Potato Cakes

Bogen MXM Tube preamp/mixer questions
« on: January 12, 2021, 09:23:25 PM »
Hello, everyone,

I am helping a friend with a Bogen MXM preamp/mixer that he is wanting to use for tracking. When he got it someone else had did some mods on it like replacing the filter switch with a pad and adding unbalanced direct outs on all of the channels which interrupts signal to the master. When tracking drums he said the inputs sounded overdriven which is fun on certain applications but not all. Since it's just going to be using as a mic preamp, I want reinstall the HPF and see about getting more headroom or padding the input to better accommodate recording drums. I've attached the schematic of the MXM-A which has a couple more features like the Hi-Z selectors, which this unit does not have. But the rest of it should be the same.

I was looking over the schematic and I noticed that the plate voltage to the 12AX7s on mic inputs 1-4 is 40V and I thought this was very low and that maybe this might be causing them to sound overloaded so easily if the plate is being starved of voltage. Would there a problem if I jumped R2, 8, 14, and 20 to get 185V for the plate like is shows for V3B?

I also noticed that there is no summing resistor network. Shouldn't there be one?

There is one problem that needs to be addressed before I get too far in the project. The master output has 4VDC between pin 3 and ground. There is also a very present 60 hum and the audio is severely high passed. The audio direct from each channel is correct, minus the headroom. The fun part is when I power the unit off it blasted my converter inputs with signal and I measure 53VDC and was dropping with the capacitors. I do not see how that is possible per the schematic. Granted I haven't begun to trace to see if the person whoever modded this unit missed something. It sounds almost like a bad transformer but I still don't know how DC is getting through. I may need to add an electrolytic inline to block DC but that won't fix the high pass issue. Any see something like this before?


« Last Edit: January 13, 2021, 12:17:13 AM by Potato Cakes »


Re: Bogen MXM Tube preamp/mixer questions
« Reply #1 on: January 13, 2021, 03:24:28 PM »
Thats a pretty Michael Mouse design, you either need to mod it significantly or not bother. The channels do mix by a passive method with the volume controls wired backwards.

Potato Cakes

Re: Bogen MXM Tube preamp/mixer questions
« Reply #2 on: January 13, 2021, 05:16:44 PM »
I'm definitely not looking to overhaul this thing, just get the gain more usable to work with drums, which really would can be achieved using a pad at the inputs and call it good. It would be great to get the master section working but it's not critical.

The one thing I found is that the direct out mod picks off from the CW connection on the gain pot, making it unbalanced at the output. It seems to be fine as far as output is concerned but I don't know if it's the half of the 12AX7 is being loaded too much. From doing my own research it seem the dual tubes use one gain stage to feed the other for better headroom (I think) so just using half of a tube seems to be not as optimal. Again, I'm not rebuilding this guy. I'm just trying to make some fairly simple improvements.

I did think about the the volume pots creating the summing network but I didn't know if that would be a problem since when you change the volume.




Re: Bogen MXM Tube preamp/mixer questions
« Reply #3 on: January 13, 2021, 07:23:38 PM »
There is nothing you can really do to improve the output from a single 12AX7. its output impedance will be in excess of 50K no matter what you do and the direct out will be unbalanced because the whole of the mixer is internally unbalanced (which is true for just about every mixer. If you could wangle it a supply, about the best you could do would be to use an ac coupled TL072 to buffer each direct out and create a balanced output.

Do you have the mic input transformers fitted? Any pad you add should go before the transformer. You can buy 20dB pads built into an XLR which you just plug into the mixer input.



'The only people not making mistakes are the people doing nothing'

Re: Bogen MXM Tube preamp/mixer questions
« Reply #4 on: January 13, 2021, 07:41:31 PM »
If you want to take outputs direct from the 12ax7 input stages they'll want to see hi-z (1megohm) active DI boxes might be your best bet . There seems to be a variety of octal transformers from Bogen available quite cheaply, might be worth having a few to try out . 100k ohm anode load resistors might work out better for relatively hot signals from drums close mic'd with dynamics . Id also go with resistor/cap bias
PSU is a bit skanky for a preamp too , halfwave ht rectification and series supplied heaters  :-\
No easy way to turn a sows ear into a silk purse unless the owner wants to throw money at it .


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