Choosing the right pot for radio project - Page 1
Forum section: electronics / projects

author: spilihps
author_rank: Regular Contributor
timestamp: September 26, 2018, 05:58:18 pm
content: Hello!I am currently experimenting with a small FM radio.I am using the Si4703 FM-chip, and TDA7266M.I want to use a analog pot for volume control. Even though the Si4703 have a digital one built in.I cannot find much info, (or not sure were to look) in the datasheet for the output of the FM-chip itself. It only states "Line-level output"Any tips on this?Thanks!

author: 6PTsocket
author_rank: Regular Contributor
timestamp: September 26, 2018, 07:58:45 pm
content: Quote from: spilihps on September 26, 2018, 05:58:18 pmMaybe you already know this but from what I saw online, it takes other modules to control and tune this chip. It does not appear to be a stand alone tuner, aside for a need for an audio amp.Sent from my SM-G900V using Tapatalk

author: spilihps
author_rank: Regular Contributor
timestamp: September 26, 2018, 08:24:35 pm
content: Thanks, but yes i know this. Its currently controlled by an arduino..

author: Audioguru
author_rank: Super Contributor
timestamp: September 27, 2018, 01:54:11 pm
content: Quote from: spilihps on September 26, 2018, 05:58:18 pmThe datasheet says all you need to know:Output Load Resistance (your volume control)= 10k.Audio Output Voltage (1/3rd max modulation)= 80mV.The datasheet for the TDA7266 says its gain is 26dB which is 20 times.Then with a 10k pot at maximum and fairly loud FM audio, the output of the amplifier into an 8 ohm speaker will be (80mV x 20) squared/8 ohms= 0.32W. At maximum FM modulation the output is 2.88W if the TDA7266 has a supply voltage of at least 9.5V.I think there is not enough gain since the volume control will be near maximum all the time and cannot be turned up more to make low level sounds louder. The amplifier is large with a maximum useable output power of about 12W so then a preamp with gain of about 12 times is needed.

author: spilihps
author_rank: Regular Contributor
timestamp: September 27, 2018, 03:21:04 pm
content: Quote from: Audioguru on September 27, 2018, 01:54:11 pmHello!And thank you very much for your help. It sure was due to not knowing were to look, and what to look for!Also, this is very valuable information for me as a novice in audio applications.I set out to find a pre-amp, and my dealer closest to me does offer one SSM2019BNZ It have selectable gain up to 70dB.Do you think that could be a good match?

author: dmills
author_rank: Super Contributor
timestamp: September 27, 2018, 05:56:16 pm
content: Nope, that is a microphone preamp, you probably only need somewhere in the 10 to 20dB gain region and are already at pretty much line level, so I would be thinking more on the lines of an opamp, maybe a TL072 or such setup for a gain of maybe 5 or 10 times.Regards, Dan.

author: spilihps
author_rank: Regular Contributor
timestamp: October 01, 2018, 06:44:06 pm
content: Quote from: Audioguru on September 27, 2018, 01:54:11 pmHello again!I was just wondering if we can clarify the calculations a bit.I if understand correctly, we want to gain the 0.08v from the FM chip by 12 times: 0.08 * 12 = 0.96If we plug that straight in to the previous calculation:(0.96*20)^2 / 8 = 46wSo i clearly miss some very important knowledge about the calculations on this.I thought that we were shooting for about 12W at maximum modulation?

author: Audioguru
author_rank: Super Contributor
timestamp: October 02, 2018, 01:42:36 am
content: You play a radio at full volume ONLY when the sound level is very low then you can turn up the volume.46W/12W is extra voltage gain of only almost 2 times. You need more gain so you can turn it up when the sound levels are low.Our hearing and volume controls are logarithmic. Double the power sounds only a little louder. 10 times the power sounds twice as loud.When a volume control is set to halfway then it cuts the input level to 1/10th.

author: spilihps
author_rank: Regular Contributor
timestamp: October 02, 2018, 02:11:56 pm
content: Quote from: Audioguru on October 02, 2018, 01:42:36 amThanks.I see. I thought that the equation were to help not exceeding the rated power of the amplifier.Ie, at maximum it would of been 2.88 without the pre-amp, and was supposed to shoot for 12 as the maximum.