what type of audio output amp (solid state) is this?  - Page 1
Forum section: electronics / beginners

author: Deactivated-1
author_rank: Frequent Contributor
timestamp: March 13, 2021, 03:02:59 pm
content: I started reverse engineering this Elta 8791 EQ/amp I have (might know already when I posted about the mechanical LED EQ potentiometer switches)so I pretty much have the output completely drawn, and I started wondering what type of amp is that, it looks pretty weird... I've never seen something like that, ... but I guess that could be because I'm a total dumby when it comes to solid-state electronicsI posted an SVG file on dropbox of the current schematic here:https://www.dropbox.com/s/jhxgtw5ewohjhh0/Schematic_Elta_Realistic%208791%20Main%20Board_2021-03-12.svg?dl=0and only the output (for the rear speakers) here as an image. (excuse my perhaps bad schematic drawing, or the part where I drew where the output goes :/ )[ Attachment Invalid Or Does Not Exist ]also it seems quite weird that Q8, Q4, and Q10 bases don't have any reference to anything it seems. but I'm like 99% sure I got it right as it is in the amp...same with the base of Q6, doesn't seem to have any ground reference, except maybe from Q8 and Q10? idkok I think the ground reference for Q4, Q6, Q8, and Q10 is gotten trough Q6.

author: bob91343
author_rank: Super Contributor
timestamp: March 13, 2021, 11:32:33 pm
content: It's too hard to follow the diagram. IC5 seems to be the main amplifier so pull a data sheet and see how it's meant to be used.

author: Benta
author_rank: Super Contributor
timestamp: March 13, 2021, 11:45:37 pm
content: Classic push-pull class B. C67 seems to be placed wrong, the "transformer" connections are suspicious and make little sense, and the schematic layout is in general miserable.In short: failed reverse engineering.

author: Cerebus
author_rank: Super Contributor
timestamp: March 14, 2021, 03:12:30 am
content: Quote from: bob91343 on March 13, 2021, 11:32:33 pmEven without knowing the part one ought to be able to spot U5 as being an op amp configured as a unity gain buffer. So "main amplifier"? No, it's doing no voltage amplification and that 1K resistor immediately after it quite clearly indicates that it isn't there for any significant current gain.

author: Cerebus
author_rank: Super Contributor
timestamp: March 14, 2021, 03:33:45 am
content: Quote from: Benta on March 13, 2021, 11:45:37 pmA little harsh, but there are clearly some errors there as things don't quite make sense while parts do make sense. Q4 and attendant resistors are obviously meant to be a Vbemultiplier (but aren't quite right), and if they are then the connections of Q8 and Q10 don't make sense. Q6 looks plausible as the main voltage amplifier with miller feedback and bypassed emitter degeneration. There's definitely something screwy with the connections of Q12, Q14, C67 and the transformer - they look like a class B output pair but connected up very much the wrong way around.There's enough to hint that it's a classic class B amplifier, but enough wrong that one can't really be certain of anything.I'd suggest to the OP that they try again while being a bit more careful, especially of transistor pinouts. (If you're using the datasheets to get the pinouts make sure whether you're looking a TOP view or a BOTTOM view.)

author: magic
author_rank: Super Contributor
timestamp: March 14, 2021, 09:42:35 am
content: 4558 is surely an opamp and PNPs should be 2SB/2SA, not 2SD.