BA283 and BA284

Hello guys, I am trying to build a 1073 based on the original vintage design and I found a pdf drawing that states it is accurate. However in the drawing, I found BA283 and BA284 boards. What is the difference? Which one should I make? Or are they both needed?
Sorry for being a noob but I really need your help. Also in the pdf file it is mentioned that "Presence switch" and two other sitches are missing. What are these? are they like buttons on the preamp or what?
Again I apologize for my noobness and your help would be greatly appreciated.

Sam,

Hey Adam! Thank you for your reply

That really helped me.

Just to understand better, what do you mean by BA284 being a make up?


Peace

80hinhiding said:

Hi Sam,

A few important documents for me at least have been:
http://www.technicalaudio.com/neve/neve_pdf/1073-fullpak.pdf

Page 2 is an overall look at the schematic.  Page 6 shows the BA283 circuits, two of them.  Put your mic input transformer before the first circuit. 

Another look at it is this: http://auroraaudio.net/wp-content/uploads/2011/06/1272-s.jpg

And from the forum here:
https://groupdiy.com/index.php?topic=6576.0

I've only been building the BA283 output amplifier section in certain cases, and in other cases, built the full BA283 AM and AV portions.  The full 1073 strip includes an elaborate inductor based EQ and presence filter, with stepped controls for both and it sounds nice, but I didn't get into building that yet.

The output amp alone (second circuit on Page 6) has a great sound in my opinion.  You could build a cheap op amp based mic preamp then hit that Neve line amp to get a certain sound.  Just an idea.

Others are much more experienced here than me, but just figured I'd share a few things that I've found helpful.  Oh the BA284 sections, those are make up gain circuits for the EQ and Presence, as far as I know.  I haven't needed them for what I'm doing so far.. but they'd be needed if you're doing a true to the 1073 strip.

Adam

Click to expand...

Hey there, I know it has been a while since I posted this thread. I have a few more questions though.  As you said, page 6 has two BA283 circuits. What are their differences? And which one should I use? also you said to put my microphone input transformer before the first circuit. Which of the two circuits on page 6 are you referring t0?

The block diagram is probably a good place to start.  The 1073 is basically a series of similar, three transistor fixed gain blocks, each with their own function.

The first amplifier block brings the mic signal up. It's only used when you need very high gain. Otherwise, it's bypassed.
The second amplifier block accepts a now-higher mic signal or a line input signal.
The sensitivity switch controls resistor pads before the second amplifier block,  and a little bit of gain control of the second amplifier, to get the desired gain.
The  third and fourth are part of the EQ.
The fifth is a line amp.

The signal flow is something like:

- Mic input transformer
- Sensitivity / gain switch part 1
- BA284 amplifier #1
((- Line input transformer))
- Sensitivity / gain switch part 2
- BA283 amplifier #1
- Low and high shelf EQ using BA284 amplifier #2
- Mid EQ using BA284 amplifier #3
-High pass filter
-BA283  amplifier #2
- Line output transformer


When you're talking about filters you're using something that has a different impedance at different frequencies. One way to filter is by connecting the filter to ground - when it has very high impedance, those frequencies pass it by. Frequencies where the filter has low impedance pass easily to ground and are subtracted from the rest.

If you can imagine taking this same filter and connecting the input of it to the signal coming in,  the center of it to the input of an amplifier, and another part to the output of the amplifier, using a potentiometer to control this connection, you are now thinking of one way to make an active filter. By moving the potentiometer, you adjust the balance of the filter that is between the output and input of the gain stage. Anything connected between the output and input is feedback.  By controlling the feedback, you control how much of the filter is subtracted or added, net, to the signal.

This loosely is how the 1073 EQ works. The BA284s by themselves have fixed gain, around 50 dB. This gain is used for feedback, with the filters, to change the frequency response of the signal.