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 Forum index » DIY Hardware and Software » fonik's place
fonitronik Thomas Henry VCO Maximus mods part 1
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ixtern



Joined: Jun 25, 2018
Posts: 101
Location: Poland

PostPosted: Tue Apr 09, 2019 2:09 am    Post subject: fonitronik Thomas Henry VCO Maximus mods part 1
Subject description: Mods for fonitronik Thomas Henry VCO Maximus to get better stability
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Thomas Henry VCO Maximus is a great design but suited rather for single-VCO setup or for making effects. In it's initial state it is not a good choice for multi-VCO setups.

When I have built fonitronik version of VCO Maximus (bought in Thonk, I like the Thonk) I have realized that it needs one fix and some mods to get better stability. Thankfully fonitronik PCB allows to do it due to an "easy", not densely packed THT design.

Mods/fix in short summary:
- positive voltage supply circuit with TL431 needs a fix (resistor value),
- negative voltage supply needs a mod (-5V voltage regulator),
- pitch control pots needs a mod (stable reference voltages + lower range),
- additional pitch trim is needed for setting initial frequency,
- pitch-sensitive resistors needs to be connected to a stable voltage rather than to supply,
- PWM control is too wide, needs limiting.

1. Positive supply fix.
In fonitronik version, 3340 chip may be supplied from more stable source than +12V bus - from TL431 shunt regulator by provided jumper. The problem is that with showed resistor values TL431 may not stabilize at all.

Lets look closer how TL431 here works (schematic in attachment).
From the datasheet we know that typical current draw for CEM3340 is 5 mA.
As TL431 is connected to the 12V through 100 Ohm resistor (RS1) - it means that at TL431 there will be a voltage no higher than 11.5V.
And it's even not including RS1+RS2 and TL431 currents.

Formula for TL431 voltage is:
Vout = 2.495*(1+RS1/RS2). When RS1 is 11k and RS2 is 3k, we get:
Vout = 2.495*(1+ 3.667) = 2.495*4.667 = 11.64V.
We can see that TL431 cannot stabilize output voltage. And if you use 5% resistors with some wrong deviation, things may go worse.

So the solution is to change RS1 resistor to lower value. 10k should be enough:
Vout = 2.495*(1+ 10/3) = 2.495*4.333 = 10.81V

So the current through RS3 will be:
(12V - 10.81V)/100 = 1.19/100 = 0.0119A = 11.9 mA

11.9mA - 5mA (3340) - 0.83mA (RS1+RS1) = 6.9mA.
This is current for TL431 - enough to stabilize (TL431 starts from 1 mA) and to have some reserve.

A notice: TL431 datasheet shows that TL431 may be unstable with output capacitor less than 2uF. Recommended value is 10uF. This circuit have 0.1uF only (CS) what may cause some trouble. I have not observed any oscillations in my VCO from the TL431 with 0.1uF value but I am planning also to replace CS with bigger one (tantalum perhaps).

To be continued...


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Last edited by ixtern on Thu Apr 11, 2019 12:28 am; edited 3 times in total
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ixtern



Joined: Jun 25, 2018
Posts: 101
Location: Poland

PostPosted: Tue Apr 09, 2019 2:36 am    Post subject: fonitronik Thomas Henry VCO Maximus mods part 2 Reply with quote  Mark this post and the followings unread

fonitronik Thomas Henry VCO Maximus mods part 2: a negative supply.

Almost every DIY 3340 VCO duplicates poor reference design from datasheet, especially with minus voltage supply. I think this design was made for marketing purposes, in times when voltage regulators were expensive, to show how cheap and simple may be a using CEM3340. Most of professional designs however used stabilized -5V sources. Also AS3340 datasheet, although copying reference design, at least mentions about recommended external -5V supply.

-5V external supply improves stability in two ways - stabilized by voltage regulator is usually better than by Zener diode and there is a less power lost in 3340 chip (so less IC heating).

There is nothing special about -5V design here - simple 79L05 with capacitor used (schematic included). Only placing it on a PCB needed some thinking (photos included) - regulator circuit was placed instead R4 resistor using socket and one additional hole drilled in the PCB (look at the photo)
.


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Last edited by ixtern on Thu Apr 11, 2019 12:29 am; edited 1 time in total
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ixtern



Joined: Jun 25, 2018
Posts: 101
Location: Poland

PostPosted: Tue Apr 09, 2019 4:14 am    Post subject: fonitronik Thomas Henry VCO Maximus mods part 3 Reply with quote  Mark this post and the followings unread

fonitronik Thomas Henry VCO Maximus mods part 3: reference voltages.

Every stable VCO needs stable reference voltages. In basic design Maximus VCO "reference" voltages are simple supply voltages. I've decided to add +-10V reference voltages to be independent from +-12V bus noise and instability.

Reference voltages are built on the TL431 shunt regulators (schematic in attachment). The reason is they are cheap (or even very cheap, when you buy 50 or more) and they are moderately stable (50 ppm temperature coefficient). No need for greater stability here as Maximus is not top-stability design. To get really good stability you need to use in sensitive places 0.1% resistors with at least 15ppm and of course good reference sources (good means 10ppm or better).

Circuit was built on a separate mini-pcb and mounted on a sockets (photos in attachment). As the load is rather low and constant, no output capacitors were included). Paths on VCO PCB providing +12V and -12V for the pots PCB were cut and instead +10V and -10 V connections were made.


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Last edited by ixtern on Thu Apr 11, 2019 12:32 am; edited 1 time in total
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ixtern



Joined: Jun 25, 2018
Posts: 101
Location: Poland

PostPosted: Tue Apr 09, 2019 4:35 am    Post subject: fonitronik Thomas Henry VCO Maximus mods part 4 Reply with quote  Mark this post and the followings unread

fonitronik Thomas Henry VCO Maximus mods part 4: coarse and fine range, pitch trimpot and pitch sensitive connections.

Coarse pot range in VCO Maximus is incredibly large - from sub-Hz to the the ultrasonic area. Such span is not only unnecessary but also greatly affects stability. Even several mV change on +-12V bus results in audible pitch change.
For multi-VCO setup my choice was to limit coarse range to about 1 octave and fine range to about 1 semitone. So some resistor values were changes.

Also pitch-sensitive resistors were in original connected to +12V bus (although there was more stable voltage available from the TL431). They were reconnected to TL431 output (10.8V). A PCB path were cut and wire connection added also. It may be connected to +10V ref instead, don't know if it made any substantial difference here.

Pitch trimpot was added to set initial VCO pitch (to
the such frequency that +1V from V/Oct input gives C1 note - 32.7Hz).
I have used 100 kOhm trimpot but 50 kOhm would do probably better.
It was placed instead of R55 (300k) resistor.


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ixtern



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Posts: 101
Location: Poland

PostPosted: Tue Apr 09, 2019 4:40 am    Post subject: fonitronik Thomas Henry VCO Maximus mods part 5 Reply with quote  Mark this post and the followings unread

fonitronik Thomas Henry VCO Maximus mods part 5: final view

Photos of top VCO view without and with added modules attached.

Remains topic of limiting PWM control - maybe later.


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Rob Kam



Joined: Jul 26, 2016
Posts: 6
Location: London, UK

PostPosted: Sun May 26, 2019 1:48 pm    Post subject: Reply with quote  Mark this post and the followings unread

Many thanks for this, very interesting and helpful.
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ixtern



Joined: Jun 25, 2018
Posts: 101
Location: Poland

PostPosted: Sun May 26, 2019 11:02 pm    Post subject: Reply with quote  Mark this post and the followings unread

Rob Kam wrote:
Many thanks for this, very interesting and helpful.

You are welcome.
Must to mention that some of the add-on modules (trim pitch, ref voltages) I have soldered to the board finally - connectors were not very reliable.
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