My Gainclone PCB - Problems?

Hi guys, I just finished builing my gainclone on a PCB i designed in Protel.

I tested the amp before connecting it to a heatsink for DC offset and it was at ~20mV for both amps. Then I connected it to a speaker and this is when I noticed a hum sound which increased to a higher pitched sound over about 5 seconds. I figured I could test out the amp at low volumes before I attached it to a heatsink (which I am yet to purchase).

Can you check this amp at all without some kind of heatsinking or is it not going to work? Could this cause this weird sound I am hearing or is it my PCB design (which is attached)?

Any help would be great thanks

-Mikey
 

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  • amplifier overlay.pdf
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I used the LM3875

The schematic is attached.

-Mikey
 

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    gainclone.gif
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fedde said:
I would advise to not use that chip without heatsink! It can easily damage that way, even considering the internal protection. Just use a metal bar or an old pentium cooler with a hole drilled in for testing.

Fedde

Fedde is too modest -- look at his Chill Amp webpage to see a very nicely designed board -- http://jwg.student.utwente.nl/fedde/chill-amp.html
 
http://www.decdun.fsnet.co.uk/gainclonecircuits.html

That's the schematic my amp is based on. Maybe the minimised approach is suitable for point-to-point wiring only?

I will try tying the non-inverting input to gruond with a resistor and attach a source. I will go buy some heatsinks too, which I will need anyway and try this PCB and amp out again. Thanks for the help once again guys.

-Mikey
 
The circuit shown on Decibel Dungeon continues to work well two years after it was built! ;)

With DC offset measured at ony 20 mV, my preference is not to add a resistor between the non-inverting input and ground. Although it won't hurt, I can't see it solving any problems like oscillation!

The circuit should work perfectly well whether hard-wired or using a PCB.

I would never even test a chip amp without adequate heat-sinking!

Is the sound Mikey describes motor-boating?

And Mikey, what speakers are you using?
 
was testing the amp with a ****** pair of Phillips speakers. I realised my Jensen speakers are 6ohm so I can't use those

What's so bad about the Philips speakers? The Jensens would probably work OK at 6 ohm!

I was testing the amp with a ****** pair of Phillips speakers. I realised my Jensen speakers are 6ohm so I can't use those

I think it is some kid of oscillation but it sounds similar to what you describe!

Another question, I am using the insulated chips, can/should I use some thermal paste?

I've not used them myself but I don't think that thermal compound is necessary! ;)

BTW - what are you using for PSU?
 

Greg Erskine

Member
Paid Member
2002-01-05 11:56 pm
Sydney
shyfx said:
I was testing the amp with a shitty pair of Phillips speakers. I realised my Jensen speakers are 6ohm so I can't use those :(

May I ask what motor-boating is?

I will attach a heatsink ASAP and try it again.

Another question, I am using the insulated chips, can/should I use some thermal paste?

-Mikey

Once you get it going I don't see why 6ohm speakers are a problem. Try them, they may be OK if they are reasonably efficicent speakers. My speakers are 6 ohm and they are OK.

I have found a piece of 40mm x 40mm x 3mm x 180mm aluminium angle is adequate for chipamps.

I use thermal paste on insulated chips.
 
Nuuk said:
What's so bad about the Philips speakers? The Jensens would probably work OK at 6 ohm!
They are those micro system speakers and sound terrible. The enclosure is thin chipboard, I'd much prefer to use my Jensen's if I could.

BTW - what are you using for PSU?

I am using 25-0-25 160VA torioidal transformer, two inline rectifier bridges, 1000uF caps and that's it. I didn't go too complex as this is just my first PCB attempt and I was just after a working system for now.

Greg Erskine said:
I have found a piece of 40mm x 40mm x 3mm x 180mm aluminium angle is adequate for chipamps.

I use thermal paste on insulated chips.

I'll have to try both. Bunnings has plenty of angle :D
I'll skip the thermal paste for this amp as I don't have enough money, but I will use it on other ones if anyone thinks it'd make an appreciable difference.

-Mikey
 
shyfx said:
Any help would be great thanks

-Mikey

You don't have a separated signal ground and power ground arrangement.
You are connecting the ground to the NI pin directly from the middle point of the two PSU caps.
And you are connecting the ground on the input signal connector directly to the power ground at the input of the board.
You should take the input (signal) ground directly to NI on the chip, pick a central point to connect the input resistor (56k) to this ground and use a small resistor (1~2.7R) from this star to the power ground.

And don't switch-on the amp without any heatsink:att'n:
 
Re: Re: My Gainclone PCB - Problems?

carlosfm said:


You don't have a separated signal ground and power ground arrangement.
You are connecting the ground to the NI pin directly from the middle point of the two PSU caps.
And you are connecting the ground on the input signal connector directly to the power ground at the input of the board.
You should take the input (signal) ground directly to NI on the chip, pick a central point to connect the input resistor (56k) to this ground and use a small resistor (1~2.7R) from this star to the power ground.

And don't switch-on the amp without any heatsink:att'n:

Yeah, in my new PCB design, I created a virtual component (which is just a short circuit) which makes doing the PCB in two seperate stars a lot easier. I am also using double sided board for my new PCB and it's a lot easier to route.

For this amp, a friend who I was building it with is coming around today with some heatsinks. We will see how it goes after today.

-Mikey
 
Tested the amp today. ABSOLUTELY AWESOME!
This is my first GC so I don't know what to expect in sound quality, except I can say that it is far better than my mum's old Pioneer SA-9500 II. It just has a sound I can't describe.

With my building, today proved some things:
- without an input present, oscillating or some weird noise was inevitable
- the Phillips speakers have an impedance of 5.4ohms made the amp go curly. It was distorted sound from the lowest input to the highest. Two of these in series however was fine
- my Jensen speakers with 6ohms impedance were great
- even though my grounding isn't done the best on the PCB, there is hardly any noise whatsoever, well, comparing to my mum's amp and my Technics amp

From here on, I am going to build a new PCB, double sided and arrange the parts a little better. I am also going to try a 5 channel gainclone to test my skills in multi-channel design within Protel and for the need of a 5 channel amplifier as I don't have one any more.

Thanks for the help guys.

-Mikey