McIntosh MA5100

This amp was acquired in the early to mid 80's. I've used it a few times but not often over the years mainly for testing speakers and to run a second system in another room for a year or so. It spent most of its time here on a shelf in my music room among my other cherished collectibles. A few years ago, I put it on the bench and just checked the voltages and drove it to full power while monitoring the output on a scope. It passed the tests but I did order capacitors for it. Recently, like in January 2016, I put it back on the bench just for the heck of it. This time, I ran extensive tests, including total harmonic distortion tests and it came out much better than expected. (See THD+N chart below) It was decided to leave well enough alone based on the expression, 'If it works, don't fix it.' I did, however, replace a lamp and spent a lot of time flushing out the front panel switches and I do mean FLUSH OUT. These things were full of whatever it is that accumulates in switches. I used about half a can of spray switch cleaner and a whole can of compressed air (the kind used for dusting keyboards and such) The potentiometers were also cleaned but these weren't anywhere as dirty.

This little fella is old; the sticker on the back says, Performance Certified 4-2-'70. It's over 46 years old and ALL original with the exception of one lamp. Another McIntosh testimonial.

The charts/graphs below are run with LMS, Loudspeaker Management System by LinearX. They conform to within 2 dB of specification. All were run with both channels simultaneously loaded with 8 ohm resistive loads.

The PicoScope was used as a signal source as it's distortion is far less (about 300%) than the Tektronix SG502.

The high figures at 20 hz are attributed to the old electrolytics in the power supply. At just under 40 watts, a 20 hz sine wave is perfect on the scope but

begins to distort as 45 watts is approached. This amplifier is all original, another testimonial to McIntosh Laboratories.

Since the acquisition of the CLIO about 2 years ago, it was decided to run another test on this fella with that device, which includes Total Harmonic Distortion, THD.

All measurements below were made into an 8 ohms resistive load.

Conversions from THD in dB to % were made at sengpielaudio

Manufacture dates 1966-1972. This fella is 50+ years old and is all original. I do have a full compliment of capacitors, resistors and transistors but considering its current performance, it was decided that is was better to leave well enough alone.

You know the saying, if it works, don't fix it.

FIGURE 4

1W GRN-left BLU THD

RED-right ORN THD

Here, THD runs between 0.03% at the low end and 0.1% at the high end

FIGURE 5

8W GRN-left BLU THD

RED-right ORN THD

THD at low end 0.56%; at high end 0.1%

A little high in the low end; spec is less than 0.25%

FIGURE 6

45W GRN-left BLU THD

RED-right ORN THD

THD at 30hz about 3%; at 50hz about 0.6%

from 200hz and up, 0.1%

again, high at the low end and within spec (0.25%) above about 100hz

The innards of this amp are all original.

Power (W)	20 hz		2 khz		20 khz
into 8 ohms	THD +N (%)		THD +N (%)		THD +N (%)
resistive	L ch	R ch	L ch	R ch	L ch	R ch

1 (2.83V)	0.244	0.256	0.110	0.109	0.121	0.116

10 (8.94V)	0.248	0.260	0.108	0.109	0.105	0.100

45 (18.97V)	0.254	0.265	0.118	0.120	0.100	0.097