Wednesday, 29 November 2017

Tone Control Finished

After much waiting on parts, the tone control is now finished and in service.

The 250K Potentiometers took three weeks from order to arrival, in the meantime I'd been using components of the wrong value, so the characteristics were not correct.

Also the front panel has been an epic test of the patience to get the printing onto it. Several techniques were tried:

  • Using thermal transfer film - the same method I use for making PCBs - with the iron. Result: Design and lettering failed to transfer cleanly.
  • Using a cold-transfer method with a laser-printed design and chemistry (mix of alcohol and acetone). Result: A highly flammable and volatile mix of chemicals, complete failure to transfer lettering
  • Print onto paper, transfer paper, transparency (smooth and rough side), experiment with printer settings regarding toner etc - all to no avail.

In the end, the method that was the least dreadful involved covering the front panel with adhesive masking tape, and using a laser-cutter to cut the outline of the letters, then peeling them off with the tweezers to create a stencil, through which several coats of black spray paint were applied, before peeling off the adhesive then applying several coats of clear lacquer to protect the paint.

The results are not fantastic, but they are tolerable in the face of the spectacular failure of the previous methods attempted.

High on my To Do list is to devise a better method of printing onto aluminium.

Anyway, this is the device as completed

All assembled and in service

Full frontal. Yeah a bit of a Star Trek vibe in the labelling. The LED is just a power indicator

The transparent acrylic top gives a nice view of the insides.

Also there's a few extra photos here if you need to see more.

The circuit as built in the end. Note I changed the R and C values in the treble arm to even up the response on both sides

What LTSpice (Circuit Simulator) says this circuit should do at various control positions

  • This circuit works extremely well; listening tests reveal a completely neutral sonic signature, and that is using the cheap Shuguang 12AX7 tubes (all I had to hand)
  • The circuit is "quiet as the grave" - hum and hiss are inaudible even with the amplifier on maximum volume and ears pressed right up to speakers
  • The boost and cut levels measured on the oscilloscope (see earlier post) match closely with the predictions in LTSpice
I am very pleased with this circuit since it was my first attempt at designing an audio circuit on a PCB. Previously, my PCBs were limited to power supplies.

Waiting to be done: Distortion and noise floor measurements. Rainy Day activity.

This unit is now in service in my listening room, sitting between my RIAA stage and integrated amplifier.

This unit is fitted with the same very important SAF* Modifier as the integrated amp: A power-pass port, to allow the main amp to turn on the tone control and RIAA stage, so that multiple power switches don't need to be toggled to play some vinyl

* SAF := Spousal Acceptance Factor

Thursday, 9 November 2017

FFT Tests on the tone control

With the tone control board and power supply built, but the case still in the machine shop, it seemed a good opportunity to run some performance tests. The site I got the circuit from only had Spice simulations rather than measured results. So I don't know if this is the first time actual test results from this circuit are available.

Anyway, my testing method was to run a sine sweep into the input from 50Hz to 40kHz, then connect input and output to the oscilloscope, running the output into a 100K dummy load (to simulate the volume control of the amplifier it would be running into)

I ran FFT transforms at tone flat, full treble lift, full treble cut, full bass lift and full bass cut. The results are below. 

Please ignore anything below around 70Hz; my FFT process loses all resolution at that frequency.

There are two traces on each plot; the yellow trace is the input signal, for reference. The blue trace is the output from the tone control.

First up - the testing setup

Ain't it beautiful? Careful where you put your fingers!
The 9V is for the signal relay; if 9V is present the contacts close and the circuit is engaged. If no 9V, then the relay bypasses the circuit and bridges the input and output. When the case is made I'll wire up the 9V supply properly but for now it's Energizer-power

Tone controls flat. As mentioned in the text, ignore the region below 70Hz; my FFT process doesn't have resolution there

Bass full cut

Bass full lift

Treble full cut

Treble full lift

In each graph, scale is dB on the millivolt, 5dB per vertical division, yellow is input trace (for reference) and blue is output. (My FFT math capability to reference one signal off the other doesn't work properly, hence the presentation of both signals).

For completeness, I ran the same tests on the other channel. Results were identical.


This tone control is working exactly as anticipated, and is not far from the predictions on the source site.

Something seems to have gone right.

Tuesday, 7 November 2017

Gestation photos of the Tone Control project

Just a few photos of the project's gestation. Click each to make it bigger :)

The circuit I decided on. Simple tone control (bass and treble) with feedback. Uses an initial cathode follower and a final gain stage to compensate for the losses in the tone control section. Because it's an Active tone control, it doesn't need audio taper pots. Linear ones work fine. I managed to find some with a centre detent as well!

I decided to make this on a PCB just because I hadn't done this before. So being a complete novice at PCB design, everything is done by hand. No auto routing or anything else. This was my initial sketch

Then I designed the board. Measuring the components I intended to use by hand with the micrometer. Yeah, when I said "First Principles" I meant it.

The board, ready for etching. The design was made according to the size of the board blanks I had to hand, to avoid having to do any cutting. The transfer process involves printing the design with a laser printer onto a plastic sheet, then using a clothes iron to transfer it on to the board blank. Usually this requires a bit of touch-up with a Sharpie-pen but in this case it was almost perfect.

Work in progress. Starting to build the board. I discovered to my dismay that I did not have all the resistors I need, so there are some unfilled holes in this board. 

Also there's the power supply board which I made the same way, it's very simple and boring.

This whole project started because I bought the power transformer from the local auction site for $9. It has 213 - 0 - 213v secondaries, plus 6.3v. This gives a nice B+ of around 300VDC.

Also I had two 12AX7 tubes left over from a previous project - these are horrible cheap Chinese ones, but they work OK. Sufficient to test it, if I like it I might put some JJ or EH ones in.

More photos later when I get the case back from the CNC and laser etching...