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Old 19th March 2016, 04:14 AM   #1
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Default LM4780 parallel amp finished - PSU question

Hi there,

I finally finished my LM4780 parallel amp using Audio Sector boards (Fig1). I use Avel 25VAC 500VA transformer, so I get about 35VDC for pos/neg rails. I first tested the amp with the fast recovery diodes provided by AS, but it gets really hot for two channels at 35V, so I decided to use two CRC boards lying around. They have 8x2,200uF each. As can be seen from Fig2, bridges become really hot.

When doing this, I found something strange. See Fig. 3.
I first tried Configuration 1, because I have two CRC boards and only one transformer. It produced very loud hum/buzz. compared to using only one CRC board. I don't understand why this happens.

When I series connected to CRCCRC, there was absolutely no hum. (Conf2)

I got curious and tested Conf3, connecting R's of positive and negative rails together, hoping it just parallels C's, and it produced as loud hum/buzz as Conf1. I don't understand this either.

I haven't tried Conf4 because it was hard to access bottom side of the board (and I am lazy to disassemble). It's basically using only one bridge, but use separate CRC boards. What will happen?

Thanks,
Doug
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Old 19th March 2016, 06:51 AM   #2
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Your drawings don't show what is going on with the earthing - this is very important, and most often at the root of hum issues.

Also, when you say hot, how hot do you mean?

Aside: I don't see any fuses in there - or am I missing something?
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Old 19th March 2016, 11:18 AM   #3
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Quote:
Originally Posted by googlyone View Post
Your drawings don't show what is going on with the earthing - this is very important, and most often at the root of hum issues.

Also, when you say hot, how hot do you mean?

Aside: I don't see any fuses in there - or am I missing something?
Ground/earth are connected from each CRC board to one star ground point. I will add it to the figure later.

The heatsinks become about 45-50 C at full power output.

I don't use fuses in DC rails. I know it's more safe, but since papa (Nelson Pass) doesn't use them for his FirstWatt circuits. I have a proper main fuse.
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Old 19th March 2016, 11:44 AM   #4
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Default ATL Hi-Fi DC & Ripple Blocker x4 ME

In my second system I have ATL Hi-Fi DC & Ripple Blocker x4 ME (aka Maty Edition). It uses four DC blockers (I have up to 3 Vdc!!! ) connected in series, with four bridges.

DC Trap Blocker Filter for Toroidal Transformers Assembled in Case | eBay

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Old 19th March 2016, 11:49 AM   #5
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You can try one in the line and one in the neutral.

AC power connection schematic explanation needed

Click the image to open in full size.

Variations of DC Main Filter against buzzing Toroid Transformers - what is the right?

I know, they are before the transformers but the idea is the series connection.

Last edited by maty tinman; 19th March 2016 at 12:02 PM.
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Old 19th March 2016, 01:47 PM   #6
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My toroid buzzes a little at full power, but otherwise it is silent.
I said hum/buzz, because some people say:

Hum : 60 (or 50) Hz noise from main AC
Buzz or ripple: 120 (or 100) Hz noise from rectified DC

I will measure if I have DC component in the main.
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Old 19th March 2016, 01:54 PM   #7
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Default Ripple

Click the image to open in full size.

If you have 100 mVdc (230 Vac / 50 Hz) or more you have problems. Maybe 50 mVdc at 115 Vac.

https://en.wikipedia.org/wiki/Ripple_%28electrical%29

Quote:
Effects of ripple

Ripple is undesirable in many electronic applications for a variety of reasons:
  • The ripple frequency and its harmonics are within the audio band and will therefore be audible on equipment such as radio receivers, equipment for playing recordings and professional studio equipment.
  • The ripple frequency is within television video bandwidth. Analogue TV receivers will exhibit a pattern of moving wavy lines if too much ripple is present.[7]
  • The presence of ripple can reduce the resolution of electronic test and measurement instruments. On an oscilloscope it will manifest itself as a visible pattern on screen.
  • Within digital circuits, it reduces the threshold, as does any form of supply rail noise, at which logic circuits give incorrect outputs and data is corrupted.
  • High-amplitude ripple currents shorten the life of electrolytic capacitors.[8]
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