Hello,
Before I build a circuit I like to understand how and why it's constructed as is it. Lately I've read "The audiophile's project sourcebook" by Randy Slone, and one of his preamp designs catch my attention. It's really nothing more than a non-inverting opamp (voltage gain about 13,5), see the attached schematic. But what bothers me is why I can's tell the reasons for all the components.
- Capacitor C3 in the feedback loop is there for limit the bandwidth of the amp, I assume? But to which frequency (value of C3 is 36pF)?
- The purpose of capacitor C2 should be to make DC gain zero, since it will act as an open circuit for DC. Therefore no need for input capacitor?
- C1 and R2 and can't tell why he put them there.
- R5 is there to isolate the opamp from eventually capacitive loads (?).
Appreciate some answer, unfortonetly the book doesn't explain this, for some, obvious things.
Before I build a circuit I like to understand how and why it's constructed as is it. Lately I've read "The audiophile's project sourcebook" by Randy Slone, and one of his preamp designs catch my attention. It's really nothing more than a non-inverting opamp (voltage gain about 13,5), see the attached schematic. But what bothers me is why I can's tell the reasons for all the components.
- Capacitor C3 in the feedback loop is there for limit the bandwidth of the amp, I assume? But to which frequency (value of C3 is 36pF)?
- The purpose of capacitor C2 should be to make DC gain zero, since it will act as an open circuit for DC. Therefore no need for input capacitor?
- C1 and R2 and can't tell why he put them there.
- R5 is there to isolate the opamp from eventually capacitive loads (?).
Appreciate some answer, unfortonetly the book doesn't explain this, for some, obvious things.
fh = 1 / ( 2 x Pi x 1.5k x 36pF ) or 2.95MHz.
C2 makes the DC gain unity ( 0dB ), not zero.
C1 is part of an input LP filter.
R2 ensures a DC path in the event of pot wiper failure.
R5 is for cable impedance, load short protection, reverse noise stopper.
C2 makes the DC gain unity ( 0dB ), not zero.
C1 is part of an input LP filter.
R2 ensures a DC path in the event of pot wiper failure.
R5 is for cable impedance, load short protection, reverse noise stopper.
Thanks for all replies.
Ok, I see. So a filter capacitor may still be needed to filter off any unwanted dc offset.
Aha, R1 and C1 forms a RC LP filter or (?). Same formula applies here I guess, fh = 1 / (2*pi*4,7k*36pF)=941kHz. Assuming the volume pot is turned at max, but with it at say only 10 %, the series resistance of 90k should be added to total R in formula. fh = 1 / (2*pi*94,7k*36pF)=46kHz. Or am I totally wrong?
Ok, I see. So a filter capacitor may still be needed to filter off any unwanted dc offset.
Aha, R1 and C1 forms a RC LP filter or (?). Same formula applies here I guess, fh = 1 / (2*pi*4,7k*36pF)=941kHz. Assuming the volume pot is turned at max, but with it at say only 10 %, the series resistance of 90k should be added to total R in formula. fh = 1 / (2*pi*94,7k*36pF)=46kHz. Or am I totally wrong?
Nice reading.
You're close. You're right for the pot at max volume. At any other position, instead of just using the series resistance of the pot, you have to use the parallel resistance, and include the 1Meg. So the resistor in the LPF formula is [(Rs[Vol]||Rg[Vol])+R1]||R2. The minimum cutoff for the LPF is 153kHz at 50%.
Yes, and even when certain pot settings give you a near 1MHz lowpass, "way above" Audio frequencies (even 150kHz is so), remember we are surrounded by strong RF fields, anything from Broadcasting or TV in Randy Sloan´s time (think 80s) to modern Cellphones, Bluetooth, Class D amps, SMPS, etc.
Randy was a heck of a Design Engineer, with Tons of practical experience.
Randy was a heck of a Design Engineer, with Tons of practical experience.
Yes, interesting point of view. In fact I've seen a lot of preamp design that somehow includes a lowpass filter.
I got caught out with that on a differential amplifier I designed.
I was getting oscillation on the output.
With higher value feedback resistors the input capacitance and feedback resistor cause a large phase shift and cause oscillation.
So where possible use lower value feedback resistors or as mentioned used a feedback capacitor.