The Super-Simple ChipAmp, Pt. II

Spasticteapot · 3rd November 2006, 01:41 AM

After having my first attempt not work very well, I analysed the problems.

1. I forgot to add a place to solder the output lead.
2. The leads were oddly positioned.
3. The "jumper" on the top side had to go.
4. I needed bigger traces.
5. I needed bigger holes.

After a little work in Eagle, I have the PCB below.

Aside from a little neatening, I have one modification to make before I get to etching. I'm not sure of where to put the 0.1uf MKP bypass caps : Right next to the LM3875, or right next to the 100uf caps?

AndrewT · 5th November 2006, 10:55 AM

Hi,
the bypass cap is the MAIN route in for the signal (also applies to NFB blocking cap). All the high frequency and most of the mid are determined by this cap.

Forget all else and get this cap on the shortest direct route in to the chip pin.

Then try to wrap the electrolytic route around it. It is only there to try and boost the bass response and nothing else.

Based on the above the plastic cap should be as large a value as you can afford (both PCB space and cost). 2u2F is a good start but maybe aim for 10uF or a //pair of 4u7F.

Spasticteapot · 5th November 2006, 04:19 PM

Quote:

Originally posted by AndrewT
Hi,
the bypass cap is the MAIN route in for the signal (also applies to NFB blocking cap). All the high frequency and most of the mid are determined by this cap.

Forget all else and get this cap on the shortest direct route in to the chip pin.

Then try to wrap the electrolytic route around it. It is only there to try and boost the bass response and nothing else.

Based on the above the plastic cap should be as large a value as you can afford (both PCB space and cost). 2u2F is a good start but maybe aim for 10uF or a //pair of 4u7F.

Most people don't use them at all. (I, however, have a fondness for overkill.)

Also, 2uf plastic caps are expensive!

AndrewT · 5th November 2006, 05:31 PM

Hi,
which don't get used?

DC blocking caps in general?
or
Electrolytic caps?
or
Plastic film caps?

And which location?

input filter,
or
NFB loop lower leg?

My 2u2F cost about £0.35~=$0.60 & 1uF about £0.16, both PES 100Vdc

Spasticteapot · 6th November 2006, 12:43 AM

Do you mean the DC bypass caps (from v+ to gnd to V-) over the main 100uf power caps, or the DC blocking input cap?
I'm using a 4uf 200v metallized polypropylene cap, not shown on the board. I could mount it on the bottom to save space, I guess.

5th November 2006, 10:55 AM	#2
AndrewT diyAudio Member Join Date: Jul 2004 Location: Scottish Borders	Hi, the bypass cap is the MAIN route in for the signal (also applies to NFB blocking cap). All the high frequency and most of the mid are determined by this cap. Forget all else and get this cap on the shortest direct route in to the chip pin. Then try to wrap the electrolytic route around it. It is only there to try and boost the bass response and nothing else. Based on the above the plastic cap should be as large a value as you can afford (both PCB space and cost). 2u2F is a good start but maybe aim for 10uF or a //pair of 4u7F. __________________ regards Andrew T.

5th November 2006, 05:31 PM	#4
AndrewT diyAudio Member Join Date: Jul 2004 Location: Scottish Borders	Hi, which don't get used? DC blocking caps in general? or Electrolytic caps? or Plastic film caps? And which location? input filter, or NFB loop lower leg? My 2u2F cost about £0.35~=$0.60 & 1uF about £0.16, both PES 100Vdc __________________ regards Andrew T.

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6th November 2006, 12:43 AM	#5
Spasticteapot diyAudio Member Join Date: Jan 2006 Location: Wisconsin....what did you expect?	Do you mean the DC bypass caps (from v+ to gnd to V-) over the main 100uf power caps, or the DC blocking input cap? I'm using a 4uf 200v metallized polypropylene cap, not shown on the board. I could mount it on the bottom to save space, I guess.

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