Technical Description, con’t
Autoformers
All solid state power amplifier output circuits work best into what is called an optimum load. This optimum load may vary considerably from what a loudspeaker requires. In the case of more than one loudspeaker connected in parallel, the load to the power amplifier may drop to two ohms or even less. A power amplifier connected to a load that is lower than optimum, causes more output current to flow, which results in extra heat being generated in the power output stage. This increase in temperature will result in a reduced life expectancy for the amplifier.
The Autoformer creates an ideal match between the power amplifier output stage and the loudspeaker. Refer to figure 13. There is absolutely
no performance limitation with an Autoformer. Its fre- quency response exceeds that of the output circuit itself, and extends well beyond the audible range. Its distortion level is so low it is virtually impossible to measure. In the rare event of a power ampli- fier output circuit failure, the McIntosh Autoformer provides absolute protection
from possible damage to your valuable loudspeakers.
Protection Circuits
The MA7000 incorporates its version of the McIntosh
Sentry Monitor output transis- |
|
|
tor protection circuit. Refer to |
|
|
figure 14. There is absolutely |
|
|
no compromise in sonic per- | Normal Operating Area | |
formance with this circuit, and |
|
|
it ensures safe operation of the | Sentry Monitor | Output |
| Transistor | |
amplifier under even the most | Safety Area | Failure |
| ||
|
|
extreme operating conditions. The different types of protec-
tion circuits incorporated in the MA7000 insure a long and safe operating life.
The MA7000 also includes the unique patented McIn- tosh Power Guard circuit. Power Guard eliminates the pos- sibility of ever overdriving the ampli-
fier into clipping. Refer to figures 15, 16 and 17. An overdriven amplifier can produce both audible and inaudi- ble distortion levels exceeding 40%. The audible distortion is unpleasant to hear, but the inaudible ultrasonic distortion is also undesirable, since
it can damage valuable loudspeaker system tweeters. You will never experience the harsh and damaging distortion due to clipping.
The Power Guard circuit is a waveform comparator, monitoring both the input and output wave- forms. Under normal operating con- ditions, there are no differences be- tween the shape of these waveforms. If an amplifier channel is overdriven, there will be a difference between the two signal waveforms. When the difference exceeds 0.3%, the Power Guard activates the PG light and a dynamic electronic attenuator at the amplifier input reduces the input volume just enough to prevent any further increase in distortion. The
Power Guard circuit acts so fast that there are absolutely no audible side effects and the sonic purity of the music reproduction is perfectly preserved. The MA7000 Ampli- fier with Power Guard is not limited to just the rated power output, but will actually produce distortion free output well above its rated power due to the McIntosh philosophy of conservative design.
Power Supply Circuits
To compliment the design of the MA7000, there is a high voltage power supply for both channels. The power amplifiers draw high current from the AC power line.
Therefore, it is important that they plug directly into the wall outlet. Turn on inrush current is cushioned by therm- istors in the power transformer’s primary circuit. This soft start eliminates component stress during
The MA7000 can provide greater than 50 amperes peak output current to drive uneven speaker loads. Some poor speaker designs have input impedance that dip to 1 or 2 ohms at various frequencies and the MA7000 has the out- put current reserve to drive them. The MA7000 has main filter capacitors that guarantee an excellent signal to noise ratio and the energy storage necessary for a wide dynamic range that music demands.
17
