Balancing an unbalanced circuit

[sahib]

It was pretty common to find errors in old hand drawn schematics. Product data sheets should be reliable but sometime imprecise qualifications (like weighting or bandwidth for noise) make numbers hard to compare.

JR

Just reading a data sheet for MC74C922 encoder from Microcell

 

[Newmarket]

Just reading a data sheet for MC74C922 encoder from Microcell

View attachment 95699

Yeah - Data Sheet proof reading is often weak

 

[ccaudle]

Just reading a data sheet for MC74C922 encoder from Microcell

View attachment 95699

That particular error is a combination of case and font error. Uppercase omega is Ω, but lowercase omega is ω. If the typeface you are using does not have Greek letters sometimes ω gets converted to W when you try to change to upper case. Or possibly an editor without the technical background to catch it mistakes ω for w, and sees that kw is not an accepted abbreviation for anything, but kW is, so "helpfully" changes kω to kW instead of kΩ.

 

[gyraf]

Thanks - this clears up why this ohm/watt swap happens, I was curious why we seem to see it quite often

 

[sahib]

50kΩ turn on resistance would still be alarmingly wrong though, as opposed to possibly mΩ.

 

[ruffrecords]

Getting back to the OP's orifginal question, he is building guitar pedal type circuits operated from a 9V supply. I would suggest simple impedance balancing is all that is required.

Cheers

Ian

 

[Newmarket]

Thanks - this clears up why this ohm/watt swap happens, I was curious why we seem to see it quite often

tbh I can't recall ever seeing the lower case letter used - at least not for resistivity where afaik it isn't recognised

50kΩ turn on resistance would still be alarmingly wrong though, as opposed to possibly mΩ.

I was thinking the same re the symbol. But I think it was meant that you might get it by accident if you don't have the proper symbols on your keyboard. Or something. It's quite easy to insert symbols on a PC keyboard though tbh. It's def not a recognised symbol for a unit of resistance.

Re "turn on resistance" and value :
Now you mention that - yes it's all wrong

 

[moamps]

50kΩ turn on resistance would still be alarmingly wrong though, as opposed to possibly mΩ.

Re "turn on resistance" and value :
Now you mention that - yes it's all wrong

"The MM74C922 and MM74C923 CMOS key encoders provide all the necessary logic to fully encode an array of SPST switches. The keyboard scan can be implemented by either an external clock or external capacitor. These encoders also have on-chip pull-up devices which permit switches with up to 50 kΩ on resistance to be used."

The explanation seems quite correct to me. Some switches, especially on soft keyboards, have significant resistance (I once measured more than 1kohm) in the on state.

 

[Newmarket]

"The MM74C922 and MM74C923 CMOS key encoders provide all the necessary logic to fully encode an array of SPST switches. The keyboard scan can be implemented by either an external clock or external capacitor. These encoders also have on-chip pull-up devices which permit switches with up to 50 kΩ on resistance to be used."

The explanation seems quite correct to me. Some switches, especially on soft keyboards, have significant resistance (I once measured more than 1kohm) in the on state.

I see (I think). It's an external switch spec' and not a physical characteristic of the part itself.
I guess it's basically a function of the on chip pull up resistor.
I'd stay away from such external switch if possible. Big difference between your measured >1K and 50K. Thanks for the input (no pun intended )

 

[sahib]

On the top left corner of the first page it states that it is Beijing company. The data sheet is likely to be written by a non-native English speaking person. So, they used the wrong terminology.

That text actually means the pull-up resistor value. Switch-On/Turn-On resistance is not the same thing.

 

[moamps]

On the top left corner of the first page it states that it is Beijing company. The data sheet is likely to be written by a non-native English speaking person. So, they used the wrong terminology.

The Chinese just copied the datasheet from some famous company (compare Fairchild vs. Microcell below). They didn't translate anything, only the omega sign was obviously not known to them so they put W. And what they wrote makes sense, not terminologically wrong except for that confusion with the omega sign.

Fairchild
https://www.soemtron.org/downloads/disposals/74c922.pdf"The MM74C922 and MM74C923 CMOS key encoders provide all the necessary logic to fully encode an array of SPST switches. The keyboard scan can be implemented by either an external clock or external capacitor. These encoders also have on-chip pull-up devices which permit switches with up to 50 kΩ on resistance to be used."

Microcell
http://www.microcell-ic.com/webmanage/kindeditor/attached/file/20180514/20180514125435133513.pdf"The MC74C922 and MC74C923 CMOS key encoders provide all the necessary logic to fully encode an array of SPST switches. The keyboard scan can be implemented by either an external clock or external capacitor. These encoders also have on-chip pull-up devices which permit switches with up to 50 kW on resistance to be used."

That text actually means the pull-up resistor value. Switch-On/Turn-On resistance is not the same thing.

Watch out for "These encoders also have on-chip pull-up devices ..." This means that the encoders have such a large pul-up "resistor" that even turning on a switch with Ron = 50kohm will initiate a change in logic state.

 

[moamps]

That particular error is a combination of case and font error. Uppercase omega is Ω, but lowercase omega is ω. If the typeface you are using does not have Greek letters sometimes ω gets converted to W when you try to change to upper case. Or possibly an editor without the technical background to catch it mistakes ω for w, and sees that kw is not an accepted abbreviation for anything, but kW is, so "helpfully" changes kω to kW instead of kΩ.

It reminded me of the times when I had to do OCR on some scientific texts. Very often the OCR sign of omega would translate into w. There were other confusions with Greek letters.

 

[sahib]

I got it. Thank you.

 

[abbey road d enfer]

It reminds me of my first ever "radio" book, from the early 20's, where they described how to make resistors with paper and pencil. There were only two types of resistors; one was IIRC 400 ohms and the other 100 000 ohms. The former was written with a lower case omega and the latter with a capital omega. Never understood why...

 

[Tubetec]

I had an electronics lab kit as a small boy , one project created an audio oscillator , you then drew with a soft pencil a series of different lenght lines from a centre point , ,one wire remained connected to the intersection , the other wire you touched on the far end of one of the lines to create a tone . My cousin got the same present ,but he managed to figure out how to eaves drop on his neighbours via their FM baby monitor .
The kit below you even get lucky bag input and output transformers , I know its off topic but these things are great for not just kids but anyone trying to learn the principles involved in electronics.

 

[k brown]

Getting back to the OP's orifginal question, he is building guitar pedal type circuits operated from a 9V supply. I would suggest simple impedance balancing is all that is required.

Cheers

Ian

And did the OP ever say why he wanted a balanced output from a guitar pedal??
Most balanced-input devices can accept balanced or unbalanced sources.

Might this be a solution in search of a problem?

 

[tedsorvino]

Getting back to the OP's orifginal question, he is building guitar pedal type circuits operated from a 9V supply. I would suggest simple impedance balancing is all that is required.

Cheers

Ian

Thanks Ian for thinking about my original question.
I have to admit that I'm lost with all this extremely technical turn this thread has taken. Not complaining or anything. I know how fora work. I just can't follow the thread any more - I mean IC technology is beyond me. Maybe I will one day.
Could you please clarify with plain words what you mean by "simple impedance balancing". In my case (and I guess many other future and current readers) simpler terms help way more than precise technical terminology. For example instr. signal impedence (hi z), or line signal impedence etc.

 

[tedsorvino]

And did the OP ever say why he wanted a balanced output from a guitar pedal??
Most balanced-input devices can accept balanced or unbalanced sources.

Might this be a solution in search of a problem?

I think balanced has a better signal to noise ratio (at least for studio use)

 

[abbey road d enfer]

Getting back to the OP's orifginal question, he is building guitar pedal type circuits operated from a 9V supply. I would suggest simple impedance balancing is all that is required.

Thank you for putting us back on rails.
The main issue is that most pedals have a variable output impedance, simply because the output is from the wiper of a pot, which implies in any case adding at least a buffer, which is not as simple as adding a resistor to create a "cold" output.

 

[k brown]

Pedals include all sorts of simple-to-complex devices nowadays - more needs to be known about the specific pedal in question - not all have pots on their outputs.

I've run up to 100' of unbalanced cable - under all sorts of comditions in many different locations - utterly silent; if high quality, tightly shielded, properly terminated cable is used. Too many think noise-free transfer of signals requires balanced lines. In many cases, not so.