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    | Introduction | Specifications | Construction | Electronics | Supplies | Photos | Pluto-2.1


Power amplifier, crossover and equalizer 
electronics module

The electronics are key elements for the sonic performance of PLUTO. A 50 W amplifier drives the tweeter and two 50 W amplifiers in bridge configuration drive the woofer. The amplifiers run off a +/-30 V unregulated power supply with large storage capacitors to handle large bursts of current or voltage demanded by the audio signal. The line level equalizer and crossover circuitry is shielded from this activity by a +/15 V regulated power supply. Equalization of the acoustic response of the mounted drivers is an important function that the electronic circuitry provides.

The raw on-axis response of the pipe mounted tweeter which obviously must be equalized.
The raw frequency response of the upward pointing woofer when mounted in the pipe and measured at 90 degrees from the vertical direction as a listener would hear it. The fine grain is not of concern at this point. Most of it is due to the test environment.
The frequency response of the woofer after it has been equalized, when measured close to the dust cap.
The frequency response of the final electronics, when measured at the amplifier outputs. 6 dB must be added to the woofer curve to see the true woofer terminal response. Only one side of the bridged amplifier output was measured with respect to 0 V. The other side has the same response, but is 180 degree phase shifted.

The complete circuitry is shown below. The input buffer stage drives tweeter and woofer channels. It can handle low level signals from a preamplifier output or high level signals from a power amplifier output. The tweeter channel has a gain control stage which is linear in dB. It is followed by the two stages of a 4th order Linkwitz-Riley highpass filter and then an allpass for correcting the phase shift due offset in acoustic centers between woofer and tweeter. The notch filter equalizes a peak in the tweeter response. The last stage is the power amplifier.

The woofer channel starts out with two LR4 lowpass filter stages, They are followed by a biquad circuit which in this case is only partially utilized for equalization. It was included on the circuit board for other possible applications. Next is a notch filter which shunts the signal going to the buffer stage before the bridged power amplifiers. This buffer stage drives one of the power amplifiers directly and the other one through a unity gain inverting stage. Thus the voltage across the woofer terminals can swing up to +/-60 V peak. 
All power amplifier stages utilize the high-performance National Semiconductor LM3886 integrated circuit. This IC clips cleanly and is protected thermally and against current overload. Not only that, but a well designed chip amplifier has an inherent performance advantage over a discrete component solid-state amplifier, because its output bias circuitry is directly on the silicon chip and thermally tracks output stage parameter changes due to fluctuating amounts of program power and continuous chip temperature change. Such accurate tracking is very difficult to obtain with discrete designs where temperature sensing can only be accomplished outside the output device packages. The thermal time constants of sensor and chip must be equal. Lack of thermal tracking is the prime source for increased crossover distortion at low output power levels and results in high order distortion products which have given many class A/B solid-state power amplifiers a bad name as sounding gritty. The distortion should be measured with dynamic test signals, not steady-state signals as is common practice when characterizing amplifiers.  
The operational amplifiers are Burr-Brown/Texas Instrument OPA2134. They are quiet and have very low distortion.

The power supply utilizes a 50 W transformer that can be wired for either 115 VAC, 60 Hz operation with the two primary windings in parallel, or for 230 VAC, 50 Hz line voltage with the windings in series. The presence of power line voltage on the circuit board requires caution when testing to avoid dangerous electrical shock. 
The return currents of the three power stages go to the 0 V terminal and are run separately from the signal ground of the line level stages in order to avoid amplifier low frequency instabilities. Positive and negative regulators provide stable and clean voltage to the operational amplifiers.


    | Introduction | Specifications | Construction | Electronics | Supplies | Photos | Pluto-2.1




What you hear is not the air pressure variation in itself 
but what has drawn your attention
in the streams of superimposed air pressure variations 
at your eardrums

An acoustic event has dimensions of Time, Tone, Loudness and Space
Have they been recorded and rendered sensibly?

Last revised: 01/11/2017   -  1999-2017 LINKWITZ LAB, All Rights Reserved