A dream came true - R2R.
- Thread starter byrdparis
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Is there something which documents this project? Clearly the PCB is part of a larger build. Thanks.
Yes, this is all textbook stuff. I wonder if it has occurred to you that your claims apply to transistors too, even opamps, yet somehow they can get some pretty decent numbers out of them (which suggests that something else is afoot...). How much schooling are you asking for here? Do you want me to walk you through it?
One great mind once wrote here:
"tubes benefit from that with regard to CMRR so that value can be decent- well into the 90dB region."
It clearly says: TUBES. There is absolutely no way under the Sun that the tubes can get even close to it. The discussion had to do with tube amplifiers.
For those not familiar with the situation (back to the same textbook stuff again), in order for a circuit to get 90dB of CMRR it must use the resistors with .001% tolerance. When was last time anyone purchased those?
That if you disregard the contribution of active components. Just the basics.
So I would be extremely impressed if a tube differential amplifier had anything higher than, say, 10dB.
"tubes benefit from that with regard to CMRR so that value can be decent- well into the 90dB region."
It clearly says: TUBES. There is absolutely no way under the Sun that the tubes can get even close to it. The discussion had to do with tube amplifiers.
For those not familiar with the situation (back to the same textbook stuff again), in order for a circuit to get 90dB of CMRR it must use the resistors with .001% tolerance. When was last time anyone purchased those?
That if you disregard the contribution of active components. Just the basics.
So I would be extremely impressed if a tube differential amplifier had anything higher than, say, 10dB.
OK: a bit of schooling. There is a simple way to get around the need for high ultra high precision resistors. FWIW, such resistors would be needed for a solid state circuit too so this is not a tube or transistor thing, its a good practice for differential amplifiers.
Here is the problem: Let's take the example of two 5K resistors to ground. A 1 Ohm difference between the two can degrade CMRR performance by as much as 60dB!
So that really isn't the way to set up a differential input although many designers do (although the input balance is not the only way to arrive at a good CMRR value...). To fix that, instead of two resistors to ground off of each input (pin 2 and pin 3 of the XLR) the two resistors instead tie together. A third resistor is then tied to that point and it goes to ground instead. This helps force common mode operation and allows the input resistors to simply be an impedance between the two inputs of the differential amplifier. In this manner matching the two input resistors is far less important. Obviously the higher the value of the common mode resistor, the better the CMRR, within the limits of the input devices, whether tube or solid state.
Hm. It seems as if you ignored most of my last post.
Your closing statement in the post above is false; there's a flaw in your argument. I alluded to it in my last post.
The passive approach then.
I measured 78dB by shorting pins 2 and 3 of the input XLR of our phono section together and driving that and ground. Then drove pins 2 and 3 normally. This kind of test tends to be inaccurate as any difference outside of 0.1% with the input resistors will typically limit the reading to about 60dB.
At the output of the phono with pins 2 and 3 tied together the output was about 0.005mV. When driven normally the output was 40V. This works out to 78dB, which suggests an excellent match of the source resistors. A more sophisticated test and be done (but takes a lot of setup time)which can measure the CMRR more accurately but it will simply show a larger value, in keeping with what I previously stated.
If this circuit were discrete semiconductors it would have a similar CMRR reading for exactly the same reasons, as CMRR has nothing to do with tube or solid state. It has to do with input matching and gain, which is why opamps can obtain high values of CMRR despite poor internal matching.
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