Bogen MXM Tube preamp/mixer questions

[Potato Cakes]

Thanks to Tubetec, went looking some more for an MXM and not an MXM-A schematic, which are more readily available and I thought that they should for the most part be the same. They are similar but quite different as I found out when I found just the MXM schematic which is attached. It clears up quite a bit of what I was seeing when looking over it, but it still has the same issues. However there are resistors for cathodes on the tubes which I can now easily add the bypass caps. There is also a summing network and the HPF filter makes more sense to me. Nevertheless, everyone's input regarding this matter is still applicable and very helpful and I will proceed accordingly.

Thanks!

Paul

 

[abbey road d enfer]

Thanks to Tubetec, went looking some more for an MXM and not an MXM-A schematic,

Aah! But it's a completely different animal, compared to the previous one, that looked like itwas cobbled together by someone who was willing to show he was capable of originality, at the detriment of performance and durability.
Here the tubes are correctly biased, mixing is done with resistors and the pots are connected the usual way, they got rid of the weird LF filter, and the HPF's are 2nd order-ish. The PSU is also more robust, with proper windings for HT, heaters and  pilot lights.
What puzzles me is that I would think the A version was later than the no-grade version.
Was it that the A version was a revision made for penny-pinching?

 

[Tubetec]

Agreed,
definately a much better starting point than the MXM-A for a project. Theres even a quite nice Baxandall eq in there . EF86 is an excellent tube for mic input stage, for some extra info including reducing gain by triode connecting it  see the article below , theres also a nice explantion of the use of feedback .

http://douglas-self.com/ampins/wwarchive/wwarchive.htm#baxpre55

 

[emrr]

Second use of triode connection here, pentode gain in many old things is unnecessary today. 

 

[Potato Cakes]

Was it that the A version was a revision made for penny-pinching?

No idea. I thought the A version was the same as the regular one with the selectable Hi-Z inputs. I did not have any reason to believe that they would be so different.

Ordering parts later today to do the cathode bypass.

Thanks!

Paul

 

[abbey road d enfer]

Ordering parts later today to do the cathode bypass.

Why? You'll have more gain and less headroom.

 

[Potato Cakes]

Why? You'll have more gain and less headroom.

I thought that it was desired to have the cathode bypass caps in there. I may have crossed my reading comprehension wires.

Thanks!

Paul

 

[emrr]

cathode resistor yes, bypass cap for you to decide.

 

[Tubetec]

The EF86 stages in the MXM already have resistors and bypass caps at the cathodes.
As was previously mentioned depending on the mics/source/levels you may well end up with to much gain, removing the bypass cap is an easy way to reduce gain a bit .

 

[Potato Cakes]

I've received the active DI boards and have run some tests and it passes signal just fine when putting a mic on a drum. I didn't change anything with the bypass caps or the cathode resistors.

The thing I have yet to wire back up is the HPF. I'd like it to be second order at 70Hz but I'm not sure if the 500k pot has to be taken into consideration when calculating the capacitor needed as it is parallel to ground to the 470k resistor that is used for the filter per the schematic. If so, wouldn't this mean the filter frequency would shift with the gain knob?

Thanks!

Paul

 

[Potato Cakes]

I've taken another look at the mixer output section to see where the voltage coming from and apparently it's passing through the cap right before the output transformer. It also does this for any other cap I try, be it electrolytic or film. removed connections to isolate just the output tube where it connects to output cap and still it passes voltage through all the way to the XLR connection. When I remove the connection to the transformer the voltage disappears but it is still on the opposite end of the cap, even with the polarity flipped. How is this even possible? Film caps let less voltage through but when I power the unit off there is a spike in negative voltage (referenced to ground) and it drains in sync with the power caps. The other caps that supposed to be blocking DC are doing their job. This is the only place that isn't and I'm quite confused. I could try adding more capacitors after the transformer but then how is DC getting through the transformer? I haven't tested to see if the massive HPF cut is due the excess voltage getting through or if the transformer is bad. First I have to find out why caps are doing what caps naturally do.

Thanks!

Paul

 

[abbey road d enfer]

I've taken another look at the mixer output section to see where the voltage coming from and apparently it's passing through the cap right before the output transformer. It also does this for any other cap I try,
[/quote] This is utterly normal. You should see the voltage decreasing as the capacitor charges (turn-on) or discharges (turn-off).
I don't know how you measure the voltage, but if you use a digital multimeter, with a 10Megohm impedance, the time constant with a 5uF cap is 50 seconds, that means after 50 seconds, the voltage at the output is still 37%.
Typically, the output should be loaded with an impedance of about 100k-500k, resulting in a time-constant of 0.5-2.5 s, so the voltage there would become undetectable after about one minute.

 

[Bill Wilson]

I am not trying to be unkind when I say this; the Bogen stuff was not designed to have audio quality for even semi pro recording. It was designed for PA & background music systems. Altec equipment was designed to a higher standard, high quality sound reinforcement; with the exception of their microphones & 250 SU & 350 mixing consoles. I was the sole technician for an Altec Engineering Contractor, also sold & serviced Bogen for PA & bar sound systems, 1958-1965. Would not recommend this piece.

 

[Potato Cakes]

I am not trying to be unkind when I say this; the Bogen stuff was not designed to have audio quality for even semi pro recording. It was designed for PA & background music systems. Altec equipment was designed to a higher standard, high quality sound reinforcement; with the exception of their microphones & 250 SU & 350 mixing consoles. I was the sole technician for an Altec Engineering Contractor, also sold & serviced Bogen for PA & bar sound systems, 1958-1965. Would not recommend this piece.

Bill, thanks for your insight. I know this is just a pretty okay sounding design from running my own audio tests and I wouldn't recommend it either. I'm just helping a friend get a couple more miles out of something he already bought, so the damage is already done. I will pass along to him that one of their tech guys said this was a bad idea, especially what he told me he paid for the unit.

The two concerns I have with DC on the mixer output is when it is powered off, there is a massive blast of DC on pin 3 of the XLR, like -60VDC, which could damage a converter downstream or blow up a speaker, so before I hand this off I'd like to either sort it out or disconnect it. There is no voltage on pin 2. The audio on the mixer output (not the preamps) also is severely high passed. I may add caps at the XLR and see if it helps and if not just say no mixer output. I'm not going to spend too much more time on this but I did enjoy learning more.

Still waiting for a delayed shipment from Mouser to finish up the DI cards and button this thing up.

Thanks!

Paul

 

[abbey road d enfer]

The two concerns I have with DC on the mixer output is when it is powered off, there is a massive blast of DC on pin 3 of the XLR, like -60VDC, which could damage a converter downstream or blow up a speaker,

I already explained why, but you don't seem to read. If you hasd a standard load, like 100kohms at teh output, you wouldn't have this problem. This is my conclusion base on teh fact that you said it happens whatever teh capacitor there.

 

[Potato Cakes]

I already explained why, but you don't seem to read. If you hasd a standard load, like 100kohms at teh output, you wouldn't have this problem. This is my conclusion base on teh fact that you said it happens whatever teh capacitor there.

Yes, sir, I did read it but not quite understand. I am measuring DC from the opposite side of the voltage entering the capacitor with reference to ground. The active DI cards I am using to do the balancing on the preamps are putting voltage on the XLR output since +48V is being used to power the cards via the output connection per normal active DI design. I put two electrolytic caps in series on both + and - connections before the output XLR and there is no DC on the XLR without connecting it to another device for loading. I am confused because DC is blocked in this instance and not the other. The most typical way the mixer section would be used to be connected to a modern audio interface, which I believe is between 10k-15k ohm impedance, which would not be enough to the recommended 100k load on the mixer output unless I am missing something. I do know that when testing this with my interface that there was about 4VDC on pin 3 of the XLR output, generating a buzz and when powered off it caused the massive voltage spike that decreased in sync with the caps draining, enough so I was worried that it damaged that input. That and the audio being high passed is ultimately what I am looking to remedy if it's fairly simple fix.

I'll do more tests with the recommended 100k load and see what happens.

Thanks!

Paul

 

[Potato Cakes]

I've wrapped this guy up finally. I have 30dB pads permanently installed on the inputs and redid the HPF to be a first order at about 75Hz, which is fine for doing drums. Here's the links the test tracks:

https://drive.google.com/file/d/1SVpgH_lueEXuFm8s9_V0JE7dAz2jzJDW/view?usp=sharing

https://drive.google.com/file/d/1LekQ8bRpKSQCBuvc6vokE4zGrSBQFuZg/view?usp=sharing

https://drive.google.com/file/d/1o1bzqws8HzZIyoJEEXYKEHsKaYwHnVb_/view?usp=sharing

I recorded everything with a Heil PR20. I do not play drums but I happen to have some in my basement. I was focused on distortion and noise floor and getting those things balanced out so that drum tracks would be clean enough rather than tone. I used Kevin's AD72DI PCBs (sold in the White Market) to do the impedance balancing. The input signal had to be shunted to ground as it would be overloaded and create unpleasant clipping on the transients. But then I also had to adjust the feedback resistors on the TL072's to give me a little more gas for the output.

I tested the mixer section again and I'm not getting the signal being exploded when powering off the unit when connected to a different interface used that earlier in the testing, so I don't know if it's being loaded different as was brought up by abbey road d enfer. I do not doubt the information communicated regarding this topic, I just don't fully understand it as of right now. It also does not seem have the high pass issue as before. I do not know what I did to fix that. It still has a small buzz on mixer output but for this project I am not worried about it as the preamps were the objective and those are very usable especially considering the how the added direct out circuit is being implemented.

Thanks for everyone's input here. I was able to learn a couple more things and successfully mod an old piece of gear, so this project goes in the win column.

Thanks!

Paul

 

[mjrippe]

I don't know who Kevin is and a search for "AD72DI" only brought up this article, so I may be off base here...But in general TL072 is not the best choice for driving an output.  They simply can't handle much of a load, unlike a 5532.  Or for about the same overall cost you could get THAT 1646 which balance and drive.

If what you have works, then leave it be, but if you have issues down the line it is something to consider.

 

[Potato Cakes]

I don't know who Kevin is and a search for "AD72DI" only brought up this article, so I may be off base here...But in general TL072 is not the best choice for driving an output.  They simply can't handle much of a load, unlike a 5532.  Or for about the same overall cost you could get THAT 1646 which balance and drive.

If what you have works, then leave it be, but if you have issues down the line it is something to consider.

The AD72DI is listed in the Link Audio Design in the White Market. The guy's name is Kevin. He's a good dude.

I went with the TL072 as that is what the DI was designed to do, even though a 5532 probably would have been better. However I'm satisfied with the results and you can hear from the audio samples that it will be just fine for recording drums. As discussed in this thread, this is not a great audio unit to begin with and it would have been better to build some proper mic preamps. This was just me helping out a friend who already had the unit which had been modified (not well, mind you) by someone else and I was just trying to make it more usable than what it was, which it is. For me, I learned some things, which is half of the reason why I am here.

Thanks!

Paul

 

[Tubetec]

The drum samples sounded good to me anyway, was a job worth doing afterall.