i think this will become a running post.
ok, i left off “part 1” with a brief explanation of how and why an “affordable” (let’s just say it: cheap) commercial tube mic pre came to be. some notion of the design issues, along with some of the issues of amplifying balanced signals, staying balanced (in order to reject unwanted environmental noise and interference sources), and above all economy. an effort to design “good bones” into the gadget also leaves us a chance to explore the possibility of building upon this design as an affordable way to get somewhere substantially better. this idea is something that can quite often be expanded to many older professional products, as well as some unusual currently produced things.
from the previous segment, if we simplify the circuit so we can see the “bones”, it is fairly obvious that we have a differential cascode gain stage, followed by a pair of cathode follower buffers. a very simple 2 stage circuit. a constant current source in the common cathode forces a high degree of balance between the two halves. the gain of the circuit using the original parts is about 56dB in practice. this is an “average” and was arrived at using the audio precision “portable one” owned by EH. the spice version predicts 60dB. i take this as a home run and proof that the models are pretty good! the gain is largely determined by the transconductance of the FETs, which is modest. this is enough gain for most mics, but definitely not for ribbon mics, or some older condensors in any job other than close placement. i am not a fan of close placement, but it is a lucky thing that almost every other recording engineer is. still, gain is an area of improvement.
small offsets and AC common mode stuff between the inputs of the diff can be passed along this circuit if one “side” of the output gets loaded down more than the other. as mentioned previously, the 12AU7 buffer does not have a low enough impedance to be “immune” from loading effect. this is an open loop design, so there is no error correction at all. a lower output impedance would be an improvement. it is the case that some “prosumer” gear, namely recording interfaces, that have economical methods of gain adjustment at their inputs, do not have completely symmetrical input impedances because the gain is adjusted with negative feedback, and the “balanced” inputs are subtley different at certain gain settings. this is typical where the input circuit uses an inverting opamp stage although why is beyond the scope of this installment. suffice it to say that as the amount of feedback goes up (the gain goes down), one input’s Z goes down (the inverting) and the other (non) stays the same. if no effort has been taken to account for this in the design (a sign of economy), offsets and common mode can be coupled through by the gain of whatever it is in front of the recording interface (the mic pre). this economy is acceptable generally because the impedances commonly encountered are isolated from the outside world with instrumentation amp circuits and other methods to extend the CMRR of typical op amp based mic pres. but this simple tube pre has none of that! it’s an open loop, “let it rip” kinda thing. it works fine as long as the output sees a balanced load, but if it doesn’t, you can get hum and ground born noise coupled through from input to output, as well as from the power supply. so another area of improvement would have to be isolation.
and finally, noise is a pet peeve of mine… way more than harmonic distortion. nonlinear and enharmonic distortion is rarely nice in pre/amps either. and it is so often the case in cheap gear the S/N ratio and distortion spectrum are roughly considered. it is “cheap” after all. as long as these specifications fall into a certain range, no further effort is spent and the “sound” becomes fixed by this limit, plus any nonlinear/harmonic attributes part of the system. most of the op amps used in the business have a class B output stage and are also run LEAN at the input. there is so much feedback, it is assumed that all forms of nonlinear distortion are “taken care of” and the noise floor is determined by the Gm of the input transistors (the bias) and the various resistive elements (all impedance creates noise) of the design. this is fantasy. of course useful, but still not what many consider to be the case. many of the most commonly used op amps add crossover distortion particularly to small signals (the output stage has to turn on…), such as those encountered in a mic pre. and of course the noise floor can be impacted negatively with small signals (noise) that just cross the threshold of “on”.
another more controversial issue concerns the classic op amp, which has a gain – bandwidth relationship determined roughly by the miller “compensation” between the first 2 stages. this means that if the gain is 100dB at 10Hz, it is often only 32dB at 20KHz open loop (6dB per octave drop). the feedback error correction is not uniform for the bandwidth of the particular arrangement. there is much less feedback at higher frequencies always… also more distortion and noise. the general opinion is that this is unimportant, as far as the audio bandwidth is concerned. and for cheap stuff, doubly so. it is a good idea to look at the noise and interference sensitivity of the design very closely so as to reduce these things… a wideband open loop design can equal or improve upon this, and need not involve the complexity required for having even more gain and then losing it with error correction. do it right from the beginning and you don’t need to fix it afterwards.
you can see an open loop op amp (no criticism of it… it is typical) above. compare with the open loop sweep of the EH12AY7 mic pre below…
the gain is MUCH less in the tube gadget, but the bandwidth isn’t bad at all. the S/N for a 4558 or TL074 diff with the same gain and a 6.8K input impedance isn’t much different! it’s still 70dB roughly, for a 3mV RMS input signal.
ok, so the fourth area of improvement would be the noise and distortion spectrum, which ain’t bad already… this does imply one important last improvement: the power supply. these five areas are where i will focus my energy. below is a simplified starting point. i have removed the phantom supply, bass cut, phase reverse and monitior sidechain temporarily, just for visual simplicity. next installment will explain what has been changed and why.
