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How to wire up an Amplifier
How to wire up an Amplifier
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Old 21st April 2019, 11:01 AM   #1
Bonsai is offline Bonsai  Europe
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Default How to wire up an Amplifier

I've started this thread so the various approaches to wiring up an amplifier can be discussed and debated.

There are two primary concerns when wiring up an audio amplifier:-

1. minimizing loop areas to prevent magnetic coupling - of which the classic ground loop is the best example, but cross-channel ground loops are a related phenomena as well.
2. common impedance coupling - an example of this would be failing to use a 'T' or a 'Star' grounding scheme to connect the rectifier(s), reservoir capacitors and the amp module grounds together.

Clearly, some have their preferences which will most often be driven around what they consider to be the worst of the various evils.

To get the discussion going, here are some presentations - others can be added to the list so that there is a more complete overview on the subject.

We should set ourselves the goal here on DIY audio of building amplifiers with hum and related noise levels of at least -100 dB ref 2.83 volts pk to pk out, but a better target is better than -110 dB.

http://hifisonix.com/wordpress/wp-co...ound-Loops.pdf

http://hifisonix.com/wordpress/wp-co...ng-Hum-1-1.pdf

Here are DIY audio member ilimzn's Excellent Posts #50 and #51 on Ground Loops

http://hifisonix.com/wordpress/wp-co...ound-Loops.pdf

(I've hosted this on my website for the time being since I don't seem to be able to load the doc up into this first post. Maybe one of the mods could help out here with some directions?)

Last edited by Bonsai; 23rd April 2019 at 09:35 PM.
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Old 21st April 2019, 11:06 AM   #2
Bonsai is offline Bonsai  Europe
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The first topic to discuss is what do you do with the speaker return (ie the speaker -ve wire).

Do you take it back to the amplifier module and let it share the same 0V line back to the PSU as the amplifier module 0V wire (easy to minimize loop areas with this approach)

or

is it better to run it directly back to the power supply 0V. You avoid voltage drops across the common 0V wire back to the PSU with this method so the 0V to the module essentially only carries the module bulk decoupling capacitor smoothing currents
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Old 21st April 2019, 11:13 AM   #3
scottjoplin is offline scottjoplin  Wales
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How about both? Run it close coupled back to the amplifier module, but don't connect it to PSU 0V there, instead run it close coupled with the PSU supply leads back to the PSU 0V. Just a thought.
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Old 21st April 2019, 11:18 AM   #4
keantoken is offline keantoken  United States
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Quote:
Originally Posted by Bonsai View Post
is it better to run it directly back to the power supply 0V. You avoid voltage drops across the common 0V wire back to the PSU with this method so the 0V to the module essentially only carries the module bulk decoupling capacitor smoothing currents
If you do that you move the speaker return reference away from the signal ground reference and the PSU return currents through the umbilical ground wire cause a noise voltage between the references.
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Old 21st April 2019, 11:24 AM   #5
keantoken is offline keantoken  United States
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Quote:
Originally Posted by scottjoplin View Post
How about both? Run it close coupled back to the amplifier module, but don't connect it to PSU 0V there, instead run it close coupled with the PSU supply leads back to the PSU 0V. Just a thought.
I would say he addressed this by writing

Quote:
(easy to minimize loop areas with this approach)
In other words it could be done, but it's harder to do and he wants an easier solution.

BUT, while you reduce the loop area the PSU return currents still create a noise voltage across the impedance of the PSU return conductor. One step forward, two steps back.
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Old 21st April 2019, 12:34 PM   #6
Bonsai is offline Bonsai  Europe
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But, if you decouple suitably on the amp module PCB and the loops are smaller, the only currents in the 0 V wire back to the amp module are LF.

How much signal coupling do you expect between the speaker return and the amplifier module?

The question is, do you want wideband currents being radiated around inside you amplifier or do you think the common impedance between the amp board and the PSU is a more manageable problem?
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Old 21st April 2019, 12:53 PM   #7
keantoken is offline keantoken  United States
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Well let's say the onboard decoupling is 10mF 10mohm ESR capacitors, and the umbilical wires are 10mohm (much larger than would actually be expected). HF current is equally split between the onboard capacitors and the umbilical, minus a small amount from the umbilical inductance, which will be very low if it's routed correctly. Even a local 1uF film cap only kicks in at 15KHz across the 10mohm ESR.

Furthermore, the rectifier pulses aren't perfectly symmetrical, there is a residual ground pulse especially on asymmetrical music or frequencies below 120Hz.
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Old 21st April 2019, 01:10 PM   #8
keantoken is offline keantoken  United States
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In fact if HF currents are confined to the amp board, that just proves that the umbilical resistance is enough to cause a noise voltage between board ground and the main reservoir tee.
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Old 21st April 2019, 01:14 PM   #9
Mark Whitney is online now Mark Whitney  Netherlands
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The local decoupling capacitors kick in every time the current passes zero in a class AB amp. The resulting (ground) currents have a phase shift forward compared to the output current because of the recharge and supply.
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Old 21st April 2019, 01:55 PM   #10
Bonsai is offline Bonsai  Europe
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Quote:
Originally Posted by keantoken View Post
In fact if HF currents are confined to the amp board, that just proves that the umbilical resistance is enough to cause a noise voltage between board ground and the main reservoir tee.
They are confined to the amp module board because the loop is smaller through the local on board decoupling.

I would argue that as long as any noise voltage across the umbilical ground is below the noise floor, you’re ok.

I control the umbilical 0V connection by keeping it short and very thick (easy to do) and then the radiated wideband field from the speaker by ensuring the signal and return are twisted and the return goes back to the amplifier board.
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