Help with first Power Supply for LM1875 - Page 4

simonlarusso · 4th November 2008, 08:07 AM

Quote:

Originally posted by 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.

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?

Ted205 · 4th November 2008, 08:37 AM

yes the chassis will make the connection. Just make sure you have good contact. Scrape off any paint and clean the surfaces with sand paper

simonlarusso · 4th November 2008, 09:44 AM

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

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?

Ted205 · 4th November 2008, 10:07 AM

add a 100nf bypass cap parrallel to the smoothing cap.

connect each ground to the star ground individually.

I used crimped ring connectors and solder for each wire.

AndrewT · 4th November 2008, 11:08 AM

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.

simonlarusso · 4th November 2008, 12:16 PM

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?

simonlarusso · 5th November 2008, 07:53 PM

Finished the back panel, next stage is the rectifier board.

simonlarusso · 10th November 2008, 03:00 PM

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?

AndrewT · 10th November 2008, 04:18 PM

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?

simonlarusso · 10th November 2008, 06:08 PM

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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4th November 2008, 08:37 AM	#32
Ted205 diyAudio Member Join Date: Jun 2008	yes the chassis will make the connection. Just make sure you have good contact. Scrape off any paint and clean the surfaces with sand paper

4th November 2008, 09:44 AM	#33
simonlarusso diyAudio Member Join Date: Aug 2008	Ok thanks. Here is my new schematic with my smoothing caps which are on my rectifier board: 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?

4th November 2008, 10:07 AM	#34
Ted205 diyAudio Member Join Date: Jun 2008	add a 100nf bypass cap parrallel to the smoothing cap. connect each ground to the star ground individually. I used crimped ring connectors and solder for each wire.

4th November 2008, 11:08 AM	#35
AndrewT diyAudio Member Join Date: Jul 2004 Location: Scottish Borders	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.

4th November 2008, 12:16 PM	#36
simonlarusso diyAudio Member Join Date: Aug 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?

5th November 2008, 07:53 PM	#37
simonlarusso diyAudio Member Join Date: Aug 2008	Finished the back panel, next stage is the rectifier board.

10th November 2008, 03:00 PM	#38
simonlarusso diyAudio Member Join Date: Aug 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?

10th November 2008, 04:18 PM	#39
AndrewT diyAudio Member Join Date: Jul 2004 Location: Scottish Borders	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?

10th November 2008, 06:08 PM	#40
simonlarusso diyAudio Member Join Date: Aug 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?