Joe Malone wrote:Second half of OPA2604AP is used as the inverted -out adding 6dB gain like a 1:2 output transformer would normally do.
I'd like to make sure I understand this.
R11 and R12 are both 10K, so the opamp is simply unity inverting, yes? BUT, when you apply this inverted signal as the -ve in a balanced line transmitter, the differential between the +ve and -ve signals is effectively doubled, and hence the 6db gain. Think of it this way - if you have a +2V signal on the +ve line, invert it (to a -2V signal) and apply the inverted signal to the balanced -ve, the differential on the balanced line is now +2 to -2, ie +4 V. Magic!
I think I've seen this little trick before in schematics, and never had thought it through as being the opamp equivalent of a 1:2 output balancing transformer.
Yep that is 100% correct
The BA BAD BAN and dINgO PCB does this as well when using normal dual OPA2604AP for the output opamp but not when using a JLM99v. The MONO does this with OPA2604AP or 2 x JLM99v.
Can anyone recommend a good microphone to record a group of 14 people? I need to record a conversation between fourteen people on a normal cassette dictaphone. Can anyone recommend what make/model/type of microphone I need to record everyone in the room?
To nicely record a group that size, you need a microphone with an omni pattern, and good sensitivity, and that means a condenser microphone - but condensers need to be powered either from a preamp or from a battery, and a cassette dictaphone can't do that. You'll also need a special cable to connect most good microphones (XLR socket) to your dictaphone.
There are omni dynamic mics out there on ebay which do not require external power, but I'm not sure they will be much better than the dictaphone's internal mic. Where do you live? Its entirely possible that someone in this forum might be able to lend you a mic that can do the job.
Chris P
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I do lots of things. I believe eclectic skills are best.
Of course it is possible to put in a send/return loop, but what would be the point? And I'm not sure what you mean by parallel - parallel to what in the preamp circuit?
This is a single variable gain stage design (the second half of the opamp is effectively a 2:1 gain but its role is really to drive the negative side of the balanced output - think of it as being a 2:1 output transformer and you get the picture). Any send / return loop would be after the gain stage, so you might as well just take the output of this preamp and run it into your desired external loop, whether that be parallel or whatever. There's then no point in bring the signal back to this preamp, you'd just patch it up to your mixer / DAW interface or whatever.
Send return loops only really make functional sense in a preamp where there is either more than one variable gain stage (eg the JLM BA Dual, which does have an insert loop on the PCB) or else some other processing going on inside a box (eg it has a compressor or EQ) that you may wish to sidechain or pre- or post-process. Mixers have insert loops so that you can process a channel signal prior to it being mixed. Some DAW converter preamps have inserts so that you can process the signal before its gets converted to digital. In the case of a standalone preamp like this, however, there's really no point. Its a one trick pony that is just really good at doing its one trick. Build it and enjoy it for what it can do!
Chris P
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I do lots of things. I believe eclectic skills are best.
The adapter will not help things unfortunately. As on it side the Carnhill is about twice the size of the OEP so there should not be enough space for it to fit.
I will get one of the original 10468 Marinar transformers we have out and try and see if there is a way to make it fit but I don't hold much hope that it will.