- Tue Apr 26, 2016 9:32 pm
#189583
1st post here and I need help, I searched around did not see anything like my issue.
I picked up one of the STA540 Kits https://www.sparkfun.com/products/9612
I have it together and seems to work fine with one exception, I will explain below.
First here is my environment:
I am running it on 12.5 volts and have 2x 4ohm speakers hooked up.
I don't have anything truly line level to use as an input.
What I do have to use for input is:
Sony Walkman
Sansa MP3 Player
Samsung Tab2
Samsung S5
I was checking to see how loud the amp is, I turned both pots on the STA540 up to about 90% and used volume on input device to see where it clips, when it distorts.
Here is the issue: When I use the Walkman or the Mp3 Player at about and around 40-50% of the full volume of input device I get an awful thumping pulse from speakers with kind of a midrange squawk. I have to turn off STA540 to get it to stop just turning down volume of either amp or input device does nothing to stop it. I notice a bit of voltage drop on my power supply in synch with sound I am hearing. In creasing or decreasing voltage anywhere between 9 and 14 volts has no effect.
The 2 samsung devices only do this if fully cranked and/or on certain songs or parts of them. They are not near as loud with just headphones so I am guessing they dont put out as much power.
I do notice certain parts of songs or songs themselves with throw it into this feedback oscillation, like say a crescendo or louder part of the song, especially lower frequencies. For example on the walkman it will do it at a lower volume if I use the mega bass switch.
I figured I am over driving the amp, looking at the schematic of the STA540https://www.sparkfun.com/datasheets/Kits/STA540 ... %20v11.pdf and it looks to me that since the pots are at the input they are pregain of the amp and they are attenuating the input signal already and not really the control of the volume of the amp itself.
I built up a circuit to attenuate the input using a 1k and 10k resistor on each channel, it did very little to help my problem. Do I need higher values?
I hope I am not chasing after the wrong solution but I am thinking I just need a better way to regulate the incoming signal.
My end goal is to put this in a box with both rca jacks for line level input and a 3.5mm jack for input of phones, walkmans, ipods etc.
I have a few arduino projects I want to use this as a base to expand.
Any help would be appreciated - I could make sound file of the problem to upload if that would help.
Thanks again,
Magnetoman
I picked up one of the STA540 Kits https://www.sparkfun.com/products/9612
I have it together and seems to work fine with one exception, I will explain below.
First here is my environment:
I am running it on 12.5 volts and have 2x 4ohm speakers hooked up.
I don't have anything truly line level to use as an input.
What I do have to use for input is:
Sony Walkman
Sansa MP3 Player
Samsung Tab2
Samsung S5
I was checking to see how loud the amp is, I turned both pots on the STA540 up to about 90% and used volume on input device to see where it clips, when it distorts.
Here is the issue: When I use the Walkman or the Mp3 Player at about and around 40-50% of the full volume of input device I get an awful thumping pulse from speakers with kind of a midrange squawk. I have to turn off STA540 to get it to stop just turning down volume of either amp or input device does nothing to stop it. I notice a bit of voltage drop on my power supply in synch with sound I am hearing. In creasing or decreasing voltage anywhere between 9 and 14 volts has no effect.
The 2 samsung devices only do this if fully cranked and/or on certain songs or parts of them. They are not near as loud with just headphones so I am guessing they dont put out as much power.
I do notice certain parts of songs or songs themselves with throw it into this feedback oscillation, like say a crescendo or louder part of the song, especially lower frequencies. For example on the walkman it will do it at a lower volume if I use the mega bass switch.
I figured I am over driving the amp, looking at the schematic of the STA540https://www.sparkfun.com/datasheets/Kits/STA540 ... %20v11.pdf and it looks to me that since the pots are at the input they are pregain of the amp and they are attenuating the input signal already and not really the control of the volume of the amp itself.
I built up a circuit to attenuate the input using a 1k and 10k resistor on each channel, it did very little to help my problem. Do I need higher values?
I hope I am not chasing after the wrong solution but I am thinking I just need a better way to regulate the incoming signal.
My end goal is to put this in a box with both rca jacks for line level input and a 3.5mm jack for input of phones, walkmans, ipods etc.
I have a few arduino projects I want to use this as a base to expand.
Any help would be appreciated - I could make sound file of the problem to upload if that would help.
Thanks again,
Magnetoman