Hi i have build a gainclone as must of you know. I have measured the dc offset i get around 89mv. The setup is stereo gainclone with caps at rectifer bridge board which are 2 per rail one is elna 50v 3.3 and one is 50v 10uf panasonic. The ch is connected to the ground for both channels . pg- and pg + are connected to the sg on both amp boards to ground them. I do not use volume pot i connect the input wire straight to the output of my pc sound card.
I read on decibel degeon (http://myweb.tiscali.co.uk/nuukspot/decdun/gainclone.html) that a carbon 56 k resister must be connected to the input wire to the ground chassis to lower dc offset.
After one hour my chip get quite hot i have on full volume most of the time. if it get too hot it starts crackle. anyone know why this happens??? stability problems??.
I read on decibel degeon (http://myweb.tiscali.co.uk/nuukspot/decdun/gainclone.html) that a carbon 56 k resister must be connected to the input wire to the ground chassis to lower dc offset.
After one hour my chip get quite hot i have on full volume most of the time. if it get too hot it starts crackle. anyone know why this happens??? stability problems??.
what circuit you use?
http://myweb.tiscali.co.uk/nuukspot/decdun/gc/nixgccd.gif
If is like this one, inverted, in this image
you can experiment with
put one resistor from pin7
to Signal Ground Star
** put this resistor close to pin7 and other end to ground
Depending on the value of such a resistor
you may get another DC-offset at output.
Try with:
10kohm, 22kohm, 47kohm, 100kohm
If the offset not get any smaller with such a resistor,
then connect pin7 to signal ground again.
http://myweb.tiscali.co.uk/nuukspot/decdun/gc/nixgccd.gif
If is like this one, inverted, in this image
you can experiment with
put one resistor from pin7
to Signal Ground Star
** put this resistor close to pin7 and other end to ground
Depending on the value of such a resistor
you may get another DC-offset at output.
Try with:
10kohm, 22kohm, 47kohm, 100kohm
If the offset not get any smaller with such a resistor,
then connect pin7 to signal ground again.
joka22 said:
That's a non-inverted. Details are here: http://www.diyaudio.com/forums/showthread.php?s=&threadid=123003
1. 89 mV ( 0.089 Volt DC ) is not very big offset.
It is one you can live with. But we do not wish to have close to 0.1 Volt
2. See attachment. Typical non-inverted LM3875 circuit.
it is taken from The Datasheet
http://www.national.com/ds.cgi/LM/LM3875.pdf
3. The cap *Ci = 22 uF is to block DC feedback to GND.
When using Ci, we can often get close to 0.0 Volt offset.
4. When NOT using Ci, then we can get such DC-offset at Output.
To remove this offset is not so simple as only use 1 resistor.
There are several ways to fix the offset when not using capacitor Ci.
But simplest way is to ad a very good quality capacitor.
There are good electrolytic:
22, 47, 100 or even 220 uF may work.
regards
It is one you can live with. But we do not wish to have close to 0.1 Volt
2. See attachment. Typical non-inverted LM3875 circuit.
it is taken from The Datasheet
http://www.national.com/ds.cgi/LM/LM3875.pdf
3. The cap *Ci = 22 uF is to block DC feedback to GND.
When using Ci, we can often get close to 0.0 Volt offset.
4. When NOT using Ci, then we can get such DC-offset at Output.
To remove this offset is not so simple as only use 1 resistor.
There are several ways to fix the offset when not using capacitor Ci.
But simplest way is to ad a very good quality capacitor.
There are good electrolytic:
22, 47, 100 or even 220 uF may work.
regards
Attachments
By R2 I mean input shunt resistor: http://www.diyaudio.com/forums/attachment.php?s=&postid=1508778&stamp=1210693738
Kudos Peter! That works like a charm. 
A stability problem is somewhat unlikely on such a nice circuit board. More information is needed:
How's your heatsink size in C/W?
Did you use thermal compound?
Can cool air get into the bottom of the enclosure and then somehow also let the now-hot air out of the amplifier enclosure?
What's the operating voltage?
Did you use flux for nice smooth soldering or are there issues?
Are your speakers 4 ohms or 8 ohms (what's the load)?
How long are your speaker cables?
Did you use the Audiosector recommended capacitor models at the amplifier board, or did you use something else?
And, can you post a photo of the amplifier view as from inside the enclosure?
joka22 said:
A stability problem is somewhat unlikely on such a nice circuit board. More information is needed:
How's your heatsink size in C/W?
Did you use thermal compound?
Can cool air get into the bottom of the enclosure and then somehow also let the now-hot air out of the amplifier enclosure?
What's the operating voltage?
Did you use flux for nice smooth soldering or are there issues?
Are your speakers 4 ohms or 8 ohms (what's the load)?
How long are your speaker cables?
Did you use the Audiosector recommended capacitor models at the amplifier board, or did you use something else?
And, can you post a photo of the amplifier view as from inside the enclosure?
I had a LM4780 amp with Ci (47uF) and my offset was ~0,1mV on each channel but sound was boring!
Then I decided to bypass Ci caps, the sound has improved incredible! Especially the bass stage has upgraded to another league (this is an English idiom). May be it was a bit psychological. Anyway, after that operation the DC offset was increased to 90-100mV
However, in my gainclone the input shunt resistor (Rs) was 47K while the Rf was 20K. So.... As a friends recommendation, I've replaced Rs with a 20K (= Rf). Then my DC offset was reduced to 40-50mV on each channel! I think these values are more acceptable and I've never heard any kind of click or plop sound in on-off operations.
IMO: Dont use Ci capacitors, reduce the input shunt resistor values.. 50mV is not too bad for DC offset! Trust me it worths.
Then I decided to bypass Ci caps, the sound has improved incredible! Especially the bass stage has upgraded to another league (this is an English idiom). May be it was a bit psychological. Anyway, after that operation the DC offset was increased to 90-100mV
However, in my gainclone the input shunt resistor (Rs) was 47K while the Rf was 20K. So.... As a friends recommendation, I've replaced Rs with a 20K (= Rf). Then my DC offset was reduced to 40-50mV on each channel! I think these values are more acceptable and I've never heard any kind of click or plop sound in on-off operations.
IMO: Dont use Ci capacitors, reduce the input shunt resistor values.. 50mV is not too bad for DC offset! Trust me it worths.
I would not avoid to use Ci capacitor.
National techniqal lab workers recommend this for low offset.
And through the history of audio
we have had so many good and even superb power amplifiers using DC gain = 1 + blocking caps.
AC blocking caps arefound most everywhere in good proper audio.
Input, output, feedback.
If you design your amplifier per sibjective listening test,
you will get a subjective result.
Which is not necessarily the good/best amplifier.
whatever crazy things.
Now, if those less good things work for you, keep it that way.
But do not say to others that audio perfromance REALLY get's better,
by such not really verified methods as subjective istening.
It can be true .. but most of the time it is only a product of imagination of the human mind and brain.
And remember that good sound is nothing universal.
We all have different ears, different music and different taste.
So, the listening criteria is nothing we can go by.
We just have to go by numbers and some measurings.
Unless there is something really bad output through speakers.
In this case everybody would agree, subjectively, that there is not good sound.
Good audio regards to all Lineup
Lineup Audio website - for correct info - no golden ears tests whatever
National techniqal lab workers recommend this for low offset.
And through the history of audio
we have had so many good and even superb power amplifiers using DC gain = 1 + blocking caps.
AC blocking caps arefound most everywhere in good proper audio.
Input, output, feedback.
If you design your amplifier per sibjective listening test,
you will get a subjective result.
Which is not necessarily the good/best amplifier.
Nobody can stop diy members here, to recommend to others
whatever crazy things.
Now, if those less good things work for you, keep it that way.
But do not say to others that audio perfromance REALLY get's better,
by such not really verified methods as subjective istening.
It can be true .. but most of the time it is only a product of imagination of the human mind and brain.
And remember that good sound is nothing universal.
We all have different ears, different music and different taste.
So, the listening criteria is nothing we can go by.
We just have to go by numbers and some measurings.
And what is wrong with numbers?
If you ride your car.
And spedometer say you ride too fast (above speedlimit).
Then you can not subjectively say:
I do not feel like I am going too fast.
So I do not care what speed meter tells
You would probably discover this wasn't so good & the meter was right,
when you try get up from the side of the road
after crashing into the terrain or after crashing into one other car.
Unless there is something really bad output through speakers.
In this case everybody would agree, subjectively, that there is not good sound.
Good audio regards to all
LineupLineup Audio website - for correct info - no golden ears tests whatever
it seems weird since i chnaged the resister from 22k at the r2 on peter daniel kit to 10k now when i switch the amp off i hear pop sound after like ten seconds when shutting off the amp.
test 1: i switch the amp on with the input connected to the pc sound card and switched of the amp no pop sound at all
test2: tried without pc sound car input heard pop sound at the speakers after ten seconds.
must be something related to the input groudning or chassis grouding.
test 1: i switch the amp on with the input connected to the pc sound card and switched of the amp no pop sound at all
test2: tried without pc sound car input heard pop sound at the speakers after ten seconds.
must be something related to the input groudning or chassis grouding.
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