Get thy bearings

Merry Christmas to all!

When I was still a teenager and learning saxophone I came across this wonderful version of “Get thy bearings” by King Crimson. The power of the electrified alto sax  blew me out my mind. Ian McDonald clearly found a way to drive the saxophone there and bring a new sound to the 1969’s progressive rock. Surely he didn’t experience any of this slew rate thing 🙂

In my previous post, I explored the slew rate challenge of the 01a preamp:

The first workaround was the output follower as shown in the below diagram:

The MOSFET follower can be any one of your choice, either depletion or enhancement types. I use the STP3NK60ZFP which has a small reverse capacitance (Crss) of 8pF and is a handy plastic TO-220 device which can handle 600V.  It also comes with input protection diodes which is very handy.  Of course you can use what you have at hand.

The second option here is to add a sink CCS to provide the current needed to charge or discharge the load capacitance as we discussed in my previous post. A simple DN2540 CCS can be added to sink 10mA but we will also need to change the lower jFET to a BF862 which can handle higher currents. A J310 can also work here but the BF862 (albeit SMD) can be easily obtainable these days and is a better device overall. The other minor point to bear in mind is the top MOSFET (M1) will need a small heatsink:

The  performance of this circuit can be seen below. The output swinging at 10V peak to peak into a 2,000pF load will need 2.4mA peak of current at 20kHz. The 2,000pF presents an impedance lower thank 4KΩ to the hybrid mu-follower:

The interesting point to highlight here is that despite the CCS sinks 10mA, it’s a stiff CCS and in AC it will only swing 200μA at 20kHZ/20Vpp. The burden is still on the follower:

The sink CCS helps here but not much above 10kHz which cannot match the performance of the isolation provided by the follower. However, it’d be interesting to listen to both options. The sink CCS is likely to be less audible (if any of them are) than the FET follower. I have implemented the FET follower in the past with great success so I can bet that either option is good here.