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?