Hey Santucci thanks for your reply.

Ive opened it up a few times to look around, which is what surprises me because everything looks pristine. The multi colored ribbon type cable connecting the rear input panel to the main board in the compressor section has a white wire that I accidentally pulled too hard during inspection but after putting it back in, its still the same. All the caps, resistors, look pristine. Soldering job looks solid as well...

Remember when I said the random buttons stop the buzzing? Well before it was the 10x on the EQ section, but that no longer works. It is now disengaging the compressor by pressing the Bypass Comp button. Seems to kill the buzz completely.

Who knows which buttons il have to press tomorrow.

I would do continuity tests to establish if a ground is dis or mis connected.

Any idea where I could find the schematic ?

Ok after doing some fine troubleshooting late at night...ive come down to this.

art balanced output > Ur22mkii line input = buzz
art unbalanced output > ur22mkii line input = no buzz

So it has to do with the balanced output of the Art not playing nice with the UR22mkii Line inputs.

Headphone connected to preamp directly is no buzzing either.

Have to ask the simple things. Are you using TRS cables from the ART to the Ur22?

The hum problem with the Pro Channel II is caused by a small design "error" in the power supply, not something you can fix by messing around with cables, power filters or grounding. I tried all of that to no avail and realised I had to get into the circuitry and find the real cause at the source. Sure enough, it's there.

It appears that as part of the design "update" from the original Pro Channel to the Pro Channel II, two power resistors were added to the 15V section of the power supply. (The original Pro Channel uses an almost identical power supply, with the exception of an additional 150V supply added for the PCII). Those added resistors bypass the 15V regulators and allow 120 Hz ripple to get directly onto the 15V supply, causing noticeable 120 Hz hum at high gain. Pressing buttons on the EQ section can change the hum level (even with the EQ bypassed), but it will always be there.

But luckily, it's a very easy fix. You don't even have to take the board out. Just pop the top and snip the two big 47ohm/5W resistors on the pcb (R245 and R41) and it's done - hum gone and the unit is quiet even at max gain, other than some reasonable hiss you'd expect with max gain.

Actually, with that done, it's hard to find any serious fault with the PCII - it's a brilliant channel strip, especially considering the price.

But is there a reason for those resistors to be there. Except being hum generators?

They were probably intended as bleed resistors for the 1000uF caps right after the rectifier diodes. That's all I can see.

After the power supply rectifier diodes there are two 1000uF caps. The voltage at that point is 25V. Then there are two 15/-15 Vdc regulators which convert that to a clean 15/-15 volts for the circuitry and tube heater filaments. Two 47 Ohm resistors were added across those regulators, directly bypassing them (with just 47 Ohms) and therefore putting the 120Hz ripple from the rectifier directly onto the 15/-15 Vdc supply.

Heating tubes from dc, rather than ac (as in most instrument amps), is a "feature" of hi-fi gear because it's the quietest way to run them - IF the dc is very clean. But if there's any significant ripple at all it will show up when the tubes are cranked. That's exactly what's happening in the PCII - 120Hz right on the 15Vdc heater supply thanks to those two 47 Ohm bypass resistors (R245 & R41).

If I had to hazard a guess, I'd say that the ART techs got tired of having to deal with the charged caps when working on the original Pro Channel, so the resistors were added to the PCII to allow those caps to drain quickly when the unit is turned off. But, in doing so, they also inadvertently added a very audible hum to the PCII when the gain is cranked.

For the sake of bleeding the caps when the power is off, two larger resistors should have been added directly across C78 and C74, not bypassing the dc regulators... but there may not have been space on the board for them, or it could have been done as a last minute revision or something. I have my PCII apart at the moment for something else (mods, not a "problem" with the unit) - I might add two 47k resistors across C78 and C74 while I'm in there. That will allow the caps to drain in a few minutes, rather than seconds like the stock PCII, but that's the way it should be done.

But, if you don't speak electronics and all that just sounded like babble, look at it this way... those 47 Ohm resistors are what's causing the hum in the PCII, and they were not present in the original PC, which used an otherwise identical 15V supply. The original PC functioned just fine without them... with no hum.

47 ohm bleeders? no I think either someone botched the pc board and it got out (like they wanted to add an extra stage of pi filtering) or if they were actually supposed to be bleeders, someone got the decimal place wrong and it didn't get caught (47K would be more reasonable size for that)

this is a very solid design. The only exception is the -15V heater run that would benefit from a current regulator to soft start the tube so there isn't an aggressive inrush current the -15V reg would have to deal with when the tubes are cold.

If it was intended to be a pi filter, why put it across the regulators? Why bypass the regulators at all? All I can think is that techs wanted an easy way to quickly drain those caps when working on the unit, so someone had them put in. Now, a decimal mistake or "typo" that didn't get caught, that would make sense... but those 47 Ohm resistors are 5 Watters, obviously intended to carry some current... that seems deliberate to me...

I find it odd that the unit is so well designed in other ways, yet something as glaring as this slipped through. Perhaps another cook was in the kitchen?

Whatever the reason, the hum in the PCII bothered me for a long time. Nice that it's an easy fix.

I think the board layout/etching was botched. 47 ohm resistor is low even for a regulator bypass and that is not the common adopted method for these regulators.

I think they were going to add a pi filter stage, Because it doesn't make sense to bleed a power supply like this. I've mostly seen bleed resistors in 100VDC and up circuits.

47/5W would make sense for a pi-filter, and the resistors are across two filter caps, so a botched layout/etching would be a definite possibility. However, ART sent me the schematic, and those resistors are on there - implying it was by design... unless there was a mistake somewhere along the lines of communication.

There's a mounting stand-off on the chassis fairly close to the filter caps. When you tip the board to access it you can easily touch the bottom of the cap joint to the stand-off and get a "snap". That's why I'm suspecting the techs might have wanted them self-draining. But I think you may also be right about the botched pi-filter attempt, otherwise 47/5W resistors there don't make any sense at all.

for a problem like this, it would be cheaper to fix it mechanically (with nylon parts).

Hi
Adding a 'bypass' resistor is a legitimate way of 'boosting' the current available from a regulator (and is mentioned in data sheets) and especially as this is trying to run valve heaters which are very low resistance when cold, assists the regulators in starting. There MAY have been issues at switch on with high mains and cold valves making some regulators fold back or switch off.
Although it CAN put 'ripple' past the regulator you have to ensure the bypass current is not too near the required current otherwise it will make the 15 Volt rail 'ripple'. Maybe 68 or 75 Ohm resistor (s) would have been more appropriate.
Further power decoupling should have been done or at least ensured any ripple does not appear on the output.
Matt S

I think you might be onto something here. The 47/5W resistors are obviously there to provide bypass current straight from the rectifier, but in doing so, they also provide the ripple that comes with it. If they were going to do that, they should have upped those 1000uF caps to something high enough to keep the hum out of the tubes... but then they might run into other issues.

However, there are 27 Ohm resistors in series with the filaments that limit cold-start current. Even if the filaments had zero start-up impedance, the current draw by the heaters wouldn't exceed the 1A ratings of the regulators... unless people cranking the gain through the unit before the tubes had warmed up was exceeding it. In that case, I'd rather just wait another minute for the tubes to warm up than put up with 120 Hz hum every time I turn the gain up.

That's perhaps what puzzles me the most. The PCII has a significant, noticeable hum at high gain. How did that slip by in an otherwise excellently designed unit?

Hi
If you look at the startup current available from a regulator when you have maximum input (supply) and something approaching a short circuit at the output (few tens of Ohms and large capacitors) there may not always be enough to 'kick' the thing into operation. A LM317 or similar can be 'slowed down' to rise over a longer time and some (LM338) have a 12 Amp startup capability for a few tens of milliseconds before reverting to a 5 Amp limit.
This scheme does not HAVE to ripple if you make sure the load is always greater than can be provided by the resistor at ALL times. Granted the 'ripple' performance of the regulator may be tested and the exact circuit layout. It could be that a 2 or 3 Amp version of the regulator may not need this 'boost', it is all down to design choices. Having a 'bypass' certainly reduces the heat burden on the regulator, tending to make it more reliable and consistent voltage.
Matt S

Obviously they deemed it desirable, but at operating temp, the filaments are pulling less than 300mA from the regulators; add to that the draw from the solid-state circuitry and you're still well under the 1A steady-state spec of the regulators. So that leaves start-up as the potential issue... and I think the 10uF caps to ground on the output might be more of a potential issue there than the low start-up resistance of the filaments. Even then, I think it's a long-shot and wasn't a known issue with the original PC - hardly worth the perpetual resultant hum at high gain with those added bypass resistors in place. DC heated tubes are very sensitive to ripple, but if they were going to go that route, then 47 Ohms is just too small (though larger may not allow enough current to be worthwhile), as is 1000uF on the mains... obviously, as the 'revised' power supply causes hum. In any case, it slipped by the company and those units are shipped with a consistent 120Hz hum at high gain that people have been complaining about since they came out... something that's easily fixed.

well they should had did a current reg. method. It works very well and no ripple. but I guess, the resistors being a 1/3 less in cost to include compared to the current regulator circuit.

but as far as their sizing, i think they did go a little bit too less in value in resistance. 100-150 ohms would have sufficed

I had some time today to take a better look at what the circuit is doing...

The regulator inputs have 1000uF caps on them, the outputs caps are 10uF. Even with wildly different ESRs the input caps will charge much more slowly than the output, ensuring that the output of the regulator cannot be shorted to ground at start-up while full supply is applied to the input. This arrangement is consistent with the regulator datasheet. Also, the 27 Ohm resistors in series with the filaments, in conjunction with the 1000uF/10uF in/out cap combination, limit the inrush to 555 mA, well below the regulator's 1A steady-state capacity (2.2A peak) and also providing something of an inherent soft-start. It's an efficient design.

The 47 Ohm bypass resistors take about 213mA worth of burden off the regulators at steady-state. So they were either put in for that purpose, were a botched attempt at a pi filter, or they were intended as bleeders, or some combination.

Looking at the original PC, they were running three tubes off those regulators with no bypass resistors (two tubes on one, one on the other). So it's possible they were running into problems, particularly on the regulator heating two tubes, and put those resistors in the PCII as a safe-guard to relieve some of the burden. The PCII has only two tubes though (one on each regulator), not three, so it's automatically presenting only 1/2 the heater load on one regulator as compared to the original PC.

Hi
What is your measured pre regulator voltage?
Of course since the 'bypass' current will have some ripple associated with it, the point where this gets injected into the main 15 Volt rail may be a bit 'resistive', making it a 'layout' thing rather than the concept.
10uF is woefully small if there is no other R/C decoupling of the supplies for a mic amp.
Unless Thom or others from ART join in it is all speculation as to WHY this was done.
Many people make unfortunate mistakes!
Matt S

I would like to thank this person. The "nightmare humming thread" also has my thank you to Brynner737 for describing a remedy for the ProChannel II humming problem.

Sincerely,
J. Toole..

I dont know if this helps but the first version of this ProChannel had 3 tubes, it was a Tube Compressor also, the following versions removed that tube compressor is my understanding. Didnt ART get spanked for mentioning Vari-Mu or something?

Hi,

I'm potentially interested in this channel strip and am wondering if someone could clarify if this has exactly the same preamp circuit as one channel of the MPA Pro II?

Also, what are the best tubes to get as an upgrade over the stock tubes for reducing noise and having a nice tone when being driven without losing any output?

Old post but I found that if you plug the Pro Channel II into a pro voltage regulator the buzz and hum problems go away, other users mentioned using a variac but using a good regulator fixed the problem for me, I was about to return mine and I'm glad I found a solution 'cause i really like how the Pro Channel II sounds, I'm using a Furman AR voltage regulator, it's expensive but it works great.

The resistor snip mod caused mine to overheat and shut down, I'm curious about just snipping one resistor and see if that's enough.

I have the Pro Channel III and love it. No issues.

Is it tube preamp?

Yes it has a tube in the compressor section. You can switch between "opto" which employs a 12AX7 tube (like an LA-2A compressor), or "FET" mode which is solid-state like an 1176 compressor.

The actual pre-amp section is not tube. Which is fine for me because my main mic is a Warm WA-67 tube microphone.