Just finished my GC, I tried to read DC offset from the output (speaker) channels.
started with the left channel...
1. no source connected.
2. hooked up one speaker on the left channel
3. powered on the amp
4. probed the output, red+ ; black- with multimeter
5. reading is -0.69V
now the right channel...
1. reading is -0.98V
What is this? I being a noob, am I doing things wrong?
what I read from this forum, ideally I should not get more than +0.5V. or should be near 0V.
what does these reading tell me?
I checked the polarities, OUT is + and OG is -.
please advise a newbie.
thanks
started with the left channel...
1. no source connected.
2. hooked up one speaker on the left channel
3. powered on the amp
4. probed the output, red+ ; black- with multimeter
5. reading is -0.69V
now the right channel...
1. reading is -0.98V
What is this? I being a noob, am I doing things wrong?
what I read from this forum, ideally I should not get more than +0.5V. or should be near 0V.
what does these reading tell me?
I checked the polarities, OUT is + and OG is -.
please advise a newbie.
thanks
I actually got a rev3 LM3875 kit from Mad About Sound. and finished assembbling it.janneman said:
22K NFB came with the kit. I did not connect any source yet.
I just tried to check the DC offset after finishing the kit. I don't know if I am doing the right thing in measuring the DC offset.
thanks for the reply.
Well, I don't have schematics, I just got the kit for a non-inverting GC.
Being a noob and just trying to get information through the forum made me wonder why this is so...
I just hope later on, after a lot of reading will tell me what these things are all about. I believe I will soon find out.
Being a noob and just trying to get information through the forum made me wonder why this is so...
I just hope later on, after a lot of reading will tell me what these things are all about. I believe I will soon find out.
The offset voltage is caused by imperfections in the chip: internal bias currents, offset voltages etc.
Your reading of -0.13V is more like what I would expect and is acceptable, IMHO. What is the reading for the other channel?
For reference, I built an eight channel non-inverting LM3886 amplifier of my own design and the DC offsets measure between -34mV and -79mV.
Your reading of -0.13V is more like what I would expect and is acceptable, IMHO. What is the reading for the other channel?
For reference, I built an eight channel non-inverting LM3886 amplifier of my own design and the DC offsets measure between -34mV and -79mV.
keps said:
I wouldn't be happy with anything outside of +/- 0.1 Volts.
Your chip amp output stage will be running unbalanced currents through the output transistors by about 0.7 / 6 = 116mA, which changes their distortion and cross-over characteristic. (Assuming a DC resistance of about 6 Ohms for the speaker system)
Running DC through the speakers is not a good idea. It heats up the woofer voice coil a little and moves the cone away from its normal resting position.
Most SS amps have very low output offset voltages, +/- 0.05 Volts or less is the accepted norm. A servo can reduce it to below +/- 0.001 Volts.
It sounds like a lot of people are comfortable with offsets less than 0.1 VDC. The feeling is that this is normal for this chip. Is that really true? 100 mV makes me squeamish.
I'd check for leakage through the shunt leg capacitance in the feedback circuit. If the loop gain is set very high, things like input bias errors and drift become more problematic. A schematic would really help.
I'd check for leakage through the shunt leg capacitance in the feedback circuit. If the loop gain is set very high, things like input bias errors and drift become more problematic. A schematic would really help.
Since this seems to be solved I thought that I would add a question.
Can a DC-offset above 100mV cause a turn-off thump?
I happen to have that om my amps left channel and the DC-offset is approx 111mV under normal use while the other channel has about 22mV and no thump.
The amp is p2p and using the same componentvalues as The Chipamp.com LM3875-kit
kind regards!
Oh, the thump is only present when preamp is connected and running, no thum with the amp connected direcctly to a laptop or similar.
Can a DC-offset above 100mV cause a turn-off thump?
I happen to have that om my amps left channel and the DC-offset is approx 111mV under normal use while the other channel has about 22mV and no thump.
The amp is p2p and using the same componentvalues as The Chipamp.com LM3875-kit
kind regards!
Oh, the thump is only present when preamp is connected and running, no thum with the amp connected direcctly to a laptop or similar.
the Mad About Sound kit is AudioSector non-inverted design -- there's a schematic on the PDF -- this is a direct coupled design so (as guru Janneman points out) the DC offset may be owing to a DC voltage from the input source, or the difference in bias currents flowing from the input pins. The offset voltage is equal to the bias current times the value of the resistors to ground, then multiplied by the gain of the amplifer.
The audiosector design is pretty high gain -- 33x if I recall -- the LM3875 is specified bias current of 0.2 to 1 uA -- the value of the input resistor is pretty high ---
Try this -- couple your source to the kitamp with a 4.7uF capacitor -- doesn't matter what type since this is just a test -- if the d.c. offset remains your source is OK.
If the offset is still there you will have to play around with the values of the resistors -- raise the value of the resistor from the inverting input to ground (which will also reduce the gain) as a first suggestion.
The audiosector design is pretty high gain -- 33x if I recall -- the LM3875 is specified bias current of 0.2 to 1 uA -- the value of the input resistor is pretty high ---
Try this -- couple your source to the kitamp with a 4.7uF capacitor -- doesn't matter what type since this is just a test -- if the d.c. offset remains your source is OK.
If the offset is still there you will have to play around with the values of the resistors -- raise the value of the resistor from the inverting input to ground (which will also reduce the gain) as a first suggestion.
Built in undervoltage protection should realy take care of that thump....
You first have to eliminate the sources you have connected as culprits.. 100mv is quite alot and may be pointing to something funny or less than 100% standard chip, remember the chips are not all the same... so unless you did some chip matching on top of matching caps and resistors expecting the same output is unreasonable...
You first have to eliminate the sources you have connected as culprits.. 100mv is quite alot and may be pointing to something funny or less than 100% standard chip, remember the chips are not all the same... so unless you did some chip matching on top of matching caps and resistors expecting the same output is unreasonable...
The source can be switched of and there was still a thump.
I tried different sources and there was still thumping, as long as the Preamp was swithed on at least. In any case it's all gone now.
Here's the sourcethread that solved the problem:
LM3875: Switch off thump
I tried different sources and there was still thumping, as long as the Preamp was swithed on at least. In any case it's all gone now.
Here's the sourcethread that solved the problem:
LM3875: Switch off thump
Hi,
so much conflicting advice!
I pity the poor chap trying to make sense of this.
Here's my attempt to clear or muddy the water.
Measure the power amplifier output offset with the load disconnected and with the input socket shorted. This is the reference to which all other measurements refer.
I would try to get the amplifier offset (that reference above) to below 50mV for all operating conditions and temperatures (except the first few seconds after switch on).
I would just about accept upto 100mV if all else fails but would seriously consider changing the schematic to get below that 50mV recommendation. There are simple ways to achieve this, but we'll consider that later.
Now connect your source and set volume to minimum then measure offset. Set volume to mid and to max and repeat the measurements.
Are they different from the reference? If yes, then sort the source.
If they are the same then effective DC cancellation or DC blocking has been employed at source.
Now connect the speakers. Switch on and listen for a switch on transient. Try this for various combinations of unit switch on order. Is the switch on transient acceptable? If not then consider either an input mute on the power amp or an output mute on the source or an output mute on the power amp.
Now back to adjustment of the output offset of the standard power amplifier.
The amp can be fully DC coupled or fully AC coupled. It is asking for trouble to have partial DC and AC coupling.
The input can have a DC blocking cap, this is AC coupling or just have an RF filter on the input (DC coupling). The negative feedback loop can have a DC blocking cap on the lower leg to signal ground (this is AC coupling) or just a resistor to ground (DC coupling).
The NFB and input filter should be the same. Either DC or AC not a mix of both.
However, there is a complication.
Is the source AC or DC coupled?
Some sources have DC coupling with a DC servo to reduce the output offset.
Most others have a DC blocking cap on the output, these are AC coupled.
A DC coupled amplifier (or opamp) sees the source DC blocking cap as an AC coupling and it now becomes a mixed DC/AC coupled amplifier. Remember that warning above?
The solution is easy: ensure the source AND amplifier are compatible, if the source is DC coupled then the amp can be either DC or AC coupled.
If the SOURCE is AC COUPLED then the amplifier MUST NOT BE DC COUPLED!!!!!!!! this can be corrected by fitting a DC blocking cap into the lower leg of the NFB loop. The use of a DC blocking cap on the amplifier input is optional since a DC blocking cap is already fitted at source.
If the reference offset is too high, is it caused by mixed AC/DC coupling? Then sort the power amp, EITHER fully DC coupled or fully AC coupled. This decision must be made in conjunction with the source coupling type.
If your amplifier topology is OK, then the offset can (usually) be adjusted by altering the DC resistance seen by the ampifier inverting and non-inverting input terminals. There is an exception with input bias compensated opamps and some chip amps may adopt similar topology.
The adjustment requires the changing of ONE resistor on either the inverting pin or the non-inverting pin. I'll explain the method when you come back with some test results.
so much conflicting advice!
I pity the poor chap trying to make sense of this.
Here's my attempt to clear or muddy the water.
Measure the power amplifier output offset with the load disconnected and with the input socket shorted. This is the reference to which all other measurements refer.
I would try to get the amplifier offset (that reference above) to below 50mV for all operating conditions and temperatures (except the first few seconds after switch on).
I would just about accept upto 100mV if all else fails but would seriously consider changing the schematic to get below that 50mV recommendation. There are simple ways to achieve this, but we'll consider that later.
Now connect your source and set volume to minimum then measure offset. Set volume to mid and to max and repeat the measurements.
Are they different from the reference? If yes, then sort the source.
If they are the same then effective DC cancellation or DC blocking has been employed at source.
Now connect the speakers. Switch on and listen for a switch on transient. Try this for various combinations of unit switch on order. Is the switch on transient acceptable? If not then consider either an input mute on the power amp or an output mute on the source or an output mute on the power amp.
Now back to adjustment of the output offset of the standard power amplifier.
The amp can be fully DC coupled or fully AC coupled. It is asking for trouble to have partial DC and AC coupling.
The input can have a DC blocking cap, this is AC coupling or just have an RF filter on the input (DC coupling). The negative feedback loop can have a DC blocking cap on the lower leg to signal ground (this is AC coupling) or just a resistor to ground (DC coupling).
The NFB and input filter should be the same. Either DC or AC not a mix of both.
However, there is a complication.
Is the source AC or DC coupled?
Some sources have DC coupling with a DC servo to reduce the output offset.
Most others have a DC blocking cap on the output, these are AC coupled.
A DC coupled amplifier (or opamp) sees the source DC blocking cap as an AC coupling and it now becomes a mixed DC/AC coupled amplifier. Remember that warning above?
The solution is easy: ensure the source AND amplifier are compatible, if the source is DC coupled then the amp can be either DC or AC coupled.
If the SOURCE is AC COUPLED then the amplifier MUST NOT BE DC COUPLED!!!!!!!! this can be corrected by fitting a DC blocking cap into the lower leg of the NFB loop. The use of a DC blocking cap on the amplifier input is optional since a DC blocking cap is already fitted at source.
If the reference offset is too high, is it caused by mixed AC/DC coupling? Then sort the power amp, EITHER fully DC coupled or fully AC coupled. This decision must be made in conjunction with the source coupling type.
If your amplifier topology is OK, then the offset can (usually) be adjusted by altering the DC resistance seen by the ampifier inverting and non-inverting input terminals. There is an exception with input bias compensated opamps and some chip amps may adopt similar topology.
The adjustment requires the changing of ONE resistor on either the inverting pin or the non-inverting pin. I'll explain the method when you come back with some test results.
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