[gracky]

Maybe this is a replicated question, I apologize if so,

It's well known that Tangent's META42 home page says that registers for the inner loop are to be higher than outer ones, and recommends R5 to be 5-10 times higher than R3. While in the PPA schematic, R5 = 3k and R3 = 1k, so it's 3 times highter.
Would you tell me which considerations are implied in these configurations? What will happen if R5/R3 ratio were lowered than PPA configuration, e.g. 2 or 2.5 times?

Thanks in advance.

 

[tangent]

Those are general rules, not requirements. The META42 schematic isn't intended to be ideal; it's just a setup that works for many purposes. We never tried to figure out what the ideal values are for any particular situation, and there isn't an ideal set that works in all situations, anyway, so no matter what values I use on the schematic, they'll be wrong for someone somewhere.

You have to apply some thought to parts selection when you build the META42. You're even welcome to disregard my advice entirely if you want.

 

[jeffreyj]

The idea behind using multiple feedback loops is to limit bandwidth to prevent oscillation! I feel there are some serious engineering tradeoffs that ought to be considered before implementing a multi-loop design or even just changing the inner and outer loop gains of one.

 

[jcx]

Walt Jung’s Op Amp Audio series [ see the Walt Jung Archive: http://www.elecdesign.com/Articles/Index.cfm ] doesn’t make claims about inner loop feedback with respect to loop oscillation stability, Walt was writing for engineers in a technical magazine and must have assumed the issues would be clear to his audience. It is only in these forums that I have seen the claim that (high values of) the inner feedback loop gain (~=R6/R5) improve stability, in fact the inner feedback resistor R6 value is not very important for stability, its parasitic capacitance is much more important. R5 should be set to match the impedance of the feedback network as seen at the minus input to the source&attenuator impedance at the plus input.

I believe that stability against loop oscillation is not usefully improved by the Jung multiloop inner feedback loop connection for the commonly recommended resistive feedback values. The stability (phase margin) can never be better than that of the input op amp running at the same overall closed loop gain (with the load isolation of the output buffer)

Start with a voltage mode op amp driving a unity gain buffer stage and consider the inner and outer feedback separately:

Either all the feedback is inner loop feedback from the op amp output setting our desired closed loop gain and we simply have a well understood positive gain op amp circuit followed by a open loop unity gain buffer, or we can place all of the feedback in the outer loop, ie from the buffer’s output and now have added any phase shift from the buffer (and op amp out/buffer input series RC) to the op amp phase shift, the added phase shift around the loop will reduce stability.

The two cases above are the limiting cases and since the multiloop feedback network linearly combines op amp and buffer outputs we can say that the general multiloop amplifier response is the linear combination of the two cases above for the purposes for which linear system analysis and the superposition principle applies such as frequency response and loop oscillation stability (phase and gain margin calculation)

This shows that more inner loop feedback does reduce the buffer phase shift added to the loop, but only to the extent that the inner feedback loop controls the overall closed loop gain, in the PPA circuit (typical of Jung multiloop values) the inner/outer loop resistive divider values only reduces the added buffer phase shift contribution by ~4%. Unless the inner loop feedback is so high (= low inner loop gain) that the (low) inner loop gain strongly affects (reduces) the closed loop gain, the inner loop doesn’t usefully effect or improve loop stability, which can never be better than the loop stability of the input op amp operating at the same overall closed loop gain setting.

Capacitive effects are larger than inner loop resistance at the closed loop corner frequency in most Jung multiloop circuits on these forums. The input capacitance of the op amp (large for fet op amps, common&diff mode input capacitance 7&8 pF and 15&8pF for OPA637, AD8610) must be considered with layout and component parasitics to determine the actual feedback conditions above a few MHz, in the case of the OPA637 in a gain +2 circuit the closed loop corner frequency is ~30MHz and the feedback conditions are dominated by real world circuit capacitance that didn’t make it on to Tangent’s schematic – Z(7 pF)@30 MHz ~ 750 ohms. The input capacitance issues are discussed on nearly all higher speed op amp data sheets and app notes, the OPA637 data sheet fig 5 pulse response shows a 4 pF lead cap to match capacitive to resistive effects at a gain of +5. This suggests that the counter examples to my analysis offered by Tangent of METAs using OPA637s at a gain of +2 are in fact noise gain compensated by these capacitive effects to a higher and more stable closed loop gain.

I believe that virtually all Jung multiloops on these forums with unity gain output buffers will be equally stable with or without the inner feedback resistor R6, some may require a 1-2 pF cap in the inner loop feedback resistor position to provide capacitive feedback equivalent to the missing resistor’s parasitic capacitance. The measured distortion will of course be less without the inner loop resistive feedback as well.

 

[jeffreyj]

Quote:

Originally posted by jcx Walt Jung’s Op Amp Audio series [ see the Walt Jung Archive: http://www.elecdesign.com/Articles/Index.cfm ] doesn’t make claims about inner loop feedback with respect to loop oscillation stability, ... , in fact the inner feedback resistor R6 value is not very important for stability, its parasitic capacitance is much more important.

Indeed, he doesn't. Not directly, anyway. But why on earth else would you want to limit the bandwidth of your amplifier??

I implied it previously, but will so again, that there are a lot of serious engineering issues that should be considered before implementing a multi-loop feedback topology. It is certainly no panacea for whatever ails your circuit! Note, also, that the multi-loop configuration seeks to reduce thd to vanishingly small levels; there are some that argue, and with both sonic and technical merit, that this goal is almost always at odds with the linearity of the the amplifier's transfer curve, especially in the vicinity of input overdrive.

Quote:

I believe that virtually all Jung multiloops on these forums with unity gain output buffers will be equally stable with or without the inner feedback resistor R6, some may require a 1-2 pF cap in the inner loop feedback resistor position to provide capacitive feedback equivalent to the missing resistor’s parasitic capacitance...

It is not possible to accurately implement a 1-2pF capacitor on virtually any type of pc board material. Stray capacitances can easily amount to 4+ pF in and around the area of the op-amp. Attempting to balance the "missing" feedback capacitance, then, is very much like Don Quixote swordfighting windmills.

 

[KurtW]

I eariler published some Haromonic Distortion data for a couple of different values of R6 in the META42 circuit (you can do a search if interested). What this showed is that decreasing the inner loop gain by raising the value of R6 lowered the THD. As jeffreyj points out, lower THD levels don't necessarily result in a better sounding circuit. However I also posted the levels for the individual harmonics and showed that decreased interloop gain resulted in not only lower THD but more importantly lower distortion levels of the higher harmonics, especially the odd harmornics (3rd the 5th). Studies have shown that reducing the odd harmonics relative to the even harmonics correlates to better sound. This by the way was done at levels far below the input overdrive levels. I believe that this is the real value of adjusting the inner loop gain, rather than trying to get a stable circuit as it can be pretty stable with any values.

My recomendation to gracky and anyone else is to try some higher values of R6 (closer to the 1M value) to lower the inner loop gain. This seems especially important for lower speed buffers like the EL2001.