9993	2014-04-30 18:54:27	bob_ledoux	How Do You Attack Audio Amp Motorboating?
Message	Date	From	Subject
I've built a number of audio amps, even from existing circuits, and seem to have problems with motorboating. If I Google the issue I read individual exploits and the black magic that was involved in a particular problem. I'm looking for a general schema for locating the causes, isolating the source of the feedback, measurement methods, and problem solving. Does a scope have utility here? What about circuit board layout techniques? Are there solution techniques, like poking a high value cap into the circuit until the problem ceases? Are there articles that address a general set of guidelines for this issue?
9995	2014-04-30 19:44:46	Russell Shaw	Re: How Do You Attack Audio Amp Motorboating?

9996	2014-05-01 06:15:31	bob_ledoux	Re: How Do You Attack Audio Amp Motorboating?
I just found this: http://www.analog.com/library/analogdialogue/archives/35-02/avoiding/index.html
9997	2014-05-01 06:35:17	Nick Kennedy	Re: How Do You Attack Audio Amp Motorboating?
I recall a warning that motorboating might occur with the Norcal Tin Ear receiver, IIRC, if you used a "heavy duty" 9V battery instead of an alkaline. I think this receiver uses a discrete version of the LM386 as is shown in EMRFD. I did some measurements and found that an alkaline 9V battery had an internal resistance of 1.5 ohms but a heavy duty battery's internal resistance was 45 ohms. I believe putting a large value electrolytic across the lower quality battery (if used) might help with the problem. 73- Nick, WA5BDU
9998	2014-05-01 09:40:49	Sandeep Lohia	Re: How Do You Attack Audio Amp Motorboating?
I was using LM386 with a 100 ohm resistance at it's positive supply pin no 6, wth 10 ohm it used to motorboat...
9999	2014-05-01 12:29:38	arfghans	Re: How Do You Attack Audio Amp Motorboating?
Your most likely culprit is current feedback via the load. High current that circulates thru the load, if allowed to return to an arbitrary point near the input of the amplifier, can result in a small I*R drop (voltage) that is then coupled into the amplifier input. This is an unwanted feedback loop. What you want to do instead is return the load to the power supply common as directly as possible. Then you can follow the circuit for high current and it is very clear: From PS+ to the amplifier, out of the amplifier, thru the load, and back to PS-. Low-level signals cannot intermix with this high-current loop, ever! The input signal common for the amplifier is also PS-, but via a separate conductor path. Power supply return is always a single-point ground in a well-designed audio power amplifier system. Gary, WB9JPS
10000	2014-05-01 15:58:16	bob_ledoux	Re: How Do You Attack Audio Amp Motorboating?
The circuit is my SMT layout of the Norcal 2030 receiver audio stage. It consists of a quad TLV2464 op amp. The first amp is a three pole high pass filter. The second stage is a 43 db audio amp. The third and fourth amps are parallel drivers for earphones. Before those last two amps is a MAX7427 SCAF filter. This is the circuit found on page 3 of the Norcal published schematic document. I've taken the proven circuit from my 2030 and redrawn the diagram slightly larger with my components moved a bit. An LP2985-2.9 volt regulator drives the audio portion. One side of the board has the etched circuit and the other side is a ground plane. Vias connect front to back when a ground is indicated. The ground plane is continuous. There are no ground traces on the trace side of the board. I assumed a ground layer in a circuit board qualifies as a single point ground. So this is in error. But I don't see a single point ground in the original 2030 board. The circuit has a 100uf bypass cap at the op amp power supply and four additional 22uf bypass caps at various points in the circuit. The motorboating exists at high gain levels. In my current test configuration I have 6 inch wires to the volume control that might contribute to the problem. I didn't expect a proven circuit to be this sensitive to layout changes.
10001	2014-05-01 16:27:31	David	Re: How Do You Attack Audio Amp Motorboating?
Ground planes do not count as single point grounds if the gain is high enough or the common mode or power supply rejection is low enough. In some cases it is necessary to cut gaps in a ground plane to route high currents away from sensitive nodes. On 01 May 2014 15:58:16 -0700, you wrote: >I assumed a ground layer in a circuit board qualifies as a single point ground. So this is in error. But I don't see a single point ground in the original 2030 board.
10002	2014-05-01 19:04:53	vasilyivanenko	Re: How Do You Attack Audio Amp Motorboating?
Also.... decouple plus bypass to filter the DC lines in your various AF stages, especially the AF power amp. For example.................... From http://www.qrp.pops.net/general-electronics-2011.asp LM386 Motorboaters Expect AF feedback motorboating via your DC power lines as you increase the AF gain pot in many LM386 circuits. If this happens, try better bypass and decoupling on the LM386 power line. 100 to 470 μF shunt bypass on the DC line + a 10-22 series decoupling resistor plus 100 - 470 μF shunt bypass on pin 6. Typically 470 μF bypass is required if you suffer 60 hertz hum. You may model your simple RC filter with application E on the javascript applet page — to see the 3 dB cut off frequency of your particular DC line low-pass filter................................ ..................