[ACMP investiupgradifications] All things PREAMP

Another company worth looking into is Tab-Funkenwerk. http://www.tab-funkenwerk.com/id2.html
I think Terry and Chance have a foot in the door with them since they are making custom transformers for the ACM mics.

I'm not sure if what they are doing is making xfmrs for the pcbs that
they are making , that can be used with many mics including the TNC
Is it an exclusive ?

doesn't hurt to see what they say , so whoever has the best shot at getting a realistic price but they may be too busy with their own products [ the microphone stuff doesn't seem to be happening too quickly ]
why don't you propose the idea to chance

Sorry, another rookie question -- what's the best way to test for preamp noise? Is it valid to run the gain up to max with nothing plugged into the input? I've seen other posts that mentioned shorting two of the pins together; is this necessary/recommended? Can I just jam a paperclip into the female end of an XLR cable, or do I need a special shorting plug? Or should I have a quiet mic plugged in (in a quiet room)?

200 ohm resistor between pins 2 and 3 of the mic input then you can mess with the gain to see where noise is.

Okgb,

I spoke with David at Cinemag. Nice guy I've done business before , but maybe I'm not the right person to
do the negotiating on these inductors. No clout/pull with Cinemag yet.

Let me know how you do with it, and/or PM me otherwise.

Roddy,

Sowter would be ok if I was dealing in pounds ,but the exchage rate is never the selling point in USD.
Can's would be easy maybe cheaper, but your still left with a chinese made inductor so not sure about that.

Hey Maxwell , email me
but i believe we'll be o.k.
[ find a way  ]

Where is anybody getting the transisters ?

okgb said:

Where is anybody getting the transisters ?

Click to expand...

If you mean the Q4's and Q5's for the eq boards, I used 2n2222 and 2n2907's instead of the recommended ones - they work just fine, and are only about $0.50 apiece in TO-18 cans at Mouser.

I got the BC441 and BC461 (but without the -6 extension) at newark.com -- more than what crazydoc paid.

Hey guys  , what do you make of this
while investigating my ACMP 81 pre [ the eq section ] noises
i stumbled on a gain setting where all the noise
disappears , has this been reported before ?

it's on the gain control of the board 5 with the  220 -1,200 freqs
at max gain the noise changes and gets a little more nasty
but right before that point there is a little area where it gets quiet
but still passes signal , you can use this frequency band but stuck with that
[ or use the off position ] gain setting all other bands become o.k.
max gain cut produces a simular effect
but the noise doesn't completely go away . regardless of the freq setting or off pos .
Also the board 4 becomes a little less noisy  [ still unusable ] when i touch
the 3 conductor cable that goes to it's gain pot .

ideas?


Curiously enough [ and i believe coincidental ] some of the inductor boards
on the 81's have had loose screws , hmmm

Hey guys,

been travelling for a bit without much internet and not keeping up with this thread.. Glad (sort of) to see it's still going, and just wanted to mention that another source for inductors would be Altran. They are quite a bit more economical than Cinemag on trafos so I would guess inductors would be the same. Plus they are really good guys and try to help out. I've been meaning to call them lately so I could ask about this too, but someone would have to tell me what exactly to ask for cuz I'm quite clueless on inductors.

Thanks to everyone who's been helping out here.

The 1081 inductors should be :

8 pin package (5 connected), core material not specified
Shielded can

Mid-Hi :  inductance taps at  767mH,  504mH,  296mH, 205mH
            resistance at taps      105R,      57R,      46R,  24R

Mid-Lo : inductance taps at 5.11H,  3.56H, 2.06H, 1.43H
            resistance at taps  1036R,    804R,  611R,  500R

Source : Carnhill part # VTB9048, VTB9047 measured resistance
            DIY 1081 inductance specification
         
Physical package is :
circular outside diameter 28mm
circular pins arranged on diameter 26.2mm

Would need to send a sample to show physical arrangement.

Interesting observation re the gain setting.

This is with the transistor mod?

No  , totally as i recieved them
it's almost like you find a null point , but it
must have something to do with the resistence
value at that point , if you still have a stock one try it ,
Imean it's be a drag to lose a band but it does allow you to use
the eq

fwiw , the Carnhill input transformers are bigger than the chinese ones
and don't look like they fit on the PCB , but the 73 has enough space
on the back panel , if that isn't too close to the output xfmr

Yes - I remember before the transistor swap, I could set the gain on the two inductor boards at a point just above all the way down, and the hum (apparently that part of it from the board I was operating) would greatly diminish -- but then if I turned it all the way down the hum  came back some.

I made an mp3 of it - not that you'd want to listen to it - in the first post of this thread:

http://homerecording.com/bbs/showthread.php?t=275506

After the swap, of course, it became a moot point for me.

the transisitors totally cleaned up yours?

the hum / buzz totally goes away but for me it's at the just before MAX gain
[ or right as roddabod points out maybe only the symptom

just got them this week so , of course i will do the xister mods
just gotta get some
thanks Zmix

as i say i wouldn't trust them for a paying gig , but good enough for home

Hum/buzz disappearing at certain points doesn't necessarily mean that he oscillation has stopped, despite the hum/buzz not being noticeable.

okgb said:

the transisitors totally cleaned up yours?

Click to expand...

I have two 81s and two 73s --

On one of the 81s, replacing the transistors in the two inductor boards *and* shielding/rotating the toroid completely got rid of the hum -  the other had some board related hum even after the treatment of the inductor boards, so I did the transistor swap on the two other boards with a similar config, and then along with rotation/shielding of the toroid, the hum was completely gone (I went ahead and treated the other two boards on the first 81 for good measure).

On both 73s, a mere shielding/rotation of the torioid got rid of all the hum (which is good, because the transistors are different, and you can't do the same swap).

alexc said:

The 1081 inductors should be :
................
Would need to send a sample to show physical arrangement.

Click to expand...

Or maybe post a couple small pics??

Here are a couple of pics

- mechanical sketch
http://www.users.on.net/~alexc/res/inductor.jpg

- snaps
http://www.users.on.net/~alexc/res/inductor1.jpg
http://www.users.on.net/~alexc/res/inductor2.jpg
http://www.users.on.net/~alexc/res/inductor3.jpg

That would help to get started. I can send one of each of the mid-lo and mid-hi inductor.
They are completely useless to me   
(not quite true - you could make a very useful 'gauss meter' out of these to test
for radiated mains noise. They are exquisitely tuned for that)

(failing getting replacement made, I will use Carnhill + OEP cans (which seem to fit OK))

Mid-Lo and Mid-Hi have same pin-out (looking at bottom of inductor)

Starting at Pin 1 and tapping to Pin 5 (max Inductance, Max Resistance)
Pins 6,7,8 are not connected.

Inductance and Resistance restated :

Mid-Hi :  inductance taps at   767mH,  504mH,   296mH, 205mH
            resistance at taps        105R,       57R,       46R,   24R

Pin 1   - Start of tapped inductor
      2   - 205mH,  24ohms
      3   - 296mH,  46ohms
      4   - 504mH,  57ohms
      5   - 767mH,  105ohms and end of tapped inductor


Mid-Lo : inductance taps at 5.11H,   3.56H, 2.06H, 1.43H
            resistance at taps  1036R,    804R,  611R,   500R

Pin 1   - Start of tapped inductor
      2   - 1.43H,   500ohms
      3   - 2.06H,   611ohms
      4   - 3.56H,   804ohms
      5   - 5.11H,  1036ohms and end of tapped inductor

And a pin to the shield would be nice - to connect to ground.

I'm pretty sure that's correct, but always nice for a second set of eyes!

Hope this helps....

On my lo freq inductor, pins 7,8,and 1 were no connection, IIRC.