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Chip Amps Amplifiers based on integrated circuits

Beginner's Gainclone, HiFi LM1875, The Amplifier Board
Beginner's Gainclone, HiFi LM1875, The Amplifier Board
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Old 17th April 2017, 11:25 AM   #751
AndrewT is offline AndrewT  Scotland
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C6 & C8 can be any size from 100uF to 1mF
C5 & C7 must be located very close to the chipamp power pins and must have a low loop area back into the Zobel Return and Speaker Return.
C1 must be your input high pass filter.
Be careful that it limits the signal that C3 needs to handle. There should be zero AC voltage across C3, if it is to do it's DC blocking job properly.
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Old 17th April 2017, 08:58 PM   #752
otc is offline otc  Denmark
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Beginner's Gainclone, HiFi LM1875, The Amplifier Board
Quote:
Originally Posted by AndrewT View Post
C6 & C8 can be any size from 100uF to 1mF
C5 & C7 must be located very close to the chipamp power pins and must have a low loop area back into the Zobel Return and Speaker Return.
C1 must be your input high pass filter.
Be careful that it limits the signal that C3 needs to handle. There should be zero AC voltage across C3, if it is to do it's DC blocking job properly.
Thank, You. I get Your points.
Specifically I read the thread about Prof. Cherry's LF phase-correction in the feedback loop and understand that C1 in my circuit can not go higher in value, it is mandatory in the circuit to cut LF at same or a higher frequency than C3. So will only use value 0,47 uF or maybe smaller, but different types of film-caps.
I suppose the Prof. Cherry phase-correction is audible and not just measurable?
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Old 18th April 2017, 07:31 AM   #753
AndrewT is offline AndrewT  Scotland
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I think it is audible. But maybe I am being fooled.
It does create a small hump in the response just before the roll-off. Maybe that is more audible than the phase correction.
I consider the input filter as mandatory to remove that LF hump.
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Old 5th May 2017, 11:09 PM   #754
fortney is offline fortney  United States
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Hi,
I build an LM 1875 mono amp using a Paia.com kit and modifying it according to suggestions in this thread some years ago. I am using an 8 Ohm speaker and my P.S. is 12-0-12. I reviewed pages from the end to about p. 68. I see one discussion about the resistor/inductor network at the amp's output, but I don't understand what purpose this serves. Fill me in, please.
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Old 6th May 2017, 07:02 AM   #755
AndrewT is offline AndrewT  Scotland
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an amplifier normally has a load. The gain of the amp is stabilised by that load.
Stability margins can change when the load is removed.

At low frequency removing the speakers or removing the speaker cable usually has no effect.
At high frequencies the amplifier is very intolerant of removing the load. So much so that an open output can change the stability margins so much that the amplifier oscillates.
The cure for this risk of reduced stability margins is to add on a load that only operates at high frequency and remains there no matter what is hung on the output.
That HF load is the amplifier Output Zobel. Often you see 10r+100nF but many other values are seen.
Don't omit the output Zobel unless you have proved the amplifier is stable AND has adequate stability margins when the output is left open.
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Old 6th May 2017, 02:15 PM   #756
fortney is offline fortney  United States
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Thanks for your assistance.
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Old 14th July 2017, 02:31 PM   #757
Robrobbe is offline Robrobbe
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Ho Daniel, Thank you for this project.

I don't understand what does the pot 200R in front of your Circuit .

And then how to regulate it?

TI A
Roberto
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Old 18th July 2017, 09:15 AM   #758
Pixelbrick is offline Pixelbrick
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Hi all,

I've been lurking about here for a while now & would like to thank all the contributors to this thread (and others on the board...) for all the excellent information they've provided.

I'm about to have a go at building an LM1875 based amp & would really appreciate it if you guys could have a look at my schematic and board layout & let me know if I'm doing anything particularly stupid.

I've a centre tapped antek toroid (2x15V @ 2.67A) that I'm looking at for the power supply, am I right in thinking that this is going to be not nearly enough to feed two of these things in stereo?

Much appreciated
Jay.
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Old 18th July 2017, 09:59 AM   #759
Pixelbrick is offline Pixelbrick
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Oops.

I meant to say that the top copper ground traces on the board are "indicative". I plan to etch the first boards single sided & ground input, output and power grounds separately at power ground.

Cheers,
Jay.
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Old 19th July 2017, 08:10 AM   #760
AndrewT is offline AndrewT  Scotland
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Your 2x15Vac 2.67Aac is an 80VA transformer.
That is good enough for between 40W to 80W of total maximum power.
That easily covers a stereo pair of lm1875
Personally I prefer at least 160VA for a power amplifier due to the lower regulation offered and thus the smaller range of voltage between no load and quiescent load and maximum power load.

Your -IN input sees 47k for the DC current .
But you have 23k at the +IN input. Why?
Change that to 47k to match the -IN input and then reduce the input capacitor C1, to 2u2F
I seriously doubt you will be able to hear any difference between an MKT/MKS and an MKP. In my book smaller and cheaper is better.
A physically smaller capacitor here is more tolerant of interference.

C8 (the Zobel route) MUST tie into the decoupling ground between C6 and C7. Do not take it on a meandering route around the PCB. That long route adds inductance/impedance and ruins the performance of the Zobel.

+32dB (40times) is quite a high gain. Do you have a reason for going this high?
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Last edited by AndrewT; 19th July 2017 at 08:16 AM.
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