Enhanced Output Stage for the Phono Preamplifier
Please see below note concerning the OPA603
Although the Burr-Brown OPA637 and Lundahl LL1570XL transformers are an excellent combination to drive all normal loads I designed an output stage with enhanced dynamic performance even at heavy loads. This output stage should comprise the following specifications:
- high gain
- high output current
- wide bandwidth
- low distortion
High gain is advantageous to avoid redundant amplifying stages, high currents enable the output stage to drive heavy loads. But if both gain and currents are high there will be thermal feedback between output transistors and the front-end differential pair within the op amp causing distortion. This is an inherent limitation of single chip op amps.
Furthermore increasing closed-loop gain decreases the error reducing loop gain. Additionally, starting at relatively low frequencies, the loop gain rolls-off at 20dB/decade of signal frequency increase. In combination these effects can produce significant errors, especially at higher frequencies where the loop gain is low.
Current-feedback op amps, such as the OPA603, have good dynamic performance at both low and high gains. Unfortunately, DC performance of these op amps is poor. A composite amplifier using an OPA627 (see data sheet) and an OPA603 (see data sheet) can combine the qualities of both. According to the Burr-Brown specifications the composite amplifier can drive 150Ohm loads to ±10V with no thermal feedback to the OPA627.
Fig.1: schematic of a balanced composite audio output stage. The gain of the OPA603 is set to 37 while the feedback loop of the OPA627 controls the overall gain.
Please note that the capacitors and resistors at the input may have different values in the phonostages because they are part of the RIAA network.
See important note concerning the OPA603 at the end of this page.
Figure 1 shows the schematic of the composite amplifier. The gain of the OPA603 is set to 37. The overall gain is set to 10000 / 20 = 500 (54db) or 10000 / 10 = 1000 (60db). Thus the OPA627 has a gain of 13.5 or 27 respectively. Since the OPA603 is in the feedback loop of the OPA627, the composite amplifier retains the excellent DC characteristics of the OPA627 at these relatively low gains. The typical offset measured is close to 50mV.
Contrary to the original Burr-Brown data sheets I used OPA627 instead of the recommended OPA637 (see Burr Brown application bulletin), because I was unable to control oscillation of an OPA637/603 compound, while the OPA627/603 compound runs smooth and stable in my application.
For optimal distortion behavior both the + and the - inputs of FET op amps should see the same impedance values. The 1.5k and 2.0k resistors minimize adverse effects of the existing imbalance of source impedances. While the - inputs see constant source impedances of 20Ohm, the + inputs see the RIAA network impedance varying between 1.5kOhm and 17Ohm across the audio band. Thus the impedance ratio is between 60 and 0.7 (see plot of impedance ratios). With the resistors added, the impedance ratio is between 1.5 and 0.75 from 20Hz and 20kHz.
The balanced design doubles the maximum voltage swing compared to a single ended op amp design and ensures an absolutely constant load of the power supply, independent from heavy bass notes or transients.
The Lundahl LL7902’s seem to be the right choice as suitable output transformers. They combine very high level capacity (+28dbU at 50Hz) with low copper resistance (see data sheet). Primary and secondary windings are separated by electrostatic shields. Depending on the winding connections, impedances can be transformed 1:1, 4:1 or 16:1. Figure 2a and 2b show the prototype of the output stage.
