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Old 2nd November 2012, 10:30 AM   #1
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Default Turntable selector

I have three turntables and only one phonostage. I made a source selector using a 4P3T rotary switch connected by shielded wire to four pairs of RCA female terminals. However, there is a humming sound. It's not my turntables since when I connected each to the phonostage, there is no noise. What could the problem be? Thanks.
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Old 2nd November 2012, 05:30 PM   #2
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Ground issue? Are there separate ground wires on the turntables that can be connected to the amp? Or a shielding issue? Is the switch inside a shielded box?
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Old 2nd November 2012, 05:34 PM   #3
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A broken cable or an incomplete soldering in any of them?
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Old 2nd November 2012, 06:13 PM   #4
w1jim is offline w1jim  United States
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The phono signals are extremely low and prone to picking up noise. This is especially true with MC cartridges though i suspect you have the higher (but still low) signal MM cartridges.
The shielding and grounding of the switch box is important. Also, do you have the hum if you just run one signal through the switch (unplug the other TT RCAs)?
You could be picking up noise through the 2 unused pairs of RCA cables.

Keep in mind that as you lengthen the TT signal cables the capacitance goes up which will roll off the high frequencies.
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Old 5th November 2012, 06:48 AM   #5
h_a is offline h_a  Europe
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Quote:
I have three turntables and only one phonostage.
Keep in mind that each cartridge needs its own optimum load set by the phonostage input; unless you have three times the same cartridge, at least 2 of them will work not optimally.

There are a couple of decent opamp based phonostage designs around here, so you could go all the way of building another 2 phonostages for small money
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Old 10th November 2012, 08:10 AM   #6
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I have the same problem. I need to switch these 3 turntables into the 1 phono input on the preamp.

I E-mailed the tech. people at phonopreamps.com about this. They advised that due to the very small voltage output of phono cartridges, any mechanical switch contacts would quickly corrode.

That made me thing about using Mercury Wetted relays to do the switching. That would eliminate the corrosion problem.

I've found lots of SPDT mercury relays on E-bay. 2 E-bay sellers stores also had DPST mercury relays, but they were around $30 each (w/o sockets)!!!

Next problem is the pwr. sup. I think it would be easier to build a divorced power supply, & simply have a DC input to the switcher. That takes care of the whole pwr. sup. hum issue.

With this type of project, you could even include MC step up XMFR's in the design.

Back in the '70's Pioneer made a huge switcher (model U-24) that many high end stereo stores used to switch multiple inputs, to multiple preamps/receivers for testing. The U-24 could switch 3 phono's. It also switched a lot more stuff, that doesn't apply to this thread.

Anybody else have any ideas on building a Phono Switcher?

MLStrand56
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Old 10th November 2012, 12:07 PM   #7
adason is offline adason  United States
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I would use three different phono preamps, ideally external with its own power supplies, and than swith the signal at the line level
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Old 10th November 2012, 12:31 PM   #8
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Default Re: Line Level switching

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Originally Posted by adason View Post
I would use three different phono preamps, ideally external with its own power supplies, and than swith the signal at the line level
That is the easy way & what the tech. guys at phonopreamps.com suggested. But it also precludes using the phono ckt. in the pre-amp.

I could use several straight amps (NO phono EQ), then a regular line level selector, THEN Pad the output back down to the 5mv range. That sounds like a recipe for disaster.

I think it's easier to build a straight switcher & let the preamp phono ckt handle the EQ part.

MLStrand56
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Old 10th November 2012, 01:13 PM   #9
adason is offline adason  United States
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"But it also precludes using the phono ckt. in the pre-amp."

no it does not...
if you have three turntables, one can go to the phono input of your preamp, than you actually need only two external phono sections, and plug them into line level of you preamp, easy...
if all your line inputs are occupied, make simple external line switcher, with spare inputs, and output of this box will go to your preamp
boy, why are you trying hard to make this as complicated as possible?
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