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

[kuba]

This is the scheme, right? The condenser acts like a crossover (6dB/oct), right?

I read somewhere that 2.2uF capacity and 10kOhms resistance lead to 90Hz roll-off. Well, I wanted to verify this fact and used this formula: f = 0.159 / (R*C)  .... for 2.2uF and 10kOhm I get 7.2Hz 

What's wrong?

I was also thinking of designing batt. box with variable roll-off, but I need the correct formula... Anyone?

[jk labs]

I read somewhere that 2.2uF capacity and 10kOhms resistance lead to 90Hz roll-off. Well, I wanted to verify this fact and used this formula: f = 0.159 / (R*C)  .... for 2.2uF and 10kOhm I get 7.2Hz 

What's wrong?

I was also thinking of designing batt. box with variable roll-off, but I need the correct formula... Anyone?

It's a matter of who you are going to trust: Electrical Science or "what you read somewhere".
You choose :-)

 

[kuba]

well, it's here: http://www.taperssection.com/yabbse/index.php?topic=23711.0

I'm really curious where the truth is...

[kuba]

I made a real mess of it.

f = 0.159 / (R*C) ... R is not the resistance used in the diagram but input impedance od the device.  You shouldn't do anything brain demanding in the night 

However, I'm not able to find input impedance of JB3 anywhere. Is it 4.7kOhm for mic-in and 47kOhm for line-in??

[hexyjones]

I made a real mess of it.

f = 0.159 / (R*C) ... R is not the resistance used in the diagram but input impedance od the device.  You shouldn't do anything brain demanding in the night 

However, I'm not able to find input impedance of JB3 anywhere. Is it 4.7kOhm for mic-in and 47kOhm for line-in??

Creative has keep the input specs a secret - no one knows...

[kuba]

Creative has keep the input specs a secret - no one knows...

Great 

So any bb roll-off parameters aren't worth damn when used with jb3 because no one knows the impedance... Splendid.

[kuba]

If you have the jb3 measure it (dynamically. A static Ohms reading isn't likely to measure through the capacitors in
signal path).  But you'd need a steady or variable frequency test tone. Like from audacity say.

Good idea. I'll give it a try as soon as my jb3 arrives.

[dklein]

Hey JK - I've been wanting to measure the JB3s input impedance for a while but may need some help?

I've got an analog multimeter and lotsa resistors kicking around 

I believe one approach was to drive a test tone to full scale and then determine what resistance value drops the signal in half (6 dB down) but that sounds like a pain since you can't monitor the JB3 live digitally (but now that I think about it I suppose I could monitor the analog out).

On the right track?  thanks.

[hexyjones]

Doesn't the fact that it is "Line Level" specify a certain input impedence? Isn't that some sort of standard spec?

[dklein]

Doesn't the fact that it is "Line Level" specify a certain input impedence? Isn't that some sort of standard spec?

no 

[kuba]

I believe one approach was to drive a test tone to full scale and then determine what resistance value drops the signal in half (6 dB down) but that sounds like a pain since you can't monitor the JB3 live digitally (but now that I think about it I suppose I could monitor the analog out).

Providing the signal from input can be send directly to output (dunno if JB3 supports this feature), I think it could work.

[jk labs]

Hey JK - I've been wanting to measure the JB3s input impedance for a while but may need some help?

I've got an analog multimeter and lotsa resistors kicking around 

I believe one approach was to drive a test tone to full scale and then determine what resistance value drops the signal in half (6 dB down) but that sounds like a pain since you can't monitor the JB3 live digitally (but now that I think about it I suppose I could monitor the analog out).

On the right track?  thanks.

Hi there!

Very good idea!

Hmmm a drawing would have made this easier to discuss though (kuba makes nice ones ....  ).

On the AC voltage source pick a frequency above the jb3's highpass filter (20 Hz?) but below the max frequency your analog meter handles (500 Hz?). Preferably one giving a steady secure reading. A good level is called for but keep it within the normal range of the jb3.
 
I suggest feeding the fixed-level AC through a 47 kOhms resistor and to the input of the jb3 (with the jb3 turned on). Then you have a nice 47k/Rin voltage divider right at the input. Now measure the divided voltage right at the INPUT terminal. It out to be V_source * Rin/(Rin + 47k).  If the nomad input is 47k the voltage dropped to half. If it's something else it might be nice to redo the measurement with a drop resistor close in value to get maximum accuracy in the estimate of the value).

Possibly also check for dependencies in the measured voltage (ie. Rin) with the setting of the jb3 gain control (just in case there is such a dependency. Probably it's not but it's nice to know for sure).

I think this will do. Looking forward to your results!!

Jon

[dklein]

cool - thanks JK.  I'll take a run at it on the weekend and report back.

[Weazel]

isn't it possible to use a dummy impedance?

[dklein]

isn't it possible to use a dummy impedance?

who you callin' dummy? 

[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.