Bottlehead Forum
Bottlehead Kits => Eros Phono => Topic started by: Jim R. on October 09, 2013, 05:45:51 AM
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Is there a good, safe way to add a mono switch to the Eros?
If there is, I'd like to plan in advance of building the kit in case it needs any mods to things that will be difficult to get to afterwards.
Thanks,
Jim
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I would do it at the output, after the output capacitors. (highest voltage, least noise susceptibility) Others will surely disagree. Just take a SPST switch and use it to short the left to the right in one position. In the other position it leaves everything as it is.
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Well, I'm just not sure if that's the best way to go but I have considered it. If I did take that approach I'd only use a SPST switch though.
-- Jim
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I misspoke, you need a DPST. You need to be able to combine the right and left channel, so DP is the way to go. You only need DPST but could, of course, use a DPDT.
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I misspoke, you need a DPST. You need to be able to combine the right and left channel, so DP is the way to go. You only need DPST but could, of course, use a DPDT.
Nah, you can just switch one channel into the other, the DPDT switch isn't necessary.
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Now after coffee I see that you could feed the right to one lug of a SPST switch and the left to the other. When the switch is on you have mono. I just wasn't seeing that.
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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!
Edited 2018 May 26:
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.