Thanks for the answer Alfred - but if i now want to "over-do" it and use shielded wire inside - would it be perfered be to have the shield connected to chassi gnd vs signal/common? i'm thinking chassi/mains ground would be better?
Also - the ground connector that goes to the tonearm. I wonder in most phono stages i guess it's usually connected to chassis/mains gnd? I won't rewire my tonearm with two separate ground, and the motor is clearaudio separate unit. So i'm just thinking connecting to chassis/main gnd would be fine instead of common?
By the way - where to find a good ground connector for the case?
Well, a lot has been said about grounding, and while some principles seem to be universal, some stuff is opinions, myths, and things that worked in one system but maybe not in another.
What worked in my system is described in the assembly guide. There is also the diagram from Frans, that he developed together with Ricardo, that is a very good and detailed description.
1. Grounding of the tonearm - I guess most turntables have a ground wire, that is connected to the tonearm metal. The idea is that this metal helps to shield the pickup signals, so it should not be connected to the cartridge wires anywhere inside the turntable. Instead, it should connect to the ground connection of the phono preamp.
2. As the signal from a MC cartridge has a rather low source impedance, electrostatic pickup is not so important, it is rather the electromagnetic pickup of noise that disturbs more. Hence, twisting the wire is more efficient than shielding it, but both can be done. Best to keep the wire short.
3. In my case, I put electrostatic shields around each of the two PCBs, and then another shield around both (the case, that is), and all are isolated to each other, but connected at the star ground which is on the back panel, in the middle between the two input connectors. (There is a book "Shielding and Grounding in Instrumentation Amplifiers" that explains this in great detail - sorry I forgot the author)
4. Usually, the ground connection for the turntable ground is a miserable screw that works more or less..... not with me. I put a 4mm female plug with gold surface, and put a male plug on the turntable wire - clean solution that works well and looks good, and does not deteriorate when undoing a few times e.g. when testing modifications on your phono preamp
5. Chassis GND vs signal GND - For safety reasons, to have the system floating (means, the cases are not connected to the mains GND anywhere) is IMHO not a good idea. This is potentially lethal. The main issue with connecting mains GND with the chassis GND and signal GND is the noise that may be introduced through ground loops. What works fine in my system is the following setup:
- Signal GND (tonearm GND) and chassis GND connected at the star ground of the phono preamp
- Mains GND connected to power supply GND inside the power supply of the phono preamp, through two power diodes connected back to back (Nelson Pass style). This power supply GND then connects to the star GND.
- All other devices are connected to the phono preamp GND and hence mains GND through the ground connections of the signal cables.
- (OK the last point is not super-safe as the ground leads of the cables may not be up to the task. Also, in my system there are additional ground connections in the other devices, potentially leading to ground loops but so far no problem. )
Lots of text, I hope this explains it better.....
Usually when i have built stuff i have always connected the mains earth directly to the chassis - and then all other grounds have gone through twin-diode/10R/0,1uF gnd lift to the point where mains is connected to the chassis. It have worked fine with my preamp and HP amp i have built. But i have never built anything as noise sensitive as a Phono pre which is why i ask.
Looking at schematic Frans have from the guide it looks like the mains gnd is lifted with diodes/cap/resistor then connected to chassis. and from chassis there is 10R to common, then everything else is connected through 10R resistors that should go to common.
So the "big" difference between that and how i have done is that i never lift the chassi ground from mains - is this recommended to do?
Also i asked the question the wrong way - in commercial built phono stages is the tonearm ground post usually connected to chassi ground (unlifted) or to the common ground?
For tonearm grounding i ordered a Vampire ground plug, i don't think i need something fancier - http://www.partsconnexion.com/VAMPIRE-55449.html
Also i have Clearaudio tangential arm, so i don't really have a "armtube".
Looking at schematic Frans have from the guide it looks like the mains gnd is lifted with diodes/cap/resistor then connected to chassis. and from chassis there is 10R to common, then everything else is connected through 10R resistors that should go to common.
So the "big" difference between that and how i have done is that i never lift the chassi ground from mains - is this recommended to do?
Also i asked the question the wrong way - in commercial built phono stages is the tonearm ground post usually connected to chassi ground (unlifted) or to the common ground?
For tonearm grounding i ordered a Vampire ground plug, i don't think i need something fancier
- http://www.partsconnexion.com/VAMPIRE-55449.htmlAlso i have Clearaudio tangential arm, so i don't really have a "armtube".
Thanks for the link, that looks nice! As for commercial stages, I think all of them *have to* connect the chassis to mains gnd, certainly in Europe (CE certification requires it). As mentioned, I have done it and no harm yet - but then I did not test what happens when I don't lift it. Sorry....
Makes me wonder what the grounding story of their latest outboard input-stage product is.
It gets even stranger... quote"The circuitry of the Absolute Phono is designed to work perfectly with any moving coil cartridge from any manufacturer. This is ensured due to the selected current amplification control which eliminates the need to set the correct load resistance and load capacitance. The optimum operating condition is always set automatically."
Today, after a long period of matching transistors and burning in I fired it up for the first time ...
The photo is just taken by a mobile, sorry for the bad quality ...
Thank you, for sharing this wonderful design.
In one channel there are really good matched transistors, the other one is ... hmm, soso ... I have a set of good matched ones laying ready but I don't heare a big difference betwene the channels, anyway I'm suffering from a tinitus by now, more about the sound after I got rid of this sh....
The photo is just taken by a mobile, sorry for the bad quality ...
Thank you, for sharing this wonderful design.
In one channel there are really good matched transistors, the other one is ... hmm, soso ... I have a set of good matched ones laying ready but I don't heare a big difference betwene the channels, anyway I'm suffering from a tinitus by now, more about the sound after I got rid of this sh....
I'l be there (on Sunday)