The Secrets Blu-ray Player HDMI Benchmark - Part 2


HK 990 Phono Stage

In his first preamp for Harman (Citation XXP), Otala addressed the challenge of achieving a flat return-loop gain across the frequency band. In a typical amplifier with a flat closed-loop response, this is readily achieved in a discrete circuit at the cost of distortion (attempts are made in the discrete opamp design to reduce open-loop distortion when the return-loop gain is purposely set to a low value as we saw in the power amp).

The problem is more difficult in a phono preamp with a varying frequency response (it follows the RIAA curve). The HK 990 uses a topology dating to Otala's tenure. Twelve transistors form a transconductance amplifier. The transconductance amplifier differs from an opamp since it has a high output impedance. Think of the transconductance amplifier as an opamp without the common emitter output stage. It is called a transconductance amplifier because a change in voltage at the input (open loop) results in a change of current at the output.

Iout=VinG where G is conductance which is the reciprocal of resistance.

The RIAA network is connected around the transconductance stage. As the frequency increases, the passive RIAA network loads the transconductance amplifier and lowers its open loop voltage gain (the current output of the transconductance amplifier flows in the RIAA network giving rise to a voltage). Since the transfer function of the passive RIAA network is increasing with frequency, the total return loop remains constant at the desired level chosen by the designer.

The plot below was taken from the original literature for the Citation XXP preamp.

Low frequencies pose a problem with this approach. Here, a closed-loop voltage gain of 60dB is required to match the inverse RIAA curve for a moving magnet cartridge. The open-loop gain of the transconductance circuit loaded by the RIAA network should be 80dB at a minimum. This may not be achievable because the intrinsic output resistance of a real transconductance amplifier limits the gain. The result can be a drop in the gain at low frequencies and increase an distortion.

I do not have measurements of the HK 990 to confirm if it exhibits this problem. Some earlier HK phono stages hinted at the problem, but the transconductance amplifier on the HK 990 is significantly enhanced. It remains fully complementary. Current sources replace a resistor to bias the differential input stage. A buffer between the first and second gain stages prevents the second stage from loading the first. The first voltage gain stage design trades off degraded noise performance for improved open-loop distortion.

The DC servo used in the phono stage eliminates all bypass and coupling capacitors. Surprisingly, no DC blocking capacitor is at the input to the phono stage. Since a bipolar transistor is used at the input, a small current always flows in the cartridge. I have rarely seen this direct connection implemented unless the input transistor is a FET with an extremely low gate-current flow.

An open-loop emitter-follower buffers the transconductance amplifier from the load presented by the line stage. Almost all phono stages are in the non-inverting feedback configuration. Such a configuration is limited to a gain no lower than one. The RIAA curve should continue rolling off and not stop at unity gain. A passive low-pass filter in this stage corrects the problem (The Audio Critic, Issue 18, page 16). This and other phono stage design issues are addressed in Chapter 7 of Small Signal Audio Design by Douglas Self (Focal Press, 2010).

A 330pf capacitor is at the input. This high value exceeds the specifications of most moving magnet cartridges when the turntable wiring is included. At this price, I would like to see a switch on the back to offer different loading options. In the absence of that, a low-valued capacitor is advisable since the value can be raised externally. In contrast, nothing can compensate for too much capacitance at the phono input.

Moving coil cartridges see a pre-preamplifier that is a two-stage open-loop design (no feedback). Since the signals from the moving coil cartridge are so small, the transistors almost act as linear devices; hence, no feedback is required to linearize the moving coil stage. Other designers might have employed feedback to reduce the distortion to lower levels. The pre-preamp has no DC servo. A standard DC blocking capacitor is at its output. In the moving magnet stage no blocking capacitor is at the stages input. Again its absence causes a small DC current flow in the cartridge.