Friday, January 27, 2017

A Test Amplifier for Tube Circuits

Have you ever just needed to test a driver-type tube stage but your sound card output can't put out enough voltage drive to get it where it needs to go? Well I have, and here is how I solved my problems.

I searched around and looked at high voltage op amps and found a few candidates. The one with the lowest distortion and noise specs that I could find was the ADA4700. I built a simple non-inverting stage with a gain of 20 and a 100k feedback resistor, powered by a couple of medical isolated 48V wall-wart power supplies (Meanwell GSM25U48-P1J) and got some pretty good performance. See below:

The above results are for 10Vrms, 20Vrms, and 30Vrms respectively. I was pretty happy with that performance, but I was getting more distortion than I expected from reading the ADA4700 datasheet.

Later, I was reading a small signal audio design book by Douglas Self and he brought up the point that some (most?) op amps suffer from much higher distortion when used in a series-feedback configuration than if used in a shunt-feedback configuration. He notes that the LM4562 is a notable exception.

I wanted to have the high input impedance of a series feedback configuration but the low distortion of shunt-feedback so I decided to try a two-stage version with an LM4562 inverting amplifier as the input stage and the ADA4700 inverting amp to get the voltage output up to 30Vrms. Particularly, I wanted to minimize higher-order harmonics so that I could recognize them in the tube stages that I wanted to test.

Here is the schematic of what I came up with:

I had the idea that I could put filter capacitors across the feedback resistors and that might have some positive effect on suppressing higher order harmonics. I added a bit of gain to make up for the loss and set the -3dB point at 1kHz for the filtering of each stage. Here is a picture of the circuit as built:

The ADA4700 only comes in a surface-mount package with a thermal pad on the bottom so I bought a prototype adapter, bent the leads, and flipped it on its back. I make a makeshift heatsink out of a couple of pieces of solid-core wire. I don't think this is really needed at the loads I am driving but it was easy to do, so I did it. I stuck with a surface-mount LM4562 because it fit well right next to the ADA4700 on the prototype board. The filter capacitors are placed in parallel to the feedback resistors and easily soldered/unsoldered on one side to take them in or out of the circuit. How does it perform?

First, here is what the distortion of my test setup looks like (including an M-Audio M-trak II plus and a high-impedance input version of Pete Millett's soundcard interface):

Here is distortion of the LM4562/ADA4700 circuit at 10Vrms, 20Vrms, and 30Vrms output, without the filter capacitors in circuit:

I was happy with the reduction in distortion of the new circuit. Let's see what happens with the filter capacitors in:

The first thing I noticed is that the noise floor is significantly lower at frequencies above mid-band. This could be helpful in detecting small levels of higher order harmonics in testing. As you can see, THD is also slightly lower. The main reduction comes from suppression of 2nd harmonic. Third and higher harmonics seem to come up a little bit. Noise floor seems unaffected in the 30Vrms plot but that is because I have to switch ranges on my Pete Millett sound card interface to a setting with a higher noise floor. The sound card interface becomes the dominant noise source in that plot.

I think I will stick with the unfiltered configuration mostly but I can see situations arising where I might want to put the capacitors back into the circuit.

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