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Printed Circuit Boards by Emissionlabs

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EE31 Printed Circuit Board, for L1531, LL1532, LL1593

EE20 BoardThis little transformer board works excellent, we use it as standard product to isolate HiFi signal inputs. Though pictured here with a lower cost ceramic capacitor, we supply the board with a audio grade. 1.2 nF ''Green Drop" foil capacitor.

Order Nr EE31 PCB: : 310-031-27

Transformer used:

Some EE31 Board Applications.

  1. Construct an XLR input, into an RCA input amplifier
  2. Construct a floating RCA input, into an normal RCA input amplifier
  3. Conversion of RCA input to XLR output.
  4. Dual Input XLR and RCA
1) Construct an XLR input into a normal RCA input amplifier

Purpose: Upgrade a standard RCA to a balanced XLR input. Eliminating hum loops in the interconnection cables.

  1. The center wire, which was previously mounted to the RCA connector, is now connected to Out+ of the PCB. Out- is connected to the signal ground.
  2. If there was a shielded cable connected to the RCA connector, the shield of the cable is connected to Out-, the center to Out+. After this is done, verify with an Ohms meter, if the cable shield seems grounded now. If yes, there is nothing else to do. This means, the cable shield is already grounded at the other end. If the cable shield seems floating, Out- has to be connected to the amplifier chassis. This is done by closing the ''Gnd'' bridge at the back side of the board. Use provisionally a piece of wire, to connect the screw hole to ground, when you have the EE32 board not attached to the chassis, but want to use it this way for testing.
  3. Make sure the mounting screw is grounded, either to the metal chassis directly, or with wire, in case of a wooden chassis. Check with an ohms meter for low impedance of the connection.
  4. Now connect the XLR connector to the EE31 board, as shown above.
  5. Mildly twist the wire pair connected to XLR2 and XLR3. Do not twist it with ground. The ground wire is connected untwisted to ''1"
  6. The metal cap of XLR connectors is usually kept floating, so not connected to anything. The cable ground is only soldered to Pin1, and not to the metal cap.
  7. Experiment with 1:1 or 1:2, what you like most. 1:2 gives more gain, but if this gain is not needed, 1:1 is probably better.
2) Construct a balanced RCA input, in an unbalanced (normal) RCA input amplifier

Click here for more info about this

3 Conversion of unbalanced (RCA) OUPUT signal to balanced (XLR) OUTPUT.

  1. INPUT: Connect RCA connector center pin to Plus (+), and RCA connector Ground to Minus (-)
  2. It is recommended to detach the RCA connector ground from the chassis, though this is not strictly needed. This requires an RCA connector type, with so called ''isolated ground".
  3. XLR ground. Attach a solder lug under the screw, and connect XLR Ground (1) to it.
  4. XLR 2 and 3, connect to + and -.
  5. Use 1:1 connection. Experiment with 1:2 connection.

4) DUAL Connector option (RCA and XLR at the same time)

Purpose:The amplifier already has a standard RCA input, which is mounted nicely, and it can be used together with the (new attached) XLR input. After this, alsp the RCA input will be balanced.

  1. In case the RCA connector was a ''grounded type", so the outside ring is grounded to the chassis, remove this connector. It must be replaced by a ''floating ground" type.
  2. Sometimes (or often...) a floating ground connector was already used, but the outside ring was grounded with a small piece of wire. This makes it easier. Just discconnect everything from this connector, and the connector can stay in.
  3. First connect the XLR connector like above, and make sure it is working. Connecting the RCA connector to it, comes as a second step.
  4. After the above is ok, the center of the RCA connector is connected to''2" of the XLR connector. The RCA floating ground ring is connected to ''3" of the XLR connector.
  5. Now you can use the RCA connector, or the XLR.

Examples

schematicThese are some examples of how the EE31 board could be build into an amplifier. For a multiple input amplifier you need only one board (per channel) provided you can switch the inputs with a double pole switch. Large drawing here.

 


Position of the board inside the amplifier

Since the input of the EE31 board is symmetric (also called balanced), placement inside the amplifier, length and path of the wiring becomes uncritical. This is the first advantage of balanced inputs. Yet it is good practice still, to mount any low signal devices not too close to mains transformers or chokes.

The EE31 board can be used with two gain factors, using the solder jumpers.

For building it inside existing amplifiers, 1:1 is advised.

For new to be designed amplifiers, it is very useful, to create already gain with a factor two, directly at the input. Unlike with tube or transistors, this gain is achieved noise free and hum free.

Background Information

With RCA inputs, unless a transformer is used, we have an unbalanced (unsymmetrical) input. This works free of hum in many cases, just not in all cases. In case there is some residual hum, it has most of the time of of the two following causes. One cause is, the signal cables may form a ground loop, which picks up hum signal, radiated by mains transformers and power supply cables. Also called a ground loop. Another cause is capacitive AC current, simply from one equipment to another. Mains transformers always have some capacitance from input winding to output winding, it is unavoidable. With the mains voltage across this capacitor, by definition a small AC current flows, which leaves the equipment via the mains ground wire. So this capacitive current flows from the hot side of the mains, to the safety ground of the mains. Since safety ground by law, must be grounded to the chassis as well, a small AC voltage on the chassis results from this. Theoretically this would play no role, as long as the safety ground of your house as ZERO volts on it, if the ground wires of your mains cables have ZERO ohms, and mains plug resistance is ZERO ohms. All of this does not apply, and so this capacitive current is going to have some effect. The equipment with the higher capacitive current, will build up a small AC voltage on the chassis. This is very little, but it is there, because we have no ZERO ohms at the above places. Equipment with a lower capacitive current will build a smaller AC voltage on the chassis. These voltages are small, but they are present. Now comes an issue, when we connect the one chassis to the other with a grounded RCA cable. This capacitively generated voltage between the two chassis, is now shorted by the RCA shield wire. Since this can not be a zero ohms cable, some voltage may result. In other words: Hum. So this is when people try to reverse mains connectors, work with gold plated mains plugs, best quality RCA plugs, etc. But it is only "dealing" with the problem instead of removing it.

WIth an XLR input, the situation becomes TOTALLY another. This bad situation, where signal wire is ground wire at the same time, is avoided with XLR, but it does cost another wire. Here, we have TWO high impedance inputs, and one ground wire. This ground wire, carries no sound signal, and the two signal wires carry no ground current. Problem solved! It is hum free by definition, and the standard in all professional equipment. Besides (don't tell the HiFi nuts...) contact quality of XLR cables is hardly important. So the cheapest XLR cable works better than the most expensive RCA cable.

An XLR input can be created electronically, these are two input lines which respond only to a differential signal. This can be done with tubes or transistors, but it increases costs very much. When the amplifier is already made, 99% have only RCA inputs, but we can create a real XLR input with an input transformer.

A third option would be to create a semi XLR input with a ''floating ground" RCA connector. This has all the virtues of XLR, and it means we need almost no change the looks of the amplifier, and there is no need to replace any RCA cables by XLR.

A fully universal amplifier can be made, by connecting an isolated ground RCA connector in parallel to the XLR.

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 quite expensive, so you find it only used with tone transformers).

LL1531 is a very small, but very high quality audio transformer. This is the normal High Impedance Input type for HiFi Audio, and the most sold version. Suited ideally for 10k...47k inputs. It gives 1:1 conversion for the signal, and does not change the original input impedance. So if the amplifier was 10k or 47k before, it will also be 10k or 47k after. It can do signal levels up to 10....20dbU, which is more then enough for any HiFi amplifier.

Use LL1531 1:1 or 1:2. 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 low impedance type, intended to be driven from a 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. We do not recommend it, but it is intended for high volume, very price sensitive applications, in a low-hum environment.

Some final hints:

 

Resonance tuning

Use of the small RC network is recommended. Though it works also without, it widens the frequency range, to a fully linear characteristic up to 25kHz for the high impedance transformer LL1531, and 50kHz for the medium impedance transformer LL1532. The only thing you need to do for this, is attach the ''green drop'' audio capacitor, which we supply with the board, and use a resistor of 8k2 for LL1531, or 2k7 for LL1532.

1. Important. When experimenting with the board, provisionally connect a wire from the screw hole, to the chassis. Otherwise the transformer shield is unconnected, and the board will not function as well as it can.

2. Important: Use no Ohms meter to measure the windings! Such meters use 1mA test current. Even when 1mA seems low, this will create excessive magnetism due to the very high number of windings. This would magnetize the core permanently, and the transformer would distort afterwards. If there is any concern about an 'open' winding, it is the wrong approach trying to find this out with an ohms meter. There will be no open windings or shorts with a Lundahl transformer! If you suspect this, the mistake is in the wiring somewhere.

Use of the "Gnd" Link on the PCB

Normally the "Gnd" link is left open. It connects the Signal Ground to Chassis ground, which connection is always made in every amplifier. The question is only WHERE it is made. In special cases, where it is desired to have to connection directly near the input transformer, insert the Gnd link. This presumes of course, this connection is not made at another place in the amplifier as well. Otherwise you would create a ground loop, for no reason. So use this option only if you want to do so. Otherwise, leave the link open .