Tempco...

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snaper
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Tempco...

Post by snaper » Thu Aug 28, 2014 9:01 am

Hi Wigglers,

As Im getting deeper and deeper into DIY, I've encountered several times with tempcos.

My question is.
Are these special temperature compensator resistors only for ...argm ... temperature compensation?
I mean, if "tune" or v/oct stuff is not important for me (my modular is basically a big noise source, won't use it for "melodies...") can I replace them with regular, simple resistors?

I'm asking this, cause Im building 3 VCO-s right now, 2 Arp Axxe and a 258 (2 pcbs actually)
Both designs have 2k or 1 k tempcos.

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Post by baloo » Thu Aug 28, 2014 9:05 am

Yes, you can use regular resistors if you don´t care about good tracking of your VCO.

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Post by ehochstrasser » Thu Aug 28, 2014 9:07 am

You can replace tempco's with regular old resistors without a problem, you just won't have accurate pitch tracking and you may get some frequency drifting during use.

That being said, most of the synth parts dealers sell them for fair prices at the moment. I know synthcube has a pretty good price on tempcos if you decide to go that route.

Best of luck!

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Post by CLee » Thu Aug 28, 2014 9:24 am

My understanding, please correct me if I'm wrong...

The tempco won't effect your tracking, what it controls is the frequency drift with temperature. You can have the VCO track 1v/oct, but the drift with temperature will make it hard to keep it in tune.

Your VCO is going to drift around (quite a bit actually) as it warms up or with any random changes in ambient temperature. So the question is... is it important that when you set the frequency of the VCO that it stay there? If so, spend a few dollars on the tempco resistor. Like it's been mentioned, they're not that expensive these days.

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Post by Randy » Thu Aug 28, 2014 9:46 am

Does the tuning eventually stabilize when the operating temperature is reached? There are VCO circuits out there that do not use tempcos and "in the old days" I would head down to the bar to turn my synth (Oberheim OB-X) on at least 1/2 hour before the show to let it warm up before tuning it.

I still do that with my Anushri and my Eurorack, although maybe 10 minutes instead of 1/2 hour.

Randy

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Post by CJ Miller » Thu Aug 28, 2014 10:11 am

Randy wrote:Does the tuning eventually stabilize when the operating temperature is reached? There are VCO circuits out there that do not use tempcos and "in the old days" I would head down to the bar to turn my synth (Oberheim OB-X) on at least 1/2 hour before the show to let it warm up before tuning it.
It depends upon the ambient temperature. If the temperature does not change, the frequency does not either. But it is a complex scenario - ambient temperature does change, and lots of gear dissipating its own heat even affects the ambient temperature of the room. You can tune for whatever the temperature is now, and this will work, but the temperature will change sooner or later.

This is what expo converter "ovens" take advantage of - they heat the differential pair in excess of the ambient temperature, so minor fluctuations do not affect it at all. But this is inefficient, as it depends upon always wasting energy to maintain the temperature. And it is hard on the trannies, as chips are generally not made to be heated.

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Post by Dr. Sketch-n-Etch » Thu Aug 28, 2014 10:56 am

Just wanted to mention two things:

1) Proper tempco requires that the temperature-compensating resistor be in thermal contact with the expo converting transistor pair -- sitting on it and glued to it with thermal paste is the preferred way. Just having a tempco resistor sitting somewhere else on the PCB is not terribly effective.

2) There are other ways to get tempco. I use one quarter of the 2164 quad VCA chip as an expo converter, and another quarter of the same chip as a temperature compensating current amplifier to the CV summer. This obviates the need for a tempco resistor and the inconvenient circuit layout this often necessitates.
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Post by snaper » Thu Aug 28, 2014 11:01 am

Now I see...
Honestly, as I wrote in my starter post, I'm not using my rack as "normal" synth.
Basically its for generating organic random sequences, I never care about tuning or something like this...
In the other hands, I've got a great FC System X VCO, without tempco...never experience any drifts...

The reason for the question was that my L-1 PCB artwork for the 258 VCO needs a daugterboard, with SMD tempco and tranny...

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Post by Graham Hinton » Thu Aug 28, 2014 11:10 am

CLee wrote:please correct me if I'm wrong...

The tempco won't effect your tracking, what it controls is the frequency drift with temperature. You can have the VCO track 1v/oct, but the drift with temperature will make it hard to keep it in tune.
Sorry, you are wrong.

Frequency drift is caused by the two transistors in the exponential converter being at different temperatures and minimised by them being on the same substrate. This is temperature balancing. Temperature compensation with tempco resistors tries to stop the Volts/oct changing by matching the exponential term which is proportional to absolute temperature. At 25 deg C the amount of change for 1 deg is 1/(273 + 25) = 0.33557% or 3355.7ppm. That is where the figure comes from, it is not 3300pmm or 3000ppm or 3500ppm or anything else.

Note that the normal tolerance of tempco resistors is ±10% and changes with every reel of wire that they are made from. 3300pm -10% is way off (2970ppm). I have had some special ±1% tempcos made and 3316ppm to 3389ppm is not close enough either, but that was the best the manufacturers could guarantee.

If you buy a keyboard synthesizer the tempcos are probably from the same batch and equally wrong so the VCOs are also equally wrong and will track each other, but may not be absolutely in tune. If you take two synthesizer modules of different design and different tempco types, or even the same design with different batches of tempcos, the chances of them matching are very small unless each have been calibrated exactly to 3355ppm, e.g. using Ian Fritz's method. Just sticking a 10% part in doesn't cut it, it's just better than not putting it in at all.

Randy wrote:Does the tuning eventually stabilize when the operating temperature is reached?
No, the transistor temperature changes every time you play another note. If you go up an octave the current through the transistor doubles and its temperature rises.
There are VCO circuits out there that do not use tempcos and "in the old days" I would head down to the bar to turn my synth (Oberheim OB-X) on at least 1/2 hour before the show to let it warm up before tuning it.
That would have had Curtis VCO ICs which did not use tempcos, but they were still compensated, just by a different method. There were very few classic synthesizers that did not have some form of temperature compensating.

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Post by snaper » Thu Aug 28, 2014 12:16 pm

Graham Hinton wrote:
CLee wrote:please correct me if I'm wrong...

The tempco won't effect your tracking, what it controls is the frequency drift with temperature. You can have the VCO track 1v/oct, but the drift with temperature will make it hard to keep it in tune.
Sorry, you are wrong.

Frequency drift is caused by the two transistors in the exponential converter being at different temperatures and minimised by them being on the same substrate. This is temperature balancing. Temperature compensation with tempco resistors tries to stop the Volts/oct changing by matching the exponential term which is proportional to absolute temperature. At 25 deg C the amount of change for 1 deg is 1/(273 + 25) = 0.33557% or 3355.7ppm. That is where the figure comes from, it is not 3300pmm or 3000ppm or 3500ppm or anything else.

Note that the normal tolerance of tempco resistors is ±10% and changes with every reel of wire that they are made from. 3300pm -10% is way off (2970ppm). I have had some special ±1% tempcos made and 3316ppm to 3389ppm is not close enough either, but that was the best the manufacturers could guarantee.

If you buy a keyboard synthesizer the tempcos are probably from the same batch and equally wrong so the VCOs are also equally wrong and will track each other, but may not be absolutely in tune. If you take two synthesizer modules of different design and different tempco types, or even the same design with different batches of tempcos, the chances of them matching are very small unless each have been calibrated exactly to 3355ppm, e.g. using Ian Fritz's method. Just sticking a 10% part in doesn't cut it, it's just better than not putting it in at all.

Randy wrote:Does the tuning eventually stabilize when the operating temperature is reached?
No, the transistor temperature changes every time you play another note. If you go up an octave the current through the transistor doubles and its temperature rises.
There are VCO circuits out there that do not use tempcos and "in the old days" I would head down to the bar to turn my synth (Oberheim OB-X) on at least 1/2 hour before the show to let it warm up before tuning it.
That would have had Curtis VCO ICs which did not use tempcos, but they were still compensated, just by a different method. There were very few classic synthesizers that did not have some form of temperature compensating.
Now that is what I call explanation ;)

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Post by Randy » Thu Aug 28, 2014 1:06 pm

It was quite a detailed explanation but does it really matter? We seem to go to great pains to design exacting circuits to emulate the analogue "warmth" that was really caused by the inaccuracies of the old circuits.

When does drift stop being "warm" and start becoming a nuisance?

Randy

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Post by euromorcego » Thu Aug 28, 2014 1:14 pm

hum, what is the difference between thermistor and tempco. The former are available at many asian retailers for a few cents: http://www.taydaelectronics.com/thermistors.html

But these are probably not what is needed. As usual, I believe the internet is always right, but it seems to be a bit ambiguous what the actual difference is (the answer so far: both words mean the same thing, only their properties are different).

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Post by burdij » Thu Aug 28, 2014 1:56 pm

Graham Hinton wrote:At 25 deg C the amount of change for 1 deg is 1/(273 + 25) = 0.33557% or 3355.7ppm. That is where the figure comes from, it is not 3300pmm or 3000ppm or 3500ppm or anything else.
I was under the impression, perhaps false of course, that the tempco resistor compensated for the change in Vbe of the transistor junction as the ambient (or device) temperature changes. The rate of change of voltage is in the negative direction and is approximately 2.1mV/deg. C. So if you take the average junction voltage of lets say 645mV (forward bias for silicon), then the rate of change as a percent would be -.0021/.645 or -3255.8 ppm. The junction forward drop can vary from transistor to transistor and will also vary with the amount of current flowing in the base-emitter circuit which contributes to the variance in the value of the rate of change.
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Post by Graham Hinton » Thu Aug 28, 2014 4:00 pm

Randy wrote: When does drift stop being "warm" and start becoming a nuisance?
When you can't get in tune. Drift does not equate to warmth anyway.
euromorcego wrote: what is the difference between thermistor and tempco
Thermistors can be made with various technologies and are non-linear. Tempcos are usually wirewound resistors, sometimes a foil, but have a precise temperature coefficient proportional to absolute temperature.
burdij wrote:I was under the impression, perhaps false of course, that the tempco resistor compensated for the change in Vbe of the transistor junction as the ambient (or device) temperature changes. The rate of change of voltage is in the negative direction and is approximately 2.1mV/deg. C. So if you take the average junction voltage of lets say 645mV (forward bias for silicon), then the rate of change as a percent would be -.0021/.645 or -3255.8 ppm. The junction forward drop can vary from transistor to transistor and will also vary with the amount of current flowing in the base-emitter circuit which contributes to the variance in the value of the rate of change.
I can't imagine where you got all that from, but, as you say, it's completely false. Very inventive though, just nothing to do with transistors or exponential conversion.

Tempco resistors compensate for the change in temperature of a transistor where the relationship between collector current and Vbe is given by exp( q.Vbe/k.T). Vbe is the applied CV attenuated and to get an octave needs to be ~18mV. q is the electron charge and a standard constant, k is Boltzman's constant, the only other variable is T in degrees K. As T increases the exponent term gets smaller, so to compensate Vbe needs to be multiplied by T. This is usually done by having a tempco resistor as part of the attenuator that drops volts to 18mV.

Does is make a difference? You've got the equation, do the maths with real figures...
OK, most people would rather gnaw their legs off, so here's the result:
For a five octave keyboard and a 25 deg C change in temperature (not unreasonable real world changes) without compensation the error would be 464 cents! An unusable keyboard. With a tempco of various coefficients the error would be:
3000ppm: -49 cents
3300ppm: -7.7 cents
3400ppm: +6.1 cents
3500ppm: +19.9 cents
So you can see that standard common 3000 and 3500ppm parts are still going to be out of tune and you need very close to 3355 ppm to be immune to temperature changes. Don't forget that these coefficients have a +/-10% variation too, some may be the value you want, but try finding them...

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Post by frijitz » Thu Aug 28, 2014 4:18 pm

CLee wrote:My understanding, please correct me if I'm wrong...
OK. :grin:

Theory is explained here:
http://home.comcast.net/~ijfritz/sy_cir7.htm

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Post by slow_riot » Thu Aug 28, 2014 5:35 pm

About "warmth", looking at the temperature compensation scheme of the exponentiation is really just assigning value arbitrarily based on sentimentality.

I would look closer at choice of timing capacitor, and other errors in the main integrating node, the timing comparator and clamping schema.

All oscillators form a closed loop, the most obvious instance is the state variable sine and cosine core. But the classic triangle core follows the same layout exactly, but instead of 2 complimentary sines, 1 side is making diagonals, and the other is making squares, but the 2 halves are still forming an endless feedback cycle.

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Post by CLee » Thu Aug 28, 2014 5:55 pm

frijitz wrote:
CLee wrote:My understanding, please correct me if I'm wrong...
OK. :grin:

Theory is explained here:
http://home.comcast.net/~ijfritz/sy_cir7.htm

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Post by The Real MC » Fri Aug 29, 2014 11:24 am

All exponential VCOs need some form of temperature compensation. Most use tempcos, some do not (CEM VCOs, any using ua726).

Linear VCOs don't need temperature compensation because they do not require the exponential converter.

So why aren't linear VCOs used in more designs? Because 90% of synthesizer models are simpler component-wise with exponential VCOs, and expo VCOs can do more tricks and has a wider frequency range. Anybody who has ever tried frequency modulation such as mod wheel or pitch bend with a linear VCO will understand why.
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Post by tojpeters » Fri Aug 29, 2014 12:20 pm

One more thing-You may want to sell those modules in the future and they will sell much quicker if built correctly.

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Post by daverj » Fri Aug 29, 2014 12:42 pm

The Real MC wrote:So why aren't linear VCOs used in more designs? Because 90% of synthesizer models are simpler component-wise with exponential VCOs, and expo VCOs can do more tricks and has a wider frequency range. Anybody who has ever tried frequency modulation such as mod wheel or pitch bend with a linear VCO will understand why.
Linear VCOs and Expo VCOs have the same frequency range. Most Expo VCOs include a linear input as well as expo input. The linear input is often marked "FM".

Most VC LFOs are linear only, because you don't need 1v/oct control. Most audio VCOs are expo because you do want that 1v/oct control. That allows you to play notes from different octaves with a simple step up in voltage. To try and play those same notes with a linear VCO control becomes very difficult at one end of the scale as the distance between notes becomes tinier and tinier, making it very difficult to play a given note with any accuracy.

For example, if we consider a 10 volt control range, that gives us 11 octaves (the initial octave at zero volts, plus 10 additional ones). If we start with 20Hz at the bottom we end up with 20,480Hz at the top. With 1v/oct it is a 1 volt step from octave to octave. But with linear voltage for the same frequency range, if 20Hz is zero volts, then 40Hz is at 0.00977 volts, 80Hz is at 0.029 volts, 160Hz is at 0.068 volts. We haven't even gotten to 1/10th of a volt yet. At the other end, one octave down from the top is at 5 volts and from 5v to 10v is all within that one top octave.

Hence the reason audio VCOs use expo (1v/oct) CV instead of linear.

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Post by benbradley » Mon Apr 18, 2016 1:19 pm

Does anyone have a copy of this webpage? It's gone from the URL, and archive.org doesn't have a copy.
frijitz wrote:
CLee wrote:My understanding, please correct me if I'm wrong...
OK. :grin:

Theory is explained here:
http://home.comcast.net/~ijfritz/sy_cir7.htm

Ian

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Post by SynthBaron » Wed Apr 27, 2016 4:32 pm

posted by accident

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Post by masterofstuff124 » Wed Apr 27, 2016 8:42 pm

So, im building an arp VCO. it uses the famous? 1.87k tempco. those are expensive. what is an alternative. I have several Of the "thermistors" (1k,10k etc) from tayda. What is a cheaper alternative. I dont need perfect tuning but I am a musician and itll prolly drive me insane randomly going out of tune(like a bad guitar) This will be my first VCO. and this is the VCO in question. http://electro-music.com/wiki/pmwiki.ph ... KII-IIIVCO


thanks for any and all help!

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Post by guest » Wed Apr 27, 2016 8:57 pm

you can either tack together a few of the thermistors you have (2k//10k = 1.66k, which is probably close enough), or just use a 1k and change the input value resistors to get the modulation depths you want. the exact value doesnt matter so much, as you adjust the volts/octave with a trimmer anyways. so as long as youre within your trimmers range, you will be fine.
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