Help with first Power Supply for LM1875

[pacificblue]

Use a single bridge rectifier with a center-tapped transformer. A dual secondary transformer (as drawn) is better combined with one rectifier per winding.

[Professor]

How about this?

[AndrewT]

post 12 shows the Audio Ground combined with the smoothing cap zero volt. DON'T.
Keep the smoothing cap charging pulses within their own (figure of 8) loop that runs from transformer to rectifier to caps and back again. Take a single short wire from zero volt to the Audio Ground.

Post 11 is wrong. A single rectifier works just as well with either a centre tapped or a dual secondary wired as a centre tapped.
The advantage of the dual rectifier on a dual secondary is the ability to keep each channel's Audio Ground separate and use a disconnecting network to connect each Audio Ground back to the Safety Earth. This option is not shown in any of your diagrams.

Start with a smaller bulb (40W) and progressively increase to what is required to stay unlit as each piece is added on. Starting with the big wattage could damage something if the wiring or semiconductors are wrong.

[simonlarusso]

Originally posted by pacificblue
Use a single bridge rectifier with a center-tapped transformer. A dual secondary transformer (as drawn) is better combined with one rectifier per winding.

Oh ok, but does a dual secondary type still work with one brdge rectifier?

Also can you get centre tapped toroidal type transformers as I can't seem to find one on the website I will be ordering from?

[simonlarusso]

Thanks for all the information so far guys. I have been reading Peter Daniel's tutorial/thread and think I am going to use two bridge recifiers. I have changed my schematic to use a star grounding:



Is this one ok and where would I put the smoothing capacitors in this configuration? In fact do I need smoothing caps or are they recommended?

[pacificblue]

Post #11 is correct. It recommends the two out of four possible choices that make most sense.

You need smoothing capacitors. Their placement is as recommended in post #3.

Your choices are

• - gainclone style with only one 1000 µF smoothing capacitor per rail (can vary from 220 to 2200 µF according to taste) as near to the LM3886 as possible.
  - datasheet style with 0,1 µF directly at the LM3886's power supply pins, 10 µF as near to them as possible on the PCB, 1000 µF not too far away also on the PCB, and greater than or equal to 10.000 µF off the PCB in your PSU.
  - high-end style with lots of capacitors in all shapes and sizes, lovingly placed along a complex pattern, supported by bleeder resistors and snubber networks, all components chosen after careful listening tests, yet everybody uses different ones according to personal taste. For an example search for CarlosFM snubberized power supply.

[AndrewT]

Originally posted by pacificblue
A dual secondary transformer (as drawn) is better combined with one rectifier per winding.

justify this statement.

[pacificblue]

The use of a single bridge rectifier for both windings makes it a full-wave-rectifier for each rail. In German that is called "two-way-rectifier", which makes its working principle clearer. The other application is a bridge rectifier for each rail.

With bridge rectifiers the effective load the secondaries pose for the primaries is 1,41 times smaller than with full-wave-rectification. The reason is that in the full-wave-circuit current is only flowing through each winding during one half-cycle, while with bridge rectification current flows during both half-cycles. The resulting lower load for the primaries is good for the transformer's regulation. It leaves more headroom until saturation. And it compensates for the efficiency loss that double as many diodes in the current path produce through their forward voltage drop.

Another advantage was explained by Mr. Nelson Pass.

Originally posted by Nelson Pass
Most customers want total silence from the amplifier, including
mechanical noise. If there is not complete matching between the
secondary coils and only 1 rectifier bridge, any net DC imbalance
between the current of the + supply and the - will tend to
saturate the core of the transformer and create noise. This is
seen for quite low current differences and can also show up with
low frequency output. Using two bridges eliminates the problem.

A possible third advantage was described by yourself in post #13.

Regards
David

[simonlarusso]

Ordered the parts to build the powersupply today so hope to start to put it all together in the coming week. Going to take it slow as its my first time and test it all stage by stage with the light bulb.

Will post some photos as I go along with questions most probably!

[Professor]

Originally posted by AndrewT
post 12 shows the Audio Ground combined with the smoothing cap zero volt. DON'T.

Explain why? It is the checked up scheme. I have made many amplifiers on LM under this scheme, they work wery well and sound is good.