[ACMP investiupgradifications] All things PREAMP

[crazydoc]

I'm starting to wonder if I made an all mighty mistake getting involved here with a large order for friends... based on some internet statements which appear to be BS.

-T

Well, you know the rule: No good deed shall go unpunished. 

 

[TomWaterman]

Tell me about it!

 

[okgb]

I still think it was a stock design that they already had [ possibly rejected by someone else ]
Hey we want a english design preamp with inductor eqs ,
O.k. we make that for you
Can you change that ?
Yes we will change that
Can you fix that ?
Yes it will be o.k. thanks for payment we send them now thank you for business
we can make more again if you pay again

One possible advantage here is that modding things one at a time , will still leave you with a working
unit and be cheaper than building from scratch [ maybe ], depend who you are and what skill you have .

but there always was a vagueness and lack of concrete info , oh maybe this is the one Calmillion labs was working on , and what will the chinese do about warrenty & QC now ? fa i think
offer to sell a improved  , fixed version is my guess . I never expected it to be as good as a neve but did think they'd work
just not to a 2k perf level

but thanks again to the good people doing good work here

 

[maxwall]

TomWaterman,

To be fair here, Chance claimed all pres were heard by a tech and they were good to go before leaving china, and a batch of 1081s had been pulled as they didn't pass QC specs.

You took the words right out of my mouth.

I to am stuck in a similiar situation based on Chances statements and units on hand for friends.

Complete BS.

 

[audioforge]

"Every fatterer lives at the expense of the one who listens to him" .
the crow and the fox.
Jean de la fontaine 1668.....

nothing new under the sun...
audioforge

 

[alexc]

I think that a 'repair' kit is required, consisting of :

REPAIR KIT
=========

- 4x BC441 transistors
- 4x BC461 transistors
- 2x shielded inductors

would just about get this to a reasonable state with about 3hrs work, consisting of :

i)  open up the unit (top, back, side screws and remove front panel selector knobs/nuts)
ii)  for each of 4 EQ boards
    iii) remove boards (screws, unplug connectors, unplug bus)
    iv) de-solder Q4/Q5 and resolder correct parts
    v)  prepare shielded inductors and replace stock ones
    vi) replace board (screws, connectors,replug bus)
iii) modify resistor(s) in the gain selector switch to remove pop
iv) close up unit (ensure no push-switch binding)
v)  test

 

[jdbakker]

[...] star grounding the individual boards
[  cutting their ground bus traces ] shouldn't be risky
6 or 7 lines back to the input or psu ground point

The individual boards have unbalanced ground-referenced signal I/O. If you star ground each board, the current consumption fluctuations of each board multiplied by the inductance of your star grounding get added to the signal. Best case this injects both PSU noise and signal-related components (possibly overtones); worst case it turns the system into one big oscillator.

You need either ground-follows-signal (current situation) here, or a whole bunch of interstage transformers.

JDB.

 

[crazydoc]

I got rid of almost all the hum from that low freq inductor! Read all about it: http://homerecording.com/bbs/showthread.php?t=275506 posts 22 - 25

;D ;D

 

[alexc]

Another couple of analyser shots.

http://www.users.on.net/~alexc/res/acmp81_mid-lo_inductor_test.pdf   (208KB)


ACMP81 with Q4/Q5 mod, mid-lo inductor Stock and Carnhill
Note 50Hz harmonic at 200Hz down 15dBu in the Carnhill.

Based on other's comments, shielding Carnhill inductor should reduce maybe another 8dBu

I'm hoping to try a shielded approach soon.

PS - my dead EQ boards all had the Q4 BC461 pnp failure. Real mysterious. No shorting of power
or anything like that. Literally I was looking at the scope then dead. One after the other clocked off.
Oh well - back in business now.

PPS - I can confirm that the 'clicking' also occurs in my toroid+PSU replaced on-board unit.
so it's not that.

So I think that there is definitely a forward path here. High quality MuMetal seems to be easy to get in
a number of formats in the US. Maybe a group buy of some mu metal  or damn the torpedoes and go Sowter/Cinemag

 

[zmix]

Someone on the homerecording.com thread noticed that if you get the get the knob to sit in between the 8 and 9 o'clock detents it produces a "horrendous tone."  He also noted it doesn't happen when set to line-in.

When I get back from this trip I'll post photos of the inside of the 84 for comparison, since we have the insides of a 73 and an 81 posted.

Yep, I was just coming over here to ask about that. My two 84's do that. Here is what I typed elsewhere, it was addressed to Zmix (Chuck):

Hey Zmix, the gain knob on the 84 exhibits a weird issue at about the mid point where you can get the knob between detents and it will send a crazy loud tone to the output of the pre, pegs the needle, and all that. It only does it in mic mode with or without a mic attached. You seen anything weird there? If you blow through the detent it sounds more like a pop. Other than this issue, I have not seen any other oddities with the 84. Somone else mentioned this before too but I couldn't remember which of the 3 forums it might have been on.

Somone else had posted about this besides me as well, so it is more than an isolated incident.

So sorry for the late reply, I've been off this forum for the past few months!!!


This is a point on the switch where N£V£ put in an additional "OFF" position on the switch. This is where the additional preamp stage gets switched in..

7th position up from fully counterclockwise.

This ONLY happens when the mic/line switch is in the "mic" position, so you may want to switch to the "line" position when rotating the switch past this point.

The additional stage gives another 20dB of gain to the circuit, allowing the -80 position. It's a standard N£V£ circuit trick.

 

[okgb]

Good & bad report

Recieved two of my three parcels today ,
[ 2 73's , 2 amc - 100's , 1- amc 900
and 1 amc 200 [ subed for the 300 i ordered ]


One of the 73's seems good out of the box , the other has a slight broadband buzz
almost like flourescent lighting [ will do the xister mod ]

It's a little goofy the way they jammed the hi pass filter close to the
LM gain control so you think they're related at first because they're closer together than
the gain control is to the lm freq control .
on the good one there may be some  slight buzz & hash but it's under the system hiss
level , so seems like nothing good gain staging wouldn't get rid of .

 

[maxwall]

crazydoc,

the photo's posted over at your links only work for registered users on that forum.

Could you post links here that don't require a password and username just to see them ?

thanks

______________________________________________________________________

Also talked with Cinemag today. I will be shipping the 81 inductors down to them for analysis and recommendations on having new ones made up that should reject noise a lot better than these
China examples. If were successful in getting the right solution , then I will be offering a group buy  on quantity order of high quality inductors to solve this problem. I will have mu metal can sheilding option added as well to the design. First , some prototypes will be tested before production of the replacement inductors.

any comments or suggestions will be gladly taken from members at this time.

 

[alexc]

Hey Maxwall - that's a great initiative. I'm certain lots of 81 owners could
benefit from a good quality shielded unit with the correct taps.

I think the frequency points on these is a little all over the place.
Am still confirming certainly at least some don't seem right.

 

[crazydoc]

crazydoc,

the photo's posted over at your links only work for registered users on that forum.

Could you post links here that don't require a password and username just to see them ?

thanks

Sorry - the pics were uploaded to the hr.com site

I'll try to re-create the postings here.

________________________________________________________________


I've been able to do some work on the 81 this weekend.

I grounded each board a la dgatwood, including board 1. I also added a 22uF filter cap to board 2 (which was what I had on hand.) Though all their ground traces are now within 0.1 mV of chassis potential, I hear no difference in the hash noise from the preamp board and eq boards, or hum from that damned lo freq inductor on board 5.

Here's a pic after grounding, with Mumetal shields in place on inductors and PS transformer.

So I removed the inductor from board 5 to make it remote, to see if that diminished the hum. There was so much noise from the 3 feet of unshielded wiring I couldn't tell.

I then thought to try to reorient the inductor with respect to the PSU transformer. This helped a little, but what made a major difference was being able to extend the shielding on the lower part of the inductor, so it is now enclosed in a long cylinder. (I'd tried capping the tops of all the shields last week - it made no audible difference.)

So now I've got the major noise under control - Zmix's suggestions for reorienting the power transformer and swapping the transistors on the eq boards (I only did boards 4 and 5, and used different transistors), shielding the inductors, and now being able to extend the shield on that pesky low frequency one.

I've socketed the 1Q1 and 1Q2 transistors in anticipation of receiving a bunch of low noise transistors this week, to see if replacing these, or some on the the eq boards, will help resolve the remaining hash noise at high gain.

But I think what I have now is very usable.

 

[okgb]

Err , maybe you don't want to send Cinemag the chinese ones
[ or at least not to copy ] but send the specs of the sowter and
Carnhill

Yeah sorry of course i didn't think you were going to copy it ,
suppose it does make a good example of how not to make it ,
my point was , incorperate the good things of the carnhill & sowter

and yes , good energy Maxwell , we all shall surely benifit

 

[maxwall]

okgb

I'd be sending these to Cinemag to get the footprint copied and correct inductance points for the taps. And We might get to find out why these China inductors are nothing but crap compared to a quality one.

I was advised that if the China inductors are made using certain common pot core material then they will be noisy by design, and no way to clean them up unless replaced with something better.

Crazydoc
thanks for reposting the photo's .

 

[maxwall]

Talked with Cinemag again today......

Seems the Chinese recipe for good transformers is part of everyone's bad inductor experience so far. And the use of the Carnhill inductor already supports that evidence.

However , Is there any evidence to suggest that we are getting a oscillation problem when the inductor hits its saturation level at a particular frequency ?

Also, Is there any evidence to suggest that the power transformer is not being run without sufficient reserve current or power rating to prevent it from inducing more noise than a transformer being run with a conservative power reserve rating. I saying that if the transformer is run to its near limits or ratings it will be a noise generator as compared to one for example that is being run at half its power and current rating. hence giving lower noise or EMI to the mid upper and lower eq section of theis preamp.

 

[okgb]

My first impression on opening the lid , was that the power transformer looked a little puny , so that & chinese cheapness
i wouldn't bet on much current reserve / headroom

 

[maxwall]

Makes more sense now does'nt it., Greg

AlexC , whats your thoughts on this ?

 

[alexc]

On the question of the toroid running close to it's VA limit and so adding noise, I do not believe this is the case.
It is puny and possibly underpowered but I don't think it is at it's limits (in the Q4/Q5 modded case)

One of my units has a 30VA toroid and JLM PSU and it exhibits all the same noise. It is definitely no where near it's limits.

On the question of whether the the inductor is saturating and causing oscillation, I do not believe this is the case,
at least on noise floor tests, where there is no applied signal.

Running the unit at no signal , full Input Gain, all EQ bands on, max boost and full Output gain does not seem to change
the spectra of the noise, just adds more, as would be expected. It doesn't seem like the noise is 'unstable' or
jumping around, as I would expect if it were oscillating.

There is some oscillation but I don't think it is due to saturation.

Changing the frequency of operation away from the 50Hz harmonic region gives a steady decrease in the
noise - no sudden reductions which I would expect if it was oscillating at some frequencies and not others.