Vintage John Linsley-Hood ‘1969’ amplifier build

The interesting bit is how he figured out how to look at the response of my amp. First we looked at the maximum output by putting an 8 Ohm load on the output, attaching a scope and pushing it until it clipped. We found that despite the bias (voltage at VE in the circuit diagram) being set halfway, it actually clipped the bottom of the waveform about 3 Volts earlier than the top. That means the bias can be moved up a couple of volts.

Then we had a look at the frequency response with a sine wave. Completely flat at the top end up to 20khz with no evidence of roll-off. At the bottom end it starts to roll off at about 50hz (despite it sounding like it has a pretty strong bottom end), which is probably due to the small input-coupling capacitor. If I increased the value from 0.47uF to 1uF, it would probably drop the low-end roll off down towards 20-25hz.

There was a tiny bit of ripple present in the power supply which I could faintly hear on my speakers. We could see this on the scope. We could also see a bit of harmonic ripple in the supply voltage at the same frequency that the amp was driving. So that was not visible at low outputs but became just visible at very high outputs. Very interesting.

Then we switched to square waves to look at the transient response. Essentially, it holds an almost vertical rise and fall on the square wave up to about 100khz. A little bit of ringing noticeable at 20khz. Down at the bottom end it shows the usual voltage slope at the top of the waveform due to having input and output coupling capacitors. This is because the capacitors block DC, and the top of a square wave is essentially DC so the capacitors block it from getting through.

This was where the clever bit began. That soundcard and REW software he used can also be used to compare the signals going in an out of an amplifier. That tells you not only what the frequency response is, but also what the total harmonic distortion is and whether it’s second order, third order etc. First he connected the soundcard output to the soundcard input and ran a frequency sweep. That generated a curve which showed the sound card’s deviation from a perfect flat response. The software used that as a correction factor to flatten the response. Then he connected the sound card output to the amp input, amp output to the soundcard input (across a voltage divider) and repeated the frequency sweep. This showed my amp had a completely flat response throughout the audio spectrum, give or take a half a decibel which was more than likely error in the correction factors etc. Total harmonic distortion 0.046%, so it’s better than the original design brief of 0.1% or better.

I’m really happy with that and totally surprised. I think if there are any tweaks to do it might be to replace the input coupling capacitors with 1uF caps and maybe put some small inductors inline with the power supply lines to filter out the ripple. Move the bias point up a little and I think that’s really all they need in terms of electronics. We did also stick a thermocouple on the heatsinks and found the PSU heatsink sits at around 60°C and the output stage transistor heatsinks sit at around 75°C. I don’t mind the PSU running at that temperature, but I do know older transistor designs are less tolerant of running that hot. So I might look at increasing the cooling since these are very rare transistors. I have one spare, but it is likely I might not be able to replace them like-for-like if I pop them.