Hi there,
I'm looking for an easy option to preset the DC-offset in solid state amplifier, implemented into the DC-servo circuit. This to further lower the RC time constant for the servo circuit and to minimise the impact of the servo to the amp, but then it will take much longer to reach 0V dc offset on the amp's output.
Now, if we look for example to the datasheet of the AD711: https://www.analog.com/media/en/technical-documentation/data-sheets/AD711.pdf
One can trim the DC-offset using a trimmer between pins 1 and 5. I can't find anywhere information to what levels of output voltage the AD711 can be trimmed. I need to have about 3Vdc on the dc-servo opamp output pin to have the DC-offset on the main amp at 0Vdc.
Regards
I'm looking for an easy option to preset the DC-offset in solid state amplifier, implemented into the DC-servo circuit. This to further lower the RC time constant for the servo circuit and to minimise the impact of the servo to the amp, but then it will take much longer to reach 0V dc offset on the amp's output.
Now, if we look for example to the datasheet of the AD711: https://www.analog.com/media/en/technical-documentation/data-sheets/AD711.pdf
One can trim the DC-offset using a trimmer between pins 1 and 5. I can't find anywhere information to what levels of output voltage the AD711 can be trimmed. I need to have about 3Vdc on the dc-servo opamp output pin to have the DC-offset on the main amp at 0Vdc.
Regards
Offset trim on opamps is purely to trim tiny internal offsets, not to set up some wanted DC value on the opamp output.
Without knowing details of the amp its hard to advise, but typically you should arrange for the amp itself to have minimal offset by design and then wrap the servo around it ensuring the servo has only a minimal range of pull.
3 volts is a large value to need for a servo output, at least for a normal DC coupled amp with differential input stage. If this were a single ended input stage design then that figure of 3 volts (or even higher) could apply.
Without knowing details of the amp its hard to advise, but typically you should arrange for the amp itself to have minimal offset by design and then wrap the servo around it ensuring the servo has only a minimal range of pull.
3 volts is a large value to need for a servo output, at least for a normal DC coupled amp with differential input stage. If this were a single ended input stage design then that figure of 3 volts (or even higher) could apply.
Thanks Mooly, that's clear.
DC-servo opamp output voltage depends mostly on the dc-servo output resistor (R4 in circuit below) and shunt feedback resistor.
As I'm using an inverting servo with 2 opamp's, I could be using the 2nd opamp to compensate dc-offset at startup.
Amp and schematic can be found here (post 68)
https://www.diyaudio.com/forums/sol...-low-nfb-fet-front-bjt-ops-7.html#post5787186
Regards
DC-servo opamp output voltage depends mostly on the dc-servo output resistor (R4 in circuit below) and shunt feedback resistor.
As I'm using an inverting servo with 2 opamp's, I could be using the 2nd opamp to compensate dc-offset at startup.
Amp and schematic can be found here (post 68)
https://www.diyaudio.com/forums/sol...-low-nfb-fet-front-bjt-ops-7.html#post5787186
Regards
Attachments
The higher you make R4, the higher the opamp correction voltage needs to be to achieve the same level of correction on the main amp.
I would say the best approach is to minimise the DC offset of the main amp (without using a servo) by looking to trim and balance the front end as needed. Only then wrap the servo around the amp (and without those Trim a and b resistors). That should give you a servo that nominally hovers around zero volts.
If the amp is prone to thermal drift then you are always going to see significant output from the servo. I wouldn't worry overmuch over all this, just use a speaker delay (relay or solid state relay) and let the servo do its thing.
I would say the best approach is to minimise the DC offset of the main amp (without using a servo) by looking to trim and balance the front end as needed. Only then wrap the servo around the amp (and without those Trim a and b resistors). That should give you a servo that nominally hovers around zero volts.
If the amp is prone to thermal drift then you are always going to see significant output from the servo. I wouldn't worry overmuch over all this, just use a speaker delay (relay or solid state relay) and let the servo do its thing.
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