Last night I did some more detailed tests using my Lindos test set. The set up was that the first stage is loaded only with the 10K pot. The second stage drives an Edcor 2.4K/600 transformer loaded with 600 ohms.
I fed a -10dBu 1KHz sine wave into the DI input and adjusted the 10K pot for 0dBu at the output. The measured distortion was 0.08%. I then increased the input level to 0dBu and again adjusted the pot for 0dBu at the output. This time the distortion was about 3 times higher at 0.23%. This is the normal distortion proportional to level you expect with a triode. However, as the first stage has about 29dB gain, the level across the 10K resistor is about +29dBu (about 22V rms), the first stage must be getting close to clipping. I then increased the input to +6dBu and the distortion rose to 0.57% so still roughly proportional to level. So I increased the input to +12dBu and the distortion shot up to 7.1%. With +12dBu input, the level at the 10K pot should be +41dBu which is 87V rms!!! I measured across the pot with my DVM and it was 80V rms which is +40dBu so it is within a dB of what was expected.
The bottom line is that the mu follower loaded with 10K can output a huge level before it really starts to distort.
The next test was with low input levels but with the gain pot turned up so we get higher levels form the output. So I started again with a -10dBu input and the gain pot set for 0dBu at the output and a distortion of 0.08%. I increased the gain pot so the output level rose to +10dBu and the distortion rose to 0.29%. At +15dBu out it rose to 0.43% . At +17dBu it was 0.63,and at +20 it was 6%. At+21dBu this shoots up to 24% so clearly this is where clipping occurs.
All this means is that, as expected, the distortion you get and the level at which you get it depends mostly on the load. With a 10K load, each stage can output a phenomenal +40dBu before clipping. However, with a 2K4 load (the 600 ohms reflected back via the Edcor transformer) it starts to clips at +20dBu output. For a single stage with a transformer, as in a dual DI unit, you need an input of about -10dBu or 300mV to reach clipping. Most modern guitar pick ups will achieve this level. SO, I would recommend loading the output with 600 ohms and adding an output attenuator to set the level and use your 1Meg as the DI input to set the drive level.
if you find you need more distortion at lower levels than there are other options. The mu follower is designed to give as little distortion as possible. If you change it to an SRPP stage and lower the quiescent current then you should get quite a bit more distortion at lower levels. If that is of interest I will add the details of how to do it to the build document.
Cheers
Ian
