SchickeRIAA
Shunt Feedback Phono Preamp for MC pickups with INA head amplifier and ANF voltage regulator - compact THT design
- the first stage uses a THAT 1510 instrumentation amplifier, used in many mic preamps as well as in severall phono stages with good reputation. This stage linear amplifies the MC output signal by 40dB. This is for an active feedback circuit higher than normal, so we benefit of lower noise and the signal leaves the first stage with reasonable signal level. Passive circuits have usally a gain of 50dB in the first stage, because of the 20dB loss in the passive network. So the Job for the INA is not so compelling inthis design.The INA is also used to invert phase to have no overall phase shift
- the second stage is the shunt feedback RIAA stage. Compared to series feedback our gain can go lower than 1, so no additional HF correction pole is needed. At 20kHz we have a gain of one, so the Job for the opamp is easier than in passive designs, where this stage has to deliver about 30dB at higher levels over the whole freuency band. Also i filter out the lowest frequencies (disc warp rumble) before doing the RIAA with its high gain at low frequencies. After some listening tests i stick with an OPA227 in this stage. Compared to modern parts it has a lower gain-bandwith-roduct (which we do not need here, see above), but has a very high open loop gain and low 1/f noise. This is what we primaly need in this stage
Very Important for me was good RIAA accuracy with not too many parts in the network. For the type C network the ratio betwen capacitors is appr. 2.9, the combinations 2*2.2/1.5, 2*10/6.8 or 2*6.8/4.7 come pretty close, with the advantage of two equal capacitors in parallel. The finaly choosen values give a very low deviation from the RIAA curve.
The Power supply consists of an external trafo in combination with onboard delon voltage double and regulation. After starting with the good old 317/337 pair with zener as reference i have now tested a little more complex, but in terms of noise and regulation better version based on an active negative feedback circuit. it is a modification of the well known two bjt CCS.
Have fun building this smart little phono preamp!
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