In addition to their inherent noise, tubes have one other drawback when used for mic pres and that is microphony. When you tap them, an audible sound can sometimes be heard. Again, the degree to which this happens varies a lot from tube to tube of the same type so again it is necessary to select tubes for low microphony.
Now I have cured the hum issues with the demo mixer I am in a position to measure the noise of the mic pre and also to test various tube types for noise and microphony. I used the Helios 69 input channel with the EQ switched out. First I fed in a signal at -60dBu into the microphone input , set the gain to maximum (60dB), the channel fader fully up and then adjusted the second stage gain for 0dBu at the channel direct output. This means there was then a total of 60dB of gain from the mic input to the direct out. To measure noise I then plugged in a shorting XLR with pins 2 and 3 shorted,into the mic input. I then used my Lindos audio test set plugged into the direct out to measure the noise levels at the output.
A brief aside is necessary here to understand the results that will be presented. Noise in microphone preamps is usually evaluated relative to the thermal noise generated by the resistance of a typical microphone that might be plugged into it. A typical mic has a source resistance of around 150 ohms and this seems to be the standard by which mic pres are measured today. The rms thermal noise due to a 150 ohm resistor at 20 degrees C in a 20KHz bandwidth is close to -131 dBu. If we amplify this by 60 dB then the noise we measure at the output should be 60dB higher or -71dBu. The noise measured will in fact be higher than this due to the additional noise generated by the mic pre itself. If you measure the rms output noise and subtract the gain then you get the noise level at the input of the mic pre. This noise level is called the equivalent input noise or EIN for short. In a perfect mic pre with a 150 ohm source, the rms EIN should be -131dBu. In a real mic pre you would expect it to be a few dBs higher.
Notice I said the 'rms noise' from the resistor. Most measuring systems cannot directly measure rms noise. The simplest are average reading and are calibrated to read the correct rms value when presented with a sine wave, but noise is not a sine wave so these can produce inaccurate readings for noise. In addition, weighting curves are often added to the output signal supposedly to more accurately reflect the response of the ear. The A weighting curve is one of the most popular and is often favoured by marketing departments because it tends to improve the EIN figure by a few dB. There is an international standard for measuring noise (ITU-R 468) which includes a curve that adjusts for the response of the ear and also includes a quasi-peak detector to account for the ear's sensitivity to short, spiky bursts of noise that simply would not show up on an rms reading device.
The bottom line is that the value of EIN you get depends a lot on the type of measuring device you use and the weighting you apply to it. This article by the Institiute of Sound and Communications Engineers lists the various methods in common use and the differences in the the results obtained. Many manufacturers quote EIN figures without giving any further information about how the measurement was made and what weighting was applied. It is not surprising that many of these quote an EIN close to -131dBu. The more honest ones achieve figures in the -128dBu region. For comparison, back in the 70s, Neve used to quote the EIN of its mic pres as -126dBu.
The tests I conducted used my Lindos test set which meausres noise to ITU-R 468 including the quasi-peak reading meter. This gives a more realistic figure for EIN but it can be anything from 4 to 7dB worse than the figure obtained using the simpler and conveniently weighted measuring methods. If you look at the published specifications for classic tube mic pres you will generally find that they quote an EIN figure of 'better than -120dBu' but usually without specifying how the measurement was made. We would like to do better than this. In the EZ Tube Mixer mic pre, the noise is determined principally by the 12AX7 tube used at the input. I tried three different types of 12AX7 in my tests:
- 12AX7LPS made by Sovtek in Russia. I tested 8 of these.
- 12AX7EH by Electro Harmonix and also made in Russia. I tested 9 of these.
- 12AX7WA another variant made by Sovtek. I tested 11 of these.
- Microphonic - these tubes were judged to be so badly microphonic as to be unusable. With tubes in this category you got a very loud pinging noise when they were tapped and you could clearly hear your finger gently rubbing the glass envelope of the tube.
- Good - these tubes produced an audible ping but it was less than 20dB above the noise level
- Very good - these tubes produced a barely audible ping which was less than 10dB above the noise level
All of these are of course subjective measurements which is why I did not feel able to split them into more than three categories. Note that none of the tubes was completely lacking in microphony. Every tube tested was microphonic to some degree. The results were as follows:
12AX7LPS - 6 of the tubes measured -63dBu for noise; one measured -62dBu and one was -58dBu; 2 tubes were microphonic, 5 were good and 1 was very good.
12AX7EH - 2 of these measured -63dBu for noise and 7 measured -62dBu; 8 had good microphonics and 1 was very good
12AX7WA - 9 of these measured -63dBu for noise and 2 measured -62dBu; 1 was microphonic, 2 were good and 7 were very good.
In conclusion, the 12AX7WA was the best all round with many measuring -63dBu noise level and plenty of very good from the microphonic point of view. Both the 12AX7LPS and the 12AX7EH turned in good noise performances at around -62dBu to -63dBu but their microphonic performance was not as good as the 12AX7WA.
I think the 12AZ7WA is the best choice from the noise and microphonics point of view. This is interesting because the internal construction is much shorter than the EH and LPS versions so perhaps this makes them more rigid and accounts for their better microphonic performance. What is also interesting is that I had previously discounted them for use as mu followers because they had rather higher levels of intrinsic distortion compared to other versions of the 12AX7, the LPS in particular producing very low levels of distortion.
The EZ Tube Mixer mic pre topology uses an SRPP output stage which I expect to be the main contributor to the overall distortion level. The 20dB or so of negative feedback in the mic pre is designed to reduce the SRPP output stage distortion by tenfold. This means I do not expect the distortion of the input stage 12AX7 to make much of a contribution to the overall mic pre distortion. Just to make sure, I measured the 1KHz distortion for the WA, LPS and EH types fitted in the first stage. The distortion measurements were made with the gain set to 30dB and with an input level of -20dB. This means the input to the 12AX7 is 0dBu and the output from the direct out transformer is at +10dBu. This means the SRPP output stage is running at +16dBu since the transform steps down by 6dB.
The results were remarkably consistent. The LPS type produced 0.094% distortion, the EH type produced 0.096% and the WA produced 0.094% distortion. This simply confirms that the vast majority of the distortion occurs in the output stage as expected.
It seems that the best noise, microphony and distortion is obtained with selected tubes of the WA type. The typical -62dBu measured output noise represents and EIN of -122dBu. This meets our target of better than the classic tube mic pres and if we add only the minimum 4dB to convert this measurment into its rms equivalent we are achieving an EIN in the region of -126dBu which is as good as a classic Neve.