ESP two way x over offset prob.

[tade]

I built this circuit from esp audio:
http://sound.westhost.com/project09.htm
I am using a +-12v psu, and imput buffer, crossover unit and output buffer.

It has run fine for a week but just now I noticed the cone of my subwoofer was pushed out. I took a look at the circuit and measured some voltages. the voltage with no input coming out of the input buffer is four and a half volts. this goes to the low and high crossover. the voltage after the high pass is .2 milivolts, and the voltage after the low pass is four and a half volts! the offset is faithfully maintained through the output buffer as well.

Should the input buffer be raising the voltage like it has? Why is the offset close to zero after the high pass but not the lowpass? Have I blown an opamp?

THanks

 

[tade]

the low pass offset is not reading 192mv, which is much smaller than four and a half volts, but a hundred times larger than the high pass...
Thanks

 

[tade]

bump

 

[AndrewT]

Hi,
this won't solve your offset problem but it will remove the symptoms.
Put a DC block into the input of your power amps. 1uF 100k or similar RC timeconstant.

 

[tade]

Good idea thanks. So the resistor in paralell with the amp and the cap in series?

I am gonna do this the right way; Bust out the multimeter, a schematic, and my calculator. i think it may be due to improperly matched resistors. Ill post again if this mystery doesnt get solved.
Many thanks!

 

[AndrewT]

Hi,
if my expnation is not clear then look up just about any poweramp schematic for the layout. You can mount the extra components either on the RCA terminals or on the PCB.

Yes cap in series and after cap resistor to ground:-
The centre pin RCA to 1uF cap, then cap to poweramp input.
100k from input to signal ground.

Looking at ESP again I notice that the low pass has DC passing all the way through to the bass output. The high pass has an effective DC block at each opamp preventing any DC getting to the treble unit.

Is it possible that the input opamp (buffer) has gone wrong and generating the DC offset? I can't tell from your description.

 

[richie00boy]

I'm with the faulty input buffer section.

I disagree with the 1uF cap and 100k resistor for the amp though. The amp input impedance will likely be about 20-50k as it is, so a 1uF cap is too small, and the 100k resistor will be doing sod all. I'd try 10uF on it's own just in series with the input of the amp. If you can find the schematic of your amp, that will help fine tune the values.

However, you really should look to correcting the real problem why there is such a lot of DC in the first place.

 

[AndrewT]

Hi Richie,
are you suggesting he fit an electro (bipolar) as his DC block?

 

[richie00boy]

Yes. If he puts his DC voltmeter on the output of the crossover red probe to output and black probe to ground and measures the polarity of the DC. If the voltage is positive he should connect the +ve side of the cap to the x-over and the -ve side to the amp. And vice versa.

 

[tade]

but wont the voltage still reach into the negative? It wont at idle, but with music it should.

Somethine is wrong with my input buffer then. Ill take some voltage readings and put them on the schematic and post that.

Thanks for the help. To me it seems like putting a circuit in to block dc is worse than not having it in the first place.

btw it is down to 80 mv

 

[richie00boy]

Yes at idle it will have DC but with music it will go 'negative' but this is not a problem because it still has the DC bias superimposed on the signal.

 

[tade]

but what if the musical signal actually makes it negative, the bias is not very large.

 

[AndrewT]

Hi,
a few manufacturers state that their polarised electros are not affected by +- signals upto 1500mV.

This allows upto 1Vrms at the input of the power amp before adding your DC bias.