Boat Amp! Sanyo LA4508 8.5w chip

[oublie]

Hi Folks,

I just picked up a little amp of ebay for £6 ! i was hoping i'd be lucky and it would have a tripath in it, unfortunately it wasn't to be.

However, I've had it running for a day now using my computer as source to a zhalou d3 dac , the amp and then on the a stax srd7 mk2 with a set of Stax SR007A headphones.

It's great! Previously i've been using an old nad 3020b 30w amp which has seen better days but has been working perfectly and this is definately a step up.

The amp is advertise as a 500w boat amp! This think is tiny and is actually dwarfed by my stax energizer!

Ebay listing

Anyways i opened it up when i first got it as i had no led light when connected up to my power supply 12v and found it was just a solder connection had come loose.

I stripped it out of it's little aluminium case which is also the heatsink and its a Sanyo LA4508 chip

Sanyo LA4508

After reading the specs its now connected to a 16v 2amp psu and it really rocks. The bass is leaps and bound better than previously and it definately outshines my old nad which i'm really really surprised at.

So down to the problems.

Last night toward the end of my extended listening session the bass started to get very distorted with some uber bass tracks. The amp itself was still stone cold as i'm not running anywhere near maximum power level (100k pot is set to 9 o'clock) and i wonder if it could be the internal caps and if so any ideas about the best way to improve this.

Before i post any images of the insides don't have my camera a the moment can i ask has anyone else tried one of these and are there any tweaks- i could't find anything on the site - btw not sure is this is chip amp or digital

Based on the specs and suggested circuit can anyone recommend anything to try.

I would add that the only noise from this little thing when in use is from a hum issue i have using a coax connection from my computer which i plan to sort with a switch to optical.

btw i have a zero ta2020 amp on the way so it will be nice to compare the two.

please bear in mind that i'm probably using about 1w of this thing's power so distortion is none existant well at least in comparison with my nad.

[oublie]

ok so now i want to do some tweaks im pretty noobish at this kind of thing so a little help / advice would be much appreciated.

From the specs there are a number of different things i'd like to try and would like some confimation.

1. feedback capacitor.

the specs say 100uf but the bandwidth is determined by the value of this any idea of what would be an optimum value for these 2 caps?

2. supressing caps

these apparently stop high frequency distortions which i would assume is caused by the chip itself so if these frequencies are outside the audio range could this be removed completely. it specifies a value of a 0.01 uf ceramic.

3. bootstrap caps

this controls low frequencies and if decreased the spec say the low frequencies go lower. would this mean that a higer value gives improved bass or less bass ? min value 47uf

4. output caps

higer value improve the low frequency rolloff. The amp currently has 1000uf on each channel I think i might go crazy and try 4700uf to see what diffence it makes.

5 power cap

the power source cap is currently 2200uf i was thinking of maybe making a little capacitor bank of around 10000uf on a seperate board and attaching this to the main board where the current
cap is. Am i right in thinking this should remove a lot of power line ripple.

6 diode d1

a diode is used to stop saturation during switch on if i remove this will it have any detrimental effect since i planning to increase the power cap size.


thanks.

[pacificblue]

Originally posted by oublie
1. feedback capacitor.

the specs say 100uf but the bandwidth is determined by the value of this any idea of what would be an optimum value for these 2 caps?

Depends. The roll-off is determined by the 1/(2*PI*R*C), where R is R26(R27). You need to find the right compromise between low roll-off and the amplifier's ability to supply current. It also depends on the speaker you have connected. If it is too small, it makes no sense to make amp and speaker suffer, when the signals don't contribute anything to what you hear.

Originally posted by oublie
2. supressing caps

these apparently stop high frequency distortions which i would assume is caused by the chip itself so if these frequencies are outside the audio range could this be removed completely. it specifies a value of a 0.01 uf ceramic.

Don't remove them.

Originally posted by oublie
3. bootstrap caps

this controls low frequencies and if decreased the spec say the low frequencies go lower. would this mean that a higer value gives improved bass or less bass ? min value 47uf

No the output goes lower. That is Japanese English, for less power output. Start with 47 µF. Then increase the value and check the impact. Don't go below 47 µF.

Originally posted by oublie
4. output caps

higer value improve the low frequency rolloff. The amp currently has 1000uf on each channel I think i might go crazy and try 4700uf to see what diffence it makes.

Same as in answer 1. It must make sense. The amplifier must be able to deliver enough current. The speaker must be able to convert the additional energy into sound and withstand the additional heat. There is also an interaction between the input and output capacitor in that their filter characteristics add up and form a filter with a certain quality. That Q should be in a reasonable range, so if you increase either capacitor, increase the other by the same ratio.

Originally posted by oublie
5 power cap

the power source cap is currently 2200uf i was thinking of maybe making a little capacitor bank of around 10000uf on a seperate board and attaching this to the main board where the current
cap is. Am i right in thinking this should remove a lot of power line ripple.

It reduces ripple voltage, but increases ripple current. If you use bigger capacitors, make sure that transformer and rectifiers are able to supply enough current and that the ripple current rating of the capacitors fits.

Originally posted by oublie
6 diode d1

a diode is used to stop saturation during switch on if i remove this will it have any detrimental effect since i planning to increase the power cap size.

The other way round. The diode has no detrimental effect, when used. It is a protection.

[oublie]

Brilliant,

Thanks pacificblue,

I've a load of different caps etc lying around so ill see what i can do with it.

seriously this little thing is surprisingly good especially at the price so ill do a bit of tweaking and see what i can come up with.

ill post some pics once ive done a little tinkering.

btw regarding your first answer, i'm running this into a pair of transformers 1:40 turns ratio for a set of stax omega mkii elecrostatic headphones essentialy a reversed output transformer so its high voltage lower amps.

[pacificblue]

Originally posted by oublie
btw regarding your first answer, i'm running this into a pair of transformers 1:40 turns ratio for a set of stax omega mkii elecrostatic headphones essentialy a reversed output transformer so its high voltage lower amps.

What a combination. That makes everything different.

If the load is a transformer, omit the output capacitor, because the transformer will do the DC blocking. In that case use as big a feedback cap as you can fit. You could even skip the feedback capacitor and make the amplifier DC coupled just like Stax does.

Consider using a voltage regulator to feed the amplifier, because the Stax will reveal any ripple voltage without mercy and the LA4508 has poor ripple rejection due to its intended purpose.

Where do you get the 580 V polarisation voltage from?

Be careful with those high voltages.

[oublie]

easy, voltage multiplier with big ohm resistor on the end. running from 240v into a small isolation trafo and then on to the resistor, zeners, diodes and hv caps, zeners limit incoming voltage to 100v then this i multiplied by 6 - in a perfect world this would be 600v but with loss im getting around 580ish a small percentage difference doen't do much harm.

this is identcal to the layout of the bias supply used in stax transformers

good tips, ill give what you said a go.

regarding voltage regulation, im going to be using my old meanwell 300v 12v dc switching power supply. I used to use it when overclocking to run a couple of peltiers

any idea if this would have built in voltage regulation as its is designed for pseudo scientific use? obviously ill be ramping up the dc on it as high as i can go.

[pacificblue]

Switching supplies are usually regulated.
Some come with a potentiometer to adjust the voltage in a range of ±10 to ±20 %.