LM4780 parallel amp finished - PSU question

[Doug Kim]

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

 

[googlyone]

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?

 

[Doug Kim]

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.

 

[maty tinman]

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

An externally hosted image should be here but it was not working when we last tested it.

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An externally hosted image should be here but it was not working when we last tested it.

 

[maty tinman]

You can try one in the line and one in the neutral.

http://www.diyaudio.com/forums/pass-labs/3924-ac-power-connection-schematic-explanation-needed.html

An externally hosted image should be here but it was not working when we last tested it.

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http://www.diyaudio.com/forums/powe...id-transformers-what-right-8.html#post2735854

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

 

[Doug Kim]

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.

 

[maty tinman]

Ripple

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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

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]