I found this Pentron MM4 microphone mixer and would like to try using it for recording. It has 4 channels each going through one section of a 12ax7. I thought I would try the circuit as is and then simply wire 2 channels with cascading gain each through 2 sections of 12ax7 with 2 outputs instead of 1.
I dont have much experience with this but understand some basics. I have not tested it yet. I will definitely replace the power cord with a 3 prong and most likely the filter and cathode bias caps. should the selenium rectifier be replaced? How do you do that without changing the B+ voltage?
Any ideas or suggestions are welcome thanks for your help
I dont have much experience with this but understand some basics. I have not tested it yet. I will definitely replace the power cord with a 3 prong and most likely the filter and cathode bias caps. should the selenium rectifier be replaced? How do you do that without changing the B+ voltage?
Any ideas or suggestions are welcome thanks for your help
Selenium rectifiers are problem areas. I would at least see if it's okay but know that you'll eventually have to replace it. While it's still working, measure the B+. Replace it with a silicon diode (1N4007) and series resistor, which you adjust to restore the proper B+. In fact, set it for about 5% low because of the slow increase of power line voltage over the years.
This mixer uses high impedance mixing and should therefore only be fed into something with a high input impedance - 500K or more to be safe.
Replace the selenium rectifier. The increase in HT voltage will not materially affect the circuit operation in this case.
Cheers
Ian
Replace the selenium rectifier. The increase in HT voltage will not materially affect the circuit operation in this case.
Cheers
Ian
The diode gets matched up with striped side same side where + would be on the selenium rectifier?
I'll be plugging the output of the mixer into my Presonus firebox which has 2 xlr balanced (1300ohm) preamp ins, 2 instrument (1M) preamp ins and 2 balanced line ins (10k). So I'll use the instrument ins, correct?
If say I wanted to make a 2 channel preamp out of this, one side being XLR microphone and the other being a tube DI maybe I would want to put a transformer on the XLR side so I could plug a SM57 into it or even add phantom power...what would be a cheap but decent transformer to add? I don't want to go too crazy with this project but a 1 channel mic pre and tube DI for bass would be really cool and useful for home recording projects...
I'll be plugging the output of the mixer into my Presonus firebox which has 2 xlr balanced (1300ohm) preamp ins, 2 instrument (1M) preamp ins and 2 balanced line ins (10k). So I'll use the instrument ins, correct?
If say I wanted to make a 2 channel preamp out of this, one side being XLR microphone and the other being a tube DI maybe I would want to put a transformer on the XLR side so I could plug a SM57 into it or even add phantom power...what would be a cheap but decent transformer to add? I don't want to go too crazy with this project but a 1 channel mic pre and tube DI for bass would be really cool and useful for home recording projects...
100K pots loaded on 300K resistors, it is fine for them. If you load 330K on zero ohm you can get theoretically best summing: less dependence between channels, trade off: less gain.
Transformers, well cheap and decent don't generally coexist.
It's common to use a high input impedance regardless of source. However, low source impedance means small voltages and consequent low output with more noise. A cheaper way is to make an amplifier rather than use a transformer.
And yes that's the correct direction for the diode.
I think the output of your new preamplifier will be too much for the instrument input of the power amplifier. Best to use one of the balanced line inputs; you can safely ground one side of the balanced input.
It's common to use a high input impedance regardless of source. However, low source impedance means small voltages and consequent low output with more noise. A cheaper way is to make an amplifier rather than use a transformer.
And yes that's the correct direction for the diode.
I think the output of your new preamplifier will be too much for the instrument input of the power amplifier. Best to use one of the balanced line inputs; you can safely ground one side of the balanced input.
I'd agree with replacing the caps and the rectifier, but would probably use a better diode than a 1N4007. A BYV26E fast recovery diode will set you back a whole nineteen cents at Newark.
Don't electrocute yourself.
Jim
Don't electrocute yourself.
Jim
Should work in practice too, just by adding an anodefollower with another 12AX7 at the output. Try Rfb 200-300kohm. Cga could maybe be a problem though.
If you make this for stereo I would split the cathode connections to one 4.8k resistor for each section bypassed with a 25 mfd cap. That is 1200 ohms times 4 , for 4 tubes to maintain the same bias and reduce crosstalk between channels.
Replace the selenium rectifier too.
Hope this helps
Ed
Replace the selenium rectifier too.
Hope this helps
Ed
Thanks that seems like a good idea. Why is that a good idea exactly? does it just help to keep each section properly biased?
If I cascade the gain stages can I just take from right after the 330k resistor following gain control right into the grid of the second stage?
If I cascade the gain stages can I just take from right after the 330k resistor following gain control right into the grid of the second stage?
You should a use full wave or bridge rectifier if you transformer isn't center tapped. While having little difference on the performance of the circuits after the power supply other than an increase in voltage which can be offset with some series resistance it does offer many great advantages.
Firstly, it will half the amplitude of the power supply ripple because the supply will be switching twice as fast. This is better for the capacitors and the rest of pre amp because it will be receiving a more smooth supply.
Secondly it encourages better transformer performance because it will be more balanced preventing DC magnetization of the core. It will also run cooler at the power output because of this.
Firstly, it will half the amplitude of the power supply ripple because the supply will be switching twice as fast. This is better for the capacitors and the rest of pre amp because it will be receiving a more smooth supply.
Secondly it encourages better transformer performance because it will be more balanced preventing DC magnetization of the core. It will also run cooler at the power output because of this.
High impedance summing works surprisingly well with less interaction than you might expect even with as few as 4 sources. The source impedance feeding the pot is about 40K (80K ra // 82K plate resistor). Worst case source impedance from pot is when pot wiper is at 70K from bottom (with 30K to top plus tube source of 40K = 70K up) and is 70K//70K or 35K. This in series with a 330K gives 365K bus source. Four of these in parallel give a 91K bus impedance. Since there are four sources the bus loss is 12dB. Changing one pot from worst case source of 35K to fully off (zero ohms) means the bus impedance now becomes 365/3 // 330 = 121.7 // 330 = 88K5. Bus loss is now 365/88.5 = 12.3dB. So worst case interaction of a single pot changes the output by a mere 0.3dB.
Zero ohm loading does reduce interaction but in practice you don't need to get anywhare near zero to get interaction below 0.1dB.
Cheers
Ian
It still may be too much when you work on live sound. Think of 3 faders moved simultaneously, while the 4'th one is on microphone on the edge of feedback.
I just calculated that moving 3 out of four faders at once results in at most 0.7dB change in the fourth with the circuit as it stands. I don't do any PA work but I would have thought that there were many other acoustic factors that could easily change the signal reaching the mic by a dB - artists moving about, placing hands around mic, removing mic from stand etc.
Cheers
Ian
If you use it as it is you will need a fairly short output cable unless you are not worried about HF. The output impedance is about 45K, and this will not drive a lot of capacitance.
I do, and I know well what I am talking about.
Exactly. That's why theoretical demand to have very high input impedance of the amp is very theoretical. 😉
And I just had one of these walk in the door today! Even the pilot light still works. I will check the filter cap to see if it is close to spec. The pot shafts on inputs 2 and 3 are very stiff.
Looking at the way it is wired is interesting. The low impedance unbalanced input goes straight to the volume control! No clue why they expect a low impedance microphone to have higher output than a high impedance unit.
Do not change the rectifier to a silicon diode! The power transformer is almost a 1:1 ratio on the B+ with a silicon diode that would provide 170 volts to the first filter cap. I measured mine at 151 Volts not the 160 as on the schematic. So a new silicon diode would raise that to 170 volts which is probably more than the filter capacitor wants to see. Mine is rated 20/20/20 at 150 volts!
For those who haven't seen one carbon comp resistors and wax paper capacitors!
The popular use for this item is to mix drum kits. The resulting sound is very desirable to some. The high pass inherent to this mixer is actually around 25 hertz.
Looking at the way it is wired is interesting. The low impedance unbalanced input goes straight to the volume control! No clue why they expect a low impedance microphone to have higher output than a high impedance unit.
Do not change the rectifier to a silicon diode! The power transformer is almost a 1:1 ratio on the B+ with a silicon diode that would provide 170 volts to the first filter cap. I measured mine at 151 Volts not the 160 as on the schematic. So a new silicon diode would raise that to 170 volts which is probably more than the filter capacitor wants to see. Mine is rated 20/20/20 at 150 volts!
For those who haven't seen one carbon comp resistors and wax paper capacitors!
The popular use for this item is to mix drum kits. The resulting sound is very desirable to some. The high pass inherent to this mixer is actually around 25 hertz.
Do not put a three wire power cord on this unit. It was designed without one and all you will get is lots of hum.
I measured the gain on all four channels of mine and found no two the same. The gain runs from just below unity to about 1.6 x. If you want to use it with a low impedance microphone use a step up matching transformer. That will give you about 20db of gain with a decent unit. If you build it yourself try a Hammond 140K if you want cheaper try a Mouser 42TM117 with the low impedance side to the microphone and the high side to the preamp. For an enclosure check out the Hammond 1550Z111 cast aluminum box or similar.
The power supply ripple was so low with the three stages of filtering that I did not make any changes to the power supply. With a 6' output cord of plastic microphone coax the high frequency response began to roll off at 10,000hz going into my 10M scope probe.
This should be an ideal unit to soften sound sources for digital recordings.
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