# Help needed reading an audio transformer wiring schematic & impedance table

I am trying to link a transformer's schematic to its impedance table. I have trouble linking the values to the indivual pins.

For now I would like to use it exactly like in the picture. My understanding is that pins 3 & 4, 9 & 10, 7 & 8 are linked. That I need two wire to connect to the primaries to pins 2 & 5. And that I need a capacitor between the 9 & 10 pair and the 7 & 8 pair.

My question is about the secondaries wiring, I am sure that I need two wires also, surely one from the 7 & 8 pair, but I don't understand where the second one is coming from.

I would like to be able to read the schematic & associate the impedance table so I can later use the transformer differently than on the schematics, any help would be greatly appreciated, I'm a complete rookie at this!

Thanks

I would like to be able to read the schematic & associate the impedance table so I can later use the transformer differently than on the schematics, any help would be greatly appreciated, I'm a complete rookie at this!

• With primaries in series you get a 10 k input impedance to that combined primary.
• With primaries in parallel, the net inductance of the primary drops by the reduction in turns squared.
• This is because parallel primaries act the same as a single primary of half the number of turns.

So, the reduction in the impedance is proportional to the reduction in turns squared hence, 10k should reduce to 2.5 kohm. OK they say 2k4 and that accounts for a slight bit of imperfect magnetic coupling.

It's a similar story for the secondaries; just use the factor of turns-squared to predict impedance changes. This applies to all transformers.

My question is about the secondaries wiring, I am sure that I need two wires also, surely one from the 7 & 8 pair, but I don't understand where the second one is coming from.

The second secondary wire comes from the join of pins 9 and 10 and uses a cable shield: -

• Thank you very much for helping me read the chart, it makes a lot of sense regarding the values now! What would the capacitor do in that schematics? If I'm reading everything correctly it is connected between two 600Ohm loads is that correct? I am still waiting for my copy of Practical Electronics and do not yet know what a capacitor would do connected like this. I guess reduce the load? Jan 7 at 18:36
• I have no idea about the capacitor because I have no idea what the transformer actually is or where the circuit came from. The secondaries are in parallel in your circuit and that would be a 600 ohm impedance. Jan 7 at 18:38
• This is a line level input audio transformer, connected to a stepped potentiometer. Jan 7 at 18:41
• I still have no idea why a capacitor is needed. But, does it really matter? I mean it's not hard to fit it and remove it later should it prove to be a problem. Jan 7 at 18:43
• Yes you're right, I will try it anyway! How would the capacitor affect the impedance? Jan 7 at 18:44

The circle around the wire from terminals 7,8 indicates a shielded wire, with the shield connected to terminals 9,10. Further right on the 7,8 wire, a half-circle and vertical wire indicates the other end of the shield is connected to the top of R2.

A circle on a wire is a common way of indicating a shield around a wire or group of wires.

• This is very very helpful, thanks a lot! If I understand correctly, that means both cables connected to pins 7,8 and 9,10 need to be inside a rubber shield together to travel? How can I know where they separate and which is which when they separate from the shield? EDIT: I think I see it! The straight line from 7,8 goes to the bottom of R2 and the half circle represents the other wire from 9,10 to the top of R2! Correct? Jan 7 at 18:22
• @Wizzardzz "need to be inside a rubber shield" No, a metallic braided shield or screen. The output of the transformer is connected to an unbalanced shielded cable. The centre conductor is connected to pins 7 + 8 and the braided outer screen is connected to pins 9 + 10. On the primary side V is connected to the cable screen and the balanced signal is connected to F + H, which will typically be a pair of conductors twisted together for interference reduction. Jan 7 at 19:38
• @GrahamNye That's very interesting, I have never seen that kind of cable before in an audio product where a physical connector (a pin our case) is connected to the braid of a cable. I know that Neve does that for "floating grounds" with "pigtails" but I've never seen that done for signal wires. What does V, F & H mean. I am used to ring, tip, 1, 2, 3 etc but not these notations. Jan 7 at 19:45
• @Wizzardzz "I am used to ring, tip, 1, 2, 3 etc but not these notations." Neither am I but it's clear from the circuit diagram what is going on. Jan 7 at 21:36
• @GrahamNye I believe the letters are refering to the output of a selector. You can select either a microphone input or line input which do not share the same transformer. Jan 7 at 22:19

The schematic shows the primary wired in series and the secondary wired in parallel.

There are two secondary windings: 7 to 9, and 8 to 10. They were lazy and didn't draw both windings in the secondary.

• Thank you for your answer Matt, that does not mean that pins 9 & 10 and 7 & 8 are linked like 3 & 4 then? Jan 7 at 18:07

The secondary outputs are the pins going to capacitor. The wire is drawn as a shielded wire like coaxial cable, pins 9&10 are the wire shield and 7&8 are the wire center conductor.

The primaries are about 5k per coil. In series that is 10k and in parallel that is 2.5k.

Same with secondaries. 1200 ohms per coil. 2400 in series, 600 in parallel.

• Very misleading way of getting the right answer. Each primary is 2k5 and in series, the turns increases by 2 and, as a result the impedance increases by 2 squared. Jan 7 at 18:11