Bonsai Monkey

Master Volume/Control box (ala Mackie Big Knob)
« on: February 07, 2005, 12:46:43 PM »
I was wondering if anyone had any plans for something similar to the Mackie Big Knob box.  I am using an old mixer to monitor off of right now, but I would like to switch over to something better, and if I can make it that would be great.  It seems like I could make a box with 3 inputs that go to a master volume knob and out to the speakers passively, using switches to select which inputs and outputs are used.  I need something that can split the signal so I can send it out to both the speakers and a headphone amp, and have seperate volume control of each.  Other features would be neat, but that is the bare minimum that I need.  Is there a project that does this already that someone can point me to?  If not, what type of variable resister should I use for the volume controls, and would it be OK for me to just split the inputs to the two controls (headphone amp and speakers) without any sort of active electronics?


radiance

Master Volume/Control box (ala Mackie Big Knob)
« Reply #1 on: February 07, 2005, 01:07:18 PM »
Check this link for info on stepped volume controls.
Is the headphone amp part of the project?
"Knowing that you are dreaming, however, does not automatically guarantee full rationality.
Then again, being awake doesn't ensure good thinking, either." -  Lynne Levitan

Bonsai Monkey

Master Volume/Control box (ala Mackie Big Knob)
« Reply #2 on: February 07, 2005, 01:51:08 PM »
I will take a closer look at that link, but that does seem to cover a passive solution.  I already have a headphone amp, and I have built a few of the CMOY portable amps, so I can put together a headphone amp if I need to.  I guess at this point the question that I am most interested in getting an answer to is if I could split the signal to get it to the headphone amp and the speakers seperately, but both at the same time.  I don't really need a volume control for the headphone send (I can use the one on the amp), but I do need it to be independant of the master volume.  Can I just split the cables coming from the input, sending one send to the volume control and the other to the headphone amp?  Will this give me a loss in sound quality, or just a volume loss?

I am also interested in suggestions for the volume control resistor.  I do not want to use a stepped volume control, so a regular pot would work, I just have no idea what value (a digikey part number would be perfect).  On that same note, if someone has suggestions for switches (with part numbers if possible) that would also be great.  I think I will need a 3 input switch for the inputs, and the same for the outputs.

Thanks for your help in this.  I am new to this stuff, so any help is appreciated.

radiance

Master Volume/Control box (ala Mackie Big Knob)
« Reply #3 on: February 07, 2005, 06:50:14 PM »
Well, I'm also a bit of a newbe so I can't answer your question regarding the splitting of the signal.
"Knowing that you are dreaming, however, does not automatically guarantee full rationality.
Then again, being awake doesn't ensure good thinking, either." -  Lynne Levitan

Bonsai Monkey

Master Volume/Control box (ala Mackie Big Knob)
« Reply #4 on: February 07, 2005, 06:59:57 PM »
I have heard good things about the blue velvet pots.  Are these a good choice, and where would I buy one?

Anyone else have any input on this subject?  I know that mic pre's and compressors are all fancy-shmancy, but the monitoring chain is just as important, and I would really like to improve mine (as would a lot of other people I am sure)

SonsOfThunder

Master Volume/Control box (ala Mackie Big Knob)
« Reply #5 on: February 07, 2005, 07:00:14 PM »
You "could" buy the stuff to make a passive volume control and selector at Radio Shack.  :thumb:

I have a project in the works to do remote volume via a "low THD" VCA arrangement with multiple inputs and mute and dim functions.  But i've been busy at my day job and doing recording lately.

I like the idea of the BK, but I don't want all that cable running up to my work area and back.  I like the idea of the Central Station, but i don't want the passive signal running thru a serial cable that far either.

Here was a previous thread on that subject: here

There are several threads and projects on this same subject as I just found when I did a search for the old thread that I started.  I have the circuit built up but have not tested yet....

HTH!
"The sow would rather have her ear than a purse." - PRR

mcs

Master Volume/Control box (ala Mackie Big Knob)
« Reply #6 on: February 07, 2005, 07:28:53 PM »
Quote from: "SonsOfThunder"
I have a project in the works to do remote volume via a "low THD" VCA arrangement with multiple inputs and mute and dim functions.

Why do it with something "bad" like a VCA, when you could use the nice TI attenuator chips (PGA2310 for instance) or a relay attenuator?

Best regards,

Mikkel C. Simonsen

lictme

Master Volume/Control box (ala Mackie Big Knob)
« Reply #7 on: February 07, 2005, 08:13:54 PM »
I'm working on one right now.

Here's the faceplate layout - I'm using a allen-bradley wirewound right now for the attenuator while I work out the active bits. I'll post everything to the board when there's some additional value...and pretty pictures.

http://www.lichthaus.com/diy/master_monitor_concept.pdf


-james
Hooray For Everything!

PRR

Master Volume/Control box (ala Mackie Big Knob)
« Reply #8 on: February 08, 2005, 01:40:08 PM »
> You "could" buy the stuff to make a passive volume control and selector at Radio Shack.

You could, and my main-room monitoring system is about that level of stuff.

Here's the glitch. RS only stocks stereo-audio in 100K value. Assuming low source Z, the wiper impedance varies from 0Ω to 25K, and will be 5K to 25K over a broad range of likely settings (-26dB to -1dB). Worst-case 25K, just 250pFd of cable and amp-in capacitance will give -3dB at 20KHz, which isn't good monitoring. It may work with a very short 1-2ft cable, which means the knob has to be practically on the amp. If driving two amps (speaker and headphone), each cable shoudl be less than a foot. Since it is not unknown for amps to have 220pFd-1,000pFd RFI caps on the inputs, even this may not give full bandwidth at the -6dB worst-case setting. (I actually used a lower-value pot, and checked the response in rack.)

Also if you have 25K source and switch-in/out a 250K load, you get a 1dB shift of level to the other load. Some power amps have lower input impedances: 47K is popular and will give around 3dB shift, some gold-ear amps have very low input impedance. This may or may not matter in a speaker/headphone selector.

Going to a 10K pot (if your source can drive it well) reduces the problems a lot. Now you can run cable to the bottom of the rack without big 20KHz droop, and switching 47K or even 22K loads on/off causes small level shift.

> both the speakers and a headphone amp, and have seperate volume control of each.

That's a better plan, just one load on each pot. Of course now your source has to drive two pots, but any recent source will drive 100K/2= 50K with ease, and most would drive 10K/2= 5K without strain.

In some ways, it would be better to buffer. Since a headphone amp can probably fit inside the box of a Big Knob, that fits sweet. Something like a low-gain CMoy behind the Speaker pot would allow use of a 100K pot for light loading of the source and low-low output Z to drive amps on the far side of the room or active-monitors. Of course that adds all the sins of a buffer amplifier. Needs power (though synergistic with a headphone amp), adds THD (but can be very low), non-infinite frequency response (though much better than pot-wiper driving yards of cable).

Kev

Master Volume/Control box (ala Mackie Big Knob)
« Reply #9 on: February 08, 2005, 02:56:10 PM »
yeah I have a project in a couple of forms.
It is at the DIY factory but for now only a few people know where it is.

... and I'm sure the accusations of keeping secrets will come.
  :roll:  
nuh

I just won't be hurried this time and my spare time is zero at the moment.
I know the DIY Factory has been a very slow build but the old Group DIY pages have remained up far longer than expected considering the tone of the RO/TT war.

If new people don't understand the history, tough ... just accept that there is history and people will navigate it around it as best they can.


I can tell you that one of the Kev monitoring units contains the THAT Corp VCA we know and love and LM833 ... probably will keep the SSM2143 and 2142 for I/O.  An optional PCB for 5532's and similar may come later.

It will also have a Dim/Cut/Mono facility and the ability for multiple controlled outputs will also be part of the project.

This is all aimed at the DAW user that want to run headphones from the main stereo output mix.

The second unit will be based around Mikkel's Digitally controlled relay boards ... or a similar system.
see this thread ... yes it is long
http://www.groupdiy.com/index.php?topic=655

oh yeah
both project are about remote control
I don't see the point of bringing balanced lines up to the work desk only to send them back to the main rack.
All audio paths can be kept short and contained within the main rack.
The DAW user only needs keyboard/mouse/screen and simple remote on the work surface.
If you use a projector then you don't need the screen on the desk.

I'm currently experimenting with ... no work desk at all ...  :shock:



... that's all I have for now and if that's not enough ... bite me ...  :cool:
Kev
DIY Factory


Bonsai Monkey

Master Volume/Control box (ala Mackie Big Knob)
« Reply #10 on: February 08, 2005, 03:38:30 PM »
OK, here is where I think I am going:

I will run all of the input signals into the control box, which will sit on my desk (that is where the current mixer sits, and everything is going to run that direction anyway).  All of the inputs will run into a selector switch.  This will send only one of the signals into the volume control (Blue Velvet 50K pot), which will be sent to a second switch.  This will select where the signal is going (headphone apm, speakers, or mute).  It looks like greyhill switches will be used for the switching on both ends.  I will not be splitting the signal at all at this point, so that should solve that problem.

Currently the input sources are a digi001 which sits 1 foot to the left of my desk, a CD player which sits on the desk, and a mini plug to plug in my MP3 player (which will sit on the desk).  The outputs sources will be my speakers (powered speakers sitting on the desk) and the headphone amp (sits on the desk as well).  As you can see I am not concerned with cables running to the desk.

Is there anything in this setup which may cause me poroblems?  I do not know a lot about electronics, so if you think this will not work please try to explain why.  Also, would it be problematic to setup a second pot just on the CD player input to match levels when comparing my recordings to CD's?  Thanks

ebeam

Master Volume/Control box (ala Mackie Big Knob)
« Reply #11 on: February 08, 2005, 06:34:50 PM »
I've built two of these passive volume controls now.  The first one (and my first attempt at diy) used cheap parts from radio shack, including the 100K pot.  Besides the loading issue, this pot also has really crappy tracking.  The stereo image shifts considerably at low volumes.  So, I rebuilt it with better parts and a TKD 10K conductive plastic pot and now I'm very happy with it.


 

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