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Old 2nd January 2011, 07:21 PM   #91
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Worked on the circuit card over Christmas, made some significant changes:

1) moved power input/output pads to the rear of the board so that all connections are at the rear, and control pots and indicator LED are at the front.

2) rearranged the voltage regulator circuitry to accomodate 1).

3) separated potentiometer pads to maximum separation so that pots can be soldered directly to board and used to secure the card to front of panel if desired.

I am not certain that the decoupling capacitors are adequate:

220uF 25V & .1uF at power input (C1, C2)
440uF 10V & .1uF at regulator output (C3, C4)
.1uF bypass at control chip power pins (C6)
.1uF bypass at current limiting resistors (C11)
1uF tantalum across each LDR-LED CX1~CX4) (optional)

Separate power supplies for the control chip and the LDRs would be ideal, but I doubt that it will make an audible difference. The control circuit will draw maybe 5ma, and the LDRs will draw maybe a maximum of 40ma total at maximum attenuation. With the pots set in the middle of their ranges, the control voltages are virtually invisible on a scope, they are so small, and the LEDs are drawing negligible current. I've put a 1K resistor (R3) to draw a continuous 5ma to stabilize the regulator. This regulator is going to loaf.

LED current changes during turn on/off affecting the power supply is a concern, but transitions will only happen when the circuit goes in to, or out of, 'mute' mode, so should not be audible.

If anyone has ideas for better decoupling of the control circuit from the supply to the LDRs, I'd be interested to hear your idea.
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Old 2nd January 2011, 07:45 PM   #92
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1) The leads of a decoupling capacitor can act like a radiator (antenna) so they should be as short as possible. I don't know if it's possible to "pour copper" for connection of the caps to the ground plane with this program, however.
2) Analog and digital grounds should be kept separate.
etc., etc. See analog devices apnote on grounding and decoupling
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Old 2nd January 2011, 08:12 PM   #93
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Originally Posted by jackinnj View Post
1) The leads of a decoupling capacitor can act like a radiator (antenna) so they should be as short as possible. I don't know if it's possible to "pour copper" for connection of the caps to the ground plane with this program, however.
2) Analog and digital grounds should be kept separate.
etc., etc. See analog devices apnote on grounding and decoupling
The capacitors are all radials, so leads are very short to the foil. Yes, I can do a ground plane on this board, but I'm not sure if it's useful -- the "audio" is restricted to the lower right corner of the board, and the audio grounds (and all audio connections) are very short and completely isolated from the power supply and control circuitry.

I plan to use a wall wart with floating ground so there will never be physical contact between power ground, signal ground, and the chassis.

While the chip is indeed a digital chip and the 'control' signals are digital, when the potentiometers are not moving the control signals are vanishingly small and very occasional -- barely visible on a scope, and they stop at the mosfets Q1~Q4. In reality, there is no digital activity going on outside of the chip itself (the control circuits are mostly "off"). The mosfets act as variable resistors, and the voltages beyond the mosfets to the LEDs is pure DC from the power supply. The only noise that could possibly get there would be through the power delivered through the current limiting resistors to the LEDs.

Having said all that, I could pour a ground plane. I assume that would be around the power supply and control circuits, and not the audio corner, correct? And that ground plane would be connected to the chassis, but not to power ground? Or two ground planes -- one audio and one control circuit -- not connected to each other. (I'll have a look for that application note you mentioned.)
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Old 2nd January 2011, 08:45 PM   #94
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you can see the clock signal with a spectrum analzyser, it's always clocking. Most scopes don't have the sensitivity to see the radiated clock signal. I would suggest pouring copper under the MCU and connect the decoupling cap from the Vdd pin to the plane, using a 100nF ceramic. Keep the analog and signal grounds separate, meeting at only one point. If you have to run an analog signal over the clock or control signal, do so at right angles.

our sensitivity to noise isn't like that of a hydrophone, ekg or seismic instrument so I don't want you to get your knickers too twisted up. Just save some money before sending the design off to PCBExpress.
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Old 2nd January 2011, 08:49 PM   #95
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Originally Posted by jackinnj View Post
you can see the clock signal with a spectrum analzyser, it's always clocking. Most scopes don't have the sensitivity to see the radiated clock signal. I would suggest pouring copper under the MCU and connect the decoupling cap from the Vdd pin to the plane, using a 100nF ceramic. Keep the analog and signal grounds separate, meeting at only one point. If you have to run an analog signal over the clock or control signal, do so at right angles.

our sensitivity to noise isn't like that of a hydrophone, ekg or seismic instrument so I don't want you to get your knickers too twisted up. Just save some money before sending the design off to PCBExpress.
Thanks for the input, I'm going to give this some thought.
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Old 7th January 2011, 09:19 PM   #96
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Got my latest boards back with the changes listed above.

Should get the components from digi-key and mouser early next week.

I've decided that 1/8W resistors are too small for comfortable handling, I've redrawn the board to take 1/4W resistors. All else stays the same.
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Old 8th January 2011, 10:34 AM   #97
AndrewT is offline AndrewT  Scotland
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Hi,
the lands around each hole are very narrow. The land is too small for direct contact with the tip of the soldering iron. This makes for more heating of the component while trying to use the component leg to heat the PCB trace.
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Old 8th January 2011, 02:33 PM   #98
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Hi,
the lands around each hole are very narrow. The land is too small for direct contact with the tip of the soldering iron. This makes for more heating of the component while trying to use the component leg to heat the PCB trace.
Hi, Andrew,

I went back and looked at the board, I kinda see what you mean that some of the lands look small. But I have been using this software and this production company for years, and I don't remember experiencing problems soldering. I'll try to be aware and watch for the problem as I solder up this board. It occurs to me to wonder if the land sizes are not intended to be optimal for wave soldering. I never thought of that. To change the land sizes would involve a lot of work changing the component drawings in the software!

There may be some 'optical illusion' going on because the holes for the two IC sockets are oversized (the last board I had made using standard sized holes didn't work for me because I wanted to use machined-pin IC sockets and the pins were too big for the holes). Also, the traces may be unusually wide compared to what you are accustomed to -- I used .04" for signal traces and .06" for power. I've always wondered if there was an optimal trace size for audio signals, but never tried to find an answer to that.

Also, the smallest lands running down the middle of the 20-pin IC outline and attached to the large lands at the edges of the board plus the six on the inside of the 24-pin IC outline are not intended to be soldered to, they are intended to accept the spring-loaded pins of a bed-of-nails board.

The lands other than the IC sockets are all the standard size that come with the component outlines included in the ExpressPCB pcb design software, I haven't changed any of them.

BTW, in order for folks to have the easiest assembling job, I've converted the board from the 1/8W resistors (which are very small) to 1/4W. So the board has changed to the attached design, just a slight rearrangement to accomodate the larger resistors but no increase in overall size. I haven't had one actually made up yet.
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Old 8th January 2011, 09:29 PM   #99
gootee is offline gootee  United States
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Quote:
Originally Posted by wapo54001 View Post
Got my latest boards back with the changes listed above.

Should get the components from digi-key and mouser early next week.

I've decided that 1/8W resistors are too small for comfortable handling, I've redrawn the board to take 1/4W resistors. All else stays the same.
I remember getting 0.6-Watt resistors through Digikey that were much shorter than 1/4-Watt resistors, but about the same diameter, if that would help.
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Old 8th January 2011, 09:58 PM   #100
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I remember getting 0.6-Watt resistors through Digikey that were much shorter than 1/4-Watt resistors, but about the same diameter, if that would help.
Well, length of the resistors on this board are not a problem. There are a few places where the 1/4W devices will be cheek-to-jowl in width, but should be OK. I'll stick with the standard 1/4W 1% devices as long as I can. For a while last fall there seemed to be a shortage of these resistors, don't know why, but they're back available now.
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