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Chip Amps Amplifiers based on integrated circuits

LM1875 Amp layout
LM1875 Amp layout
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Old 2nd June 2019, 02:00 AM   #1
Dunk02 is offline Dunk02  Australia
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Default LM1875 Amp layout

Hi Peeps

I have been gathering parts for a stereo amp build and am at the construction stage. This is my second diy amp, using two LM1875 boards.

I wanted a compact unit to go with my DIY bookshelf speakers.

Can I get some advice on component layout, what do I need, what donít I need, can it all be squeezed in?

Here is a picture of the parts in the case.
Components:
Toroidal 120VA
DC power supply board
2 x LM1875 amplifier boards
Preamp board for iPhone (if needed) and separate transformer (blue)
Speaker protection board (not yet assembled) and separate AC transformer (blue)
Anodized aluminium case with side heat sinks from Ali-express
Copper sheet to make a shielding can for the transformer if needed

It is very tight with all components used and I am not sure how to best arrange it.

Thanks
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Old 2nd June 2019, 02:28 AM   #2
rayma is offline rayma  United States
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Quote:
Originally Posted by Dunk02 View Post
Can I get some advice on component layout, what do I need, what don’t I need, can it all be squeezed in?
Can the power supply board go vertically on the right heat sink, or is it too big (wide)?
Don't bother about the copper, it won't magnetically shield the transformer.

Can you swap the RCAs with the rightmost binding posts? They're very close to the toroid.
Will the lid clear the transformer bolt? You don't want that to touch the top (shorted turn).

Last edited by rayma; 2nd June 2019 at 02:37 AM.
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Old 2nd June 2019, 02:40 AM   #3
Dunk02 is offline Dunk02  Australia
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Hi
Thanks for the comments.
Unfortunately the PS PCB is just too wide to go vertically.
Now you point it out, not sure why I got copper, do I need to shield the transformer?
Yea I can switch the RCA over.
I will have to trim the bolt when the toroid is installed.
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Old 2nd June 2019, 02:49 AM   #4
rayma is offline rayma  United States
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Quote:
Originally Posted by Dunk02 View Post
do I need to shield the transformer?
Yea I can switch the RCA over.
I will have to trim the bolt when the toroid is installed.
It's gonna be tight in there. Forget the copper, though.
There may be no room for proper steel shield similar to this:
https://www.ebay.com/i/251787507844?chn=ps
Make certain that the bolt and large mounting washer cannot touch anything.
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Old 2nd June 2019, 02:53 AM   #5
Dunk02 is offline Dunk02  Australia
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Alright, ditching the magnetic shielding idea
RCAs moved over.
Do I put the volume pot before or after the preamp?
Do I only use shielded cable for the wiring between the signal input and the main amplifier board input?
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Old 2nd June 2019, 02:58 AM   #6
rayma is offline rayma  United States
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Quote:
Originally Posted by Dunk02 View Post
Do I put the volume pot before or after the preamp?
Do I only use shielded cable for the wiring between the
signal input and the main amplifier board input?
Volume control goes between the preamp and the power amp.
Twisted pairs carry the signals in/out of the volume control.
I'd run tightly twisted wire pairs straight along the RCA side of the heat sink,
and then over to the preamp. Many prefer shielded cables, though.
They'd be much more difficult to work with for this.
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Old 2nd June 2019, 03:04 AM   #7
Dunk02 is offline Dunk02  Australia
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Thanks for the advice
The preamp is fixed 5x gain, will this help with iphone as a source, or is it not needed?
The gain these eBay amp boards is 23

Last edited by Dunk02; 2nd June 2019 at 03:08 AM.
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Old 2nd June 2019, 03:12 AM   #8
rayma is offline rayma  United States
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Quote:
Originally Posted by Dunk02 View Post
Thanks for the advice
The preamp is fixed 5x gain, will this help with iphone as a source, or is it not needed?
I am not sure what the gain has been set to for these eBay amp boards.
Typical LM1875 gain seems to be around x21 (26dB),
so you'd need around 0.75Vrms input to drive the amps to clipping.
Check the phone specs to see if you can ditch the preamp.
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Old 2nd June 2019, 03:18 AM   #9
Dunk02 is offline Dunk02  Australia
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Does this information from Apple's Lightning Headphone Adapter: Analog or Digital? | InnerFidelity say that it is about 0.5v for the iphones?
Not sure how to apply 5x gain to this, is it dB or a voltage multiplier?
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Old 2nd June 2019, 03:23 AM   #10
rayma is offline rayma  United States
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Quote:
Originally Posted by Dunk02 View Post
it is about 0.5v for the iphones?
Not sure how to apply 5x gain to this, is it dB or a voltage multiplier?
Looks like that's close enough to 0.75V that I'd lose the preamp.
A gain of x5 is 14dB. No units always means a factor (which has no units).
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