Electrolytics for AC signals - Page 1
Forum section: electronics / projects

author: Balthazar
author_rank: Contributor
timestamp: June 10, 2025, 11:08:50 pm
content: HiInthis circuiton the Altium website, it appears that electrolytic capacitors are used with AC, and not with a DC bias that ensures the polarity across the capacitor pins is never reversed. I thought this was a big no-no.In the schematic (Figure 3), the capacitors C29 and C30 appears to get AC across them.In the accompanying video, he mentions something about "these are perfectly fine in the signal path, as long as they are large enough" (17:21).However, this seems to refer to concerns about distortion, not about reverse polarity. We see that as the signal enters the schematic that is shown on the Altium page, it is already biased around +VBL, and the signal at the output is also biased around +VBL.I tried to simulate the circuit in QSPICE, and it appears the caps do get some reverse voltage across the pins. In AC simulation, C29 can get about -16dB across its pins at 1.5Hz, and C30 gets about -36dB, and transient analysis shows that the voltage does swing negative. Input signal for my use case might be in the range of 1Vrms or more, so -16dB might be in the ballpark of 100 to 200mV.My QSPICE schematic is part of a headphone amplifier I'm making for myself (hobby project), so I'd like to figure it out if I'm doing something wrong here, cause it sounds scary when people say these things can explode if reverse biased.Most searches about the topic just say that you should NEVER do this, but this guy has an impressive CV so I reckon he knows what he's doing. When, if ever, can you use electrolytic capacitors for AC signals, without DC bias?

author: Benta
author_rank: Super Contributor
timestamp: June 10, 2025, 11:27:11 pm
content: Show the relevant schematic, please. I don't watch videos.

author: moffy
author_rank: Super Contributor
timestamp: June 10, 2025, 11:33:37 pm
content: C29 does apear wrong and quite possibly unnecessary, not the best design I've seen, but then again not the worst.

author: MariuszD
author_rank: Regular Contributor
timestamp: June 11, 2025, 06:38:37 am
content: Quote from TDK Aluminum Electrolytic Capacitors - General technical information:QuoteQuoteAudio band begins at 20Hz

author: Andy Chee
author_rank: Super Contributor
timestamp: June 11, 2025, 06:51:29 am
content: Quote from: Benta on June 10, 2025, 11:27:11 pmYou must've skimmed over the OP's introductory words.The relevant schematic appears in Figure 3 on the hyperlinked websiteQuote from: Balthazar on June 10, 2025, 11:08:50 pm

author: Messtechniker
author_rank: Frequent Contributor
timestamp: June 11, 2025, 07:24:44 am
content: I have used this configuration in the past. Needs a low noise negative supply for biasing.Never tested it in depth for possibly improved distortion levels.

author: Zero999
author_rank: Super Contributor
timestamp: June 11, 2025, 08:50:48 am
content: Quote from: Messtechniker on June 11, 2025, 07:24:44 amIt's probably not a bad thing, but you don't need the biasing resistor.Electrolytic capacitors have parasitic diodes, so all you need to do is connect two back-to-back.

author: Balthazar
author_rank: Contributor
timestamp: June 11, 2025, 08:45:27 pm
content: Thank you guys for all your help.Quote from: MariuszD on June 11, 2025, 06:38:37 amI know. I didn't mention that it was supposed to be a headphone amplifier made from components I already have (in my hobby "lab"), to test audio circuits with various inputs. Therefore I couldn't guarantee that the headphone amplifier circuit would not receive signals outside the audio frequency range.Come to think about it, I couldn't guarantee that it wouldn't receive signals outside the voltage range either, so it would be nice not to need electrolytics in there.I do not understand completely the purpose of those capacitors in the circuit. I thought perhaps it was related to the following, quoted fromthe article: "in a volume control schematic we need to avoid DC current passing through the potentiometer wiper, as this can lead to audible scratching noises when rotating the potentiometer". If that's the only reason, I think I'd rather drop them, it's better to have a little scratching noise in the ears than an exploding capacitor.I have attached the circuit, changed to work from a dual battery supply and omitted the capacitors.It simulates fine in SPICE, will it work in the real world, and do you think I should change anything? The original schematic is inthis article, figure 3. The omitted capacitors are C29 and C30.

author: Balthazar
author_rank: Contributor
timestamp: June 11, 2025, 08:52:19 pm
content: I'm sorry but something went wrong when trying to insert attachment. I will try again.

author: Vovk_Z
author_rank: Super Contributor
timestamp: June 12, 2025, 11:39:27 am
content: Quote from: Balthazar on June 11, 2025, 08:52:19 pm1. Which capacitor in this diagram is electrolyte type?2. Polar electrolyte capacitors work quite fine with a zero DC bias if AC voltage across capacitor is small enough (I mean less than 0.1..0.2 VAC).3. Electrolyte capacitors C26, C29 in that Youtube video (which I've seen briefly) work with a DC bias. It is called VBL, and it is a half of a power supply voltage, as it is typically expected.Look at 17:52 (output amplifier with a wire to VBL) and 19:42 (VBL bias circuit).

author: Doctorandus_P
author_rank: Super Contributor
timestamp: June 12, 2025, 11:56:49 am
content: Quote from: Zero999 on June 11, 2025, 08:50:48 amI've also read a high DC bias reduces distortion, but it was a long time ago and I don't do much audio.The bias resistor can be of a quite high value. it only has to compensate for elco leakage. But of course there is the initial charging at startup that may cause some trouble. However, if the output has (for example) a 20Hz high pass filter, would it then be a problem if the capacitors take a few minutes to fully charge to the bias voltage? The constant changing voltage will cause a slight DC offset during the charging time. This may be a problem for "high end" audio, but that does not use this construction in the first place.

author: Zero999
author_rank: Super Contributor
timestamp: June 12, 2025, 07:47:43 pm
content: Quote from: Doctorandus_P on June 12, 2025, 11:56:49 amMaybe, maybe not? There are lots of myths andsurrounding audio.Anyway, for completeness, the positive supply can also be used for the bias resistor, which might be beneficial for some op-amps such as the TL072 who's common mode range extends to +V but not -V.

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: June 13, 2025, 07:54:19 pm
content: Actually, it's probably not a problem. You'll never have significant AC voltage across large value electrolytic capacitors unless the cap value is too small for the application. Coupling capacitors are supposed to pass the entire signal, so there should be almost no AC voltage across it.