smd resistor in power audio amplifiers - Page 1
Forum section: electronics / beginners

author: mrjoda
author_rank: Regular Contributor
timestamp: October 27, 2019, 07:13:34 pm
content: Hi guys, as HW designer i should know it but... i need advice.Are there any "no go" for SMD resistors/capacitors in power audio amplifiers ? Maybe some noise, power capabilities ... ? At input stage. Power resistor will be wounded welwyn or so.I am currently working on update of one of my projects and i need to decrease dimensions under the 100x100mm. And is it quite old project made for DIY production (i made almost 100pcbs at home of this) so it needs some modernisation.Based on this pagehttps://www.beis.de/Elektronik/ResistorCurrentNoise/CurrentInducedResistorNoise.html, the difference between 1206 smd and tht metal film resistor is almost 10dB.The graph shows that noise is voltage dependend and voltage of that amplifier is around +/- 30V . I uploaded schematics, i was thinking about usage of smd parts up to VAS. Overcurent sould be OK with SMD parts, bias controller also.I am not sure about it. What do you think ?Thanks

author: magic
author_rank: Super Contributor
timestamp: October 27, 2019, 09:25:53 pm
content: Look for teardowns of audio gear, it seems everybody uses SMD these days.There are two SMD resistor technologies: thick film and thin film. The former are noisy and nonlinear, the latter are similar to through hole metal film resistors, although IIRC there were some complaints about silght voltage coefficient in the smallest package sizes. Maybe thermal modulation? Stronger field gradient?As for capacitors, small values in C0G/NP0 dielectric are OK, all the X7R, Y5V stuff is highly nonlinear and microphonic so not really suitable for coupling, filters and such.

author: exe
author_rank: Unknown
timestamp: October 29, 2019, 01:26:20 pm
content: I'd say resistors totally fine if they can handle power you need. As of smd caps, depends where and what they do, DC bias being, probably, the biggest problem. I wouldn't use smd caps smaller than 0805, but bigger packages, while having better specs, are more fragile and can cause a short.Anyway, I wouldn't care much about distortion, etc. Unless you have "golden ears", I doubt it's audible to have 0.1% or 0.01% distortion, let alone making a difference in experience. I know the pressure in audio community for "special" cables, resistors, capacitors, opamps, etc. Probably that's why you are here asking this question. But look critically at the plots you showed. Are you really care about resistor noise of <-110db?My advice: get an audio analyzer if you really care and see yourself how different components affect distortion and noise. Otherwise just use normal components and save yourself a lot of time and trouble.

author: mrjoda
author_rank: Regular Contributor
timestamp: October 29, 2019, 02:04:39 pm
content: thank you guys.My question was pretty much general about power amplifiers. Audio is not my field of interest, i don't care about seventh place of distortion. My scope is to publish my old project as open source in smaller form factor with the newest parts.I decided to use 1206 resistors and parallel combination if power rating is too high for one. For capacitors, the best choice is NP0 for stability and X7R as general for power rails blocking.I don't have access to audio analysers, just bode100 and i am not sure, if input dynamic range is sufficient for audio measurements. My knowledge in this field is minimal.

author: mariush
author_rank: Super Contributor
timestamp: October 29, 2019, 02:55:20 pm
content: If you just want to reduce the size, you could place the resistors on some small boards and connect them like SIMM sticks to the board (like ram sticks but without connectors).You could solder a right angle 0.1" (or 2mm or 2.5mm) spaced header and then plug the header on the base board.With double sided circuit boards you could route things quite nicely that way.

author: exe
author_rank: Unknown
timestamp: October 29, 2019, 03:57:03 pm
content: When putting multiple smd resistors together, make sure there is enough spacing and surrounding board area for heat dissipation. In other words, stacking two 1/4W resistors won't make it a 1/2W resistor. For smd parts the board itself is considered a heatsink. This also have two side effects: cooling efficiency is not that great 2) heat can be transfered to parts that should not get hot. Also potential temperature gradients, etc.Another reason to put resistor in series can be to increase voltage handling. Two 100V resistors in series make a 200V resistor (but only if they have same resistance, afaik).

author: mrjoda
author_rank: Regular Contributor
timestamp: October 29, 2019, 04:46:25 pm
content: exe,Thank you. This is closer to my field. I design SIL4 devices so... there is no space for a mistake and thermal spots and thermal considerations of all cases are one big topick

author: ratatax
author_rank: Regular Contributor
timestamp: October 29, 2019, 07:31:31 pm
content: They make 10kW+ car audio amps with full SMD so no problem at all, it just depends on how you design your amplifier

author: dom0
author_rank: Super Contributor
timestamp: October 29, 2019, 07:40:04 pm
content: Quote from: ratatax on October 29, 2019, 07:31:31 pm10 kW on the plate, 10 W at the output.

author: mrjoda
author_rank: Regular Contributor
timestamp: October 29, 2019, 08:00:20 pm
content: One of the most powerfull car amps that i am aware of is Phoenix Gold The One and there are not SMD parts.But if they are, it is completely different case. Car audio is not about the quality, it is just mumble jumble dB with lot of blinky lights. Nobody cares about noise level.But, i already started with redesign. I choose 1206 because footprint is very similar to MELF so there is posibility to use MELFs. Lets see.

author: mariush
author_rank: Super Contributor
timestamp: October 29, 2019, 08:21:01 pm
content: I personally don't like those to-92 packages... hope you replace those with something better. Difficult to passively cool..Maybe replace with sot-23 or sot-223 ?You should probably make the footprints 0805 but extend them to accept 1206 as well. 0805 is probably cheaper as it's more common.It still leaves you enough room to get a trace between the pads, so you may want to consider 0 ohm resistos / jumper links to simplify your layout, if needed.

author: exe
author_rank: Unknown
timestamp: October 29, 2019, 08:22:44 pm
content: Quote from: mrjoda on October 29, 2019, 08:00:20 pmInteresting, why do you want to use melfs? 1206 seems to be quite a big footprint, just by mounting TH resistors vertically you may get a similar density. I'd say 0603 is a sweet spot. Or 0805, if it needs to be beginner-friendly.

author: dmills
author_rank: Super Contributor
timestamp: October 31, 2019, 05:32:18 pm
content: 1206 has more thermal mass (clearly) then the smaller parts, which makes for less low frequency distortion due to the heating at twice the input frequency.It is small, but is a real thing.MELF are sometimes preferred for much the same reason, but for me 0805 or 1206 thin film generally get it done, MELF is just such a pain in the arse and frankly well into audophoolery tweaksville.Regards, Dan.