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promixe

Well-known member
Joined
Jun 9, 2008
Messages
193
Location
Midwest, US
Hi All,

I've got a couple of basic questions that I would like your input on:

nvam1.jpg


1) Do R56/C7/C8 serve only for power supply filtering? If so, if I have clean power nearby on the board can I omit them and run 24V directly into M pin of the BA283AM section? Or is R56 there for current limiting (2A?)? If so, can I still omit just the caps?

2) Is there a sonic advantage of inserting a switchable BALANCED pad (say -20dB) in front of the input transformer, as opposed to having an unbalanced pad between secondary of the transformer and the gain switch?

Thanks!
 
promixe said:
1) Do R56/C7/C8 serve only for power supply filtering? If so, if I have clean power nearby on the board can I omit them and run 24V directly into M pin of the BA283AM section? Or is R56 there for current limiting (2A?)? If so, can I still omit just the caps?

They are local power supply filtering.  I have experienced hum without these local to the pcb in the past.
 
I think C12 and C11 was added later after they had some trubble with popping/clicking (DC) in the EQ bypas switch.
In my EQ I omitted them, and I have no clicks. But I guess if you are making a large series like Neve, you want to be shure there is no popps.

For the local filtering I would keep it, like Rob said.
As for the line pad, Im not shure. But I know it sounds nice the way it is.
In some routing modules I have, they have added two 470R on the primarys on the line in transformer,
But I guess we are not so affraid anymore of some distortion in the input transformer, like some BBC folks was in 1975.
j
 
> Do R56/C7/C8

It's, what? $1.37 of parts?

When you mis-bias that single-ended module, R56 smokes instead of the precious module.

Assuming that R56 is worthwhile, then the caps are necessary, else R56 aborbs some audio power and diverts other audio power into pin "M" which may upset the system.

Neve didn't put excess parts in. Sometimes it is astonishing how much he did with so few parts.

> pad (say -20dB) in front of the input transformer

Input transformer is expensive; some of that is proportional to power handing. A pre-xformer pad keeps high levels out of the xformer.
 
Regarding the pad, Neve pads the signal after the inputtransformer in the 1063, it can handle +10dB through the mic input. Schematics can be found here: http://danalexanderaudio.com/neve.html
 
PRR said:
> Do R56/C7/C8

It's, what? $1.37 of parts?

=) I should have mentioned that my primary consideration was PCB space saving. It's about an extra sq. inch with those caps if you look at it this way. I'd like to challenge myself and miniaturize the design as much as possible without sacrificing sound. And component price is way down on my list (for now). =)

When you mis-bias that single-ended module, R56 smokes instead of the precious module.

Assuming that R56 is worthwhile, then the caps are necessary, else R56 aborbs some audio power and diverts other audio power into pin "M" which may upset the system.
Pardon my ignorance, but if I make an effort to bias the TR3 correctly right away and there is no "user access" to fiddling around with it later, then I could omit R56 and caps assuming I ran clean +24V to the board? I know it's not ideal safety wise for the AM stage, but is the sound going to be affected by NOT having R56 (then it won't absorb audio, as you mention above, thus filter caps are not needed). Or is there an unwanted crosstalk of some kind between NV and AM gain stages?

> pad (say -20dB) in front of the input transformer

Input transformer is expensive; some of that is proportional to power handing. A pre-xformer pad keeps high levels out of the xformer.

That makes total sense! Thanks for your input, PRR!
 
Tekay said:
Regarding the pad, Neve pads the signal after the inputtransformer in the 1063, it can handle +10dB through the mic input. Schematics can be found here: http://danalexanderaudio.com/neve.html
Yes, but isn't that pad wired to the gain switch? It gradually "unpads" the signal up until the next gain stage kicks in. What I was talking about is an extra dedicated pad that is engaged with a pushbutton regardless of the gain switch position. I think PRR above gave a useful point of view about U-padding it before the input xformer.
 
After studying this REALLY cool thread - http://www.groupdiy.com/index.php?topic=6576.0, I still have a big question:

How do you calculate the input impedance of the NV preamp?

JPK HERE mentions that its impedance varies with gain and is 12K without gain boost resistor and then decreases with additional gain. I'd like to know how I can calculate the impedance at various gain settings. Thanks!
 

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