Phono switching with relays.

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Another one of those questions where I have been blissfully ignorant for years as never needed to know :). I'm building a new preamp, which includes consolidating various functions that have been in seperate boxes into one. I have two turntables and, although most of the time one will be MM and one MC, I want to be able to support all combinations without replugging. And to be difficult I have seperate MM and MC phono stages. Both of these are firsts for me. Looking at the switching requirements to be able to be totally flexible relays seemed to be a good option. Then I started looking at specifications and realised that it's hard to find relays rated for Initial signals under 10mV, fine for a beefy MM but a tad high for a weedy MC, so I run the (worry of the) gauntlet of the contacts not whetting somewhere down the line. And herein is the ignorance. In a domestic environment with maybe 500 cycles a year max is there anything to worry about?

If anyone has recommendations for relays that work in this application and are not billet unobtanium?
 
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Pickering reed relays from old blighty.. ;)

Pickering Reed Relay - General Instrumentation Reed Relays

Form B and Form C relays are much less reliable than Form A (NO)

Make sure you select relays suitable for low current/low voltage applications and make sure you put no significant current through them. Where an input capacitor is involved include a small amount of series resistance to limit charge/discharge current during contact closure.
 
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Any advantage in the wet contact? I see Pickering make it hard to buy, hopefully they will deal with the great unwashed directly.

No capacitors in the signal path, balanced and servo'd all the way. Horrible 8-legs abound :) . I think I have mission creep down to a point where I can build it in non-glacial time!
 
One of the better small-signal relays I know of is the Panasonic TX-S series. The p/n's with the -1 suffix (see datasheet) are designed for low-level signal switching. I'm not a fan of switching cartridge-level signals; I'd much rather have the phono stage located in the turntable 'plinth'/base. I realize that's not your intent, but I'd consider it

www.panasonic-electric-works.com/eu-asset/en/downloads/ds_61022_en_txs.pdf

I doubt you'll be able to get Mercury whetted relays in your country.
 
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I was historically of that opinion, it's only been in the last year where I have decided to start playing with MMs again. I even have a turntable with a cutout for a pre-amp!

But the wish to be flexible combined with the Memsahib requesting a reduction in number of boxes makes it an avenue worth pursuing.
 
Why relays? About 35 years ago I've built a control amplifier according to Elektor's Prelude articles. It featured three phono inputs, one MC and two MM's. The MC RCA jacks had fixed connections to the pre-pre amplifier's inputs, whose outputs were wired to a very common Lorlin dual pole triple throw rotary switch, as were the MM inputs. The switche's shaft was prolonged by a rod protruding through the face plate. Still works well without any issues.
Best regards!
 
I use a lot of relays for my phono amp, 3 inputs, mc or mm amplication and loading all switchable with relays. No problems with signal lost.
 

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Somehow I fail to see why sealed, gold contact relays would be any worse the switches, - without checking the realities, capacitance may be an issue for MMS.... OTOH- separate RIAAs would certainly be the better option...

I remember Mooly mentioning a few weeks back that most switches were not really fit for the purpose either, but at least you get some wiping action with a switch. In my case doing what I want with switches would be a pain.

Capacitance is not as issue for the MM stage as I'm using a form of trans amp.
 
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Any advantage in the wet contact? I see Pickering make it hard to buy, hopefully they will deal with the great unwashed directly.

No capacitors in the signal path, balanced and servo'd all the way. Horrible 8-legs abound :) . I think I have mission creep down to a point where I can build it in non-glacial time!

Sealed reeds are pretty good without wetting. I used Pickering Form A types in ATE hardware doing audio measurements at very low levels and had no problems.

Hopefully the relays offered are suitable. The Pickering stuff is very good as long as you design to avoid welding. (This is true of all reeds, no indictment of Pickering intended) No input capacitors goes a long way towards that goal.They will never degrade and will operate forever in this application. I got service lives of >2 x 1e6 contact closures without failure during test.

Relays are great if you get the right ones because they generally offer better isolation than the typical low level dpst or dpdt and won't require Bill to run wires all over the place with extremely low signal levels. Also allows for control by a mpu if desired. I use CMOS switches in amplifier circuits designed for measurements but I have not really disabused myself of the notion that they don't belong in audio designs (they probably do).
 
While I would shy away from Pickering in a production product as I haven't experienced the best reliability with them, Pickering does have the advantage of having options for the contact plating. IIRC, Ruthenium is their recommendation for low-level signals.

However, see the article on reed relay distortion by Samuel Groner and Bruno Putzeys in the last issue of Linear Audio. Fortunately for your application, I think signal levels are low enough that this wouldn't be an issue.
 
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While I would shy away from Pickering in a production product as I haven't experienced the best reliability with them, Pickering does have the advantage of having options for the contact plating. IIRC, Ruthenium is their recommendation for low-level signals.

However, see the article on reed relay distortion by Samuel Groner and Bruno Putzeys in the last issue of Linear Audio. Fortunately for your application, I think signal levels are low enough that this wouldn't be an issue.


Odd, we used thousands of them in semiconductor ATE test hardware and had very few problems unless we destroyed them by running them beyond the recommended operating conditions. Over extended periods of time in the lab we had less than 0.25% failure rate with lots of mistakes and abuse going on.

I will admit they are very unforgiving if you put significant current through them when the contacts are opening or closing.. (Welded contacts)
 
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