[ACMP investiupgradifications] All things PREAMP

[crazydoc]

I didn't check for oscillations before replacement, since I was on deadline to finalize the transformer winding voltages, so I will have to observe the problem on one of the other '81s here for repair/upgrade, since I have no desire to replace the new parts with the old. I have a couple of questions from those who have already tackled this problem:

1. Is the oscillation problem consistent and repeatable?
2. Is it a low frequency (motorboating-type) oscillation or a high frequency oscillation?
3. Anyone put a scope on it so I'm sure that I am identifying and solving the problem thoroughly and completely?

Steve

Here's a post I made on the big GB thread at HR a couple of months ago:

http://homerecording.com/bbs/showpost.php?p=3084168&postcount=7286

The oscillations were about 70MHz. There must have been some type of intermodulation/beating for this to be made audible of course. Swapping out the transistors as described resolved the problem. I think the residual noise that I describe afterwards with the eq sections engaged, was, in retrospect, the inductor resonances we're having so much trouble with.

I'll be very interested to hear of your degree of success in eliminating these resonances with the new power transformer, so that shielding might not be necessary.

 

[okgb]

mine had a higher freq broadband hum/buzz as soon as the eq switch was in
regardless of eq settings , once the q4/5 were changed that went away and
the more mains related [ or so sounding ] hum became apparent .
Alex's testing as mentioned in the thread showed this eliminated by removing
the inductor totally or replacing it

 

[Steve Hogan]

I forgot to mention that in addition to the magnetic shield band built into the new replacement transformer, I am also going to experiment with the effect of a copper band around the perimeter of the toroidal transformer.  In theory the thick copper band makes a shorted turn that intercepts only the stray flux and lowers it by loading it with a short.  I will report later on whether or not it makes any difference.

Toroidal power transformer stray field not reduced by copper band around circumference

I'm sure many of you have seen conventional EI power transformers with a big copper strap around the outside of the transformer.  That copper band outside of the core acts as a shorted turn which intercepts stray flux and reduces it by converting it to heat generated by the current flowing in the low-resistance turn.

I was thinking of trying the same thing on the toroidal power transformer in order to further reduce its flux emissions, but I realize that a copper band around the perimeter of the toroid is at right angles to the direction that a shorted turn must be oriented to pick up the unwanted flux.  So the copper band is effective with EI cores, but no help with toroids.  Like Edison, I will add that to my very large collection of ideas that I know don't work.

 

[Steve Hogan]

The oscillations were about 70MHz. There must have been some type of intermodulation/beating for this to be made audible of course. Swapping out the transistors as described resolved the problem.

Somehow I thought you might have exactly what I was looking for.  I read your linked post, looked at your scope photo, and now will check out the oscillation and its cure here.

Thanks!

 

[alexc]

>All those bugfixes aside, did anyone get a warm feeling already from the sound of these preamps ?

Definitely! They do sound really, really good. I've never used a an N1081 and so can't compare with the
real thing. But soundwise, they are up there with any of the diys I've done.

 

[clintrubber]

>All those bugfixes aside, did anyone get a warm feeling already from the sound of these preamps ?

Definitely! They do sound really, really good. I've never used a an N1081 and so can't compare with the
real thing. But soundwise, they are up there with any of the diys I've done.

Hi Alex,

Nice, since i recall the main concerns initially went to the quality/sound of the signal transformers used.
... concerns which have been pretty much taken over by the other magnetics (power-TX & EQ-coils).

Can't imagine they'll be spot on or even pretty close, but decently close would be nice.
Once that's completed, I'll compare them to a BA283-project with the groupbuy-Carnhill-iron for an indication of that.

Bye,

  Peter

 

[0dbfs]

Hi,

Does anyone know whether Max's RM8 inductors would be a drop-in?
http://www.groupdiy.com/index.php?topic=32475.0

Alex, Sorry, I don't recall now but did the Carnhil's fit or did you have to rework the connections and/or mounting?

Many Thanks,
jonathan

 

[okgb]

it looks like you can use the " white cup "  that the original inductor is in
from the chinese pre so that the pins will line up ,
whichever inductor you choose may or may not fit inside the cup , but
you may be able to cable tie it on and the cup may help for putting a shield around it

good luck , this is what i plan to do

 

[0dbfs]

it looks like you can use the " white cup "  that the original inductor is in
from the chinese pre so that the pins will line up ,
whichever inductor you choose may or may not fit inside the cup , but
you may be able to cable tie it on and the cup may help for putting a shield around it

good luck , this is what i plan to do

Cool. I haven't removed the stock inductors to inspect them at all yet.

Are you planning to use Max's or is there any news from Cinemag about a possible replacement?

Thanks,
jonathan

 

[okgb]

I'm waiting for Cinemag prototypes right and will report when
i install them .
I think  [ only my personal opinion ] they didn't want people
spending money if this would not solve the problem but
i can't see how it wouldn't be an improvement , the value
judgement however [ is it worth it ? ] is up to the individual

the carnhills have the advantage of already being inexisitence and used
for neve type circuits , the british pound is low but they are still a little more
than cinemag

Alex do you think your carnhills would fit in the white cup ?

 

[0dbfs]

I think they will most likely be an improvement. Apparently, removing the stock devices was an improvement though.

If they have mu-cans or something and the same or similar spec's then I'm sure they will be great.

Cheers,
jonathan

 

[alexc]

Max's RM8 will work electrically, but not as a drop in replacement.

You could use the stock inductor cup, but you need to watch the height -
to avoid ending up too tall for the 1RU case.

Mine with the OEP shield, no cup, just fit into the height.

Without trying the Cinemags, I can't know how they will work.
But if they are shielded, I can't see them *not* being a massive improvement on the stock.
Only one way to know for sure!

You can see the pics I posted for the OEP shield hack - need to insulate the inductor
with heatshrink to prevent any shorts to the can. I insulated the inside of the can with a piece of heatshrink and
then hot glued the inductor into the can.
I used short wires and then hot glue ;D to pcb.

There's enough pcb footprint space there, but I also insulated a couple of resistors there with hot glue
to ensure they don't short to the can.

Also, I ran grounding wires from the inductor core clips to can to star ground. May not be
necessary but I did it anyway.

With respect to changing audio transformers - there's no room in there. On the unit where I
replaced the psu with a JLM one, there is room to move things around and putting a Carnhill
output traffo (narrow form gapped), but the 2 inputs would need to be wired in -
the carnhills are too big to fit on the pcb. Some rearrangement of the output amp could free
up space on the left hand side to wire in Carnhill input traffos off the pcb. Not easy, though.

EQ aside, they do sound quite similar to my fabio n72s with Sowter iron. Definately the same
sort of 'crisp with a slight crunchiness' that makes discrete transistor + transformer different
from an IC based desk channel.

With EQ, there's massive bottom end available and as much high end as you could ever want.

I've been using these every night for 3 or 4 hours on guitars for the last week now
There's a ton of EQ possibilities there to explore.

I'll have these running thru a dual g1176 box shortly.
Thru the LA2A, I can't get over just how fab the combination is.

 

[clintrubber]

EQ aside, they do sound quite similar to my fabio n72s with Sowter iron. Definately the same
sort of 'crisp with a slight crunchiness' that makes discrete transistor + transformer different
from an IC based desk channel.

Hi,

That's still with the stock transformers ?

Bye,

  Peter

 

[alexc]

yep

 

[clintrubber]

yep

Nice, thanks.

 

[clintrubber]

[QC]I got a ACMP-81 that's missing the copper bolt for it's rightside
inductor (the lower-mid band).
I don't expect any influence on the coil-value, but sloppy assembly it is.

Missing bolts more common ? I saw a pic on the net of an ACMP-84 that had one
of the three missing as well, but still can't imagine this is anything deliberate.

Haven't removed PCBs yet, is the bolt essential for holding the coils in place ?
Didn't check yet if the coils are glued to the PCB as well, or maybe it's now
just the wires holding it in place...[/QC]

Bye,

  Peter

 

[alexc]

The bolt attaches the core to the holder (cup)
The core is also glued to the holder.
The winding leads are wrapped and soldered to holder pins
The holder pins are soldered to the pcb

The unbolted ones still look pretty strongly attached with the glue.
1 out of 4 had a bolt in mine. They unsolder pretty easy.

Other qc stuff - lighted switch not lighting, signal led not lighting

I found a  shorted solder on the Q switch led. Hairline solder short.
And one of the leds had a leg not soldered to the pcb

Glad to read you have finally received! Look forward to hearing your impressions.

 

[okgb]

plenty of the pcbs and xlr connectors had screws loose in mine
don't think i have any bolts but would that affect inductance ?

Another thing i mentioned was that the soldering on my inductors looked
different  [ more shiney  ] as if done at a different time .

likely a good idea to eventualy take out every board and inspect it
i still don't really use any of mine yet , too bitter [ and shitty ]

go  peter go

 

[0dbfs]

about half and half with my inductor bolts/screws... Makes me wonder. But then so do a lot of things...

I haven't noticed any loose connections although I've had mine apart and back together again a couple times so far... Also no degraded/damaged resistors here.

I do like the sound of the pre's in the shop but also will not put them into service (and critically compare to other pre-amps) until I get the inductor resonance issue sorted out along with some of the PSU issues.

Cheers,
j

 

[clintrubber]

Thanks Alex for the coil-info. Weird actually, sounds like they could have saved on all bolts then.
But in case there's a possibility to make the bolt go through the PCB (traces permitting) it'll have amore sturdy connection of course
- depending on which inductors will finally end up in these boxes I'll change that.

Thanks Greg, sounds like we all better inspect before powering on, will do that before I'll fire up my other one.

But about the issues with the oscillating transistors & inductors, if I'm not mistaken simply hitting
the EQ-bypass button takes them both away from the signal-path (unless they spoil the rest of
the stages by means of the common supply).
That leaves us with a box that has a pre-amp function that's now minus two troubles, so more usable until coils & replacement-transistors are in.

Bye,

  Peter