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 Forum index » DIY Hardware and Software » Developers' Corner
Expo converter trouble - need some help
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metal_head_82



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PostPosted: Mon Jan 02, 2012 5:50 am    Post subject: Expo converter trouble - need some help Reply with quote  Mark this post and the followings unread

Hey everybody!

I tried toget my single opamp SAW VCO to 1V/Oct (thread here: http://electro-music.com/forum/topic-49901.html ).

I swapped the opamp with an inverting schmitt-trigger from a 40106 IC. Works just like the opamp and is already a single supply part (basically it's the same circuit - the opamp is used as schmitt trigger).

The idea was to keep this design as simple as possible. So I tried the expo converter from René Schmitz's 4069 VCO. It can be found here:
http://www.uni-bonn.de/~uzs159/vco4069.htm
I just put both transistors to ground (0V) because I don't have a negative rail in my circuit. I can get the converter to work over 3 octaves. After that the response of the VCO seems to flatten. So if I try to adjust the 1V/Oct trimmer further I can get that 4th octave working but this will mess up the setting for the other octaves. Perhaps the expo converter needs some "high frequency compensation".

Anyone any ideas? I'm stuck here...
Thanks in advance!

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umschmitt



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PostPosted: Fri Jan 06, 2012 7:11 pm    Post subject: Reply with quote  Mark this post and the followings unread

Wow cool ! Three octaves are already nice for such a small number of components ! (Now you understand I'm not going to be of any help)
Actually I have a couple of questions : did you try the converter also with a regular op-amp ? And what did you do with that 22k resistor (initial CV ?) to negative rail in R. Schmitz design ?

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beep



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PostPosted: Mon Oct 07, 2013 10:49 am    Post subject: Reply with quote  Mark this post and the followings unread

PUSH!
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JingleJoe



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PostPosted: Sun Oct 13, 2013 1:49 pm    Post subject: Reply with quote  Mark this post and the followings unread

You may be reaching the limit of the VCO, use a smaller cap. This will also mean that lower CV is required to acheive the same frequencies you were hearing in your test.
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beep



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PostPosted: Tue Oct 15, 2013 7:25 am    Post subject: Reply with quote  Mark this post and the followings unread

that would just mean bigger CV input resistors, right?
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JingleJoe



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PostPosted: Tue Oct 15, 2013 12:42 pm    Post subject: Reply with quote  Mark this post and the followings unread

That would either make the range smaller or when combined with a smaller cap, decrease the availiable range.
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beep



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PostPosted: Tue Oct 15, 2013 4:41 pm    Post subject: Reply with quote  Mark this post and the followings unread

I tried the expo stuff today using the original single op amp saw vco circuit.
In the lower frequencies it works well but when entering the upper range,
the response gets weaker, just like metal_head described.

I changed the resistor then from 100nF to 10nF and noticed
that the oscillator is still at a high pitch even if the CV is at 0V.

I lowered the initial pitch then by replacing the 1M resistor at the
first transistor (the pnp) with a 10M resistor, but that fucks up the response.

looks like I need to have a negative voltage rail to get it working
properly, but I really want to get it working on a single supply.

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JingleJoe



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PostPosted: Thu Oct 17, 2013 12:07 pm    Post subject: Reply with quote  Mark this post and the followings unread

here's a crazy idea Smile rather then use a dual rail power supply, make a 1 or 2 volt power supply (you can use an buffer on a voltage divider) and use it to bias up the emitter of T2 (going by this schematic http://www.schmitzbits.de/vco4069.htm you'd just connect the emitter of T2 to the output of the buffer at about 2V).
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beep



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PostPosted: Tue Oct 22, 2013 1:30 pm    Post subject: Reply with quote  Mark this post and the followings unread

*__* I did not watch the forum for some time and didn'T recognize your answer. Thank you, that sounds interesting. I'm gonna give it a try tomorrow, don't want to upset my girlfriend tonight Smile

I'll tell the results immediately. Working on things together. I love this community Smile

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beep



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PostPosted: Thu Oct 24, 2013 2:35 am    Post subject: Reply with quote  Mark this post and the followings unread

I tried the 2V emitter biasing thing, but it didn't work. It works with a voltage of around 0,2V. SCale is still messed up, but I used BC547B/5547B transistors, maybe it'll work better when I have the suggested ones (3904/06)
and I need a proper 1K trimmer.

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rene_schmitz



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PostPosted: Sun Oct 27, 2013 10:04 am    Post subject: Reply with quote  Mark this post and the followings unread

Hi Beep and all,

I think the current that runs into the transistor buffer is the main problem. A FET would be a better choice here. And ultimately some form of reset time compensation is needed.

Cheers,
René

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beep



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PostPosted: Mon Oct 28, 2013 8:22 am    Post subject: Reply with quote  Mark this post and the followings unread

Hey Thanks for the answer Smile

since I'm a noob, I don't have any experiences with a FET and how
it could be integrated in the circuit. I have a few BF245C.
Do I use them just like NPN?

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rene_schmitz



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PostPosted: Tue Oct 29, 2013 5:25 am    Post subject: Reply with quote  Mark this post and the followings unread

A JFET source follower is superficially very similar to the BJT emitter follower. You might also need to change the resistor, 100k is probably too big. I'd start with 5-10k.

(Alternatively you can just use a FET opamp as a follower.)

Cheers,
René

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beep



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PostPosted: Tue Oct 29, 2013 9:07 am    Post subject: Reply with quote  Mark this post and the followings unread

I have no idea what to do... Very Happy

according to the VCO4069 schematic,
do I have to replace T1 or T2 or both?

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rene_schmitz



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PostPosted: Wed Oct 30, 2013 3:57 am    Post subject: Reply with quote  Mark this post and the followings unread

I had meant the emitter follower at the saw output in metal_heads schematic.

To clarify it, I've drawn this little schematic.
Posted Image, might have been reduced in size. Click Image to view fullscreen.

Its to be taken with a grain of salt, e.g. I've not drawn a trimmer for v/oct. And one probably needs to add a reset time compensation.
(Edit: thats now included in the drawing)

Cheers,
René

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Last edited by rene_schmitz on Wed Dec 04, 2013 6:59 am; edited 1 time in total
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beep



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PostPosted: Wed Oct 30, 2013 4:41 am    Post subject: Reply with quote  Mark this post and the followings unread

Hey, thank you for this schematic! Smile)

I'm actually doing this with Opamps and I have an op amp buffer at the sawtooth output.

I'm sorry if I steal your time, but I don't know what a reset time compensation is and how to build it.

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elektrouwe



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PostPosted: Wed Oct 30, 2013 6:22 am    Post subject: Reply with quote  Mark this post and the followings unread

beep wrote:

I'm sorry if I steal your time, but I don't know what a reset time compensation is and how to build it.


let me answer for René, enough time stolen from him Wink

when the sawtooth (Vc1) is ramping down it will reach the lower switching threshold of the 40106. The 40106 will then charge C1 as fast as possible until Vc1 reaches the upper threshold of the 40106 again. This action takes time - between some 100ns and some microsecs. Let's assume this delay time Tdly is 1us and the VCO frequency is 100Hz = 10000us.
The error caused by Tdly is 1us/10000us which can be ignored.
If the VCO is tuned to 20kHz=50kHz ,the error is 1us/50us =2% which could be a problem.
The solution would be to add a compensation voltage to Vc1, so that the 40106 always switches Tdly BEFORE Vc1 reaches the lower threshold !
In our 20kHz example the sawtooth ramp time should be 49us, @ 10kHz it should be 99us...
For CV=0 this compensation voltage is 0, for the highest CV this compensation voltage reaches the highest value: Vcomp is proportional to Fvco which is proportional to the sawtooth slope dVc1/dt which is fast for high charging currents and small capacitors:

dVc1/dt = I/C1

for a compensation dt =Tdly and dVC=Vcomp !
( draw a sketch and think about if you don't get it immediately Wink

the we can rearrange the equation:

Tdly/C = Vcomp/I

because Vcomp/I is a resistor (Ohms law) and
both Tdly and C are constant, it is obvious that we can just put a series resistor between C1 and the 40106 input with the value
R = Tdly/C
to completly cancel out any Schmitt-Trigger delays.
this is the reason why many sawtooth VCOs even work with slow TL061-opamps as Schmitt-Triggers: just the right R in series with the integrating C and you are done !
I discovered this compensation scheme a couple of years ago and when I checked other VCO designs I saw that this was a really old story and has been done since decades, but it seems that it is still not very well known among DIY-VCO builders Smile
PS: in high end VCOs you can often find another high frequency compensation. It is for compensation of the expo transistors bulk resistance. But that is another story...
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rene_schmitz



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PostPosted: Thu Oct 31, 2013 6:07 am    Post subject: Reply with quote  Mark this post and the followings unread

Thanks elektrouwe!

I just have two minor things to add. When you do reset time compensation in a schmitt trigger oscillator with a resistor in series with the cap (which is called a Franco Resistor, after Sergio Franco btw.), you will affect both thresholds, effectively just DC-shifting the signal instead of changing the amplitude. So you can to put a diode over the resistor to make it only affect one side.
(which explains why there's a 1n4148 across the 680 ohms resistor in the VCO4069 Smile)

The emitter bulk resistance compensation was developed by Dave Rossum.
There's some reference in the Oakley One of Three VCO builders Guide:
http://www.oakleysound.com/vco5-bg.pdf

Cheers,
René

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beep



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PostPosted: Thu Nov 28, 2013 3:03 am    Post subject: Reply with quote  Mark this post and the followings unread

eeeer... the main problem was to get this work on single supply without -12V rail Very Happy
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rene_schmitz



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PostPosted: Wed Dec 04, 2013 6:58 am    Post subject: Reply with quote  Mark this post and the followings unread

Hi Beep and all,

I think the reason for the bad performance of the expo on the positive side is that at some point there is not enough current available to open the npn. So one should buffer the voltage first. For this I've drawn now a compensated emitter follower between the pnp and the npn. (A single supply opamp could perhaps also do, but that would be cheating. Smile ) It looks good in simulations, I haven't built it however. When you add this buffering the input voltage range can be shifted towards the positive supply, and the negative supply is not needed anymore.

You'll find the image above in this thread updated. Also included are the HFT components now.

Cheers,
René

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beep



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PostPosted: Wed Dec 04, 2013 7:55 am    Post subject: Reply with quote  Mark this post and the followings unread

Thank you very much! You're great!! Will this circuit work the same when using an op-amp (single opamp saw vco) instead of the 40106?
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rene_schmitz



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PostPosted: Thu Dec 05, 2013 6:13 am    Post subject: Reply with quote  Mark this post and the followings unread

HI Beep,

if that opamp/CCO also needs it's current pulled to GND then it should work.
Do you have a link?

Cheers,
René

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beep



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PostPosted: Thu Dec 05, 2013 7:18 am    Post subject: Reply with quote  Mark this post and the followings unread

Posted Image, might have been reduced in size. Click Image to view fullscreen.

it's the vco design from the guy who started this thread.

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rene_schmitz



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PostPosted: Fri Dec 06, 2013 12:09 pm    Post subject: Reply with quote  Mark this post and the followings unread

This also sinks the current to GND, so it ought to work too.
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beep



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PostPosted: Sat Dec 07, 2013 3:07 am    Post subject: Reply with quote  Mark this post and the followings unread

Smile Thank you very much for your advice and the schematics!

Can't wait to breadboard it. Already bought the 10K NTCs. Smile

I know, dual supply is a much better solution for proper synth applications,
but I want to build something similar to the SH101,
just a playable synth, which can be powered from batteries.

Good for traveling.

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