Jim Williams wrote on Sun, 06 February 2011 10:46 |
I do have faster stuff if interested. I can load up the High Speed mic preamp (rack version) with TI THS3061 current feedback opamps. Those run at 300 mhz and slew at 7000 v/us. I also have those parts designed into a custom DAC designed by Michal J. of Mytek and me for Mark Levinson Labs using a BurrBrown PCM1704 DAC chipset, it sounds wonderful and very open and clear. I used the current feedback THS3061 as a current to voltage converter and as a 2 pole Sallen Key low pass bessel filter set to 65k hz. |
John Roberts {JR} wrote on Sun, 06 February 2011 18:25 |
In the specific case of driving A/Ds or CODECs you do need gain bandwidth well in excess of normal audio bandwidth. While you don't literally need to slew any faster, you do need to damp very high edge rate current spikes dumped out the input pins by internal S/H circuitry or whatever. At some point it seems there may be some value in shunting the opamp buffer output with a HF cap to ground. Do these uber-fast opamps tolerate much (any?) capacitance on their output ? |
bruno putzeys wrote on Mon, 07 February 2011 02:12 | ||
Ahem. |
Jim Williams wrote on Mon, 07 February 2011 18:25 |
It's needed to allow that 1 pole passive LPF used to feed the ADC to operate without destroying the opamp's phase margin. |
Jim Williams wrote on Mon, 07 February 2011 11:25 |
I used Sallen Key filter topologies with the THS3061 as they don't include a capacitor in the feedback loop, a no no for transconductance or current feedback opamps. For voltage feedback opamps I prefer the Multiple Feedback (MFB) filter topology. As one can see in Bruno's schematic, there is a 22 ohm resistor placed into the feedback loop in series with the opamp's output. That isolates any capacitive load from the opamp's output pins and prevents ringing, or worse. It's needed to allow that 1 pole passive LPF used to feed the ADC to operate without destroying the opamp's phase margin. The same concept was used on the Mark Levinson DAC, it has series resistors isolating the opamp from the load. Since that TI part is class A and has mondo output current, driving loads is not a problem. Transconductance opamps have a different sound than voltage feedback opamps, I can't really put my finger on it, but they do sound more like tubes than voltage feedback devices. FFT sweeps show differences, but they are so far down in the noise floor I'm not sure if they are the source of those differences. This is a subject I've not seen explored fully yet, the application of transconductance opamps in audio. I began using them in the early 1990's as they are fun to mess with as all those old opamp rules are re-written. |
John Roberts {JR} wrote on Mon, 07 February 2011 19:48 |
There is a phase shift associated with that isolation technique that needs to be factored into your phase response if the pole is close to your passband of interest. |
Jim Williams wrote on Tue, 08 February 2011 10:16 |
I missed that 100 pf feedback cap as another pole, most ADC's used for audio use a passive 1 pole filter in front of the ADC, after the opamp buffer. In this example it's included into the feedback path to create a 2 pole response. As to current feedback opamps for audio, there is one designed for audio, the National LME49713. It's rather linear for a CFA and as usual has low input voltage noise. The problem with using these for audio is two fold, first the inputs are not matched, the inverting input feeds the emitter of the input transistor rather than the traditional base. The other audio problem is current noise, it's through the roof. If used with low value resistors in a non-inverting configuration it can work quite well. Bandwidth is set by the feedback resistor value instead of the usual RC ratio. The original Audio Upgrades High Speed mic preamp design used Analog Devices AD811's, a CFA opamp. Other than that, I know of no commercial audio products using this class of amplifiers. |
John Roberts {JR} wrote on Tue, 08 February 2011 17:18 |
I am inclined to speculate that this topology has indeed been used in professional audio products for decades. |
John Roberts {JR} wrote on Tue, 08 February 2011 09:18 |
PS: While I haven't paid close attention the the early mic preamp ICs, it seems they too used variations on this circuit topology. |