EZ Tube Mixer

[ruffrecords]

Now I am reasonably happy with the poor man's tube mic pre design I thought I might use it as the basis of a simple, easy to build tube mixer project. It would be an N channel into 2 with each channel module sporting:

mic/line inputs
switched mic/line gain (up to 60dB on mic)
20dB pad, phase and phantom supply switches
3 band EQ based on the Helios type 69
post EQ direct out
pan pot and L/R/mute routing switches

The simplified signal flow means there would be no aux sends or solo.

The fader would be external to the module so you can use anything from cheap rotary pots through to P&G faders.

The only other module would be a mix/monitor module. This would include the L&R mix bus amps, monitor switching (between L&R and 2T tape return) and a monitor gain control. You could tack a couple of VUs across the monitor switch output for visual monitoring.

All the modules would be 6U high and 14HP wide so 6 will fit into a standard 19inch sub-rack. So you could make a 5/2 in one sub-rack or a 11/2 in two sub-racks (you might like to make it 10/2 and use the spare slot for other features) or a 17/2 in 3 sub-racks (this is a natural for a 16/2 with a spare slot).

The whole would be powered from an external 19inch rack mounting power supply or if you are feeling lucky you could try building one inside the mixer itself.

Since most of the smarts are in the PCBs I'll be offering them via the white market (so no one gets upset) for a year assuming there is enough interest to cover the costs of doing this.

I'll also be doing front panel layouts in front panel designer and I'll make the files for these freely available - you can order them as cheaply as I can and this gives you the opportunity to customise the look.

How you enclose the whole is entirely up to you and I guess there will be quite a few who would happy  just building a rack of 6 tube mic pres with Helios EQ.

The design is in its early stages right now so don't expect anything to be available for some months yet.

Cheers

Ian

[G-Sun]

You rock Ian!

[spase]

great looking project! I think you could make a big console layout from this ) maybe someone will make a bigger project,more then 19 inch rack so its good to have a good layout for like 16 channel console.

[leadbreath]

 

[tomas1808]

yeyy

[bieckmusic]

this is awesome!!!!!!!!   

[kante1603]

Hello Ian,

This sounds awesome!
I thought a bit about that having a channel insert point might be good to have for dynamics etc..
N channels in a stereobus is absolutely enough I think.If they have direct outs then everything is cool.This way one could have it as tracking channels in front of e.g. a daw or can mix otb by using the consoles line inputs.
In such a setup even auxes are not necessary since you can generate them from the daw.

Very nice and another piece of gear to dream of,hahaha....

Cheers,

Udo.

[bieckmusic]

I say this with the most respect, +1 for insert point, sourced pre/post eq if at all possible.  (not wanting to make this too complicated, I don't know if this would require additional transformers, etc..)  You can send the eq'ed tracks to out board compressors and limiters before they hit the mix buss, or eq after the compression.  So important in controlling how the compressor reacts.  I've been wanting to build a tube mixer, and this sounds incredible.  The 32track TLAudio console I used to use had such a great sound, really fat and clean, this would be a great tube sidecar summing mixer, as well great for tracking...

best!

greg

[ruffrecords]

I am trying to keep this simple and cheap which possibly presents a problem for inserts depending on exactly what you mean by an insert and where you want it. The basic signal flow is

(mic input to 1:10 transformer)                                                                           (transformer)(direct output)
                              (mic/line relay)(10 to 40dB amp)(fader)(passive EQ)(gain make up/output amp)
(line input to 4:1 transformer)                                                                                                                                                       (pan/bus select)(L&R buses)

The only safe places to have an insert is where the signal is balanced. So, if you are tracking and using a single channel routed via the direct out to a tape track then you can insert an outboard device between the direct out and the tape input - this would be post fader and post EQ. You might well wire all the direct outs via half normaled patch inserts to the tape machine inputs. If you did a tracking sub-mix using the L or R bus you could insert an outboard device at the L or R output but not on individual channels of the sub-mix.

On mix down, you can insert outboard devices between the tape outs and the line inputs - these would be pre fader and pre EQ.

The beauty of this approach is it uses only two amps, three tubes, an input transformer and a single Eurocard sized PCB - the output transformer is external and hence optional if you are prepared to live with unbalanced outs. You can even dispense with the separate line input transformer by putting a 30dB pad front of the mic transformer instead. In other words it is reasonably cheap and works also as a stand alone tube mic pre with EQ.

If you want to do a true per channel insert the place to do it is between the (10 to 40dB amp) and the (fader). If you are prepared to accept an unbalanced insert a la Mackie then you can just wire the amp output and the fader input to a TRS and you are done. If you want a proper balanced insert then you need to add (external) output and input transformers.

Cheers

Ian

[ruffrecords]

To clarify things a little I have drawn a basic signal flow diagram for the EZ Tube Mixer. I have dispensed with the separate line input transformer (to save cost) which I had in there primarily out of a concern to minimise noise on mix down. However, I realised that a 32dB pad on the mic input gives a line input impedance of about 6K (which should be high enough for most gear) and needs only 12dB of gain from the mic pre tube stage so its noise should me minimal.I have also shown the pre-fader optional insert point, the master bus and a simple monitor section.

Cheers

Ian

[lassoharp]

Nice!


Agreed that conventional insert points aren't worth the trouble - DirOut already has EQ in path.  Add outboard dynamics and you're there.

[alexc]

A real exciting project for sure Ian 

How hugely cool of you contribute such an awesome thing.

Look forward to see how this develops.
Of course I already  *have*  a mixer some mixers, so  *I*  won't be going there.

That's what I'm telling myself.

Over and over again   

[flaheu]

I have just found some vintage 1:10 Sowter mic input TX, now, I know what to do with them 
Looking forward the progress on this excellent project. 

[kante1603]

Hello Ian,

o.k.,I understand the troubles with that insert point.It was just a thought like "nice-to-have",nothing else.Totally agree that there is no good point here to implement it (balanced) because of the rising costs for additional transformers etc..
So I think you can leave it as is and omit it ( a mackie style insert is not what we want to have,so wrong ballpark I think ).
Keep up your excellent work Ian,I´m highly interested in it anyway!

Best regards from germany,

Udo.

[ruffrecords]

At the moment there can be up to4 switches between the mic input and the input transformer, viz:

1. Phantom power on/off
2. 20dB pad
3. Phase change/Inversion
4. Mic/Line select

My initial PCB layout has 4 ALPS SPUJ push-buttons fulfilling these functions but it occurred to me that not everyone will want all four switches and not everyone will want to to use push-buttons - they might want to use toggles instead and I expect a lot of people will want to position them differently. I have also included a 12 way PCB mounting single gang Grayhill 71 series switch on the PCB plus a bunch of resistors to allow 12 stepped values of gain (11 steps does 30 to 60dB nicely in 3dB steps).

So the question is what do I include on the PCB? The connections between these four switches is quite simple so if you want to use toggles you could fit them wherever you like and just point to point them. The same goes for the gain switch - you could use a regular Grayhill or a Lorlin and hand wire the gain resistors to it (just like we did at Neve in the 70s).

Having not long ago completed a 6 channel mixer I found the hand wiring per channel quite tedious which is why I initially went for PCB mounted switches. I can leave the layout like that because it is easy not to fit these parts and the pads provide convenient hook up points if you choose not to use them. So I think I have convinced myself what to do but I would value your thoughts.

Cheers

Ian

[bieckmusic]

Hi Ian,

I hope I'm not chiming in too much here, but I'm super excited about your mixer design.  I'm for the board mounted push button switches and rotary, just for the convenience.  Will the line in pad provide enough attenuation to accept a pro-tools signal?  (probably a stupid question) I'm guessing that the resistor values could be tweaked to allow further padding, if necessary.

Also, looking at your signal flow diagram.  It looks like more tape returns could be added if necessary...

I'm just ordering the BOM for my poor man's eq now, can't wait. 

Bless,
greg

[ruffrecords]

Hi Ian,

I hope I'm not chiming in too much here, but I'm super excited about your mixer design.  I'm for the board mounted push button switches and rotary, just for the convenience.  Will the line in pad provide enough attenuation to accept a pro-tools signal?  (probably a stupid question) I'm guessing that the resistor values could be tweaked to allow further padding, if necessary.

Yes it is easy to change the padding to cater for higher levels. The Sowter microphone transformer I use will accept a maximum input level of +9dBu. The current design uses a 33dB pad for line in before the input transformer so as far as the transformer is concerned you could input a level of up to 33 + 9 = +42dBu before it runs out of headroom. In fact the first stage amplifier runs out of headroom first. At a push it will output +30dBu and since its minimum gain is 10dB that means its maximum input is +20dBu. Since the mic transformer gain is 20dB the maximum level at the mic transformer input can therefore be no more than 0dBu and hence no more than +33dBu at the line input. Since the maximum output from most DtoA converters driven by Pro-tools is only +22dBu then there's more than a 10dB margin even with the standard input pad. With the first stage gain set to 13dB gain the line input gain is -32 +20 + 13 = 0dB.

Also, looking at your signal flow diagram.  It looks like more tape returns could be added if necessary...

Yes, the monitor section is pretty much up to you. I just showed the minimum necessary to monitor the mixer output and the return from the master 'tape' machine

Cheers

Ian

[alexc]

I'd like to see  an EQ bypass switch if possible.

Also, pushing the boundaries perhaps,

but a HiZ DI connector to grid would be of great benefit.

Just something simple so those who wish to do so can wire in
a switched jack which interrupts the signal to grid and replaces with HiZ DI signal.
Or if really adventurous, with a relay and additional front panel switch position.

Something like this would cry out for at least a few direct injected instruments.

Great stuff

[ruffrecords]

I'd like to see  an EQ bypass switch if possible.

Also, pushing the boundaries perhaps,

but a HiZ DI connector to grid would be of great benefit.

Just something simple so those who wish to do so can wire in
a switched jack which interrupts the signal to grid and replaces with HiZ DI signal.
Or if really adventurous, with a relay and additional front panel switch position.

Something like this would cry out for at least a few direct injected instruments.

Great stuff

Until the other day it did have a HI-Z input via a relay! I don't like conventional HI-Z inputs that are switched by the jack socket because it means the mic signal from the input transformer secondary has to travel all the way to the front panel jack socket and back again. This part if the circuit it the most sensitive to external interference and I prefer to keep it as short as possible. In the original design I had a relay right by the transformer and the first tube grid so a HI-Z input could be switched in but the transformer to first grid length was kept small. Anyway, because this was turning into a mixer I took out the relay a couple of days ago. Looks like I need to put it back in again!

Cheers

Ian

[alexc]

Yes please! What you said   

Totally agree - I prefer the relay method for sure. Much more civilized