Solved: How does a NJM2073 audio amplifier works? - Page 1
Forum section: electronics / beginners

author: Frazatto
author_rank: Contributor
timestamp: April 05, 2022, 07:09:22 pm
content: I "finished" upgrading some vintage pc speakers and it does all that I wanted to, except that the power is disappointing now that I'm using a bluetooth receiver instead of the original audio jack. I wander if I could still take this project a little further and fix this issue.Well, the original circuit uses a NJM2073 on each box, I already looked at the datasheet but I can't understand how it works exactly.https://datasheetspdf.com/pdf-file/606742/NewJapanRadio/2073D/1Poking the board with the multimeter, V+/Gnd is constant ~7V, I was expecting this to change as I turn the volume knob, but it doesn't.The volume knob is a 2.4K pot, at least that is what I get measuring the pins as I turn it.All other components are resistances and capacitors.It would be VERY nice if I could just change the pot for a lower resistance one and miraculously get what I want hahahaSo, is there hope for my little project?

author: magic
author_rank: Super Contributor
timestamp: April 05, 2022, 07:42:52 pm
content: Per the datasheet, the feedback network is fully internal, you have no access to the IN- input of the actual amplifier, and the INPUT- pin is used to ground the feedback network (through an external capacitor - see application examples).You can't change the gain of this stage. (Well, you could decrease it, by adding a resistor in series with the capacitor. It's not clear if the amplifier would remain stable.)If there are no opamps in there before the NJM2073 then you will need to add one, assuming that the bluetooth receiver itself can't be set for louder output.

author: Frazatto
author_rank: Contributor
timestamp: April 05, 2022, 08:23:02 pm
content: Quote from: magic on April 05, 2022, 07:42:52 pmHum....that is a shame.No, the bluetooth receiver doesn't have any regulation that I can see either (documentations is scarce though).https://www.aliexpress.com/item/1005001830804948.html?mp=1

author: Frazatto
author_rank: Contributor
timestamp: April 05, 2022, 08:46:39 pm
content: Quote from: magic on April 05, 2022, 07:42:52 pmNo...wait, this makes no sense....How does the volume knob works as intended than???There are no other ICs there or.....hummmmmmAlright, I think we are on to something, there is a transistor on the other side of the board:https://html.alldatasheet.com/html-pdf/54742/FAIRCHILD/S9014/404/1/S9014.htmlI can't quite follow the trail though, it jumps around too much.Is it pre-amplifying the signal itself before the IC than?

author: magic
author_rank: Super Contributor
timestamp: April 05, 2022, 08:50:20 pm
content: A volume knob simply attenuates the signal, or passes it unchanged if set to maximum.A transistor may be doing just about anything. No way to know without understanding the surrounding circuit.

author: Zero999
author_rank: Super Contributor
timestamp: April 06, 2022, 08:56:53 am
content: Quote from: magic on April 05, 2022, 07:42:52 pmReducing the gain is clearly shown in the data sheet.https://www.mouser.com/datasheet/2/294/NJM2073_E-275459.pdfTrue no minimum gain is given for stability, bu that can be fixed by increasing the gain at ultrasonic frequencies.

author: Frazatto
author_rank: Contributor
timestamp: April 06, 2022, 07:17:49 pm
content: Good news everybody!Observing the board, I noticed a 33ohm resistor in parallel with the pot, it become a simple test.If removing the resistor decreased the volume, it means the pot was just attenuating the signal.But it didn't, it has a very adequate power output now.I guess it means the pot drives the pre-amplification transistor.....Thanks for your help o/