PCB vs matrix vs strip board for phono pre-amp - diyAudio
Go Back   Home > Forums > Design & Build > Parts

Parts Where to get, and how to make the best bits. PCB's, caps, transformers, etc.

Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Reply
 
Thread Tools Search this Thread
Old 30th January 2012, 07:19 PM   #1
diyAudio Member
 
Join Date: Dec 2011
Default PCB vs matrix vs strip board for phono pre-amp

Hello again. Did a quick search and couldn't find this discussion so starting a new thread...

I am planning to build an opamp-based phono pre-amp in a re-cycled full-size chassis/enclosure using an existing transformer with +/- 16.5V centre tapped. As there will be a lot of space left I plan to include a microphone pre-amp running off the same PSU. The transformer also has a +/- 38.5V tap which I will experiment with as a PSU for my TT motor. This last bit may never get beyond taking the AC out via a screened lead (captive lead, no connector) direct to the motor.

The question is: how much better will the noise performance be if I build on a PCB vs. matrix or strip board and _why_ will it be better. My gut feel is that, for the phono pre at least, a sensitive layout on stripboard would be as good as a single-sided PCB. What I mean is that all of the things you need to be careful of when doing a PCB layout can mostly be accommodated on strip board can't they?

Also, what would be the technical justification (physics) for needing a ground plane for an audio frequency circuit in an earthed metal enclosure?

If it all goes horribly wrong then I don't need anything more than a crude metal box around the PCB to exclude induced noise from stray transformer EM fields , right?

I'd welcome your thoughts/advice. Thanks.

PS PSU will be LM317 based with RC LPF on the input with smoothing cap on the voltage divider + diodes.
  Reply With Quote
Old 31st January 2012, 12:00 PM   #2
marce is offline marce  United Kingdom
diyAudio Member
 
Join Date: Jun 2007
Location: Blackburn, Lancs
RF noise.
For small signal designs a ground plane will always produce a quiteter design.
I would reccomend these sites, regarding EMC.
EMC Information Centre - The EMC Journal (Free in the UK)
And the Godfather of EMC design and noise reduction:
home page
  Reply With Quote
Old 31st January 2012, 02:00 PM   #3
DF96 is offline DF96  England
diyAudio Member
 
Join Date: May 2007
To keep transformer magnetic fields away from a PCB the main thing is distance. If that is not possible, use thick iron/steel sheet. Personally, I would keep motor supplies well away from a phono preamp.

Ground planes are good for RF, but for audio you may need more control over where the currents flow. Grounding topology is the main thing. Bus, star or plane is secondary; all can work if done properly.

Noise performance is decided mainly by circuit design, secondly by component selection (e.g. low noise opamps).
  Reply With Quote
Old 31st January 2012, 02:14 PM   #4
AndrewT is offline AndrewT  Scotland
diyAudio Member
 
Join Date: Jul 2004
Location: Scottish Borders
A crude metal box will make an excellent electrostatic shield. But you must connect it to your circuit correctly.
That box will do almost nothing for electromagnetic shielding.

Marce,
can you jump in here with a set of rules on how to integrate the box with the circuit?

Can you also advise on adding a copper shield plane to the top (or bottom) of a perf board assembly to mimic a ground plane?
__________________
regards Andrew T.
  Reply With Quote
Old 31st January 2012, 03:15 PM   #5
marce is offline marce  United Kingdom
diyAudio Member
 
Join Date: Jun 2007
Location: Blackburn, Lancs
I'll have a dig around tonight, as shielding is mainly (to quote Keith Armstrong and others) is seams and seals!!!
There are notes by Mr Ott that show some advantages of having what they call a mirror plane for emc. I believe that it will help terminate the h and e fields of the signals on the vero-board as well as providing some shielding.
Just been chosen for redundancy, so a bit manic at the moment (its not that bad, I am a lucky one I have a job to go to. And I will get to play with more varid designs as its beureau work, including a couple of pof audio companies He He)
For small signal designs be they any sort of analogue, ground planes are best and produces the quietist circuits, and allow returns to find there own way home. Messing about with stars busses etc is not the best way, especially if you dont understand return currents etc. For POWER designs ie AMPS etc, then yes keeping high currents away from low current is essential. And yes the other schemes work, but for the best results a gnd plane is best. In the early days of layout I would do digital boards on two layers but ALL analogue designs (low signal) I did where a minimum of 4 layers with a contigous ground plane.
Anyway I'll dig out the info and get back.
  Reply With Quote
Old 31st January 2012, 04:41 PM   #6
diyAudio Member
 
Join Date: Dec 2011
Thanks guys this is great. I'll research those links and come back with a few questions I'm sure.

Let's see if I've got it so far:
source - stray EM fields from internal transformer and nearby equipment
distance - keep small signal cct as far away as possible from EM source
signal tracks - as short as possible to minimise induced voltage from EM fields
metal shielding - won't do anything for magnetic fields (how about soft iron?)
PSU ripple - not a problem if I use opamps due to PSRR

OK so here's a question...

If shielding won't do much for me how come a ground plane will? Wouldn't these two techniques work the same way in terms of protection from EMI?

Don't get me wrong: I'm not disagreeing or arguing against what you are telling me, I just want to understand how and why these techniques work. I do DIY in order to learn stuff as much as get decent sounding audio at minimal cost.

Thanks again.
  Reply With Quote
Old 31st January 2012, 04:42 PM   #7
diyAudio Member
 
Join Date: Dec 2011
PS Marce, sorry to hear about your redundancy - I hope it works out for you.
  Reply With Quote
Old 31st January 2012, 04:55 PM   #8
marce is offline marce  United Kingdom
diyAudio Member
 
Join Date: Jun 2007
Location: Blackburn, Lancs
Its not totaly expected, this is the fourth round of them in the last 12 months, defence cut backs here and in the US.
I've got Henry Ott Electromagnetic compatability engineering sat next to me, also what little knowledge I've picked up over the last 25 years doing PCB's, I'll answer in more detail in a bit.
Sheilding does woek, rip open your mobile phone and look at the metal cans in there, I'll dig out some examples of how thats done. The problem with shielding though can be gaps such as slots in the shielding, these can act as waveguides and actually increase interference, whereas having no shielding could be a better solution, strange but true.
With a shielding can on a board you can shield the areas you want 100%, where as if you put a metal box around it you have to have proper terminated cable enty etc to make a complete shield, and that is the key having a complete shield. More in a bit.
  Reply With Quote
Old 31st January 2012, 05:16 PM   #9
DF96 is offline DF96  England
diyAudio Member
 
Join Date: May 2007
You need to be clear what you are guarding against. Techniques for stopping 1GHz and 50/60Hz can be quite different. Small circuit loop size will help with both, but a slot in a screen which 1GHz sees as an open window will look like a closed shutter to 50Hz electrostatic fields.
  Reply With Quote
Old 31st January 2012, 07:00 PM   #10
marce is offline marce  United Kingdom
diyAudio Member
 
Join Date: Jun 2007
Location: Blackburn, Lancs
Yes that is correct. The trouble is there is a lot of VHF rf mush about these days and adding a shield for lower frequencys can work for them but concentrate the VHF mush if it is present. Its another complex and interesting subject (similar to transmission lines at low frequencymore questions on that soon, in another thread of course, the info and reading material has been an education).
The problem wih partial screening is also the cables coming out, acting as antennas, I am probably being over cautious but I think it is worth looking at and discussing.
As to ground planes I am putting some thoughts and pictures together, which I will start a new thread on, we should have some interesting discourse on them, from previos posts youv'e all probably gathered I'm not a fan of excessive star grounding.
As it is a phono pre-amp we are talking about low level signals so any noise is going to have a big effect.

Quote:
Don't get me wrong: I'm not disagreeing or arguing against what you are telling me, I just want to understand how and why these techniques work. I do DIY in order to learn stuff as much as get decent sounding audio at minimal cost.
Wot you doing on DIY Audio then never start a thread on how long a bit of string is!Joking apart, there are so many views that a definitive answer is probably impossible to get, but that is a big part of the education, your beliefs will be challenged, but it is always an education. Dont expect two answers the same, on any point
  Reply With Quote

Reply


Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
DIY LM1875 chip amp on strip board made simple dannyjmf Chip Amps 7 5th October 2011 03:23 AM
Phono-pre-amp+power supply pcb Roushon Analogue Source 41 15th August 2011 12:23 AM
Red board add-on phono/RIAA pre-amp rsumperl Tubes / Valves 5 14th May 2011 04:40 PM
A PCB-design for Phono-pre-amp Roushon Analogue Source 19 17th May 2010 05:41 AM
Circuit board for phono pre-amp peterb Analogue Source 1 4th February 2004 04:56 PM


New To Site? Need Help?

All times are GMT. The time now is 09:49 AM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright 1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2