this thing is a pos. I picked one up because its cheap and now I know why.
after looking at the schematics they did the line in jack wrong. it does have a lot of tube noise. will try tube rolling some tubes. when I pulled the tube out I noticed that there is a row of resistors under the tube. these are cheap ones that will dift in resistance when the tube heats up! My diy stuff is better engineered than this crap, but what do I expect for $125. I am going to list the engineering flaws as I see them.
1. CAUTION!!! THE TRS INPUT JACKS ARE PHANTOM POWERED!!! YOU HAVE BEEN WARN...
2. the resistor layout under the tube will cause bias drift in the circuit by 30%
3. the power for the heater of the tube is tied off the -15V rail ! not only this is bad practice, as we inject heater noise back in 1/2 of the op amp supply, typically, the negative voltage regulator is the weakest regulator in the series. why burden the poor thing?
4. there are unnessary diodes in the signal line and the phantom power protection diodes are not matching!!! so, there adding leakage currents on the diodes and if the inputs get to 15V p-p inside, there is too much gain.
5. can not bypass the tube stage.
6. half of the "tube character control" lands into a gyrator op amp eq.
after swapping tube it improved it a little, but the noise floor is too high.
so the next thing is to pull the caps that are in R26 and R124 (yes they are screen printed wrong) c22&c69 and remove the tube and bypass the BS tube circuit.
while taking a coffie break to let the soldering iron heat up, I observed the poor solder flow.
going to reflow board and redue tuberoll test. crystallized joints can increase noise floor.
after dealing with normal stuff I got back to this thing.
so far today, I changed out the tube that sound like handling noise to a jj ecc83s
I tried a different "r26" coupling cap on one channel just to a/b it tomarrow
I touched up the diodes because I need to get stuff to clean the nasty board. I removed that piece of weatherstripping on the back of the tube base board. Just because I don't trust the adhesive's insulation properties....ha!
The regulators were changed on this board. so some one has repaired it and I baught it supossebly "new" at musicians freind and opened this thing up 14Hr after I ran it for 10 min,...... dude, behringer takes longer before I take a screw driver to it.. But what is the shocking thing I saw was the unwashed board under it. I also like the diode tacked badly under the board
I will try it out and see. hope it doesn't die from the flux shorting things out..... ...oh that is what already happened....I guess
not bad... after changing the tube the noise floor went to -70db. when the piece of weather stripping was removed the heater hum went away and the noise floor dropped to -90
the use of a 4.7 uf cap in replace of the 1uf changed some sublties with the tone, and when I strapped the inputs (tied the mic inputs together in the back, mic plugged in the front) and panned 100% apart, the stereo signal has a nice presence.
after looking at the schematics they did the line in jack wrong. it does have a lot of tube noise. will try tube rolling some tubes. when I pulled the tube out I noticed that there is a row of resistors under the tube. these are cheap ones that will dift in resistance when the tube heats up! My diy stuff is better engineered than this crap, but what do I expect for $125. I am going to list the engineering flaws as I see them.
1. CAUTION!!! THE TRS INPUT JACKS ARE PHANTOM POWERED!!! YOU HAVE BEEN WARN...
2. the resistor layout under the tube will cause bias drift in the circuit by 30%
3. the power for the heater of the tube is tied off the -15V rail ! not only this is bad practice, as we inject heater noise back in 1/2 of the op amp supply, typically, the negative voltage regulator is the weakest regulator in the series. why burden the poor thing?
4. there are unnessary diodes in the signal line and the phantom power protection diodes are not matching!!! so, there adding leakage currents on the diodes and if the inputs get to 15V p-p inside, there is too much gain.
5. can not bypass the tube stage.
6. half of the "tube character control" lands into a gyrator op amp eq.
after swapping tube it improved it a little, but the noise floor is too high.
so the next thing is to pull the caps that are in R26 and R124 (yes they are screen printed wrong) c22&c69 and remove the tube and bypass the BS tube circuit.
while taking a coffie break to let the soldering iron heat up, I observed the poor solder flow.
going to reflow board and redue tuberoll test. crystallized joints can increase noise floor.
after dealing with normal stuff I got back to this thing.
so far today, I changed out the tube that sound like handling noise to a jj ecc83s
I tried a different "r26" coupling cap on one channel just to a/b it tomarrow
I touched up the diodes because I need to get stuff to clean the nasty board. I removed that piece of weatherstripping on the back of the tube base board. Just because I don't trust the adhesive's insulation properties....ha!
The regulators were changed on this board. so some one has repaired it and I baught it supossebly "new" at musicians freind and opened this thing up 14Hr after I ran it for 10 min,...... dude, behringer takes longer before I take a screw driver to it.. But what is the shocking thing I saw was the unwashed board under it. I also like the diode tacked badly under the board
I will try it out and see. hope it doesn't die from the flux shorting things out..... ...oh that is what already happened....I guess
not bad... after changing the tube the noise floor went to -70db. when the piece of weather stripping was removed the heater hum went away and the noise floor dropped to -90
the use of a 4.7 uf cap in replace of the 1uf changed some sublties with the tone, and when I strapped the inputs (tied the mic inputs together in the back, mic plugged in the front) and panned 100% apart, the stereo signal has a nice presence.
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