Turning down the bass of my makita site radio - Page 1
Forum section: electronics / projects

author: iwtommo
author_rank: Contributor
timestamp: July 30, 2018, 02:51:43 pm
content: I debated putting this in the newbie section as it's my first toe-dip into the world of 'proper' analogue electronics, but this is more of a quick project than anything else.https://www.toolstop.co.uk/makita-dmr104w-white-job-site-radio-stereo-with-dab-and-fm-replaces-bmr104w-p71185This beast is my new makita site radio, and its the good one, i splashed out. £80 buys you a new unit with DAB, and with a 5 quid chinesium dongle in the rear aux port, it accepts bluetooth too. It's a really nice bit of kit - durable, loud, easy to transport, runs off the makita 18v batteries - as a result its a common sight to any tradesman on a jobsite around the uk. If you dont have one, one of your mates does.And im getting on great with it, with one major exception. Its WAAY too bassy. As in, what were they thinking levels of bass. Anything above maybe 33% volume and the speakers start bottoming our and distorting. No problem i thought, ill just turn the bass down. Ha. ITS NOT ADJUSTABLE. The EQ levels seem to be hard coded into the firmware of the thing. I asked the guys... "yeah they are all like that mate, just turn them down when the customer is nearby"Heres a thread on a plasterers forum with similar complaintshttps://www.plasterersforum.com/threads/radio.59144/What to do.The internet didnt help - there are tons of videos on youtube, ranging from turning the input devices EQ levels down or ripping out the guts of the unit and installing car stereos, but as far as i can see noone has tried the obvious. Installing high pass filters on the speakers. Obviously this doesnt fix the root problem of a bad eq setup, but i think this solution is the best combination of subtle, cheap, quick and non-invasiveness of the other possible fixes.A high pass filter. No problem says i - Just an R and C per side. Peasy. Ha! My problem here is choosing the values - sure i could just start throwing parts at it and eventually find something that works but its a good excuse to try and learn how to do things properly - i have never played with filtering (or, anything proper analogue...) before.To start with, i connected the stereo up to my computer and, using my audio players EQ, dialled in a curve that made the stereo sound usable. The built in setting of 'bass reducer' did pretty good.Pic attached - The preamp level is just to get my laptop audio up to a level similar to the inbuilt DAB receiver's output volume in the unit.This got me the sound ive always wanted. Score. I found the equation f = 1/(2pi)RC, and then hit a roadblock. I have no idea how to actually relate the EQ curve i need into the equation to generate the values of component i actually need.Any hints? Am i headed in the right direction?SpecsSpeaker output: 3.5w using the 18v makita battery2 speakers (2 channel stereo)Speaker resistance: 4ohm

author: Ian.M
author_rank: Super Contributor
timestamp: July 30, 2018, 03:23:33 pm
content: Its not designable unless you have a plot of the actual complex speaker impedance over the lower half of the audio frequency range. That 4 ohms is purely nominal. You also need to know the amp's effective output impedance.However *if* the amp had effectively zero output resistance and the speaker presented a pure resistive load, adding 330uF || 10R in series with the speaker would at least get you 'in the ballpark', with a -3dB corner frequency of 155Hz, rolling off to -9.5dB at 32Hz, so that's a good place to start. Worst case dissipation in the 10R resistor is well under 0.1W.

author: james_s
author_rank: Super Contributor
timestamp: July 31, 2018, 07:07:35 pm
content: How about inserting an active filter just prior to the audio amp IC? Then you can easily tune the frequency. Or you could just mess around with a passive filter until it sounds good to you and call it done.