Amplifier Schematic Question - Page 1
Forum section: electronics / rf-microwave

author: bostonman
author_rank: Super Contributor
timestamp: October 08, 2018, 04:19:17 am
content: In the following service manual on page 26 is a schematic and I have a question about the PRE-O input stage.https://www.manualslib.com/man...E250p.htmlU6-A seems to be a transimpedance circuit I think, and U6-B seems to be an oscillator running at approximately 178kHz.Why exactly is this circuit being used (assuming I'm correct about what it's doing)?

author: coppercone2
author_rank: Super Contributor
timestamp: October 08, 2018, 04:31:35 am
content: fix link

author: bostonman
author_rank: Super Contributor
timestamp: October 08, 2018, 04:35:21 am
content: Sorry about that.https://www.manualslib.com/manual/762771/Jbl-E250p.html

author: bsfeechannel
author_rank: Super Contributor
timestamp: October 08, 2018, 06:44:36 am
content: U6-B is an oscillator around 200kHz, alright. U6-A is at the same time an integrator (through C58 and R80) to the square wave coming from U6-B, turning it into a triangle wave, and an error amplifier taking the feedback signal coming through R93 and co. U6-A also takes these two signals, adds them to the input signal, and compares the result to ground. Pin 1 should give you a nice PWM signal.EDIT: better wording.

author: bostonman
author_rank: Super Contributor
timestamp: October 08, 2018, 12:36:52 pm
content: U6-A is at the same time an integrator (through C58 and R80) to the square wave coming from U6-BI guess my two questions are: R80 and C58 would be more of a roll off than an RC, correct? From what I can tell, U6-A is a transimpedance amplifier, and, as you stated, comparing it to ground.The more involved question: why the need to make a triangle wave?

author: glarsson
author_rank: Frequent Contributor
timestamp: October 08, 2018, 12:46:24 pm
content: Hint: Class D

author: bostonman
author_rank: Super Contributor
timestamp: October 08, 2018, 12:54:42 pm
content: I'm reading now... makes more sense.So C58 and R80 are an RC or just a feedback with roll off?

author: bsfeechannel
author_rank: Super Contributor
timestamp: October 08, 2018, 10:00:05 pm
content: R77 and C58 make the integrator actually. R80 is a leakage resistor so that the integrator does not accumulate a DC level.

author: NiHaoMike
author_rank: Super Contributor
timestamp: October 09, 2018, 02:36:06 am
content: It's basically an implementation of Delta Sigma.http://www.beis.de/Elektronik/DeltaSigma/DeltaSigma.html

author: TheUnnamedNewbie
author_rank: Super Contributor
timestamp: October 09, 2018, 04:49:22 am
content: No offense, but I'm not sure if this belongs on the RF section of EEVBlog. Wouldn't this be more suited on the projects page?

author: bsfeechannel
author_rank: Super Contributor
timestamp: October 09, 2018, 05:24:08 pm
content: I noticed that too, but it's an interesting question, nevertheless.However, if I'm not mistaken, the OP themselves can move a thread to another board if they sense appropriate. The admin/moderators can too on request.

author: bostonman
author_rank: Super Contributor
timestamp: October 09, 2018, 05:27:48 pm
content: I considered an amplifier RF and why I posted my question here.

author: bsfeechannel
author_rank: Super Contributor
timestamp: October 09, 2018, 06:31:07 pm
content: Maybe it's time for an audio electronics board. Although I think the Admin/Mods may have their reasons for not having one.

author: TheUnnamedNewbie
author_rank: Super Contributor
timestamp: October 10, 2018, 06:37:30 am
content: Quote from: bostonman on October 09, 2018, 05:27:48 pmIf we go by that logic, an opamp would also be RF...I'm not trying to sound angry or anything, just that I think a subwoofer amplifier doesn't belong on the RF pageyou will probably get way more response on the projects or repair pages (you never mentioned if you were repairing this device, or just studying the schematics to learn something)

author: bostonman
author_rank: Super Contributor
timestamp: October 10, 2018, 12:43:48 pm
content: I'm not disagreeing with you. It's an amplifier my buddy is replacing a bad capacitor in, but I was curious about the input section.Since I'm not 'repairing' it, I assumed my question didn't belong under 'repair'. Since it's 'RF', I thought my question was better suited for RF.