So I am the proud owner of a McIntosh MX-113 tuner-preamp, which I had hoped to pair with my MC-2505 amp. I thought they would look nice and play well together. Before the MX-113, I was using an Adcom GFP-565 preamp for CD and Phono, and feeding in the tuner section from an old Pioneer receiver for FM listening. I was hoping to replace the Adcom and Pioneer setup with the MX-113. Not expecting the same sound, but something synergistic at least.
Although the MX-113 looks really nice for its age and has most of it's functionality, I think there is something lacking to the sound. One thing I noticed immediately was the background noise level, even on line level input (white/pink noise, not so much hum). Not super-loud, but noticeable. I am thinking the old transistors might be contributing to this type of noise.
The other thing I noticed was the cross-talk of the CD that was still playing in the background when I switched over to the Phono 1 input. I imagine the selector switch is not providing a solid ground to the de-selected inputs so maybe some cleaning/Deoxit would help there.
The only function that seems not to work at all is when I switch the selectivity to narrow - the reception disappears altogether. I haven't explored this issue any further yet as I would probably listen in 'wide' even if the narrow setting worked, due to my lack of competing stations here in a rural area, and the typically better sound quality that wide usually offers. Eventually I would want to fix this issue anyway.
I downloaded a copy of the MX-113 service manual and took a look at the preamp section schematic. I have read that the MX-113 sends the FM Multiplex output into the Phono section of the preamp board. This is correct as far as I can see. The trick here is that when you select the FM input, one of the wafer switch sections is used to bypass the frequency-selective parts of the feedback loop around the phono stage with a resistor that lowers the overall gain and flattens out the response. I assume this is done because the output from the Multiplex section is too low to compete with line level sources and needs some flat boost.
I modeled the phono section with LT Spice and the response of the Phono preamp in the "flat mode" is flat within a dB from 20Hz to 20kHz and gain is about 20dB midband. Using a Laplace function to create a reverse-RIAA voltage controlled-voltage source as a simulated input, the response of the Phono section in Phono mode is correct within a few tenths of a dB across most of the audio band.
Interestingly, there is a serious out-of-band resonance peak around 500kHz that is due to the cumulative phase shift reaching 180 degrees inside the loop. It should be noted that I modeled the circuit with the added Service Bulletin components in place (the one that was designed to suppress RFI).
Back to reality - I measured the actual voltages of the power supply on my MX-113, and they were pretty much right on spec, and there is very little ripple on the 75V supply (the 10V and 9.5V are filtered derivatives thereof) - which would explain the lack of hum.
So - my main sonic complaints with this little gem seem to be some excess noise and and overall lackluster sound quality. Maybe part of what I am hearing is age, and part is inherent to the design.
I am guessing that some modern parts could help in the first case. I would be interested to hear what people more experienced in these matters would have to contribute to this pursuit.
Parts I am thinking of replacing:
All of the transistors in the preamp boards -
132-095 = KSC1845
132-096 = KSA992
132-092 = BC549
132-042 = 2N4401
These are subs I have read about on the forum. I would particularly like to hear feedback about these subs; are there commonly-available subs other than the ones listed that you prefer in this case, and why?
There are some electrolytics in the audio path used in emitter bypass and even feedback coupling functions. I would be interested to hear ideas about modern replacements for these (I am thinking something like the Elna SILMIC series?)
I am assuming for now the box film caps should probably be left alone - maybe not?
What about the encapsulated component clusters in the bass/treble section - should they be adapted over to discrete? The pinout should allow you to test the values of the internal components individually to check for out of spec/drifted values.
Perhaps I need to check all the values (R's, C's) for drift - I do have a reverse RIAA circuit somewhere I can use to plot the Phono section flatness - just haven't gotten around to it yet. I confess, all of my observations thus far have been admittedly quite subjective.
What other ideas does anyone have?
My appologies for the rambling post...
Regards,
Dave
Although the MX-113 looks really nice for its age and has most of it's functionality, I think there is something lacking to the sound. One thing I noticed immediately was the background noise level, even on line level input (white/pink noise, not so much hum). Not super-loud, but noticeable. I am thinking the old transistors might be contributing to this type of noise.
The other thing I noticed was the cross-talk of the CD that was still playing in the background when I switched over to the Phono 1 input. I imagine the selector switch is not providing a solid ground to the de-selected inputs so maybe some cleaning/Deoxit would help there.
The only function that seems not to work at all is when I switch the selectivity to narrow - the reception disappears altogether. I haven't explored this issue any further yet as I would probably listen in 'wide' even if the narrow setting worked, due to my lack of competing stations here in a rural area, and the typically better sound quality that wide usually offers. Eventually I would want to fix this issue anyway.
I downloaded a copy of the MX-113 service manual and took a look at the preamp section schematic. I have read that the MX-113 sends the FM Multiplex output into the Phono section of the preamp board. This is correct as far as I can see. The trick here is that when you select the FM input, one of the wafer switch sections is used to bypass the frequency-selective parts of the feedback loop around the phono stage with a resistor that lowers the overall gain and flattens out the response. I assume this is done because the output from the Multiplex section is too low to compete with line level sources and needs some flat boost.
I modeled the phono section with LT Spice and the response of the Phono preamp in the "flat mode" is flat within a dB from 20Hz to 20kHz and gain is about 20dB midband. Using a Laplace function to create a reverse-RIAA voltage controlled-voltage source as a simulated input, the response of the Phono section in Phono mode is correct within a few tenths of a dB across most of the audio band.
Interestingly, there is a serious out-of-band resonance peak around 500kHz that is due to the cumulative phase shift reaching 180 degrees inside the loop. It should be noted that I modeled the circuit with the added Service Bulletin components in place (the one that was designed to suppress RFI).
Back to reality - I measured the actual voltages of the power supply on my MX-113, and they were pretty much right on spec, and there is very little ripple on the 75V supply (the 10V and 9.5V are filtered derivatives thereof) - which would explain the lack of hum.
So - my main sonic complaints with this little gem seem to be some excess noise and and overall lackluster sound quality. Maybe part of what I am hearing is age, and part is inherent to the design.
I am guessing that some modern parts could help in the first case. I would be interested to hear what people more experienced in these matters would have to contribute to this pursuit.
Parts I am thinking of replacing:
All of the transistors in the preamp boards -
132-095 = KSC1845
132-096 = KSA992
132-092 = BC549
132-042 = 2N4401
These are subs I have read about on the forum. I would particularly like to hear feedback about these subs; are there commonly-available subs other than the ones listed that you prefer in this case, and why?
There are some electrolytics in the audio path used in emitter bypass and even feedback coupling functions. I would be interested to hear ideas about modern replacements for these (I am thinking something like the Elna SILMIC series?)
I am assuming for now the box film caps should probably be left alone - maybe not?
What about the encapsulated component clusters in the bass/treble section - should they be adapted over to discrete? The pinout should allow you to test the values of the internal components individually to check for out of spec/drifted values.
Perhaps I need to check all the values (R's, C's) for drift - I do have a reverse RIAA circuit somewhere I can use to plot the Phono section flatness - just haven't gotten around to it yet. I confess, all of my observations thus far have been admittedly quite subjective.
What other ideas does anyone have?
My appologies for the rambling post...
Regards,
Dave