[ACMP investiupgradifications] All things PREAMP

[crazydoc]

I think there is something deeper going on.

I think you're right, and I think it's simple - he's been unable to fix the inductor hum in the eq section of these preamps (which is what made them unusable in the first place, after the eq buzz on the 81's was fixed with the transistor swap.) He assumed (probably correctly) it was due to the power supply transformer, and he thought that redesigning this would fix the problem, and it didn't. So he's been unable to return any fixed preamps, or a kit of parts that would fix them. He's spent the money we sent on parts and the manufacture of new transformers, and doesn't have much to show for it.

I have emails from him that tend to confirm this.

 

[okgb]

Hey Frank , which ones are going for 400.00 ?
from the same batch ? new ones ?
or ones that actually work ?

 

[TB-AV]

how much was he charging for the kits? If you don't mind me asking...

At this point in time, I would just like to say it was a reasonable and fair price imo. I'm not sure what direction I'm taking with respect to getting my money back and don't want to compromise my ability to do so should he think I'm passing along his private business info.

I figure after a year and half I've given him every opportunity to do the right thing. Like I said he doesn't have to fix anything just refund my money or drop some parts in a box and mail to me. We're talking 20 minutes time either way.

 

[maxwall]

okgb , selling for $400 for the un-modified 1084's -same batch, thats right !!!
they used to be $175, yes ? are they worth $400 , maybe
maybe not , I think $250 is fair considering they need to be
reworked. Now that the ACMP81 is reworked its now worth $1.5K USD
about the same as a BAE or Vintech re-issue , aye ?

Also, the inductor noise issue is fixed on the ACMP81 unit.
just , relocate the effected inductor. It needs to be moved
away from the power transformer. Shielding will not fix the
RFI problem , only the electromechanical proximity noise. Also,
a new inductor from Cinemag or Sowter will not fix the problem.
There is a very tidy way to relocate the inductor and mount it
securely in the chassis. I hear the Golden Age units have a
corrected gain switch circuit unlike these units.

I have measured the inductance values on these cheaply
made inductors , and the taps are quite surprisingly accurate
to within 5% of the values taken off the schematics.

 

[alexc]

>Shielding will not fix the RFI problem , only the electromechanical proximity noise.
>Also, a new inductor from Cinemag or Sowter will not fix the problem.

Don't quite agree with this. I didn't test relocating the inductors and so can't comment on the effect of this.

But I did do extensive testing on the ACMP81 and found that in my units,
replacing the mid-low inductor in-situ with a Carnhill part reduced the mains-harmonic noise substantially,
and shielding it reduced it further again to an acceptable level.

The mid-high inductor did not exhibit the same degree of mains-harmonic noise but
replacing with Carnhill part and shield also reduced the magnitude of the 100-1K noise somewhat.

My measurements on the stock inductors indicated the dcr of the taps was way out.

Additionally, I noted that replacing the mains transformer (with another cheap sino unit)
and psu made no appreciable difference to the mains-harmonic noise problem.
Nor did setting the transformer and psu external to the unit. Nor did re-earthing the unit
with heavier guage wire. Nor did increasing/adding psu rail capacitance.

I tested these conditions with the unit rack mounted in close proximity to other
rack units, such as compressors, mic pres etc, powered on and with the ACMP 81 lid on.

At the end of replacing the inductors with properly specified parts my units performed
and continue to do so, very well with respect to mains-harmonic noise.


-------------

I think there are probably several ways to fix this issue.

I have read reports of successes involving replacement+shielding inductors, shielding inductors and relocating inductors.

As well as reports of shielding psu traffos, replacing psu traffos, correcting cap networks, tweaking grounds ...

and then there's your sure-fire fixTM hyper-permeable, virgin-cold-rolled-iron, extra-flux, multi-$, super self-shielding toroids with redesigns large and small  from a certain deposit taking individual who was happy to spend man years writing forum tome-epics at the 'spruiking stage' but appears 'mia' when it comes to deliverables or even a simple HNY (#2) msg to his financial sponsors..

and last but not least, the 'return to base and we will surely, definately (eventually) fix it under warranty and return it right to you'.
'No really. We surely Will. c/o Emmett's Fixit Shop ' option.

I guess it comes down to 'different strokes for different folks'   or   'your mileage may vary'  or  'Still Waiting...'

or

"a mystery wrapped in a riddle inside an enigma"

 

[diyfanatic]

On my 1081, I've just made the transistor replacement and used some shielding sheet from don-audio.com on the power tx and the inductors.
worked pretty well for me, after that I've made no mesurement, I recorded some vocal demo with it and I felt that its very quiet and clean.
no hum, no buzz...

cheers.
Jr.

 

[maxwall]

Alexc

On the whole , you might find a Carnhill or Sowter to work for you, but
why spend the money if you don't need to, this was my point . In my case,
I did'nt find it necessary to do this after finding a suitable postion and huge drop in
noise with the simple relocation. So as far as the inductors not working at all , well
I failed to clarify my meaning. They will work , although I would not put them
back in the same postion on the EQ board. If you can make the original inductor
work by relocating it , why replace it. Unless the original inductor is completely
and utterly useless. In my findings,  I have not proved this to be the case.

The proximity of the power transformer to the inductors has been
one of the major problems of the ACMP81 design all along. Having a inductor
that is internally grounded will help , but inductors should not be
in close proximity to a power transformer from a simple design
principal- nothing new here. The most cost effective solution is to relocate the
inductors away from the power transformer as far as possible.
Additionally , I found that rotating the power transformer so the primaries
are facing away from the EQ boards was a significant move. Also , taking advantage
of angular positioning of the newly placed inductors helped too.

I found that the tall standoffs (2) , postioned on the eq boards showed further
ground noise improvement by isolating them from the top of the case. I was
discovering some ground loop issues in this area

Now the DCR of the inductors may be a topic of discussion , the actual
printed inductance taps on my units inductors measured very nicely. It
might be possible that there are variances in the quality from one unit
to the next. To this date, I have not seen any published DCR for these inductor
taps, and I cannot determine if the ACMP design tried to re-create the real
1081 values regarding DCR. It seems this is not the case and is no cause of surprise.


DIYfanatic,
Your solution makes sense since you dealt directly with the RFI radiating off
the power transformer which injected noise into the inductors. However, What was the
cost of those grain oriented sheets compared to relocating the stock inductors ?
The net result would be the same. It seems combining the sheets with a relocated inductor
might give even better results.


Lastly,
At the end of the day, how much money will you decide to throw at your ACMP before it is
not a ACMP anymore, might as well build the real thing.

My take is , If you can do a lot with a little , you 've done a lot.

 

[alexc]

I'm very glad that more solutions have been found and reported. 
Especially cost effective ones.

My moto is "whatever gets you thru the night..."

For my part, I still can't explain my observation that remoting the psu+toroid made no difference.
Logically I expected it to solve the problem. I'm talking 3 feet distant.

But I tested it a lot. I was not imagining it. I *wanted* to see improvement. But didn't.
Got quite desparing that I could ever get these to work right at that point.

I think that the rfi is from *any* nearby mains transformer not just the internal one.
The unit definately showed a sensitivity to nearby rack units, soldering irons and so on even after replacing and shielding.

I don't claim to be able to explain it all - it is not a simple area, rfi/noise/grounding etc, and defies analytical thought, imho.
I mean I have seen huge improvements by twisting a toroid 2cm or making seemingly insignificant ground wire changes.

And I have units with lots of windings close to quite strong rfi from mains traffos that are extremely quiet, so
I don't think it's a one size fits all thing.

I know I usually spend at least 1/3 of my build time again at then end of each project doing 'noise optimisation'.
Just reducing noise floor by tweaking traffo positions, toroid rotations, ground wires and so on.

It's all good.

 

[maxwall]

alexc,

Did I mention that my power transformer and power supply board have undergone
all the Steve Hogan changes, as well as all eq boards and amplifier boards. maybe this
can be attributted to my improved ACMP. I followed the whole gamet of updates
that okgb compiled and completed all of them. I'm certainly surprised to see that
when you moved the power transformer you did not expereince any RFI noise
reduction. When I performed this , I had instant results with a stock unit and no
Hogan updates. Further rework with the updates only improved the ACMP's
noise performance.

I'm also running on 118VAC mains power , you may be running on 220VAC
this might have some influence , not sure.

Have you performed all the Hogan updates compiled by okgb ?



If you look at the ACMP line your really have to agree the power transformer does
really belong in the same chassis. Also , with the reduction in RFI noise I have noticed the
ACMP has a higher than normal white noise level, not heard in more common made
preamps like 1272 for example. This may be due to the higher gain levels of the ACMP or
different PCB board layout and design issues. I can certainly say I do not like the power rails
that run on the back side of all the EQ boards passing near the power transformer, the power rails
have the jumper boards connecting each eq board then the amp board. This seems a likely place
for unwanted pickup of noise riding on the power bus.

 

[okgb]

I think we brought it up ?
but , that more people [ more ] problems
at N.A. 117 line voltage than people in
countries with 220 ?

 

[maxwall]

What could be the difference between my ACMP81 and Alexc ACMP81 with regards
to power transformer positioning besides our global addresses, seems unlikely to have different results , but
I might have to go down under(AUS) to find out ? Or Alexc your welcome over here in the USA,  our dollar is
very week right now , good time for vacationing and shopping for audio gear maybe.

Alexc , can you describe your the grounding at your mains outlet , or is it two prong , no safety ground ?

 

[okgb]

Yeah , Alex , looks like we need come over to your place
to investigate , let us know when the best weather is .
I anticipate a 2 week stay unless the beach is nice then 3 weeks

 

[crazydoc]

Did I mention that my power transformer and power supply board have undergone
all the Steve Hogan changes,

How did this happen? Specifically, how did you get a transformer from Steve?

 

[maxwall]

Crazydoc,

you had to ask. ok it goes like this....

I purchased it , waited 8 months, then I contacted the district attorney's office
after that, I went over there unannounced with three big Samoan dudes, surrounded the house
and said its time to hand over the parts. I must say, what a change of heart.
Reminded me of the Alamo in Texas , you know what happened there, right.
you can only hold out for so long.

Needless, to say, everything is back to normal now.

never mess with a mans DIY bits , ever !!!

 

[alexc]

You guys are keen  ;D

3 prongs - active, neutral and earth. Case is mandatory grounded.

US travel not likely for me. Too far from my little farmhouse.
You're welcome to visit here, tho!

Like I said - I can't explain it. And I'm not reopening these for love or money!
They work well, with a low noise floor and that's good enough for me.

I don't doubt the effects people have reported. My feeling is simply that
peoples issues and solution vary. Very small differences in scenario can have
large differences.

Anyway, don't want to over emphasize all this - just wanted to mention that
in my case, my solution does work.

And I didn't apply all the improvements that have been subsequently noted.
I'm sure they all help - with the hiss that is there, and the ringing and so on.

I'm glad to hear your units were, shall we say, "returned"  

"never mess with a mans DIY parts , ever !!!"

Amen to that!

Cheers

 

[crazydoc]

Crazydoc,

you had to ask. ok it goes like this....

I purchased it , waited 8 months, then I contacted the district attorney's office
after that, I went over there unannounced with three big Samoan dudes, surrounded the house
and said its time to hand over the parts. I must say, what a change of heart.
Reminded me of the Alamo in Texas , you know what happened there, right.
you can only hold out for so long.

Needless, to say, everything is back to normal now.

never mess with a mans DIY bits , ever !!!

Glad to hear it was successful.

I did contact the Fullerton Police Department a few months ago, and they said they would be glad to send a policeman with me to his house so we could sort this out, but I would have to do it in person, and unfortunately I live 600 miles away and have no plans to visit SoCal in the foreseeable future.

 

[stringofpoloponies]

glad to see the units got back . i spoke with steve a week ago and was told he is working on other projects . he told me to give him another week and i still didnt get a call from him as agreed to. im in the process of getting them back. the group buy has a free fix program so i guess im gonna get in line for that. if i ever get my units back.

 

[dustbro]

the group buy has a free fix program so i guess im gonna get in line for that.

Anyone have a link to this "free fix" thing?

 

[mitsos]

The "Repair Station?"  good luck with that. See the last three pages in this thread:

http://recforums.prosoundweb.com/index.php/mv/msg/16069/0/2944/0/

seems more like a circle jerk between terry, chance and now oliver archut... Terry posted back in September that the test units were fixed and undergoing "long burn-in period" to make sure the fixes worked.  Apparently they are still being burned in.  More recently, though, he started being an a-hole to those asking questions and making them feel as if he's doing them a favor.

 

[alexc]

That 'repair station' thread recent pic of the extraterrestrial with the acmp81 cracked me up.  ;D
Now *thats* Art.

Watch out for the alchemical, unobtainable but *dangerous* shields 'soon' to settle the nations.

Sounds like Excalibur.
"Listen - strange women, lying in ponds, distributing swords, is no basis for a government!"

I put my money on the magic toroids 'materialising' sooner.

Good luck indeed! (circa 2011)

Gimme a break.