DIY
Factory
Daw
Monitor Switcher Project MkI
... and
discussion paper
A project brought to you by Kev.
Updated 28 December
2005
General background
This discussion paper, although aimed at the new DAW user that has a basic two channel sound card or interface with or without headphone monitoring the building blocks and ideas, can be applied to more complex setups.
The project presented here is not a one size fits all but is presented to purely fill the need and desire to do some DIY. A very simple project for any beginner that, in it's basic form, is very cheap on parts and doesn't require a great deal of experience. Used as a building block into a multiple rack mounted unit, it can grown to a larger more expensive project as your needs demand. This project also serves as a stepping stone to the more complex DAW monitor switcher MkII and MKIII
An older or very basic DAW set up my only consist of a simple PCI sound card like a SoundBlaster from Creative Laboratories or an Audiomedia-III from Digidesign (AMIII or AM3). To adequately use this to record original music or do voice over and narration work for video some audio monitoring and a microphone and microphone preamplifier will be required, at least. So the very basic setup will include Mic and Mic-pre and a pair of self powered computer speakers typical with any computer purchase today. The addition of some headphones and a Headphone amp would complete a typical simple setup.
Please also refer to the simple Headphone Amp Project
A very basic soundcard set up
The above diagram shows this very basic old style DAW set up and these days you will find many products that attempt to put all this into the one package. This is typical of many of the USB products like Digidesign's m-Box, which is a new product that effectively replaced the AMIII card. Creative Labs have also introduced a USB device with more facilities than the original SoundBlaster card and so to have many audio companies.
Although it is very easy to get started with these products but I find they a lack a couple of very key features. If you were to always use a single set of headphones and never use the main speakers, you may never notice a problem, however if you do begin to grow there are a number of traps.
The first problem you are likely to meet is feedback and this can be quite a shock the first time. As a soloist sitting in front of the computer and of course the main speakers, when you set up a microphone for a vocal record the sound from the main speakers will get into the microphone and so be applied back out to the speakers again ... this is feed back and is too often a full volume squeal which is bad for speakers and for you. If the headphones are also plugged in, they too would get the full volume. This full volume squeal is likely to be a clipped signal, so it's all bad.
Sure you could use the volume control on the speakers or use the A /B speaker selector if you are using an old Hi-Fi Amp but it can all get a bit messy and tedious. Added to that is the likelihood that some of you may have purchased a pair of small powered studio monitors that are so popular today. These often have the volume control on the back plate and don't have a mute switch anyway.
I propose that you need at least a mute switch for the main monitoring and that it should be in a very handy location. That way, as you are about to open a mic channel in front of the main speakers a simple press of the mute switch solves the feedback problem.
The second issue is not so obvious and the solution is not so much required as it is missed when it is not available or removed. Not all of the USB audio interfaces like the m-Box have a volume control for both Main Speakers and headphones out. Even then the m-Box has a strange input to playback control and so is not a true speaker volume control. (The mono switch is a nice touch but only available on the headphones)
Kev's basic solution
A simple passive solution is to have a mute and stereo volume control for both the Main Speakers and the Headphones. You may find that once set, the two volumes need not be changed. This will take the load of continually changing volume, off the volume pot on the interface which could be of dubious quality for constant changes for every overdub. With a DIY unit this is not as issue as, you made it - you can fix it. In addition it would be possible to set up a second switch for a preset dim level. A very handy feature that is too easy to get used to.
Initially we will look at unbalanced signals but it isn't too long a stretch to incorporate some balanced ideas.
A simple mute switch can be just that, a switch with enough contacts to get the job done. Unbalanced stereo is usually two signals and a shared ground so a two pole single throw switch will suffice. DPST - Double Pole Single Throw. In pass mode the signal is fed through the switch to the amplifier and in mute mode the switch takes the amp input to ground and so therefore silence. This switch can be toggle, rotary or simple push button. They can be configured to light a lamp in mute mode or handle a stereo balanced signal. In the end it is all just switches and the number of contacts required.
Simple SPST switch will mute one single signal source
The above diagram shows a simple contact slider of SPST switch and there will need to be two link units to give a stereo mute. It might be a good time to mention Break Before Make and Make Before Break (BBM and MBB). The length of the slider can allow for the mute position to take effect before the signal source is released. For line level type devices this may not be an issue and is probably preferred for a rotary attenuator with many positions but for speaker level devices the Power Amplifier would be shorted for an instant as the switch throws. This is not good and BBM is preferred. To achieve this the manufacturer would fit a slider of shorter length to allow the signal to be released before the short to ground is taken. There are configurations and circuits where it is not important but it is worth considering the consequences of a given circuit or task before you purchase.
A volume control for a stereo our stereo signal can consist of either a dual ganged logarithmic potentiometer or a dual layer rotary switch with resistors fitted to emulate the logarithmic potentiometer. Why use a rotary switch ? It does offer a method to gain better control of the volume curve and the stereo tracking. It also allows for repeatability but the disadvantage is it can not be used for a continuos fade out. We will look at this in more detail later.
A simple approach is to use a dual ganged logarithmic pot of any where from 5k to 50k. To know for sure we would need to know the source and destination impedance but for now we will just keep to the concept and call it a dual ganged pot. The diagram above does look a lot like the switch and the presentation was deliberate. The pot or rotary switch can be view in a similar light. The switch provides two states of On and OFF, the rotary switch provides a number of positions from On down to OFF and they are set be resistor value and the POT provides a continuous slide from ON to OFF.
A basic project
From the above information be can set out to scan the catalogs to find a DPDT switch and a dual ganged log pot.
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dual ganged pot ......... rotary switch ......
SPDT ........... DPDT .................
luminated push button ......
For our first project a dual ganged pot and a DPDT switch with your choice of input and output connectors. I like to use RCAs for the simple projects as there is enough RCA adapters and standard leads in singles, doubles, Y-cords and more, all in off the shelf and fully molded. These include the typical SoundBlaster cable. Excellent for experimenting and setting things up in a hurry.
The schematic below shows our first project as a while and although it is drawn as separate pots and switches we will used ganged versions. This allows for a single stereo volume and a single switch to mute both left and right channels with one movement. The schematic does suggest a possibility of having separate controls for left and right. This is important to note as this project has the scope to get complicated with many options and combinations. The ability to mute just one speaker is desirable and to be able to trim the volume of each channel to balance the stereo image may also be desirable. For the most part the basic ideas don't change and nor do the parts used.
more building instructions and pictures to come
Commercial products
A commercially made product may be the fastest solution to a given problem and as with many things passive they will work in both directions. Some without noticeable differences but some may need a reconfigure. The pot or stepped attentuator may need input (top) and output (wiper) reversed. The units that are wired point to point lend themselves to modification and variation. Not so with the PCB built stuff.
A very famous company specialising in stepped attenuators is Goldpoint Switches. Although expensive these do contain quality switches and very select resistors. Whether they warrant the extra expense is for you to decide but they do highlight the basic idea and present it with the best components, attention to detail and a neatness of construction that we can learn from.
Please do visit http://www.goldpt.com/ as it is one of the best sites on the net. The DIY section is a must and I suggest you download the whole site for future reference. Use a download program like Sitesnagger or set IE to make the site available off-line.
One of their products is a stereo attenuator or volume control with a two input selector. Not quite the same circuit as mine above but for the task at hand you will not notice any difference in operation. By using a shorted RCA connector the second input can become a mute. This without any internal mods. This input can always be used later for the task intended ... say a CD or MP3 player. You don't always want to import audio to your computer just to have a quick listen.
The rotary switch is the heart of the Goldpoint product and it is a very choice switch indeed. Many positions and the resistors can be tailored to suit a variety of configurations. The unit pictured here has individual gold plated RCA connectors and a very similar change over switch to the one we used in the above project
The box is steel and will help to keep out electrical noises from the outside world. The wiring is neat but nothing you can't do your self with a bit of care and a few simple tools. Note the ground bus-bar that runs between the RCAs. This bus-bar is tied via the black wires to the chassis and to a connection on each switch wafer. As I said above to pairs of wires could be reversed and the addition of two shorted RCAs and this is my circuit above. The difference is subtle and although it is not worth any changes in this simple configuration, as things get more complicated and more features are added the order of switches and attenuators will have more meaning.
Never forget that it's all just switches and resistors and connecting wires.
Another mod that could be relevant to the task at hand would be to link four of the RCA's in such a way as to give a loop out. This means that the signal from the DAW can be looped directly to to the second unit. Off-The-Shelf RCA leads provide a quick means to get the signal from one box to the other. Especially no different to a Y-cord but very neat and convenient. This will have completed Kev's basic solution above.
The Goldpoint company offer a variety of product from parts and kits to completed units.
Parts List
if there is one
Downloads
if there are any
Pics
none yet
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A very simple 3
in 1 out - balanced stereo switcher
using a typical 4 pole 3 position Lorlin
Gold Point Switches
Goldpoint switches are expensive. However they do offer a variety of products from parts and kits to completed units. Do give their web site, a visit as is truly excellent, packed with drawings and pictures and circuit ideas that should help to explain much of the above. Their pictures clearly show what is available and their build quality. The site may even inspire you to heat up the soldering iron.
UN-balanced or balanced, it's all just switches, resistors, connectors and wires
ERRATA
So far so good. Check back for any updates in this area.
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