Yes.not sure I follow,,, do you mean virtual earth?
I think I recall from the SSL discussions that broadcast/mono compatibility drove the compromise to -4.5 dB.
Yes.not sure I follow,,, do you mean virtual earth?
It's what I meant by current summation - combining the LCR "pan" switch circuit with virtual ground summing.or 4 resistors off of the sp3t switch, two center resistors adjusted to whatever pan law I want. The latter foregoes a buffer amp but I think I'd have to switch over to virtual earth summing from the current passive mix bus plus gain.
Probably not... The phase shift which I alluded to (in addition to noise), is related to loop gain margin and with only 6 stems not a problem for modern SOTA op amps. Further, passive summing requires make up gain in the same ballpark as the VE noise gain, so same-same....My strategy of summing current sources, did not have the noise gain component so less noise and less phase shift.weroflu wrote:No headohone amp planned
re: cmos switches, I thought about those for phantom, pan and phase as was intended from the 5171 designers but I don't know what ground noise problems await so I went for analog. Plus, the intended use for this box is set and forget, set gain, pan, phantom and use it to record. There will be no gain riding or on the fly panning.
Metering maybe could be useful but the downsteam box will have meters so it's not really essential.
I'm still studying the panning you suggested. You were not the 1st, 5th or 10th person to tell me to use virtual ground. Initially my reason for staying with passive bus was my panning scheme was incorrect and I thought there would be 4 channels per bus which would be somewhat quieter with a noise gain of 4 vs. v.e. noise gain of 7. With 6 channels per bus I may as well go ahead with v.e. summing. JR, in some other posts I think brought up the point that it's not just a noise issue but phase errors as well from v.e. if I understood correctly.
So that coupled with expected ground noise led me away from v.e. summing. Maybe it's possible to keep the summing topology modular/isolated to the summing card alone. I'll need an inverting stage before/after the v.e. summing.
There is one other issue. I actually wanted to burn a little power to increase efficiency on the ltm8049. Too low a current draw and I might be heating things up too much. Will check datasheet later. I really need to pick a number for the rails and stick with that, but it's going to require a new p/s board and parts since I used fixed voltages on the regulators. I wonder if I should put some mini trims on the ltm8049 fbx V output resistors and use TI matching TPS7A4700 and TPS7A33 regulators and mini trims there too.
On the subject of powering That preamps, do you think using the ltc3265 charge pumps would create any audible problems due to weak/slow current supply. I've heard various opinions on that topic. If I do wind up lowering the current draw I might switch over to using 2-3 ltc3265's for the rails, or one per preamp card.
I've been trying to learn basic measurements with the picoscope. Sweep and spectrum plot is working but the in-built signal generator is unbalanced 600ohm output. I didn't realize it would output down to 1mV, no pad needed. For now I'm just going to tie signal generator V+ to Mic input+, sig gen gnd to Mic input-, and float ground. I'm working on a test jig board that plugs into the backplane with a drv135 on it so i can just use a 3.5mm cable out from the test board into a preamp. It seems like there are a lot of very useful things I could put on this test board, any ideas? Maybe some resistors and a switch to simulate different microphone impedances.
You may not.I'll need an inverting stage before/after the v.e. summing.
Well you do need to burn power.No headphone amp planned
I actually wanted to burn a little power to increase efficiency on the ltm8049.
For the balanced input Phono Preamp I use a balanced H-pad.I'm working on a test jig board that plugs into the backplane with a drv135 on it so i can just use a 3.5mm cable out from the test board into a preamp. It seems like there are a lot of very useful things I could put on this test board, any ideas? Maybe some resistors and a switch to simulate different microphone impedances.
Was thinking you might say that.I'm on a linux box.