LM1875 amplifier design - Page 1
Forum section: electronics / projects

author: dosensuppe
author_rank: Contributor
timestamp: August 29, 2025, 09:23:09 am
content: Hello. I just want to make sure I made absolutely no mistakes in my simple gainchip design.Should I add output caps just to be sure there is absolutely no DC coming to my speakers?best regards

author: Roehrenonkel
author_rank: Frequent Contributor
timestamp: August 29, 2025, 09:52:01 am
content: Hi dosensuppe,no need for output-caps, you have C12 and C13.Maybe add a DC-protection with a cut-off-relay for the speakers.Are you shure you want that much gain?(R9/7 R13/12)Why are C1+8 and C2+9 in series?What's the use of R8 and R14?Good luck

author: dosensuppe
author_rank: Contributor
timestamp: August 29, 2025, 11:06:34 am
content: Thanks for the feedback!QuoteNope, I'm not.Which gain would you suggest for typical audio source ~1.5V ?QuoteThey are tiny MLCC smd caps which can only take 16v max.QuoteIt was suggested by this tutorial:Quotehttps://www.instructables.com/Tales-From-the-Chip-LM1875-Audio-Amplifier/

author: Circlotron
author_rank: Super Contributor
timestamp: August 29, 2025, 12:11:36 pm
content: Quote from: Roehrenonkel on August 29, 2025, 09:52:01 amDatasheet says:QuoteSo not like many low signal opamps that are okay with a gain of 1.

author: themadhippy
author_rank: Super Contributor
timestamp: August 29, 2025, 01:06:32 pm
content: Quotewhat sot of output voltage are you after? most chip amps have a gain around 20-30 times, a gain of 20 with 1.5V input would give 30 volts output,at 8 ohms thats around 110 watts.

author: gbaddeley
author_rank: Regular Contributor
timestamp: August 30, 2025, 05:58:14 am
content: R after input cap does not need to be that high 220k, good bass response would still be had at 22k.Feedback 220k is also too high. 22k or even lower, down to 15k would provide sufficient gain.

author: dosensuppe
author_rank: Contributor
timestamp: September 01, 2025, 08:08:22 am
content: Yup. Just noticed that I fucked up a decimal digit on the feedback resistor.Also went with gain of 16. That should be more than enough. The actual supply voltage with the trafo I'll be using will propably not be more than 20v.But not really sure. The 1.5V input voltage was only an estimation.Maybe someone could educate me on how much the average PC soundcard/ smartphone outoputs...

author: dosensuppe
author_rank: Contributor
timestamp: September 01, 2025, 09:57:10 am
content: What about the Thiele/Zobel Network. Are those values suitable for the LM1875? Took this from the lm3886 datasheet. the LM1875 datasheet doesn't say anything about it.

author: Gyro
author_rank: Super Contributor
timestamp: September 01, 2025, 10:57:15 am
content: The Zobel network looks rather power hungry. 2R7 and 100nF for R3 and C7 would be more appropriate (recommended values for the LM3875 and LM3886, my datasheet version).

author: mawyatt
author_rank: Super Contributor
timestamp: September 01, 2025, 01:05:58 pm
content: This article might be useful.https://www.eetimes.com/ic-audio-power-amplifiers-and-zobel-networks-one-size-does-not-fit-all/Best

author: dobsonr741
author_rank: Frequent Contributor
timestamp: September 01, 2025, 01:46:58 pm
content: What protection role does the relay between the rectifiers and bulk capacitors serve? If its purpose is speaker protection, wouldn’t it need to switch the speaker load rather than the supply rails?

author: dosensuppe
author_rank: Contributor
timestamp: September 02, 2025, 07:14:45 am
content: A microcontroller turns it off when the chip or transformer overheats

author: dosensuppe
author_rank: Contributor
timestamp: September 02, 2025, 10:50:31 am
content: Quote from: mawyatt on September 01, 2025, 01:05:58 pmMy amplifier "only" has around 20W per channel. So I guess it shouldn't be that crucial.

author: dobsonr741
author_rank: Frequent Contributor
timestamp: September 02, 2025, 10:51:53 am
content: Suggestion still applies - overheat is probably due to the load. The most expensive single component is the speaker, worth protecting against amp on/off transients.

author: dosensuppe
author_rank: Contributor
timestamp: September 02, 2025, 10:52:38 am
content: Quote from: dosensuppe on September 02, 2025, 07:14:45 amSee my other post.https://www.eevblog.com/forum/projects/digitally-controlled-preamplifier/I made it so I can stack the boards atop one another.Quote from: dobsonr741 on September 02, 2025, 10:51:53 amThe TDA1875 has it's own short circuit protection etc. built in.

author: dosensuppe
author_rank: Contributor
timestamp: September 02, 2025, 11:29:51 am
content: I also want to implement some kind of standby function when there's no signal coming in

author: dosensuppe
author_rank: Contributor
timestamp: September 02, 2025, 12:35:21 pm
content: any idea how to automatically stitch vias over the whole opposing GND planes in KiCAD?Kinda tiresome to copy them all manually.

author: dosensuppe
author_rank: Contributor
timestamp: September 02, 2025, 01:42:28 pm
content: Please tell me what I can improve on the layout

author: dobsonr741
author_rank: Frequent Contributor
timestamp: September 02, 2025, 02:36:44 pm
content: QuoteStitched ground plane does not belong to audio. Your rails are skinny, make them as thick as ground. It’s the same current that causes ohmic losses, that you wanted to protect against with the ground plane. The GNDs should run into a star topology to avoid unwanted feedback of these losses into the real feedback loop.Build your circuit on a breadboard, connect and disconnect the power, use a cheap speaker. Observe the pops.The standby is not done by disconnecting the power, you need a proper low power mode built into the amp IC.Look athttps://www.ti.com/lit/an/snaa057c/snaa057c.pdftoo.

author: dosensuppe
author_rank: Contributor
timestamp: September 03, 2025, 06:31:48 am
content: Quotewhy??QuoteMy whole top and buttom layer is set to GND.QuoteI have been advised against this here.https://www.eevblog.com/forum/projects/digitally-controlled-baxandall-tone-control-preamplifier/QuoteI just automatically assumed the chip would have built in pop protection. But yes, it's definetely a good idea to test that out and perhaps I will add relays with some kind of delay circuitry at the output.QuoteIt's more of an afterthought. I think being able to completely turn the board off with my controller board could be quite handy.

author: dosensuppe
author_rank: Contributor
timestamp: September 03, 2025, 08:57:59 am
content: I just built it on a breadboard and it has quite some popping going on. I'll definetely need to implement some kind of delay with a relay on the output.

author: Gyro
author_rank: Super Contributor
timestamp: September 03, 2025, 10:50:17 am
content: You can expect some crackles etc. on a breadboard but, looking at the LM1875 datasheet (always a good thing to keep going back to), unlike the later ICs, it does not include invalid supply rail muting - just thermal and output SOA protection.An externally controlled output muting relay would be preferable to supply rail relays for pop and speaker protection, particularly if you have a lot of reservoir capacitance.

author: dosensuppe
author_rank: Contributor
timestamp: September 03, 2025, 12:04:54 pm
content: I found this circuit, but it is only for single rail supplies. I would be very grateful if someone knows a suitable DC protection circuit for dual rail amps.https://sound-au.com/appnotes/an017.htm

author: themadhippy
author_rank: Super Contributor
timestamp: September 03, 2025, 12:23:51 pm
content: Quoteyou didnt look very far ,on the very same site for example dont even touch the amps psuhttps://sound-au.com/project208.htm

author: dosensuppe
author_rank: Contributor
timestamp: September 04, 2025, 12:06:24 pm
content: Oh well. Now I need to get this simulated in spice. somehow...

author: dosensuppe
author_rank: Contributor
timestamp: September 04, 2025, 01:04:52 pm
content: Comparing the specs the AA118 and SB360 I found to the UF4004 used in the circuit. They seem quite similar. Can they work as a replacement?