Introduction: AT-LP120 Turntable RCA Mod and Direct PHONO Out

By Barney's website

The AT-LP120 is a direct drive turntable manufactured by AudioTechnica. Although it looks exactly like the very popular high end Technics SL-1200 turntables, one must not forget that this is a 300$ budget turntable. However is one of the best for it's price range:

  • it has a removable cartridge, therefore you can upgrade it later with a better one.There is no such thing as 'best cartridge'. There will always be a better - and more expensive - cartridge that will perform better than the previous one. However, in order to get the best music experience, the whole audio chain (cartridge, preamp, amplifier, speakers and the cables!) must also be suitable, as well. This turntable has a built-in preamplifier and RIAA corrector, therefore you can connect it directly to the LINE INPUT of your receiver/amplifier. Note: Modern amplifiers/receivers do not have PHONO input stage. The output of the cartridge is very weak compared to a LINE level - millivolts versus hundreds of millivolts, therefore it must be pre-amplified to LINE level. Also , RIAA correction (sort of equalization) must be applied to the input. This is necessary, as vinyl recording is made with the low frequencies reduced and the high frequencies boosted, and on playback the opposite occurs. The net result is a flat frequency response, but with attenuation of high frequency noise such as hiss and clicks that arise from the recording medium.
  • you can adjust tonearm height, tracking force and anti-skating. A turntable, unlike modern digital music is a fully analog device, as the records store the music in concentrical grooves (not that you didn't already know that) and the stylus tracks the grooves on the record: this causes vibrations that are translated to electrical wave-forms via 2 moving magnets inside the cartridge. In order to allow they stylus to accurately track the grooves of the disc, one must make sure that proper force is applied to the record (usually 1.2 g - 2.5 g). This is called vertical tracking force, or VTS. Each cartridge has the VTS specified by it's manufacturer. If you track the groove with a too high force, it will cause premature wearing of your discs and to the stylus, and also the sound quality will not be the best. A too weak force will also get you to poor audio quality and sometimes the stylus will not be able to track the grooves of the record and will lead to skipping. Therefore a VTS is mandatory to be set properly. Anti-skating is also important: as the record rotates, the tonearm will be dragged towards the inside of the record. To compensate this, there is a spring that will hold against it, dragging the tone-arm towards the outside of the record. The best is when the force applied to the arm is balanced, therefore it won't apply a bigger force to one channel than to the other. Also, just to note here is important to have a good cartridge alignment, this means that the stylus has to track the outer side of the record in the same way as the inner side.This can be done with a proper protractor, suitable for your turntable's geometry.
  • you can digitize your records, as it has an USB connector on the backside. Connect it to a computer and it will act as an audio device, allowing you to record the discs.

The purpose of this instructable is not to explain how to do the adjustments of the turntable, but I wanted to note that one must pay attention to these before even placing the first vinyl record on it.

Step 1: Introduction

Now, like stated before, the AT-LP120 is a good turntable. If you search for the best budget turntable, most of the websites will list this one as the best, but due to it's price range, it has also some flaws, that can be corrected with some additional effort (and money). Some people blame it calling it "full of plastic", "rubbish". Don't listen to them. This turntable looks and also can share some components with the Technics SL-1200, therefore people are usually comparing it with this one. But the Technics is another class: that is a high range audio equipment and costs more than 1000$ also when buying it used. (Being discontinued, you will only get used ones).

What can you do then, to improve your turntable?

  • Replace the cartridge. The at-lp94 cartridgeis a very good budget cartridge that will provide you great sound quality. Like said earlier, there is no such thing as 'best cartridge', you can always buy a better one that will have 'better sound quality'. Before spending too much money on it, a little bit of advice: it's important to have the whole audio equipment at the level of the 'cartridge'. There is no need to invest a lot of money in a cartridge when you have poor speakers or amplifiers. Do the investment gradually. A good cartridge would be Ortofon's 2M series, the Red one costs around 150 $ and will greatly improve the sound quality, if you're using an external phono preamplifier (More on this later.). This cartridge line also has a blue, a silver and a black cartridge, each of them will improve the sound quality more and more.
  • Get yourself another mat. A cork or rubber one will be better, as it will eliminate static better. Don't forget to adjust the arm height after you raise or lower the disc's place by adding/replacing the mat.
  • Use an external pre-amplifier. Although this turntable has a built-in pre-amplifier, due to it's price range, this one is not one of the best one. Using an external one will improve the sound quality. Phono pre-amplifiers also have a wide price range. You can get a good one around 150$ (I'm using a NAD PP2e) that is build with transistors, however the audiophile people are always using gear that was built with tubes. These have lower noise than ones with transistors, however they also cost more. Like I've said before, do the upgrades gradually. Now we've come to the problem itself:

In case you wish to use an external Phono pre-amplifier, there is a switch on the back of the turntable that can bypass the built-in pre-amplifier and allow you to use an external one. This sounds good, however the guys at AudioTechnica made a little mistake when designing the PCB: Also in case you switch to PHONO, bypassing the internal pre-amplifier and RIAA corrector, there's still an LC filter applied to the input. This filter is necessary to elliminate turntable rumble, the problem is that also the external preamplifiers will have this filter built-in. Applying it twice to the signal will result in a very mudded sound, even with a great cartridge.

The internet is full of instructables in how to remove the PCB board completely.

Here's one explaining it in detail:

However, if you remove it completely, you will also remove the USB out, therefore you won't be able to digitize your records.

My idea was not to remove the internal pre-amplifier completely. Instead, I've added another switch that will really bypass the internal circuitry completely, routing the signal from the cartridge directly to the RCA outs on the back. When the switch is moved to the other position, the signal will go from the cartridge into the original PCB, therefore you won't lose the possibility to use the internal pre-amplifier or the USB output.

Step 2: Disassembling the Unit and Locating the PCB

First, before even touching any screws, make sure that:

  • you've removed the cartridge, by unscrewing it from the tone-arm.

    and store it somewhere safe during the operation. Also, place it's protective cover on it .

  • Secure the tone-arm
  • Remove the platter and mat
  • Place a soft material on a desk and carefully turn the turntable upside down and place it on it (I've used a soft pillow for this)

Now, before removing the screws, watch this video and also the picture attached to this step:

Please note, that underneath the screws they've placed some metallic placeholders, that will fall out, or worse, fall inside the turntable once you remove the back panel. It will be very difficult to put it back together, unless you glue them with super-glue or something similar to hold them in place. I've marked these with red in the attached pictures. Also, locate the PCB where the 4 wires from the cartridge are connected. This corner of the PCB is only used to connect together the 2 ground wires coming from the cartridge, and then the grey cable will carry the signal further to the pre-amplifier PCB (the one a the bottom-centre). Also, please note in the attached picture the 3 wires from the grey cable: Red is right channel, White is Left channel and black is Ground.

Step 3: Drilling the Holes

Before drilling the holes, you will have to carefully check and calculate where you can fit all the connectors:

  • the 3 pole, ON-ON switch that will be used to switch the Left and Right signal between the PCB and direct RCA out connector
  • the2 RCA connectors for the direct out
  • the ground wire (needed to connect together the grounds of the turntable and the external phono pre-amplifier to eliminate ground loops and possible hum.
  • [OPTIONAL]: I've eliminated the original cable, and instead used also 2 RCA connectors, in case you still want to use the built-in pre-amplifier circuitry.
  • Use tools suited for woodworking when drilling. These have a sharp edge, and will help you do the job easier.

Step 4: Connecting the Original Output to RCA Connectors

When Audio Technica designed this turntable, they've tried to keep the costs down, but still providing a well built product. Why did they eliminate the RCA connectors and used a direct cable that's not replaceable is beyond my knowledge, but so they did. Also, this cable is not one of the best one, so, if one would like to use a better cable to connect the turntable with the amplifier (or external phono pre-amp), he just can't, and has to stick with the provided cable.

To eliminate also this problem, I've cut the original cable and used 2 additional RCA connectors. This way you can use your own cable also in case you would like to use the internal pre-amplifier circuitry.

Step 5: Soldering the Switch and the 2 RCA Connectors

As a first, you'll have to de-solder the red, white and black wires from grey cable that goes from the corner of the big PCB to the pre-amplifier board. (This cable connects the signal from the tone-arm with the internal pre-amplifier board). To the same place you will have to solder a new cable, preferable a shielded cable to carry the Left and Right channels to the switch that we will add to the back panel. One side of the switch will be connected to the RCA connectors. This way, if the switch is in this position, the signal from the tonearm (cartridge) will be connected directly to the RCA connectors, bypassing the pre-amplifier PCB board completely. The grey cable that was de-soldered earlier will be soldered this time to other side of the switch. If the switch is in this position, the signal will be routed via the original (grey) cable to the pre-amplifier PCB board. Therefore, it's possible to use the internal pre-amplifier and also the USB part.

The ground connection from the corner of the big PCB board has to be connected together with the ground poles of both the RCA connectors and also with the ground wire of the grey cable that originally connected the big PCB with the pre-amplifier PCB board.

To better understand the connections, I've created a small wiring diagram. Hopefully it helps.

Step 6: Adding a Ground Wire and Connector

As the external phono pre-amplifier is outside the turntable, it could be supplied from a different socket. To prevent so-called ground loops and to eliminate the 50hz hum or other interference caused by ground level differences, the ground of the external phono pre-amplifier must be connected via an additional wire with the ground of the turntable. Originally, AT did not provide such a connection possibility even in case PHONO level output is being used, but it's advised to do so. Also almost every external pre-amplifier will have such a connector on the back.

Step 7: Testing the Connections

Before you re-assemble the back panel, it's good to use a multimeter and check the signal path in both switch positions. Make sure that Left and Right channels have continuity and also check that the Left, Right and GND lines never connect between each other.

In case everything is ok, head to the next step.

Step 8: Assembling the Turntable

Now here comes the most difficult part of this instructable: re-mounting the back panel of the turntable. If you remember, at step 1 I've told you that AT has used some metallic spacers between the back panel and the front panel. Why on earth did this, I have no idea, but this makes it very difficult to re-assemble the unit: these spacers will always fall out, or even worse, fall inside the turntable and you'll have to start over. After almost an hour spent trying to carefully put the back panel in place, I've glued these tiny spacers to the pillars on the front panels. Still, you will need to take great care when putting the back panel in place, and also make sure to check in the end that all of these metallic spacers are in still in place (you can see them if you look inside the holes where you'll put back the screws).

If everything is in place, carefully turn the turntable over, re-attach the platter, the mat, the cartridge, measure the tracking force again, as it could have moved during the operation and place a record on it to try out your work.

Step 9: Conclusion

If you did everything right, after making the connections, you should have a more vivid, more "bright" sound when you bypass completely the internal pre-amplifier board. Check carefully that both channels work. Also, try out the other RCA connectors, where the signal still goes through the pre-amplifier board. If you're still using the external phono pe-amp, make sure that the original LINE-PHONO switch is in PHONO position, otherwise the LINE-OUT level will be too high for the external pre-amplifier.

Congratulations!

With this mod, you noew have a better turntable. You can use high quality RCA cables and you'll be able to use the cartridge as it was originally intended. The turntable won't alter the sound in any way in case you're using an external pre-amplifier. Also, if you decide to use the USB connection or the internal pre-amplifier, you can still do it.

https://photos.app.goo.gl/kEsqG3VEnQJ4iQV5A

Disclaimer: I've included some youtube links. These were not made by me, but I've considered they contain useful information that will provide help in understanding why this mod is needed and also provides information regarding how to disassemble the unit.