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Lexicon MC-1
Analogue Bypass Solution by Shawn Fogg


Introduction

Lexicon Analog Bypass ProjectSome Lexicon owners would like to have an analog bypass feature in their processors, allowing a 2-channel input to be passed through the Lexicon directly to the volume control and then on to the power amp. This path would bypass the A/D conversion, all the digital processing (crossovers, EQ, surround sound) and the final D/A conversion. In other words the Lexicon would act as a traditional pre-amp. For vinyl lovers who just can't stand the thought of digitizing their preferred medium or for those with 2-channel SACD players the feature could be considered desirable.

The DC-1, DC-2 and MC-1 actually do have an analog pass-thru in the form of the Zone 2 output. With the proper system configuration the Lexicon will keep an analog input in the analog domain through the volume control and out of the Zone 2 outputs. The problem is this set of outputs do not run to your main L/R amplifier channels, use of the Zone 2 pass-thru would require you to manually swap cables around whenever you want to use the feature. Not something most people will stand for, which is why most Lexicon owners don't bother.

However, it is possible to make this area operation much simpler and in some setups completely seamless to the user. In other words the Lexicon will automatically switch to the analog pass-thru when you hit the 'Aux' input for example.

To make this work involves a few steps.

First we need to build a switch box that will switch the signal going to your L/R amp channels from the Main L/R outputs to the Zone 2 outputs. In essence this box will rewire your system as needed when you want to use the analog pass-thru.

The second step involves configuring the Lexicon to use the analog pass-thru for whatever input(s) you desire. This will tell the switch box to change inputs and will have the Lexicon mute the other outputs when the analog pass-thru is engaged.

Lastly, if you have a flexible programmable remote control (500T/700t, Pronto, etc.) you need to make a small change to its programming to make the pass-thru completely seamless to the user.


Bypass Box Construction

Lexicon Analog Bypass ProjectThis is a DIY project which requires some soldering skills but not much more. At your local Radio/Shack you need to pick up the following parts:

  • 4PDT relay Part #275-214 $6.49
  • Socket for relay Part #275-221 $1.99 (optional)
  • RCA jacks gold plated Part # 274-852 $2.59 (you need 3 or 4 packages... see below)
  • Wire 22 gauge 3 spools/pkg Part # 278-1221 $4.49

If you have a DC-1 and are not already using the trigger you need a 5 pin connector Part #274-003 $1.89. On the DC-2 and MC-1 you can use the bare wire trigger block or you can use this connector.

Suitable project box. The plastic boxes are easier to drill holes in and they isolate the ground of the audio cables. Pick whichever sized box you feel comfortable working in. The relay needs to fit inside and you will need to wire and mount at least 6 RCA plugs on the box and possibly 8 if you build all the features of the box. Bigger is easier to work in, smaller is easier to hide. I like the 6 × 3 × 2" enclosure Part # 270-1805 $3.69.

Readers based in the United Kingdom can substitute the following parts from Maplin:

  • RCA chassis mount plug red DE77J £0.99 each (need 4)
  • RCA chassis mount plug black FT95D £0.99 each (need 4)
  • 4PDT 12v DC relay QC82D £5.59
  • Wire red BL53H £1.29
  • Wire black BL46A £1.29
  • 5 pin din FM44X 0.89
  • Enclosure 152 × 89 × 47mm BZ74R £2.79

The Radio Shack relay above requires 12v at 130mA to operate. All but early production DC-1s can supply 12v at 140mA on the triggers so this stays within spec. If you have an early DC-1, check with Lexicon to make sure the trigger output current of your particular unit isn't limited to 100mA (you'll need your serial number handy when calling). The DC-2 and MC-1 both supply 12v at 500mA so they are fine.

Lexicon Analog Bypass ProjectIn the project box on one side we need to mount the 2 pairs of inputs to the switcher box. Drill 7/64" holes in the box in whatever location you are comfortable with. Mark one pair of holes as Main IN (Left and Right), and the other as Zone 2 IN (Left and Right).

Now we need to drill the holes for the output(s) of the box. If you do not currently use the Lexicons Zone 2 output, for a reason other then the analog pass-thru, and have no plans on ever using it you only drill 2 holes in the box for a single pair of outputs and therefore you only need to purchase three pairs of the RCA plugs listed above. Mark this set of holes MAIN OUT (Left and Right).

If you currently use the Zone 2 for audio in another room, or think you may use it in the future, drill two pairs of holes to mount the outputs. Mark the second set of holes Zone 2 OUT (Left and Right).

Next drill a small hole in the box where you can run wire out of the box to the programmable trigger on the Lexicon.

Lexicon Analog Bypass ProjectWe need to solder short lengths of red wire onto all of the inner posts of the RCA plugs. Make them long enough to easily reach the relay when both are mounted in the box with a little slack so we can reposition the relay if needed. This is easier to do before the plugs are mounted in the box. If you are planning on adding the optional Zone 2 output solder 2 wires to one white and one red RCA plugs.

Use some of the black wire to connect all of the ground rings together as shown. Try to keep their lengths as short as possible, just enough to reach between the mounts when they are installed in the box. Be sure to route the wires around the inner edges of the box in a neat manor so the relay, and its wiring, will fit in.

Lexicon Analog Bypass ProjectNow mount all the RCA plugs in the box. Be sure to keep L/R positions consistent. If you soldered two wires to a pair of RCA plugs then make sure these are positioned at the Zone 2 IN. Pass the red wires through the ground rings and nuts. After you tighten down the nut on the inside of the box it isn't a bad idea to apply a little nail polish or similar to the threads to keep the nuts from loosening up in the future.

Now comes the most difficult part of the project, wiring the relay itself.

On the package for the relay, or on the socket if you purchased one, you will see the 12 pins for the relay as well as the 2 coil connectors. The pins are numbered and need to be wired as follows to the red wires coming off the appropriate RCA plug. As you are going along be careful not to burn the other wires with your soldering iron and be sure you don't short across any of the pins. If you are soldering to the socket do not plug the relay in yet. The connections as are follows:

  1. Left Main IN
  2. Left Zone 2 IN (optional second wire)
  3. Right Zone 2 IN (optional second wire)
  4. Right Main IN
  5. Left Zone 2 IN
  6. no connection
  7. no connection
  8. Right Zone 2 IN
  9. Left Main OUT
  10. Left Zone 2 OUT (optional)
  11. Right Zone 2 OUT (optional)
  12. Right Main OUT

Triple check all your connections to be sure everything is wired properly.

Now you need to make the cable to run to the programmable trigger. Using the red and black spools of wire twist the two wires together until you have a couple of feet of cable. If you think the switch box is going to be located further away from your Lexicon make the cable longer. Cut the wire off and feed one end through the hole for it in the box. Feed a little extra wire into the box then take that wire and tie it in a loop as a strain relief. Now solder one wire to one side of the relay coil connector and the other wire to the other side, pins 13 and 14 on the socket. Polarity doesn't matter. If you used the socket plug the relay into it. You can secure the relay in the box with silicon glue, hot glue or similar.

Lexicon Analog Bypass ProjectThe pins on the socket can move around a fair amount so it would be a good idea to apply a liquid electrical tape to prevent them from shorting across each other. Wrapping each pin in electrical tape or using heat shrink tubing would work well too.

As all the internal work is done you can now put the top on your box.

If you are connecting the trigger cable to the 5 pin plug you need to connect one wire to pin 1 or 2 (both are grounds) and the other wire to pin 5. Again polarity doesn't matter. A pin assignment diagram is available here. Be sure you don't short across the pins in the plug. If you are using the bare wire trigger you connect one wire to ground and the other to the PGM trigger.


Connections

Turn your power amplifiers off. Turn the Lexicon OFF using the back panel switch. The Lexicon's Main L/R outputs plug into the switch box's Main L/R Inputs. Lexicon's Zone 2 L/R outputs plug into the switch box's Zone 2 L/R Inputs. The switch box's Main L/R Output plugs into your main L/R amps inputs. The switch box's Zone 2 L/R Outputs plugs into your Zone 2 L/R amp or is left unused. You can turn the Lexicon back on at the back panel now.


Bypass Box Operation

Lexicon Analog Bypass ProjectOK, so you have built the box. What is it doing? Basically, there are 4 switches inside, each switch can connect to one of two things. Pins 1, 5 and 9 make up the first switch. Pin 9 is the output of the switch which is what routes audio to you left channel amp. When no power is applied to the relay (normally closed) Pin 1 (L Main IN) is connected to Pin 9 and Pin 5 (L Zone 2 In) is floating. When 12v is applied to the switch Pin 1 is disconnected from Pin 9 and instead Pin 5 (normally open) is connected to Pin 9. Pins 4, 8 and 12 make up another switch which acts in the same way for the right channel.

The optional Zone 2 output switches are made up of Pins 2, 6, and 10 and Pin 3, 7, and 11. The Zone 2 input is on Pins 2 and 3 on the switches. When there is no power applied to the switch Pins 2 and 3 are connected to Pins 10 and 11 which are the Zone 2 outputs on the box. This has the effect off passing the Zone 2 audio directly through the box when we aren't using the analog pass-thru. In other words it lets you keep using Zone 2 to feed another zone in your system if you want. However when we want to use the analog pass-thru, Pins 2 and 3 get connected to pins 6 and 7 which aren't connected to anything. This basically mutes the signal to your regular Zone 2 amps and in combination with the first two switches re-routes the Zone 2 signal to your main L/R amps.... Presto! A true analog pass-thru.


Lexicon Programming

Now we need to set up some programming in the Lexicon. In the Setup Menu-->Input Config go through all of your inputs and turn TRIGGER to DISABLED except for whichever input(s) you want to use the analog pass-thru in.

For an example, we will set up the AUX input as an analog pass-thru input. After you have turned off the TRIGGER in all the other inputs go back into the AUX input configuration as we need to make a few more changes.

Most of the settings are unimportant but the following parameter adjustments are necessary.

Set the AUDIO IN to an unused DIGITAL input. If you do not have an unused Digital input set it to a PCM only input (CD, MD, etc...) and set FX Format to AC-3 or DTS only. If you do not do this when you select the AUX input (which will cause the switch box to change your inputs) you will still get output from the Center, Sides, Rears and Sub (main outputs) and you would have to manually reduce the master volume. By assigning an invalid digital input the Lexicon will Mute those channels automatically when you select the specified input.

Next, the Rec/Zone2 setting must be ANALOG. This is so the Lexicon will pass the analog input on the AUX input directly out the Zone 2 outputs.

The trigger must also be ENABLED. If you have several Inputs you want to use the analog pass-thru with configure them in the same way. Of course you must have an analog signal plugged into each of the inputs.

Lexicon Analog Bypass ProjectIf you are not using a programmable remote control you are done. When you want to use the analog pass-thru you simply press the AUX input on the remote control then Zone 2+AUX and the Lexicon and the switch box do the rest. The only additional step you must remember is to use the ZONE 2 volume control to control the output volume. On a V3+ DC-1 (or all DC-2/MC-1) remote you press and hold the Zone 2 button while using the volume key. From the front panel you press and hold Zone 2 and turn the volume dial. If you are only using Zone 2 for the pass-thru you can leave Zone 2 engaged on AUX. If not, press Zone 2+OFF to disengage those outputs when you are finished but see the warning below.

If you are using a good programmable remote, for example the Lexicon 500/700t, you can make the above process much easier. For the key to switch to the AUX input of the Lexicon program a macro which does the following:

Send Volume -30
Send Zone 2 Volume -30
Send AUX input
Send Zone 2+Aux input

Now on the volume keys in your AUX device page program them to send the Zone 2 volume control commands instead of the normal volume controls. This way the operation of the analog pass-thru will require no extra effort when you are using that remote. You lower both volumes upon entry into the analog pass-thru mode so you don't accidentally surprise yourself with a damaging full volume signal. Upon leaving the AUX device page repeat the two volume commands and turn off the Zone 2 output.


Warnings!

First, remember that all processing functions of the Lexicon are performed digitally. Our scheme bypassing them completely. In other words when you are in the analog bypass mode your main L/R speakers are going to be run FULL RANGE. If you have small satellite speakers be careful not to overload them with bass material and remember if your subwoofer is connected to the Sub output, it won't be active in the analog bypass mode.

Second, and most importantly, the system now has two volume controls. The main volume and the Zone 2/Analog pass-thru volume. Be careful not to switch between modes when either is above -30dB as you don't want to overload your speakers and potentially damage them. I highly recommend specifying a default power-up -30dB Zone 2 volume in the output levels setup menus. If you are able to program the macros listed above you should to be safe. Otherwise remember to turn down both volumes when changing modes, Zone 2 volume is retained until the Lexicon is placed into Standby or turned off completely.


© 2001.

 

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