Phase control circuit?

[DavidJE]

Hi,

I'd like to make a simple circuit that allows me to control the phase of a mono line level input. This would sit between the sub output on my AV amp and my subwoofer's line level input. I'd then put this circuit in a small metal project box.

I'd like the phase control to be constantly variable from 0 to 180 degrees by simply turning a knob. I want the design to be as transparent as possible to the signal so that it doesn't effect the sound quality. So I'd like to use the best quality components for audio applications.

I know practically nothing about designing electronics but I can use a soldering iron pretty well. I also don't have access to any proper PCB production, so I'd probably have to use stripboard.

Can anyone provide me with a suitable design and component recommendations?

Many thanks,

David.

[RobWells]

Rod Elliot does one over at Elliot sound products.

[DavidJE]

Hi Rob,

Thanks for the info. I found the design HERE and I've already managed to gather most of the components from work. We have lots of old stuff stashed away. I should have the last couple of bit's soon. I also found an old breadboard thingy with built in PSU that will be perfect for testing the circuit. I've also managed to talk one of my colleagues into giving me some help. He has some electronics experience so that will be helpful.

I'll hopefully have a test circuit setup by the middle of next week, so I'll post more info once I've got that setup.

Cheers.

[DavidJE]

I managed to get a test circuit setup on some breadboard. It seems to partially work but I'm having a couple of problems I'm hoping someone can help me with.

When the switch is open, I get the same phase and amplitude out of the circuit as I'm putting in, so that works as expected. When I then close the switch, the phase is flipped 180° as it should be, but the amplitude at the output also drops by about half. According to the design description, the amplitude should be unaffected. Does anyone know why this happens?

Also, I don't seem to be getting much control out of the variable resistor. It only causes a very small phase shift over a small part of it's range. I will admit that I have substituted the 47k variable resistor with a 50k model. To compensate for this a substituted R6 for 7K. This was just to get the test circuit working. I will order the 47K later. I'm not sure if this is the reason for the lack of control. Does anyone have any ideas on this.

To test the circuit I have been using a 1KHz tone generator and a 2 channel digital scope to compare the input to the output.

Thanks.

[DavidJE]

Forget the first problem, I think I've fixed that. I simply boosted the output of the tone generator from 50mV to 100mV (it's maximum). I guess I just wasn't giving the opamp enough signal.

The other problem still remains though.

[DavidJE]

Well, I think I've figured out what's causing the second problem. After reading through the circuit description more closely, it seems that the circuit will affect the phase of different frequencies by different amounts. So as I'm currently using a 1KHz test frequency, the phase of this will be affected differently to the frequencies a sub is likely to be playing. So I guess the circuit has been tuned accordingly.

However, from what the description is saying, it seems that when the variable resistor is turned, not only does the phase vary, but also the frequency to this phase change is being applied to. That's the way it reads to me anyway. So that sounds a bit pointless.

I'm going to read into this more tonight to make sure I understand it properly.