Why can"t I get this circuit to work???

Minion · 14th December 2007, 04:55 PM

Hi Folks, Yesterday I quickly did up a basic Guitar amp design useing a LM3886 and a Dual op amp which I thought was pretty basic but for the life of me I can"t get it to work.....

The basic idea was I would use the Input opamp to add gain in an inverting config with some Clipping diodes in the feedback loop to add an Overdrive sound , The second opamp would be used as an active inverting One knob tone controll , and after this it would go into a Non inverting LM3886 in the Basic datasheet implementation.....

The PCB I did has the PSU on the same board ,It puts out an unregulated +/-26v for the LM3886 and a regulated +/-15v for the TL072 opamp useing LM7815/7915 regulators with 10,000uF per rail and 2200uF on each rail close to the LM3886 and 10uF per rail for the tlo72 close to the IC.....

I have tested the PSU part and it puts out the correct Voltages and I am getting the correct Voltages at the pins of the IC"s , I am not getting anything at all at the output ,Not even DC and the LM3886 isn"t getting hot at all but I know it is getting the correct voltages at the power pins.....

I have attached the schematic and the PCB design and I weas wondering if someone could tah=ke a look at it and see if they can see any Glareing errors that i might have missed that would cause the amp to put out nothing....

I"m hopeing I just made a stupid mistake and another few sets of eyes might help me find the problem....

Thanx a Lot!!!

markiemrboo · 14th December 2007, 04:58 PM

The LM3886 needs a resistor (~10k should be OK) from pin 8 to the negative rail to unmute it.

Minion · 14th December 2007, 05:27 PM

Thanx a Lot....You solved my problem(s)...i knew it had to be something simple...

I just soldered a 10K resistor to the bottom of the PCB between the Mute and -V pin and I am getting a pretty good sound...

I think this will also solve a problem I am haveing with another LM3886 amp I built but this time a 100w Br amp....

Anyone can feel free to Use this design if they want, I just played with the amp for a few minutes and I has a Really nice sound and no Hum, just remember to add the Un-Mute resistor.....

Thanx again....

markiemrboo · 14th December 2007, 05:30 PM

No problem. Glad you got it working and it sounds good!

peranders · 14th December 2007, 05:56 PM

Pin6 U2, has no DC path from the input!

teemuk · 14th December 2007, 09:57 PM

The output stage’s input should have a ground reference to prevent floating of the DC level. I guess a Zobel network in the output would do no harm either.

Is guitar going to connect directly to P1? If yes, this is a serious impedance mismatch.

The PCB traces carrying rail and ground return currents are awfully thin.

kikikaka · 14th December 2007, 10:22 PM

this circuit is bleeding from many wounds...

1. P1: 5k gives very low input impedance, and this potentiometer is put there with wrong pinout (the source is shorted at zero volume level)

2. U1: the input impedance of this stage is approximately 10k, which is low...

3: P2: wrong pinout... (see (1))

4: U2: pin 6 should have been connected to ground by a resistor, because of two reasons:

a, this one-band eq isn't working properly
b, bias problems...

5: LM3886: pin 10, so the non-inverting input is floating, this aplifier has serious DC-offset issues, if it's working at all

regards

kikikaka · 14th December 2007, 10:41 PM

6: lack of capacitors (~100nF) at voltage regulators, and LM3886. danger of high frequency oscillation...

7: lack of zobel-network and inductur at the output gives unstable operation

8: grounding on the pcb...

Minion · 16th December 2007, 06:55 PM

LM3886: pin 10, so the non-inverting input is floating, this aplifier has serious DC-offset issues, if it's working at all

Maybe there are problems But I don"t notice them...I got about 10mv of Dc offset at the output and amp sounds great and goes really damn Loud...

1. P1: 5k gives very low input impedance, and this potentiometer is put there with wrong pinout (the source is shorted at zero volume level)

Don"t opamps work better with a Low input impedance, Like in Mic preamps that deal with 200 Ohm source impedances??

3: P2: wrong pinout... (see (1))

and I fixed the pot, I allways work with Trial and error with pots...

and I usually put a Pedal in front of it...

a, this one-band eq isn't working properly

Seems to be working for me....

The output stage’s input should have a ground reference to prevent floating of the DC level...

How do I fix that???

And there is an Inductor at the output....
Thanx

Minion · 16th December 2007, 08:12 PM

Can I fix the Dc and Bias problems by adding a resistor to ground after the input caps?? Is that what is meant by a DC return path??

Thanx

14th December 2007, 04:55 PM	#1
Minion diyAudio Member Join Date: May 2006	Why can"t I get this circuit to work??? Hi Folks, Yesterday I quickly did up a basic Guitar amp design useing a LM3886 and a Dual op amp which I thought was pretty basic but for the life of me I can"t get it to work..... The basic idea was I would use the Input opamp to add gain in an inverting config with some Clipping diodes in the feedback loop to add an Overdrive sound , The second opamp would be used as an active inverting One knob tone controll , and after this it would go into a Non inverting LM3886 in the Basic datasheet implementation..... The PCB I did has the PSU on the same board ,It puts out an unregulated +/-26v for the LM3886 and a regulated +/-15v for the TL072 opamp useing LM7815/7915 regulators with 10,000uF per rail and 2200uF on each rail close to the LM3886 and 10uF per rail for the tlo72 close to the IC..... I have tested the PSU part and it puts out the correct Voltages and I am getting the correct Voltages at the pins of the IC"s , I am not getting anything at all at the output ,Not even DC and the LM3886 isn"t getting hot at all but I know it is getting the correct voltages at the power pins..... I have attached the schematic and the PCB design and I weas wondering if someone could tah=ke a look at it and see if they can see any Glareing errors that i might have missed that would cause the amp to put out nothing.... I"m hopeing I just made a stupid mistake and another few sets of eyes might help me find the problem.... Thanx a Lot!!!

14th December 2007, 05:56 PM	#5
peranders Electrons are yellow and more is better! diyAudio Member Join Date: Apr 2002 Location: Göteborg, Sweden Blog Entries: 4	Pin6 U2, has no DC path from the input! __________________ /Per-Anders (my first name) or P-A as my friends call me

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14th December 2007, 04:58 PM	#2
markiemrboo diyAudio Member Join Date: Mar 2006 Location: Great Yarmouth, UK	The LM3886 needs a resistor (~10k should be OK) from pin 8 to the negative rail to unmute it.

14th December 2007, 05:27 PM	#3
Minion diyAudio Member Join Date: May 2006	Thanx a Lot....You solved my problem(s)...i knew it had to be something simple... I just soldered a 10K resistor to the bottom of the PCB between the Mute and -V pin and I am getting a pretty good sound... I think this will also solve a problem I am haveing with another LM3886 amp I built but this time a 100w Br amp.... Anyone can feel free to Use this design if they want, I just played with the amp for a few minutes and I has a Really nice sound and no Hum, just remember to add the Un-Mute resistor..... Thanx again....

14th December 2007, 05:30 PM	#4
markiemrboo diyAudio Member Join Date: Mar 2006 Location: Great Yarmouth, UK	No problem. Glad you got it working and it sounds good!

14th December 2007, 09:57 PM	#6
teemuk diyAudio Member Join Date: Nov 2004 Location: Suomi, Finland	The output stage’s input should have a ground reference to prevent floating of the DC level. I guess a Zobel network in the output would do no harm either. Is guitar going to connect directly to P1? If yes, this is a serious impedance mismatch. The PCB traces carrying rail and ground return currents are awfully thin.

14th December 2007, 10:22 PM	#7
kikikaka diyAudio Member Join Date: Mar 2006 Location: Budapest	this circuit is bleeding from many wounds... 1. P1: 5k gives very low input impedance, and this potentiometer is put there with wrong pinout (the source is shorted at zero volume level) 2. U1: the input impedance of this stage is approximately 10k, which is low... 3: P2: wrong pinout... (see (1)) 4: U2: pin 6 should have been connected to ground by a resistor, because of two reasons: a, this one-band eq isn't working properly b, bias problems... 5: LM3886: pin 10, so the non-inverting input is floating, this aplifier has serious DC-offset issues, if it's working at all regards

14th December 2007, 10:41 PM	#8
kikikaka diyAudio Member Join Date: Mar 2006 Location: Budapest	6: lack of capacitors (~100nF) at voltage regulators, and LM3886. danger of high frequency oscillation... 7: lack of zobel-network and inductur at the output gives unstable operation 8: grounding on the pcb...

16th December 2007, 06:55 PM	#9
Minion diyAudio Member Join Date: May 2006	LM3886: pin 10, so the non-inverting input is floating, this aplifier has serious DC-offset issues, if it's working at all Maybe there are problems But I don"t notice them...I got about 10mv of Dc offset at the output and amp sounds great and goes really damn Loud... 1. P1: 5k gives very low input impedance, and this potentiometer is put there with wrong pinout (the source is shorted at zero volume level) Don"t opamps work better with a Low input impedance, Like in Mic preamps that deal with 200 Ohm source impedances?? 3: P2: wrong pinout... (see (1)) and I fixed the pot, I allways work with Trial and error with pots... and I usually put a Pedal in front of it... a, this one-band eq isn't working properly Seems to be working for me.... The output stage’s input should have a ground reference to prevent floating of the DC level... How do I fix that??? And there is an Inductor at the output.... Thanx

16th December 2007, 08:12 PM	#10
Minion diyAudio Member Join Date: May 2006	Can I fix the Dc and Bias problems by adding a resistor to ground after the input caps?? Is that what is meant by a DC return path?? Thanx

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