Small valve mixing console

[C.B. - Boudio]

I have in the last couple of months been trying to design some equipment. My plan is to set up a ultra-low-budget studio with later 1950s-early 1960s-inspired equipment. I want to do most of it by myself, which translates into 'all except microphones and recorders', so the preamp/mixer, EQs, and compressors. For the compressor, I will probably use the 'mix bus compressor' as now being built on the forum.
My last - and maybe least-embarrasing! - attempt to design the mentioned equipment is this small 4-into-2 mixer.
It features an EF86 and an ECC82 per channel, plus a ECC81 in the master-section and an EM87 as a peak indicator (I will use also normal VU-meters, maybe I wil just make a scale on a regular mA-meter).
Per channel, there is also a rudimentary equalizer, not as a precise kind of adjustment but more like some knobs that may affect the frequency response, to put it that way.
Transformers will be bridgeable, so I can leave them out (because of budget - it must still be decent!), by means of a 7-pin DIN connector; each audio transformer will have its own subchassis, connected by such a plug.
Every connection between 'modules' will be patchable.
Here are the schematic drawings. If there is something you don't understand, feel free to ask of course.

View attachment 1710526725764.jpeg
View attachment 1710526769936.jpeg

 

[C.B. - Boudio]

I did not design any negative feedback. Would this be a problem in such a circuit, or would the benefits in this case be big enough to maybe even include extra valves (to keep gain up)?
Or other improvements, inside the limits of an ultra-low budget valve mixer (well, I will of course start with just one preamp module)?

 

[ruffrecords]

Several points come to mind.

Your input transformer is labelled 600 and 200. It is not clear to me if this is the actual mic impedance or the input impedance. Note that it is common these days for the input impedance to be about 10 times the mic impedance. The mic transformer secondary impedance is shown as 5K which implies a 1:5 step up and the 15K resistor on the EF86 grid will be reflected to the primary as 600 ohms, So the first question is which input transformer do you intend using?

The mic transformer gain is 14dB and the EF86 stage will probably have about 40 db of gain. This might mean you channel gain pot (vol mixer) will be a
long way down on loud sources. You might like to consider adding a switchable 20dB pad on the mic inputs. It will only cost you a toggle switch and three resistors. Also this 500K pot will be heavily slugged by the 40K grid resistor of the cathode follower it feeds.

The direct out uses half an ECC82/12AU7 with a 100K plate resistor and 10K cathode resistor. I think you need to review these values as I suspect you will end up with a very low quiescent current. I would suggest you need at least 5mA quiescent in this stage to provide sufficient drive capability. What output transformer do you plan to use?

The 22K channel fader? (vol )has dc passing through it. This is a recipe for scratchiness. I would suggest adding a 1Meg grid resistor and 100nF coupling capacitor.

I cannot get my head around how mixing occurs in the master section. Any particular reason for choosing that circuit. Also the ECC81 output stage with 100K plate resistors needs a lot more quiescent current. Any reason to use an ECC81 rather than an ECC82. An ECC888 would pobably be a better choice.

Great to see another tube mixer builder.

Cheers

Ian

 

[C.B. - Boudio]

The input transformer's impedances should be 200/600 primary (or something like that; some of the old equipment I intend to use have only loudspeaker outputs, and I also really the German broadcast/recording things).
At the moment, I will probably avoid transformers as much as possible.
I know they can be very useful and that they play a role in the sound of the preamp, but since I am at the moment using not too much professional equipment yet, and I do not really want to emulate any 'classic sound', they do not matter. But I wanted to keep the options open.

The pad is a very good idea indeed. And I may already use the console with some small passive mixers, so that will help as well.
The grid resistor of the first half of the ECC82 was, in fact, a 10K, even worse I guess... I will adjust it to 100K then.

What values do you recommend for the output stage of the ECC82?
The output transformer is the same case as the input; I will probably use there the same transformer as for the input, if there will be one in the long run.

The 22k channel fader does not have DC passing through; before the tonestack, at the cathode follower, there is already a 100nF capacitor and since the tonestack is entirely passive, there is no DC. Or is there something else I have to take into account here?

As drawn on the schematic, the master section gets the 4 mono signals, from the second half of their ECC82s, and those are divided to stereo by their respective panpots - this will in real life be kept at the channels, not at the master stage.
So, the stereo outputs of the four channels then get grouped left and right at the respective master faders, and each of them goes in one half of the ECC81 (or ECC82, if that will be a better choice). I hope that this is a bit clear, I am not to great in explaining these things in English (I'm just some quite mad Belgian teenager).
I chose that circuit just because it looked simple to me...
You meant an ECC88 with an 'ECC888'?

Thank you for your feedback, it was very helpful.

 

[audio_fil]

hi
like this https://www.preservationsound.com/wp-content/uploads/2015/01/1954-Audio-Mixer.pdf

 

[C.B. - Boudio]

hi
like this https://www.preservationsound.com/wp-content/uploads/2015/01/1954-Audio-Mixer.pdf

Quite interesting!
Mine should be even more modular, and stereo, but indeed...

 

[ruffrecords]

The grid resistor of the first half of the ECC82 was, in fact, a 10K, even worse I guess... I will adjust it to 100K then.

You need to consider the total load on the output of the EF86. Its output impedance is essentially the 220K anode resistor. Its load is the 500K mixer vol pot in parallel with the ECC82 grid resistor. If this is 100 K then with vol fully up the parallel combination of the two is 71K which throws away 10dB of gain. I would change the 100K to 1Meg which makes the worst case load 330K and you lose only 4dB of gain

What values do you recommend for the output stage of the ECC82?

The ECC82 is a dreadful audio valve, especially when used single ended. I would recommend using the ECC88 for all instances of it.

The output transformer is the same case as the input; I will probably use there the same transformer as for the input, if there will be one in the long run.

Any idea which one you plan to use?

The 22k channel fader does not have DC passing through; before the tonestack, at the cathode follower, there is already a 100nF capacitor and since the tonestack is entirely passive, there is no DC. Or is there something else I have to take into account here?

There will be grid current flowing to ground from the grid via the wiper of the pot. It is very small but enough to cause crackling when the pot is turned.

As drawn on the schematic, the master section gets the 4 mono signals, from the second half of their ECC82s, and those are divided to stereo by their respective panpots - this will in real life be kept at the channels, not at the master stage.
So, the stereo outputs of the four channels then get grouped left and r

Ah, now I understand - they are panpots!!

ight at the respective master faders, and each of them goes in one half of the ECC81 (or ECC82, if that will be a better choice). I hope that this is a bit clear, I am not to great in explaining these things in English (I'm just some quite mad Belgian teenager)

No problem. I was messing with valves and pirate radio transmitter.ters when I was your age

I chose that circuit just because it looked simple to me...
You meant an ECC88 with an 'ECC888'?

Yes

Thank you for your feedback, it was very helpful.

My pleaseure.

Cheers

Ian

 

[ruffrecords]

By the way, your HT supply could be easily improved and made cheaper. You do not need the big, heavy, expensive choke. Just use four stages of CR smoothing. Also, because you have a semiconductor rectifier you can increase the smoothing caps to 220uF or even 470uF each. A 470uF reservoir capacitor follow by three RC stages with 470uF caps will produce a very quiet HT supply.

Cheers

Ian

 

[C.B. - Boudio]

Any idea which one you plan to use?

Which transformers do you recommend?

The ECC82 is a dreadful audio valve, especially when used single ended. I would recommend using the ECC88 for all instances of it.

That's nice, because the PCC88 (300mA series heating for TV sets, I may combine them then with PF86s) are at least half the price, too.

You need to consider the total load on the output of the EF86. Its output impedance is essentially the 220K anode resistor. Its load is the 500K mixer vol pot in parallel with the ECC82 grid resistor. If this is 100 K then with vol fully up the parallel combination of the two is 71K which throws away 10dB of gain. I would change the 100K to 1Meg which makes the worst case load 330K and you lose only 4dB of gain

I didn't want to make the load to big, indeed. But I'll adjust it that way, then!

There will be grid current flowing to ground from the grid via the wiper of the pot. It is very small but enough to cause crackling when the pot is turned.

I didn't know that. I am learning quite a bit at the moment!

 

[ruffrecords]

Which transformers do you recommend?

Transformers are probably the biggest variable in terms of performance and cost of any component in a mixer design so it is difficult to make firm recommendations but my guess would be cost would be your number one consideration. In which case you have perhaps three choices of supplier: OEP (UK) , Edcor (USA) and UTM (Poland). assuming your prefer an EU source the you could try the UTM3510 for the output mand the UTM2555 for the input - but I have not used either of these myself

That's nice, because the PCC88 (300mA series heating for TV sets, I may combine them then with PF86s) are at least half the price, too.

I checked out the ECC88/PCC88 datasheet and for your output stages you could try a 22K anode load and a 680 ohm cathode resistor. This should get you close to 5mA quiescent. This means you cannot really feed the master from the cathode but you should be able to feed it from the anode instead.

I didn't want to make the load to big, indeed. But I'll adjust it that way, then!


I didn't know that. I am learning quite a bit at the moment!

That's what we are all here for.

Cheers

Ian

 

[C.B. - Boudio]

Ok, then I'll look for those transformers.

I checked out the ECC88/PCC88 datasheet and for your output stages you could try a 22K anode load and a 680 ohm cathode resistor. This should get you close to 5mA quiescent. This means you cannot really feed the master from the cathode but you should be able to feed it from the anode instead.

So, transformer primary 22k, 680 cathode, and what about the anode resistor?
But I will probably be able to use the cathode signal in a (relatively) high-impedance transformerless amplifier, or not? (doesn't matter all that much, but may be convenient at some point)
And as for the master, I will do the same, but then I will have to adjust the 680 ohm to the DC resistance of the meters, so to get a resistance of 680 ohms between cathode and ground in total, isn't it?

 

[MidnightArrakis]

Quite interesting!
Mine should be even more modular, and stereo, but indeed...

>> And.....I even have the easily modifiable 3D CAD-modeled rack-mount chassis design already to go!!!




Unless you are planning on -- manually hard-wiring -- this mixer together, possibly using "Point-To-Point" turret boards like are used in vintage guitar amplifiers of yesteryear, you're going to need to learn how to use a modern schematic and PCB-design program like KiCAD for all of your electronics. In addition and in a similar, yet different manner, unless you are planning on salvaging/scavenging all of the required sheet-metal parts yourself and going through your own painstaking process of -- hacking/whacking/cutting/drilling/filing/etc./etc./etc. all of your sheet-metal parts, you will also need to learn to use a modern 3D Mechanical Design Program like FreeCAD to create and design all of your sheet-metal parts and have them cleanly fabricated by a sheet-metal shop. Either that, or..........you could possibly work with someone who already has all of required CAD software and knows how to use them. hint...hint...hint

(FREE) KiCAD download: https://www.kicad.org/download/

(FREE) FreeCAD download: https://www.freecad.org/downloads.php



And, then.....should you decide to go the "PCB-design" route, you will need to keep in mind to specify your components, including any transformers, to be a "PCB-Mount" type. This will allow and help you keep any superfluous wiring down to a minimum while making the overall assembly easier. But, by doing things this way, it will require you to "mentally imagine" how your audio mixer project will be designed, layed-out, assembled and eventually used. As you can (kinda) see in the image above, the small PCB on the left-side has the AC-mains input connector, a DC-output connector and an RJ-45 connector all PCB-mounted. The large PCB has all of the audio I/O connectors (GREEN = terminal blocks, GRAY = Quad RCA connectors) and a pair of (BLUE) level-setting posts that are also all PCB-mounted. NO WIRES!!! The front-panel switches are all PCB-mounted with only a small ribbon-cable jumping between its PCB and the large I/O PCB. And.....that's about it!!!

And, finally.....please take-to-heart the comments you receive from Ian (RuffRecords), as you won't find anybody else here on this forum who is more knowledgeable about not only designing valve circuitry, but -- valve audio mixers -- more specifically!!!

GOOD LUCK!!! on your daunting project!!!

/

 

[C.B. - Boudio]

/

It will be all point-to-point wiring. It is in my case the easiest and cheapest way. I would like to do it the old Revox way:

... although I am not sure how close I will get to something like that.

It will all be two-valve modules, probably on parallel printer connectors (my local electronics shop should still have a small truckload of them, never touched in ten years or so...), with some aluminium panels and angles, all just screwed in place, and the whole should be quite similar construction-wise to the 'poor-man's tube mixer' (except in my case, it'll probably be the 'Boudio homeless man's audio scrambler M1' or so )

And indeed, Ian's comments are gold to me.

 

[MidnightArrakis]

It will be all point-to-point wiring. It is in my case the easiest and cheapest way. I would like to do it the old Revox way:
View attachment 124666
... although I am not sure how close I will get to something like that.

It will all be two-valve modules, probably on parallel printer connectors (my local electronics shop should still have a small truckload of them, never touched in ten years or so...), with some aluminium panels and angles, all just screwed in place, and the whole should be quite similar construction-wise to the 'poor-man's tube mixer' (except in my case, it'll probably be the 'Boudio homeless man's audio scrambler M1' or so )

And indeed, Ian's comments are gold to me.

WHE-E-E-E-E-E-E-W-W-W-W-W DOGGY!!!

/

 

[ruffrecords]

Ok, then I'll look for those transformers.

So, transformer primary 22k, 680 cathode, and what about the anode resistor?

Sorry I did not make myself clear. The anode resistor is 22K. Keep the existing 15K:600 transformer or use a 10K:600 one (which is the one i suggested)

But I will probably be able to use the cathode signal in a (relatively) high-impedance transformerless amplifier, or not? (doesn't matter all that much, but may be convenient at some point)
And as for the master, I will do the same, but then I will have to adjust the 680 ohm to the DC resistance of the meters, so to get a resistance of 680 ohms between cathode and ground in total, isn't it?

Yes, you can use it but since the cathode voltage will only be about 3.5V, the output signal swing will be limited but the signal level will be lower anyway so maybe it will be fine. Thing is you may now find you have not enough gain. The input transformer and EF86 have probably 56dB of gain. You lose 4dB connecting to the cathode follower and maybe 10 to 20dB in the EQ (the loss will be at least equal to the maximum boost available. So maybe you have 36dB gain overall at the second vol pot. Then you will have about 26dB in the output stage less the 12dB loss in the output transformer which gets you back to 50dB overall. Probably about right for most situations.

Cheers

Ian

 

[C.B. - Boudio]

Thank you! I think the schematic should now be more or less complete. One of these days, I will scan the now-updated version in.
Good that I now have got an idea of the gain structure, not something I have otherwise learnt much about yet.

I have a couple of projects running at the moment, but this summer, I may get there!

 

[C.B. - Boudio]

I'm thinking... Microphone (input) transformers are quite small and thus don't require all that much power to drive.
Could 'your average cathode follower' be sufficient to drive such a transformer backwards?

Alongside this mixer, I'd like to make some extra preamps (by example to amplify the output of the plate reverb, headphone capsule, that I am also building, I'll post a thread on that as well), a parametric equalizer (more or less with said preamp, for makeup gain),... based on the design of the preamps for the mixer.
Those will have an EF80 with switchable gain (the anode resistor will be switchable, I'll post the schematic if I see time to scan) and an EC92, which is, quite litteraly, half an ECC81/12AT7, as a cathode follower. A Very Poor Man's V76, let's say.
To balance things if nescessary, the backwards mic transformer would by far be the simplest way.

 

[ruffrecords]

I'm thinking... Microphone (input) transformers are quite small and thus don't require all that much power to drive.
Could 'your average cathode follower' be sufficient to drive such a transformer backwards?

It is not really a question of power, more a question of signal level. Transformers saturate when the signal voltage reaches a certain point. Microphone input transformers are small because the voltage fed to them by the microphone is tiny (millivolts) so there is not much chance of saturating even a tiny core.. In an output stage the voltage fed to the transformer is much higher (about 1 volt or 1000 times bigger than a microphone signal) so they will saturate very easily when used as an output transformer.

Cheers

Ian

 

[C.B. - Boudio]

It is not really a question of power, more a question of signal level. Transformers saturate when the signal voltage reaches a certain point. Microphone input transformers are small because the voltage fed to them by the microphone is tiny (millivolts) so there is not much chance of saturating even a tiny core.. In an output stage the voltage fed to the transformer is much higher (about 1 volt or 1000 times bigger than a microphone signal) so they will saturate very easily when used as an output transformer.

Cheers

Ian

Of course... I must have overlooked that aspect. Thank you!

 

[Matt Syson]

Having ANY DC current through transformer windings changes the game as well. Studer* recommend 'demagnetising' input transformers periodically to maintain the quoted distortion characteristics.
* And others, for which I vaguely remember a discussion I had with Mr Lundahl many years ago about the use of 'line level' transformers breing used as a mic input for galvanic isolation purposes (not specifically the sound).