Help with first Power Supply for LM1875

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AndrewT

R.I.P.
Joined 2004
Professor said:
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.
Click to expand...
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

Professor

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Joined 2005
mymegahobby.jimdo.com
AndrewT said:
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.
Click to expand...
Hi, big thanks
 
S

simonlarusso

Member
Joined 2008
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
 
Scratchy

Scratchy

Member
Joined 2008
pacificblue said:
Use a single bridge rectifier with a center-tapped transformer. A dual secondary transformer (as drawn) is better combined with one rectifier per winding.
Click to expand...

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

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


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.
 
A

AndrewT

R.I.P.
Joined 2004
Ted205 said:
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.
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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.
 
S

simonlarusso

Member
Joined 2008
Ted205 said:
i would do the ground :

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


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.
Click to expand...


I am going to keep my first build quite simplistic being new to building amps.

So I don't need a physical wire to the mains ground point on the chassis from the 'star' ground point (blue blob)? Would the chassis act as this connection?
 
S

simonlarusso

Member
Joined 2008
Ok thanks. Here is my new schematic with my smoothing caps which are on my rectifier board:

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


Another couple of questions though:

1. Should the ground wires coming from my rectifier board connect together before being connected to the star ground point?

2. Is it best to use ring connectors crimped to all my ground wires (and then all bolted to the chassis) for my star ground, or should I just solder the wires all together?
 
A

AndrewT

R.I.P.
Joined 2004
I use a bolted connection using separate solder tag on evry wire.
This allows experimentation.
You will find that the order of the solder tags makes a difference to the various unwanted noises coming from the output.

Use a separate bolted connection (not a solder tag) for the chassis connection to Safety Earth.
 
S

simonlarusso

Member
Joined 2008
Thanks guys, I really want to crack on with this build now but finding it hard to get spare time and don't want to rush and make mistakes.

Are the bypass caps essential parallel to the smoothing caps or can I add them later? I have bypass caps on the amp PCBs but I guess having them on the powersupply and the amp boards are a good idea, yes?
 
S

simonlarusso

Member
Joined 2008
I finished the amplifier yesterday and the sound I get out is very good indeed. Very pleased I took the DIY route have I have learnt a lot in the process.

I just have a couple questions:

My transformer is outputting around 20V AC which when rectifies is around +/- 27V DC. The transformer says it has 18V AC secondaries (therefore giving me +/- 25V DC). Is there any reason why the tranformer is outputting a slightly higher voltage?

Secondly the left channel is cutting in and out when the amp has been going for a while. I think this must be because its getting to its thermal limit so I am going to check its tight to the heat sink and maybe buy a different one to prevent this. I assume the chips will run hotter as they have 27V through them not 25V I was aiming for, is that correct?
 
A

AndrewT

R.I.P.
Joined 2004
almost all manufacturers quote the secondary voltage when all the windings are resistively loaded to their rated current and when the primary is fed with the rated input voltage.
If your 18Vac measures 20Vac when on open circuit, then that includes both the regulation and mains voltage error at the time of measurement.

Does the +-27Vdc drop slightly when you connect the amplifier? Or is that with the amplifier connected? Does it rise even higher with all loads disconnected?.
How high will it rise if the supply rail fuses blow and your mains is at maximum tolerance. In the UK that is 254Vac from the mains. That determines the minimum voltage rating of the caps before the fuses.

How hot do the chips get?
Can you hold your finger on them continuously & comfortably?
Do they get hotter when delivering music to your speakers?
 
S

simonlarusso

Member
Joined 2008
Tested the voltages tonight and they do indeed drop when then ampifier is running, +/- 26 V.

The chips are getting very hot and no I cannot keep my fingers on them comfortably! They will burn me. Not sure if they get even hotter when connected to my speakers cause I cant touch them. Are my heatsinks to weedy for these chips then or do I have another problem?
 
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