How to change internal preamp on AT-LP120 USB (and improve sound...) !!!

FileFixer

Well-Known Member
Hello again...
Before I did one useful guide about your AT-LP120 USB turntable to be more dead on vibration.This guide you can find into this DIY section on AK or just simple click on this direct link...
http://audiokarma.org/forums/index....lp120-usb-turntable-and-all-other-too.799039/

Now I want to show you how to improve sound on AT-LP120 USB... This is not simple to do but if you have any experience with soldering and you have time and good nerve then you can to improve this turntable to be very nice and decent with pleasure sound then "plastic junk" as many people say.
My statement about AT-LP120 USB is then this turntable have many manufacturer's mistakes but this turntable have huge space to be great turntable...
So, how to do this, I will to explain... with many pictures. ;)
For this modification I try to found some preamplifier what have similar dimensions to fit inside turntable on same place where is located old internal preamplifier but with relative good sound and I found that Audiowind A-300 satisfies my criteria for dimension, sound and future modifications on it. It`s based on OPA2134 Burr-Brown Op-Amps.

AT_LP120_USB_Preamp_mod_01.jpg


Basic schematics for complete connections looks like this...

AT_LP120_USB_Preamp_mod_00.jpg


First you must to remove headshell, counterweight, platter and 78RPM adapter from turntable. After that you must to secure tonearm and all upper parts on the turntable.
I`m pinned dust cover and turntable together with scotch tape around dust cover and edge of the turntable.
Pictures show how to use scotch tape to do this (not AT-LP120 on this pictures)...

Dust_cover_secure_01.jpg


Dust_cover_secure_02.jpg


Prepare some soft cloth as a base because it will turn turntable upside down on its dust cover.
When you prepare your turntable and workspace then you can to remove back cover from it. Unscrew eight screws, labeled with arrows on back cover plastic and easy remove back cover.
Be careful of the small rollers that are inside of the screws holes !!!
After you remove back cover, you can to start to modify your AT-LP120 USB turntable...
Lets begin...

First unplug any connectors from internal preamp then cut ground wire what come from GND spring. Also you must to desolder three wires on controller board where is connected GND shim from tonearm. These three wires is connected on 2,3,4 pins from left side shown on picture. On the right side from shim is wires which comes from headshel and tonearm and that wires dont need to desolder from board, only wires on left side what goes to internal preamplifier..

AT_LP120_USB_Preamp_mod_01_1.jpg


Then place new preamplifier when it has been him future place to see how can fit and how can be attached. I am attached preamp on that place with scotch tape but you can find any other way to fix it on that place.

AT_LP120_USB_Preamp_mod_02.jpg


Remove new preamp and back to another task... power source...
For that purpose I`m use +/- 18V 500mA transformer and Velleman K8042 power supply kit.
My idea was that this power supply must be on power when put power switch to ON and power off when power switch put to OFF position, so we must to find points where curren comes after power switch and this is point where we must to connect transformer. This place is located on little electronic board near existing transformer.

AT_LP120_USB_Preamp_mod_03.jpg


AT_LP120_USB_Preamp_mod_04.jpg


This picture shows where is placed that two poins where current goes to transformer (DANGER high voltage 110/220V !!!)

AT_LP120_USB_Preamp_mod_05.jpg
 
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Next problem goes with new transformer and its place but I decide to place it somewhere in oposite side from tonearm to be less affected by the signal path. Also you need to find some transformer with proper dimension there not higher then 45mm. If transformer has to be higher then 45mm then is not chance to put them between back cover and front plastic. this 45mm is higher space what I measure. No more space then 45mm.
This specifications satisfied this transformer as an example...
Myrra transformers ref. number 44323/117V or 44311/230V

Mirra_2x18_V_Trafo.jpg


And this is place where it placed inside turntable... I`m use double adhesive construction tape to fix it transformer to the plastic and scotch tape to isolate contacts on it.

AT_LP120_USB_Preamp_mod_06.jpg


AT_LP120_USB_Preamp_mod_07.jpg


Only free place where power supply (Velleman K8042) may to stay is here... Dont forget to secure power suply on this place. I`m use previous double adhesive construction tape to fix it under it.

AT_LP120_USB_Preamp_mod_08.jpg


AT_LP120_USB_Preamp_mod_09.jpg


AT_LP120_USB_Preamp_mod_10.jpg


AT_LP120_USB_Preamp_mod_10_1.jpg


Before fixing power supply to this place I decide to add two additional capacitors on it for better current filtering and lower ripple on power supply output. This is also only place where it can to fit.

AT_LP120_USB_Preamp_mod_10_2.jpg


AT_LP120_USB_Preamp_mod_10_3.jpg


When is all connected and secured on its place, it looks like here...

AT_LP120_USB_Preamp_mod_11.jpg


AT_LP120_USB_Preamp_mod_11_1.jpg


AT_LP120_USB_Preamp_mod_11_2.jpg


AT_LP120_USB_Preamp_mod_11_3.jpg


AT_LP120_USB_Preamp_mod_11_4.jpg


How all this work you can see on this video...

 
Next work to do is to find place for RCA connectors and switch what we uses to select between internal and external preamplifier...

AT_LP120_USB_Preamp_mod_12.jpg


AT_LP120_USB_Preamp_mod_13.jpg


AT_LP120_USB_Preamp_mod_14.jpg


Switch what I use is simple 4PDT switch...

4_PDT_Switch.jpg


And all connections where wires on new premplifier must be connected on this connectors...

OPA2134_PA_preamp_connections.jpg


On the next pictures you can see how my wire connection looks inside turntable...
For all interconnect wires inside turntable I`m use Tasker C-286 cables with 160pF/m capacitance but late I find Tasker C-121 cable with 96pF/m and change all interconnect cables with lower capacitance cables. You dont needd more than 45cm long cables per channel if you have good path plan for internal cables. in that case you can get it no more then 60-70pF overall capacitance to the end of RCA connectors and preamp inputs.

AT_LP120_USB_Preamp_mod_15.jpg


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AT_LP120_USB_Preamp_mod_18.jpg


AT_LP120_USB_Preamp_mod_18_1.jpg


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AT_LP120_USB_Preamp_mod_18_3.jpg


AT_LP120_USB_Preamp_mod_18_4.jpg


AT_LP120_USB_Preamp_mod_19.jpg


So, when we finish all that hard work you have new dimension on your speakers and your can to screw back panel on turntable...

AT_LP120_USB_Preamp_mod_20.jpg


AT_LP120_USB_Preamp_mod_21.jpg


I hope this guide will help to improve your "plastic junk" to be very nice and decent turntable with more then very good sound. OPA2134 have warm and pleasure sound and many users prefer op-amps with "tube" sound touch. If you need to tune your cartridge with different resistance on inputs in that case you can to change resistors with R5 and R6 mark on preamplifier board.

Best Regards,
FileFixer
 
Before few days I`m go to do some improvement on this modification...
So, what is reason to do this?! The reason is that this modification have ground loop and too much noise if volume potentiometer be on maximum on power amplifier, to be precise something about 3/4 or 4/5 at maximum scale on amplifier volume.
For that reason I`m go to resolve this and see how to shut off these iritating sound when I have unusual requirements for the volume of the sound.
First I want to say that Audio Technica or maybe Hanpin decide to put internal preamplifier and some othes components in totaly wrong place, eg. Internal preamplifier is close to AC inlet, transformer and some transistor who have more radiation from itself then Chernobyl nuclear power plant.
After few hours to determinate where is problem and after use not too much standard procedure from electronics, finally I do this completely and shut these sound forever from my TT. Now this is quieter then dead itself. :)
After all I measured where is placed floor noise from RCA outputs on TT and that noise is under -96dB. Now what I only hear is this quiet noise when is volume under 9/10 on volume scale, ground loop is gone forever and I dont hear it at maximum volume on my amplifier. Anything else is the same from preamplifier, transformer, power source and cables. Trust me, this is only place where I can put this components without any usurpation of unnecessary signals.
What I make you can to see on pictures... Any question are welcome.

Preamp_mod_1.jpg


Preamp_mod_2.jpg


Preamp_mod_3.jpg


Preamp_mod_4.jpg


Preamp_mod_5.jpg
 
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