Why is Falstad circuit not working - Differential to Single-Ended Audio Signal - Page 1
Forum section: electronics / beginners

author: LooseJunkHater
author_rank: Frequent Contributor
timestamp: April 20, 2025, 09:54:01 pm
content: Falstad Circuit:Reference Circuit (frompage 12 of this document):_____________________________The purpose of this circuit will be to convert a differential audio signal (from the driver of bluetooth headphones) to single-ended audio signal (to feed into the input of aTPA3110 module). I'm aware that the TPA3110 supports *either* a single ended/differential audio signal, however, I don't want to modify the pre-built module (but I have done it before without issue).I'm not exactly sure what I'm doing wrong for the Falstad circuit to not be working?

author: Andy Watson
author_rank: Super Contributor
timestamp: April 20, 2025, 11:17:57 pm
content: What is "unity gain buffer"? If it is intended to be an op-amp, then it has no provision for biasing. Also, it will struggle to drive a 32load to ground.What is the "differential input" - why are there two different frequencies at the input? Is it A.C. coupled?

author: LooseJunkHater
author_rank: Frequent Contributor
timestamp: April 21, 2025, 12:54:19 am
content: Quote from: Andy Watson on April 20, 2025, 11:17:57 pmIt's to simply illustrate that the op-amps are feeding a TPA3110 module, with the TPA3110 module driving an (example) load of 32ohms.Quote from: Andy Watson on April 20, 2025, 11:17:57 pmI wasn't really sure how to produce a differential input in Falstad lol. Aside from the failed differential signal, will the circuit work?

author: LooseJunkHater
author_rank: Frequent Contributor
timestamp: April 21, 2025, 01:43:45 am
content: Figured it outhere. Looks like I only needed to set the signals 180 degrees out of phase. However, it's clear that the signal generated is asquarewave andnotthe intended sine wave that I was hoping for.

author: LooseJunkHater
author_rank: Frequent Contributor
timestamp: April 22, 2025, 09:07:53 pm
content: Anyone have any idea on why Falstad is turning my sinusodial Differential Audio Signal to asquare wave, when converted to Single-Ended? Anyone know what I can do about this?

author: LooseJunkHater
author_rank: Frequent Contributor
timestamp: July 06, 2025, 01:02:47 am
content: Quote from: LooseJunkHater on April 22, 2025, 09:07:53 pmBeen a little while but I figured out the various problems with my circuit.Here's the fixed Falstad circuit. Had some silly mistakes, like how the simulated opamps weren't rail to rail, the output resistor of 32ohms was drawing too much current from the op-amp, and more.

author: Zero999
author_rank: Super Contributor
timestamp: July 06, 2025, 11:34:45 am
content: Quote from: LooseJunkHater on July 06, 2025, 01:02:47 amThat's still not right. Look at some of those numbers: -2.27kA, -18.35MV. Complete nonsense!The circuit still contains errors. The inputs should be AC coupled, in order to keep the op-amps' inputs around the bias voltage.What are you trying to achieve? That Falstad model won't be anything like the real SSM2135.Download the SPICE models for the SSM2135 from the Analog Devices website.https://www.analog.com/en/products/ssm2135.htmlAnd use a real SPICE simulator, such as LTSpice or Micro-Cap.Falstad is a great visualisation and educational tool but it has limitations which make it unsuitable for use serious design work.