Tweaking the QED MA34 Passive Pre Amp

Regular visitors to Hi-Fi insight will know already how much we rate this simple but outstanding CD passive pre-amp from our original review but what most people are not are aware of is how easlily and cheaply these amps performance can be taken to a whole new level.

The QED MA34 although very well made has two basic (cost driven) weaknesses namely the gold plated phono sockets and the PCB. By repacing the phono sockets and bypassing the printed circuit board and hardwiring the connections with high quality cable, the sound quality from CD to power amp is placed in a much higher echelon.

Valve enthusiasts have long argued that PCBs sonically degrade audio and have long preferred to use thick silver plated tag boards to securely mount and hard wire electronic components. If anyone needs a good example of how PCBs degrade sound quality then the QED MA34 is a good place to begin as it is such a simple PCB layout yet it adds distortion to the final sound quality without question.

For this project we are going to use 6 chassis mounted teflon insulated gold plated phono sockets and pure silver cable interconnect cable. I have tested the amp with silver plated PFTE cable and pure silver cable, the latter being slightly and subtely superior although both offer significant improvement compared to the PCB. Pure silver cable as you would expect is not cheap and the two core twin axial Shark Wire (#SL07282GB) weighs in at £20 per half metre from Maplin Electronics but is well worth it and no more than that length is needed to complete the project. The phono sockets can also be bought from Maplins and cost £2.13 each. You could use the silver plated variant if you can source them, but the gold plated sockets are very high quality and vastly superior to the original ones.

The first step is to remove the four long screws from the base and pull the volume knob off and carefully take off the top without damaging the input switch. The phono sockets are held in place by two small black screws. The PCB is mounted on four small screws. Once these have all been unscrewed the PCB can taken out of the case.

The next thing to do is to turn the PCB upside down and draw the circuit diagram on a piece of A4 paper so you understand the how it works and where to hard wire (it is very straight forward and will reduce the chance of any mistakes being made later). Once you have drawn the diagram, you should have something that resembles this.

The next step is to carefully unsolder the two pairs of phono sockets from the PCB. Then using a sharp knife, carefully scrape away about 5mm of the PCB circuit tracs immdiately around the 12 solder points of the switch and the Alps volume pot so that both are now isolated from the PCB. This can be a bit fiddly but if you are going to hardwire then none of the signal must on the PCB.

We then attach the 3 sets of phono sockets to the side chassis, the fourth set of unused holes can be blocked off using grommets or insulation tape. The ground connections of the three left and right plugs need to overlap each other so that one ground connection can be made to each of the left and right pre-amp grounds and provide grounding for all the other connections on that channel. It is important that these phono sockets are tight and secure so as to provide good grounding and will not work loose. No additional drilling should be necessary to mount the new sockets. Once the sockets are in place, some of the PCB may need removing so as it can be mounted in its original place. Use a hacksaw to facilitate this and there are some holes in the PCB to follow and help you.

The first conection to solder is the Left and Right switch output connections to the volume pot on the underside of the board. Cut about 2" of cable to do this and remove the two cores from the outer sheath of the silver cable as this will make soldering easier. Then we need to use two runs of cable to connect the volume pot to the Left & Right Signal output of the power amp sockets and complete the ground circuit. For each of these you will need 4-5 inches of cable leaving the sheath and shielding on the cable although not grounded but making sure it does not come in contact with the phono sockets or the volume pot output connectors.

The PCB can then be mounted in its orignal place and the silver cables soldered to the power amp phono sockets and the common grounds . All that then needs to be done on the top of the PCB is to solder the CD and pre-amp signal points to the switch using the points on top of the switch. Screw the top case back on and replace the volume knob and that is the project finished. All you need to do is reconnect the interconnects and be amazed at the improvement you have just achieved for £30 !

This entry was posted on Friday March 10th, 2006 at 12:30 PM and is filed under Amplifiers, Tweaks. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response.

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