Saturday, 28 October 2017

KT88 Amp Freq Response measurements

With the big KT88 amplifier nearing one year old, it occurred to me that I had never performed a frequency response measurement on the whole amplifier.... I'd done plenty of them when I was prototyping the preamp and driver stages, but I hadn't done anything other than cursory checks with the oscilloscope over the entire amplifier.

Today I set about rectifying that omission. My method was simple. Move the amplifier to my (newly tidied) workbench, attach the function generator to the input, dummy load to the output, set up a sine sweep from 50Hz to 50hKz at 200mV RMS, open the taps until the amp is producing about 5v RMS on the output, then start the FFT process.

This resulted in the following:

So the yellow trace reveals that the input signal is not exactly flat... about 1dB down by 30kHz. This is why I showed the input signal, for a reference.

The blue plot is the speaker terminals. the load is an 8 Ohm carbon resistor on a heatsink.

So what does this tell us? I read this as the amplifier being reasonably flat (within 1dB) to 40kHz whereafter it drops rather quickly. 

My 'scope does have a "Math" facility where I can supposedly reference one signal against the other. This functionality seems buggy however, which is why I avoided it.

I am happy with this frequency response.

Monday, 16 October 2017

New project - tone control

After building the last two amplifiers, I've had several months of not building anything, while I wait for the next firm customer. Lots of people expressing interest, but no-one ready to put any money down... yet.

So in the meanwhile I've been keeping an eye on the local auction site and pouncing whenever anything that looked suitable for a future project came up.

Latest score was a small power transformer with 213-0-213 secondaries (so good for 300V B+) and a handy 6.3v also.

Size suitable only for a preamp, this transformer cost the grand total of $8.00

With a couple of 12AX7s left over from my last project, thoughts turned to the possibility of making a headphone amplifier, nice idea except I didn't really need one.

Then I saw a site with some tone controls. My phono cartridge is a little down on treble so it seemed like a good idea to build a tone control based around a pair of 12AX7s. Idea being to put this between my RIAA stage and the integrated amp.

This is the schematic:

(Heaters don't need elevating; it's an error in the notes)

Just to challenge myself, I plan to build this on a PCB rather than on a chassis with point-to-point. The reason for this decision is so that I can at some future point take this PCB and transplant it into something else, if needed. Or alternatively, make up another one quickly if needed.

The next challenge is to arrive at a PCB layout. Being a complete novice at PCB layout design, my preferred design method is the same as the constipated accountant: Work it out with a pen and paper.

Or in my case, because I am not a complete luddite - my Surface Pro computer with the drawing pen.

So. This is the concept drawing of the PCB - obviously I'll duplicate it for stereo - and there will also be a pair of relays to disconnect the inputs and outputs from the circuit and tie them together for a tone bypass.

Next step will be to measure the components and design the PCB.

Work in progress.

Just for reference, my Phono preamp I am using is the 3-triode "Little Bear"

This machine uses 6N2 tubes, two for gain and one on the output as a cathode follower.

I've tested its RIAA response using the FFT function on my USB oscilloscope:

Blue is input 5mV RMS Sine sweep,  yellow is output. Scale is dB on the millivolt. Ignore the input below 70Hz, hum due to unshielded leads.

From this it's clear that the RIAA stage is behaving itself, so the blame for the slight treble loss from the turntable must be with the cartridge. An Audio-Technica ATS-11 with a band new Shibata stylus that cost considerably more than this tone control will.

(The treble loss is ascertained by listening tests by doing an A/B comparison with the same music played from vinyl and a digital source, synchronized at playback. Other than that, the vinyl sounds fantastic.)