In figure 3.2 on the left, the feedback resistor R1 is put in parallel with a capacitor C1 resulting in a low-pass filter. In this example all frequencies on the output of the opamp are equally fed back to the negative input and therefore the resulting amplification factor is equal for all frequencies resulting in a flat curve.īy changing R_2 from a fixed resistor to whatever type of impedance that is dependent on frequency, we construct a filter.Ī simple way of building an active filter is to replace the single feedback resistor R1 with the right mix of filter components such as resistors and capacitors. (* see OpAmp background article for relevant formulas) as it is a non-inverting design. In this case`the amplification of this design is given by V = 1+R2/R1. On the right is a figure with a basic amplifier circuit for opamps. These unwanted signals could be certain frequencies. Apart from filtering unwanted frequencies, feedback loops have a positive impact on noise and distortion as well. The idea behind the feedback is as follows, design an amplifier with more gain (and bandwith) than necessary and use the feedback loop to the amplifier to selectively reduce amplification. Anyway, I'll describe the principle using opamps. In fact, the Hounddog amplifier was based on the same principle of feedback. Of-course the active filtering (or negative feedback) principle is usable for tube projects as well. Both Phonoclone and GainPre have hybrid filtering: Part of the RIAA filter is included in the feedback loop and the highest pole is passive in between the first and the second Opamp stage. On this page I want to give some more background based on my experience with building the PhonoClone and GainPre phono amplifiers that were built with opamps. But in this case I was inspired by the gainclone principles, building amplifiers with monolithic power Opamps and a matching phono amp using opamps as well. Of-course this was nothing new, since almost every tube based amplifier is built around that concept. Some time ago I decided to build some amplifiers according to the principles of "less is more". On this page I want to give some background on active filter design for RIAA amps.