Input Sensitivity?

rickmoen

Member
Hello,

Having searched and not found anything I can understand, I thought I'd ask here. What is input sensitivity? What happens when it is changed? I have an MC7200 amp. On the back of the amp is a slider switch labeled 1.4v and 2.5v. Mine is set at 1.4v (with gains set at 12:00). The owner's manual recommends the switch be set at 2.5v and the gains be turned clockwise all the way to achieve quietest operation. The amp is quiet now. What effect will this have - louder, softer, something else, not something else? Obviously, input sensitivity is increased by 1.1v, but then what? Thank you in advance for any explanation.

Rick
 
Should be set to 2.5V if you're using a McIntosh preamplifier for best S/N ratio.

Higher sensitivity setting increases the gain of the input section which will elevate the noise floor. McIntosh suggests 2.5V with the gain controls set to maximum.
 
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Before the invention of laser trimmed volume controls, controls were designed and assembled to have a range were the level between channels would track together.

This range was typically between 9:00 and 1:00 in the rotational range of the control.

The idea was to set your amp gain controls to have you system play it loudest at around 1:00 so most all the time you would be within that rotational sweet section.

Mac and other high end, typically American, manufactureres also set their input sensitivity high at 2.5 volts since typically a preamp was quieter than a amp.

So to they the full 200 watts in your case out of the amp you would need to get 2.5 volts out of your preamp. This was no problem for most American designed preamps, however many Asian popular brands especially their receivers, struggled to put out in some cases even 2 volts. So Mac started to build their amps with both voltage options....what they wanted and a accommodation for those entry level units.

In 40 years I think the lowest output Mac preamp I ever measured was 8 volts. A C33 would put out a max near 15 volts. Obviously these units are over built just like your amp that most likely would bench test out a near 250 watts not the 200 you paid for.
 
The old Mac amps apparently could accommodate the 0.5 VRMS British line level standard as well as the American 2.0 VRMS line level standard. Those amps could also take almost any single source sans preamp for applications not requiring the versatility options of a preamp control center.
 
Older Mac power amps without the switch the 12:00 position was the recommended 2.5 volt setting and wide open was the .5 volt sensitivity setting. With the introduction of more modern pre-amps by all manufacturers the .5 volt sensitivity wasn't needed anymore. With commercial versions of the MC 2005 and MC 2125 having a switch to select the voltage was very important as it kept knob twiddlers from making unauthorized adjustments which could damage drivers connected to the more powerful amplifiers and compromise distortion and signal to noise ratios.

So with the switch set at the 1.4 volt setting and the volume controls set wide open it takes 1.4 volt to drive the preamp to full power. Now actually you need a bit more voltage to drive to full power where the Power guard engages. With the controls set at the 12:00 position with the switch set in the 1.4 position you will require close to 14 to 16 db more voltage to reach full power. or about 7 volts. Most Mac pre-amps can handle those requirements without to much added distortion. A lot of other Pre-amps cannot. What you are doing insures very quiet operation but you are increasing the distortion levels from the pre-amp, in addition that can cause the pre-amp to clip destroying drivers that the power amp Power guard has no control of; so set the switch to 2.5 volts and turn the controls wide open. It provides the best compromise between distortion and signal to noise parameters. It also insures clipping will not occur if you slightly over drive the amp as the Power Guard will work properly to prevent clipping.

One thing some owners don't understand. Power Guard prevents clipping which can destroy drivers. But if you use speakers that can't handle the continuous out put of a Mac amp they will eventually fail. Music is a very complex sound with very little continuous high level sounds. The speaker can handle a lot more peak power than they can continuous power. So finding the right match of power for the peaks with out having to much power for continuous operation can be be quite a challenge. With the increased dynamic range capability of digital recordings the issue can be even more dynamic.
 
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