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Old 29th December 2011, 06:49 PM   #121
AndrewT is offline AndrewT  Scotland
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Swap the two RF filter components.
The C follows the R for a low pass filter.
The trace under the C8 label should be disconnected from the top of the 1k0 and moved to the bottom of the 1k0. This is after you have swapped the R&C.
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Old 30th December 2011, 05:35 AM   #122
Grega is offline Grega  Australia
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cool - will do andrew. thanks
i didn't think they were right when i was penning them into the diagram
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Old 30th December 2011, 12:52 PM   #123
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Default Section of Quad board

I wanted to show a layout section that would fit 8 or 12 of LM1875 amplifier into 100x100mm. I was new to using Eagle (#&%@!!), and while experiencing the torture of the slowest way to promote bad layout ever invented, I am also quitting smoking and was going postal. Eagle? No. Snail. Egads! Is there not something a bit quicker and more layout centric?

Shown are normal size components.
35v 330uF for LM1875 fits as does 35v 220uF for alternative chips; and in either case, the 100nF are under board.
The feedback resistor is under board as is typical of chip amplifier applications.
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Old 30th December 2011, 03:51 PM   #124
AndrewT is offline AndrewT  Scotland
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I reckon one could hand draw 10 copies of your PCB and etch all 10 copies quicker than you could use Eagle to create a matching layout for your through hole discrete components.
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Old 30th December 2011, 04:08 PM   #125
GloBug is offline GloBug  Canada
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I started using Google Sketchup, I never could find a fast easy way to draw electronics until now.

Pro's - fast, you can draw your whole project in 3D.

Cons- I don't know if their is a direct way to print off PCBs, you would have to convert the file somehow to another format, if needed. Example, a sketchup file would have to be converted to run a CNC machine. Not much of a con as is is doable.
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Old 30th December 2011, 04:48 PM   #126
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Default Feedback values, and current effect

1). Feedback-shunt resistor range: 820R to 3.3k (more commonly 820R to 2.7k)
2). NFB cap range corresponding to functional value: Based on 2.7k feedback-shunt with 220uF NFB cap, if the resistor value goes to half, double the cap value.
Nearby figures work conditionally (trim primarily with input cap, not NFB cap).
3). Possible feedback resistor values range from 7.5k to 115k, (more commonly 7.5k to 68k) depending on other factors, such as. . .
4). gain between 10 and 43 (more commonly 10 to 35).
It would be nice to see designs where the above 4 conditions are all true, unless there's a good reason for an exception.
For example LM675 is inherently an exception due to low gain.

More discussion (effect on current, sound):
The effects on audio from using a voltage divider, L-Pad on a tweeter are exactly the same as or similar to effects on audio possible from the gain setting resistors of the audio amplifier, which is also a voltage divider.
As can happen with a tweeter, some of the possibilities are unfortunately dull with more impact, some are pretty with unfortunately less impact, some have a higher crossover point, and some have an open but low resolution that doesn't promote a sense of space. The voltage divider also adjusts series current--low value resistors for more current, high value resistors for less current. A wide variety of effects are possible, but it seems that the most desired collection of effects isn't simultaneously available all at the same time.
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Old 30th December 2011, 05:01 PM   #127
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Quote:
Originally Posted by AndrewT View Post
I reckon one could hand draw 10 copies of your PCB and etch all 10 copies quicker than you could use Eagle to create a matching layout for your through hole discrete components.
Thanks man!
Economies of scale?
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Old 30th December 2011, 10:04 PM   #128
Grega is offline Grega  Australia
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.....until you want to get a couple properly made then you have to do them up anyway
have to admit it does take a while to get all the traces correct
maybe have a look at sprint DWB - thats for penning boards minus schematics
horses for courses i guess

got your point DWB - will see what i can brew up
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Old 15th April 2013, 03:43 AM   #129
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It is time for me to explore the 1875 chipamp. I will be ordering 10 of them plus a bunch of 0.1% resistors from Mouser. It is my intention to build a parallel 1875 amplifier to take advantage of lower distortion from less loading of the chip and ability to drive "difficult" loads.

The circuit in post number 5 is a starting point. I intend to reduce gain to around 12. I thrashed out compensating them for unity gain in passband but the 1875 just doesn't have enough GBP (area under Bode plot) to make it worthwhile. I couldn't work out a practical, theoretically stable unity gain configuration; everything I tried (starting compensation @ 100 kHz to not interfere with the audio band) intersected the Bode plot at 40 dB/decade slope. But if anybody disagrees with my conclusion, please let me know!

So I have a couple of simple questions.

1) Is there any advantage (or disadvantage) to using an opamp front end and enclosing the circuit in a global feedback loop with a global gain of around 15 while keeping the local nested gain of the outputs at 12?

2) Is there any advantage (or disadvantage) to DC nulling each chip individually?

Any other tips or omissions are much appreciated. Thank you.
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Old 15th April 2013, 11:19 AM   #130
AndrewT is offline AndrewT  Scotland
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From what I have read, you must DC null each chipamp individually. For this to be effective you must also DC block the input of each chipamp.

That is probably why National show separate DC servos attached to each chipamp in some implementations.
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