[ACMP investiupgradifications] All things PREAMP

[rodabod]

Hi Peter,

I'm looking into this just now, and trying to do some investigative work with what information we have so far. I will report back if I can figure anything out from the pictures, although my Neve knowledge is limited!

 

[plumsolly]

[quote author="reg presley"][quote author="rodabod"]This gets marginally interesting again:

http://homerecording.com/bbs/showthread.php?t=180964&page=196

That forum has a pretty bad s/n ratio though. In fact, it's awful. I think it's fairly obvious what's going on though (despite the number of people who seem to think otherwise).[/quote]

!!!!!ouch!!!!! makes for a painful read.[/quote]


http://www.theonion.com/content/news/local_idiot_to_post_comment_on

-Ben

 

[rodabod]

[quote author="plumsolly"]
http://www.theonion.com/content/news/local_idiot_to_post_comment_on
[/quote]

I love the Onion. So true.


Well, here are my (not particularly valuable) thoughts so far.

Having compared the Chinese '73 clone schematic from Alan Hyatt, it appears to be a direct copy from what I can see from a few minutes of comparing.

The '81 appears to have adopted the class-A output section from the BA283. I'm fine with that. Good.

Now to the rest of the circuit, it appears that they have not used the same component numbering for the '81 as they have with the '73 schematic. So thia does not help us here. I'm not sure about this suggestion of the ICs seen on the board being used as replacements for the BA338 and other amps. You can see that the transistors are laid out the same on each separate EQ board. In total, I count the correct number of transistors for each opamp in the original circuit.

What I'd like to know if anyone else knows off the top of their head is, what is the purpose of the two BA306 opamps in the original schematic?

There is a twelve transistor arrangment on the low-pass/high pass board in the Chinese '81 unit, and I'm wondering if these could possibly account for two BA306 circuits.

Right, that's enough mad speculation for the moment!

Roddy

 

[clintrubber]

[quote author="rodabod"]What I'd like to know if anyone else knows off the top of their head is, what is the purpose of the two BA306 opamps in the original schematic?

There is a twelve transistor arrangment on the low-pass/high pass board in the Chinese '81 unit, and I'm wondering if these could possibly account for two BA306 circuits.
[/quote]
Hi,
Little bit of RE for now, I checked the BA306-schematic (it's for instance in the 1081 manual here) and twelve BJTs is exactly right for the hi/lo-pass-section :thumb: :

each BA306 is a dual unity-gain buffer: 2 BJTs for a dual-output current source and 2*2 BJTs forming the actual buffer-stage (Sziklai-topol.)
So four buffers in total.

Bye,

Peter

 

[rodabod]

Not quite enough detail, but here is the '81 schematic courtesy of Chance, and enlarged, flipped and sharpened to get a little more detail:

http://www.sendspace.com/file/1iiv6n

I'm going to compare in a minute. Looks promising.

 

[ChuckD]

Nope still can't make out any values in this one either.

-Chuck

 

[rodabod]

Guys, there seems to be a noise issue with the '81 preamps:

http://recforums.prosoundweb.com/index.php/m/380587/13216/#msg_380586

Can anyone offer any help? Perhaps Chance could ship to a buyer who has some background with 1081 channel strips. I'd be happy to look over one myself, but there are many people here who have built and serviced N*eve gear who would be more efficient.

Roddy

 

[clintrubber]

[quote author="rodabod"]http://recforums.prosoundweb.com/index.php/m/380587/13216/#msg_380586[/quote]
Hmm, curious to this but the link won't give anything else than a blank page... problem at my side ?
Either way, thanks for spotting Roddy.

Bye,

Peter

 

[MATTI]

The link is working again

Matti

 

[clintrubber]

[quote author="MATTI"]The link is working again

Matti[/quote]
Thanks, works here as well now.

Let's discern between 'noise' (hiss) & 'hum' (AC-induced-nasties), as I understood now it's the latter.

Cheers all,

Peter

 

[okgb]

it looks like there is room to [ drill a hole ] move the xfmr an inch or so,
although may make no difference , anyone try turning the xfmr ?
Or as mentioned [ and never a bad idea ] shielding the inductors ?

we all know what they cost and where they came from , at worst
could convert to external xfmr / supplies

 

[CHANCE]

I contacted China and they can't duplicate the buzz we are getting (that figures) They sent me a very detailed schem in a "RAR" file that has many PDF's in it. I'd be glad to share it with anyone.

 

[ChuckD]

Chance ,

Chuck here, we met on the weekend when I picked up some mics. Please send me the schematics for the 1081 to cdickinson66 (at) hotmail (dot) com
I built a stereo 1081 as I mentioned before as well as a stereo 1073 preamp. Maybe I could have a look at the design in detail.

-Chuck

 

[CHANCE]

[quote author="ChuckD"]Chance ,

Chuck here, we met on the weekend when I picked up some mics. Please send me the schematics for the 1081 to cdickinson66 (at) hotmail (dot) com
I built a stereo 1081 as I mentioned before as well as a stereo 1073 preamp. Maybe I could have a look at the design in detail.

-Chuck[/quote]

Thanks Chuck. I'll send it now


I'm going to the wearhouse tomorrow
If anyone is POSITIVELY coming over on Sunday for your gear PM me by noon Friday and I'll bring your gear back with me.

 

[rodabod]

Here are the schematics which I have converted to JPEG:

"Amp"

"Filter"

"Out Power"

"Upper"

"Bass"

"Low"

"Treble"

 

[tommypiper]

Roddy, thanks. But I can barely read the values at that scan size, at least on my screen. I've asked Chuck for a copy of the PDFs. In meantime, any chance you can post them with higher line screen?

cheers.

 

[rodabod]

Ah. I've got a large screen so overlooked that. I could double the resolution if that helps.

Roddy

 

[Svart]

hum can be caused by a number of reasons, mainly poor decoupling and/or poor bulk capacitance. It can also be caused by poor grounding or large currents flowing over ground.

If anyone wants to donate a preamp to me, I'll figure out what's wrong..

:green:

 

[ChuckD]

Looking at the schematics now. Not much time spent on them yet.

But I can say ... The EQ section is a close 1081 EQ copy.


They have copied the B338 correctly it seems however they are using BC557 and BC547 for the constant current emitter follower output stage. Also they are using 1N4148 diodes instead of the correct ones BAX13 and the AA144

I don't like all that LED junk in there! 50% of the components are to support the different LEDs. You don't need that crap in there. Plus it is tapping off the output signal in each stage I think noise can get injected there easy.

Has anyone gotten the center frequency of the hum? 120 or 60 maybe?


More to come later

-Chuck

 

[MATTI]

http://www.sendspace.com/file/osjykd
81
hope this helps

Matti

P.S. didn´t notice these were already in use ;-)