[CButterworth]

Hello,

I built my own passive preamp using an Elma switch and Alps 50K pot wired as a shunt attenuator.  I get great sound from the preamp - well, the sound is likely not degraded much by the preamp and wiring the pot into shunt mode did vastly improve sonics especially at the lower end of the spectrum.

Anyway, I have the grounds from the inputs bussed to the output and the ground from the pot.  As such, LEFT and RIGHT signals are totally separate.  However, I then wired the ground to the chassis.  After doing this, I noticed that using a DMM, I got connectivity from LEFT signal to RIGHT signal (obviously connection through the chassis0, and depending upon where the volume is set, the resistance is the same for LEFT to RIGHT that for RIGHT IN to RIGHT OUT through the pot.

OK, in spite of this, I get very good soundstaging, and instruments can be placed to stage LEFT or RIGHT, so stereo is still functioning.  I use a DIY AKSA (Aspen) amp which has separate PSU toroids and boards for LEFT and RIGHT.

So, my question - should I not wire the chassis at all?  This would ensure that R and L signals are kept totally independant.  Or does it not matter?

Thank-you,
Charlie

[mgalusha]

Charlie,

Since the is not mains connection to the passive I would leave it ungrounded. I would keep the L and R gounds seperate if possible. Of course they may be tied together in your source but at least your piece won't be connecting them.

mike g.

[mb]

Hi Charlie,

Yes, I also think that the ground for your passive should not be connected to the chassis. In the end there will be grounding at the AKSA input (actually slightly lifted from chassis ground if I recall) and at the source.

FWIW, I think a 50k pot for the AKSA's 47k input impedance may not be optimal. When I made my 20k stepped attenuator active via a battery powered buffer, the performance difference in terms of control, body and speed  was very large. Done carefully the buffer adds none of the grain or 'transistory' sound you might fear from a chip.

[Dan Banquer]

It appears that the folks on this thread either have not read my post on grounding practices or read it and did not understand it.
Charlie you have a few choices here on your passive pre amp. The first is that right and left grounds do not meet in the pre amp and therefore do not connect to the chassis. If you leave the chassis floating than you have given yourself an antenna to pick up stray interference.  You can earth ground the chassis with the audio grounds isolated from the chassis. This however will give a potential difference across the volume control due to the stray capacitance and you may well get some noise pick up. I think your best option is to connect the left and right grounds at one point to the chassis. This should not degrade stereo separation and it also gives you a reasonable ground plane for all of your components to connect to. In addition you may wish to experiment with connecting the chassis to earth ground. This going to be trial and error here because there are no standards for grounding in consumer audio. If you find that you get more 60 Hz hum from the earth ground connection or that any component appears to be behaving strangely disconnect the chassis form earth ground. If the opposite happens than you should get a quieter system that is less prone to interference.
Hope this helps.
           d.b.
P.S. Here's another suggestion. Connect all the right channel grounds on you RCA inputs with some !/8 inch flat braid, and do the same with the left. Connect both to one point on the chassis.
P.P. S. Here's the link: http://www.audiocircle.com/index.php?topic=8780

[CButterworth]

Well,  I did disconnect the signal ground from the chassis.  I have not, so far, picked up any stray RF interference.  I have no hum in the system, but somehow the sound did not appear quite a clean as before.  I'll need to have another listen tonight.  What I may do is to connect spades to my chassis ground wires so that they can be connected and removed without recourse to the soldering iron.

Considering that I am also a HAM operator, a properly grounded passive preamp may be necessary in order to reduce self-generated RF interference!

Thanks for the insight.
Charlie

[Dan Banquer]

Hi Charlie;
    I use a very simple technique for verifying best possible grounding. Turn on the entire system (all source components) with no signal applied and turn the volume all the way up. The grounding scheme that produces the least amount of hum and hiss wins.
   BTW: One of the major reasons I got to be real concious about grounding, shielding, and filtering is that I live less than 100 yards from a very large and powerful ham radio antenna. Thanks to guy's like you I even use shielded speaker cable.  
                     d.b.

[CButterworth]

Thanks for all the replies.  I rewired my chassis ground, this time using a couple of spade lugs to connect the ground to the chassis.  This does allow them to be removed.

I agree on the potential problem regarding HAM RF interference.  As an operator myself, I was taught that most consumer electrical equipment may be subject to interference (read the FCC statement on the equipment). However, as a consientious HAM operator, I usually enquire with neighbors, etc about interference caused by me before problems arise.

Anyway, this is just a side-point, but if you do experience interference, your local HAM operator should be able to offer advice on how to reduce it.  This is considered "good practice" by the Amateur Radio Relay League (ARRL).

Regards,
Charlie

[Dan Banquer]

Charlie:
       Did you get less noise with the method you described?
                        d.b.

[CButterworth]

Dan,

I think that overall noise was reduced when I wired the ground to the chassis.  However, I cannot say for sure as I am likely still burning-in the amp after upgrading some of the parts.

Thanks,
Charlie