=&priceMin=&priceMax=&SUBCATID=305
Optical said:
It seems to me that you can safely go to 6.8k/220k and still have something like <10mV on the output. That will make the sch. very much like the IGC you build before. Did you try removing the input cap to see if that'll give you more HF and MF.
Maybe you just have mjore base with this configuration compare to what you build before. Did you measure the frequency response?
Aaaaaarghhh
A few weeks ago I followed te schematics of the 'ultimate gainclonce'. It turned out to be a hit and no no 'nasty side effects'.
I could not measure any dc anywhere and the thingies (insulated on an apex heatsink, that was it) played for weeks.
So when my brother in law came by yesterday he took them home after leaving behind some hard earned dollaaaars.
I made one last night, istarted whel after midnight till about 3 pm.
Same config, same hardware (I have stock for 5 stereo's) same toroid etc...
and every. let's say 45 seconds to one minute i get a non defined noise accompaning the music. Something i would describe as oscillating and sometomes a very present HUMM.
What did I do wrong??? I thought I did exactly the same thing. I use the simple schematic-ultimate gainclone with very few components (2 caps 1000uf, 2 resistors 680 & 22k and one pot 22k)
A few weeks ago I followed te schematics of the 'ultimate gainclonce'. It turned out to be a hit and no no 'nasty side effects'.
I could not measure any dc anywhere and the thingies (insulated on an apex heatsink, that was it) played for weeks.
So when my brother in law came by yesterday he took them home after leaving behind some hard earned dollaaaars.
I made one last night, istarted whel after midnight till about 3 pm.
Same config, same hardware (I have stock for 5 stereo's) same toroid etc...
and every. let's say 45 seconds to one minute i get a non defined noise accompaning the music. Something i would describe as oscillating and sometomes a very present HUMM.
What did I do wrong??? I thought I did exactly the same thing. I use the simple schematic-ultimate gainclone with very few components (2 caps 1000uf, 2 resistors 680 & 22k and one pot 22k)
GregGC said:
Yes it seems the input cap was dictating the bass responce.. i changed a few around and now it sounds good..
I havent measured the freq responce, how do you do that?
i have some ideas but im not sure on how reliable my soundcard is for a flat responce..
I have 2 working good sounding GC's now so im not sure if i should test my luck
but i think i'll try out the 6k8/220k config you suggest.. would i need to change the resistor from -ve to ground to a 6k8 too?- Status
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