amplifier KA-5090R - strange circuit in the Kenwood KA-5090R amplifier - Repair - Page 1
Forum section: electronics / repair

author: max-bit
author_rank: Frequent Contributor
timestamp: December 25, 2021, 09:27:59 am
content: I'm repairing Kenwood KA-5090RIt has on one DC channel at the output ...Interestingly, it seems that the transistors in the OK and DC path are at the output - a topic for further analysisBut this is still a small problem, because there are two operational amplifiers IC7 and IC8 in the feedback circuit.But the circuit is strange, the amp outputs are (cascaded) shorted to ground! What is this amp supposed to do? linearize gain? but how?I don't get it ....Anyone seen something like this?

author: bob91343
author_rank: Super Contributor
timestamp: December 25, 2021, 06:20:10 pm
content: It's almost as if those are circuit buffers to measure some point. Maybe an artifical test point. I don't know.

author: max-bit
author_rank: Frequent Contributor
timestamp: December 25, 2021, 06:26:16 pm
content: This is not a virtual mass (there are such suggestions)Or rather, it is not intended to reduce the supply voltage of operational amplifiers

author: coromonadalix
author_rank: Super Contributor
timestamp: December 25, 2021, 06:45:20 pm
content: Could be a constant current sinkSince they are in the first differential tansistors pairs circuit on the amp input ....

author: max-bit
author_rank: Frequent Contributor
timestamp: December 25, 2021, 07:04:49 pm
content: No .... And do you see the connection somewhere?

author: m k
author_rank: Super Contributor
timestamp: December 25, 2021, 07:06:09 pm
content: I'm with Bob.Maybe it's factory Idle Adj zero check.

author: max-bit
author_rank: Frequent Contributor
timestamp: December 25, 2021, 07:10:34 pm
content: How is it regulated?Because that's the problem that this amplifier is not connected anywhere (its output)

author: m k
author_rank: Super Contributor
timestamp: December 25, 2021, 07:15:24 pm
content: It's a comparator.Output is that resistor connected to its output.

author: max-bit
author_rank: Frequent Contributor
timestamp: December 25, 2021, 07:37:24 pm
content: Can you explain how it works in this amp?

author: m k
author_rank: Super Contributor
timestamp: December 26, 2021, 11:08:44 am
content: It must be filtering the power or balancing it somehow.Still, never seen any of its kind.Upper pair.When pin 5 goes a bit away from GND the OpAmp pin 7 flips.Then its partner flips and pin 1 starts heating R179.When power is on that is pretty irrelevant.(typical input resistance of NJM4565 is 5M)But when power is turned off the situation changes, pin 5 is 270/75k of L-Out.Maybe then the circuit starts doing its thing.

author: coromonadalix
author_rank: Super Contributor
timestamp: December 26, 2021, 02:37:06 pm
content: Check carefully the circuit how its connected, and you seem kinda close minded to answer rapidly without checking every possibilityIt is not a virtual ground, and a line goes to the muting section ... hum could be the mentioned 30db attenuator ??The Ka-5010 and the KA-5020 doesn't have this attenuator and you can see the amplifier differences in the amp inputs section

author: m k
author_rank: Super Contributor
timestamp: December 26, 2021, 04:00:08 pm
content: I don't get it.Be a teacher and explain.

author: floobydust
author_rank: Super Contributor
timestamp: December 27, 2021, 04:16:41 am
content: The circuit looks like it's trying to cancel ground-currents on the PC board. Samples R205 and inverts and drives that into ground with R179, both are 270R.Sometimes that is a marketing gimmick, "distortion cancelling" or some wild name given to it. Does the user manual mention anything revolutionary lol.What is strange though, the DC feedback looks +ve. R155 is also seeing output DC and some AC signal.

author: max-bit
author_rank: Frequent Contributor
timestamp: December 28, 2021, 11:31:58 am
content: Perhaps ... this is it.In fact, the amplifier has quite low noise. But I haven't experimented with desoldering these (IC7.op-amps.Interestingly, the C143 220uF / 35V capacitor was damaged (breakdown) and this caused the appearance of DC voltage on the Left channel.