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Not another phono preamp thread, aggh!
fewtch
post Jun 26 2002, 09:17
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I've noticed many outboard preamps are subject to RFI, even if it's below the level of audibility... when recorded as a wave & normalized, I can hear a radio station in the background.

Anyway, it seems to me most of the RFI is coming from the turntable cables, which are acting like antennas (I've *never* heard RFI in a preamp built into a turntable). Anyone know of a basic RFI filter that can be connected between audio equipment and blocks out anything in the RF range? Or how something simple could be made with a couple capacitors?

The Music Hall MMF-1 I have (yah I know it's a turntable, apparently Music Hall briefly made an outboard preamp too) is actually not bad at all, sounds very good and after taking a peek inside, the components look to be of high quality -- but it does pick up some RF. The noise floor isn't that great either, instead of a light "fluttering" when normalized, what I hear is pink noise, like the sound of a waterfall -- but it's still below the level of normal audibility.

Thanks.


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JonPike
post Jun 26 2002, 09:46
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Go over all your grounds.. and I mean close visual inspection, take them apart and clean surfaces.. rub connectors together, etc..

Chassis to preamp, grounds and signals at the cartridge, where they connect to the external cables inside the TT, your RCA jack ends.. make sure your'e shielded as well as you can be. Possibly use some corrosion cleaning liquid if you have some..

A tiny bit of corrosion can act like a diode.. making a leeeeetle crystal radio for your preamp to play.

The other thing, your new preamp could be more sensitive to stray RF.. Improving the shielding can help a lot.. but I'd be leery of putting anything in series with your phono outputs. Too easy to screw with the proper resistive/capacitive load, and screw your sound up.

That said, in desperation, you could add small capacitors across either the TT side of the cable (inside, where the arm wiring goes to the shielded output cables) or on the input jacks of your preamp. Small enough caps can act as near shorts to RF, but still be fairly high impedences to audio.

Another thing.. perhaps the TT cables are so-so with their shielding.. if they seem lousy (test by seeing if you can pick up hum by running your hands up and down them) or have bad shield connections to the RCA's, you might think of replacing them.

The question I should have asked first.. Is this with the same TT/cables setup you used without noise before? As in, only the preamp has changed?

Try ground improvement first.. rub those connectors around in their sockets, make sure the chassis ground wire has good connections on both ends..

Good Luck!

Jon
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fewtch
post Jun 26 2002, 19:15
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Grounds, what grounds? biggrin.gif

The MMF-1 preamp has no grounding whatsoever. Plugs in via a 12vdc wall wart, and the case is made of plastic and has no grounding post. Would simply hooking the turntable's ground to the "-" terminal (electrical "ground") constitute grounding in that case? What kind of grounding can you expect anyway with this kind of setup?

I see grounding posts on other preamps that also use a 2-prong type plug, what are they trying to do?

There's nothing to hook the turntable ground to... but I've used a Sony turntable quite awhile with a built-in preamp that also isn't grounded and never had a single issue, no hum, no RF, surprisingly clean output. You would be amazed, this is a $150 turntable that Sony discontinued several years ago (only trouble is proprietary cartridge, and *extremely* sensitive to vibrations, even light touch on the chassis sounds like a "thump").

Here's the real question maybe -- having found an adequate (if massively unspectactular) setup in the Sony turntable, why am I putting myself through this sh*t? I'll take this as an advisory to leave well enough alone from now on.


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Pio2001
post Jun 26 2002, 20:28
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I just had, after 6 monthes of delivery from Arcam, the phono input for my ampli !
I recorded a vinyl with both the old and new preamps (both integrated into the amplis).
As it was already the case with a custom made phono preamp I tried once, the audible difference is small. I couldn't ABX the two recordings.
Would you be interested in the sound files, with background noise and all ?
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fewtch
post Jun 26 2002, 20:48
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Naah, that's OK. I guess it's the mental idea of recording an LP to digital, while down at -60dB (inaudible under the music) a radio station is playing. It's a little disconcerting, isn't it? smile.gif


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Pio2001
post Jun 26 2002, 21:13
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My speakers are definitely better than my headphones ! I just got 7/8 at ABX ;-) on the same files I failed with the headphones.

Headphones : AKG K-400
Speakers : Dynaudio Gemini
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fewtch
post Jun 26 2002, 21:19
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Speakers present a soundstage... headphones barely do. That could be part of the explanation.


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Pio2001
post Jun 27 2002, 01:34
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I don't think so, it's the texture of the treble that allowed me to make the difference. They have good tweeters.
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fewtch
post Jun 27 2002, 02:31
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P.S... here are pics of the MMF-1 internals, any 'electronics whizzes' have any particular commentary on it?




Looks to me like a pretty simple IC-based op-amp circuit, altho it sounds pretty good to my ears (what little I've listened so far). I probably paid too much for it, tho...


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JonPike
post Jul 2 2002, 20:37
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QUOTE
Originally posted by fewtch
Grounds, what grounds? biggrin.gif

The MMF-1 preamp has no grounding whatsoever.  Plugs in via a 12vdc wall wart, and the case is made of plastic and has no grounding post.  Would simply hooking the turntable's ground to the "-" terminal (electrical "ground") constitute grounding in that case?  What kind of grounding can you expect anyway with this kind of setup?

I see grounding posts on other preamps that also use a 2-prong type plug, what are they trying to do?


Ehhhh.. you probably want to work up a ground in that thing. The minus on the supply
MIGHT be the true chassis ground, but might not. BTW, I saw no pics, or link, of your pre on that last post..

The separate ground wire, found on all TT that have no preamp built inside, is to provide a different path for possible AC leakage currents from the motor or whatever.
If those currents have to flow down the shield of the phono cables.. you probably will induce tiny voltages on your phono signal wires.. and tiny voltages are what we're dealing in here.. so you hear hum. Or, the whole metal chassis of the TT is "humming" a bit from leakage, causing the shields to go up and down AC wise in voltage, and therby providing a differential voltage that the phono pre will end up "hearing". Slightly different mechanisim, same resullt. Tying that chassis hard to the ground of the preamp (or stereo with preamp inside) eliminates the difference.

We need to figure out where your ground is.. get me some nice close pics and maybe I can tell..

QUOTE
There's nothing to hook the turntable ground to... but I've used a Sony turntable quite awhile with a built-in preamp that also isn't grounded and never had a single issue, no hum, no RF, surprisingly clean output.  You would be amazed, this is a $150 turntable that Sony discontinued several years ago (only trouble is proprietary cartridge, and *extremely* sensitive to vibrations, even light touch on the chassis sounds like a "thump").

Here's the real question maybe -- having found an adequate (if massively unspectactular) setup in the Sony turntable, why am I putting myself through this sh*t?  I'll take this as an advisory to leave well enough alone from now on.


Heh.. well, you do have a point! The reason you have no problem there, is exactly because the pre is "embedded" in the TT chassis.. it's surely well grounded to the chassis, and the power supply for it as well.. It's only when you create this normal but challanging situation of a very high gain amp, way out on the end of some cables, and AC noise sources of varying amounts of shielding, attached to chassis and or nearby...

If you can measure continuity with an ohmmeter, from the RCA jack shield to the minus, yes you can us it.. or find a place on circuit board where those cable shields attach and make a connection.. you should be fine.. But KNOW you have the right connection, so you don't blow the thing!

Get me your pics..

Jon

Get me your pics!
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fewtch
post Jul 2 2002, 21:00
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I figured it out, appears the "ground" on the outside part (not inside connector) of the phono plug outputs seems to get rid of any hum from the turntable.

Anyway, I uploaded those pics again, try reloading the page (they should display now).


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JonPike
post Jul 8 2002, 03:56
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QUOTE
Originally posted by fewtch
I figured it out, appears the "ground" on the outside part (not inside connector) of the phono plug outputs seems to get rid of any hum from the turntable.

Anyway, I uploaded those pics again, try reloading the page (they should display now).


Back from Alfa Romeo Convention.. biggrin.gif

Aha.. yep, can see the pics now, and it sounds like you got the hum quieting you need.
If you need more, you can run that wire separately from the TT chassis to the OTHER end of the RCA shields (actually find where they go into the circuit board, make a connection there) and you should lower it even more by making that hum current flow
away from the shields.

But, it sounds like it's already too quiet to hear.. so, not worth the effort.

Jon - if a preamp hums in the woods, and nobody hears it... does it make a noise?
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JonPike
post Jul 8 2002, 04:02
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Hmmm.. I am wrecked from this convention.. meant to say from looking at the pics, you could put in a ground if you wanted to, by finding which "tab" the outer shells of the connectors go to, on the backs of those connectors. They're probably all together..

You could possibly put a binding post in the cover (that was taken off for the pic) and solder a short wire to the ground at the back of the connectors.. your'e done.

I'll change my last statement.. it might be worth it, since you could change your setup someday, and the hum could get worse... but then again.. you could wait till you have a problem to worry about it.

Jon
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fewtch
post Jul 8 2002, 14:12
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Thanks for the responses. I'm looking again for a better phono stage, and bidding on a Technics SU-C01 preamp (ca. 1979) on Ebay. This one is OK, but I don't like the RFI issue. If the bidding doesn't work out, well... the phono stage in a separate amplifier I already have is very quiet (although RIAA response isn't great, too much emphasis on bass/treble & not enough on midrange, and it appears to cut out around 18KHz).


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fewtch
post Jul 20 2002, 23:36
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I just found a really interesting looking preamp, if anyone's still looking for one:

http://www.elusivedisc.com/products.asp?dept=613

Specs look good, includes MM and MC cartridge support, and costs $99 bucks! Wish I had known about this one earlier... Music Hall is famous for low cost, high performing gear.


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