Mobile audio mixer, w/ high level inputs

[Earthshaker]

I am looking for a mixer that will have the capability to tolerate high level inputs, (5 watts or so) it will need to be a split supply to avoid ground problems (most radio gear does not use a common ground speaker output) 4-6 inputs will be ok, 1 output, to audio amplifier.
Any ideas?

 

[Nigel Goodwin]

I am looking for a mixer that will have the capability to tolerate high level inputs, (5 watts or so) it will need to be a split supply to avoid ground problems (most radio gear does not use a common ground speaker output) 4-6 inputs will be ok, 1 output, to audio amplifier.
Any ideas?

What exactly are you trying to do?.

Car radios (presuming you are talking about something like that!) are either single ended outputs (where one side of the speaker is grounded), or bridged outputs (where neither is) - in the second case you could simply use either one of the speaker connections to feed a mixer.

It's not a very 'nice' way to feed a mixer, and HiFi buffs will be fainting in their thousands - but you would only need a very crude mixer, with the speaker outputs feeding directly to pots (perhaps even with a series resistor to drop it further), then a simple single opamp mixer.

 

[Earthshaker]

Let me clarify, communications radio gear (two-way) I am building a storm chase vehicle and I just as soon not have a ton of different speakers around, I am planning on using a simple op amp mixer, but I do need to reduce the level to safe limits for the op amp.

 

[Nigel Goodwin]

Let me clarify, communications radio gear (two-way) I am building a storm chase vehicle and I just as soon not have a ton of different speakers around, I am planning on using a simple op amp mixer, but I do need to reduce the level to safe limits for the op amp.

OK - you could either add resistors feeding to the top of the pots, or configure the gain of the virtual earth opamp mixer to give attenuation. Either way would work fine.

 

[Earthshaker]

OK - you could either add resistors feeding to the top of the pots, or configure the gain of the virtual earth opamp mixer to give attenuation. Either way would work fine.

Nigel, I am totally drawing a blank, I understand the concept of the virtual earth but the actual doing of it is escaping me. I ahve been out of touch for several years with the electronics hobby, let me try this, as far as full device connections, we have +Vcc, -Vcc and ground (earth) in a dual supply arrangement. I have a intercom schematic that appears to use the virtual ground arrangement (I think) Basically what it is is a system that uses a TL074 quad op-amp, the first op-amp, amplifies the signal coming in, then the second op-amp is used as what appears to me as a unity gain follower, followed toward the output amp with another unity gain follower, at this point the flow also goes to the 4th op-amp
which may be a "virtual ground" op-amp I am not sure. If you want I have a gif file of the schematic I can post. Posting it may clear some things up, and I also have a question on converting that circuit to a dual supply. Let me know.

 

[Nigel Goodwin]

Nigel, I am totally drawing a blank, I understand the concept of the virtual earth but the actual doing of it is escaping me.

It sounds a lot more complicated than it is, it's simply an op-amp in inverting configuration - so you have a resistor from the output back to the inverting input, then each mixer input has a resistor feeding to the inverting input. The other end of the input resistors go to the sliders of the input level controls. It's called 'virtual earth' because the inverting input pin of the op-amp has no signal on it (it's cancelled out by the output feeding back), so it looks like it's at earth potential.

I would draw you a circuit when I've got time, but there are already loads of mixer circuits out on the web, there's a nice simple example here **broken link removed**, you could easily use that - just ignore the microphone preamps at the top, and duplicate the bottom line inputs. This design gives a gain of about two, you probably don't require that - changing the feedback resistor from 100K to 22K would reduce the gain to about 0.5, you could even replace it with a variable resistor and make the gain variable.

 

[Earthshaker]

Ok I ve got it, thanks for your help, it looks like I have some brain updating to , oddly enough the mixer link is the same one I want to use . 73 (best regards, I am also a ham operator)

 

[Nigel Goodwin]

73 (best regards, I am also a ham operator)

I used to be, my callsign was G8MMV, but I've not been active for a number of years.