1272 LINE INPUT ?

Hi

would appreciate if someone would check this up. I've merged a few schematics together, , to achive what I thought might be useful for me, and hopefully someone else. I'm not sure about few things though (the line input in whole, mic/line select switch),Also could I use 1 pole/bank/deck 8 position switch for the line input or for some reason, 2 deck/bank sw, is needed?
Thanks
Updated 05.05.2006 with a kind help from rascalseven.I hope it looks better now.I'll do a better schemo once i'm sure that is all o.k.Sorry for the drawing it is still a "demo".

HI resolution for download: http://geocities.com/bvebyz/img/MicInLineIn5.jpg

I haven't looked over the switch yet, but i do see few things to address in the amplifier section:

1) the 1000uF/35 from the 1.5k resistor on pin K to ground should be 100uF
2) swap the caps on pins N and M, that is M should have thte 4700uF and N should have the 1000uF
3) Make sure the 12 ohm resistor that couples the 24 volts to pin M is a 1 watt resistor
4) I see no indication of shielding on the drawing, though you may very well intend to do this. Make sure any wire carrying an audio signal should be shielded to audio ground. This includes leads from the input transformers to the cards, the connection between pins P and L (preamp stage to output stage), and the leads from pins M and B to the output transformer and to the output from the transformer. Shielding is critical for proper noise performance.

Also, make sure that you connect audio ground (pin V) to chassis ground at only one spot, otherwise you're going to have noise issues. The power supply ground should be connected to audio ground, not to the chassis. The transformer cores should be connected to the chassis as shown in the drawing, not to audio ground.

JC

Hopefully I corrected the mistakes from the previous version. Schematics above. I plan to fit as many components on the pcb, (including the psu) as I can, and on that point I will see about shielding. Thank you rascalseven for pointing me in to that direction I forgot about it. Thank you very much on other corrections too,,,.
1.Any ideas on the “line in” switched attenuator
2.How about mic/line switch? Might introduce clicks when switching on that point in circuit?

this makes it pretty easy-

http://www.glideonfade.com/electronics.htm

dave

It is a great solution, however I am trying to do something a bit different...
I know about other great swch. kits and they are all fine, but I'd love to have separate atten. for mic and line, and also would like to have FUN building it (hopefully will learn something along the way).cheers :grin:

I must admit in all my building of Neve preamps, I've never actually built the proper line input as used in the 1073, etc... I'm actually about to do just this to upgrade a pair of mic input-only modules that I currently have, so this is just the exercise I need to get ready.

I looked over the 1073 drawing and have a couple of comments regarding yours:

1) The line input position of the "Mic/Line" switch should have no connection whatsoever to pin T on the card (right now it is connected to what looks like the pole of the line switch (at the bottom in the drawing). Lose this connection.

2) the top of the line switch in the drawing (at 'R24', the 27 ohm resistor) should be the only pole of this switch. A 1x12 Lorlin switch, shorting (also referred to as 'MBB', Make Before Break), would be perfect for the line input gain (assuming you're using separate mic and line gain switches as the drawing shows). This switch can be set for only 8 positions. Perfect for this application.

3) You have connected R7 (430 ohms) at the bottom of the line switch to audio ground, which should not happen. Instead, the connection to audio ground should be made at the point where R19 (220 ohms) R9 (33 ohms) and R1 (2k2) connect. By connecting R7 to ground, as you currently have drawn, you completely short the signal from the line input transformer directly to ground = no output. The ground connection should be on the OTHER SIDE of R1 (2k2) for the circuit to function properly. If I were building this drawing I would simply connect the junction of R19, R9 and R1 to the audio ground of the mic gain switch (assuming the switches will be side-by-side on your front panel) and then just use one lead to connect both gain switches to audio ground on the card.

4) The shields you've drawn (I believe that's what the big black dots that connect to the audio ground are, anyway) are connected to pin V at the bottom of the line input transformer on your drawing. I realize the shield for this transformer's primaries are connected to pin V, but on an actual Neve module, pin V is the chassis connection (look at the mic input transformer on a 1073 drawing, and you will see the core of the transformer is connected to the chassis AND pin V). I realize you are not working with an actual Neve module here, so this chassis connection is not established, but I just thought this might be worth mentioning assuming you also refer to Neve drawings and wind up connecting the transformer cores to this as well, in which case you will wind up with considerable noise issues, particularly at higher gains.

This is something you absolutely must watch out for when building Neve stuff. Using the chassis for a shield connection is fine, so long as the other end of that shield doesn't connect to the audio ground ("B-" on Neve schematics). They shielded the input of the line transformer and the outputs of the output transformers to chassis (again, looking at the 1073 schematic), but notice that there is no connection of these shields to B-/audio ground anywhere in the module. In Neve consoles the connection of audio ground/B- to chassis ground/Earth was only made at the power supply.

5) remember to shield the send from pin P to the 5k audio taper pot as well as the send from the pot's wiper to pin L.

6) I know it's just the way your drawing makes it look, but just to be sure, the mic input connection to pins U/S on the card should be from the first pole of the mic gain switch. Right now it looks as if it is connected to that pole's shield. Like I say, I'm sure it is just the drawing, but make sure it is connected to the pole, and be sure also that this lead IS shielded to ground somewhere. This is a very low-level signal, so the shield is critical here.

I think that should just about do it for you. Please let us know how it turns out.

:thumb:

JC

Edit - oh yeah, as for clicks, I don't think you will experience anything significant when switching from mic to line inputs and back. Remember to use 'shorting' (MBB) switches for your two gain controls.

Also, I just realized that you don't need the "OFF" position on the line input. This was to avoid pops, I believe, when switching from mic to line inputs on a single, shorting, gain switch like Neve used in their channel amps. You are doing this by way of a separate mic/line switch (most likely non-shorting), so this isn't necessary. You can leave this position out and go with just the 7 line input gain positions.

Schem. updated above not checked

Thank you very much, i understand the circuit now much better although it is still a big :roll: to me. Thanks for the great info on shielding. Some things i am not sure of again:
1.What is T.P. on 1073 ( mic in/line out transformer) schematic?
2.Does phantom look ok?
3.Pin T no connection at all? I am looking for 50dB of gain.
5.Connect only one end of the shield to the audio GND.?
6.Atten.& Switch in/outs now ok?

I will post on results for sure.

Thanks :roll:

Answers:

1) I don't know what 'TP' means. This portion of the circuit is the impedance switch on the back of the 1073 (switches between 300 and 1200 ohm input impedance for the mic input)

2) I would use a 6.81k resistor at pin 2 and another at pin 3 of the input XLR instead. Just connect the remaining ends together an apply phantom there. That willl work regardless of which input impedance you have selected. What you have drawn currently will work with the 1200 ohm position, though I'm not sure about the other. The dual 6.81k's will work with both, I'm sure.

3) Pin T is necessary for the MIC gain portion the way you had previously drawn. My comment for no connection was only for the LINE gain. In other words, based on your previous drawing, you simply needed to remove the 'T' connection between the mic/line switch and the line gain switch. The mic version was fine, and indeed is necessary to achieve 50dB gain for the mic circuit.

4) (you don't have a question 4)

5) You can connect both ends of the shield to audio ground if you want. My comment about only one side being used is twofold: 1) it's less work, since both sides don't need to connect and 2) connecting on only one side will ensure that you won't accidentally connect one end to audio ground/B- and the other to chassis/Earth. The connection between these two 'grounds' should be made once only, preferably on the other side of the power supply as close to the AC mains input as possible. Having two sides of one shield connecting the two as well will create noise hassles like you wouldn't believe.

6) You need to put the mic/line switch back the way it was in the previous drawing (two-pole) with the only exception being to eliminate the 'T' link between this switch and the LINE gain switch.... that is, leave the connection of pin T to the mic/line switch intact, just don't connect the pole that would send this to the line gain switch. Leave it for the Mic section, however. Also you need to go back to the previous mic gain switch drawing as well... it was correct, including it's usage of pin T.

JC

Rascalseven thank you very much. I think I'll go JLM hotrod+line in. Shielding and GND. like on EH10023. Now I'm waiting for the parts to come in...
Updated the drawing.
:grin:

[quote author="syn"]Updated the drawing.
:grin:[/quote]

Your file was so big.
It might be very useful.
But now it is gone.

:roll:

It's close, but there are still a couple of things to address:

1) you need to move your ground connection on the line input section from the bottom of R7 to the left side of R9. If you leave it where you have it, you'll get no signal, because it is shorted straight to audio ground

2) you currently are only using one pole of the two-pole mic/line switch. You need to wire the second pole in the middle of the connection of SW1b and Pin T. In the current drawing the gain of the preamp stage is still dependent on the position of the mic gain switch even when you select the line input (SW1b affects the actual gain of the preamp stage).

Attach the lead from SW1b to the "mic" position of the second pole (the top pole on your drawing) and connect a lead from the actuator of that pole to Pin T, just like you did from the actuator of the other pole to Pin U. This way, when you select the line input, you will deselect gain control of the preamp stage (by breaking the connection between SW1b and Pin T).

There will be no connection for the "line" position of this pole.

3) This one I hesitate to share, because I'm sure I"m stating the obvious... however, it's probably just the drawing, but your drawing shows the four contacts on the left side of the mic/line switch drawing connected together. Do not connect these together. Each of the four should be completely independent connections.

If you would like, I have a PDF that I made from your drawing that shows these corrections. If you PM me your email address I'll forward it to you.

Peace,

JC

It took me a while to update, (top of the page), hope to get into the wires soon. I think, thanks to rascalseven, this should be O.K. now. This is a basic concept and probably could be applied to any 283 family based circuit. Thanks to JLM for his hot rod schematic and Duka for 1073 schemo.
:grin:

[quote author="syn"]It took me a while to update, (top of the page), hope to get into the wires soon. I think, thanks to rascalseven, this should be O.K. now. This is a basic concept and probably could be applied to any 283 family based circuit. Thanks to JLM for his hot rod schematic.
:grin:[/quote]

Anybody else able to see the update ? Here it's not working I'm afraid.

Thanks

Excuses to all, who could not see the image from IE and Ffox, Opera worked though.I added the download link for hi reso image. :green:

2) swap the caps on pins N and M, that is M should have thte 4700uF and N should have the 1000uF

Click to expand...

The hotrod circuit is right. 470uF on N and 1000uF on M like in a normal Neve 1272 and 1073 modules.

Also the 12k across the secondary of the mic input transformer needs to be removed. As well as the PAD needs to be changed to a different design as it has changed the 1200ohm input impedance to 600ohm due to its extra load all the time.

You do realise that you can have a seperate mic & line inputs and level controls with only 1 input transformer.