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Large motional feedback system

HF MOSFET amplifier with soft clipping:

The HF section consists of a 110W Power MOSFET amplifier with special lateral MOSFETS, the almost ideally complementary N-channel 2SK135 and P-channel 2 SJ 50, once manufactured by Hitachi. Their unique property is that they can be tuned to an almost temperature independant idle current of ≈ 100 mA by means of a constant DC Gate-to-Source voltage, positive for the N-channel version and negative for the P-channel version. With the exponential characteristics around zero current the transfer characteristics of these P- and N-channel MOSFeTs add almost ideally to an almost linear gain in a class A-B configuration. Unfortunately they are no longer available from Hitachi but there are approximate replacements by Exicon (ECF10N16 and ECF10P16), now abtainable at Profusion plc. I personally have no experience with these replacements as I once bought 10 of each and after building 5 amplifiers I have just used them all. They did not yet fail on me fortunately after many years.

The basic circuit of the amplifier has been published by Hitachi and introduced in the Netherlands by Elektuur many years ago. Starting with the Elektuur version I changed several things. The input circuit was replaced by an operational amplifier, the open-loop frequency compensation was adapted and I added a soft-clipping circuit. First the basic amplifier circuit will be explained without the soft clipping circuit and the capacitors that determine the feedback stability.

General working principle

OPA 1,2 and 4 determine a balanced purely differential instrumentation amplifier with a gain of 3. This balanced input is necessary to avoid groundloops as all three amplifiers are supplied from the same power supply with connected grounds. Also the filtersection shares the shame ground, starting at the signal input circuit. The high currents to the power amplifier will mainly contain AF components due to the single-ended configuration of this amplifier. This AF current in the ground-wires creates voltages over the impedance of these wires, resulting in a common mode AC voltage at the signal inputs of the power amplifiers. These common-mode voltages can cause instability because they correspond with the input signal but with different phase. Further they can be caused by magnetic fields from the power transformers giving hum and noise. These negative effects are effectively cancelled by means of a pure differential input when the negative input is connected to the ground of the filter section via the shielding of the signal cable between the filter section output and the positive signal input of the power amplifier.

The power section starts after the output of OPA3 with two high voltage level shifters T1 and T4 that act as variable current sources and create a voltage over the 5.1 k resistors with a value of 1/3 times the output voltage of OPA3. This voltage is fed into the Gates of the high voltage amplifier MOSFETs M1 respectively M2 of which the Drains are mutually connected through a variable voltage source network around T3. This network serves as bias setting for the idle current of the output power MOSFETs. M1 and M2 act as variable current sources with a value determined by the voltage at the Gate, the forward transfer admittance of the MOSFETs (can be neglected) and the 120 Ohm source resistors. The signal from the Drains of M1 resp. M2 depends on the current by M1, M2 and the total impedance between the Drains and ground. The resulting signal is fed into the Gates of pair M3,M4 resp. M5, M6 and these pairs act as a low output-impedance Source followers to supply the driver with current through a series resistor of 1 Ohm and a resettable thermal fuse of 1 Ohm. The total resistor value of 2 Ohm is chosen in order to limit the maximum current at short circuit, as there is no current protection provided in the amplifier. Also only for that reason the thermal fuse is applied.

HF amplifier

Feedback and stability

The low frequency voltage gain of the path between the output of OPA3 and the Sense node where the tweeter is connected is equal to a summation of three gains: the level shifters, the gain stage with M1,M2 and the output stage. The level shifters have a gain of -10 dB, determined by the 5k1 and 15 k resistors. The LF gain of the gain stage is determined by the 22k resistors to ground and the current that is determined by the 120 Ohm resistors in the source of the MOSFETs. This gives a gain of 22k/120=183 → 45 dB. The output stage gives -5 dB attenuation by the internal impedance of the power MOSFEts and the 2 Ω total series resistor that together are loaded by the driver impedance of 3.6 Ω. This all results in a total forward gain of 30 dB. The feedback gain from the Sense node to the output of OPA3 equals 5.6\3.67 → 4.5dB resulting in a total LF open-loop gain of 35.5 dB.

For DC protection of the tweeters a series capacitance of 12 μF is used. This capacitor gives a high-pass characteristic with a corner frequency at approximately 2.5 kHz. This frequency is much lower in closed-loop by taking the feedback from after the capacitors. This also reduced negative effects from the capacitors.

For stability the phase lag at the open-loop unity-gain cross-over frequency (0dB) should be less than -180o while -145o is a value with enough phase margin to be sufficiently robust against phase delays by higher frequency parasitic poles. The open-loop is designed with a frequency independent gain over the audio band. The main pole at 20 kHz is placed in the high gain stage around M1 and M2, thereby minimising ringing after overload. This first pole is determined by the 22 k Drain resistors of M1 and M2 (parallel for AC voltages → 11 k total) with their parallel capacitance. This capacitance is the sum of the two 150 pF capacitances and the input capacitances of the power MOSFETS. These capacitances consist of the Drain-to-Gate capacitances of ≈30 pF per MOSFET and the combined Drain-to-Source capacitance of ≈1200pF for the N-channel MOSFETs and ≈2000 pF for the P-channel MOSFETs. Additionally four 330 pF capacitors are placed over the Gate-to-Source terminals of eacht MOSFET and this all amounts to a total Gate-to-Source capacitance of 4500 pF. This value is divided by the ratio between the Gate-to-Source voltage and the Output voltage. With a Forward Transfer transmittance of ≈2 S for two MOSFETs this becomes equal to one over twice the load impedance =1/(2×5.6)≈0.09, resulting in a total MOSFET related load capacitance of 0.09×4500≈400pF that is placed parallel to the two 150 pF capacitors resulting in a total capacitance of 700 pf. This value gives an RC time with 11 k of 7.7 μs which corresponds with a corner frequency of approximately 20 kHz. Note that the fixed capacitors (150 and 330 pF) are added to compensate for the temperature and voltage dependency of the internal capacitances of the power MOSFETs that otherwise could cause oscillations in the cross-over area around 0V output voltage.

An additional pole is created in the loop around 500 kHz by a feedback capacitor over OPA3. This is done to keep the Op-Amp happy even in case of occasional parasitic capacitences at the input by bad wiring. In the loop this pole is compensated by two small capacitors of 22 pF over the 15 k resistors in the emitters of T1 and T4. Note that the emitters are supplied with an RC filter (470 Ohm plus 1 μF) to reduce HF noise.

The result is a fast and stable amplifier extremely well suited for HF sound reproduction. In the figure below the results of an LT-spice model is shown where the open loop gain is taken between the Sense node and the output of the amplifier. The closed loop gain is shown between the positive input and the output of the amplifier. The closed loop -3 dB bandwidth corresponds with the open-loop unity-gain cross-over frequency of 1.5 MHz. Note that the difference in nodes causes the closed-loop gain to be equal to ≈21 dB instead of 0 dB.

loop stability

Soft clipping and maximum power

Soft clipping is achieved by reducing the gain of the gain stage when the peak voltage of the gain stage reaches a level of +/-8.6V below the +/-60V supply voltage of the output stage. This level is set by the zener diodes at the base of T2 and T5. The additional diodes and resistors create a nice rounding of the clipping. This method reduces effectively the gain at clipping which fully cancels all ringing after clipping, which is aimed to mimic the "tube-like" sound of a low feedback-gain amplifier. In fact the clipping is only audible at a factor two in power over the clipping level increasing the perceived dynamic headroom with +3dB.

The maximum power before clipping can be calculated starting with the power supply voltage of +/-60V. Clipping occurs when the Gate voltage of the output power MOSFETs exceeds approximately 51V. The -5 dB attenuation by the Power MOSFET's and the 2 Ohm series resistor at the output results in a maximum voltage of Vmax=28V at the driver terminals. With an impedance of R=3.6 Ohm this voltage is equivalent to a power level of Vmax2/2R=110 WRMS. With higher levels, during clipping, this level can easily reach values of 200WRMS which largely exceeds the power handling capacity of a tweeter being not much more then 4 W. Normally these extreme power levels are seldomly reached with high frequencies in music reproduction and if so, they are only present for a very short time. For reasons of safety in case of a non-natural sound source a special protection circuit is used to limit the average power to approximately 4W. This circuit will be presented in the section about Protections. The Thermal fuse is unfortunatly not fast enough for a passive protection so the overload safety has to be realised by means of an active electronic protection circuit.

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