BrianT LM4780 DC offset

[SEIJI]

Hi all,

I had just completed 6 BrianT LM4780 KITS set, I did a test Bench Test for all the 6 commpleted module with individual large heatsink by connecting a toroidal transformer with 2 secondary 0-18 VAC (5A) to the module each at a time, the DC offset for the 6 module read 150mVDC +/- 10mVDC with the signal cable open, with the signal cable shorted to ground, all 6 module DC offset read at 15mVDC +/- 1mVDC........

When connected a high efficientcy speaker driver (100db/w/m) to each individual module, they is no audioble noise at all...........and when I touch the signal cable, they is some noise on the speaker driver which show the amp are able to amplifier signal.

Is the above mention DC offset symptom normal????????

Any input from you would be much appreciated..........

Many Thanks ...............

 

[maxw]

150mVDC +/- 10mVDC with the signal cable open

I am pretty sure thats normal and you should not measure the offset with with open signal anyway.

with the signal cable shorted to ground, all 6 module DC offset read at 15mVDC +/- 1mVDC........
Is the above mention DC offset symptom normal????????

15mV is pretty good!! alot of people have upto 75mV and are not concerned at all
If it bothers you, you can add Ci caps whioh is mentioned here along with plenty of other info about DC-offset:
http://www.diyaudio.com/forums/showthread.php?s=&threadid=41133

 

[peranders]

Your values are normal. Worst case if you read the datasheet is 300 mV (10 mV x gain). The changes when your have different DC resistance's, this is also normal becasue you have input bias currents which are some.

 

[Russ White]

Your offset sounds close to what I get on my LM4780s which do not have input caps or the "Ci" capacitors as per the datasheet. If you add "Ci" which should be at least 47uf you will get 0 offset and I did not notic any ill effects. In fact my amps ran cooler after the mod. I think it helped tame some oscillation.

 

[SEIJI]

Thank you so much for all the kind reply, the chasis will be fabricate in 5 days time to accomodate the 6 module, is the CI cap need's to be place right at the input of the PCB board or it can be place at the input connector (in my case the neutrik XLR jack) which is about 6-8 inch away form the PCB board?

As the LM4780 will be connected to the output of electronic crossover which already has a output 22uF cap, do I still need the CI cap in this case?

Thanks agian for your kind reply...........

 

[Russ White]

The input cap and the "Ci" caps are two different caps. The input cap can be a lower value like 2.2uf or 4.7uf Ci is the capacitor that goes between the negative feedback resistor to ground and ground. Look at the schematic for the parallel LM4780 on national's site as it will be more clear than I am being.

As for the input caps I put them right on the input jacks, but it is not important. You could also mount them to the PCB, or even inline.

 

[Russ White]

If your crossover has output caps you do not need input caps. But if you plan to use the amp with other sources it is a good idea to use them.

 

[SpittinLLama]

As noted in the datasheet, the Ci cap is there so that the DC gain is 1. Without the cap your DC gain is equal to your AC gain so if you have DC present on the input it will be gained up at the output. The input capacitor is there to block any DC from a source. It is not needed if your source is already about GND. The purpose of both of these caps is to keep DC out and from being gained up. That's all they are there for.

-SL

 

[SEIJI]

Thank you so much for all the kind reply, I will be building the module up with a chasis soon, and see whats the result of DC offset after conneted to the electronic crossover without the input and Ci cap and I will post some picture here........

Thanks agian.........