Fourmix Issue 1 Parts List discrepancies

[eljay]

Hi Tony, I've just built an Issue 2 Fourmix using the V2.1 Builders Guide for PCB Isuues 1 & 2. I have been checking performance against an earlier Fourmix build.

I have previously built an Issue 1 PCB using the V1.0.0 Builders Guide. I have noticed that this earlier Fourmix outputs a slightly lower voltage offset (+/- 4.5V) from channels 3 and 4 than the Issue 2 variant and that the version I have lights the 5V level LED when channel 3 is turned up to full i.e at a value significantly less than the 5V it is supposed to indicate.

Looking at the Issue 1 parts list in the V2.1 builders I notice that there are a small number of component differences to the earlier V1.0.0 guide. These are : R18 now 120R, R23 now 11k, R22 now 22k, D6 and 7 now 2v7 Zener diodes, Q1 change from BC559 to BC560.

Are these changes correct and if so if I implement them will the result be that the 5V level LED behaves correctly and the Channel 3 and 4 voltage offsets will be the same as the Issue 2 PCB at approx +/- 4.7V?

[Synthbuilder]

I have noticed that this earlier Fourmix outputs a slightly lower voltage offset (+/- 4.5V) from channels 3 and 4 than the Issue 2 variant and that the version I have lights the 5V level LED when channel 3 is turned up to full i.e at a value significantly less than the 5V it is supposed to indicate.

Which output are you using to measure the output voltage? The LEDs should indicate the output level on the main output only and not the soft output. That said, the LED indicators are not precision meters and only offer a rough indication of output level. Wave shape and frequency will affect the actual values detected. DC accuracy will be affected by supply voltage and slightly by temperature. And the actual output voltage will depend on the pot's resistance which does vary a lot from pot to pot.

Issue 1 and 2 of the Fourmix have identical LED metering. Issue 2, however, uses a different soft clipping circuit to issue 1, hence the different values to the original issue 1 boards. The circuit used in issue 2 was developed when I did the soft clipping circuit for the Eurorack COTA and SVF modules and I thought it sounded better than the older design.

There was a subtle revision to the soft clipping circuit values on issue 1 boards over the lifetime of the Fourmix board. The schematic shows the original values. There is no right or wrong with these values though. They are just small tweaks I made over the eight or so years the older board was around.

The change from BC559 to BC560 reflects the sad demise of the BC560 in through hole.

[eljay]

Thanks for the reply Tony, I'm not sure I explained my query clearly but you have covered most of it.

I am getting a range of +/- 4.47V at the main output on my Issue 1 PCB with no external inputs and using channels 3 and 4 to provide a DC offset voltage. This is consistent with V1.0.0 of the builders guide and I just wondered why in the V2.1 builders guide the expected voltage range is slightly higher at +/- 4.7V (page 13 Testing). I do see a value of +/- 4.7V from the Soft Out though.

The +5V LED comes on when the main out shows a constant DC voltage of + or - 4.3V and I wondered if I needed to tweak this to get this value closer to 5V.

[Synthbuilder]

I just wondered why in the V2.1 builders guide the expected voltage range is slightly higher at +/- 4.7V (page 13 Testing).

It should read +/-4.5V. I'm not sure why I put 4.7 there. I'll get that changed.

Reducing R38 to 20K will allow the output to reach a maximum of +/-5V if required.

The +5V LED comes on when the main out shows a constant DC voltage of + or - 4.3V and I wondered if I needed to tweak this to get this value closer to 5V.

You can try playing with R16. Lowering to say 130K will make the metering more sensitive. However, I think I chose a value that would suit a typical audio signal rather than a steady state DC voltage. The meter needs to be fast enough to register short bursts of peaking signals but stay lit long enough to allow the user to notice it.

One other thing to note for both LEDs is that the turn on voltage is different to the turn off voltage. So the voltage has to drop lower to turn off the LED than it was to turn it on.

[eljay]

Thanks Tony, that clears up all of my queries, I think on reflection that I will leave things unchanged but thank-you for your feedback.