Provided Two Transistor Audio Booster Circuit In HDMI To RCA Converter
My second son, who came home to provide aid for my wife who is in the recovery stage after a mild attack of stroke, mentioned that in the LED TV recently bought for his home at Bangalore, did not have RCA output and he was not able to play the 5.1 Speaker System through the HDMI to RCA converter he bought. Even the optical output did not work with the converters bought from the market. So, he requested whether anything can be done, upon which I stuck up with the idea of trying to boost the audio signal with a preamp so that we can have stronger audio output analogue signals. So, I applied Petrol on the sides of a similar converter that I had with me and opened the cover, which was fixed with glue.
If any of you are under the impression that such devices which are available for a throw away price may have some resistor capacitor network with some small digi-ana ICs, you are in for a surprise, as you can see the kind of complicated circuit with a few ICs, consisting of Video Processor, Memory IC, digital to analogue converter of Audio and Video Signals and other slave circuits before we can see the outputs reaching the RCA sockets! All assembled on a double sided PCB, with an external power supply port:
The next step for me was to obtain a booster circuit consisting of an op-Amp that should work on 5V drawing very minimum current. Though I tried my best to look for such circuits in the web, I noticed that almost all the circuits that I could get were using 9V to 12V power supply. Then I looked for transistor circuits. But unfortunately even that turned out to be working on higher voltages. So, as usual, I turned to my techie friends, one of whom shared the following circuit, which I thought of assembling on a small board using SMD components:
But, unfortunately once again I did not have the SMD version of a couple of resistors and capacitors. So, I decided to assemble it on a vero board and fix it on top of the gadget, wiring it through a couple of holes on both sides to solder the power, input & output required for its proper function. I used 549 instead of 149 mentioned in the circuit, as I was not having stock of that transistor.
I used a Zero Emery (No.100) paper to scrape the soldering surface very clean for an easy solder, without applying too much heat which might peel away the copper dots. Let us have a look of this board after assembly:
Then tested the audio boost by applying 5V power from an adaptor and injecting audio signals to the input and measuring it on my oscilloscope. There was a bit of noise found, which I assumed could be due to unshielded long wires used. After making sure that the circuit was working and the purpose could be achieved, I kept the board on top of the bottom cover pasting it centered on a double sided tape and made two holes on the cover by the sides of the PCB and inserted the wires through it:
Then cut the output tracks of the audio outputs that come to the Red and White Pins of the RCA. Then gave the input connections of the PCB to the output tracks of the mother board and output of the PCB to the RCA pins, so that the audio signals would get re-routed through this booster. Then gave two grounds, one for power and another for the input and gave the positive power to the board from the charging port. Then pulled the wires to inside and placed the mother board on its holding legs.
The wires were also routed (before soldering) through the holes on the mother board so that it will seat properly. Then tested it connecting it to my Laptop through HDMI and connecting the output to a testing amplifier. The output was good. Then I placed a cover of an AC adaptor with a shied earthed to protect the board from mischief mongers! (LOL)
Mission accomplished with great satisfaction which got qualified to get entry into the collection bag!
This article was prepared for you by Parasuraman Subramanian from India. He is 74 years old and has more than 30 years’ experience in handling antique equipment like Valve Radio, Amps, Reel Tape Recorders and currently studying latest tech-classes conducted by Kerala State Electronics Technicians’ Association. He has done graduation in BBA degree, private diploma in Radio Engineering and retired as MD of a USA company. Presently working as Consultant to Hospital and other institutions.
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Albert van Bemmelen
February 2, 2025 at 6:30 pm
I guess that the BC149 is the much older version of which the BC549 is the newer equivalent replacement transistor.
And nice job on the single side vero board assembly! Of which the "without applying too much heat which might peel away the copper dots" sounds very familiar to me.
Parasuraman S
February 2, 2025 at 9:10 pm
Many thanks for your keen and expert comments!
Yogesh Panchal
February 3, 2025 at 12:33 am
Good Modification! Sir
Parasuraman S
February 3, 2025 at 3:21 pm
Many thanks, dear Yogesh Bai!
Mark J
February 3, 2025 at 1:20 am
Parasuraman great job once again.
Parasuraman S
February 3, 2025 at 3:22 pm
Many thanks, dear Mark!
Waleed Rishmawi
February 4, 2025 at 3:03 pm
good job my friend on the repair success and thanks for including the circuit diagram in the article. I am so glad you were able to help out your son..it has a great warm feeling where we are able to help family members. thanks for sharing and have a blessed day
Parasuraman S
February 4, 2025 at 5:21 pm
Yes, I was remembering you when I got the chance! Many thanks for your encouraging and supporting comments, dear friend!
moshe jacobson
February 11, 2025 at 8:17 am
an excellent modification to the device...and a great article. thanks for sharing it.
Parasuraman S
February 11, 2025 at 11:46 am
Many thanks!