Topic: DIY battery box (JB3 input impedance ??)

[Weazel]

i saw some schematics once on http://groups.yahoo.com/group/micbuilders/
see if i can find them again


maybe this link is usefull too.
http://www.tangentsoft.net/audio/input-cap.html

[Weazel]

ok i found the message about a dummy impedenace. good i remembered.

this is taken from http://groups.yahoo.com/group/micbuilders


From:  "dayVel" <dayvel@f...>
Date:  Thu Nov 11, 2004  9:28 am
Subject:  Re: basic battery box - what does the resistor do?

 

--- In micbuilders@yahoogroups.com, "thepompousdwarf" <sanaka@j...>
wrote:
>
> DayVel said, in part:
> > 1.8Kohms isn't all that low for a balanced mic preamp. Most
nominal
> > 600 ohm mics have an actual impedance of 150-300 ohms or so.
> > and most modern mic pres have an input impedance of about 1k-5k
> ohms.
>
> He's right. My mixer is a Spirit F1, fairly decent. Their better
> E-Series mic ins are 2.4k, Yamaha tends around 3k, and a Mackie
> 1202VLZ is 1.3K.
>
> I am studying up on transistors, thanks for all the info and
guidance.
> Bob, Hans, and DayVel seem to concur that if R1 is "in the drain
lead"
> Zo would follow R1, but if it's "in the source lead" Zo doesn't
follow
> R1 and is much lower, like a regular mic.


>
> Soooo...
>
> I'm not sure the question is settled of _which_ situation is in
effect
> given the WM61 capsule and the typical battery box.
>
> Is it known for sure how the WM61's internal FET is configured?
>
>
> Here's a schematic I did from my understanding of KenBob's
suggestion
> about buffering the signal with a transistor:
>
> http://sanaka.smugmug.com/photos/11178253-M.jpg
>
> Is it correct? I thought perhaps the collector should also get it's
> voltage through the 10K resistor...?
>
> DayVel said, in part:
> > Am I correct in assuming you want to run this into a balanced mic
> pre
> > and run the whole thing off a 9v battery? I'd be happy to suggest
a
> > few ways to do this, all of them simple, like me.
>
> Yes! Though the signal can just stay unbalanced. I've been using a
> basic battery box, see circuit in my first post in this thread, and
> it's been 'ok.' I unfortunately don't have much to compare to, but I
> suspect I can up the performance I'm getting now. I'd be very
grateful
> for your suggestions :-)
>
> Pece,
> Sanaka


That circuit looks correct as is, though you may want to use a
slightly lower value for R2, say 1.5k-4.7k Ohms. Use a 47uF
electrolytic for C1 with the positive end going to the transistor. If
you want to use it balanced, connect a dummy impedance (100 ohm
resistor in series with a 47uF cap) from the cold side (pin 3) to
ground on the mic end of the cable.
 

[Weazel]

okidokie

[jk labs]

bump so dklein doesn't forget ......

[dklein]

damn! 

[Weazel]

any progress? i wonder if it diffeence much from dat or md.

[dklein]

Finally got to it.  Here's the hookup:
laptop sound card out > JB3 line-in > JB3 line-out > pc soundcard in

I sent sine wave tones out of the laptop and adjusted the output to drive the JB3 to near full scale.  Monitored levels on the pc in Wavelab and played with series resistance on the line-in signal until the tone dropped 6 dB.  Tested both channels separately just to be sure.  I was subbing in 1% resistors and swapping them around just to make sure they weren't off.

Here's the -6 dB points:
100 Hz    37k
2 KHz      37k
10 KHz    27k

I ran the tests at a couple of gain settings on the JB3 to verify there was no effect (and there wasn't).  I checked the values several times - there's probably some slope that occurs between 2k and 10k but it wasn't worth chasing.  So I might guess the working number for input impedance is 35k.  I attempted to verify this by putting caps across the 100 Hz signal but the smallest one I have on hand is a 220 pf so it's still too big (rolloff ~ 21 Hz).  I'll pick up a few caps and pop 'em in and see where the rolloff really occurs.

A couple other confirmations along the way.  The analog front end has 1-2dB of headroom beyond the a>d.  I confirmed this by getting the JB3 to clip and then turning up the JB3 gain further.  The analog line out didn't brickwall until 1-2dB after the a>d.  The other thing that means is that the line-out is a straight, post-gain analog feed from the line-in (unlike the UA-5).  I suppose that's only interesting because it's possible you could clip your recording but an analog patcher could have a clean one (but 2dB ain't much).

Well...that was fun!

[dklein]

Still gotta confirm but the capacitor values look like
1 microfarad for flat
68 pf nf for ~60 Hz
47 pf nf for ~90 Hz
33 pf nf for ~130 Hz

These are smaller values than you'll find in standard battery boxes.  For most boxes the bass rolloff will be ineffective. 
e.g. if I recall correctly, CSBs use a 220 pf nf for rolloff - down 3 dB at 20 Hz when plugged into a JB3.  Nada.

edited for wrong prefixes

[Weazel]

then the resistors need to be 35 kohm right?

[dklein]

then the resistors need to be 35 kohm right?

No - the 35k is the input impedance that you need to use when calculating which capacitor value to use for bass rolloff.  I saved you the trouble and posted the values above.

The resistors in the original diagram at the top of this thread are there to provide 'plug-in' power (bias) to the electret.  10k is probably still the right value for an electret capsule like the panasonic .  Also - Kuba - the diagram needs one small correction (or I'm not reading it properly).  It looks like you have both resistors terminating on the R+.  One should terminate on L+.

btw, the rolloff frequency is where you're already down 3 dB (in a 6 dB/octave slope).  It's not where the rolloff starts!

Any corrections welcome...

[dklein]

Just did a couple edits above on the capacitor values...I got sloppy and confused my nanos with my picos 

If anyone is using that little spreadsheet I did a while ago it's got the same error - the column that reads µF is correct but the next one over should be titled nF, not pF.  Thanks for catching that JK.

[kuba]

Kuba - the diagram needs one small correction (or I'm not reading it properly).  It looks like you have both resistors terminating on the R+.  One should terminate on L+.

Yeah, that's pretty obvious, I'll vorrect it as soon as possible.

btw, the rolloff frequency is where you're already down 3 dB (in a 6 dB/octave slope).  It's not where the rolloff starts!

Good point, I didn't know that and it sounds a bit weird. Still, you can calculate the frequency where the rolloff starts...

[Weazel]

need the capricators to  be electrolytic  as i read some other schematics? or what kinds are best to use?

[dklein]

my universal translator thinks you're asking about electrolytic capacitors.  Not the best choice for audio.  Go for some kind of film cap.  Bigger and pricier but still under $1.

The golden ears say:
Polypropylene best, polyester & polyethylene next, then mylar et.al. Electrolytics and tantalum are to be avoided.