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EE31 Printed Circuit Board

Tweak #2 for Yamamoto amplifiers

We called this tweak #2 some years ago, because there was also a tweak #1, which however had to do with the power supply. So even when it's 'tweak #2' here, there is only one such a tweak.

 

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Cut the Ground Loop. Some Yamamoto amplifiers hum a little bit, so we use these here as an example Hum has to do with several factors, and it is not always possible to say, what exactly has to be done, and what exactly it will achieve. But several times I found a ground loop was the cause. Such a ground loop can not always be physically pointed at. A ground loop is always created when two pieces of equipment are grounded via the mains, and also their ground is externally connected via the shield of the tone cable. One way or another, there is now a physical loop in the room, created by all these ground wires. Now, such a loop technically is one winding of a transformer. This single winding is shorted electrically, because it is round going. Any magnetic stray field from mains transformers, will also stray into this loop, and if that happens, a small current will be generated, because that's what transformers so. You may think, it is only one winding, but it is a very large winding, several meters in length. What counts actually, is the loop surface, but that is quite large as well. So yes, as small AC current will flow, through this loop always, and here is the point: The tone cable shield is part of it, and that's where the hum is coming from. Since with an asymmetric (standard RCA) connector the signal is between center pin and ground. So when you have music signal on the center pin, and a hum signal on the ground connection of the RCA, you will hear both. Cutting the ground loop, in this case means disconnect the ground of the RCA cable. But with asymmetric (standard RCA) that will make it only worse

It is not recommended to remove the ground connection from amplifier and pre-amplifier, to achieve the same. People do it, but this is dangerous and illegal, and even so that will not completely cure it, since a capacitive path of the ground loop still exists. The one and only cure is to go for a symmetric input.

So how is that done? Some amplifiers have differential inputs. That involves a lot of circuitry. A much more practical way to eliminate a ground loop at the input, using an input transformer, because it has also differential inputs. For this I made this little board EE31. For a small cost, you have a good chance this solves the hum problem, and fur sure prevents it. As you can see from the picture it is amazingly small size, and it's mounted with a single screw.

Link to EE31 board

How to mount the EE31 board.

The principle for the this tweak is, to keep the original RCA connector of the Yamamoto, since it has already a floating ground type. So to say, we are almost done. We only need to re-wire it for the EE31 Board. If the connector in your amplifier has no floating ground, you need to replace it.

  1. For the Yamamoto, with a soldering iron, lift off the ground bar from the RCA input jack, but leave the ground wire undamaged and as it is, for the rest of it.
  2. Both terminals of the RCA jack are first disconnected, and it should be checked with an ohms meter vs. ground, if this is really so. Unplug the audio input cable for this!
  3. What is left now is only the center wire, which went to the RCA connector. Take it from here, connect the EE31 board, and it already works.
  4. For this, connect it like in the drawing. At the back side of the board are jumpers, configure for 1:1 by closing the according Jumper. Leave "Gnd" Jumper open. Solder the green drop capacitor, and an 8k2 resistor on the board (Or 10k if you have no 8k2 around)
  5. Since we mount the EE31 Board in the Yamamoto amplifier, most likely on a wooden part of the chassis, we have no chassis ground. For this reason, the Jumper Gnd can be left open. The Ground connection is now made, by connecting ''Out-'' to the ground bar, at the place where we lifter it off the RCA connector. So THREE wires go to the RCA connector (indeed!) and these have the same function as the three wires of an XLR connector. This method works elegant and mechanically easy. Mildly drill the two wires In+ and In-. Do not drill the ground wire with this.
  6. Configuration 1:2 is not advised in the first place, but can be tried later. This gives extra gain.
  7. You will see, this works quick, easy, and very good.

How to connect this with cables?

Actually there is little to know or do. There are two cases.

  1. If the pre amplifier has RCA connectors, just use any standard RCA cable.
  2. If the pre amplifier has XLR connectors, a special cable has to be made, once side XLR, one side RCA. The XLR cable is connected the normal way. The RCA connector is connected with the center pin to XLR2 and the RCA ground to XLR3. The cable ground (XLR1) is only connected at the XLR side.