EE31 Printed Circuit Board
for High Impedance Audio Input (LL1531)
for Medium Impedance Audio Input (LL1532 and LL1593)
This is the smallest PCB I ever made :) This takes very little space and quickly changes an unbalanced input to a hum-free balanced input. For a balanced input, an isolated ground RCA connector, or XLR connector must be used. Moreover, because it is a transformer input, the RCA and XLR can be simply connected in parallel, without loosing the balanced function. Despite low cost and small size, this little transformer works really excellent. Though pictured here with a lower cost ceramic capacitor, we supply the board with a 1.2 nF Foil capacitor included.
The EE31 Board has two applications:
1) High Impedance Audio Input (47k). This is the most used version.
Order# 218-350-19. LL1531 transformer + EE31 Board + mounting hardware.
Order# 218-405-99. LL1532 transformer + EE31 Board + mounting hardware.
Order# 218-438-79. LL1593 transformer + EE31 Board + mounting hardware.
In each application the board can be used with two gain factors, with solder jumpers.
For amplifiers, this is extremely useful, to create already at the input a gain factor two. Unlike with tube or transistors, we achieve it at this sensitive level, noise free and hum free. Moreover it breaks the ground loop, so any strange and hard to understand hum, may suddenly be gone as well.
For building it inside existing amplifiers, probably 1:1 is to be used.
The board in this picture has Jumpers J1 and J3 closed, for 1:1 configuration
- J1 + J3: for 1:2 Step up configuration
- J2 for 1:1 configuration
- J4 connects the screw terminal (See Picture) to Ground of the output. (So not the ground of the input which we recommend to keep floating). This is probably the most common way to use it. It means the ground level is now at the place where the chassis screw is. Like this, after connecting it, there remains only one signal wire, which is "Out+"
- If a ground connection is needed at another place like the mounting screw, leave J4 open and ground "Out-" at the preferred place.
- If a symmetric (floating) output is needed, leave J4 open, and use "Out+ and "Out-"
With both transformers, it can also be choose between 1: 1 transfer, or 1:2 Step up. This can be quickly done with solder Jumpers.
About LL1531, LL1532, LL1593.
Despite their small size, LL1531 and LL1532 are offering very high noise suppression, due to the electrostatic shield between primary and secondary, and the use of a Mu-Metal housing. (Mu metal is the best shielding material, but it is expensive).
LL1531 is the normal High Impedance Input type for HiFi Audio. This is the most sold version. Suited ideally for 47k Ohms input, doing simply 1:1 conversion for the signal, and no impedance change. It can do signal levels up to 10....20dbU, which is more then enough for any HiFi amplifier.
The official Lundahl data sheet writes, LL1531 is recommended only as a 1:1 device. So officially we say the same. But...when driving Ll1531 with a low impedance source, it worked very nice in 1:2 mode just as well, and no reduction of the frequency range at all. At least you could try it with this EE31 board, just set the Jumpers for 1:2. Generally though, the sensitivity of any pre-amp should be set as low as possible. When 1:1 works good, this is the best choice.
LL1532 is more a medium impedance type, intended for 600 Ohms source. This one can do officially 1:1 or 1:2
LL1593 is a low-cost version of the LL1532, with the same winding structure but without internal Faraday shield, and it has a lower grade metal housing. (no mu metal). This is for applications needing less noise suppression.
In order to remove self resonance ringing, a small RC network may be used. Though it works also good without, it is mainly needed when there is no, or a high impedance load.
Included if ordered with PCB Board option:
- High Quality "green drop" Foil Capacitor 1.2nF
- Mounting stud with 3mm thread, and one 3mm screw.
This is not necesary! Frequency range is very high and recommended termination is correct. These little transformers work so well. Yet for your interest and for fun, you can still do individual tuning of the RC network when using an oscilloscope. It has to be done with the intended load on the output, and the intended driver. To change the effect, change the value of R1 experimental. But you will notice, the recommended values are well chosen already. For tuning, use a 5kHz square wave, and tune for 10% overshoot on the edge at the output signal. This is so called crirical damping. Like this, frequency behaviour with a sine wave, will be linear best possible way, and phase error the smallest. Alternative you can of course also use a sine wave generator, sweep it through the intended frequency range, and judge the phase error and frequency behaviour like this. Do not go below 3Hz, because at some point, a very low frequency may as well be called a changing DC voltage, and this is not good for any tone transformer. But as said, this is for interested users only, and technically it is not needed.