gridcurrent said:
1. the 990 has large output current capability so why not take advantage of it ? use a 1:2 step-up output transformer. think "free" gain. you can even consider revising the summing gain structure.
2. if the output is transformer coupled, why bother with a servo? if the output DC is low enough no capacitor is needed. think API 312. if there is more DC than you are comfortable with, use a capacitor as in API 325. that method was employed in maybe a billion successful recordings.
3. gain control. as mentioned, put a rheostat in the feedback loop.
The problem with the DC offset, is that will change depending on the amount of channels assigned, and a little DC offset through the low resistance winding of an output transformer will result in a current that may be problematic getting higher THD at the transformer, not as a lack of headroom in the amplifier. In the API 312 the gain trim resistor has a DC blocking cap, no change in the DC impedance presented at the inputs, so you are just with the DC circuit as it is, the offset could be trimmed or at least know how much it will be and will be constant. On the other hand the transformer used in the 312 has M6 core, the one in the schematics 80% Ni, which is much easily upset with DC current. I wouldn't chose that exact transformer for this circuit, I would look for something with lower DC resistance on the windings, to get lower distortion when driven by a 990.
Actually the API 512 has both, a DC servo and a coupling capacitor, the DC servo is there so the reversing polarity switch doesn't pop, which did sometimes before it had the DC servo. The coupling capacitor is no longer needed but it was left there to make the change in the circuit as minimal as possible, so no costumer would complain about it. I think I would just put the switch after the cap, but they know what they are doing.
bjoneson said:
abbey road d enfer said:
Gain control could be achieved passively by putting the pots after the summing stage but you would need a buffer stage to drive the xfmr.
I've been scouring the web for hours trying to answer this question on my own... but, can someone help me understand why the 990 output couldn't drive the 600 ohm output transformer with a 10k pot in line for signal attenuation without the use of a buffer? I feel like this is a fairly rudimentary concept, but I can't wrap my head around it.
The problem is that the output impedance of the pot is too high to drive the transformer, and you would be driving the transformer from the pot, not the 990, the 990 would just be driving the pot and the feedback loop. you would have to account for the worst case, which is in this case something of daily use at -6dB at the pot, where you will have 2.5kΩ output impedance from the pot, so pretty bad, against the recommended 50Ω for driving this transformer, or the output impedance of the 990 which may be around a few mΩ or even less.
The resistor on the + input is good to take care of the offset if all the inputs of the summing amp are connected all the time, and when not used grounded, this means noise gain is always the worst case. I do like the idea of the servo and gain rheostat on the NFB as you are going for since it allows you to get the minimum noise gain you need at that time, so the design in this way is really well optimized in noise performance, then you need good implementation and layout, but that's another story, which isn't really easy neither. The only problem comes with the low gains again, which make the amp to be loaded by a very low impedance on the NFB loop, but as you are expecting always line levels signals when you have low gain you expect low output level, so the current isn't any bigger and no problem, and if you have a bigger input, the 990 is a tough guy which can take under 100Ω without much trouble, if the output is loaded with 600Ω you still can have about 100Ω on the feedback loop and drive it to high output levels, probably the pot himself won't be happy when that happens.
In the 990 data sheet, is recommended for summing amp applications to use an OLI on the input, that's to prevent oscillations because of the capacitance on the bus, if you have all the summing resistors close, in the same board of the summing amp, that's not needed. If you have a long bus in a mixer it's probably good to have it there.
JS