Passive Monitor Controller, just a pot?

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buildafriend

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could a passive monitor controller just be one correctly sized double pot with a switch on the input for input select and a switch on the output for output select?? ??? that would be so much cheaper than a lot of the stuff i see out there. how is it that these people are achieving building their extremely transparent attenuators?
 
Pots may not track well from channel to channel over the full wiper range like can be done with discrete stepped attenuator/gain circuits, unless very expensive pots are used; even though stepped attenuators can be made better than this...  You may get some unintended panning as you change the volume with a dual stacked pot, or if used in more precise circuits like M/S matrixing can change stereo imaging...

Check out some of the lifetime specs for pots... how many rotations they can handle, what is the degradation in performance over time etc...

Stepped attenuators can be had to be within 0.01dB ~ 0.02dB or so with 0.1% tolerance resistors over the entire range...

Good switches / relays (more than 1 Million cycles; e.g., telecomm relays) can be placed on the inputs / outputs, but notice that their contact resistance may be less than 0.05 ohms (somewhere between 0 and 0.050 ohms, and maybe not matched in a balanced configuration), which when stacked up against 10k on each leg of a balanced circuit is a tiny percentage of possible mismatch with minimal effect to CMRR....

But start stacking lots of relays and switches in series and there may be some skew of contact resistance into the perfectly matched 10k resistance on each leg of balanced line receiver affecting CMRR... 

Especially if there is not enough wiping current to clean the contacts every time...  Yes, most switches have a minimum current that should be applied before switching to wipe the contacts... contact material and construction may vary for this regard...

The devil is in the details...
 
Well, the first thing I'm noticing is that you're talking about all super low tolerance components. transparency must have a lot to do with accuracy of your parts.
 
The main reason for high accuracy resistors/pots in the attenuator circuit is to prevent the stereo image from "wandering" left to right as you vary the attenuator.  Inexpensive dual-gang pots can easily have several dB difference between channels depending where in the rotational range the pot happens to be.  Even these very expensive P&G pots (IIRC, they cost at least $100.00) are speced to be only within +/- 1 dB from channel to channel:

http://www.pennyandgiles.com/Rotary-Fader-pd-115,3,27,.php

Best,

Bri

 
Indeed... it depends on the design goals... however, the devil is in the details... Sorry for the long post below, but for the more costly, more precision, here is an example... In most DIY cases, the above PDF is just grand and perfect especially if it's design goals (including cost) are desired, which they probably are...  

Nick Franklin said:
http://proharmonic.com/articles/AT78_OTB.pdf

Enjoy :)

This is a nice design, and the binning of the pots and finding that a bunch are within 0.5dB rocks; perfect for DIY, hopefully not too many need to be purchased to get the magical part, and if both decks of the pot are from the same batch it is possible to have tracking capability...

However, if balanced gear is in a noisy environment with common mode noise coupling on the cables, then a better matched resistor on each leg for CMRR may be desirable, although relying on that over proper grounding and system design...  The good news is that this circuit in the PDF is verging on a T-network style of attenuator...  Thusly keeping the pot somewhat "out of the signal path"....  1% tolerance gives only about -40dB of CMRR... 0.1% -60dB... 0.01% -80dB (rule of thumb 20*Log(%match), e.g., 20*Log(0.01) for 1% matching)...  The higher impedance of most passive stepped attenuators does render the need for shorter cables typically...

I guess it depends on desired design goals... 0.01 ~ 0.02 ~ 0.05 dB  match +/- is what the big guys do (Dangerous Music, Manley etc...)... I have simply found that using 0.1% tolerance resistors which are still cheap for small projects can get the design to match the "specsmanship"...

Years ago, the 0.1% resistors may have been more expensive and not justifiable to commercial engineering designs (and taught that way in colleges and tech school as "verboten" - do not use them, even if your calculations determine that a 10.74635223754673858568934674 k Ohm or a 1.0001 k Ohm resistor is what is needed; been there, got the T-Shirt) unless absolutely precision is required at that circuit node, and without it, it will not meet spec...  of course if you're manufacturing/selling millions of pieces of equipment, and you can save 1 penny on a resistor, while keeping the sale price the same, you are saving 1 million pennies of profit margin...

In this case, transparency is just the design circuit topology; many op amps or completely passive, or tube/transformer or whatnot... Precision and accuracy, especially when talking of the resistors' tolerances, are two different topics...  Precision is how repeatable the settings are on the front panel, in this example, and accuracy is usually approached by calibration (of the manufacturer's measurement and binning of their resistor products) and absolute target value being reached in the design, perhaps with calibration capabilities if designed in...  If all resistors thermally track and age the same in a certain circuit topology and the ratio of resistance (like in inverting gain equation of opamps Av=-Rf/Ri) is more important than the absolute value of the resistors (within reason of course) then you should get the same setting on the front panel every time.

Passive attenuators have their issues when switching in different pieces of gear with different impedances (especially if the passive attenuator was designed for one specific set of I/O impedances)... If it is a stepped design the attenuation steps may not be what is advertised on the front panel when switching in different impedanced gear... for a pot, the same position of rotation may not be the same attenuation for all combinations of gear switched in...  For DIY purposes, this may be OK within reason of course, especially if you're expecting the outcome...

So some manufacturers put a buffer amp after the passive attenuator to get the impedances to play nice and keep the advertised gain/attenuation settings on the front panel...

My opinion, if'n yer gonna put an amp in there, why not do gain/attenuation as necessary...  Especially if your going to listen at a calibrated monitor level that's repeatable...

Here is a single ended attenuator/gain block, which I was toying with, but never built, not necessarily for monitor gain/attenuation listening, more for input gain/atten capabilities; 0.5dB steps +/- 6dB (I think)... Using two of these for a balanced configuration is probably not desirable and a T-network solution with a balanced set of opamps would probably be more elegant as in the above PDF but with precision matched resistors on each leg as desired (emphasis on the match for CMRR not necessarily the absolute value, although that helps)...  The T resistor need not be matched as it is a singleton resistor, but repeatable is always nice, as resistors age and mechanical stresses can change stability... could be a pot as shown in the PDF design above....

The relays/switches in the opamp loop can be a rotary switch and would be way cheaper than all the relays (about half price) !!!! (I just wanted recallable and automated reference settings with a micro controller)... You'll notice that there are 12 relays, similar to a 12 position Grayhill rotary (mere coincidence... or was it?)....  The 200k and 100k are there for the momentary open/closing of contacts in the switch giving a temporary -6dB when switching... The calculations were tricky for this because of the 200k and 100k resistors paralleling and took a long time in the spreadsheet (and getting values to be close enough to render 0.01dB to 0.02dB of repeatability with standard 0.1% available parts)...  The 100pF caps may be too much depending on your BW requirements...

Essentially this circuit is a variable stepped "pot" (with only one relay closed at a time in the opamp loop) where the wiper is in the virtual ground and the two legs of the pot form input resistor and feedback resistor of the inverter opamp... The Atten/Boost switch swaps the legs of the pot from input to output and vice versa rendering different input/feedback resistor values relative to the "wiper" to obtain gain or attenuation of the same value based on the front panel setting.. R151 and R152 add some trim as the swapping between boosting and attenuating is not perfect with the 200k/100k paralleling and the swap...  I use inverting opamps to reduce purported crossover distortions... Notice 2 inverting opamps maintaining polarity...

The first opamp provides buffering from balanced and a possible +10dB gain.... The ALT_GAIN switch is there for -0.25dB for those inclined for the resolution...  All resistors are 0.1% tolerance unless otherwise specified...

The opamps are specified to essentially be similar to a straight wire with gain/atten, hopefully, under most linear circumstances...  The resistor values were kept low...  I try to keep similar parts/values being reused throughout the whole design so as not to have too many different part numbers in stock and manufacturing likes that so that we are not binning a bunch of different parts or changing mindset (and hunting for different parts/locations) too many times while stuffing the board - stuffing the wrong part number in the wrong location etc.; not so evident here but it was attempted...  

Those were some of the design goals... However, the caveat is converting from balanced to unbalanced and back again if so desired....

Since I had a hard drive crash, you may want to simulate the resistor values to make sure these are the proper values... simply the loop/input resistors in the 12 relay spots and the paralleling 200k/100k should suffice...  I think this was the last version of the schematic before the crash...

Cheers,
-chris

 

Attachments

  • SingleEndedSteppedGainAtten.pdf
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Of course, there are caveats to the inverters with low input impedances in that they should be driven by a super low output impedance from the circuit prior upstream, otherwise it would appear as a kind of insertion loss... For example, with a 150 ohm output impedance upstream, into 2.2k Ohm of the above circuit, comes out to be about 0.57dB of attenuation ahead of the gain/atten block (Ri is now 2.2k Ohm plus the 150 ohm)...  Thusly the engraved settings on the front panel will not be accurate... but they will be repeatable if the 150 ohm is constant (even over frequency)....

Also, there is a 0dB setting... why not bypass it completely for the 0dB setting with a couple of relays?  I thought about it, and thought that if, for example, one channel (L or R) needs 0.5 dB of attenuation or gain using the opamps in this circuit to do that, but the other channel does not need any gain/atten, then there should be a 0dB to keep propagation delays / phasing the same on both channels...  Accordingly both channels will have this circuit switched in...  Maybe just paranoid, but this gain/atten block could be feeding a very very extremely precision M/S circuit downstream...

Design goals, detailed design, even though not perfect...  I have a fully balanced version of this circuit in the CAD system now with only one set of opamp stages for +/- 28dB, that has similar caveats, but worse, it suffers from not having the native -6dB safety when switching (0dB native only so far), and thusly may not be desirable when switching between steps at large attenuations if a source is too hot...


 
Brian Roth said:
The main reason for high accuracy resistors/pots in the attenuator circuit is to prevent the stereo image from "wandering" left to right as you vary the attenuator.  Inexpensive dual-gang pots can easily have several dB difference between channels depending where in the rotational range the pot happens to be.  Even these very expensive P&G pots (IIRC, they cost at least $100.00) are speced to be only within +/- 1 dB from channel to channel:

http://www.pennyandgiles.com/Rotary-Fader-pd-115,3,27,.php

Best,

Bri

Wow! I had no idea pots were that loose.
 
Even the expensive pec 2 watt dual gangs @ 25.00 dollars each are unreliable. Made a bunch of passive volume headphone boxes for work recentlt. Had a total of 16 pots  and out of 16 over half would easily shift the stereo image when the pot was at the far left or lowest in volume.  It would easily be 2 -3 db between left and right. However once  you get around 8-9 o'clock the image would be nice a centered and adjust evenly. Hasn't been a problem so far as everone who uses them usually has them cranked to around 12 o'clock which is a nice but not overly loud volume.

To answer your original question if you could get away with just a pot yes.  But figure four decks deep to do 2 stereo signals and that it will not will not work as well as one would like, it's better to look at other options. The easiest way to do a passive monitor volume control that is fairly inexpensive and responds properly is to build yourself or buy yourself a pre built Stepped Attenuator.

Gold point is a great way to go, they can sell you the stuff to build and also sell pre built units.

http://www.goldpt.com/index.html


 
The Alps pot referred to in the A T article are specified to have a track to track accuracy of better than 2 dB.
I've measured over a dozen of these pots and found that from a bees tit above fully CCW they measure better than + / - 0.5dB. In a practical application as a monitor volume control I reckon that is pretty acceptable. In aiming for something with a tighter specification, you'd be wanting to ensure that the rest of your chain exceeded this benchmark.
 
Sorry to resurrect an old post, but I have a couple painfully dumb questions to ask.  We're talking electronics 101 dumb!
That article (http://proharmonic.com/articles/AT78_OTB.pdf) doesn't specify what to do with the ground.  Is it just tied together before the ins/outs hit the pot?
Also, it recommends a 10k pot.  I have a 50k pot lying around I could use for this.  Any reason that would be a bad idea? (for use between a typical daw output and a pair of powered monitors)
If not, is there a particular formula I'm forgetting to calculate the proper resistor values for a 50k pot?
 
Neve had a very simple way of doing this on their lower cost mixers. The first thing to recognise is that pot tracking problems exist primarily with log law pots. With linear law pots the tracking is much better. Second, if you slug a linear pot from wiper to ground with a resistor about half the pot value you get a semi log law with the mid point 10dB down. So use a 2K linear pot with a 1K slug resistor and connect the ends  of the pot right across the incoming hot and cold. Take the output hot from the slider and the output cold from the input cold. This method has no effect on common mode performance.

Cheers

Ian
 
ruffrecords said:
.. use a 2K linear pot with a 1K slug resistor and connect the ends  of the pot right across the incoming hot and cold. Take the output hot from the slider and the output cold from the input cold. This method has no effect on common mode performance.
Du.uuh!  Sure about that?
 
ruffrecords said:
Neve had a very simple way of doing this on their lower cost mixers. The first thing to recognise is that pot tracking problems exist primarily with log law pots. With linear law pots the tracking is much better. Second, if you slug a linear pot from wiper to ground with a resistor about half the pot value you get a semi log law with the mid point 10dB down. So use a 2K linear pot with a 1K slug resistor and connect the ends  of the pot right across the incoming hot and cold. Take the output hot from the slider and the output cold from the input cold. This method has no effect on common mode performance.
Thanks, I may try that if I can find a 2k pot lying around.  Only reason I asked about a 50k log pot is because I have one of those fancy-shmancy $30 ones that I never ended up using.  From my measurements it seems that tracking is well below 0.5db which is fine for my needs.  Figured I might as well use what I've got!

*Edit/Facepalm: Figured it out. Problem solved. Carry on, nothing to see here...  ;)
 
ruffrecords said:
Neve had a very simple way of doing this on their lower cost mixers. The first thing to recognise is that pot tracking problems exist primarily with log law pots. With linear law pots the tracking is much better. Second, if you slug a linear pot from wiper to ground with a resistor about half the pot value you get a semi log law with the mid point 10dB down. So use a 2K linear pot with a 1K slug resistor and connect the ends  of the pot right across the incoming hot and cold. Take the output hot from the slider and the output cold from the input cold. This method has no effect on common mode performance.

Cheers

Ian

Thanks Ian!

So this is an early neve design for their monitor section? It's so easy, I have no choice but to test it out.
 
It was used on AUX outs and the like.  Their monitor/ SLS pots were usually stepped switches/ resistors.  That is speaking for the 80 and 54 series.
I can never get a straight answer why they decided to pad the monitor output rather than have a fader in the middle of the output module like every other section in the consoles.  Geoff T and Steve B did not know- they just said because that was how they had done it on the previous desks.  Do you have a different take, Ian?
Mike
 

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