Swapping audio amplifier IC's - Page 1
Forum section: electronics / beginners

author: andrewlapham
author_rank: Contributor
timestamp: April 24, 2020, 10:24:05 am
content: Hey Guys,I got a cheap car stereo from eBay (JDS-520) and after inspecting the internals I quickly came to realize that the advertised 60w x4 was a load of crap haha. the audio amp inside was a TDA7385 which after looking through the datasheet I found it is capable of 30w x4, not only that most of the capacitors used in the device didn't match up to the datasheet at all, the biggest difference being that the 1000uf cap used on the VCC was meant to be 2200uf. anyway, I changed out all the caps to the desired values, and then it got me thinking. i had an old stereo laying around that can output at 50w and after looking through the datasheet for the TB2910HQ I have laying around it seems that all the pinout match up completely. the only difference being this new chip has an i2c bus and the other doesn't. will this still work if i just swap them?i understand more cooling would be needed but I'm confident that there is plenty of material available to still passively cool off the case.

author: mikerj
author_rank: Super Contributor
timestamp: April 24, 2020, 11:10:38 am
content: Take all those "headline" power values with a huge pinch of salt, they are as close to lies as they can get away with.RMS power for a conventional bridged amp is = (2*V^2)/(8*R), so for a 14.4v supply and 4 ohm load, the maximum possible power is 25.9 Watts and in practice it will be somewhat less. This ties in with the TDA7385 (23W @14.4v) and I strongly suspect the TB2910HQ will be similar, though I can't find a datasheet for it. Also note that these maximum RMS power outputs are usually stated at a very Lo-Fi 10% THD.

author: wraper
author_rank: Unknown
timestamp: April 24, 2020, 11:19:26 am
content: It provides increased power only when used with 2 Ohm speakers.

author: fcb
author_rank: Super Contributor
timestamp: April 24, 2020, 11:28:11 am
content: Standby and Mute circuit looks different. You might get lucky.Output power is limited by Vcc and speaker impedance - not likely to see any improvement in 'swing' between the IC's. Wouldn't waste time swapping.

author: tooki
author_rank: Super Contributor
timestamp: April 24, 2020, 06:40:13 pm
content: Quote from: mikerj on April 24, 2020, 11:10:38 amThat's one heck of an understatement. When I was a teen, I bought a cheap "400W" car audio amp, whose amp ICs are, in fact, rated at 20W/channel.

author: jaycee
author_rank: Regular Contributor
timestamp: April 25, 2020, 07:27:09 am
content: You simply aren't going to get more than about 20-25W from an amp that takes only a low current +12V feed. If you want more output power, you have to have a step up circuit that generates higher voltages. This needs a THICK cable directly from the battery.

author: M0HZH
author_rank: Regular Contributor
timestamp: April 25, 2020, 08:51:11 am
content: Quote from: fcb on April 24, 2020, 11:28:11 amTechnically not true; output power is limited by the supply voltage of the power transistors, not the supply voltage of the power amplifier module. There are amplifier ICs that step-up the supply voltage so they can achieve much higher power with 14.4V supply. See TDA1562 for example, 70W @ 10% THD with 14.4V supply. This is called Class-H.

author: AVGresponding
author_rank: Super Contributor
timestamp: April 25, 2020, 08:58:36 am
content: Yeah, even using BTL outputs you're not going to get more than 40-50W/ch, and as pointed out, you'll be in 10% THD territory at this point (though with ambient noise levels in a moving car, this isn't the issue it would be in your living room at home.Higher power amps use SMPS power supplies to get higher voltages +> higher power outputs, and this is an approach I've used to modify cheap "200W" bass cannons.It's worth taking another look at the amp IC data sheets, as some are rated for much higher supply voltages than 14.4V (which by the time you take drop along the cables into account is more likely 13V or so). The ones in the bass cannons I mentioned are TDA2009A's, which have a maximum rated Vcc of 28V or so, which obviously doubles your potential output power.Easy enough to get cheap SMPS boost regulators powerful enough on fleabay for this, just make sure you re-cap the amp pcb to cope with the higher voltage.Another advantage of using a boost regulator is that you can get full V at the amp/head unit, no drop to worry about.In a head unit where there's other things going on besides just the amp, you might have to mod the power rails so only the amp section gets the full beans, or there will be a mass escape of magic smoke...

author: fcb
author_rank: Super Contributor
timestamp: April 25, 2020, 04:29:37 pm
content: Quote from: M0HZH on April 25, 2020, 08:51:11 amNeither of the IC's that the OP mentioned are class H.

author: james_s
author_rank: Super Contributor
timestamp: April 25, 2020, 05:54:35 pm
content: Why not just use an external amplifier? Personally I find the 20W or so per channel produced by the head unit to be more than adequate when using reasonably efficient speakers but you can get hundreds of watts easily with a separate amplifier.There's a saying for what you're trying to do, "put lipstick on a pig".

author: andrewlapham
author_rank: Contributor
timestamp: April 27, 2020, 10:09:16 am
content: Oh man that was amazing, spat my drink everywhere when I read the lipstick part. hahaha

author: andrewlapham
author_rank: Contributor
timestamp: April 27, 2020, 12:04:50 pm
content: Would it be possible to use a boost converter to up the VCC voltage to the chips limits on the datasheet?Absolute maximum ratings:VS Operating supply voltage 18 VVS (DC) DC supply voltage 28 VVS (pk) Peak supply voltage (t = 50 ms) 50 V

author: AVGresponding
author_rank: Super Contributor
timestamp: April 27, 2020, 12:10:39 pm
content: Quote from: andrewlapham on April 27, 2020, 12:04:50 pmYes, but as I said, you need to be careful to only supply the boosted voltage to parts of the circuit that can handle it.This probably means some re-capping around the amp IC itself, and not supplying it to any of the control electronics, for example the front panel display etc, as that would definitely release the magic smoke.In practice it probably means cutting the amp IC Vcc pcb tracks and just supplying them directly from the boosted supply.EDIT: Also bear in mind if you're pushing up the power output you'll probably need to look at improving the cooling arrangements.

author: wraper
author_rank: Unknown
timestamp: April 27, 2020, 02:50:55 pm
content: Quote from: AVGresponding on April 27, 2020, 12:10:39 pmYes, there is only limited amount of heat car radio can dissipate. You cannot do anything about it other than taking it outside and attaching heatsink to it's side. The only real option to increase power by a lot is using class D amplifier which needs to dissipate way less heat. But then you also need to think about wiring, since stock wiring probably is too thin.