Changing a deemphasis from 50 uS to 75 uS

[legarem]

I recently bought a Rogers Panthera tuner which has a deemphasis of 50uS. The deemphasis circuit is encapsulated in a module so I can’t reach the parts inside this module.

Can I build a RC circuit inserted in the output section so I can have a 75 uS deemphasis.

Do I have to add a 75uS deemphasis with R and C or a 25uS deemphasis because I already have a 50uS deemphasis in the tuner ?

Thanks a lot.

[ashok]

Your 50uS filter is a 1st order filter ( I assume). Adding another filter in series will increase the ultimate response to a 2nd order slope. The Q of the filter will be another problem.
To change to a 75uS filter you have to get the signal before the 50uS filter and possibly you need to remove the 50uS filter as it will also be loading the circuit.
Do you have a circuit diagram ?
Why would they encapsulate a 50uS filter ?
A simple 50uS filter is just a R and C combination.
L and C if it's 2nd order.

Are you sure what is inside the module ?
Cheers.

[legarem]

Originally posted by ashok
Your 50uS filter is a 1st order filter ( I assume). Adding another filter in series will increase the ultimate response to a 2nd order slope. The Q of the filter will be another problem.
To change to a 75uS filter you have to get the signal before the 50uS filter and possibly you need to remove the 50uS filter as it will also be loading the circuit.
Do you have a circuit diagram ?
Why would they encapsulate a 50uS filter ?
A simple 50uS filter is just a R and C combination.
L and C if it's 2nd order.

Are you sure what is inside the module ?
Cheers.

Unfortunatly, I can't get any schematic diagram for this tuner

I looked at the circuit after the MPX chip and there's this module with two output transistors.

Last night I opened the module and there's 4 chokes which can be adjusted (variable) and 6 little capacitors.

For me, this means a lot of trouble to change something. This appears more complicated than a simple R C filter.

[oshifis]

That encapsulated LC filter is the multiplex (MPX) filter, to avoid 38 kHz interference with the bias oscillator at tape recording. The deemphasis is usually a parallel RC complex at the stereo decoder chip. What is the type of the stereo decoder chip? The datasheet should show which pins are the deemphasis RC components connected to.

[legarem]

Originally posted by oshifis
That encapsulated LC filter is the multiplex (MPX) filter, to avoid 38 kHz interference with the bias oscillator at tape recording. The deemphasis is usually a parallel RC complex at the stereo decoder chip. What is the type of the stereo decoder chip? The datasheet should show which pins are the deemphasis RC components connected to.

As I will never have a tape recorder in my system should it be a great idea to get rid of this LC filter ?

Thanks

[eds65gto]

Hey:
To my lousy memory the only difference that you will hear is a lower level of high frequency response.
I think that if you leave the de emph. alone you will never know the difference except that you MAY have to reduce the treble a bit. Now if you are older like me the increased treble sounds good.
Hope this helps
Ed

[legarem]

I finally found the R and C which make the deemphasis

There's a 2.7K and a .022 uf cap for each channel as seen in the drawing.

In the National typical application sheet, they show a 3.9K and a .022 uF. I tried a 3.9K instead of the 2.7k and I think that the sound is perharps worse with more or as much treble than it was.

Can someone help me

Thanks

[hughmon]

Rather than change the resistor, which might effect bias points in the chip, why not just parallel 5.6 nF caps with the existing .022 uF caps? That should be pretty close to a 75 uS time constant.

[legarem]

I'll try 5.6nF

With the standard setting of .022 and 2.7k, I calculate a deemphasis of 59.39 uS. This is not near 50 uS I tought this tuner had.

I also tried .033 uF to replace both .022 uF and the multiplex didn't want to decode in stereo with .033 uF caps.

[oshifis]

Something is not correct here... The deemphasis pins not 1 and 2, but 4 and 5. The capacitor value of 33 nF should be OK for 75 us, since there is an internal resistor shunting the external 2.7 k.