chrissugar
Well-known member
I'm thinking about something for some days and I'm curious about your opinion and experience.
In all the classic noninverting opamp topology mic preamps like the A*P*I and co. there is always a high value capacitor in the negative feedback network. This capacitor will make sure that there is no DC component amplified by the opamp, but at the same time it is necessary a large value to have the lowest possible cutoff freq and low phase distortion.
The negative aspect is that beeing a large value it has to be some electrolyt condenser (usualy bypassed with a small value polyprop).
Look at the schem at Fred's site:
http://www.forsselltech.com/schematics/JE16%20Mike%20Preamp.PDF
this is a tipical A*P*I topology mic pre. There is the 4700micro capacitor I'm talking about.
But if you look at John Hardy's M1 at page 7:
http://www.johnhardyco.com/pdf/M1_M2_M1p_20031025.pdf
you will see there is no capacitor in the feedback network. It seems logical because the transformer will not let DC into the opamp, there is the BIAS circuit that will minimise any DC offset and there is also the DC servo that will kill any kind of DC to the lowest value.
Question. I read tons of posts at different places about the pros and cons for DC servo. Anyone tried to do some serious listening tests with a similar preamp with the servo circuit in and out, capacitor in and out?
What is the best compromise? To servo and not include the capacitors (feedback and output) or not. How do you feel it affects the sound?
Anyone has the possibility to do a listening test of the M1 with the servo and without ( JP3 jumper permits disabling the servo)
My intention is to start to do some tests in the near future but before I do anything I'm curious about your opinions.
chrissugar
In all the classic noninverting opamp topology mic preamps like the A*P*I and co. there is always a high value capacitor in the negative feedback network. This capacitor will make sure that there is no DC component amplified by the opamp, but at the same time it is necessary a large value to have the lowest possible cutoff freq and low phase distortion.
The negative aspect is that beeing a large value it has to be some electrolyt condenser (usualy bypassed with a small value polyprop).
Look at the schem at Fred's site:
http://www.forsselltech.com/schematics/JE16%20Mike%20Preamp.PDF
this is a tipical A*P*I topology mic pre. There is the 4700micro capacitor I'm talking about.
But if you look at John Hardy's M1 at page 7:
http://www.johnhardyco.com/pdf/M1_M2_M1p_20031025.pdf
you will see there is no capacitor in the feedback network. It seems logical because the transformer will not let DC into the opamp, there is the BIAS circuit that will minimise any DC offset and there is also the DC servo that will kill any kind of DC to the lowest value.
Question. I read tons of posts at different places about the pros and cons for DC servo. Anyone tried to do some serious listening tests with a similar preamp with the servo circuit in and out, capacitor in and out?
What is the best compromise? To servo and not include the capacitors (feedback and output) or not. How do you feel it affects the sound?
Anyone has the possibility to do a listening test of the M1 with the servo and without ( JP3 jumper permits disabling the servo)
My intention is to start to do some tests in the near future but before I do anything I'm curious about your opinions.
chrissugar