2 equal resistors is not the same as 1 ? - Page 1
Forum section: electronics / projects

author: JanJansen
author_rank: Frequent Contributor
timestamp: July 29, 2018, 01:47:10 pm
content: Hi, i,m building a DAC for a VCO with 17 resistors from schematic : look in the left upper corner ( R74 to R90 )http://www.martianarts.net/web/images/stories/ourgear/roland_tb303_schematics.gifI thought 2 exact the same resistors are the same as 1.If i remove the double resistors, the tones are different.Why is it different, can anyone explain ?thanksedit : by the way : i left those triangles out, i dont know what that is for, it also work without.

author: sokoloff
author_rank: Super Contributor
timestamp: July 29, 2018, 02:12:21 pm
content: Two equal resistors in parallel has an equivalent resistance of half of the resistance of each of them.https://www.swtc.edu/Ag_Power/electrical/lecture/parallel_circuits.htm

author: MarkF
author_rank: Super Contributor
timestamp: July 29, 2018, 02:13:43 pm
content: Resistors R74 thru R90 form a R-2R Digital-to-Analog ladder network. If you remove the double resistors, you no longer have the ladder network. ThisDigital to Analog Conversion – The R-2R DACtutorial may help to explain how the ladder works.If you want to build a ladder network instead of using a DAC, look at the DAC0808 chips or the R-2R resistor network ICs.This video may also help:

author: JanJansen
author_rank: Frequent Contributor
timestamp: July 29, 2018, 02:44:43 pm
content: Quote from: sokoloff on July 29, 2018, 02:12:21 pmHere are many cases, wich one is the one for this DAC ?Quote from: MarkF on July 29, 2018, 02:13:43 pmWhats in the video i understand, only here they use a 10K and 20K resistor instead of double resistor,could i use 100K instead of 200K for the double resistors ?Quote from: MarkF on July 29, 2018, 02:13:43 pmHere is also no double resistors.Quote from: MarkF on July 29, 2018, 02:13:43 pmFor now i am building the synthesizer like original ( with 180K instead of 200K should do the same, else i have 200Ks that i dont use ).The thing is that it is for the notes, so it should be spot on else it will sound false.I like to use the oscillator schematic in my own designs also, so ofcourse i would like a simpler method.Are those you suggest perfectly linear ?In PIC16 there is also 8bit resistor DAC build in, only it is not perfect linear, usable for other things, not for tuning notes.At MCP site they also adviced me external 8 bit SPI DACs, saying that they are perfect linear.I dont really need anything now just build this synth takes ages.

author: ArthurDent
author_rank: Super Contributor
timestamp: July 29, 2018, 03:19:30 pm
content: The reason for the parallel resistors is that they match R74-R90 to be 200K because that allows them to only choose 1 close tolerance part in all location to get an R-2R ladder. Because the design calls for the horizontal resistors on the schematic to be 200K and the vertical resistance to be 100K, they can just solder 200K in all locations and it makes it easier for them to construct the circuit.If you are making your own circuit and match all resistors to either 180K or 220K it probably will work o.k.. Also if you choose 180K for the horizontal resistors and a single 90K for the vertical resistors in the ladder it would probably work just fine as well but 90K isn't as common a value as 180K.If you're copying the circuit I would go with the values they used, just in case. Buying a bunch of 200K film resistors and matching them with a good handheld DMM won't be that hard or expensive.

author: JimRemington
author_rank: Regular Contributor
timestamp: July 29, 2018, 03:22:30 pm
content: QuoteNot sure what you mean by "removing". Try replacing the two paralleled resistors with the equivalent single resistor.

author: MarkF
author_rank: Super Contributor
timestamp: July 29, 2018, 03:42:54 pm
content: Quote from: JanJansen on July 29, 2018, 02:44:43 pmThe case dealing with parallel resistors.What you are calling double resistors are resistors in parallel.Two equal value resistors in parallel are the same as one resistor of half the value.Quote from: JanJansen on July 29, 2018, 02:44:43 pmThe actual values used in the resistor network depend on the circuit around it.What is important:1) The resistor network is made of R values and 2*R values2) The 100K/200K network will have a lot smaller current usage than the 10K/20K network.They can only drive a smaller load and will be susceptible to noise.3) The actual values are chosen from the drive capabilities for each input bit and the loading of the output.Quote from: JanJansen on July 29, 2018, 02:44:43 pmThe double resistors are two resistors in parallel.Two equal value resistors in parallel result in half the resistance.Your circuit uses resistors in parallel so they can use all the same value resistors to form the R-2R network.With resistors (2R=200K), the "double resistors" are in parallel and equal half their value (R=100K).Quote from: JanJansen on July 29, 2018, 02:44:43 pmI'm only suggesting that you get a resistor network package instead of individual resistors.You will have a much better DAC because there will be much less variation in their values.Look athttps://www.mouser.com/ProductDetail/Bourns/4606X-R2R-103LF?qs=sGAEpiMZZMvrmc6UYKmaNVBnzKD%252b4UiWRP2VuuROfX0%3das an option.

author: Richard Crowley
author_rank: Super Contributor
timestamp: July 29, 2018, 03:44:42 pm
content: Quote from: JanJansen on July 29, 2018, 01:47:10 pmNo. I can't think of any example in the Real World where 2 = 1 Can you?As others have observed, using those 17 equal-value resistors to make an "R-2R ladder" is less expensive and easier than finding exactly some resistor value and exactly 1/2 the value. As others have already observed, the exact value of those 17 resistors is not critical as long as they are identical and close to that value.QuoteDepending on what you mean by "remove", you have destroyed the function of the R-2R Ladder. You will never get the proper tones unless you use a proper R-2R Ladder.QuoteThose "triangles" are CMOS CD4050 Hex non-inverting drivers. There are six drivers ("hex") in the chip package. Four of them are wired in parallel and are used to buffer/drive the signal from the 4174 latch-register pin 10 (the most-significant bit or "MSB"), and the other two in parallel to buffer/drive the signal from the 4174 pin 7.The 4174 doesn't have enough current output to drive the MSB and next level of the R-2R ladder. That is why the non-inverting drivers are used. Not surprising that "the tones are different".Since the circuit appears to be a voltage-controlled oscillator (VCO), you must be quite precise in that area of the circuit. Note that you must be powering the 4174 from 5.333 Volts. THIS IS CRITICAL. And fooling around with the R-2R resistor values won't do you any favors if you are trying to get proper note frequencies out of this circuit. This is not a place to be substituting and "experimenting" if you expect proper results.This is why we don't use this kind of technology anymore. It is so much easier to synthesize waveforms digitally, we have left that kind of tedious and temperamental circuit behind decades ago. And good riddance, IMHO.

author: JanJansen
author_rank: Frequent Contributor
timestamp: July 29, 2018, 03:56:20 pm
content: Thank you Arthur, i understand now.A multimeter might measure 100K different then 200K, within the specs, so to be sure you better measure all to 200K.

author: JanJansen
author_rank: Frequent Contributor
timestamp: July 29, 2018, 04:08:53 pm
content: Quote from: Richard Crowley on July 29, 2018, 03:44:42 pmI use a 7414 hex inverter instead, not the original 4174.Quote from: Richard Crowley on July 29, 2018, 03:44:42 pmThat is right, only it dont sound so good, there are many new analog synthesizer now these days, digital is nice for additive synthesis wich is impossible in analog.

author: Richard Crowley
author_rank: Super Contributor
timestamp: July 30, 2018, 05:04:52 am
content: Quote from: JanJansen on July 29, 2018, 04:08:53 pmA 4174 is a 6-bit NON-inverting LATCHA 7414 is a hex INVERTER with NO latching function.Those chips are completely different functions and in no way are substitutes for each other.If your intention is to design a completely new circuit, then go for it.But it appears to be completely unreasonable to copy a circuit like that with completely different chips.

author: Zero999
author_rank: Super Contributor
timestamp: July 30, 2018, 07:44:26 am
content: The R2R DAC should also be driven off a CMOS push-pull output, otherwise the input voltage will be different from the 0V and 5.333V, required to generate the correct output voltages.

author: bugi
author_rank: Regular Contributor
timestamp: July 30, 2018, 09:17:48 am
content: Quote from: Hero999 on July 30, 2018, 07:44:26 amWell, even the 4050, while seeming to be push-pull, has non-symmetric output, especially at higher loads, but the 200k-based R-2R should result in low enough currents that the voltages will be close to ideal on both states (and considering the 2x or 4x parallel buffers). Just another tiny detail to be considered if tweaking things.Quote from: JanJansen on July 29, 2018, 04:08:53 pmIf only changing that DAC section (latch, buffer, R2R-combination) with a DAC + any necessary glue-changes, the sound should not change, as the rest of the circuit makes the real effects or how it sounds. The amount of distortion might even get lower, though whether that sounds better or not depends...And I think additive synthesis can also be done in analog, though that would be another topic and a can of worms...

author: Zero999
author_rank: Super Contributor
timestamp: July 30, 2018, 10:19:23 am
content: Quote from: bugi on July 30, 2018, 09:17:48 amYes the CD4050's non symmetrical output should make no difference, but a TTL output on the 7414 would.

author: JanJansen
author_rank: Frequent Contributor
timestamp: July 30, 2018, 11:26:31 am
content: I use the SN74AHCT14N to be precise, and it works.for inverting i do PORTC = ~note;SN74AHCT14N has 6 pins from MCU in, and 6 pins for resistors out, no latch/clock pin needed, btw my 20 pin MCU is also completely in use.I have the SN74AHCT14N laying around for the MIDI thru signal, so i used that instead.It does not change anything with the SN74AHCT14N + 180K from the original, just different code.

author: JanJansen
author_rank: Frequent Contributor
timestamp: July 30, 2018, 11:30:48 am
content: Quote from: bugi on July 30, 2018, 09:17:48 amTrue, i thought he was saying waveforms, not DAC, so the complete oscillator digital from a DAC.

author: Zero999
author_rank: Super Contributor
timestamp: July 30, 2018, 04:21:19 pm
content: Quote from: JanJansen on July 30, 2018, 11:26:31 amThat's not what you said originally. Seemingly minor details matter a lot. The 7414 is TTL which is completely unsuitable for driving an R2R DAC. The 74HCT14 is CMOS and will drive an R2R DAC perfectly. Please bear this in mind in future, when you talk about part numbers.

author: JanJansen
author_rank: Frequent Contributor
timestamp: July 30, 2018, 04:26:35 pm
content: Sorry, the name was to complicated to remember, besides its all CMOS these days, i buy my parts new, you cant buy a real TTL anymore it seems.

author: Zero999
author_rank: Super Contributor
timestamp: July 30, 2018, 07:20:16 pm
content: Quote from: JanJansen on July 30, 2018, 04:26:35 pmPlain old TTL, not so much, but 74LS is still widely available and has similar output levels to old TTL.