amp motorboating

[petey twofinger]

it is a2 X 25Watt Class D Audio Amplifier Board :

http://www.sure-electronics.net/download/AA-AB32165_Ver1.0_SCH.pdf

http://www.sure-electronics.net/download/AA-AB32165_Ver1.0_EN.pdf

it runs off of an internal 12 volt sealed lead acid battery . there is a stereo amz mosfetboost pushing it . it has a 7809 vreg to provide power to pedals , and another automotive style multi voltage adapter serving to reduce the current a bit to power the syth , @ 12 volts . i tried the synth right off the battery , no go ... the automotive trahnsformer is blown but it still puts out ONLY 12 volts , the other settings ( 3 to 9 volts )do not function .

the amp works well , until i power the synth off the internal supply , then it motorboats consitantly . when i tested it , it was in my room at maybe 70 degrees it was fine , outside @ 85 it motorboated .

is there a more efficient / better way to power this synth simply .

is there a simple fix to eliminate the motorboating like a capacitor . i was thinking a 1000 uf 16 volt cap connected close to the battery , like across the leads , plus to plus , minus to minus ... not sure though . i know its a goofy rig , figured it wouldnt hurt to ask .

[tca]

You should decouple the synth from the amp. Use a RC network, something like this:

(power source)-----(Amp)
                          |
                          |   R
                          --//////-----(Preamp/synth)
                                      |
                                   ------  C
                                   ------
                                      |
                                   GND


R=100R, C=100u

I guess you are not powering the synth with the 7809, you could also do that to remove motorboarding.

Cheers.

[petey twofinger]

thanks for the advice .

i got some 1 watt 100 ohm res , figuring a 1/4 watt couldnt handle it . thing is unless i went with a lower value r like a 10 the keyboard wouldnt power up .

i tried a few different transformers , settled on the one that drew the least current . i did not use the rc network though , as things were getting hot , and i also blew a fuse . it makes a bit of hetero dynie type noise .

odd thing is this, the main motorboat issue only happens when its hot , like at the woods . have to see what happens . i fused this transformer so if it happens , i can pull the fuse , then run the synth off its own dedicated battery with a nother transformer . i would love to figure this out / eliminate the motorboating .

thing is my kid reverse polaritied a transformer and fried it already ... i would like to power the synth off the amp to keep it simple for us but ...

thanks for the help man , i really do appreciate that !

[tca]

I thought that the "synth" was a small thing... if it is a keyboard that would take a lot of current... and you really need two separate power sources: one for the amp and other for the keyboard. No way around it! The motorboating, when you plug in the keyboard, is just the power source complaining not being able to provide the required DC current. What is the max current that your main transformer can deliver?

I do think that  you really need two separate power sources.

Cheers.

[Gurner]

i tried a few different transformers , settled on the one that drew the least current .

Transformers don't draw current....it's your follow on circuits that draw current.

Transformers supply current.

[GibsonGM]

i tried a few different transformers , settled on the one that drew the least current .

Transformers don't draw current....it's your follow on circuits that draw current.

Transformers supply current.

I don't want to be a snit, but they actually DO draw some current from the primary when idling.  All those eddy currents and stuff.  But not enough to be considered much of a part of the design process!  Worth keeping in mind only because (in my experience, anyway) every little thing that draws on the primary tends to have larger effects than we think...you could end up pulling a lot more current out of a primary that you suspect you are.   Most times it's not important, but once in a while....   

That said, it ALL adds up, and yup, when you demand too much from a transformer (which is operating at what, 60% efficiency, to starty with??), you get all kinds of issues that the OP is reporting - unreliability being a big one. 
Simplest to just use a 2nd power supply! 

[Gurner]

i tried a few different transformers , settled on the one that drew the least current .

Transformers don't draw current....it's your follow on circuits that draw current.

Transformers supply current.

I don't want to be a snit, but they actually DO draw some current from the primary when idling.

Ok, granted, but you'd be hard pressed to glean which transformer (from a selection of many) drew the least current at idle (unless you had the datasheet) .....& I'm pretty sure hardly anyone cares in stompbox land about how much current a transformer draws when idling!

My earlier quip was just to underline that when looking at transformers (or more so the labels on transformer/wall warts), it's not how much current the transformer draws...but how much current the transformer can supply to the follow on circuit (which is what really draws the current).

I'll ask my solicitor to cast his eye over my next post prior to posting to make sure it's 100% watertight!  

[amz-fx]

Petey,

The speaker output jacks need to be isolated from the chassis, if the chassis is metal and grounded.

The power amp ground should go directly back to your internal power supply's ground terminal.

The mosfet preamp ground should go directly back to your internal power supply's ground terminal.

The  internal power supply's ground terminal can be connected to the metal box.

Grounding schemes can be a problem in power amps, and  if there is enough resistance in a shared ground, then noise on the ground wire can set up a feedback condition that causes the motorboating.  Your  internal power supply's ground terminal can be the star-ground-point.

Best regards, Jack

[petey twofinger]

the factory supplied synth power supply ( wall wart that came with it )  is rated at 12 v 500 ma .

with my meter on it it was idling at .065 , then firing up the synth (yamaha csx1)  it skyrockets to around .600

the amp chassis is not metal , it is a kicker box style mdf or whatever carpeted box .

the amp board is directly connected to the battery , there is a 7809 for her theramin , then a seperate 12 volt transformer rated at 2 amps that powrs the synth , which is 500 ma or so . so i figured with one thing on each circuit , i was safe , especially at 1/4draw on that synth transformer .

the ground issue makes sense , and yes ,  every thing runs direct to ground . although i admit i was looking for a quick add a cap or two somewhere solution but ... unless i bring everything outside on a hot day and monkey around i can not even duplicate the issue so ... i forsee this being a permanant headache , but a really minimal one .

i tried a few things last night , including running the synth off the amps 9 volt port which it appeared to run fine off the 9 volts .

running the synth off the 9 volt OR a seperate transformer exibits a high pitched ticking which i believe is heterodyning . a seperate issue , but again i have that with all the amps when i try to run multi effects off the amps internal p.s. . one remedy is to swap p.s. , like my wifes amp 9 volt port runs my one kids effects . it works but ...

for now , i swapped out the old half blown transformer for a new good one , 12v 2 a for the synth . i heterodynes a bit when cranked so we shall see what happens when ts hot ! lol .

again , what i am trying to do , run all this equipment of sealed lead acid cells , presents many little challenges for a goof ball guy like me .

thanks everyone