Eros Mono switch

[scotth]

While I'm impatiently awaiting the arrival of my Eros kit.  I've been thinking about adding a mono switch to my system, which I've wanted for some time.  Rather than revive any old threads, I figured I'd start a new one.

I found this advice from PJ:

It would be better to do the switching at the grid of the last stage.  Otherwise the difference signal sees an unnaturally low load impedance, causing distortion in the tubes even though that difference signal is cancelled in the net output. The same goes for the Seduction/Reduction.

Such a switch is at a high impedance node and thus in danger of picking up hum and noise - keep the leads as short as possible. That may make positioning the switch difficult! Be prepared to move the switch r wires to find a suitable location - you are not guaranteed to find the best position the first try!

Has anyone successfully done this with the Eros?

Is there a risk that even if I test the lead length and general location of the switch before drilling any holes that I might find that the location is poor after drilling a hole and mounting it?

In light of the risks of switch placement that PJ alluded to above, would it be better not to add the mono switch to the Eros? Would somewhere else in the signal chain be a more ideal place to add it? Thanks!

[Paul Joppa]

It's now 5 years later, and I looked again at the circuit. I do NOT, at this point, recommend ANY mono switch in the Eros.

Back then I must have missed the problem that would occur when the EF86 plates are connected together. The bias servos would interfere with each other.

The intermediate connection (grids of the second stage) should work with Seduction/Reduction since the stages are cap-coupled and the RIAA equalization has a large series resistance at the first stage outputs, which isolates them from each other.

In some cases, it can be done with little or no sonic damage in a preamp, but that depends on the particular circuit.

A general-purpose circuit to work with any preamp/amp would necessarily involve some loss of gain, complicating the design and system.

I'll go edit my old post.

[scotth]

Thanks for the clarification. I'll hold off on a mono switch for now.

[Paul Birkeland]

Hey PJ, what about connecting the output side of the coupling caps together?

[Paul Joppa]

With the output caps connected together, you would still have a high difference signal current flowing through the low impedance of the other channel's plate, much greater current than the output current - resulting in distortion. It's the same problem as that with paralleled triodes, except the difference signal and the resulting distortion are both much larger.