Passive preamp grounding questions
Jan 1, 2011 at 3:28 PM Thread Starter Post #1 of 13

Yoga Flame

500+ Head-Fier
Joined
May 1, 2010
Posts
813
Likes
46
I'm planning to build a passive preamp-- simple thing with just an input selector and stepped attenuator. It will have no active components and no gain. Though I think I will try adding Tangent's Linkwitz crossfeed (also passive) with an additional DPDT switch to optionally  bypass it completely.
 
My question is, where should I ground the enclosure to? On my amp, the case is connected to the common L+R input ground. But on the planned preamp, there will be multiple inputs. Should I connect all of the input ground wires together and tie the case to that?
 
Jan 4, 2011 at 4:24 PM Post #3 of 13

Yoga Flame

500+ Head-Fier
Joined
May 1, 2010
Posts
813
Likes
46
Thanks.
 
Is there any advantage to going ungrounded rather than tying the case to the output ground?
 
What if I tied the preamp case to my PSU's case (i.e. AC ground)?
 
edit for clarity: The PSU is a σ11 I'm using with my headphone amp, it won't be supplying anything to the preamp.
 
Jan 6, 2011 at 3:30 AM Post #4 of 13

AmarokCZ

100+ Head-Fier
Joined
Sep 8, 2010
Posts
203
Likes
18
I would prefer grounding to output, because there's no reason to left case ungrounded (plus I never saw ungrounded metallic caase).
You can tie ground to PSU case, but it's just one more unnecessary cable because output ground will be also connected to PSU ground (through headamp).
 
Jan 28, 2011 at 8:49 PM Post #6 of 13

Yoga Flame

500+ Head-Fier
Joined
May 1, 2010
Posts
813
Likes
46
Hello again, fellow DIYers.
 
So I finally finished the casework on my preamp and will be wiring it up next. But there might be a problem. My selector switch is a rotary 2P3T type. (And not 3P3T.) Meaning I can switch between the three pairs of L + R input channels but not the input grounds.
 
3 pairs of RCA jacks --> 2P3T selector --> (optional crossfeed circuit) --> stepped attenuator --> RCA output jacks
 
Now I think I could just tie all of the input RCA ground tabs together, and still have a functional preamp. But what are the potential problems of combining the input grounds?
 
I've done some googling and these are the relevant pages I found:
http://audiokarma.org/forums/showthread.php?t=70124
http://www.diyaudio.com/forums/parts/72652-selector-switch.html
http://gilmore2.chem.northwestern.edu/projects/showfile.php?file=cmoy7_prj.htm
 
Seems to suggest that combining the ground among the various inputs is okay, or even preferable sometimes. Just want to make sure there isn't something in my planned configuration that makes it an exception.
 
Jan 28, 2011 at 9:09 PM Post #7 of 13

Avro_Arrow

MOT: Soundwerx Designs
Joined
Apr 8, 2010
Posts
2,211
Likes
54
I would tie all the grounds together anyway. I would never have run them through a switch.
If you want an option, Isolate the output RCA jacks so you can have a switch to break the ground
if you wish.
 
Feb 9, 2011 at 1:44 PM Post #9 of 13

Yoga Flame

500+ Head-Fier
Joined
May 1, 2010
Posts
813
Likes
46
UPDATE:
This is mainly for the benefit of others who might search for this in the future. But of course comments are welcome.
 
I now see a disadvantage with combining the ground of all the inputs. If any one of my sources has a noise problem, then I will hear the noise no matter which source is currently selected with the rotary switch. For example, I tried connecting my Yulong D100, AMB γ2, and Creative Xmod to the preamp. Nothing really noticeable during normal listening, but at high volume levels with no music playing, I can hear noise. It was the loudest when I switched to the Xmod, and still audible with the other two. But after I unplugged the Xmod from the preamp, the noise was gone.
 
Anyway, this is not a serious problem. The noise is not audible at normal levels. And it's really the fault of the Creative Xmod. But maybe I will get a 3P3T switch so I can try toggling the input ground too.
 
Not related to the above, but I also grounded the case to the input/output ground channel.
 
Feb 9, 2011 at 3:08 PM Post #10 of 13

googleborg

100+ Head-Fier
Joined
Nov 10, 2009
Posts
397
Likes
12
nice :) i don't understand why all preamps aren't passive...but there is probably a very good reason for active ones that i don't know :p
 
Feb 11, 2011 at 4:48 AM Post #12 of 13

Yoga Flame

500+ Head-Fier
Joined
May 1, 2010
Posts
813
Likes
46
Thanks. Okay, I managed to borrow a decent camera, so here are some more pics.
 
Here it is in between my M³ and σ11. The amp is topless because I'm currently working on adding better ventilation to the lid.

 
Lots of DIY interconnects at the rear.

 
In order to isolate all the RCA jacks from the chasis, I needed fairly large panel holes so that the shoulder washers could fit. These were larger than my hand drill could make, so I used a file. As you can see, my filing precision leaves much to be desired. :p

 
Interior of the preamp. Steel plate mounted with blu tack. Directly below in the σ11 PSU is the transformer. I didn't have any noise or hum, but the steel is there to put my magnetic interference paranoia to rest.

 
The RCA jack grounds are tied together with stripped hookup wire. I used Navships wire throughout.

 
The crossfeed is mounted onto the base of the chassis with a single standoff. The circuit's output ground is wired to the ground of the RCA jacks. The input ground is wired to the chassis. Both are directly connected on the circuit. I used two types of DPDT switches. The blue one (Mouser: 642-5646A) costs more and is larger. I used it for the master bypass switch. In hindsight, I should have used that for all three positions. The smaller red ones are harder to flip.

 
I used a 100K log stepped attenuator from Vintage Audio Labs (eBay) for volume control. The volume knob is temporary and was pulled out from my Creative Xmod. Still waiting for my eBay knob to arrive.
 
The selector switch is actually 2P5T, though only three positions are actually used. I got it from Mouser (part number: MB00L1NZQD). It's got a narrow 3.17mm shaft, so your standard volume knobs won't fit. I'm using a Kilo knob from Digikey.
 
Hope that helps.
 
Feb 11, 2011 at 10:13 AM Post #13 of 13

AmarokCZ

100+ Head-Fier
Joined
Sep 8, 2010
Posts
203
Likes
18

Quote:
nice :) i don't understand why all preamps aren't passive...but there is probably a very good reason for active ones that i don't know :p

 
There was a discussion at another forum and I was the only one who thought, that passive preamp is best solution (in certain cases), so generally people are buying active preamps and in most cases they "do not need it".
Active preamp pros: gain 0 to theoretically infinity, output impedance very low, cons: difference between input and output (every preamp has certain "sound")
Passive preamp pros: theoretically no difference between output and input, cons: gain only 0 to 1 (this is often not a problem), high input impedance so it is unable to drive long cables without distortion (long cable + passive pream = low-pass filter).
So basically the best solution is passive preamp inside power amplifier (that means just stereo potentiometer at input), but such power amp is unrealistic, because two amps (preamp + power amp) means twice as much money for manufacturer. 
rolleyes.gif

 

Users who are viewing this thread

Top