I'm at the pointy end of a LA2A build for a family member, this is my first attempt at pro style audio gear most of my previous experience had been with guitar amps.
Most of the hardware for the build has been salvaged from old junked equipment, in particular the chassis comes from an old solid state PA amp, I also made use of the input transformer which has a 15:1 turns ratio. The transformer is potted in an aluminium casting and has no terminals but instead two short lengths of coax for the primary and secondary connections The secondary coax is straight forward with one insulated conductor and the earth braid - the primary however has 2 insulated wired protruding from the earth braid, measurements with an ohm meter show that the earth braid is the centre tap of the primary winding. The circuit shows that the braid(CT) connects to what looks like pin 3 of the input XLR which is then earthed and the two insulated conductors are connected to pins 1 and 2. I must admit that this has confused me a bit as I always assumed that pin 1 was ground with 2 and 3 as the active signal pins.
My question is do I connect the braid to pin 1 (which is my first instinct) or as shown on the circuit and connect the braid to pin 3 - there is always the possibility that as the XLR pins in the circuit are not numbered that what looks to me to be pin 3 is in reality pin 1 in which case my instincts are correct. This question may seem rather basic but as the finished product will be shipped off interstate and I doubt that the family member even owns a soldering iron I need to get this right before it leaves my hands.
For those that are interested the build went well, I used a Kenetek TB4 and wound the mains transformer and also the output transformer on my homemade coil winder that has automatic traverse, for the output transformer I stripped out the windings from a choke that had been pulled from an old junked Telefunken bit of gear, it was partially dismantled when it came my way so I'm not sure what the original use was. The choke used what I call M laminations and a neat bobbin made of flat pieces of bakelite that interlocked together similar to a puzzle toy. The choke was labelled EMT which I believe is the maker of better quality transformers so I assumed the laminations would be half decent.
The output transformer was a bit of an experiment but has turned out quite well - I'm not sure if I did this the right way but with a 560Ohm resistor and scope connected across the LA2A output the response is good from 25Hz and starts to fall off slowly at around 13.5Khz, the -3db point being around 17Khz.
Most of the hardware for the build has been salvaged from old junked equipment, in particular the chassis comes from an old solid state PA amp, I also made use of the input transformer which has a 15:1 turns ratio. The transformer is potted in an aluminium casting and has no terminals but instead two short lengths of coax for the primary and secondary connections The secondary coax is straight forward with one insulated conductor and the earth braid - the primary however has 2 insulated wired protruding from the earth braid, measurements with an ohm meter show that the earth braid is the centre tap of the primary winding. The circuit shows that the braid(CT) connects to what looks like pin 3 of the input XLR which is then earthed and the two insulated conductors are connected to pins 1 and 2. I must admit that this has confused me a bit as I always assumed that pin 1 was ground with 2 and 3 as the active signal pins.
My question is do I connect the braid to pin 1 (which is my first instinct) or as shown on the circuit and connect the braid to pin 3 - there is always the possibility that as the XLR pins in the circuit are not numbered that what looks to me to be pin 3 is in reality pin 1 in which case my instincts are correct. This question may seem rather basic but as the finished product will be shipped off interstate and I doubt that the family member even owns a soldering iron I need to get this right before it leaves my hands.
For those that are interested the build went well, I used a Kenetek TB4 and wound the mains transformer and also the output transformer on my homemade coil winder that has automatic traverse, for the output transformer I stripped out the windings from a choke that had been pulled from an old junked Telefunken bit of gear, it was partially dismantled when it came my way so I'm not sure what the original use was. The choke used what I call M laminations and a neat bobbin made of flat pieces of bakelite that interlocked together similar to a puzzle toy. The choke was labelled EMT which I believe is the maker of better quality transformers so I assumed the laminations would be half decent.
The output transformer was a bit of an experiment but has turned out quite well - I'm not sure if I did this the right way but with a 560Ohm resistor and scope connected across the LA2A output the response is good from 25Hz and starts to fall off slowly at around 13.5Khz, the -3db point being around 17Khz.
