how do you know you need matched pair?

[jarthel]

I have recently built an aikido headphone amp and I'm wondering what are the criteria that determines the need for matched pairs.

not just for the aikido but other designs like PP or SE or PSE.

thank you.

[Shoog]

In PP it will help because it will reduce standing DC in the transformer, which will allow the transformer to deliver better bass.
In all other applications (apart from possibly LTP phase splitting) it marketing driven and a complete waist of money.
Remember your expensive matched pair will stay matched for less than 50% of their life- are you going to bin them after that.

I think using valves from the same batch and changing valves at the same time is more than adequate.

Shoog

[rdf]

Simple zero feedback topologies also require well matched tubes for channel balance. The second option is to have plenty of spares and keep swapping until the cumulative gain errors in the circuit cancel to an acceptable degree.

[Shoog]

Simple zero feedback topologies also require well matched tubes for channel balance. The second option is to have plenty of spares and keep swapping until the cumulative gain errors in the circuit cancel to an acceptable degree.

Have you noticed this as a practical issue ?
Balance control might be a cheaper solution.

Shoog

[rdf]

Surprisingly yes. Quite often just swapping drivers between channels gets it close enough.