[ProAudio] Muncy Revisited: Pin 1 to chassis through a capacitor?

[crispin at crookwood.com]

Hi Bob,

 

Well, there’s theory and practice…

 

In your case, there are two things to consider:

1.	There is no such thing as ground
2.	You are making essentially a power amp (big current)

 

Point 1) means that you the second you pass any current through a conductor, it ceases to be a ground reference.  Point 2) means that you are going to be dumping an amp say (if it’s a decent studio headphone amp) through your ground return – it really will stop being a ground.

 

Typically if you’ve got a nice metal box, you can treat this as a chassis ground for RFI.  Relatively low level currents will circulate around the chassis, and you can tie pin 1 to this OK.  The chassis will float to a potential due to the RFI currents, and all of the pin 1’s will mostly be shorted together.  Every input can be connected to a differential amp, balanced, or unbalanced, so the inputs are all sorted.

 

However, your power amp is unbalanced.  If you connect the return of your  headphones to this chassis by shorting the output “ground” to the chassis, it will all go Pete Tong, because you’re now dumping 1A through the chassis.

 

The current flow goes, 1A through the transformer secondary, ref the 0V of the secondary, via the regulators to the power amp, via the output transistors, via the headphones, back down the return ground. This return ground needs to be connected to two points. 1 to the power amp ground reference differential pin, because the power amp output is referenced to this, and 2 directly to the 0V tap of the secondary, and nowhere else.

 

You can connect the return ground to pin 1/ chassis via a 10N ceramic if you want some RFI attenuation however.

 

Finally, you need to connect your 0V secondary to the chassis at one point, and one point only.  This ground partitioning will give you the best RFI and audio performance.  Digital grounding is different however. 

 

Note mass production often uses ground planes.  These are convenient, but do not provide a good analogue ground, especially if you’re dumping any current through them.

 

Hope this helps.

 

 

Kind Regards

 

Crispin Herrod-Taylor

Managing Director, Crookwood

 <http://www.crookwood.com/> www.crookwood.com  

Tel: +44 (0)1672 811 649

Mobile:+44(0)7910 637 634

 

Sign up for our great newsletter  <http://crookwood.com/newsletter/> here! and keep up to date with the audio world

 

From: ProAudio <proaudio-bounces at bach.pgm.com> On Behalf Of Bob Katz via ProAudio
Sent: 02 December 2023 17:21
To: proaudio <proaudio at bach.pgm.com>
Subject: [ProAudio] Muncy Revisited: Pin 1 to chassis through a capacitor?

 

Hi, Pro audio! Happy Holidays from Bob.

Naturally, I started a firestorm on facebook asking a question about grounding in a headphone amplifier I'm building. 

I made a mistake by forgetting to show the connection between the 0 volt (transformer center tap) and chassic ground... oops, so the conversation went downhill and even after I revised my diagram some people were responding to my first diagram with the error :-(

I insisted (and still insist) that EMI/RFI interference at the audio input is ameliorrated in Muncy's "rule 2": by connecting pin 1 to chassis directly at the audio input. This is the centerpiece of Muncy's seminal article "Noise Susceptibility in Analog and Digital Signal Processing Systems"

I know this from experience as well as theory, having successfully installed a microphone preamplifier on the 101st floor of the World Trade Center in 1995 or so. 

Now on Facebook, a genuine expert in power supply regulation design said "that will make the regulator unstable cause it sees every ground current as an error".

I'm not expert enough to either verify or refute the above statement, but perhaps my response to his statement could be to recommend connecting pin 1 to chassis through a capacitor. 

Would the experts on this group please comment on these thoughts? Thanks.

I think that this reflector does not permit attachments, so here is a link to my simple block diagram, which is essentially a simplification of Muncy's Figure 1 in his paper if you'd like to refer to the diagram:

https://crush.digido.com/?u=pro_audio <https://crush.digido.com/?u=pro_audio&p=muncy> &p=muncy

 

Thanks in advance,

 

Bob

 

 

-- 

 
 
If you want good sound on your album, come to
Bob Katz 407-831-0233 DIGITAL DOMAIN MASTERING STUDIO
Author: Mastering Audio
Digital Domain Website <https://www.digido.com/> 
 
No trees were killed in the sending of this message. However a large number
of electrons were terribly inconvenienced.
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://bach.pgm.com/pipermail/proaudio/attachments/20231202/80538921/attachment.html>