Bogen MXM Direct out Mod (attached schms in reply posts)

Hey Yall

I have a Bogen Mixer model mxm with EF86 inputs and am currently trying to configure my direct out 12au7 stage.  I went with a typical low impedance output (see later posts) and am wondering if I made the right choice for this particular mixer.  Does anybody else recommend me using a different configuration?  Ill attach the original schematics for the mixer as well (see later posts, unfortunately I can only post one image at a time).  A little more info for you. Im using the stock input transformers (T155) and am using an Edcor output of 10k to 600ohms. 


Sorry this is such a broad question.  I just want some input!

Thanks!

Romp

here is the whole schematic of the MXM

Here is my output I currently have with the 10k to 600 ohm transformer.  I also have the Sta-level ouput utilized

> output of 10k to 600ohms. 

That RCA Hi-Fi line output is rated for 100000 (100K) load.

The 12AU7 output stage shown in the first post is a better bet, For some reason I get error 404 attachment not found for the complete mixer schematic.

Cheers

Ian

MXM-a  output stage attached.

PRR said:

> output of 10k to 600ohms. 

That RCA Hi-Fi line output is rated for 100000 (100K) load.

Click to expand...

Do you think I'm to mismatched impedance wise with this design? 

ruffrecords said:

The 12AU7 output stage shown in the first post is a better bet, For some reason I get error 404 attachment not found for the complete mixer schematic.

Cheers

Ian

Click to expand...

Thanks for the reply, Ian.  Would you by chance be able to explain why this would be better?  I think the circuit you're referring to may have been a part of the Bogen mixer in a later model when they switched to all 12ax7 preamp tubes.  SO this my be the best route for sure.

Romp

gridcurrent said:

MXM-a  output stage attached.

Click to expand...

Awesome,  Thank you!

I will give this a try!

Rompstomp89 said:

Thanks for the reply, Ian.  Would you by chance be able to explain why this would be better?  I think the circuit you're referring to may have been a part of the Bogen mixer in a later model when they switched to all 12ax7 preamp tubes.  SO this my be the best route for sure.

Romp

Click to expand...

The circuit posted earlier uses both halves of the 12AU7 in parallel. This allows you to double the total plate current and hence double the maximum output current which quadruples the power, The 12au7 tube produces quite a lot of distortion (compared with others) so the design uses half a 12AX7 in a negative feedback loop to reduce it. It also means the stage provides some gain.

For a direct out you don't really need any gain so you could do worse than duplicate the half a 6CG7 cathode follower used in the hand drawn schematic output stage.

Cheers

Ian

ruffrecords said:

The circuit posted earlier uses both halves of the 12AU7 in parallel. This allows you to double the total plate current and hence double the maximum output current which quadruples the power, The 12au7 tube produces quite a lot of distortion (compared with others) so the design uses half a 12AX7 in a negative feedback loop to reduce it. It also means the stage provides some gain.

For a direct out you don't really need any gain so you could do worse than duplicate the half a 6CG7 cathode follower used in the hand drawn schematic output stage.

Cheers

Ian

Click to expand...

Thank you Ian,

This is some great info for me! Im still kind of noodling around but after putting the newer 12au7 stage in the unit seems to work better.  Another question for you would be negative feedback.  Im familiar with guitar amp negative feedback and Im assuming it would be somewhat of the same approach in a mic pre but was wondering if you had any cool tricks I could try!

Thanks again!

Romp

Rompstomp89 said:

Thank you Ian,

This is some great info for me! Im still kind of noodling around but after putting the newer 12au7 stage in the unit seems to work better.  Another question for you would be negative feedback.  Im familiar with guitar amp negative feedback and Im assuming it would be somewhat of the same approach in a mic pre but was wondering if you had any cool tricks I could try!

Thanks again!

Romp

Click to expand...

Negative feedback works wonders if applied correctly but is disastrous if not. There are no 'tricks' as such.

Cheers

Ian

ruffrecords said:

Negative feedback works wonders if applied correctly but is disastrous if not. There are no 'tricks' as such.

Cheers

Ian

Click to expand...

Totally.  Ive tried a few options and may have some ideas.

I mocked up a drawing of what I have now and would love for you to take a look at it.  I ended up putting a 12ax7 in the AU slot to give it a little more gain and changed out the cathode from a 470 to 820 and changed the anode to a 100K instead of 180.  This all be a major no no but would love some more insight/critiques!

Thanks again,

Romp

even though it's a "no-no" as you say, I think it's a good experiment to learn about what's important in an output stage. (Seems like that's at least partly what you're after here.) There are people here who can explain this better than I can, but here goes.

First question is, what kind of load are you plugging this unit into and how much signal level do you need to push into it? Though some output stages do provide some extra gain to the circuit, your primary concern with this stage is pushing your signal at the required level into  the next device in the signal chain. Voltage Gain alone won't do that very well. Without adequate current, your output impedance goes up (Ohm's law) and you don't have enough power to effectively drive the signal across the load, so much of the signal will be lost to the load impedance.

The unloaded gain of your output stage will go up by putting in a high mu tube like a 12AX7, but the 12AX7 has a small max plate current, leaving you without enough power to effectively drive the signal across the load. That's why medium to lower mu triodes with higher transconductance like the 12AU7, 12BH7, 6CG7,  12B4 etc, or pentodes are usually used in output stages. Higher current also lowers your output impedance, making it easier to drive the load. High mu, lower transconductance triodes like the 12AX7, 5751, etc are best at providing voltage gain and are generally used in prior stages.

Another consequence of higher output impedance is a loss of low end. As output impedance goes up, the primary inductance of the output transformer must be higher or the -3 db point on the low end moves up. You may not know the primary inductance of a transformer at low frequencies of interest, So as a rule of thumb,  use an output transformer with a higher nominal primary impedance than the output impedance. You've got a nominal 10k primary on the Edcor, but the output impedance with the 12AX7 is much higher - I'm guessing around 38,000 ohms.  10k is much better suited to use with a 12AU7 in that configuration.

I'm sure sharper minds on this will chime in and correct/clarify as needed.

Have fun.

BT

BT pretty much summed it up. These are all class A circuits so you need sufficient quiescent current to provided the required current swing in the load. The 12AX7 is a low current tube intended for preamplification. Typical quiescent current is 1mA or less. For otuput duty you need a tube capable of a much higher quiescent current like a 12AU7 which can sit happily at 5 to 10mA.

Just remember, +20dBu into a 600 ohm load is 100mW of power and class A stages are typically 25% efficient.

Cheers

Ian

rackmonkey said:

even though it's a "no-no" as you say, I think it's a good experiment to learn about what's important in an output stage. (Seems like that's at least partly what you're after here.) There are people here who can explain this better than I can, but here goes.

First question is, what kind of load are you plugging this unit into and how much signal level do you need to push into it? Though some output stages do provide some extra gain to the circuit, your primary concern with this stage is pushing your signal at the required level into  the next device in the signal chain. Voltage Gain alone won't do that very well. Without adequate current, your output impedance goes up (Ohm's law) and you don't have enough power to effectively drive the signal across the load, so much of the signal will be lost to the load impedance.

Click to expand...

Thank you so much.  This is a huge learning experience for me.  I understand some basics with circuity from building guitar amps for a company here in Nashville but as far as the whole impedance side, it throws me for a loop. The only real thing I know about the 12a*7 tubes is the gain factor hence the reason i put an ax7 in there because I was hoping for a little more gain  :.  That being said, I am really just wanting this unit to be a decent mic pre to add some color and maybe put it on a mix bus.  Id be running it into my UA Apollo's line inputs. 

Line inputs on your Apollo are probably 10k in, which is a typical bridging load on modern recording interfaces. The fact that you're plugging into this and not trying to drive an old 600 ohm limiter input makes things easier, but the 12AX7 is still not going to do it for you for the reasons I stated above. You'll lose any extra gain you get from the unloaded 12AX7 and more, along with a lot of low end.

Use the 12AU7 and you should have plenty of power to drive a 10k load.

BT

rackmonkey said:

Line inputs on your Apollo are probably 10k in, which is a typical bridging load on modern recording interfaces. The fact that you're plugging into this and not trying to drive an old 600 ohm limiter input makes things easier, but the 12AX7 is still not going to do it for you for the reasons I stated above. You'll lose any extra gain you get from the unloaded 12AX7 and more, along with a lot of low end.

Use the 12AU7 and you should have plenty of power to drive a 10k load.

BT

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Cool.  I tried the 12au7 back in there this morning and with the channel dimmed im really only getting a max gain of -20db in protools.  Is there a way i can boost the gain in the 12au7 stage?  I suppose i could drop the 820r cathode resistor back to 470R?  Any other ideas?

Thanks,
Romp

What's your source? Dynamic mic, ribbon, signal gen, etc?

BT

rackmonkey said:

What's your source? Dynamic mic, ribbon, signal gen, etc?

BT

Click to expand...

Sm57 on an amp in an Iso.  I should report back with a signal generator test however. that seems like the easiest route.

Ill report back!

Romp