Oh, right, I'm sorry for the confusion.
Still, 100-250V is quite high here as well, isn't it? Unless the rails are quite high and some fault exposes the cap to +-V ...
Still, 100-250V is quite high here as well, isn't it? Unless the rails are quite high and some fault exposes the cap to +-V ...
Why?
The input cap is used to block DC in the input of the amp, ie comming from the source. So if you start with no DC offset and say you can tolerate any internal offset of the IC then why do you need the cap on the feedback?
Hi,
because missing out one of the DC blocking caps is half way towards DC coupling.
But seriously, look at the LTP at the front end of a power amp.
The LTP performance is determined by how well the input signals are balanced. Yes, the power amp has two inputs, although if you read Jung he shows why it actually has 5 or more inputs.
The resistance seen by the +IN should match the resistance seen by the -IN. If it does not then that generates an input offset voltage.
The amplifier gain then multiplies that input offset voltage and turns it into an output offset voltage.
Worse, if the resistances don't match the input offset is less stable with temperature changes. That makes the output offset wander.
The power amp operates in a similar manner to an opamp and follows similar rules, one of which is: The Negative Feedback tries to ensure that the voltages at the +IN and -IN are the same. Any difference becomes the signal for an output voltage. Yes, another revelation, a power amp behaves as a differential amplifier. It amplifies the difference between the two inputs.
Finally, if you omit one of the DC blocking caps you will find it very difficult to arrive at impedances/resistances that you can drive with most sources and do so quietly if you want the input offset to be near zero.
Either make the amp AC coupled or make it DC coupled, don't mix AC and DC coupling.
because missing out one of the DC blocking caps is half way towards DC coupling.
But seriously, look at the LTP at the front end of a power amp.
The LTP performance is determined by how well the input signals are balanced. Yes, the power amp has two inputs, although if you read Jung he shows why it actually has 5 or more inputs.
The resistance seen by the +IN should match the resistance seen by the -IN. If it does not then that generates an input offset voltage.
The amplifier gain then multiplies that input offset voltage and turns it into an output offset voltage.
Worse, if the resistances don't match the input offset is less stable with temperature changes. That makes the output offset wander.
The power amp operates in a similar manner to an opamp and follows similar rules, one of which is: The Negative Feedback tries to ensure that the voltages at the +IN and -IN are the same. Any difference becomes the signal for an output voltage. Yes, another revelation, a power amp behaves as a differential amplifier. It amplifies the difference between the two inputs.
Finally, if you omit one of the DC blocking caps you will find it very difficult to arrive at impedances/resistances that you can drive with most sources and do so quietly if you want the input offset to be near zero.
Either make the amp AC coupled or make it DC coupled, don't mix AC and DC coupling.
Thanks Andrew, your replies have been quite helpful.
Just one thing though - in that case, is the configuration used in some Chip Amp kits, where there's no input cap, but there's a feedback cap, wrong, from your point of view? It ensures unity gain at DC, as in - the amplifier will not push any input DC further than it already is, but you won't get rid of it either.
I don't directly see an issue with that, if a certain maximum DC offset is tolerated, as it often is.
Just one thing though - in that case, is the configuration used in some Chip Amp kits, where there's no input cap, but there's a feedback cap, wrong, from your point of view? It ensures unity gain at DC, as in - the amplifier will not push any input DC further than it already is, but you won't get rid of it either.
I don't directly see an issue with that, if a certain maximum DC offset is tolerated, as it often is.
Andrew thanks for the excellent answer.
A couple of questions though if you don't mind.
If the pre amp has an output that goes similarly through a HP filter what's the difference? (and then straight to the chipamp with DC coupling)
In other cofigurations such as the BPA, the combination of servos & input DC block cap doesn't have the same issues?
Cheers,
A couple of questions though if you don't mind.
If the pre amp has an output that goes similarly through a HP filter what's the difference? (and then straight to the chipamp with DC coupling)
In other cofigurations such as the BPA, the combination of servos & input DC block cap doesn't have the same issues?
Cheers,
I do not believe any beginner should build any power amp other than AC coupled.
I don't care if there are sound quality gains to be made by using mixed coupling or DC coupling. The risk of damaged speakers is too great for any beginners.
Once the newbie understands how the circuit operates and why one would want to start omitting components, then it's time to consider using DC coupled, but it must be combined with the protections that should be inbuilt.
I don't care if there are sound quality gains to be made by using mixed coupling or DC coupling. The risk of damaged speakers is too great for any beginners.
Once the newbie understands how the circuit operates and why one would want to start omitting components, then it's time to consider using DC coupled, but it must be combined with the protections that should be inbuilt.
Andrew i understand your comments and why someone who is not familiar with the operation of the circuit should stick to the safest path.
However when i asked why you can't use mixed coupling that was kind of my point. It's not that you can't but there are other things to consider.
However when i asked why you can't use mixed coupling that was kind of my point. It's not that you can't but there are other things to consider.
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