sek@
New Head-Fier
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- May 4, 2006
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A very interesting discussion about the servo circuit.
I read that as follows:
Under normal conditions the servo acts to minimize the output DC voltage occuring from both an offset inherent to the opamp/buffer circuit and the input signal. Depending on the choice of feedback and servo/gain resistors, the influence ("authority") can be adjusted, but is of course defined as a proportion of the signal, limited by the supply voltages. So far, so good.
Thus, if either a full scale DC voltage is applied to the input or any active device fails, introducing either rail voltage into the circuit's signal chain, the servo is incapable to effectively double as a safety feature in order to protect the headphones.
In order to account for this, among other things, wakibaki incorporates a relais based protection circuit into his design.
The question for me is:
Is the servo worth the trouble under normal conditions, i.e. is the nulling effect that significant to the sound out of the headphone (compared to, say, a fully DC coupled signal chain)?
Seeing that protection is covered by the relais circuit, isn't the servo expendable in all but the most delicate cases?
As of my understanding, the circuit is rather stable, using high quality/low drift components. All causes of output offset voltage that stem from the circuit itself should essentially be constant, making it easy to deal with them without a servo (i.e. selection, trimming) or to just live with it and leave well enough alone.
And in order to put things into perspective, I can report a maximum of approx. 50 mV DC out of THE WIRE, which is basically the circuit at hand minus the servo. That's about 80 uW into a 32 Ohm headphone or 5 uW into 600 Ohm.
Thanks,
Sebastian.
I read that as follows:
Under normal conditions the servo acts to minimize the output DC voltage occuring from both an offset inherent to the opamp/buffer circuit and the input signal. Depending on the choice of feedback and servo/gain resistors, the influence ("authority") can be adjusted, but is of course defined as a proportion of the signal, limited by the supply voltages. So far, so good.
Thus, if either a full scale DC voltage is applied to the input or any active device fails, introducing either rail voltage into the circuit's signal chain, the servo is incapable to effectively double as a safety feature in order to protect the headphones.
In order to account for this, among other things, wakibaki incorporates a relais based protection circuit into his design.
The question for me is:
Is the servo worth the trouble under normal conditions, i.e. is the nulling effect that significant to the sound out of the headphone (compared to, say, a fully DC coupled signal chain)?
Seeing that protection is covered by the relais circuit, isn't the servo expendable in all but the most delicate cases?
As of my understanding, the circuit is rather stable, using high quality/low drift components. All causes of output offset voltage that stem from the circuit itself should essentially be constant, making it easy to deal with them without a servo (i.e. selection, trimming) or to just live with it and leave well enough alone.
And in order to put things into perspective, I can report a maximum of approx. 50 mV DC out of THE WIRE, which is basically the circuit at hand minus the servo. That's about 80 uW into a 32 Ohm headphone or 5 uW into 600 Ohm.
Thanks,
Sebastian.