Passive Mastering EQ

[ruffrecords]

Got the Elma switches quote and I have ordered five. Lead time is 5 to 6 weeks so nothing is going to happen very fast.

In the meantime here is the first instalment of the write up.  It is meant to be a Design and Construction Manual but i reads more like a blog right now. I might post it as a blog at some point.

Cheers

Ian

 

[petermontg]

Fantastic work Ian!

 

[skal1]

Very informative Ian, i was wondering about the buffer amp , i guess it will have to be tailored to which EQ your building.


Skal

 

[ruffrecords]

Very informative Ian, i was wondering about the buffer amp , i guess it will have to be tailored to which EQ your building.


Skal

I think the most straightforward approach is to make all versions the 10dB type. With the resistor values given in the document you can make your EQ span any number of dB in the available range in whatever size steps you like. The output will always be suitable for a 10K balanced line input so almost any piece of pro gear will  work as a gain make up amp. Of course there is nothing to stop you building in a gain make up amp of your own.

Cheers

Ian

 

[malnatim]

this looks fantastic! thank you ian. it's great having the all the options of how to build... bands, frequencies and gain steps.

i'm thinking of 2 channels and 1dB gain increments (+/-10).
basically, a stereo, four band version of EMI RS56 with it's octave spacing plus some additional intervals.

band 1: 32, 64, 96, 128
band 2: 256, 448, 512, 768
band 3: 1024, 2048, 3072, 4096, 6144
band 4: 8192 12288, 16384, 28672

 

[ruffrecords]

An RS56 version sounds like a great idea. I have added a section to the document giving a simplified formula for calculating the required inductance for any frequency. I have also added some tables listing the required inductor values for a boost Q of 0.6  for each of the four bands if the RS56. I have copied the new version of the document to the 6dBEQ folder of the DIY tab of my web site. Here is a direct link:

https://drive.google.com/open?id=0B_n67A1hN3qtSFZTMzhucVpKU0k

Cheers

Ian

 

[malnatim]

thanks ian! is a Q of 0.6 equivalent to the sharp setting of RS56, then adding resistance (via Q connector) softens down the curve for the other bell settings of shaping control switch? they look like 0.12, 0.2, 0.5

are the switches on switch board 1.6" apart? i guess this board will change with the elma switch. might it please be possible for the 2 switches to each have their own pcb? then panel layout is also undetermined.

pic is a screen shot from this video...
https://www.youtube.com/watch?v=TX6vAD55_Bs

 

[thomasdf]

Love the idea of working with octaves! Will there be some some of control over the Q? Per band, or for the whole EQ?
Bravissimo Maestro! I'll def need this one

Edit: I agree that having separated PCBs for the switches is a good thing in order to freely design the frontpanel. Or at least to be able to wire the rotary switches to the PCBs

 

[ruffrecords]

thanks ian! is a Q of 0.6 equivalent to the sharp setting of RS56, then adding resistance (via Q connector) softens down the curve for the other bell settings of shaping control switch? they look like 0.12, 0.2, 0.5

Good question. I had not seen that graph of the RS56 before. It is clear that the RS56 can boost/cut the same amount no matter which sharpness setting is selected. This means they must use a different LC pair for for each sharpness setting at each frequency. This explains why pictures of its insides show switches with very many wafers on them and probably explains why nobody had tried to replicate one in hardware. My EQ uses the same approach as the original Pultec EQs by adding series resistance to the LC circuit to reduce the Q. However, this also reduces the amount of boost/cut, just as it does in the Pultec so you don't get the same range of boost/cut for each Q setting. You could add an extra switch to select an appropriate LC pair for each of blunt, med blunt, med sharp and sharp but this would multiply the number of inductors by 4. Not a cheap option.

It is hard to tell exactly what the Q is for each of the RS56 settings.  lot depends on the dB scale on the right hand side of the graph you posted. My best guess is that each vertical division represents 5dB because that then shows the curves operating at maximum boost which is where a Q measurement is most valid.  So I printed off the graph and drew in the -3dB line and measured as best I could the frequency at which each of the four curves intercepted it so I could estimate the bandwidth and hence calculate the Q. On this basis I estimate the Q of each setting to be:

Sharp = 2.8
Med Sharp = 1.7
Med Blunt = 1.0
Blunt = 0.6

If these are correct then it implies the 0.6 setting we have chosen corresponds to the Blunt setting.

are the switches on switch board 1.6" apart? i guess this board will change with the elma switch. might it please be possible for the 2 switches to each have their own pcb? then panel layout is also undetermined.

This has not yet been determined. However, in the interests on minimising stray capacitance, track inductance and interference I do not want to make the spacing too large. One possibility is to leave this up to the constructor. The boost/cut switch could interface to a small PCB. This would hold a 24 way ribbon cable connector and a two pin connector to connect the LC circuits for that band. These LC circuits would be on a separate PCB for each band and would also hold the frequency selection switch. The only connection between bands is then the ribbon cable which daisy chains them together and plugs into the input/output board which contains the resistor ladder. This gives you complete flexibility in spacing the controls.

pic is a screen shot from this video...
https://www.youtube.com/watch?v=TX6vAD55_Bs

Many thanks for that. A very usefull piece of information.

Cheers

Ian

 

[malnatim]

thank you ian. i see now. that does make sense of the huge switches. i guess it really was a no expense spared project.  in the video there's also some great footage demonstrating the stereo linking cogs. incredible stuff.

i'm glad i posted the pic. i thought it might be old news. i'm pretty sure during the waves promo videos there a few more momentary fragments from original documents. i'll have another look on the weekend and and take some screen shots.

the flexible scheme sounds great.

 

[ruffrecords]

I have ordered the switches. as soon as they arrive I will start designing PCBs.

Cheers

Ian

 

[5v333]

just something worth thinking about is that the inductors do like to pick up magnetic energy from mains transformers. for ex a rack unit under or above the eq.

if your eqs are high in dimension, you could get away from hum by locating the inductors in the middle of the box.

my NYDEQ has got indukturs about 1cm from the bottom and if i have it standing on top of my cranesong hedd, i will get hum picked up from a transformer in the hedd.

im thinking about getting some mumetal sheets to sheild bottom and top of my eq.



shelf/bell switches are great for a versitile eq!!! the adition of another reactive component costs money and space though...

variable Q doesnt feel neccessary since its a passive eq. many mastering techs have more than one eqs for different tasks. a passive friend is a broad  one who works in harmonics while correcting is a task for the fully parametric eq like sontec or porter. my nydeq have wide boost and narrow cut and i like it.



if your going for transformer in/out, consider m/s encode/decode.

 

[ruffrecords]

just something worth thinking about is that the inductors do like to pick up magnetic energy from mains transformers. for ex a rack unit under or above the eq.

if your eqs are high in dimension, you could get away from hum by locating the inductors in the middle of the box.

my NYDEQ has got indukturs about 1cm from the bottom and if i have it standing on top of my cranesong hedd, i will get hum picked up from a transformer in the hedd.

im thinking about getting some mumetal sheets to shield bottom and top of my eq.

Interesting. You obviously have first hand experience of this. The HEDD appears to have a chunky toroid mains transformer inside it. I agree that keeping the inductors in the centre of the box is a good idea. You should also consider their orientation. Rotating them through 90 degrees could help.

shelf/bell switches are great for a versitile eq!!! the adition of another reactive component costs money and space though...

Switching an entire band from bell to shelf is not easy. However, adding a few shelving frequencies to a set of bell ones is straightforward as in the Pultec EQP-1S3 for instance.

variable Q doesnt feel neccessary since its a passive eq. many mastering techs have more than one eqs for different tasks. a passive friend is a broad  one who works in harmonics while correcting is a task for the fully parametric eq like sontec or porter. my nydeq have wide boost and narrow cut and i like it.

The variable Q is an option. SOme people will use the EQ for tracking where it might be useful, If you don't need it you simply short it out on the PCB.

if your going for transformer in/out, consider m/s encode/decode.

I am not sure this is an option I can include. The input side is OK but the output is more difficult because of the insertion loss of the EQ. If you are building the EQ with built in gain make up then it can be done.

Cheers

Ian

 

[AudioGimmer]

Hi Ian.

On the subject of inductors and hum I can also chime in. My Buzz Audio REQ 2.2 is VERY sensitive to surrounding equipment, despite having the inductors wrapped in Mu metal cases. It took a bit of juggling of equipment position to get it quiet.

I'm enjoying your idea of a simple  passive mastering EQ and I'm following with interest.

For the record, like Ruairy I earn my crust mastering, and .5dB steps would be essential. I like the idea of half steps to +/- 3 and whole steps there after.

The Gimmer

 

[salomonander]

pikatron does some very high quality mumetal cans for inductors that are rather cheap.
ian, how much does elma want for  a switch?
and one suggestion when you are doing the layout... id opt for wider traces than on your previous boards. they always seemed very thin compared to other makers. cant cost much more i assume...

 

[ruffrecords]

pikatron does some very high quality mumetal cans for inductors that are rather cheap.
ian, how much does elma want for  a switch?

Over £50 each

and one suggestion when you are doing the layout... id opt for wider traces than on your previous boards. they always seemed very thin compared to other makers. cant cost much more i assume...

I am surprised you say that. I try not to make them any thinner than necessary and I default to 25 thou or 20 thou width wherever possible. There are occasions where you need to squeeze a track between pads so it has to be thinner but there is no getting round that. Did you have a particular board in mind?

Cheers

ian

 

[5v333]

yea your right about the toroid inside the hedd... i thought there was a EI transfomer inside responsible for this... one or two rack units up and im probobly fine.

if i only knew about the 90 degrees rotation thing half a year ago

another thing ive been wondering about is if its possible to wind inductors with two sections and make some sot od humbucking effect? or maybe there would be cancelation of sound.. if anybody smart enough could give a scheme on a inductor like this i could have a go on my hand powered winder.

a pcb with different build options is genius!

ciruit layout things to consider might be ground and sheild. i suspect that in the eq there could be benefits to ground some parts before others... also the input transfomrer for my NYDEQ didnt like having its casing connected with electrical ground but rather the chassie ground or pin 1. massive rf oscilation.


i also follow this project now since ive been bitten by the world of inductors!

 

[salomonander]

Over £50 each
I am surprised you say that. I try not to make them any thinner than necessary and I default to 25 thou or 20 thou width wherever possible. There are occasions where you need to squeeze a track between pads so it has to be thinner but there is no getting round that. Did you have a particular board in mind?

Cheers

ian

hey Ian,
you must look at the wrong places to get the pikatrons. the small version is 12€, the big one is 19€ at buerklin.

 

[thomasdf]

50£ is for the Elma Switch I suppose

Keep up the great work Ian, can't wait to read more

 

[salomonander]

oh yeah. i misread ians post. that price makes sense.