The phono preamplifier amplifies the audio signal of a MC2.1 phono pickup with a sensitivity of 150V to line level and also equalizes the frequency response according to the RIAA deemphasis curve (see [Hü97]). Additional equalization according to the IEC deemphasis curve is not done as my turntable does not produce significant drive noise and the electronics following in the signal chain is able to handle all signals produced by the MC pickup. Usually there is at least one highpass somewhere within the signal chain between pickup and speaker which has enough suppression of sub bass frequencies.
The preamplifier consists of several coupled stages (see the schematics ):
I choose two separate filters instead of one composite filter for the RIAA equalization because it is very difficult to construct a three pole filter where the poles are very close together and influences each other. If you look into the excellent paper in [Lip79] you'll understand what I mean.
Nowadays you can get extremely ultra low noise operational amplifiers which compare with transistor pairs regarding price but are more easily available. Furthermore a discrete amplifier using a transistor pair needs very high precision resistors and a very good stabilized supply voltage. Operational amplifiers on the other hand have all these parts built in. Examples of ultra low noise operational amplifiers are LT1028 by Linear Technology and AD797 by Analog Devices.
A none inverting amplifier is built with the feedback resistors and to get an amplification2.2 of about 40 dB. Usually I do not use feedback resistors with such low resistance to prevent overloading the output of the operational amplifier (it gets warm and does not sound very good). This case is an exception because the input noise current of the operational amplifier flows through which means that the output noise increases with the resistance of . Therefore its resistance should be so low that its residual noise is small compared to the input voltage noise of the amplifier.
The input impedance is set by putting in parallel to the input. Its value should be selected according to the specifications of the MC pickup. Usually this is 100-150, for low output MC pickups, high output pickups might need up to 1000 (check the owner's manual). I chose .
This means that the none inverting input sees a source impedance of and the inverting input of . As the input offset current (which is relatively high) flows through both input impedances the imbalance leads to an additional input offset voltage which is then amplified by 40 dB. Usually this can be prevented by adding an additional resistor at the inverting input to recreate the balance. I did not do this because the input noise current also flows through this additional resistor, thereby adding unwanted voltage noise. The resulting offset voltage of the first stage is later eliminated by the offset compensation circuit.
At some later time I replaced in one channel by the serial connection of a 750 resistor and a 500 adjustable resistor. This allows to balance pickups with unequal output voltage.
I chose a passive filter to realize the 75 s lowpass for the following reasons:
The capacitor should be of the highest quality both in precision and sound. The only choice is a styroflex capacitor, which is available with 1% tolerance. The biggest value is just about 15nF. I chose which leads to a resitance of according to equation 2.1, and this part is available with 1% tolerance. I got hold of a large bunch of 10nF capacitors and used a capacitance meter to select two capacitors of equal value (to improve channel balance) as close to the specification as possible.
The second amplifer stage also serves as filter for the poles 318s and 3180s. It is possible to use a passive filter-however that makes no sense as then the signal must be amplified much more which leads to increased distortion, and this is not our target.
Therefore we need an active filter where the amplification is constant between DC and 50,05 Hz, decreases by 6 dB/octave up to 500,5 Hz and stays constant again for all higher frequencies. This is easily achieved by adding the serial connection of resistor and capacitor parallel to resistor in the feedback loop. At very low frequencies the impedance of is very high so that the amplification depends on alone. At very high frequencies the impedance of is close to a short cut so that the amplification depends on .
To be able to calculate the filter exactly we must find the transfer equation of the circuit. Having done this we can compare the coefficients of the amplification equation with those of the filter equation. Let's do this assuming an ideal operational amplifier (having infinite difference amplification and input impedance and zero output impedance)2.3.
The transfer equation of the active filter is:
Subjective listening tests comparing the new preamplifier with my old one2.5 by connecting the output of to the line level amplifier showed that the sound of the new phono preamplifier was more clean, had much more resolution and was much better at the frequency extremes, but was also more cool, sterile and metallic and not that musical. After countless tests I found a solution which added the missing characteristics: a class A output stage within the feedback loop using a power Mosfet (credits go to Nelson Pass).
is a source follower using the source resistance .
The quiescent current running through is calculated according
to the equation
The signal is coupled to both the line level amplifier and the tape output via a serial resistor of each, because I use coax cables (RG58 / RG214U) both internal and external. This serial resistor prevents reflections2.7.
Both stages of the phono preamplifier together have a DC amplification of approximately . Even when using the best precision operational amplifiers money can buy today it is not possible to get rid of the DC offset without reducing the DC amplification-which makes sense anyway in a phono preamplifier. There are several possiblities how to realize this:
Approaching a corner frequency below 20 Hz requires a capacitance of 4,7 F. Subjective listening tests using a capacitance of 6,8 F (leading to a corner frequency of 13 Hz) gave the impression of reduced bass dynamic (punch) compared to a version with full DC amplification. For a corner frequency of 1 Hz a capacitance of 82 F is required, and a good one is very big in size and very expensive too. You cannot use a cheap electrolytic capacitor as there is no DC voltage applied which would lead to significant distortion.
A top view of the printed circuit board is in figure 2.2 on page .