Attenuator Reactive Load Impedance Query

[Littlewyan]

I'm currently designing a new attenuator for myself that will include a solo boost and I'm a bit concerned about the load presented to the amp when switching to the boost. The design is basically a fixed 6dB attenuation step with a reactive load (Inductor) for the amp that then gets followed by a modified airbrake. The "airbrake" section is where I can make adjustments to the amount of attenuation. To give myself a solo boost I'm using two rotary switches and a relay to go between different attenuation levels on this airbrake section. What concerns me is, when switching, the airbrake will be temporarily disconnected from the reactive attenuation stage which means the reactive stage will have no load and the amp could end up seeing as much as 56Ohms at high frequencies, compared with 32Ohms normal operation.

I've looked at the relay specs and they state the Operate/Release time as 8/6ms, which is very low, but would it be low enough to prevent a flyback spike hitting the amp hard? The other concern of course is if the relay gets stuck for whatever reason, the amp will be running flat out into a higher than normal load.

Reasons for not thinking this is an issue, the load presented back to the amp will be just under double, so would almost be like running the amp on 8Ohms into a 16Ohm speaker. So is this really an issue if the amp is seeing this for milliseconds? What are your thoughts?

[martin manning]

Why not change the attenuation by having the relay connect/disconnect a parallel resistor?

[Littlewyan]

I could add another 6dB stage that could be activated by the relay, but I was trying to keep this as flexible as possible. My original idea was to have two rotary switches, both hooked up in the same way to the airbrake, but a relay to flick between them. That way you can set one just 3dB higher than the other or even 6dB higher.

[Littlewyan]

I've just looked at Eminence Speaker impedance curves and the Private Jack curve for an 8Ohm speaker actually goes up as high as 60Ohms at 20Khz! So perhaps 56Ohms isn't that bad after all.

https://www.eminence.com/speakers/speak ... ivate_Jack

Note: I realise that this is a measurement taken with the speaker in free air, but I can't imagine it would be much different in a 1x12 Open Back cabinet.