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June 29, 2022, 06:03:54 AM

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Motor boating..but not the fun kind

Started by dogbox, December 08, 2017, 03:31:32 AM

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dogbox

Hi guys,
I've got a AN7523 Panasonic 3w 8v supply chip amp on a breadboard. Works perfectly just as is, although lacking a little sparkle. https://lib.chipdip.ru/126/DOC001126730.pdf When a guitar is plugged in isn't doesn't overload, and sound perfectly ok as a jazz amp, if thats your thing..plenty loud enough to practice at home. My plan is to add some some gain/distortio stage..experiment a bit once the amp is stabilised.

I have just added a Tillman preamp to see what that would do, and theres an improve in sound quality.. unfortunately it starts motor boating..slow thud thud thud..about 1.5 secs apart. I looked at the de-couple cap and resistor on the tillman and changed, 51k R to 5k..and the problem went away...when i power from a 9v battery only..if i try and use a wall wart multivoltage supply that i have, motorboating is still there. The problem was the same initially whether i used the battery or wall wart, untill I changed that resistor. Also have tried larger cap values. up to 470uf .no change

When I check the voltage directly from the supply..it seems stable, a slight variation by a mv or 2. Measured the breadborad rails when its under load the supply seems to vary by up to half a v..even when guitar just plugged in..also the red led i installed blinks in unison???

I plan on plugging this in so, although the battery works to a point..I ultimately want this issue sorted so i can use use the dc wallwart.

So to summarise, I geuss my questions are...
1. Can anyone suggest how to properly clean up the supply v, or send a link.
2. Why would the amp on its own not have an issue if the dc supply is unstable?
3. Why does adding a small preamp stage cause this issue.
4. Why does the battery fix the issue, but still problem with wall wart?
DB

g1

You need a better power supply.
It's giving you barely enough to power the amp, and adding the preamp is just too much for it.
Your meter is not catching the supply drop outs, but a scope would show it.  The pulsing LED is showing you the supply fluctuation.
What is the current requirement of the amp and of the preamp?

dogbox

From the AN7523 spec sheet

■ Absolute Maximum Ratings
Supply current ICC 1.0 A

■ Recommended Operating Range
■ Electrical Characteristics at VCC = 8.0 V, RL = 8 Ω, f = 1 kHz, Ta = 25°C ± 2°C
Parameter Symbol Conditions Min Typ Max Unit
Quiescent circuit current ICQ VIN = 0 mV, Vol. = 0 V  25 60 mA
Standby current ISTB VIN = 0 mV, Vol. = 0 V  1 10 µA

The supply is DC 3v, 4.5v, 6v, 7.5, 9, 12v.. multi-type. Its rated as 1200mA...cost about US$4.00 here in Indonesia, no name chinese import..got a bit of weight to it..got a clear plastic case and can see a little transformer inside.

Weird how it works on 9v batteries..i got 6pack of AA to make 9v...I thought the max current draw from batteries is about 600ma?? I dunno? Maybe the power supply is defective??

I built a fuzzface onto the breadboard..sounded best before the tillman..on batteries. On the power supplly..thum, thump, thump. Oddly enough i bypassed the tillman and ran the fuzzface direct to the amp and the motorboating stopped with the power supply!!!...just sounded crappy as there stop high pass filtering going on possibly??

I'm getting rid of the tillman will look at something else for the input, with tone controls.
If I can't get good results, then Plan B..start working on proper power supply. I got a transformer and plan to build a bigger amp using higher rated chip.

This was just to kind of experiment, with how different circuit snippets sound etc . Think I need my wife to strum the strings and get some current measurements. What's the best place on the circuit to make measurements for current? I'm a "for dummies" kinda electronics guy?? My problem is eectronics is only seasonable for me. When the rainy season hits, the surf craps out at the beach, I get back into electronics for a couple months and have try and remember everything. Rest of the year its just too bloody hot to worry about :)