how to test relays for phono signals

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JR.
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Re: how to test relays for phono signals

Post by JR. » Tue Jan 26, 2021 4:28 pm

mediatechnology wrote:
Tue Jan 26, 2021 3:29 pm
JR. wrote:
Tue Jan 26, 2021 3:08 pm
Not that I expect there to be much there there, the phono preamp input termination is typically 47k (and 100pF), and at mV level signals any rectification from oxides or dirt may be more like on/off.

Good luck

JR
As someone who has replaced thousands of ITT Schadow switches in MCI consoles I know the heartbreak of on/off rectification at line level signals.
I see your schadow p-p switches and raise you insert jacks with intermittent switching contacts. I recall some drama after a purchasing puke approved changing the plating spec for the switch contacts on hundreds of thousands of (Switchcraft) TRS insert jacks. Silver was generally reliable but looked bad because it turned dark when oxidized. Some better looking contact treatments didn't work as well.

FWIW one of my two recent TS-1 repairs was a faulty (open) P-P switch.

And I've also replaced a lot of unsealed P&B and Omron relays in MCI and Otari recorders.
I can't recall however ever having to replace a sealed relay except an occasional gas-filled Aromat "ice cube" style and that was rare.
That includes low signal, tape head level, cue relays.

The concern here may be MC levels and currents.

I'm really curious to see if there's anything to measure.
The beauty of the null test is that you can also use music and listen to the residual.
Open that contact however and you'll lose your hearing.

It might be a good idea to add a diode clamp to the output of the diff amp....
Spoken like someone who has experienced that.

JR
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mediatechnology
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Re: how to test relays for phono signals

Post by mediatechnology » Wed Jan 27, 2021 5:20 am

Spoken like someone who has experienced that.
No, I just saw that one coming...
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Re: how to test relays for phono signals

Post by crin » Wed Jan 27, 2021 9:46 pm

The 1 K Rbias might not be enough.
With the concern being the low levels of MC switching pick 10-100Ω.
If the relay load resistor is 100Ω then you only need 2 mV of common mode voltage at the input to the diff amp to produce a contact current of 20 µA.
not sure i understand you – are you suggesting to replace the 1k bias resistors with 100 ohms? i assume the bias resistors should be equal, so it's 100 ohms per leg to gnd. for the generator, they are parallel – so i'm asking the poor low-distortion gen to drive 50 ohms with little distortion… (the 1k5 Rsource was JR's suggestion and it makes sense to me.)

i will not be switching MC level signals (except for the load resistors where only one N/O contact is between the 0 and 180 signal. that contact will be critical, but at least it doesn't switch the signal). the relays will be used e.g. on the mono preamp where i have 3 different SUTs (each with its own XLR input) and use the relay to connect the secondary of the CHOSEN ONE to the preamp input i.e. the signal is at around 4 mV @ 1khz.

come to think of it, there IS one application where i switch the signal at MC levels – and that's the 1:4 / 1:12 switch that connects the 3 primary windings of the JT-346 either in parallel or in series. i have already done this with toggle switches and couldn't hear a difference (i never did this on the test bench, but with the JT built into a preamp. i used the 1:4 setting for mid to hi Z MC cartridges (aka DL-103) and the 1:12 for low Z (aka Ortofon SPU). for a real comparison i would have to use the same cartridge for both ratios. but that means that in one of the two settings, the cartridge is not optimally loaded, and the effect of different loads swamps the influence of the relay.

the diode clamp is a good idea, more to protect the measuring gear than my ears. what would you suggest - a clipper with back to back zener between signal and ground? or two normal diodes from rail to rail?
You probably could use the MC preamp as the diff amp but I doubt the CMRR will be anywhere near what a THAT1510 is.
is there a particular reason why you suggest the 1510 instead of the 1512?
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Re: how to test relays for phono signals

Post by terkio » Thu Jan 28, 2021 4:38 am

crin wrote: is there a particular reason why you suggest the 1510 instead of the 1512?
The datasheet is clear.
Here high gain is needed.

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Re: how to test relays for phono signals

Post by crin » Thu Jan 28, 2021 5:36 am

crin wrote: ↑
is there a particular reason why you suggest the 1510 instead of the 1512?
The datasheet is clear.
Here high gain is needed.
that's what puzzles me. according to the data sheets, both chips provide up to 60 dB of gain. the only difference i can see WRT gain is that the 1510 is specified for 70 dB of differential gain compared to the 1512's 64 dB.
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mediatechnology
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Re: how to test relays for phono signals

Post by mediatechnology » Thu Jan 28, 2021 5:38 am

I was trying to avoid drawing anything...

What I was thinking was a high-ish generator build-out resistor feeding the switch contact and the + input of the mic preamp.
(That resistor simulates a current source.)
The relay shunts the inputs when closed.
Then a small value resistor to ground on the - input.
When in the closed position the values of the bias resistors don't matter too much since they are seeing a dead short in parallel.

I'm not sure it makes any difference whether you use a 1510 or 1512 since what you're measuring will be so buried in the noise floor.
You may not see anything.

And, instead of a 1510, you could use an active mic preamp in an interface.

For the diode clamps put a build-out resistor on the 1510 feeding a couple of grounded back-to-back diodes.
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