Some basic questions...

Ziller

Member
2002-01-09 2:10 pm
For my first project, decided to build this active direct box:
http://sound.westhost.com/project35.htm
and have a couple of basic questions. The one I'm building is the battery operated one at the very bottom.

First, can I substitute a 10uF cap for the 1uF input coupling cap? Everything I've read seems to indicate that close is close enough.

Second, I've built a steel box to house the circuit in. I'm assuming I should ground the case to something. Should that be the -ve or the 0 volt line (going into pin 5 of opamp)?

Also, if anyone is inclined, can you explain what is happening.

Here's what I think is happening. The general idea is that the audio in is split and sent out to the XLR where, if one line is positive, the other is neg and visa versa. This allows any interference to be removed by the mixer circuit. Also, because of the low output impedence of the opamp, the output is low impedence. What I'm not clear on is how the signal is being inverted to go to pin 3, since the input is being fed into the non-inverting input 5. And, since the top opamp is using unity gain, wouldn't it be a different voltage level then the bottom one, which has some feedback? A general explination would be very helpful.

Thanks,
Ziller (a newbe)
 
Ziller

Yes, you can use a 10uF in place of the 1uF capacitor. The case should be connected to ground (the inverted triangle symbol), which in this circuit is also the negative battery connection.

The circuit is basically an unbalanced to balanced converter. The signal is applied to the upper op-amp which is operating as a non-inverting unity gain buffer. The output from this op-amp goes to the XLR connection and also to the input of the lower op-amp via the 100k resistor. This lower op-amp is connected as an inverting unity gain buffer (it is unity gain as the feedback and inverting input resistors are the same value). The inverted output from this op-amp is then also taken to the ZLR connector.

Geoff
 

Ziller

Member
2002-01-09 2:10 pm
OK...

Thanks, that clears things up. One other thing. Isn't the output of pin 7 (the out of phase signal) lower in magnitude then the in phase from pin 1, since the signal goes through the 100K resistor before going into pin 6?

Thanks alot for the responses.
 
Ziller

The output from the two op-amps is at the same level since they are both operating as unity gain buffers.

Increasing the capacitor value from 1uF to 10uF will not have adverse effect on the frequency response of the circuit. It will in fact improve it by lowering the low frequency -3dB point. However, this will not be audible.

AudioFreak is also incorrect with regard to the case connection. This should be taken to ground as I indicated before. The 0V line is not at ground potential due to the arrangement Rod has used for powering the op-amps from a battery.

Also, to correct AudioFreak's erroneous statement, the input signal is not buffered and then fed to an op-amp network which outputs two signals. The method of operation is as I explained in my first post.

Perhaps AudioFreak would like to explain how "2 signals 180degrees out of phase with each other...." can become " both of the same phase and magnitude as the input...."

Finally, the benefit of a balanced connection is that noise induced into the connecing cable is cancelled and does not appear in the final signal. If AudioFreak wishes to call this noise distortion then that is up to him, just don't be confused by this since it is not a customary definition.

Geoff
 
Geoff said:
Ziller

The output from the two op-amps is at the same level since they are both operating as unity gain buffers.

Increasing the capacitor value from 1uF to 10uF will not have adverse effect on the frequency response of the circuit. It will in fact improve it by lowering the low frequency -3dB point. However, this will not be audible.

AudioFreak is also incorrect with regard to the case connection. This should be taken to ground as I indicated before. The 0V line is not at ground potential due to the arrangement Rod has used for powering the op-amps from a battery.

Also, to correct AudioFreak's erroneous statement, the input signal is not buffered and then fed to an op-amp network which outputs two signals. The method of operation is as I explained in my first post.

Perhaps AudioFreak would like to explain how "2 signals 180degrees out of phase with each other...." can become " both of the same phase and magnitude as the input...."

Finally, the benefit of a balanced connection is that noise induced into the connecing cable is cancelled and does not appear in the final signal. If AudioFreak wishes to call this noise distortion then that is up to him, just don't be confused by this since it is not a customary definition.

Geoff

1. Ok, I am in error i admit it....

2. It was a typo

3. Yes it should be @ ground .... 0V and ground are usually but not always the same i admit i overlooked this ....

4. Yes it is noise and i would normally call it noise but i was trying to be consistent with the language Ziller had already established.... i read Ziller's post again and he used the term interference.... this would have been a more appropriate word for my posting :)

5. I removed the post in question.