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I am soldering up Music from Outer Space: Noise Toaster synthesizer. The guy who designed it said that carbon resistors should be used ("The resistors should be carbon composition or carbon film"). But I have only wire wound ones.

Searching on the internet answers lead me only to confusion. Some articles say carbon resistors are noisy, but googling images on modern guitar/synth gear PCBs I see they use carbon resistors even today.

My question: how will wire wound resistors affect the sound of MFOS Noise Toaster?

Moog Delay: enter image description here

Fulltone OCD v1.7 (2012):

enter image description here This image is uploaded in addition to answer in the comments: enter image description here enter image description here

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    \$\begingroup\$ That looks like a nice piece of equipment. Buy the right parts for it. They'll cost you a few cents each. \$\endgroup\$ – Transistor Sep 13 '20 at 20:33
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    \$\begingroup\$ In the schematic there's numerous resistors and potentiometers which have resistance 100 kOhm or more. Having them as wirewound can cause some problems. I guess you run out of the available space for them. Can you maybe post a photo of your existing 300 kOhm wirewound resistor. At least I am interested in seeing how it looks. \$\endgroup\$ – user287001 Sep 13 '20 at 23:28
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    \$\begingroup\$ These are not wirewound resistors, they have thin metal films if they are metal based. The film can be a spiral for bigger resistance. They work well in audio frequencies. In synths resistor noise shouldn't be a problem as it would be in millivolt level audio, for ex in mic preamps. \$\endgroup\$ – user287001 Sep 14 '20 at 8:51
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    \$\begingroup\$ I have seen in eastern webshop pages text "wirewound resistor" near same looking photo but a little lower in the page there's tech spec " metal film resistors". If they are wirewound I will be surprised. But the east is full of unexpected things... BTW: The text in your schematic says that metal film resistors work fine and they drift less than carbon resistors, but this project doesn't need metal film resistors, carbon resistors are good enough. \$\endgroup\$ – user287001 Sep 14 '20 at 11:35
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    \$\begingroup\$ There are also worse options than the carbon based resistors that you can get if you do not order carefully. They can be mechanically impossible. to use in this project. Carbon composite resistors are outdated stuff due their unstability and non-insulated surface. But vintage gear lookalike builders and renovators still demand them. \$\endgroup\$ – user287001 Sep 15 '20 at 7:34
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Carbon Composition:

enter image description here

These are obsolete, inaccurate, value drifts a lot with aging, also with voltage, humidity and temperature, they have excess current-dependent noise and even a little bit of distortion. The only thing they're good at is high pulse power. Your circuit doesn't use that, so don't buy carbon composition resistors.

When you replace one of these with a modern resistor in old equipment, it will probably sound different, mostly because the value of the old resistor was wildly off from what it was supposed to be.

Lots of old obsolete stuff has a mysterious aura of magic mojo and sellers will profit off the average sucker. Most of it is bullshit. Do not waste your money.

Carbon & metal film: see illustration in your post

Carbon film used to be the low cost alternative to metal film, with higher resistance variation with temperature, higher noise, etc. So, most likely, when the schematic designed, the author decided the upgrade to metal film resistors was not worth the cost. If you make a multimeter, 500ppm/°C carbon film resistors would be a bad choice, but I don't think a synth needs precision resistors, unless you see some 1%s somewhere.

Now they're pretty much the same price unless you buy really high quantity, so there's no reason to not use metal film.

Wirewounds

enter image description here

It's a wire wound around a ceramic core. That's useful when you want a resistive material with certain properties, and that material can't be used in a film resistor. Depending on the material you can get excellent stability, accuracy, or power handling.

The resistors you posted are not wirewounds, they're spiral cut film resistors, so I hope this clears it up. There's no need for wirewounds in your project unless the design calls for it.

There is no way to know what resistors were used in the equipment photos you posted just by looking at them. All sorts were made in all sorts of color.

Do not buy expensive resistors unless you need the special features that make them expensive.

About the inductance: who cares.

Impedance of an inductance is 2.pi.f. So, at 1GHz, even the tiniest 1nH inductance will add 2pi ohms reactive, which matters if the resistor value is low, like 50 ohms, so the inductive impedance screws a significant part of it. At audio frequencies though, if it's a through hole resistor, and it has 20nH inductance, at 10kHz, that will only add 0.006 ohms. Your 1k resistor (and the phase margin of your LM324s) are safe.

Now, while tweaking the resistors is a waste of time, there are other parts that will definitely influence how it sounds. For example the opamps and the FETs. LM324 is an old industrial workhorse that is probably running your neighborhood nuclear power plant, among other things. It is especially old, crummy, slow and unsuitable for hi-fi, which means all of its numerous defects will be part of the sound of the synth. If you use a modern substitute it will probably sound different.

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The guy who designed it said that carbon resistors should be used ("The resistors should be carbon composition or carbon film"). But I have only wire wound ones.

There is absolutely no reason to do this since carbon resistors cost literally cents. However if you wanted to try and do this, you'd need to go through the circuit and the datasheet for your resistors, add in the series inductance, and then evaluate the effect of each substitution individually.

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  • \$\begingroup\$ wire wound resistors add so little inductance to the system. Do they really affect the sound considering synth does not operate at high enough frequencies. \$\endgroup\$ – Qeeet Sep 13 '20 at 20:48
  • \$\begingroup\$ @Qeeet: How much inductance do they add? You should put that information in your question. \$\endgroup\$ – user1850479 Sep 13 '20 at 21:00
  • \$\begingroup\$ These are through hole components. The resistor leads will add inductance an order of magnitude higher than the winding itself. This comment sounds the designer has taken a rule of thumb out of context and applied indiscriminately (it happens all the time in EE sadly) \$\endgroup\$ – Oliver Sep 15 '20 at 10:46

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