Need help with amplifier - Page 1
Forum section: electronics / projects

author: laejf
author_rank: Regular Contributor
timestamp: October 25, 2019, 04:59:00 pm
content: I'm making an amplifier using the TPA3123 class D amplifier IC (http://www.ti.com/lit/ds/symlink/tpa3123d2.pdf) and I'm using it in a bridge tied configuration. The recommended circuit of the TPA3123 (Figure 1) and my circuit (Figure 2) can be seen below. I have used a bit different values in the output filter but those are the recommended values for a 4 Ohm load as seen in the datasheet.My problem is that when connecting the input I get a very low volume on the output regardless what the gain is set to. But when I tried connecting both of my input channels into LCH and grounding RCH, the volume is normal.Does anyone have any idea what could cause this?ThanksFigure 1: Recommended circuitFigure2: My circuit

author: themadhippy
author_rank: Super Contributor
timestamp: October 25, 2019, 05:08:42 pm
content: In bridge mode your using both half's of the amp to make 1 more powerfull amplifier instead of a less powerfull 2 channel amp.If you look at the top circuit the input and outputs are marked as + and - not left and right.

author: laejf
author_rank: Regular Contributor
timestamp: October 25, 2019, 05:19:24 pm
content: Quote from: themadhippy on October 25, 2019, 05:08:42 pmI see, then how should I connect my source with a left and right channel?

author: StillTrying
author_rank: Super Contributor
timestamp: October 25, 2019, 05:47:52 pm
content: I see, then how should I connect my source with a left and right channel?AFAICT In BTL mode you only have 1 power channel and have to use the +/- (L/R) inputs as 1 differential input channel, = one has to be inverted phase.

author: laejf
author_rank: Regular Contributor
timestamp: October 25, 2019, 06:14:01 pm
content: Quote from: StillTrying on October 25, 2019, 05:47:52 pmWhat's the go-to way of doing that?

author: Kleinstein
author_rank: Super Contributor
timestamp: October 25, 2019, 06:19:47 pm
content: In bridge tied mode one would need a second amplifier chip for the other channel. Otherwise add both signals together to create a good old mono signal and use the inverted signal für the - input.In bridge mode the amplifiers see effectively half the speaker impedance. So with 4 ohms speakers the amplifiers see only an effective 2 Ohms load and would thus need more current and less voltage.So with 4 Ohms speakers better use non bridge tied mode.

author: laejf
author_rank: Regular Contributor
timestamp: October 25, 2019, 06:34:01 pm
content: Quote from: Kleinstein on October 25, 2019, 06:19:47 pmWhich inverted signal do you mean?

author: StillTrying
author_rank: Super Contributor
timestamp: October 25, 2019, 07:03:02 pm
content: I'm not sure what you're expecting, BTL gives 2X the voltage and 2X the current through the speaker so 4X the power, but you have to make one of the +/- (L/R) inputs inverted yourself.I'm no expert in audio, I've used simple 1 transistor to get 2 out of phase signals in the past.https://www.electronics-tutorials.ws/amplifier/phase-splitter.html

author: laejf
author_rank: Regular Contributor
timestamp: October 25, 2019, 08:50:43 pm
content: Quote from: StillTrying on October 25, 2019, 07:03:02 pmThank you. I'll try to figure it out

author: Kleinstein
author_rank: Super Contributor
timestamp: October 25, 2019, 09:13:04 pm
content: The signal inverter is normally done with an OP-amp. The simple transistor tends to cause distortion and poor PSRR and thus hum.One would get 4 times the power only if the amplifiers can deliver twice the current. More normal one would use speakers with twice the impedance and than get 2 times the power from 2 amplifiers.

author: Audioguru again
author_rank: Regular Contributor
timestamp: October 26, 2019, 04:49:23 pm
content: I agree that the datasheet does not show BTL driving a 4 ohms speaker because with the much higher power then the IC will melt.It can drive a 4 ohm speaker is the supply voltage is reduced enough (try 14V) for the maximum low distortion output to be a little less than 25W.

author: laejf
author_rank: Regular Contributor
timestamp: October 28, 2019, 09:26:32 pm
content: Quote from: Audioguru again on October 26, 2019, 04:49:23 pmI have not thought about that, thanks for clarifying!