Help on Neve HPF

[SaMpLeGoD]

Hi all, I'm building an Adrianse's EZ1290 preamp, and I want to add a HPF to it... I saw a lot of schematics, even the BA182C Neve's board. Is there any way to add that to the EZ1290? if yes, how? someone has some alternative for that? I really need add the HPF to this project.
Thanks a lot guys!

Eddie.

 

[briomusic]

a schematic i once saw had a capacitor in between the 'L' and 'P' nodes on the EZ1290 board. (this is the spot where an optional volume fader could be inserted too). you could try different values, maybe start with a 220nF. (good quality poly cap as this is smack in the middle of your audiopath.) to switch the hpf off, just short the cap (i.e. connect L and P).

 

[bruno2000]

To add a Neve style HPF, you'll need an inductor and 2 caps.
Best,
Bruno2000

 

[SaMpLeGoD]

Hey thanks for the info! I would like to have a switchable HPF in 2 or 3 different frequencys, like the original Neve... i thought to make an BA182C with the rotary switch, an inductor from audiomaintenance with several taps, some caps and that's it... my question is where to spot that board! so, are you guys telling me that I can use that pads where the optional output pot are inserted? I just want to avoid any impedance issues... thanks a lot for replying!
All the best!

Eddie

 

[briomusic]

As far as I am aware, between the L and P pads is where the eq is inserted in the original neve modules, so yes, that would be the best spot. a file called "1073_1084_User_manual_issue5" has all the relevant schematics and block diagrams. If you can't find it I'll email it to you if you pm me your email address.

 

[SaMpLeGoD]

Ok! thanks a lot for the help!!!! I already downloaded it and will try to figure out how to insert it!
Best wishes!

Ed.

 

[MagnetoSound]

As far as I am aware, between the L and P pads is where the eq is inserted in the original neve modules, so yes, that would be the best spot.

HPF is the last stage of EQ (B182), and is placed in series with the whole chain as a tuned shunt before the EQ in/out switch.

 

[kambo]

why not place a basic HPF right in the front before the whole pre_amp....
whats the point of letting low frequency run all the way through to pin L-P anyway ???

 

[briomusic]

why not place a basic HPF right in the front before the whole pre_amp....
whats the point of letting low frequency run all the way through anyway ???

I don't have a scientific answer to this question, but my half-educated guess is that between the two amp stages is considered a good, nicely buffered spot. Right after the input transformer the signal is attenuated with the gain switch, I would suspect that a capacitor shunt to ground would interact with the selected resistors in that gain switch?

What I would be curious to know is, if either a inductor-shunt-type or cap-in-series HPF would interact with the output level fader, if one was fitted?

 

[kambo]

if that was the case, neve would warn people; not to use hpf on mic, but use their mic_pre hpf  ;D

i havent heard any scientific explanation why hpf is usually placed somewhere towards the output, either ...

 

[MagnetoSound]

why not place a basic HPF right in the front before the whole pre_amp....

Difficult to implement a variable corner point with constant Q (esp. given unknown source impedance)?

What I would be curious to know is, if either a inductor-shunt-type or cap-in-series HPF would interact with the output level fader, if one was fitted?

Assuming, say, a 10k fader with a buffer impedance on the wiper of, say 5 or 6 times that (B183), I would doubt it.

 

[abbey road d enfer]

i havent heard any scientific explanation why hpf is usually placed somewhere towards the output, either ...

Probably because you haven't listened enough. This subject is a basic in filter design.
Inserting an HPF in the direct microphone input path creates loading problems on the source and noise problems since the active part sees a degraded impedance. Also because the source impedance is somewhat uncontrollable (microphone have impedances that can span 50-2000 ohms), the performance of the HPF would not be guaranteed. Inserting the HPF between two active stages is a sensible way of making sure the performance is stable AND optimized.

 

[MagnetoSound]

Inserting the HPF between two active stages is a sensible way of making sure the performance is stable AND optimized.

Exactly.

Besides, with the extremely well-designed (or convoluted, if you prefer) independent MIC/LINE transformered gain stage, where would you begin?

This way, the HPF is available whichever input you happen to be using. 

 

[kambo]

ok, that makes sense, thank you...

what do you think of this hpf right in front.
http://i251.photobucket.com/albums/gg291/diy33609/PGA2500_KITS/pre_sch.gif
(Igor's pga2500 micpre schematic, http://www.groupdiy.com/index.php?topic=31783.0)

 

[MagnetoSound]

I don't see one.

I see an RF filter (L1, L2, C31-33), 48v supply (R31, R32 etc) and a pad (R5,6,7).

No HPF.

 

[kambo]

i think its my bad, i will double check it...
anyway, i learnt something good today 

 

[SaMpLeGoD]

Probably because you haven't listened enough. This subject is a basic in filter design.
Inserting an HPF in the direct microphone input path creates loading problems on the source and noise problems since the active part sees a degraded impedance. Also because the source impedance is somewhat uncontrollable (microphone have impedances that can span 50-2000 ohms), the performance of the HPF would not be guaranteed. Inserting the HPF between two active stages is a sensible way of making sure the performance is stable AND optimized.

Thanks for the explanation! That was my main question, where to fit the HPF without having impedance problems! If I understand correctly, all I need to do is simply fit the BA182 between the L P pads! isn't it?
Thanks a lot all of you!

Ed.

 

[Moses]

HPF on Igor's schematic is just to the right of r11/12, no?

Mo

 

[MagnetoSound]

where to fit the HPF without having impedance problems! If I understand correctly, all I need to do is simply fit the BA182 between the L P pads! isn't it?

Between P and the top of the fader (5k pot), if you have one.

 

[kambo]

HPF on Igor's schematic is just to the right of r11/12, no?

Which schematic is this? Link?

http://i251.photobucket.com/albums/gg291/diy33609/PGA2500_KITS/pre_sch.gif