[ACMP investiupgradifications] All things PREAMP

[rocketattack]

Someone on the homerecording.com thread noticed that if you get the get the knob to sit in between the 8 and 9 o'clock detents it produces a "horrendous tone."  He also noted it doesn't happen when set to line-in.

When I get back from this trip I'll post photos of the inside of the 84 for comparison, since we have the insides of a 73 and an 81 posted.

 

[b3groover]

The gain switch does not cut out or pop on the 73.

 

[Crash]

Someone on the homerecording.com thread noticed that if you get the get the knob to sit in between the 8 and 9 o'clock detents it produces a "horrendous tone."  He also noted it doesn't happen when set to line-in.

When I get back from this trip I'll post photos of the inside of the 84 for comparison, since we have the insides of a 73 and an 81 posted.

Yep, I was just coming over here to ask about that. My two 84's do that. Here is what I typed elsewhere, it was addressed to Zmix (Chuck):

Hey Zmix, the gain knob on the 84 exhibits a weird issue at about the mid point where you can get the knob between detents and it will send a crazy loud tone to the output of the pre, pegs the needle, and all that. It only does it in mic mode with or without a mic attached. You seen anything weird there? If you blow through the detent it sounds more like a pop. Other than this issue, I have not seen any other oddities with the 84. Somone else mentioned this before too but I couldn't remember which of the 3 forums it might have been on.

Somone else had posted about this besides me as well, so it is more than an isolated incident.

 

[b3groover]

I hemmed and hawed about which preamp to buy; looks like I made the right choice between the pops of the 84 and the hum of the 81.

 

[rocketattack]

Somone else had posted about this besides me as well, so it is more than an isolated incident.

It does seem to affect the 81s and 84s. If it is the preamp section, then I'm surprised it doesn't affect the 73 as well, but B3groover didn't notice it on his units. It happens with or without the EQ section engaged, and is noticeably worse if you turn up the gain on the last HF filter on the 84. That causes the meter to bounce. Without the HF turned up, the meter doesn't bounce, but the clipping light comes on.

 

[b3groover]

I'm surprised, too... but no pops, no hum with the 73.

 

[rodabod]

Has anyone investigated the switching to see if the amp is temporarily going to open-loop gain at this point? That might cause the symptoms that you're speaking of I guess.

I'm going to take a look at the schematics again tonight to see if I can gather anything - I assume the mic amp stage is that same as that of the '73?

Roddy

Someone on the homerecording.com thread noticed that if you get the get the knob to sit in between the 8 and 9 o'clock detents it produces a "horrendous tone."  He also noted it doesn't happen when set to line-in.

When I get back from this trip I'll post photos of the inside of the 84 for comparison, since we have the insides of a 73 and an 81 posted.

Yep, I was just coming over here to ask about that. My two 84's do that. Here is what I typed elsewhere, it was addressed to Zmix (Chuck):

Hey Zmix, the gain knob on the 84 exhibits a weird issue at about the mid point where you can get the knob between detents and it will send a crazy loud tone to the output of the pre, pegs the needle, and all that. It only does it in mic mode with or without a mic attached. You seen anything weird there? If you blow through the detent it sounds more like a pop. Other than this issue, I have not seen any other oddities with the 84. Somone else mentioned this before too but I couldn't remember which of the 3 forums it might have been on.

Somone else had posted about this besides me as well, so it is more than an isolated incident.

 

[CHANCE]

Chuck (zmix) figured that the pop position is exactly where on the original Neve the "off" position is

 

[rocketattack]

I'm going to take a look at the schematics again tonight to see if I can gather anything - I assume the mic amp stage is that same as that of the '73?

I thought this was a true statement before, but maybe it's not since no one with a 73 has reported the problem yet.
???

 

[Mylithra]

Oh.. the 73 has the pop problem in the gain switch. It really doesnt bother me much as I generally set the master out all the way down when Im messing with the gain stage so I didnt notice it till I started hearing about the problem and checked to see if mine had this problem. They did.
I havent looked deeper into this because I didnt really feel the need to.
I dont think the gain switch is of the shorting variety due to some of the noise I hear when clicking through it though as I said, I havent checked it out.
I didnt hear any humming noise in the 73 but I havent done any real critical listening yet.

 

[tarnationsauce]

Here are some noise floor tests of the 73, 84, and 81. I also did the converters alone as well as a SCA N72 as a control.

Keep in mind a SCA N72 has superior power supply and shielding. It utilizes a fully regulated PWM power supply and full Mu metal shielding.

Note: This is a non-scientific test, just a reality check. This was done without top covers on the ACM preamps! Also the scaling is slightly different from graph to graph.

Green █= EQ Out
Pink █ = EQ In

______________________________________________________________________

 

[b3groover]

Oh.. the 73 has the pop problem in the gain switch. It really doesnt bother me much as I generally set the master out all the way down when Im messing with the gain stage so I didnt notice it till I started hearing about the problem and checked to see if mine had this problem. They did.
I havent looked deeper into this because I didnt really feel the need to.
I dont think the gain switch is of the shorting variety due to some of the noise I hear when clicking through it though as I said, I havent checked it out.
I didnt hear any humming noise in the 73 but I havent done any real critical listening yet.

This post made me curious, so I went back and checked mine again.  On one unit, there is a slight noise when switching gain, but nothing more than my Quad Eights, which have a stepped attenuator on them.

However, on that particular unit, stepping the gain back down from the highest point to the next position below did cause a slightly louder noise, almost a tone.  But only when coming down and only in one spot.

On my other 73 (yes, I was able to fix it), there is the slight noise as in the other one, but that's it.  However, that one does a weird thing when you first turn it on that the other doesn't:  The meter pegs itself all the way to the right and then slowly goes back to zero.  While it's doing this, there's no noise or anything coming out of the output.  Weird.  The other 73 I have doesn't do this.

Quality control!

 

[tarnationsauce]

B3Groover, were you doing those tests with mic input? My 73's make a loud "pop" oscillation like others describe. But onlu with mic input.
I have noticed that when the 73 is first turned on the meters move all the way to the right and slowly fall. I think that's just a coupling cap charging. Maybe this was the case for you?

 

[Mylithra]

Both of mine did the VU meter peg when first started up, but even then it passes audio normally. The pop on the gain switch was mainly in 1 spot, Just past 9 o'clock, there is a loud pop going in both directions.

 

[Mylithra]

I went and did a bit of listening. Heres the setup.
ACMP - Line in,mixer - Headphone out on mixer.
Have preamp all the way down,
Swith on the EQ High band with the boost all the way up on that channel, output all the way up, I can hear what sounds like 60 cycle hum. But only on the first 3 eq settings. I might try playing with the transformer or shielding it up to see if that will solve my problem.
Might also try the transistor replace if that will do it too.

Edit---
Anyone have a source on the  BC441, 461's

 

[rocketattack]

Both of mine did the VU meter peg when first started up, but even then it passes audio normally. The pop on the gain switch was mainly in 1 spot, Just past 9 o'clock, there is a loud pop going in both directions.

fwiw, neither of my 84s had the VU meter issue.

Did Chance ever post what the issues were with the pre's that were held up due to QA issues?

 

[rocketattack]

Crash posted this over at the home recording forum. I didn't see it on either of the ACMP threads (this one or the group buy one) here.

This is a point on the switch where N£V£ put in an additional "OFF" position on the switch. I think that this is where the additional preamp stage gets switched in.. I'll glance at the schematics when I get a second.

Yes this is exactly the spot where the additional gain stage gets switched in.
7th position up from fully counterclockwise.

This ONLY happens when the mic/line switch is in the "mic" position, so you may want to switch to the "line" position when rotating the switch past this point.

While that explains how it behaves similarly to the pre's it's modeled after, such a loud pop seems to be a
design flaw here.

 

[BradM]

I'm noticing the hum/noise problem in my ACMP-84 when the EQ is engaged and the midband is set to either 360Hz or 700Hz.  The hum is there regardless of the amount of boost or cut and goes away if I switch to a higher frequency or switch the midband to off.  I am experiencing this on both 84's that I have.

Has anyone else noticed this as well? 

I'm guessing the toroidal inductor is the culprit?  Any thoughts?

thanks,
Brad

 

[Crash]

Crash posted this over at the home recording forum. I didn't see it on either of the ACMP threads (this one or the group buy one) here.

This is a point on the switch where N£V£ put in an additional "OFF" position on the switch. I think that this is where the additional preamp stage gets switched in.. I'll glance at the schematics when I get a second.

This is a point on the switch where N£V£ put in an additional "OFF" position on the switch. I think that this is where the additional preamp stage gets switched in.. I'll glance at the schematics when I get a second.

Yes this is exactly the spot where the additional gain stage gets switched in.
7th position up from fully counterclockwise.

This ONLY happens when the mic/line switch is in the "mic" position, so you may want to switch to the "line" position when rotating the switch past this point.

While that explains how it behaves similarly to the pre's it's modeled after, such a loud pop seems to be a
design flaw here.

Just so the right person is given credit for this, I quoted Zmix, that is his quote. I have no business making any sort of statements like that, I more of an end user of these things. ;D

 

[maxwall]

Brad,

Loosen the single retaining screw holding your power supply toroid and move it around  a inch or two towards the back of the chassis while you have the mid band engaged with the hum present and report back your results here.

you won't be able to move the inductor since its soldered to the board.