"Balanced" speaker outputs

[Dr. Toobz]

I'm interested in trying out the "balanced" speaker arrangement Doc has mentioned but have no real need for impedance switching boards, since I've settled the question of what taps to use with my speakers. (4 ohm seems best). I do not understand the wiring of these new OPT's, as they do not appear to be labeled. If I want to make a virtual ground with a 70 ohm resistor going from the center tap of the secondary to ground, which pin represents the center tap? Then, which pins would I take to the speakers for 4 ohms? 8 ohms?

Thanks in advance - I'm sure others have this question as well!

 

[Paul Birkeland]

There may or may not be a center tap.

Create one virtually by using one resistor from each speaker leg to ground on each channel.

-PB

 

[Doc B]

Balanced connections are about making sure the impedance at each leg (hot and cold, or signal + and signal -) is equal, relative to ground - nothing more, nothing less (this is not differential, push pull, etc., etc. ad nauseum). If you want to make a "virtual ground" (we are really talking about a virtual center tap) connection, that implies that there is no center tap to connect to ground. The 70 ohm resistors would go from each end of the secondary to ground, and that creates the virtual (grounded) center tap. Thus, if either end of the secondary is currently connected to ground that connection must be lifted for this to work.

 

[Paul Joppa]

Doc B said it, but in more general terms. Here's something more specific:

Wired fro 4 ohms, there are two secondary windings in parallel. They are tapped, but the tap is not in the center so it's no use for balanced output. Here's the best way:

===============

You will need four identical resistors; 70 ohms is a reasonable value - I'd say anything from 50 to 100, but they have to be all the same. They should be rated 1 watt or greater. I think 5% is good enough.

From each set of speaker connectors, there is a wire from the black speaker terminal to safety ground. Replace each wire with a resistor. Add a resistor from each red speaker terminal to the same safety ground point.

That's all. You may need an additional solder lug to get all those resistors in place. Remember that the safety ground is solidly connected to the chassis, which will absorb a lot of heat from the soldering iron, so there is danger of a cold solder joint unless you apply a very hot iron or hold it there for a while. If the resistor body is closer to the speaker terminal, the the resistor won't get as hot while soldering. Fortunately, resistors can take very high temperatures.

 

[4krow]

This all sounds intriguing to me. What benefit would come from this arrangement?

I did read once about a company that was taking the output from a phono cartridge(balanced by it's very nature), putting it into a balanced headphone amp, and rewiring headphones to be a balanced connection. This was to be the ultimate-fi.

 

[Grainger49]

  .  .  .    You will need four identical resistors; 70 ohms is a reasonable value - I'd say anything from 50 to 100, but they have to be all the same.  .  .  .

Use your meter.  Hand match pairs of resistors.  For instance, if you decide on 70 ohms if you get two that are 69.9 and two that are 72.5 they are matched pairs.  All four shouldn't need to be the exact number, at least I don't think so.

If your meter is off by 5 ohms it doesn't matter.  They will still be the same resistance, just a difference number in actuality.  The resistors will be matched to the full resolution of your meter.

You will have fixed the two voltages symmetrically around ground.  

Does that make sense?

 

[chard]

This all sounds intriguing to me. What benefit would come from this arrangement?

According to the Bottlehead discription for the Impedance matching selector switch, tighter more dynamic bass.

 

[4krow]

Thank you Cliff. It impresses me how such a seemingly simple arrangement can alter the sound. Since my amp is not BH, I will have to think this over a bit before making a final decision. Just like with my bass amp, that I have heard can be converted from one class of operation(A/AB), to another(A), some stuff that I have done in the past has become 'permanent' if you know what I mean.

 

[chard]

Doc B said it, but in more general terms. Here's something more specific:

Wired fro 4 ohms, there are two secondary windings in parallel. They are tapped, but the tap is not in the center so it's no use for balanced output. Here's the best way:

===============

You will need four identical resistors; 70 ohms is a reasonable value - I'd say anything from 50 to 100, but they have to be all the same. They should be rated 1 watt or greater. I think 5% is good enough.

From each set of speaker connectors, there is a wire from the black speaker terminal to safety ground. Replace each wire with a resistor. Add a resistor from each red speaker terminal to the same safety ground point.

That's all. You may need an additional solder lug to get all those resistors in place. Remember that the safety ground is solidly connected to the chassis, which will absorb a lot of heat from the soldering iron, so there is danger of a cold solder joint unless you apply a very hot iron or hold it there for a while. If the resistor body is closer to the speaker terminal, the the resistor won't get as hot while soldering. Fortunately, resistors can take very high temperatures.

I just did this mod. played Duke Ellington Film Score "Ananomy of a Murder" and the song from Star Wars were Hans Solo and others are in an alien bar listening to a 4 or 5 piece alien jazz band. The bass was better, tighter and more articulate. Sounded like an acoustic bass. The piano and a deep soulful violin solo also sounded really good. Everything sounded better then I remembered it sounding. In some parts of the Duke Ellington album I could feel the large room the album was recorded in, the thing that gave the size of the room away was the natural room echo that came out in the recording. It didn't seem like reverb or something they were trying to contrive but the result of instruments playing in a large room. I also did another mod based on another thread, so some of this improvement could have been because of that mod. What I did was switch around my interconnects between my seduction and my stereomour to get the best phase balance for my room. This was not what was suggested in the thread. I had done this by accident before and it sounded very good so after reading the thread I want back to the way that sounded best. The thread suggested that speakers terminal interconnects could be switched around to determine the best phase match for your listening room. 

 

[chard]

By the way this mod cost under $10. Just the cost of 4 resisters and a 2 terminal terminal strip.