Altec Lansing 4200A, 6125A manual Design Theory, Troubleshooting Tips, No Audio Output

Operating Manual 6125A and 4200A Multi-Channel Power Amplifier

10.DESIGN THEORY

The 6125A and 4200A amplifiers are based on stereo driver modules that use digital processing to gener- ate a spread-spectrum switching pattern between about 200KHz and 1.5MHz, depending on input signal ampli- tude and frequency. This overcomes the self-limiting and inherent weakness in traditional class D fixed-frequency PWM amplifiers, and produces an output with THD+N numbers comparable to class A and class AB linear ampli- fiers. (see Specification Notes at end of this section)

In the 6125A and 4200A, the input signal is re- ceived by a single operational amplifier configured as a difference amplifier to reduce common-mode effects from sources located at a distance. This difference amplifier has a gain of 2.74 which allows the use of less gain later in the system, improving overall system noise performance.

The amplified signal, based on the position of vari- ous selector switches, either bypasses or is processed by a two-pole high-pass filter with rolloff at 50Hz, and then passes through the attenuating potentiometer. The attenu- ated signal is buffered and passed to the amplifier module along with a zero-volt reference.

The amplifier module receives the attenuated sig- nal with a unity gain differential amplifier to eliminate com- mon-mode interference picked up within the amplifier chassis. The signal is then added to a small DC offset sig- nal opposite in polarity to any DC offset on the output bridge and adjusted by the digital processing module as described above. The digital processing module, which also generates FET drive signals, has a voltage gain of 11.7 for a total system voltage gain of 32. The drive signals generated by the processing module are fed to a pair of high-current MOSFET transistors, and the switched out- put of these transistors is filtered and applied to the speaker outputs . Fault conditions such as overvoltage, undervoltage, and output overcurrent are measured by the module and therefore affect a stereo pair of channels re- gardless of whether the fault actually exists on both chan- nels processed in the module. Since every action applied to protect the digital processing module affects both chan- nels being processed, other fault conditions are not sepa- rated by channel, but rather by amplifier module.

Specification Notes: Due to its spread spectrum out- put switching pattern, the output signal of a 6125A or 4200A amplifier contains significant dynamic frequency content* far outside the audio band, which makes no difference to audible performance, but which makes heavily bandwidth-limited** mea- surement of amplifier noise and distortion characteristics man- datory.

*variable 200KHz to 1.5MHz

**greater than 48dB/octave above 22kHz

11.TROUBLESHOOTING TIPS

11.1No Audio Output

1.) Power LED not lit:

Line fuse is blown or power outlet is dead.

IF LINE FUSE IS BLOWN, REPLACE ONLY WITH SAME TYPE AND RATING FUSE.

2.) Power LED is lit but Protect LEDs stay on:

Amp module is in protect mode. Speakers have been disconnected from amplifier output and connected to ground until the protect fault is corrected. See section 8 for a complete explanation of protect fault conditions. Out- put overcurrent, overvoltage and undervoltage faults will reset in about eight seconds once the fault condition is re- moved, while excessive DC offset or high frequency faults take about 30 seconds to reset.

Speaker Impedance: The 6125A or 4200A may go into self-protect at high output levels if the actual speaker load impedance is much less than 4 ohms. To cal- culate speaker impedance for a given combination of di- rect-coupled speakers, use Ohm's law as applied to series and/or parallel resistor networks, where each speaker (for this purpose) can be thought of as a single resistor, using DC resistance measurements.

Simply stated, speakers connected in series will add together their impedance. Conversely, two speakers con- nected in parallel will result in half the impedance, three parallel speakers a third the impedance, four speakers a fourth, and so on, assuming the speaker impedances are all the same. Don't use mismatched impedances in parallel. When using paralleled speakers, the available amplifier power for that channel is evenly divided among speakers, so 100 watts driving two parallel speakers of equal imped- ance provides 50 watts to each speaker, etc.

3.) Thermal LED and Protect LEDs stay on:

Amp module is in thermal protect mode and needs to cool. The fan will continue to run while in thermal pro- tect, and other modules will continue to function. See sec- tion 8.4 for details.

4.) Power LED lit but no Signal LED activity

There is no input signal applied or input level con- trols are turned down.

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