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A precision LED/LDR-based Attenuator
A precision LED/LDR-based Attenuator
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Old 5th July 2011, 07:43 AM   #131
gootee is offline gootee  United States
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Generally, you'd want the output impedance to be very low and the input impedance to be high, but maybe not "too high".

The other pieces of equipment that are connected to your attenuator will have their own output impedance (connected to your input) and input impedance (connected to your output). Those input and output interconnections will each act like a voltage divider.

So if your input impedance is too low, relative to their output impedance, the loss of signal level might be significant. Also, for low attenuator input impedances, their output might not be able to drive enough current at the voltages it's trying to produce.

Similarly, if your output impedance is too high, relative to their input impedance, signal level loss might be significant.

Another consideration is that there will also be capacitance and inductance facing both ends of the attenuator, in the interconnects or in components in the source or receiver or both in the interconnects and in actual components. And the attenuator's input and output impedances (mostly resistances) will always interact with those inductances and capacitances, resulting in a particular "transfer function" for that portion of the signal path (i.e. output circuitry, cabling, attenuator, cabling, input circuitry).

A transfer function describes the frequency response (gain vs frequency and phase vs frequency) through a linear time-invariant system and completely defines the system's transient and steady-state characteristics.

i.e. The attenuator's input and output impedances can cause changes in the gain and phase angle of the signal, versus frequency, when ideally you would want either no change at all or a change that is exactly constant versus frequency.

The only example that comes to mind at this hour is when the output impedance is too high. In that case, it could interact with cable capacitance, and/or an existing RC low-pass filter in the other equipment's input circuitry, to either create a low-pass filter or lower the cutoff frequency of an existing low-pass filter, possibly affecting the sound.

Additionally, it seems like it would be a really good idea to keep the attenuator's input and output impedances as constant as possible, so that the frequency response of the system wouldn't change as the volume control is changed.

Yet another separate consideration is resistor noise. All resistances generate noise, all by themselves. The higher the resistance, the more noise is generated. So lower resistances are better (as are lower currents and voltages, as it turns out), except when it makes the input impedance too low. Here is an interesting article about the different types of resistor noise, in guitar amplifiers: Resistor Types--Does It Matter? . In low-noise circuits, I try not to use resistors above 10k Ohms, and usually use much smaller ones if I have that luxury. But for an attenuator you might want the input impedance to always be more than that, for example.

Obviously, there are trade-offs to consider. I'm sure that someone will come along and give you some typical "ideal" values of input and output impedances for audio system components. You might even want to have different operating modes available for solid state and tube interconnects, for example.

Last edited by gootee; 5th July 2011 at 07:56 AM.
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Old 5th July 2011, 10:17 AM   #132
MiiB is offline MiiB  Denmark
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Here is what you're looking for....

50-Step Shunt Attenuator Resistor Calculator - Neville Roberts
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Old 5th July 2011, 05:03 PM   #133
wapo54001 is offline wapo54001  United States
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Here is a calculator that I looked at earlier.

hifiZen's Constant Impedance Attenuation Network Calculator
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Old 8th July 2011, 12:21 AM   #134
gootee is offline gootee  United States
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Aren't your LDR diodes backwards? Or am I looking at it wrong?
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Old 8th July 2011, 12:33 AM   #135
wapo54001 is offline wapo54001  United States
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The LDRs are correct, however, I had power to the chip bass-ackwards.
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Old 18th August 2011, 11:54 PM   #136
wapo54001 is offline wapo54001  United States
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Making a little progress:

1) Multi-tasking turned out to be a red herring. The chips won't run at full speed during multi-tasking, and other issues arise as well, so I'm sticking with single-task structure.

2) Switching to multi-turn pot for better control over setting regulator voltage and for setting external Vref for the ADC ports on the chip.

3) Increasing the mosfet gate control current limit resistor value from 1 meg ohm to 3 meg ohm to get finer control of steps up and down on the gate. Right now, a single minimum duration corrective pulse moves the level at the mosfet gate too far. Increasing the resistance will reduce the amount of correction applied in one step. The alternative was to increase the capacitance of the capacitors which would either make them physically much bigger and more expensive or force a switch from metal film to tantalum, and I don't like either alternative.
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Old 23rd August 2011, 04:41 PM   #137
wapo54001 is offline wapo54001  United States
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In case anyone is curious, this is my latest board with Alps motorized pot (drawn to scale).

Board will include IR receiver and a second board will carry the H-bridge controller for the Alps. H-bridge is a single 16 pin IC, almost no support required except for regulator. The motor will have it's own voltage regulator, but will draw power (100ma when running) from the 12V 1A wall wart.

In process.

Renumbering of components not complete, so don't criticize that!

Circuitry for the IR receiver and H-bridge not drawn. The IR will be only for volume UP/DOWN, nothing else. Later it won't be difficult to add another larger board for input switching tasks.

Control code is working very smoothly. LDR becomes difficult to control above about 50K due to the very low current requirement, but at 10K it is very steady. Control is so fast that adding control of a second LDR device as a simultaneous task won't make any difference at all.

I will use a separate chip for the IR receiver and H-bridge control.
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Old 24th August 2011, 03:38 AM   #138
gootee is offline gootee  United States
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Wapo54001,

I truly LOVE using audio equipment with remote-controlled motorized pots. Great idea!

Judging by the nearly-instantaneous evolution of your board layout skills, I believe that you are a very good and fast learner, and could go far and do well. (Or maybe you already have. :-)

If you don't mind, what is your age, and background? If you are still young, what are your intentions, so far, for the rest of your life?

Sorry about the non-technical quesions. But I'm old-enough, now, at age 54, to just ask what I want to ask. I finally realized that "Life is too damn short" to be hesitant or reticent, any more. (Too bad I didn't know that well-enough when I was younger...!)

By the way, if you are relatively young, or even if you are not, I hope that you will look at the world-famous Jan Didden's blog (or whatever it was) about engineers and communication skills, to which I contributed. The main point was that interpersonal communication skills are AT LEAST as important, for engineers (and others who are similar), as are technical skills. I'm only mentioning this because you seem like you might be one of the ones who is "worth saving". Go, NOW, and read it(!):

http://www.diyaudio.com/forums/blogs...scientist.html

Regards,

Tom Gootee

Last edited by gootee; 24th August 2011 at 03:48 AM.
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Old 24th August 2011, 11:19 AM   #139
wapo54001 is offline wapo54001  United States
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Quote:
Originally Posted by gootee View Post

If you don't mind, what is your age, and background? If you are still young, what are your intentions, so far, for the rest of your life?
I am retired Air Force. During Viet Nam, I was a pilot. Afterwards, I became an air traffic control officer and part-time pilot and managed ATC facilities. Later, I commanded communications and computer systems organizations (because in the Air Force, comm, computer, and ATC are combined together). Am now 66 and in my dotage. My intentions, so far as they go, is to stay above ground for as long as I can, and play with electronics and my toy airplane. :-)
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Old 24th August 2011, 12:23 PM   #140
ua100k is offline ua100k  United States
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Quote:
Originally Posted by gootee View Post
Wapo54001,

I truly LOVE using audio equipment with remote-controlled motorized pots. Great idea!

Judging by the nearly-instantaneous evolution of your board layout skills, I believe that you are a very good and fast learner, and could go far and do well. (Or maybe you already have. :-)

If you don't mind, what is your age, and background? If you are still young, what are your intentions, so far, for the rest of your life?

Sorry about the non-technical quesions. But I'm old-enough, now, at age 54, to just ask what I want to ask. I finally realized that "Life is too damn short" to be hesitant or reticent, any more. (Too bad I didn't know that well-enough when I was younger...!)

By the way, if you are relatively young, or even if you are not, I hope that you will look at the world-famous Jan Didden's blog (or whatever it was) about engineers and communication skills, to which I contributed. The main point was that interpersonal communication skills are AT LEAST as important, for engineers (and others who are similar), as are technical skills. I'm only mentioning this because you seem like you might be one of the ones who is "worth saving". Go, NOW, and read it(!):

http://www.diyaudio.com/forums/blogs...scientist.html

Regards,

Tom Gootee
I second Tom based on my experience. It is actually easy to do well in this world and not be as smart or good at what you do. I say this to convey the point that many smart and great people do not do as well as they could and the world loses.
If I may for the ambitious ones out there, and the ones who would like to make this a better place.
To excellent books to read.
1. Executivel EQ
2. Outliers.

Gives you a sense of what can be achieved.
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