Compressor board capable of NPC mod.

[Shockwave]

Everyone wants to sound louder and those who are wise would like to do so without objectionable distortion or splatter. Reducing the carrier on AM to add swing is not enough to accomplish this goal. That causes negative peak cutoff with distortion and splatter whenever the negative peaks hit 100%. There is a simple way to remove the distortion and maintain the huge positive peaks on AM. Connect this $40 compressor-limiter inline and modify it for NPC COMPRESSOR LIMITER FOR FM TRANSMITTER BROADCAST | eBay (JPL-995). I was amazed how well this device worked. Even being placed in front of many coupling caps in the audio chain it was still able to create asymmetrical modulation quite easily. The trick is those two little LED's on the board. One controls the limiting point for the positive peak and the other controls the limiting point for the negative peak. By experimenting with different LED's with various voltage drops, you can tailor the trigger points for the positive and negative peaks independently! Hint, infrared LED's drop much less voltage then standard ones and they can be wired two in series.

This is a Hi-Fi device intended for FM broadcast and you will want to remove the jumper on the board to disable the pre-emphasis otherwise you will have too much treble. There are many onboard controls to adjust things like attack and release time. In addition there are independent controls for both the low end bass and hi end treble compression levels. The limiter section is followed by several poles of audio filtering to remove any distortion created by the clipping. When the board is properly set, it is simple to achieve 200% positive peaks with no RF cutoff on the negative peaks. Its response is very smooth and only the trained ear will detect the compression effect because there are no obvious signs of the typical pumping and sucking sounds some compressors produce when they operate. The end result is approximately twice the modulation with virtually no objectionalable distortion from the transmitter. Not bad for $40 and a little install time. However, it does require line level drive so it will need to be installed after some preamplification.

 

[tecnicoloco]

Hello:

Just follow this link for The Whole Enchilada

Pira CZ Compressor/Limiter/Clipper for FM broadcasting

 

[Robb]

Optical diodes, that is interesting.
Just like the 'big kids' compressors.
Using a single operational amp IC only?

I'm no expert; but the quality of that IC is going to ultimately determine the noise level before/after compression.

Did you build one and try it out - loco?

 

[Shockwave]

Robb, I've been using this little board for about 2 months before I decided it was good enough to write about. It has the performance of something much more expensive and impressed me to say the least. I run a SM-58 XLR mic into a Shure Audio Master pre amp (very RF proof). That feeds the compressor / modified NPC board. Then it hits a 5 band graphic EQ to boost the bass, drop the mids and shape the highs so they roll off without splatter. Kind of like a "de-esser". All three boards fit inside the Shure Audio Master pre-amp. Even with all three boards I can't see the slightest bit of inside noise on the scope with no input applied to the pre-amp. I've witnessed worse results coming from an entire rack of expensive audio equipment and I don't have $100 invested into this box. Most of it came off my scrape pile and was reused for this application.

 

[RickC.]

Interesting, thanks!

I just got a Behringer Shark to use in my audio chain but have been too busy with work and other obligations to build a breakout box for my Icom yet. I could just wimp out and make up a cable, I know, but I want to be able to to switch between that and the USB AFSK interface that's in the works.

If I don't like the Shark, this is on my radar.

 

[Shockwave]

I'll share a little more on the NPC mod for this board. Most peoples voices are not symmetrical and you will want to take advantage of this fact. If you buy this board, you are likely to notice that one of the clipping LED's lights brighter and more frequently then the other on voice peaks. That will be the peak that is stronger with your voice and you will want this one as your stronger positive peak. It may require inverting the output of the compressor board if this peak happens to work out to be your negative peak. The $3 Radio Shack 1:1 isolation transformer works great to confirm the correct phase and invert it if required.

The weaker peak is the one you want as the negative peak. Meaning the LED that lights less frequently is the one you want to use the IR LED on to clamp this peak even lower so you can increase the positive peak more without distortion. Another alternative is to add a IR LED in series with the positive peak LED so it's clamped at a higher level. You will have to experiment to determine what works best with your voice and TX. I had best results with the last version explained. I added one IR LED in series with each red LED and connected a double throw center off switch to shunt out one or the other IR LED so I can switch between limiting either peak to test the results.

 

[ElectronTubesRule]

This was a popular Mod well over 5 years ago maybe more. In fact RCI had just come out with the 2950DX with all it's surface mount construction and all the other boards where still old school discrete compactness. It was mostly people that used Export Radios on CB and those that catered to them that where big on this mod. In fact you could still buy President Lincoln Radio's new from Copper and HR Electronics when this first gained lots of popularity!

In fact the forums that then Panther Electronics I think the owners name was John Mahoney and a lot of on that board back then where doing this mod in various ways on all of the RCI chassis's.

It would be cool to have a show down on the scope and on the air between the various speech compressors and NPC Mod's!

 

[Shockwave]

This was a popular Mod well over 5 years ago maybe more. In fact RCI had just come out with the 2950DX with all it's surface mount construction and all the other boards where still old school discrete compactness. It was mostly people that used Export Radios on CB and those that catered to them that where big on this mod. In fact you could still buy President Lincoln Radio's new from Copper and HR Electronics when this first gained lots of popularity!

In fact the forums that then Panther Electronics I think the owners name was John Mahoney and a lot of on that board back then where doing this mod in various ways on all of the RCI chassis's.

It would be cool to have a show down on the scope and on the air between the various speech compressors and NPC Mod's!

This board is significantly different then anything RCI or Ranger ever stuffed inside a CB. The old Ranger compressor boards and similar products were all designed around communications grade audio and were not easily modifiable to provide effective NPC. Many people who have developed an interest in NPC also have an interest in Hi-Fi. The communications grade compressors will destroy any hi fidelity sound since their filters kill all the high end.

This compressor is designed to pass broadcast quality bandwidth from 20 to 15,000 cycles. It also has more then one compression adjustment to cover this bandwidth. It so easily lends itself to modification for NPC with its two discrete LED's just asking to be swapped out to form asymmetrical modulation in AM applications. The design of this board is very cleaver to have accomplished this with a dual op amp, two transistors, two LED's and a handful of support components. Publishing the schematic was another plus.

 

[ElectronTubesRule]

I understand now ok........I some how missed the Hi-Fi part of this boards design. Thanks for restating that!