MC 7270 question

jameswei

Super Member
I have this MC 7270 for a few month now and to me it doesn't sound like 270 watts, so I figure out if I flip the switch in the back from 2.5V to 75V the amp comes alive. However, there's a downside to it, everytime I power on and off the speakers will make a high squeaking noise. What's the fix here?
 
I have not seen this with Mac amps, but with some others. In a few cases it was faulty connections, either ground or signal of the RCA jacks. It could be the condition of the sensitivity switch. Maybe a good cleaning of both. Of course it could be internal connections, out of spec caps, or bad active components. I would be very careful you could easily cause damage to your loudspeaker drivers plus further damage the amplifier.
 
So the amp with no input from your unnamed preamp when cycled on and off squeals? Most of the time these turn on noises come from not following the age old protocol of turning the amp off before the preamp as the preamp PS degrades it puts out noise.

Do you have a list of the new caps that were put in?
 
I have this MC 7270 for a few month now and to me it doesn't sound like 270 watts, so I figure out if I flip the switch in the back from 2.5V to 75V the amp comes alive. However, there's a downside to it, everytime I power on and off the speakers will make a high squeaking noise. What's the fix here?

Flicking the input sensitivity switch to .75V does not change the output power of the amplifier in any way at all.
It simply changes the input voltage required to run it at full power.....

So 0.75V means it takes less output from the pre-amp to run it to full power than the 2.5V setting.
What pre-amp are you running with it?

What upgrade?? There's no upgrade to stop it doing this....But we can check a few things, the first thing is are you turning the pre-amp off before the amp?
If you are then the noise will be coming from the pre-amp.
You need to turn the power-amp off first.....
 
So at lunch checked the sony ta2000 schematic. States preout max is 2.5 volts so it is another example of why Mac needed to put the .75 volt input as a option.

Most likely clipping the preamp output.
 
The Preamp is a C-28 and was also recapped. Now I have dial it back to 2.5V and when I power it on there's noise from the speakers. but when I turn off the amp first then the preamp there's no noise. My tech friend told me that using the .75V will hurt the amp and I probably already blown something. Is that true?
 
Old school written in stone from the days of tubes.....pre on then amp on......pre off then amp off.

The C28 makes a bit of a racket as you charge up the headphone amp. When I was doing my posted C28 update I was tring to source a headphone jack that included a power switch to tame this noise which surprisingly affect the pre amp outputs.

Also increasing the value of the multicap, especially the first section, can greatly add to the noise at the pre outs when the unit is turned on.

I am going to go out on a limb here......if your tech actually told you that, in those words,that you damaged it by possibly over driving the .75 input.......well...... maybe you need a new tech.
 
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Agree with both kev and c_dk here. I'm not sure you're understanding the difference between the .75V and 2.5V settings, nor what the function is.

.75V - with the gain controls fully CW, the amp can be driven to its rated output with just .75V of input per channel.

2.5V - with the gain controls fully CW, the amp can be driven to its rated output with 2.5V of input per channel.

Gain controls are just like the throttle on your car - they're a regulator. When they are at any position other than fully CW, they decrease the input signal from the source and thereby lessening the amplifiers output with relation to a given volume control on your preamp.

The turn on noise you have may simply be a symptom of aged contacts in the speaker relay. My MC2600 also makes a small bit of noise on turn on, but this doesn't happen every single time. Having used my MC2600 with my C28 for a short period of time, I can tell you with certainty that the C28 was unable to properly drive the input section of the MC2600 with the sensitivity switch in the 2.5V setting. The 1.4V setting was a perfect match to both the C28 and to my Integra preamp / processor.

Really what you're trying to achieve here is a good compromise of volume versus noise. I'd definitely revert back to the .75V setting and then set the gain controls on your MC7270 in such a way that you can achieve the volume you desire with minimal noise from the speakers with no signal. A good place to start would be with the gains set at 12:00 and then go from there.
 
When I first have this set up it was 2.5V and there were no speaker noise issue. After I try the .75V the powering on and off came with the high squeaking noise. I really like the way .75V sounded and wants to know what's causing this noise and how to remove this noise. Both of these units have been recapped by my tech, but he had no prior Mcintosh experiences.
 
This problem may or may not be related to your switching the sensitivity switch. You may elect to take both units to your tech so that he can replicate the problem on his bench.
 
I really like the way .75V sounded and wants to know what's causing this noise and how to remove this noise. Both of these units have been recapped by my tech, but he had no prior Mcintosh experiences.

The amplifier sensitivity doesn't change the sound of the amplifier, it only changes the amount of signal required to drive the amp to full power.....
It will change how hard you are driving the pre-amp and this may or may not affect the sonics.....

Thanks, I will do that. I might try a different preamp and see if I can isolate the problem source.

Unplug the pre-amp from the 7270 entirely and try power it up and powering it down without the pre-amp plugged into it at all, this will tell you if it is the 7270 or not.....

Let us know how you get on.....
 
Unplug the pre-amp from the 7270 entirely and try power it up and powering it down without the pre-amp plugged into it at all, this will tell you if it is the 7270 or not.....

I did this and there's no noise from the speakers, so I guess the preamp was the problem?
 
Now, if your reconnect the pre and amp, with the preamp off, when you turn the amp on and off is it still quiet?

If it is noisy with the pre off you could have a bad interconnect or a ground issue at the pre amp output jacks. If your interconnects have "arrows" make sure the ground shield is terminated at the preamp....or in other words the arrows showing signal flow towards the amp.
 
I put in a different preamp in and everything is working fine(both in 2.5V and .75V), so I guess I should just take the C-28 to my tech and have him check it out.
 
I put in a different preamp in and everything is working fine(both in 2.5V and .75V), so I guess I should just take the C-28 to my tech and have him check it out.

Hang on, are you powering the 7270 down BEFORE the C28?

You need to power the 7270 down FIRST, then power down the C28.....Have you tried this?
 
Look at it like this ... the MC7270 is an amplifier. It will amplify anything that comes into its RCA jacks - including any noises any upstream components may make on power up or power down. Therefore, it is undesirable to have the amp powered up when turning ON or turning OFF any upstream component.
 
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