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Can I get your opinion?


Eddiecurrent

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Just a little help requested, I was hunting a phono preamp for my Anthem pre-1L, and I didn't want to spend serious dollars because IMO excellent results can be obtained for modest means when you're talking about an equalization circuit.

On Ebay, I saw this little NE5532 board that appealed to me from the circuit topography, but I'm not sure where the turntable ground point is.
It looks like it's a drill-thru right between the main power supply filter caps. Am I wrong? Thanks for any input.

http://www.ebay.com/itm/390610384583?_trksid=p2055119.m1438.l2649&ssPageName=STRK%3AMEBIDX%3AIT



241vafd.jpg

Edited by Eddiecurrent
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If you are talking about your TT tonearm Gnd wire (not the TT mtr Earth) than I'd say that you'd want to Gnd it to the signal cct Gnd (not the PSU Gnd) - although they will likely link up, it's better practice to link your incoming Gnd at the signal input Gnd connection - I see 3 x 'IN' solder pads - they are likely to be +/- & Gnd. Check under to confirm the Gnd track/s.

 

How's that?

 

Cheers, Owen

Dark Lantern blog - http://darklanternforowen.wordpress.com/

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To avoid hum,the circuit board needs to be fitted inside a metal box.The power supply to the pre-amp  should be external to this enclosure,ie.,a mains transformer with 18-0-18 volt secondaries.The 0 volt lead from this has to go to the "GND" point between the two 2700 mFd filter capacitors.An earth lead from this point should go straight to the case and be firmly fixed or soldered at that point. The screen earths for the interconnecting leads  appear to have been commoned at the centre of the IN and OUT  terminal points on the pcb.  

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To avoid hum,the circuit board needs to be fitted inside a metal box.The power supply to the pre-amp  should be external to this enclosure,ie.,a mains transformer with 18-0-18 volt secondaries.The 0 volt lead from this has to go to the "GND" point between the two 2700 mFd filter capacitors.An earth lead from this point should go straight to the case and be firmly fixed or soldered at that point. The screen earths for the interconnecting leads  appear to have been commoned at the centre of the IN and OUT  terminal points on the pcb.  

 

+1 Agree, except that there is a pad for the 0V from the supply between the +/- 18 volt pads, visible in the other photos on the eBay link. IT would be easier to connect the external power supply leads there. 

 

The OP question is "where is the turntable ground point?"

 

Turntables typically have 1, 2 or 3 grounds, depending on design, and this information has not include in the original post, so it is unclear which ground the OP is referring to.

 

1. One ground is the electrical safety earth from the power connection which would normally connect to the chassis of the turntable. This is omitted in double insulated equipment, some gear with an external power supply, and in countries that do not have a safety earth, e.g. parts of Japan / USA.

 

2. The second ground are the signal grounds, or more correctly the signal "returns". They are not strictly grounds because they are carrying the return signal current. To minimise crosstalk the right and left channel returns should be kept separate to keep the return current of one channel out of the other channels return wire. Thus there is normally a return for each channel, although sometimes the turntable manufacturer has joined these two "grounds" together on the tagstrip / termination where the wire come out of the tonearm inside the turntable. As far as the preamp setup for the OP, it is best to keep the cartridge ground wires separate from the preamp all the way to the cartridge.

 

It is common for the cartridge manufacturer to incorporate a screen in the cartridge and to connect this screen to the right channel return (-), but I have seen cartridges with no screen, cartridges with the screen connected to the left channel return, and cartridges with the screen connected to the tone arm. Although extremely rare, the latter is the theoretical optimum, the reason being that draining noise current from the cartridges screen to earth thought the signal return wire, with its finite resistance, will impose a (very small) noise voltage on the channels signal, which is not ideal.

 

3. The third ground is the tonearm ground. This is ideally connected to the preamp power supply ground with a separate wire for the reasons given above about the cartage ground, namely not to impose noise voltage on the signal. Most turntable manufacturers keep the tonearm ground wire separate, though there are some notable exceptions, like Rega who use one or both (?) of the signal returns to ground the tonearm.

 

If the OP has a separate wire for the tonearm, the best place to connect it would be the GND point between the two power supply capacitors. Some turntables have internal metal covers to screen the signal wires and terminations inside. Ideally theses screens would be connected to and/or treated like the tonearm ground.

 

The GND point between the two power supply capacitors is also the correct place to connect the metal enclosure the preamp goes in for screening.

 

It is important to make sure that no "ground loops" are created. For example, if there is a separate tonearm ground wire, it should not be connected to the preamp GND and the signal returns, just one or the other. Also if mounting RCA connectors on the metal enclosure, make sure the earths are insulated from the metal enclosure or omit the wire from the metal enclosure to the GND point on the circuit board.

 

Hope this helps. Happy to clarify any unclear points.

Edited by Guest
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Thanks very much for the well-considered answers, I didn't realize there was so much to consider in grounding. I meant the tonearm ground in the OP b/c I thought that was the only ground, the one that is a 20 gage or so single black wire on most turntables, and accompanies the RCA connector cables and is normally connected to an external dedicated thumbscrew on the chassis of the main preamp/integrated/receiver, often near the phono RCA inputs..

I live in the U.S. and have a technics 1300mk2 and a thorens td160. 

I first discovered the fact that the ground came from the tonearm about 30 yrs ago when I modified an old AR turntable, beefing up the chassis, improving the 3 point "T" platter suspension and replacing the original tonearm with a grace 707b (they were relatively inexpensive then ;-)).

Apparently there are design variations when it comes to grounds.

I will take the offered advice and use the ground point between the 2 main filter caps, and if I use a metal enclosure for the phono board I'll ground that to the same place. I also will probably replace the ne5532 with a ne5532A which is certified at a lower noise level. 

I have some pure copper sheeting laying around that I salvaged from some old audio and cable tv equipment  so I'll probably use that to act as a screen between the Xsformer and the phono board in the same enclosure.

DIY is fun.

Again thanks for the responses. ...... Eddie

Edited by Eddiecurrent
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  • 1 month later...

Well, I have one more question for anyone willing to contribute...that riaa ne5532 phono board doesn,t come with a transformer, it takes 18v ct and I have a spare one that,s the right voltage but only puts out 350ma.

I asked the Chinese seller what current requirements there are for the circuit, and as usual when dealing with Chinese ebayers, he answered a completely different question. Happens every time. I guess I shouldn,t blame them when bablefish translates (where is the tobacconist) with (you have beautiful thighs).

Does anyone know a general rule of thumb for the current capabilities of a phono eq circuit? I realize it,s low, but how low?

Thanks. ....Eddie 

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Just a little help requested, I was hunting a phono preamp for my Anthem pre-1L, and I didn't want to spend serious dollars because IMO excellent results can be obtained for modest means when you're talking about an equalization circuit.

On Ebay, I saw this little NE5532 board that appealed to me from the circuit topography, but I'm not sure where the turntable ground point is.

It looks like it's a drill-thru right between the main power supply filter caps. Am I wrong? Thanks for any input.

http://www.ebay.com/itm/390610384583?_trksid=p2055119.m1438.l2649&ssPageName=STRK%3AMEBIDX%3AIT

241vafd.jpg

hi here is a good phono circuit 

post-146626-0-72134900-1416775049_thumb.

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Well, I have one more question for anyone willing to contribute...that riaa ne5532 phono board doesn,t come with a transformer, it takes 18v ct and I have a spare one that,s the right voltage but only puts out 350ma.

I asked the Chinese seller what current requirements there are for the circuit, and as usual when dealing with Chinese ebayers, he answered a completely different question. Happens every time. I guess I shouldn,t blame them when bablefish translates (where is the tobacconist) with (you have beautiful thighs).

Does anyone know a general rule of thumb for the current capabilities of a phono eq circuit? I realize it,s low, but how low?

Thanks. ....Eddie 

Hi Eddie - I'm no SS expert, but this board has a NE5532 with a couple of 7815 regs, so I'm expecting that it draws no more than a few tens of mA, quiescent current at most.

So, an 18v @ 350mA PTX should eat it  ;)

 

Cheers, Owen

Dark Lantern blog - http://darklanternforowen.wordpress.com/

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