[ACMP investiupgradifications] All things PREAMP

[0dbfs]

I did earth the kitchen sink...
Same results..

This is with the new transistors...

Move the toroid out of the box 6 inches or so and the source of the field is far enough away to not be picked up by the inductors.

Cheers,
jonathan

 

[playboss]

what about an 1U case under the 2x 1U eq , holding both supplies? Someone was about to offer expansion cables no?
That 1U case might house some ledmeters ( I sell  ;D )

 

[wmtunate]

I wonder if one external PSU (or just a torroid for that matter) is enough to power two 81s?  If I can get away with powering both of mine (whenever they arrive  :) from one external supply/transformer, that would be ideal.

 

[okgb]

Someone gave some current figures and it [ the transformer looked low ]
and knowing the chinese figure on underpowered as a cost saver

 

[clintrubber]

I wonder if one external PSU (or just a torroid for that matter) is enough to power two 81s?  If I can get away with powering both of mine (whenever they arrive  :) from one external supply/transformer, that would be ideal.

Might be doable, very doable. But why not solve the issue completely while at it ?
You might otherwise end up with potential ground-loops, at least additional thinking about it required.
And you won't be able to use those two units far away from each other. You might save a few tens of your fav. currency but introduce things you need to be aware of at the same time (but if you'll always use these in pairs then it'll be less of a potential hassle)

Bye,

  Peter

 

[okgb]

I think he was asking  [ or please clarify ]
if one of the stock transformer had enough current
to power two units

 

[clintrubber]

Either way, if wmtunate uses the original Chinese TXs or a golden one with endless power and signed by Mr. Rupert himself, he'll introduce the issues of one supply box that's common to both boxes. That one-box supply is obviously exactly what he asks, but perhaps the consequences I mentioned weren't immediately clear ?

Related: if the orig Chinese TX 'just' disturbs but is of adequate capacity then you could of course put both in one box, to supply two preamps. You could even connect them in || if you want (I'd keep them isolated, both electrically as physically but that's just my preference).

But hey, no boxes here yet, not even humming ones, so what am I talking about ?!  8)

 

[rodabod]

I may try a different trafo with a shielding can. I'd need suitable voltages though...

I was thinking a JLM 48V PSU might be an option too.

 

[okgb]

You haven't had too much of that strong Euro coffee today have you ?

Yes the grounding is the big issue , and there may be times when
you want to use the units separately  , otherwise one A.C. feeding
multiple bridges is not out of the question .

I got two greens yesterday , so hope your next

 

[wmtunate]

Now that I look again at the torroid's current specs, it does seem pretty low.  I was just curious if anybody had approached the external PSU from that standpoint yet.  I know the Chandler power supply will run two different pieces of gear at the same time.  It was just a thought that occurred to me.  Saves a little bit of space behind my rack.

 

[b3groover]

Chance did mention on the PSW thread that he would honor and deal with warranty issues after shipping is complete. I haven't heard anything else yet.

Yes, he did.  But when one of my ACMP-73 arrived non-functional (ie, not passing audio) and I emailed to ask about a replacement, he replied that I should send my unit back to China.  That's not standing behind the product, imo.

 

[okgb]

Clint has been asking how far we are in the process , but
it's not all shipped yet
My prediction is that the chinese will offer chance a
new improved model which will be a new sale

 

[clintrubber]

You haven't had too much of that strong Euro coffee today have you ?

Tokkin' to-mie ?  If it was about my previous entry sounding a bit harsh perhaps then ignore;
I never post sarcastic stuff.

Yes the grounding is the big issue , and there may be times when
you want to use the units separately  , otherwise one A.C. feeding
multiple bridges is not out of the question .

Might very well work indeed, but under weird circumstances some weird consequences are not to be excluded
(the units are DC-linked). Again, while at it I myself wouldn't cut the corner of saving one secondary winding,
but it's not up to me how everybody should make his/her boxes, I'm just suggesting.

I got two greens yesterday , so hope your next

Hey, congratulations, I'm happy for you (not in jest) ! Enjoy !

Bye,

  Peter

 

[okgb]

IF the chineve xfmr COULD supply enough current for two pres ,
that may be slightly convenient but having the second psu xfmr
as a backup would be wise , otherwise one psu going down
could put two units out of commision , so make two anyway .

Hey Clint next are you going to tell you don't smoke those
Stinky Euro cigs either 

 

[wmtunate]

If it was about my previous entry sounding a bit harsh perhaps then ignore;
I never post sarcastic stuff.

"It's off the charts!"

 

[clintrubber]

 

[antichef]

I chose to test it out by replacing a single 680R resistor (R43) with a 1.5k, disconnecting one end of that resistor and jumpering it to the position between R42 and R43.

This pretty much replaces the #7 position sensitivity setting with OFF and keeps the original remaining 4 higher gain settings.

You could do this with a different value (replace R43 closer to 620 ohms) and replace R42, 41, and 40 with different values to make the gain steps more uniform up to the original full gain if you want.

Thanks!!! and that goes out to 0dbfs and everyone who's been posting here.  I've been following this thread with interest and trying not to post, due to my chronic ignorance condition, but after enduring that pop as many times as I now have, I'm compelled to ask:

1) by "jumpering it to the position between R42 and R43" do you mean taking the lead of the resistor that's farther away from the switch (I'll call it the "far lead") and soldering it to the same place as the far lead of R42, so that R43 runs diagonally away from the switch, with the near lead soldered to the original pad, and the far lead soldered to the same pad as the far lead of R42 (or to the far lead of R42 itself, assuming there's not room on/in the pad for both leads)?

2) Since different values for R43 will serve in predictably different ways, if I'm not worried about elegance or uniformity, would it work to merely re-wire the existing resistor at R43 into the new configuration, instead of replacing it?

 

[alexc]

Received the first ACMP81 here is Oz. Another one to come and a ACM1200 mic still.

The shipping, which seemed to be Fedex, is obviously split up into 3 boxes.
Which is a good thing, because the value for customs was around USD 400 for the one pre. (much inflated)
Shipping ended up being around 70% of purchase price USD to Oz.

Anyway - the first ACMP81 out of the box - like others, I get :

- rack on  outside looks quite solid. I had to JAM it into my rack though - MUCH wider than normal.

- preamp section is very quiet. I mean *really* quiet at <-90dBu. This is with input and output connected to my Motu
  2408MkII balanced in/out, input gain at unity, output gain at unity and signal at -4.5dBFS on Cubase level meters.
  A 'normal' working level for me.

- EQ is a real stinker   
  Adds between 30dBu of 50Hz harmonics when all bands flat and eq engaged
  Lo/Hi pass - no effect on noise
  Lo and Lo-Mid add about additional 10dBu of noise  each
  Hi-Mid and Hi add about 15 dBu each.

- Mic switch engaged st the '-80' marking creates a Full Scale blast of noise - ear/speaker popper for sure!

Fix Mode ON
- rework the input selector
- change the eq transistors
- play with power grounding for kicks, then replace with external PS : toroid, JLM PSU and dc connect to preamp
FIX Mode OFF

Should be OK then.

Good thing we live to DIY around here.
Wouldn't want to have bought 8 of these and not be ready to do significant work! 

PS - makes me feel better about the stuff I throw together!  Quieter than this, for sure 
      I'll eventually be able to compare with the 2 81's I'm making from Tommy's boards with Carnhills

 

[rodabod]

That's great Alex. Thanks for posting.

How did you go about converting to use the JLM PSU - did you use the 48V model?

 

[okgb]

Hey Alex are you 50 or 60 hz line fequency in Oz ?
curious if this is general line contamination and
perhaps grounding issues