Small Dial

[chris p]

This idea is a small monitoring control box: a choice of 2 balanced inputs, then mute, dim and mono switches, an attenuator volume control and then a choice of 2 balanced outputs.

The building blocks are just linked sequentially:-

(a) the input select switch is a simple 4PDT switch, where the poles select a L+, L-, R+ and R- from one or the other input;

(b) the mute is a DPST on/off switch that interrupts the L+ and R+ signals;

(c) the dim switch is 4PDT that switches in a simple resistor U pad across L+ and L-, and across R+ and R- (I'm testing 120R for the shunt and 470R for the series resistors);

(d) the mono 4PDT switches in a resistor mixing circuit (I'm using 5k1's as the mixing resistors) that joins the L+ and R+ together and sends the mixed signal to both L+ and R+ (the combined signal thus making up the lost gain from the mixing circuit), and the same for L- and R-;

(e) the attenuator is a dual gang audio taper pot (if I were really keen, this could be a VCA instead); and

(f) the 4PDT output switch sends the L+, L-, R+ and R- pole to either the main or aux ouputs.

It would be possible to just wire up the switches and pot just like that. However, my motivation for doing this was to get some hands-on relay wiring done, so rather than all the switches, I'm going to use illuminated push on / push off buttons - SPST - to connect a 12V supply to some DPDT relays (in pairs where a 4PDT is required).

Here's my PCB layout



Not much explanation is needed: I uses these relays
http://www.electusdistribution.com.au/productView.asp?ID=3975

Each has a control V+ from the switch, and a ground connection. The other 3 pins on each side are the switch: the top one is the pole, the next is the connection when there is no control voltage, and the last is the connection when there is control voltage.

Don't be confused by my naming of the relays: in fact, the top ones deal with the + signals and the bottom ones deal with the - signals (rather than L and R respectively). I've played around withe the design and that was one of the things that changed.

The 2 rows of 3 pins towards the right are the dual gang pot connections.

COST WARNING: the relay version raises the classic issue where you can probably buy something like this for not much more than you'll pay to DIY. For example -
SM Pro Patch2 http://www.sound-music.com/product_info.php?PID=213
or else for more money get something better, like the Mackie Big Knob.

[chris p]

Kurt - you asked for this thread, so now you've got to answer a question: is that "Message in a Bottle" you're pretending to play in your mug shot?

[astrovic]

Chris,

Once again you've done the thinking for me, before I'd even thought about thinking about it. You're legend - thanks again for all the contributions you're making here.

Cheers mate.

[Kurt]

I was feeling particularly posey at a beer fuelled hot gig.... I believe the singer was screaming "Under the blackening skies of the apocolypse....etc" at the time. I've actually lost the hair now, but the beard's longer. Still use the same guitar though, BC Rich Acrylic series warlock, it's oh so very metal ;)

Kurt - you asked for this thread, so now you've got to answer a question: is that "Message in a Bottle" you're pretending to play in your mug shot?

[wez]

the monitor box that i had ross giles build me a few years ago is similar in concept... except that we were going for absolute pristine audio quality, with no unneccessary electronics getting in the way. so we opted for passive, unbalanced - 8 inputs (Elma switch), mono, separate L & R mute, an insanely expensive ALPS volume pot, & 3 outputs. 2 of the outputs are post-volume control via an A/B switch, the third is pre-volume control and is usually hooked up to a CRO.

all point to point wiring etc etc. in retrospect i could've done without the A/B switch, or the 8 inputs - 5 or 6 would've been enough. but the end result is incredibly transparent and it makes my job so much easier. oh, and the volume knob is a big blue thing from a vintage Neve broadcast console, that's my favourite bit.

generally, the features we left out are available in software anyway, ie phase invert, dim etc. adding stuff like that would've meant an entire paradigm shift - more components, forget passive etc. having said that, if i were building a new one today, and mastering wasn't my sole pursuit, then i'd definitely go for all the stuff i could squeeze into it!

i've tried a few monitor systems - and ultimately something like the crane song avocet would be great - but i keep coming back to the giles box, for sound and simplicity it can't be beat.

chris, your project looks right on the money, good luck with it. just don't compromise on the components. yes, it will absolutely end up costing more than those POS big-knob type things but A) will (potentially) dump all over them for sound quality, and 2) will be customised to your needs. what more could you want?

[chris p]

Well there you go: skies blacken during an apocalypse - I never knew that.

The BC Rich guitars - love 'em or hate 'em, but the Warlock is still one mean lookin' beast.

Astro - thanks, I still put myself in the amateur camp: its guys like Howard and Rob that we both need to listen to. And a caution - I'm still in the process of veroboarding this up as a prototype - I'll let you know if it actually works!

Wez - thanks for the encouragement - the resistors will all be 1%, the relays look good and so the only real issue for componentry is the volume pot. A nice Neve NOS would be good. Anyone got one spare???

[Chris H]

Try Dan Alexander for a neve pot
http://www.danalexanderaudio.com/parts.html

[rob]

good-o

i've built a few passive monitoring control boxes. They are a snack and being totally passive maintain pure audio quality. There is a way of doing it balanced all the way through. For pots try the Alps dual 10K log pot STRK27101 available through RS ( part # 219-9130 ). These have a gang to gang balance of 2dB. From my experience it's usually much better than that at around 0.5dB over most of the range. If you do it fully balanced you loose level overall, but this isn't a problem when driving powered monitors or amps as you usually want to get from your max DA output level of ~ +20dBu down to a max amp level of ~ +4dBu anyway.

Rob

[chris p]

Thanks for the tip, Rob:

[astrovic]

[quote="chris p"]Astro - thanks, I still put myself in the amateur camp: its guys like Howard and Rob that we both need to listen to. And a caution - I'm still in the process of veroboarding this up as a prototype - I'll let you know if it actually works![\quote]

Don't worry mate - lots still to learn before I get there! Looking forward to hearing how you go.

[rick]

i have been at the custom monitoring coalface for about 15 years and have spent an absolute fortune on various versions
the things to learn frommy mistakes are
1. stereo pots no matter what the brand what the price will not continue to track the same on both channels
so save 20 years of buying and trying and just dont bother , stepped attenuaters, or relay driven switches or vcas or any tricky combination of all three is the way to go.

2. nothing is as simple or as "transperant" as it seems

3. most "market versions " sound worse then a simple 24 step passive attn
try a goldpoint one for a "safe bet" ( goldpoint.com)

4.i had the best loudspeaker monitoring device loosely based on the neumann console monitor section at festival made for me years ago and it beat any competitor that i tried it against by far
UNTILL i tried the crane song avocet
i plugged the avocet in and bought it in 5 secs ! ( even though i did not have the bucks or was even looking for a volume control at the time )
nothing on the market comes close .
5. i am hyper critical about stereo imaging and soundstage
you may not be ( or need to be)

so build your own and learn a lot but you will not save a cent if your looking for something really special

oh yeah listen to what rob has to say about this stuff he has recently delivered some amazing relay insert +gain devices to us and
as far as stereo seperation goes in a mastering device i think rob has opened up a whole new catergory
i think i will call it class S

[Linear]

Cool idea.

It's interesting because I just spent waaay too long on a similarish mod for my console (I was amazed at how long it took for something which seemed relatively simple.)

I basically made a switch to select between 4 sets of monitors, from an already complicated monitoring section (it has 4 independent channels for each of the 8 busses). So after doing that, here are my suggestions:

- make it expandable (so when you decide you need more functionality you don't have to build a new box). I did this by having a 4 x 12 switch but only using 5 positions.

- maybe include a headphone output? I did on mine and it was easy.

- use a rotary switch that feels good with a nice click. I don't really like how the generic lorlin switches from jaycar/farnell feel. I used one of these

http://au.farnell.com/jsp/Electrical/Ro ... +(CONTROLS)/459102JMK2/displayProduct.jsp?sku=1165273

- make sure you get a nice, sturdy knob with two locking nuts - nothing worse than when it slips. I managed to get a great marconi knob from an old AWA gain measuring attenuator. Makes switching between monitors very gratifying.

- You might want to think of some internal calibration for the outputs in case your monitors don't have any adjustment. My new focals are a case in point, but thankfully everything else is adjustable.

- Make the mute switch usage really obvious somehow. SO MANY TIMES I get no signal and can't work out why. Same goes for the dim button.

Good luck!

Chris

[chris p]

The Avocet makes my price warning a little redundant, but I know its WAY beyond anything I can achieve DIY.

For those following at home, goldpoint.com points you to a neat little holiday resort in the USA. The actual reference is http://www.goldpt.com.

Thanks for the voice of experience, Rick and Chris (Linear). I understand the stepped attentuator point and think that will be a good upgrade to the design. The cost of this thing seems to be going only in one direction, but I need something that will last for quite a while, and I'm sure price now will be forgotten when quality is long valued.

[chris p]

I've been looking over the Goldpoint site, and they have confused me.

My understanding was that to attenuate a balanced signal, the voltage divider goes across the + and - signal (so for a stereo balanced pair, you need a dual gang pot or its stepped equivalent).

That site seems to suggest that the voltage divider goes from + to ground and from - to ground for each side of the stereo pair, and so you'd 4 gangs in effect to attenuate a balanced pair.

Can anyone elucidate?

[rob]

either way can work.
if you put the attenuator between + and - with no reference to ground you either need to use 2 gangs per channel ( as you would if you did refer them to ground ) or use a pair of series resistors between the input + and - and the attenuator ( single gang ) .This method means you are always loosing some level at min attenuation ( max volume ). There is a trade off between min load impedance and min attenuation. Also the load impedance varies as you change attenuation. This isn't generally a problem for pro audio and as i mentioned earlier you generally are happy to shed level between your source and the monitors with typical pro audio signal levels.
Goldpoint seem to cater more to the Hi Fi world where the ground referenced attenuator is probably more appropiate.

[chris p]

Thanks Rob - I've always used the series resistor approach and never thought about the dual gang to ground idea.

[Linear]

I've just made a 24-way stepped attenuator to replace the volume knob. It's great! Used the goldpoint website too, very helpful.

I actually have a second one here (with resistors) in case anyone is interested. 24 way/dual gang. I had to buy 50 each of all the resistors to make 1 5k attenuator, so if anyone wants a set pm me.

Lastly I also made the stereo/reverse stereo/mono/Lmono/Rmono switch. It's pretty cool, however they recommend 22k resistors for the mono sum which didn't seem quite right. chrisp I note you used 5k1 resistors.

After much experimentation and listening, I settled on 1k5 resistors. Seems to work great.

Cheers

Chris

[chris p]

Yeah, that 6 way switch is way cool - I've made one up myself, although for a balanced stereo pair you're looking at 8 poles! Its significantly useful and a work or art in its own right.

[chris p]

Latest incarnation:

It starts with a 3 pairs of balanced ins, into a 3x4 input selector (manual knob) to select one stereo pair. Relay driven phase switches then, one for each channel, with illuminated front panel switches to clearly show when they are engaged.

Then the 6 way MODE switch (manual) as per Linear Chris's post, a -20db Dim switch (illumated, realy controlled) and then the stepped attenuator for volume control. Outputs for left, right. I'm tempted to add in a second, manually switched pair of outputs (easy to do) and a mono output as well, and wire up a tricky part to the mode switch that mutes the mono when in stereo and mutes the stereo when in mono. I need to think a little further how to do this.