Tube mixer

I'm projecting a 10 Ch mixer tube, with an IC output section for 4 monitors (...where a high quality of the signalit is not necessary).

1) I'd like to know if the outputs IC sections (for the monitors) may affect the signal after the tone control section.

2) Somebody can help to answer to the questions reported into the IC section?

Many anticipated thanks
 

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SY

diyAudio Moderator Emeritus
2002-10-24 10:19 pm
Chicagoland
www.SYclotron.com
The White cathode follower should be able to deal with the tone controls. My question is, why so much gain? The way you have it set up, gain is about 250. That seems... excessive. The input cap for the tone controls is not necessary if the preamps have output capacitors. The internal coupling caps can be 2-10uF. R1 can be large- 100k will work fine (not critical). R5 can be 10k (not critical). R4 is superfluous.
 
Can you enlighten me please, what for are Baxandall shelf-type EQs in each channel strip then?

Well a Baxandall HF/LF EQ is clearly quite simple and therefore limited but it does have its uses.

Often when mixing, it is important to manage the audio spectrum. In particular, to avoid muddying the bass (guitar or synth etc) it is common practice to roll off the bass end of all other instruments (guitars, snare drums etc). In other cases bass roll off is used to achieve a particular sound e.g to make a thin sounding rythm guitar.

Equally, to add sparkle, it is common to add some HF boost to individual instruments.

Cheers

Ian
 
All instrumental amps have own tone controls, so even when miking cabs I don't see any reason to have Baxandall controls in channel strips. I've found that the better are mikes and mike preamps, the less audio engineers want any EQ in them. Do you read Gearslutz forum, Ian?
Insert sockets would be much more valuable, according to my experience.
 
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So then it would be desirable that the musician alter his amp's EQ away from his desired tone to suit your mix so your mixer needn't have adjustment? I can imagine the conversation...

"No, Jimi, you need to roll off here and boost there... What? No, I know you like the tone you are getting, but my mixer has no EQ, so you have to skew your tone for me. I know you'll understand."
 
I think the first thing you need to look at is the tone control stage because I think with the values shown it will present too heavy a load to the tube stage driving it. This will cause much more distortion than any loading effects the later IC stage might cause.
Ian,
the exact value of the load is about 3.5K (with both treble and bass control at the middle position). The output impedance of the previous cathodic stage (6DJ8) is 350 ohm, with 10 mA of current. Do you think that such a load is too little for that stage? Have you considered also that the subsequent stage (in parallel with the control tone load) is a 6DJ8 tube stage (with 50K input impedance)? How can I calculate the potential risk (distortion) you suppose?


My question is, why so much gain? The way you have it set up, gain is about 250. That seems... excessive.

Sy,
the gain is not so much. Both first (6DJ8/6DJ8) and second stage (6DJ8/6DJ8) have a gain of 14 (total = 196)... the last has then to be divided with 10 (loss in the tone control). So the total gain is less than 19.
 
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The internal coupling caps can be 2-10uF. R1 can be large- 100k will work fine (not critical). R5 can be 10k (not critical). R4 is superfluous.

I believe that 1M for R1 should be a better value (in order to not overload the previous 6DJ8 stage ... because I'll have 10 of these loads in parallel in the mixer-bus... having the mixer 10 channels).

Which ICs do you suggest? ... in particular for IC3-IC4, that have to drive a 50 mt cable?
 
Ian,
the exact value of the load is about 3.5K (with both treble and bass control at the middle position). The output impedance of the previous cathodic stage (6DJ8) is 350 ohm, with 10 mA of current. Do you think that such a load is too little for that stage? Have you considered also that the subsequent stage (in parallel with the control tone load) is a 6DJ8 tube stage (with 50K input impedance)? How can I calculate the potential risk (distortion) you suppose?
.

The worst case is less than 2K. The 'small signal' output impedance of the CF may be 350 ohms but as soon as you get to realistic signal levels this ceases to be true. 10V rms into 2K is 7mA peak which is pushing it for a 10mA standing current and will certainly give you increased distortion.

Calculating distortion is difficult but in tubes it is almost always directly proportional to signal level. Best thing is to build the stage and try it driving 10V rms into 2K and measure the distortion.

Cheers

Ian
 
All instrumental amps have own tone controls, so even when miking cabs I don't see any reason to have Baxandall controls in channel strips. I've found that the better are mikes and mike preamps, the less audio engineers want any EQ in them. Do you read Gearslutz forum, Ian?
Insert sockets would be much more valuable, according to my experience.


Drum kits, trumpets, saxophones etc etc don't have tone controls. When micing cabs there is the proximity effect to take into account which is another reason to want to roll of the bass for example. Some mics include a low cut switch for this but most dynamics (which are commonly used to mic cabs) do not.

Quite often, very little EQ is used when tracking (except perhaps to roll of bass where it is not wanted). EQ is used most often during mix down, to create tones and to separate instruments in the audio spectrum.

Yes, I do read Gearsltz.

Yes, inserts are very useful too, not just for EQ but for dynamics processing. And once again bass roll of before dynamics can be rather useful in preventing compressor/limiter thumps

Cheers

Ian
 
Anatoliy, do you have an example of something you've recorded and mixed without using EQ on individual channels?

I'd like Pultec style EQs on mine please! One per channel. And balaned inserts, one per channel.

As for the original design posted, what's the intended purpose? Cascaded Aikido gain stages with single ended 25K ohm inputs? What kind of mics do you use and how are you planning to manage the level structure with different sources from tracking to mixdown?

Also the summing is another place where the level structure is important. What's the plan there?

I think a lot depends on how this mixer is going to be used. It doesn't look much like your typical recording studio desk.

Michael
 
The general schematic of the mixer is reported here:
Mixer - general scheatic | Agorà

I'd like to inform that the mixer is to be considered a pro-mixer (i.e. for instruments and microphones).

The summing stage will include a two further tube stages, one of which with a gain of 10. This will be enough, considering that the signal levels after the tone control shoul be of 254 mV and 140 mV RMS rispectively for microphones and instruments (considering an input value of 2 mV for mic and 100 mv for instr).

S2 is utilized to select an output C having less capacitance, in order to obtain a low cut (to elminate signals below 75 Hz).

Antonio