Mic pre front end attenuator and filter design?

[rotation]

Hi!

I'm doing my new EF86-E88CC preamplifier. Schematic only shows in tx-electronics-output tx. Schematic is here:

http://img56.photobucket.com/albums/v172/gyraf/REDD47.gif

At the end i will put phase reverse switch, but problem is front end.
I though about using Jensen's standard mic input circuit, but i'm not sure it's enough flexible for all expected situations; dynamic, condenser, ribbon (in tx has 50 Ohm option) microphones and line input.
Jensen is here:
http://www.jensentransformers.com/as/as016.pdf

Solution i like best is here:
http://revolutionrecording.com/r47mkii.html (scroll down)

I like the idea of putting attenuator before the input tx.
My question here is:
Is this T pad and do you see it as enough flexible?

There is also -10dB pad.
Isn't this pad same as attenuator before it when set at -10?

About the bass lift:
Is it really needed? From what i understand it's there for figure-of-eight pick-up pattern mics. Can someone explain this?

Low cut (not on Rev. site):
I would probably need it. Where can i put it and can you give me example? I checked Gyraf's site and it's very simple filter placed between first and second stage.

What do you think about all this, am i complicating? Is there any other, simpler way to have flexible input, maybe few positions for attenuator and low cut?
I would also like to add instrument input, but that probably goes directly to the grid of first tube.
I know there was front end pcb layout here some time ago and if someone has it please let me know. If you maybe have other examples i would like to see them too. No need for a pcb if it doesn't exist, schematic will be more than fine.

Miha

 

[guavatone]

I would deff. g with a 2 to 3 step pad with bridged T up to 30 dB. These don't have much gain for a ribbon so I would just use the standard 7:1 I/P xfmr


Put this circuitry before the xfmr as wel as a roll off if u want.


5Ohm ribbon? I think forsel designed a preamp for AEA that was 8k. I thnk our friend Paul reviewed it.

I would like to see that front end pcb too, but I am going to just wire the stuff raw for now when I build one.

 

[rotation]

Ok, maybe too many question....

How to make correct pad from 0-24dB in 12 steps?

I found this site:
http://www.tkk.fi/Misc/Electronics/circuits/line_to_mic.html (mic in also)
Is the last one correct?

Btw, i searched before asking. I can find examples but they are only for one value.

Miha

 

[pstamler]

[quote author="guavatone"]5Ohm ribbon? I think forsel designed a preamp for AEA that was 8k. I thnk our friend Paul reviewed it.[/quote]

Not me, Scott Dorsey. He found that the higher-impedance loading opened out the top end a lot on classic ribbon designs like RCAs.

Peace,
Paul

 

[guavatone]

oh, sorry you wrote impedance article with the spring anology around the same time then.

 

[pstamler]

[quote author="guavatone"]oh, sorry you wrote impedance article with the spring anology around the same time then.[/quote]

Yep.

Peace,
Paul

 

[volta]

This is the design used in Hinson's proto I believe.
U-pads.This is a 6 position but you can make a 12 or whatever you need.

http://twin-x.com/groupdiy/albums/userpics/Attenuator.pdf

These sites may be helpful to calculate resistor values.
http://www.uneeda-audio.com/pads/

http://radiomagonline.com/notebook/passive_attenuators/

 

[rotation]

Ok, i have stepped attenuator from 0 to -30dB. Thanks to Dukasound!

Now i need a filter for low cut. Gyraf has it between the first and second stage, Tube Tech the same.
Can i put this kind of filter in front of input tx or it's too sensitive to have it there?
And is bass lift for figure-of-eight pick-up pattern really needed? Can you tell me how this kind of filter reacts and why all preamps don't have it?

Miha

 

[guavatone]

you could try an RC
http://www.muzique.com/schem/filter.htm

a 600R and 2.2 cap will give a lo cut. I am not sure how this will effect impedance though.

 

[rotation]

No, trafo sees 200ohm. It still don't affect the frequency.
Calculator shows 40k and 0,1u for 39.8Hz high pass.
Am i correct?
Is it correct to put this filter before tx?

 

[volta]

[quote author="rotation"]Ok, i have stepped attenuator from 0 to -30dB[/quote]

Can you post schematic?

 

[rotation]

So here is pre set attenuator i asked for:

http://twin-x.com/groupdiy/displayimage.php?album=lastup&cat=0&pos=0

Is there anyone that wants to help me with the filter?

 

[guavatone]

Sorry, I was totally wrong on Bridged T for Mic input( still learning). I hope u didn't build it. I just found out the hard way with signal sweeps to prove it. A variable U would do the trick. The preamp needs to see the 200R shunt and the mic is happy with seeing 2K.

Hinson's variable doesn't look good to me since you'd be changing what the input sees.

 

[rotation]

Yeah, i'm having problems too.
I think i've read somewhere that he done it on purpose, to change the tone of preamp.
So, is Hinson's attenuator ok for mic pre input stage or not?

Here it is:
http://twin-x.com/groupdiy/displayimage.php?album=random&cat=10184&pos=-1683

Here are attenuators from Gyraf:
http://www.gyraf.dk/schematics/bridge-attenuator.gif

And suggested calculator (from Kubi):
http://www.mcsquared.com/dbframe.htm (go to T and H)

What would you choose from those links :?

 

[JohnRoberts]

The H is the only one that is balanced.

JR

 

[rotation]

Aha, but than i come to another question: do i need balanced? Some are saying yes , some no.
So what should we use?

 

[Guest]

[quote author="rotation"]Aha, but than i come to another question: do i need balanced? Some are saying yes , some no.
So what should we use?[/quote]

Put any attenuator in balanced signal, but don't ground any components except shields.

 

[guavatone]

I think your confussion or contrasting resonses is that Pads are very application specific with regards to Impedance. If it's mic input then, unless you are running only unbalanced mics -which not many are, then u need a ballanced setup. Then u get into semantic references which some people may interchange aspacts of balanced pads and unbalanced. That last statement is a bit of a black hole and it's too late to get into.....so....um... the short version is that U=balanced L and H= balanced T(turned sideways of course)


You need to start with the impedance that are "typical" for both sides. "generaly" a mic wants to see 2K and an input transformer/preamp wants to see a 150 to 300 mic. For most cases up to 20 dB a 3 resistor network U-Pad of 2 series and one shunt will do the job for both sides. However when you need more attenuation the impedance that the mic sees may get too high so an O-Pad will be better.

I am still trying to figure when an H-Pad will be used but I am thinking it's a subtle variaton on the O-Pad.


BTW, why doesn't Batman or any superheroes ever carry a gun?

 

[JohnRoberts]

I was not familiar with all of those naming conventions but was able to figure out with a little help from google.

I have traditionally used a "U" configuration for switched pads in mic pre front ends and using both throws of a DPDT switch can maintain both nominal 2k load termination to the mic, and 200 ohm source impedance to the preamp input. Switch simultaneously removes a shunt from series input resistors while adding a shunt at preamp side of those series resistors to drop the impedance seen by following electronics.

This way the only thing that changes is level. The pad will not introduce any coloration from impedance interactions coming or going.

JR

 

[guavatone]

[quote author="guavatone"]both sides. "generaly" a mic wants to see 2K [/quote]

Sorry this value of 2K should be approx 1.5K to2K but there are deffinitely exceptions