Setting ROHM BH1414K IC Circuit to 75us Pre-emphasis Curve

[MrBruce1959]

You are mixing up the capacitors. The pre-emphasis treble frequencies boost capacitor is 1000pF for 50us and is 1500pF for 75us. The audio input coupling capacitor determines which low frequencies are passed and is 1uF polarized for the datasheet and for the circuit you have but I suggested that 470nF (almost half of 1uF) would also pass all audio frequencies.

Oh you're right, sorry I did misunderstood you, you're referring to the electrolytic type 1uF cap.

Bruce.

 

[MrBruce1959]

This I can tell you I have a 6 band processor just ahead of this thing and the 6 different bands shows higher peaks in the mid bands, but the BASS is showing very little peak, but on the transmitted signal the BASS is very dominate, makes the music sound very muffled and very BASSY, I also have to have the LOW frequency potentiometer on the mixing board almost at -30dB, so either I have the 1500pF caps shorted, which I find hard to do on both inputs, or the caps are not 1500pF. I suppose I could just put the originals back in and see what happens.
I have tried using my digital VOM meter to read the caps but, it appears my meter can not read capacitor values. This circuit is too tight to add regular sized caps so that's out of the question unfortunately.

Bruce.

 

[audioguru]

If the 1500pF capacitors are shorted or are 1.5uF then the sound will have a boomy and loud peak at about 250Hz with dropoffs at lower and higher frequencies. Maybe that is your problem.

 

[naessens yves]

I audioguru
First post for me and sorry for my bad english
Dont forget the formula Xc = (1 / 2 π f c)
if you increase c you are abel to pass more lower frequency and not more high frequency because is a paralle of two resitor r and xc, xc that change with F
Best regards

Yves

 

[audioguru]

The audio input resistor is 51k ohms. If the capacitor in parallel with it is 1000pF then pre-emphasis is 51μs and if it is 1500pF then pre-emphasis is 76.5μs. If it is 75μs then the transmitted response will be +3dB at about 2133Hz and will rise at +6dB per octave. The lower frequencies are not affected by the pre-emphasis capacitor that simply boosts high audio frequencies. Here is a simulation of the boosted high frequencies: