[ACMP investiupgradifications] All things PREAMP

[b3groover]

Wow, those look terrible.  I haven't inspected my ACMP73s close enough to determine if they have a similar issue.  I should probably get on that.

 

[Steve Hogan]

Here are some pics of the two offending resistors on my 81's preamp board. The other boards and resistors seem to be in pretty good shape, though some of the resistors have a dull rather than shiny surface. It looks like the coating was partially dissolved (and maybe dried with pressurized air?), as some of it is stuck to an adjacent resistor.

Steve - are these the same damaged resistors in your boards? (They have different values - 39 and 120k ohms.)

The Bad Resistors on the 81 I have here are the same two as yours, 1R17, 1R35 plus 1R3 (120K) is wrinkled and should be replaced as well IMHO.

The PRP Phantom feed resistors arrived today. The lowest measured 6806 Ohms and the highest measured 6812 Ohms, with the big part of the bell curve about 75% of them landing beween 6807 and 6809.  I can't help myself, I sorted them into 1 Ohm groups 6806, 6807, 6808, etc. from which I will pull the pairs for the preamps.  I predict no problems due to lack of matching in the modified mic preamps.

I have noodled on an easy, but reliable way to add the ground wire to the 48V switch.  Remember that I am trying to make all these mods able to be done in as short a time as possible to keep the labor charge down.  The Phantom filter cap on the Front-end PCB is grounded by means of the screw located just behind the mic/line switch.  I am thinking of connecting a short green wire with a serrated solder lug on one end to go under that screw in order to:
1. Make sure that the cap is securely grounded. I don't trust the PCB to standoff interface, but the teeth in the solder lug will dig into the copper PCB and the screw and make sure that the screw makes a good connection.
2. Allow the Green wire to be disconnected when the switch is unplugged from the preamp PCB without adding a connector.  This seems to me to be a simple, but effective way to add the ground contact to the Phantom switch.
The other end of the wire connects to the normally closed contacts of the switch in such a way that when the switch is on the feed resistor is connected to 52V and the filter cap charges up and the mic is powered.  When the button is in the off position that same resistor is connected to ground and the cap is discharged thru the resistor.
Should work OK I think.
 

 

[crazydoc]

Just a couple more resistor pics. I opened up my 71 again. These are on two of the inductor boards. 5R15 is another 120k resistor and R20 (at the DI input) is 100 ohms, per the schematic. The preamp board has a couple lightly pitted resistors, but minimally noticeable.

In one picture you can see spatter drops on the next resistor (and the  pc board), so it looks to me like the coating was softened, then subjected to high pressure air. I don't understand why just these few were vulnerable, and the majority were not.

 

[alexc]

Another piece of info ...

Some time ago I posted my method and measurements relating to the main noise
component of my ACMP81s in 2 units with Zmix transistor mod, one with stock
onboard toroid psu and other with replacement onboard psu and toroid.

Where I left it last was that the Carnhill inductor replacement for the low-mid
stock inductor showed a big improvement (>15dBu) to the mains-harmonic components
of the noise floor (line input, unity gain, connected to Motu interface in/out (bal +4dBu)

Well - I've just added the OEP mu-metal shield (thanks Roddy for the fab idea!)
to the Carnhill inductor replacement and retested (in the my rack with nearby units powered on).

Results are really good.

Great, in fact. Essentially low-mid eq noise  has completely gone (Eq on, Low-Mid at max +ve gain, 220Hz)
<-98dBu all individual spectral components, overall -85dBu RMS
Practically the same as with no low-mid eq engaged .  ;D ;D ;D

Now the major noiser is the mid-hi inductor. So that's being replaced shortly.

For me, the mains noise issue is over -  game, set and match.   8)

Total upgrade cost for this unit upgrade - toroid, psu, transistors, inductors and shields - 100usd.

I'll mod and test the unit with stock transformer and psu shortly.
I expect to see the noise gone also.

Pics :

 

[okgb]

Nice Alex , thanks

Cinemag has a couple of my inductors now , and
felt they'd be less than 20.00 each , i'll make sure
cans come with those

 

[0dbfs]

Alex & Greg,

This is great! Thanks!

Alex, Looking forward to seeing the results after swapping out the JLM with the original.

Greg, do you know if these inductors will be a direct order item from Cinemag or might this end up being a BM organized purchase?

Cheers,
jonathan

 

[okgb]

Tom from Cinemag was checking the inductors i sent from my 81
and mentioned that the values were close to the schematics ,
although core material is unknown .

He recomends taking care of the power transformer first
[ replacing with as steve mentioned lower gauss higher va ]
& sheilding the inductors next [ doesn't need to be Mu metal
many homemade solutions will work ]

That said  he is sending me some prototypes to check in circuit
to be sure of their role in the noise problem
if that is confirmed [ i was satisfied with Alex's work ]
he will offer them at 20.00 or less with a small min of 25
[ Tom wants to make sure no one is needlessly spending money ,
but is happy to help ]

As the world turns ..................

 

[maxwall]

Clever DIY teamwork work

Roddy, Alexc 

Great idea with the Mu Can , did a transformer sacrifice its shell to do this or can those
Mu cans be purchased empty , good to know for future projects and testing.

Alexc,
What PSU transformer did you use to replace the China version , same taps or just basic taps without Phantom supply ?

Greg,

How did those 2N2907 & 2N2222 work out in the Q4,Q5 positions ? Did you use T0-92 plastic or T0-5 metal cans ?

 

[okgb]

the xisiters got rid of the main buzz / hum when engaging the eq in switch ,
then the hum from the inductor bands became obvious ,

see above post for update

 

[alexc]

> did a transformer sacrifice its shell to do this or can those

Ha! No - you can get them separately. 15aud each for me.

>What PSU transformer did you use to replace the China version , same taps or just basic taps without Phantom supply ?

I used a chinese toroid from Altronics Australia. It's a pretty cheap-and-nasty one (22aud), but works OK.
At the 'crappier' end of the quality scale

Different taps than stock acmp81. It provides 120+120VAC primary and 24-0-24VAC secondary at 30VA.

I use it with the JLM 5Rail Power Station (around 50aud diy), which takes 24-0-24VAC and provides (all adjustable)
+48VDC phantom(up to max 300mA), +24VDC(up to max 1A), +/- 15VDC (up to max 1A)
plus an unused -V adjustable rail.

I use this setup in most of my DIY builds and find it to be reliable and trouble free.

I am going to leave them both with stock PSU and toroid and just increase the voltage of a couple of caps for safety.
And look at the gain switch pop thing as well as phantom bleed off.

That'll do it for me.

 

[alexc]

Hey Greg

Regarding the Cinemag replacement - Great stuff.

At that price and with Cinemags usual undoubted quality, these things will improve dramatically.

"Team DIY & Tom of Cinemag. Taking care of the audio *little* guy..."

 

[floxe]

just a quick question: i got my preamps and most of the microphones today.

i use mine with 230V, so i have to switch the fuses to 250V 100mA T - right?

do i also have to switch fuses in the power supplies for the acm-310 and acm-1200?

best,
floxe

 

[maxwall]

just a quick question: i got my preamps and most of the microphones today.

i use mine with 230V, so i have to switch the fuses to 250V 100mA T - right?

do i also have to switch fuses in the power supplies for the acm-310 and acm-1200?

best,
floxe

Yes, I think you need to switch them all. Since a fuse, in general, limits the power to a circuit (to protect it), and power is a function of voltage times current, if you double the voltage (115 to 230), you need to half the current (250ma to 125ma.) The T designation makes it a time delay (slow blow) fuse, to allow an initial inrush of current as the device powers up.

 

[okgb]

My latest mod ,
Now i know you're going to say , hey wait Greg
this isn't going to affect the sound one bit , I know
but it amuses me and makes me feel better
[ do you think it's an insult to the knobs ? ]

btw either it's metric or they skimped and the shaft size is smaller
than normal and the old knobs won't fit 1/4 shafts ............useless

 

[Paul G]

My latest mod ,
Now i know you're going to say , hey wait Greg
this isn't going to affect the sound one bit , I know
but it amuses me and makes me feel better
[ do you think it's an insult to the knobs ? ]

btw either it's metric or they skimped and the shaft size is smaller
than normal and the old knobs won't fit 1/4 shafts ............useless

Hey Greg.  Can't really tell from the pix what you did.

;D

 

[alexc]

It is only insulting if is red 

Black is OK.

 

[okgb]

It's subtle , you have to look , lol





hint felt marker over those offending letters in the circle
make it better

 

[floxe]

read the stamp on the end of the fuse ( metal cap), does it say AGC250 or 250 anything ? if not then you need a fuse with this designation.

are you sure? the manual says 250mA for 115V and 100mA for 230V.

best,
flox

 

[crazydoc]

just a quick question: i got my preamps and most of the microphones today.

i use mine with 230V, so i have to switch the fuses to 250V 100mA T - right?

do i also have to switch fuses in the power supplies for the acm-310 and acm-1200?

best,
floxe

Yes, I think you need to switch them all. Since a fuse, in general, limits the power to a circuit (to protect it), and power is a function of voltage times current, if you double the voltage (115 to 230), you need to half the current (250ma to 125ma.) The T designation makes it a time delay (slow blow) fuse, to allow an initial inrush of current as the device powers up.

 

[crazydoc]

Just a further note on the QC on this group buy stuff. While just now checking the value on the fuse in the ACM900 mic power supply (T160mA), I noticed the fuse holder wasn't inserted flush to the receptacle. When I took it out, the cause was immediately apparent - the prongs that hold the fuse were bent over to one side, and the fuse had been wedged in there to the side.

It's getting more apparent that we got what we paid for, but it's good we can correct most of it.