Desjet,
I don't want to derail this thread but I'm interested in your Lexicon MC12 power supply substitution. I also have an MC12, it's the HD EQ version. The power supply failed about a year ago and although it was just out of warranty Lexicon did replace it for me at no cost.
I would be interested in knowing more about what you did. I thought the smps had lower voltages, + - 5volts, in addition to the +15 and - 15 rails. Can you add anything regards what you found regards the original smps output voltages?
Regards
Mike
I don't want to derail this thread but I'm interested in your Lexicon MC12 power supply substitution. I also have an MC12, it's the HD EQ version. The power supply failed about a year ago and although it was just out of warranty Lexicon did replace it for me at no cost.
I would be interested in knowing more about what you did. I thought the smps had lower voltages, + - 5volts, in addition to the +15 and - 15 rails. Can you add anything regards what you found regards the original smps output voltages?
Regards
Mike
So you are using LM431 as voltage reference? 11.92 V is 12 V -0.7% => pretty good.
Lexicon
Hi, the Lexicon MC12HD uses an industry SMPS see EOS Power - Standard Modified Customized Power Solution with +/-5V and +/-15(16)V
The audioboard in the Lex gets with this mod +/-15V for all analog circuits from the external power supply and the SMPS remains for for the digital circuits.
The first power supply is for a good friend for his Lexicon MC-12 (non HD Version). Now i'am building a second power supply for my own SDP-40HD (same as your unit).
Now after several hours i can say that soundquality of the Lexicon is much more improved that i thought is possible.
regards.
Andreas
Hi, the Lexicon MC12HD uses an industry SMPS see EOS Power - Standard Modified Customized Power Solution with +/-5V and +/-15(16)V
The audioboard in the Lex gets with this mod +/-15V for all analog circuits from the external power supply and the SMPS remains for for the digital circuits.
The first power supply is for a good friend for his Lexicon MC-12 (non HD Version). Now i'am building a second power supply for my own SDP-40HD (same as your unit).
Now after several hours i can say that soundquality of the Lexicon is much more improved that i thought is possible.
regards.
Andreas
Attachments
I forgot to say that you have to put a 150 Ohm loadresitor across the unused +15V rail to ground in order to simulate a minimal load for the SMPS (appr. 0,1A). I made this with a TO247 power resistor, there is an unused hole in the side panel
Attachments
Thanks again Andreas,
I wish I had found that smps power supply before.
The smps unit originally installed in my MC12 was obsolete and Lexicon would not tell me the model number of the replacement module nor supply it.
Although they did the work of replacing it and did some other upgrades under warranty the shipping to Texas and back from Alberta cost me quite a lot. I've bookmarked the EOS site for future reference.
Can you quantify or qualify the improvement in the sound? I know that when I started using the SSR03 for my Linkwitz LX521 ASP there was a noticeable improvement in the higher frequencies, much cleaner.
I wish I had found that smps power supply before.
The smps unit originally installed in my MC12 was obsolete and Lexicon would not tell me the model number of the replacement module nor supply it.
Although they did the work of replacing it and did some other upgrades under warranty the shipping to Texas and back from Alberta cost me quite a lot. I've bookmarked the EOS site for future reference.
Can you quantify or qualify the improvement in the sound? I know that when I started using the SSR03 for my Linkwitz LX521 ASP there was a noticeable improvement in the higher frequencies, much cleaner.
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Hello,
the sound improvement is really dramatic, not subtle. If you ever compared a Lexicon MC12 against a good two channel preamp you know that a Lexicon is able to and NOT.
For many years i used a good two channel preamp for music and the Lex only for homecinema. The Lex with L7 and over coax digital in is really fantastic, even the newer formats over HDMI (LPCM) don't give you such good results.
But for 2-ch. the MC12 isn't any special, even in bypass mode sondquality is beaten by every good preamp for a fraction of the price. The reason for this is primarily the SMPS - it reduces the soundquality dramatic - believe me.
If you look into the schematic you also will see nothing special in the audio path, there is a lot to improve for example better coupling caps, opamps etc.
My unit got a service some years ago and Chris from Pyramid Audio replaces for me the CS3310 volume IC's with better PGA2310 with good results. He made some measurings and the improvement was not subtle. I think the MC12 have a lot of unused potential.
I can send you schematic of your MC12HD if you wish by email.
Best
Andreas
Peranders,
A question on the best way to trim the voltages. I built 2 15 volt versions and the voltages were perfect. I have just completed a 12 volt version and the positive and negative voltages are 11.85 and -11.75 respectively.
What is the best way to trim these up to 12 volts? Which resistors should I change and in which direction, increased impedance or reduced impedance>
A question on the best way to trim the voltages. I built 2 15 volt versions and the voltages were perfect. I have just completed a 12 volt version and the positive and negative voltages are 11.85 and -11.75 respectively.
What is the best way to trim these up to 12 volts? Which resistors should I change and in which direction, increased impedance or reduced impedance>
A clear case of a too low input voltage. You can adjust the series resistors in the smoothing section and also remove the LM317/337 in order to gain some volts.
Hi Peranders,
The values of the resistors you listed are as per the BOM for the 12v 431 version. I did a check on those around both of the LM431's and I found that R39 and R40 were below the supposed value. I trimmed them and the voltages came to 12.002 positive and 11.950 negative, unloaded.
However when I loaded the supply, about 250ma, the negative rail collapsed to 4.5 volts, the positive rail stayed rock solid. With just a light load, about 20ma everything is fine. Obviously something wrong in the negative regulator.
The circuit that is being powered is fine, I checked with a bench supply and the current draws were both the same at 250ma on both the positive and negative rails. and the circuit behaves as it should, a pre-amp.
I'm going to go over all components in the negative side but any thoughts or directions would be welcomed. The transformer is a 100VA 18-0-18 so should be fine, its the same transformer I used for my 15 volt supplies which worked perfectly.
The values of the resistors you listed are as per the BOM for the 12v 431 version. I did a check on those around both of the LM431's and I found that R39 and R40 were below the supposed value. I trimmed them and the voltages came to 12.002 positive and 11.950 negative, unloaded.
However when I loaded the supply, about 250ma, the negative rail collapsed to 4.5 volts, the positive rail stayed rock solid. With just a light load, about 20ma everything is fine. Obviously something wrong in the negative regulator.
The circuit that is being powered is fine, I checked with a bench supply and the current draws were both the same at 250ma on both the positive and negative rails. and the circuit behaves as it should, a pre-amp.
I'm going to go over all components in the negative side but any thoughts or directions would be welcomed. The transformer is a 100VA 18-0-18 so should be fine, its the same transformer I used for my 15 volt supplies which worked perfectly.
Thank you Peranders,
I'll take those measurements later today and post them. The pre-amp board has quite a lot of bypass capacitance, a 1000uF at the supply point and seven 10uF caps per rail at each of the op-amps on the board. There are 2 boards, could this be sufficient capacitance to cause a start up problem?
I will also try turning on the SSR03 unloaded and then turning on the regulated output to the pre-amp boards.
I'll take those measurements later today and post them. The pre-amp board has quite a lot of bypass capacitance, a 1000uF at the supply point and seven 10uF caps per rail at each of the op-amps on the board. There are 2 boards, could this be sufficient capacitance to cause a start up problem?
I will also try turning on the SSR03 unloaded and then turning on the regulated output to the pre-amp boards.
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