Group Buy Interest?

[tarnationsauce2]

I don't know how much of the GB threads you've been following.
The buzz issue is pretty well known. Someone (can't remember who) on GS claims to have the fix for it. Apparently it's not a ground loop issue, it's an induced voltage issue from the mains transformer. It's pretty obvious all the other mfr's of similar products use external power supplies. Probably for this exact reason.
I am going to mount all my transformers in an external 1u box. But I may tru some Mu metal over the transformer and inductors. I wonder if the metal over the input transfos is true Mu metal or some other metal?

 

[MichaelJoly]

I'm a little bit too busy to check in with this thread and the ones at Prodigy Pro, ("Whatever Works" and "The Lab") and Gearslutz regularly. Ground loop or induced buzz, no matter - You think maybe the buy organizer would send out an email heads-up concerning the problem and what he's going to do about fixing it?

It would be one thing if the ACMP-73 was sold "as is" - "needs troubleshooting"..."a great fixer-upper"... "get out your soldering iron, Sawzall and 1U case and have fun making an external power supply". But it was not sold that way. It was sold as the functional equivalent of a Neve preamp.

I've already written my two units off as a business loss. But what about the folks who dumped hard-earned money into these things and don't have the chops, time or money to make them right?

 

[dgatwood]

Depending on which thread you believe, it is either an induced hum or it is caused by some sort of high-frequency oscillation caused by not-up-to-spec transistors (and the post that reported the latter said that the problem still occurs even with an external PSU).

I'm reserving judgment until I get back to Cali and can study mine. I have a pair of 73s and a pair of 81s waiting for me. My gut says that there's more than one cause here....

 

[dgatwood]

I've already written my two units off as a business loss. But what about the folks who dumped hard-earned money into these things and don't have the chops, time or money to make them right?

Shana, they bought their tickets. They knew what they were getting into. I say let 'em crash.

But seriously, we all knew going into this that past group buys have had QA problems.

In the last group buy, I retensioned four ribbons, tearing one in the process (but I still had enough excess material to retension it). On at least one of the tube mics, one of the screws that holds the XLR jack was completely unscrewed with the nut rolling around inside the power supply. I had a very high stuck screw rate on pretty much all the mics. Several people had ribbon mics that were wired out of phase. And so on.

I sincerely hope that people recognized going into this that there was a good chance they'd have to do at least a little bit of work to make this hardware usable. The ability to ship DOA hardware back for replacement is the difference between a group buy and buying from an actual retail distributor.

That said, if the whole lot of 73s are defective by design, that will be a little disconcerting.

 

[redddog]

Yeah. These should have been sold as projects. They really weren't.

If there was a stipulation that these were going to need serious work to function correctly, I would have thought twice before ordering 8 of them, as would a bunch of other members. I don't have a ton of experience with electronics so I'm going to have to learn on these. the other issue that I'm having is that some of the buttons on my 84s and 73s won't disengage. How I go about fixing those is a mystery - chinese style.

I'm not chalking them up as a total business loss (and I bet you really won't either, Michael) but I am a little disappointed at the apparent TOTAL lack of any form of QC or just basic function tests. Whatever.

This stuff will work out to be great someday. After much effort from the novice like myself. I'm cool with that. If these get to where they can get, they will be FAR better what we paid. I think the bottom line is what you think your time is worth. If you can live with all the time you'll need to put into these, you'll be fine.

I do agree though, that it is remarkable that so many of these can go out that are not even considered to be functional.

 

[mshilarious]

the other issue that I'm having is that some of the buttons on my 84s and 73s won't disengage. How I go about fixing those is a mystery - chinese style.

Try reaming out the holes in the front panel (after you remove the PCBs of course)!

 

[BudgetMC]

ACM-1200 Key Test

Just spent some time testing my three 1200's. I gave them a three-day burn in, and tonight was the first real test. I used the famous Harvey Gerst key-clinking test, and all three (Huey, Lewie, and Dewie) of mine came up fairly wanting... lots of farty distortion. I then did the test on an MXL 960 (with Peluso CEK-89s), an Octavamod 319 (PE), and a CAD GXL 3000... all of which came through just dandy, thank you very much.

I then thought... 'well, maybe it's the crap tubes', so I swapped a NOS Mullard 4024 into Huey, fired it up and... nothin'... I mean nothin'. Dandy, thinks I... I killed a mic doing a freakin' tube swap. I swapped the stock tube back in... nothin'. I took a deep breath, opened it up again, and gave the tube sockets a good wiggle... put it back together and... it was alive. Alive I say!!

I'm a bit too insecure to try another tube swap just now. There may well be a bad solder joint in Huey that makes the mic iffy when the wires are jostled. When I get my mojo back, I'll try the swap in Lewie, and get back to you on the results.

On the bright side, I pulled the grill off of Huey, and popped the top dent out just as easy as you please.

 

[drBill]

Just listened to my two ACMP-73 preamps - input shorted to ground and EQ bypassed. Totally unacceptable hum and buzz.

Hi Michael!!

EQ bypassed eh??? Sounds like yours are DOA. I'm sure Chance will replace them. My 73's with EQ bypassed are absolutely acceptable. Even with the EQ in, they are acceptable for drums or loud sources, but not as clean as I'd like. Still, useable for all but the softest of instruments utilizing the LoMidFreq band. I have not tested them with the input shorted - only with mics on them, and notice no real difference between my console (D&R), my API's or the ACMP's - noise wise - until EQ is switched in.

I'd guess Chance will pop in here soon enough. Or contact him directly. I hope you figure it out. I'm thrilled with mine. One of your 205's thru the ACMP on drum room.......

Bill

 

[BudgetMC]

As regards the ACMP-73 (and others), there seems to be a lot of variation between individual units. Some of this might be attributable to variations between people's ears, but I'm pretty confident in saying that mine is nice & quiet so long as I don't engage the mid-range EQ. As a preamp it seems to work dandy... it is certainly no louder than my Meek VC3Q, ART MPA Gold, or DMP-3.

As for the apparent QC problems, I went into this with eyes wide open. I took (and take) all the enthusiasm with a grain of salt, and am not even a wee bit grumpy about the outcome. That said, I didn't mortgage anything to buy any mics or pre's. Heck, I won't even go without beer. I bought the mics mainly as cheap platforms on which to try develop some mic modding chops, and I figured the pre was a worthwhile risk. C'est la vie!... as we say in Le Pays de groupbuy!

 

[MichaelJoly]

OK, an hour of troubleshooting reveals...

1. Torroidal transformer windings induce hum into different parts of the circuit from front end to line driver output.

2. The torroid can be rotated on its mounting post to find a null...but because there are multiple windings and multiple circuit boards there is no one single best angular rotation of the torroid to eliminate induced hum at all PC board locations and thus all circuits.

3. In fact, if you unplug the blue and brown connectors, leaving just the white & orange connector attached you can listen to the just the line driver stage hum. The hum in this stage can be very effectively nulled out by rotating the torroid, but then the other stages are not in the hum null because of their physical location.

4. I lengthend the torroid output wires and moved it around the case - the only way to get acceptably low hum (to my ears any way), is to remove the torroid from the case and operate it some distance away from all the PC boards.

5. I have not yet heard anything that sounds like a rectified oscillation 'hum' - but haven't put a 'scope on it yet, not until I get the basic power supply issue taken care of.

6. If you're not too particular, and are only going to be recording guitar amps with these things just rotate the torroid and pick your favorite hum null, there are several.

7. To anybody who has one of these things and doesn't hear objectionable hum - I'd say your torroid is mounted with an angular rotation that produces an acceptible hum null for your application.

 

[guitar zero]

I was pretty excited to get these 81s, then started reading about the problems for the last 3 months while I waited. I'm not super excited right now, even though they finally arrived. Mine are sitting unopened, waiting for chance to post some kind of solution. The mics, I'll try out soon. If they sound decent, I'll think about modding. The pre's I've pretty much written off for now, to be revisited at a later date. Worse things have happened to me.

 

[BudgetMC]

Great info, Michael, thanks for the effort.

Does this mean that some sort of shielding solution, as some have suggested, might offer significant improvement, as opposed to moving the power supply to the outside of the case?

OK, an hour of troubleshooting reveals...

1. Torroidal transformer windings induce hum into different parts of the circuit from front end to line driver output.

2. The torroid can be rotated on its mounting post to find a null...but because there are multiple windings and multiple circuit boards there is no one single best angular rotation of the torroid to eliminate induced hum at all PC board locations and thus all circuits.

3. In fact, if you unplug the blue and brown connectors, leaving just the white & orange connector attached you can listen to the just the line driver stage hum. The hum in this stage can be very effectively nulled out by rotating the torroid, but then the other stages are not in the hum null because of their physical location.

4. I lengthend the torroid output wires and moved it around the case - the only way to get acceptably low hum (to my ears any way), is to remove the torroid from the case and operate it some distance away from all the PC boards.

5. I have not yet heard anything that sounds like a rectified oscillation 'hum' - but haven't put a 'scope on it yet, not until I get the basic power supply issue taken care of.

6. If you're not too particular, and are only going to be recording guitar amps with these things just rotate the torroid and pick your favorite hum null, there are several.

7. To anybody who has one of these things and doesn't hear objectionable hum - I'd say your torroid is mounted with an angular rotation that produces an acceptible hum null for your application.

 

[MichaelJoly]

Round two -

Just 'scoped it out - no oscillations in the two units I've got.

Turns out the mic pre and DI boards sit at different points in the hum field, so its possible the DI can be relatively hum free because of the rotation of the toroid. The converse can also be true - that is, the torroid can be rotated for best mic pre hum null. The difficulty with rotating the toroid for best hum null is that it can only be done with the top cover off. But this allows stray AC hum from the environment to enter the unit and swamp the hum generated by the internal toroid. It is possible to rotate the toroid in small increments and close the cover each time but what a PIA.

I really think getting the toroid out of the box is the solution - whole lot cheaper than trying to shield the toroid with mu metal - as elegant a solution (with unkown efficacy) it might be. Just what I need, more hacked gear with wiring kludges hanging out of it

 

[dgatwood]

2. The torroid can be rotated on its mounting post to find a null...but because there are multiple windings and multiple circuit boards there is no one single best angular rotation of the torroid to eliminate induced hum at all PC board locations and thus all circuits.

I assume the transformer you're talking about here is the PSU transformer. I also assume that most of the induction is occurring in chokes or other similar parts. The difference in some units may well be in either:

A. subtle difference in the orientation of the windings on the various chokes (particularly if they aren't in a package that prevents them from getting bent during installation), or

B. the quality of the transformer windings themselves (e.g. loosely wound), or possibly

C. the length of leads in some critical circuit path causing the whole thing to act like an antenna at some multiple of 60 Hz.

That said, if they're using a toroidal transformer for the PSU, the best fix for these problems might be to just rip it out and replace it with a modern, shielded off-the-shelf equivalent laminated part from Rat Shack or whatever. What's the secondary voltage(s)?

 

[MichaelJoly]

Yeah, multiple winding toroidal PSU transformer. Output voltages under load are 47v, 27v and two 15v stradling a center tap to ground.

Long output wires are still way cheaper and quicker than a new transformer or mu metal. I'll just gaffer tape the AC connector to the toroid, sit it on the floor and run a multi-conductor back up to the power supply board.

 

[P. Glaser]

OK, an hour of troubleshooting reveals...

1. Torroidal transformer windings induce hum into different parts of the circuit from front end to line driver output.

2. The torroid can be rotated on its mounting post to find a null...but because there are multiple windings and multiple circuit boards there is no one single best angular rotation of the torroid to eliminate induced hum at all PC board locations and thus all circuits.

3. In fact, if you unplug the blue and brown connectors, leaving just the white & orange connector attached you can listen to the just the line driver stage hum. The hum in this stage can be very effectively nulled out by rotating the torroid, but then the other stages are not in the hum null because of their physical location.

4. I lengthend the torroid output wires and moved it around the case - the only way to get acceptably low hum (to my ears any way), is to remove the torroid from the case and operate it some distance away from all the PC boards.

5. I have not yet heard anything that sounds like a rectified oscillation 'hum' - but haven't put a 'scope on it yet, not until I get the basic power supply issue taken care of.

6. If you're not too particular, and are only going to be recording guitar amps with these things just rotate the torroid and pick your favorite hum null, there are several.

7. To anybody who has one of these things and doesn't hear objectionable hum - I'd say your torroid is mounted with an angular rotation that produces an acceptible hum null for your application.

Hi Michael. Chuck Zwicky figured this out a while back and posted it on the PSW forum (although I think is was on an 81). I fired up my 73 yesterday morning (after a 24 hour burn-in) and it was almost completely silent until I engaged the EQ, even with the input and output wide open. Lots more gain here that I need, (although the only ribbon I have is the 900).

Now, when the EQ is engaged, I can get all sorts of different pitches of hum and buzz depending on which of the various center frequencies I select. But the input and output have to be pretty much wide open.

I was planning on doing the toroidal rotation trick to see what kind of improvement I could get. At this point I'm going to watch the forums and see what you smart guys figure out. LOL I wasn't really planning on using the EQ much anyway....

Paul

PS I've also noticed that the EQ switch wants to stick in and not disengage.

 

[MichaelJoly]

The power line spectrum of the toroid is pretty rich so engaging the EQ will cut and boost different portions of the spectrum. In addition to the input transformers there are three EQ boards with inductors - each sits at a different angular rotation to the toroid so the spectrum seen by each board is different.

Back in the old days there would be analog engineers who specialized in low-noise power supply design - both radiated EMI and ground loop current. They would actually take pride in being able to deliver DC to the "small signal guys" with battery-like purity. Ah...the lost arts...

re: Chuck Zwicky's discovery - That's awesome, did Chance do an email shout out? I must have missed it. Who has the time to wade through four different forums and thousands of posts to track this whole project from ordering through troubleshooting?

 

[drBill]

5. I have not yet heard anything that sounds like a rectified oscillation 'hum' - but haven't put a 'scope on it yet, not until I get the basic power supply issue taken care of.

Michael - I *believe* that this is only in the '81's.

Any ideas on removing the transformer from the case and going external?

Edit: Oh, I just saw your duct tape solution....

I should have book marked Chuck's transistor swap fix, but I didn't. I'm sure someone here did, but again, I believe it only applies to the 81.

BTW, Brad McGowan has got some type of shielding thing going on. It's not Mu Metal, but another solution. He says it's working well, and is going to sell a kit for around $25-30 I think. (Expensive material) And it will be a simple fix. Or should I say Band-aid. He has not had as good of luck with the 84's. Probably due to the different angles of the PCB's.

I hope you're feeling better about your purchase. While I'm sorry for your grief, I'm overwhelmed and grateful for your tech saavy and ability to sus out the problem quickly and suggest alternative fixes!!!

bp

 

[guitar zero]

Michael,

This could be the start of something really big. Michael Joly's TNCmods. Hundreds of units already lined up. Hmmmmmmmmmm. Seriously, thanks for the great mic mod earlier this year, and thanks for posting your findings here. Wish I had the chops to rip mine open and fix it like you do.

 

[MichaelJoly]

A new day, a new attitude

Nothing like a few hours of sleep to restore my normally chipper and positive attitude! I'll try and keep it that way, my apologies for the ranting. Had a severe case of bad first impression-itis.

That's cool Brad is on the problem with a shielding solution - he's a good guy with good ears. Oh, I'm really not looking to fix, mod or upgrade any TnC gear - mics included, so I'll politely decline up front before too many more people ask about it.