Printed Circuit Boards for Moving Coil and General Purpose transformers



About Grounding

Good grounding is the key to a hum free MC system. In most problem cases, the hum signal level is in the range of micro Volts, and this is too low for measurements. So more effort should be invested in a good concept. Using this board will simply most of the common mistakes.

The board has a ground plane, which is electrically connected to the mounting holes (Screw holes) of the board, the ground pads and the transformer shielding internal and external shield.

EE21 Board

The EE21 Board is supplied with a "break point" in the middle. For a stereo version, like in a MC box, they can be left attached. For building into an amplfier, they can be broken off. The order number 310-27-80 is for such a double board. So for stereo only one board needs to be ordered.

There will be some ways of use:
  1. The EE21 board is mounted inside an amplifier. These will have a metal chassis, and the EE21 board is attached to this with metal screws In that case, no further grounding is needed for the board itself. For the input signal, we must keep apart symmetrical (XLR) and unsymmetrical (Coaxial Cable). If symmetrical (XLR), the input is connected to +In and -In, which are floating inputs. The shield of he XLR cable may contain some hum signal, and it is normally electrically grounded directly at the amplifier housing input. Yet for persisting rare hum cases, it may be tried to connect this to the (input) ground pad of this EE21 board, or not connect it at all. Though the amplifier housing is the right place for this. If unsymmetrical(Coaxial RCA plugs) the best way is to use an isolated ground RCA connector. This will seldom be mounted as a standard, so it needs to be replaced for this. The ground of this (isolated-ground) RCA connector is then connected to -In, and the signal wire is connected to +In. In case, an isolated ground RCA connector is not used, we have the more commonly used grounded type. This is not the best way to go, but it can be done. In this case, connect two (mildly) drilled wires to +In and -in, which will be connected to the RCA plug. +In is connected to the center, and -In is connected to the ground of the RCA plug. If for some reason the RCA plug does not provide a ground connection, proceed as follows: Connect -In to the ground of the EE21 board with a small piece of wire, and connect +In to the RCA plug.
  2. The EE21 board is mounted in a separate plastic or wooden box. In this case, the ground pads become the ground reference, since these are connected to the board ground plane, and any other grounding we do not have. If symmetrical (XLR), the two signal wires are connected to +In and -In and ground of the E21 board. It is best to (mildly) drill +In and -In. If unsymmetrical ( Coaxial RCA plugs), connect the RCA ground to -In, and the RCA signal wire to +In.
  3. The EE21 board is mounted in separate metal box. It is the same as above, but everything becomes less critical. The connection scheme is the same as under 1) above here.
The output signal of the transformer.

This by definition floating (Symmetrical), and this signal is presented to X+ and X of the EE21 board.. To continue with a floating (symmetrical) signal, the switches 5 and 6 are both set in the 'Off' position.

When a non-symmetrical (RCA, Coaxial cable) output is required, which is the most common application, either X+ or X must be grounded, and the other end represents the non-symmetrical signal. This means also inverting of the signal is possible. Though inverting is not a target by itself. In some cases, it may be needed to solve strange problems. With the Switch 6 (marked: "Norm") in the 'On' position, the output X+ is non inverting to ground, which is the normal setting.

With the Switch#6 'Off", and Switch #5 (marked: 'Invert') in the 'On' position, the Output X will be inverted phase to ground. Do not use this, unless there is a reason.

Switch Number
at '+' and '-
for XLR connectors.
Output at '+'
For RCA connectors. This is the most common way
Unsymmetrical Inverted Phase
Output at '
For RCA connectors. This not common.
Output shorted, for hum tests.
For fault finding only



Switch Number


Recommended damping potmeter: 100k, and series Resistor 5k. In case a potentiometer is undesired, but a fixed resistor is preferred, the potentiometer should not be mounted, and a piece of wire mounted at the potentiometer position instead. (This position is marked) A useful fixed resistor value is from 10k...100k. Without potentiometer, this must be found by experiment. By means of switch #4 the result can be heard.

Pot Meter
Resistor Resistor only
5k 47k


No part of the transformer specifications however, is the reaction of the head shell to more damping. This can hardly be specified, and can only be tested.



This is a very low distortion MC transformer, with mu-Metal Core.

Switch Number


LL1681. A damping network is not mentioned in the Lundahl datasheet. It appears LL1681 is fully linear until 20kHz, at no phase error. Above 20kHz, good functioning will maintain, but some roll of begins to appear at 30...40kHz unsymmetrical used. Damping will make the LL1681 fully linear up to 50kHz, however at the cost of amplitude loss. Meaning gain is not 13 or 26 any more, but less. So it must be carefully considered if errors above 30kHz must be avoided at the cost of gain. However it can be done. To measure the difference, switch #4 can be switched on or off. For LL1681 the damping network is recommended like: 100K potentiometer, 4k7 resistor.




Extra part. Some help with failure finding.

Sometimes, an external magnetic hum field finds it's way into the transformer core, which generates a hum signal. It would be essentially wrong to blame the transforme ot the cabling for this. As all magnetic fields, also a magnetic hum field, have a field direction. The solution can be to find the source (useually a low quality mains transformer or choke), and rotate the EE21 board for a minimum disturbance. Additionally the mains transformer or chokes could be turned to achieve a better result. Before turning the transformers, it should be checked if these are the cause. For this, take off the mounting screws of the EE21 board, and ground it by means of a ground cable, using the EE21 board ground pad. Disconnect X+ and X, but leave +Out and -Out connected. Now the board acts as a direction sensitive hum-antenna, and by rotating the board in all three (!) axis directions, the source of the hum field may be found. (So turn the board also on it's side, and then turn it around all directions). Then set both switched 5 and 6 the 'on' position, and when the hum is gone now, this more or less proves the transformer is just the receiving element, and not the cause of the hum.

The above pictue is from the documentation of the Hammond B3 Organ amplifier, and it shows the angled position of the mains transformer. Though this looks a bit 'old stuff' in fact this is an extremely well made transformer, and one of the best constructed music amplifiers there is. I own a B3 myself, and I took out the transformer mounting screws, to rotate it. This confirmed what I expected, the transformer is placed at such a position, there is no interference with magnetic pick ups of the tone generator. In the pictured position of the transformer, the organ is almost free of hum. In any other position I tried, more hum is radiated into critical parts. The output transformer is the smaller one in the middle. The box on the right is a capacitve (yes!) volume control, totally free of any wear out. Note, the mains transformer and output transformer are mounted on another axis. Just try to pencil in the 'axis' of each transformer, and you will see they are 90° off, which makes interference virtually zero. So try to see where the 90° is, despite the lef one is turned 45°. Once this is understood, where the 90° is, and what this does to magnatic radiation, observe some HiFi amplifiers of the type with visible transformers. You will see this small knowdledge is known to only few designers ;).