My GainClone is Blowing up Speakers

Arunkumar14_th · 25th January 2009, 02:10 AM

I went ahead to do a GainClone Amplifier.
I took the information from this site:

http://diyaudioprojects.com/Chip/Syn...875-Gainclone/

MY Parts are exactly the same
Differences being, Transformer is an EI core , 24 0 24, 5Amp one
The Bridge was a standard bridge with Caps of value 1pF.

The First Thing I noticed was that one of the diodes were overheating, But the output voltage was in range around 47V between the +V & -V nodes.

Then When I connect the output to a speaker, the voice coil simply burns out.

It seems like the output stage has a large DC current.

Any Ideas if I'm doing anything wrong?

please guide me in the right direction.

pacificblue · 25th January 2009, 06:18 AM

Quote:

Originally posted by Arunkumar14_th
MY Parts are exactly the same
Differences being, Transformer is an EI core , 24 0 24, 5Amp one
The Bridge was a standard bridge with Caps of value 1pF.

The First Thing I noticed was that one of the diodes were overheating, But the output voltage was in range around 47V between the +V & -V nodes.

Is the diode already overheating, when no speaker is connected? Then there is probably a short to ground on that rail. Or is that diode connected the wrong way round?

47 V is very low. With a nomnal 48 V transformer the no-load voltage should be around 70 V. Is there a short somewhere that makes the voltage break in? What other capacitors do you have in the power supply? Are they all connected the right way round? Do you use a light bulb tester?

Does the IC get hot as well? Then it is probably broken, the reason for the short and needs replacement.

Quote:

Originally posted by Arunkumar14_th
It seems like the output stage has a large DC current.

Before you connect a speaker, you should measure the output voltage. If you have DC at the output, you need to find out, where it comes from. Is it from the source? Then use Cin as per National's datasheet or repair or replace the source. Is it from the IC? Then use Ci, if the IC is still okay. If it is not, find out why, repair that and replace the IC.

The best thing is to use either both Ci and Cin or a very good ( = very fast) DC protection circuit.

audio1st · 25th January 2009, 08:59 AM

Quote:

Originally posted by Arunkumar14_th
I went ahead to do a GainClone Amplifier.
I took the information from this site:

http://diyaudioprojects.com/Chip/Syn...875-Gainclone/

MY Parts are exactly the same
Differences being, Transformer is an EI core , 24 0 24, 5Amp one
The Bridge was a standard bridge with Caps of value 1pF.

The First Thing I noticed was that one of the diodes were overheating, But the output voltage was in range around 47V between the +V & -V nodes.

Then When I connect the output to a speaker, the voice coil simply burns out.

It seems like the output stage has a large DC current.

Any Ideas if I'm doing anything wrong?

please guide me in the right direction.

Hi, your V+ to V- should be nearly 70V dc. Your problem could be the 1pF caps? Also check the polarity of the power caps.. The usual mistake is the V- cap, the + side of this cap goes to Gnd. I guess that one rail, V to Gnd, has a different voltage to the other rail..
Basic layout below..

BrianGT · 25th January 2009, 10:19 AM

Did you test the power supply before connecting the amplifier channels to it? I always test the power supply before hooking up the amplifier to it. Can you unconnect the amplifier channels from your power supply, and test it out.

Have you measured the DC offset of the channels?

Post up some pictures of what you have made.

--
Brian

krips · 25th January 2009, 04:10 PM

audio1st, could you explain your picture? My understanding of the Synergy schematic was that the + side of one cap went to pin 1, the - side of the other cap goes to pin 4, and the other pins go to ground.

sangram · 25th January 2009, 05:41 PM

Hi Arun,

First off, disconnect and check the supply without the amp connected, your voltages should be all correct. As has been said, you should be getting around 35V on each rail or 70V between the rails.

Be sure to not reverse the supplies - this can be catastrophic.

Connect a dummy load to the amp. I use a wirewound 10hm, 50 watt resistor bought from the local market.

Now turn on the supply, and measure the voltage across the resistor, it should not be more than 50mV. Builders here strive for less than 5mV of offset, but even upto 100mV is not catastrophic. Anything more and your amp would be overheating and the resistor may also burn out in the time you take to read this.

Check all your connections again. Note that the input potentiometer also serves as a ground reference, and if you omit you'll have approximately 15VDC on the output, which will burn the speaker coils.

I would advise against building a DC-coupled design like this as a first attempt, try and build it according to the National schema in the datasheet and experiment from there.

Arunkumar14_th · 28th January 2009, 05:44 AM

Thanks for the reply guys...

My Bridge is not a ready made one as shown in audio1st's post. Instead I built my own bridge with four 3A diodes.
I'll Post the pics of the same by tonight...
I did check the voltages before connecting them to the amp.
The voltages were in the range of 48 Volts DC out on the Bridge rails.
The voltage between ground & V+, ground & V- are the same & is around 24 V even without the amp connected or with the amp connected.
I know it's far less that the specs, but that shouldn't cause the speaker output to have large DC current.

I checked the output Voltage at the speaker terminals..... It shows me a DC voltage of 25V... That's really nasty... & is causing the voice coils to melt away

I think the Chip is blown up.
It's funny that it happened the same with two chips. Is there any way to verify that the IC is still in good condition?

About the source: The Source cannot have any voltage.. because I'm using a small MP3 player as the source.

peranders · 28th January 2009, 06:12 AM

Was your input hanging loose or was it tied to ground?

Good advice when you start an amp for the first time:

Check the supply voltage before you'll connect it to the amp

Short the input, measure the DC at the output. Observe or measure the current consumption.

then.... connect a speaker with a small(!) fuse.

then apply music or some test signal.

Are your LM3875 isolated from the heatsink?

sangram · 28th January 2009, 06:27 AM

I don't understand how you can have a DC voltage of 25V with a supply voltage of 24V - this is not a generator we're dealing with here. There's probably something we're missing.

Arunkumar14_th · 28th January 2009, 06:43 AM

sorry there sangram... My mistake

What I actually meant was that the voltage was around 25K NOT exactly 25.

Even the input voltages (AC Input 210-230V AC) were varying, as this is not a regulated power supply & it used to change......
Hover over an average of 24

I Took the measurements last night.... and i'm posting this from my memory.... from office....

sorry if i got something wrong.. My bad

25th January 2009, 02:10 AM	#1
Arunkumar14_th diyAudio Member Join Date: Feb 2006 Location: Bangalore	My GainClone is Blowing up Speakers I went ahead to do a GainClone Amplifier. I took the information from this site: http://diyaudioprojects.com/Chip/Syn...875-Gainclone/ MY Parts are exactly the same Differences being, Transformer is an EI core , 24 0 24, 5Amp one The Bridge was a standard bridge with Caps of value 1pF. The First Thing I noticed was that one of the diodes were overheating, But the output voltage was in range around 47V between the +V & -V nodes. Then When I connect the output to a speaker, the voice coil simply burns out. It seems like the output stage has a large DC current. Any Ideas if I'm doing anything wrong? please guide me in the right direction.

28th January 2009, 06:12 AM	#8
peranders Electrons are yellow and more is better! diyAudio Member Join Date: Apr 2002 Location: Göteborg, Sweden Blog Entries: 4	Was your input hanging loose or was it tied to ground? Good advice when you start an amp for the first time: Check the supply voltage before you'll connect it to the amp Short the input, measure the DC at the output. Observe or measure the current consumption. then.... connect a speaker with a small(!) fuse. then apply music or some test signal. Are your LM3875 isolated from the heatsink? __________________ /Per-Anders (my first name) or P-A as my friends call me

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25th January 2009, 10:19 AM	#4
BrianGT diyAudio Moderator Emeritus Join Date: Jan 2002 Location: near Atlanta, GA	Did you test the power supply before connecting the amplifier channels to it? I always test the power supply before hooking up the amplifier to it. Can you unconnect the amplifier channels from your power supply, and test it out. Have you measured the DC offset of the channels? Post up some pictures of what you have made. -- Brian

25th January 2009, 04:10 PM	#5
krips diyAudio Member Join Date: Sep 2008	audio1st, could you explain your picture? My understanding of the Synergy schematic was that the + side of one cap went to pin 1, the - side of the other cap goes to pin 4, and the other pins go to ground.

25th January 2009, 05:41 PM	#6
sangram diyAudio Member Join Date: Sep 2002 Location: India	Hi Arun, First off, disconnect and check the supply without the amp connected, your voltages should be all correct. As has been said, you should be getting around 35V on each rail or 70V between the rails. Be sure to not reverse the supplies - this can be catastrophic. Connect a dummy load to the amp. I use a wirewound 10hm, 50 watt resistor bought from the local market. Now turn on the supply, and measure the voltage across the resistor, it should not be more than 50mV. Builders here strive for less than 5mV of offset, but even upto 100mV is not catastrophic. Anything more and your amp would be overheating and the resistor may also burn out in the time you take to read this. Check all your connections again. Note that the input potentiometer also serves as a ground reference, and if you omit you'll have approximately 15VDC on the output, which will burn the speaker coils. I would advise against building a DC-coupled design like this as a first attempt, try and build it according to the National schema in the datasheet and experiment from there.

28th January 2009, 05:44 AM	#7
Arunkumar14_th diyAudio Member Join Date: Feb 2006 Location: Bangalore	Thanks for the reply guys... My Bridge is not a ready made one as shown in audio1st's post. Instead I built my own bridge with four 3A diodes. I'll Post the pics of the same by tonight... I did check the voltages before connecting them to the amp. The voltages were in the range of 48 Volts DC out on the Bridge rails. The voltage between ground & V+, ground & V- are the same & is around 24 V even without the amp connected or with the amp connected. I know it's far less that the specs, but that shouldn't cause the speaker output to have large DC current. I checked the output Voltage at the speaker terminals..... It shows me a DC voltage of 25V... That's really nasty... & is causing the voice coils to melt away I think the Chip is blown up. It's funny that it happened the same with two chips. Is there any way to verify that the IC is still in good condition? About the source: The Source cannot have any voltage.. because I'm using a small MP3 player as the source.

28th January 2009, 06:27 AM	#9
sangram diyAudio Member Join Date: Sep 2002 Location: India	I don't understand how you can have a DC voltage of 25V with a supply voltage of 24V - this is not a generator we're dealing with here. There's probably something we're missing.

28th January 2009, 06:43 AM	#10
Arunkumar14_th diyAudio Member Join Date: Feb 2006 Location: Bangalore	sorry there sangram... My mistake What I actually meant was that the voltage was around 25K NOT exactly 25. Even the input voltages (AC Input 210-230V AC) were varying, as this is not a regulated power supply & it used to change...... Hover over an average of 24 I Took the measurements last night.... and i'm posting this from my memory.... from office.... sorry if i got something wrong.. My bad

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