Hello all I have an FM transmitter which uses the BH1414K IC chip. The data sheet for the BK1414K chip is here pira.cz/pdf/bh1414k.pdf
I was informed by the company that sold me the FM transmitter that the pre-emphasis defaults at 50 µs and that is how my transmitter is configured at 50 µs. I am in the USA and the pre-emphasis is not set by simple programming, instead, it is done by a resistor/capacitor configuration on the main board
I am including two photographs, one is an image that was sent to me on how the pre-emphasis is set and the other is an actual image showing the BH1414K chip and the associated resistor/capacitors on the PC board.
The seller says two capacitors have to be changed which is C3 and C4 to set the pre-emphasis to 75 µs. A few have told me there has to be more to it than that such as changing out both a capacitors and resistors at the same time. I wish to know if this is true, that both a resistor and capacitor have to be changed or if the seller is correct about only needing to change C3 and C4 to a 1500pF capacitor.
First off does anyone know anything about the BH1414K IC chip? Second, I am not able to get an actual schematic diagram as I was told that can not be given out to the public.
Please check the photos and give me as much help as you can.
Photo below was sent to me by transmitter seller who said C3 and C4 should be replaced with a 1500pF capacitor.
(click image below to blow up and scroll) Left side is full board view, right insert is zoomed in view of chip area with part numbers.
Below is the actual chip in question on the main circuit board of my transmitter. Below the BH1414K chip are the capacitors that were mentioned in the map above.
(Click image below to blow up to larger size)
This chip above, is normally hidden under a large shielding plate.
I have a thread going on at another forum board, since this topic went un-noticed.
The responses there, say to replace those two caps with 1500pF ones.
I replaced mine with 1500pF ones, since I really can not identify what value the original capacitors were for lack of any component markings on the caps.
The result has me confused though, because the frequency response is not what I expected and I am still hashing it out over there asking questions.
Before changing those caps, I felt the BASS was a little bit out of control, now since I changed those two caps, I feel the BASS is even more dominant than before and the MIDS are now more hollow sounding and hard to get sounding clear.
I was told 75us pre-emphasis is supposed to raise the MIDs and 50us is supposed to raise the LOWs, I'm not hearing this effect on the receiver end. I am hearing the exact opposite, making me think if I wasn't 50us before, I am now.
According to the datasheet for the BH1414K the example circuit shows the pre-emphasis using a 51k resistor in parallel with a 1000p capacitor, with a 1uf coupling cap on the input and a 10k resistor right before the chips L/R input for impedance padding. I wouldn't even touch the 1000p capacitor and instead focus on the 51k resistor. Change it to a higher value of 76k...
Changing that capacitors value of 1000p to 1500p is only going to let more low frequency audio content through and lower the curve point (as you noticed). Increasing the resistors value will decrease low frequency pass and overall set the curve start point closer to the 75us standard. What the exact resistor value should be I can only guess at but given most kits tell the user to switch from 10k to 15k, 10 is 66% of 15, and going from 51k to 76k comes real close to that same 66% figure. I know the similar BA1404 IC uses a pre-emphasis circuit which is almost identical and most kits that use that chip tell the owner to change that resistors value, not the capacitor.
Personally if it were my transmitter I would leave it alone except completely remove that 1000p capacitor for both L and R channels and do pre-emphasis using external audio processing like professional equipment is designed to do, and for good reason. When you can do all of this with broadcast audio processing software (or hardware if you have it) there is no good reason to do it within the transmitter, especially considering the issues it causes down the road when you do want to use good external audio processing.
I replaced that set of capacitors because that is what I was told to do by the seller and those at another forum board.
I do have 3 pieces of processing hardware. I prefer hardware, because that works best when my turntables are used for live broadcasts.
I have some issues to address here, but it is late at night and do not wish to start discussing them now, please watch this topic as I plain on publishing new issues and concerns.
I sent just the main board to the seller of my CZE-501. I had them replace the capacitors because I made a mess of the PC board trying to remove the original surface mount caps.
My transmitter was set for 50 µs, perhaps that mistake was made because the transmitter was shipped while their tech was on holiday.
I sent only the main board minus the rest of the transmitter to save on shipping costs, to which the cabinet and heat sink weigh enough to cost a heaping $30.00 shipping fee to China. Removing and shipping just the board, cost me around $13.00.
Sub, I am sorry I missed your post. Let me recheck my images and see if I can't blow one up for you so you have a clearer picture on my transmitter's main board near the BH1414K IC chip.
Okay I should be ashamed of myself for posting this picture, but this is the mess I created when I tried to solder in two 1500pf surface mount caps. As You can see I made a real mess of things. (Click Images below for a larger view)
This is a photo the seller sent me showing the repair they did and the Pre-emphasis now set to 75 µs.
Now here is a closer look at the BH1414K where the Pre-Emphasis is set by the seller to 75 µs.
As you can see, the seller cleaned up the mess I made and now things look good. To tell you the truth, I was really worried I screwed this thing up for good. I now realize, my eye sight is too poor to work on surface mount devices like this and my hands are too shaky as well.
Sub I will recheck things to see if I can tell you more about those capacitors you asked me about.
My CZE-T501 is now operating at 75 µs Pre-emphasis.