Oscillation/input issues in single-supply bridge amp - Page 1
Forum section: electronics / projects

author: Prehistoricman
author_rank: Regular Contributor
timestamp: March 19, 2017, 05:22:40 pm
content: I'm modifying an FX-15 guitar amp (brandless made-in-china job - almost no documentation online) to include two TDA2030A ICs in bridge. The transformer doesn't have a negative supply, so I looked online for single-supply bridge circuits and found this:https://www.allaboutcircuits.com/technical-articles/bridge-amplifiers-for-single-supply-applications/TDA2030A datasheet:http://www.st.com/content/ccc/resource/technical/document/datasheet/0f/a7/9c/08/83/f5/4d/6c/CD00000129.pdf/files/CD00000129.pdf/jcr:content/translations/en.CD00000129.pdfSo I went about drawing up a new circuit based on the one from allaboutcircuits:Based off of the old circuit (red is measured value, diagram from typical application in TDA2030 datasheet):And the datasheet's recommended specs:And I stick it on the power supply and it makes some very unpleasant sounds:One mode of oscillation (as it varies wildly) is for the output to be flat, and then climb in frequency up to 250kHz:Changing the dials for overdrive/volume usually change the mode of oscillation, even with no input.The 100k voltage divider you can see near the input in the 2nd image is still present on my board to raise the signal input to about near the middle of the supply (Vbias).What should I do?

author: Andy Watson
author_rank: Super Contributor
timestamp: March 19, 2017, 05:44:39 pm
content: Start by correcting the gain of the inverting amplifier (R3, R4). It is presently adding X56 to the output of the first amp.

author: Prehistoricman
author_rank: Regular Contributor
timestamp: March 19, 2017, 05:58:13 pm
content: Quote from: Andy Watson on March 19, 2017, 05:44:39 pmOh yes! Wow...However, the datasheet calls for >24dB closed loop gain (page 11). Would a divider from the output of the 1st amp into the 2nd amp work?

author: Andy Watson
author_rank: Super Contributor
timestamp: March 19, 2017, 07:14:26 pm
content: Quote from: Prehistoricman on March 19, 2017, 05:58:13 pmYes, it should work.

author: Audioguru
author_rank: Super Contributor
timestamp: March 20, 2017, 12:43:50 am
content: Why are you using the datasheet of the old TDA2030 instead of the datasheet for the newer TDA2030A that you say you have?Why aren't you using the schematic and pcb design for the bridged TDA2030A shown in its datasheet.One Chinese amplifier manufacturer made the ground trace too thin that caused oscillation on a different amplifier. Maybe the same problem with this one.The datasheet for the TDA2030A shows external frequency compensation parts that might stop high frequency oscillation.

author: Prehistoricman
author_rank: Regular Contributor
timestamp: March 20, 2017, 01:40:53 pm
content: Quote from: Audioguru on March 20, 2017, 12:43:50 amAh thank you, that explains why I could find two versions online! I didn't realise there was a difference between 2030 and 2030A. I've updated the link.Quote from: Audioguru on March 20, 2017, 12:43:50 amThat schematic uses split supply, which I don't have.Quote from: Audioguru on March 20, 2017, 12:43:50 amYes, I definitely noticed that in this design. The board doesn't appear thoughtfully laid out at all. The amp on the heatsink has its own wires that are thicker than the legs on the package, and I added wires to the bottom of the PCB for the amp there.Quote from: Audioguru on March 20, 2017, 12:43:50 amI think I've tried most of those that are applicable to the new inverting amplifier layout. If I've missed something, please tell

author: Audioguru
author_rank: Super Contributor
timestamp: March 20, 2017, 05:22:05 pm
content: You bias the amplifiers at half the supply voltage with resistors as shown here:

author: Prehistoricman
author_rank: Regular Contributor
timestamp: March 20, 2017, 05:59:48 pm
content: Isn't that just two non-inverting amps? Surely the output voltages would cancel, not combine?

author: Andy Watson
author_rank: Super Contributor
timestamp: March 20, 2017, 06:12:12 pm
content: C2 removes the A.C. signal component. The output is taken via R7 to the inverting input of the second amplifier.

author: Prehistoricman
author_rank: Regular Contributor
timestamp: March 22, 2017, 02:02:27 am
content: Quote from: Andy Watson on March 19, 2017, 05:44:39 pmSo I made this change today and it looks quite promising. Much less issues with the amp trying to drive DC into the speaker. Still a bunch of popping issues and the amp doesn't want to work before the signal level is somewhat high, leading to a lot of saturation and strangeness.I'm in the electronics lab on Tuesdays usually so if I don't have a better idea for this implementation of the circuit, I'll rip it all out and try the one from Audioguru.

author: Prehistoricman
author_rank: Regular Contributor
timestamp: March 26, 2017, 09:35:19 pm
content: I took my circuit to a simulator called TINA-TI and now I understand. The mid-supply biasing doesn't work, as current flows out of the opamp and into the potential divider. This destroys the middle bias and sends the output to the upper supply rail. If the input signal is large enough, it dips into the middle and then the output works as expected with a huge amount of gain.I put Audioguru's circuit in and it seems pretty solid except two things.I did an AC analysis and the gain over frequency is pretty poor. The datasheet says that the TDA2030A has an open-loop gain of 80dB at 1kHz. The data entry in TINA doesn't mention a frequency so I entered 10k. Would that be right? As far as I can tell, this low open loop gain is what's holding back my high frequency response.I made a few tweaks and managed to shift it over a bit:However I'd like to know if this is either a mistake or something that can be improved.Secondly, this arrangement will still have one of the flaws that my original design did: lots of DC output when initially turned on. As the input cap charges (and the others, but they seem to have less effect), the output goes to 10 volts for a small amount of time which may damage any lesser wattage speaker that I connect.Decreasing the value of R6 (Audioguru's diagram) lessens this spike but also negatively affects the bandwidth and gain. Same goes with the resistor closest to the input.Time delay relay on the output maybe?