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Chip Amps Amplifiers based on integrated circuits

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Old 8th June 2013, 01:00 AM   #1
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Default Buffered Volume Control

This design is based on the Alps "Blue Velvet" RK027 pot. I'm using OPA2134 opamps and Rod Elliot's P05B power supply board with an Antek 10VA transformer in a Hammond case. It has turn on/off muting via a shorting relay (driven by the AUX supply on the P05B). It is DC coupled, and intended to be used with AC coupled source and amplifier.

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Old 8th June 2013, 01:03 AM   #2
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Schematic:

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Old 8th June 2013, 01:07 AM   #3
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Note that the RF bypass cap C1/C5 is too large as shown. Should be more like 22pF.
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Old 9th June 2013, 12:34 PM   #4
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Is it that boring?

Any comments on the PCB design? Good, bad, ugly?
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Old 9th June 2013, 04:01 PM   #5
SoIL4x4 is offline SoIL4x4  United States
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Very cool project! Are those butt connectors on the input output connections? Its hard to tell on my phone.
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Old 9th June 2013, 04:05 PM   #6
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Quote:
Originally Posted by SoIL4x4 View Post
Very cool project! Are those butt connectors on the input output connections? Its hard to tell on my phone.
On the board? No, I used 2pin Molex connectors for the signals and a 4pin molex for power (+15V, -15V, GND, AUX for relay). I used milspec shielded wire for the signals with solder sleeves for the shield terminations.
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Old 9th June 2013, 04:27 PM   #7
SoIL4x4 is offline SoIL4x4  United States
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Sorry I was referring to what appear to be blue heat shrink connections towards the rca side of the input output wiring.
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Old 9th June 2013, 04:36 PM   #8
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Those are solder sleeves for the shield terminations. They are a small piece of blueish/clear heatshrink with a ring of solder and usually a wire that is shrunk down onto the cable. The solder melts joining the wire and shield.
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Old 9th June 2013, 04:48 PM   #9
dmills is offline dmills  United Kingdom
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What is the point of the input buffer on each channel? You have it set up for G=1, 47K input Z, but it is driving a 100K pot, so you could eliminate it and just go straight onto the top of the 100K pot.

Comes to that, why 100K? Seems more then a little on the high side.

What are the opamps? With a 100K pot you are asking for small DC level shifts as the pot is turned and worse, crackle once the pot ages if there is any significant input bias current present.
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Old 9th June 2013, 05:02 PM   #10
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Quote:
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What is the point of the input buffer on each channel? You have it set up for G=1, 47K input Z, but it is driving a 100K pot, so you could eliminate it and just go straight onto the top of the 100K pot.

Comes to that, why 100K? Seems more then a little on the high side.

What are the opamps? With a 100K pot you are asking for small DC level shifts as the pot is turned and worse, crackle once the pot ages if there is any significant input bias current present.
The opamps are OPA2134.

I used the 100K Alps because that's what was available without resorting to possible fakes on ebay, and with the JFET input opamps there is no DC on the pot, or the output.

You could eliminate the input buffer with the 100K pot, but it doesn't hurt anything.
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