Help with first Power Supply for LM1875

[AndrewT]

Originally posted by Professor
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.

Hi,
the Audio Ground must be a clean ground as reference for all the stages connected to it, particularly the Signal and Input Ground.
The link between the smoothing caps carries the charging pulses flowing from transformer through rectifier and though caps and back following a figure of eight around the two halves of the rectifier/transformer secondary/+V& -V caps.
Any tiny resistance/impedance along this link will have a commensurate volts drop due to the passing current and the current does flow in shortish pulses.
If this link also doubles up as the Audio Ground then the volts drop between the various ground connections will give rise to a variable voltage reference and this can lead to buzzes and hum at the output of the amplifier. The Audio Ground MUST be separate from the smoothing cap link. It only needs 1mm of connecting wire from the link to the Audio Ground to ensure this required separation exists, but you must not allow the various grounding wires to be spread along the link. That guarantees trouble.

[Professor]

Originally posted by AndrewT
Hi,
the Audio Ground must be a clean ground as reference for all the stages connected to it, particularly the Signal and Input Ground.
The link between the smoothing caps carries the charging pulses flowing from transformer through rectifier and though caps and back following a figure of eight around the two halves of the rectifier/transformer secondary/+V& -V caps.
Any tiny resistance/impedance along this link will have a commensurate volts drop due to the passing current and the current does flow in shortish pulses.
If this link also doubles up as the Audio Ground then the volts drop between the various ground connections will give rise to a variable voltage reference and this can lead to buzzes and hum at the output of the amplifier. The Audio Ground MUST be separate from the smoothing cap link. It only needs 1mm of connecting wire from the link to the Audio Ground to ensure this required separation exists, but you must not allow the various grounding wires to be spread along the link. That guarantees trouble.

Hi, big thanks

[simonlarusso]

All the components have arrived and I have started to work out the layout in the case.

My first silly question has come up, the transformer came with two rubber mats, for vibration reduction as you know, do both of the mats go between the case and the traffo or does one go between case and traffo and one between traffo and the plate to secure the traffo down?

Thanks

[AndrewT]

I think the soft washers are to prevent damage to the windings when it is tightly bolted into place.
Put one on each side.
Ensure that the top of the bolt/washer plate can never short to the chassis.

[simonlarusso]

Originally posted by AndrewT
I think the soft washers are to prevent damage to the windings when it is tightly bolted into place.
Put one on each side.
Ensure that the top of the bolt/washer plate can never short to the chassis.

Many thanks

[simonlarusso]

Started building tonight, so far got the transformer and amp PCBs mounted.

[Scratchy]

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.

Even with a center-tapped transformer you should be able to use 2 rectifiers:



This image is no different than a center-tapped secondary where the 18V black and 0V yellow wires are tied together internally.
It should still work the same.

[AndrewT]

Originally posted by Scratchy


Even with a center-tapped transformer you should be able to use 2 rectifiers:



This image is no different than a center-tapped secondary where the 18V black and 0V yellow wires are tied together internally.
It should still work the same.

NO!!!!

[Ted205]

i would do the ground :



i would also place a capacitor accross the switch and before and after the transformer. Its really up to you how simplistic you want the design though.

[AndrewT]

Originally posted by Ted205
I would also place a capacitor across the switch and before and after the transformer. Its really up to you how simplistic you want the design though.

no, place an interference suppressor, consisting of a series pair of C + R, across the switch.

All mains connected caps must be X or Y rated.
Mains Live to Mains Neutral are X1 or X2 type.
Mains Live to Earth and Mains Neutral to Earth are Y1 or Y2 type.