142-2000 Direct Injection

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a365
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142-2000 Direct Injection

Postby a365 » Mon Aug 14, 2017 8:48 pm

Hello Goldfinger:

426 posted the following info in another forum and I have a question


Ok here is what I got from Goldfinger:

If you want to drive the modulator directly, you will need a 50-100 watt home audio amp between the line level source and the radio as the modulator doesn't have enough gain for line level to work. I use a 10uF 50 volt tantalum, + towards the circuit board where C174 was removed, with 2 - 1Kohm 1 watt in parallel going to ground to load the audio amp.


Image


Note the blue electrolytic at the bottom of the picture? That is a 220uF 35 volt radial lead electrolytic going between PC ground and Chassis ground to allow the low frequencies a good ground return. I install this cap on either method of feed.

This is a 2000 but works with 142 also. the large cap is a .22uf 50v Teflon cap very hard to find but a Mylar will work also.
Thank you Goldfinger!!

Well since the Audio Amp is loaded by 500 ohms instead of 8 ohms, the watts drop way back. It's true it only takes 5-10 watts to drive the modulator, but my circuit is "voltage fed". If I would have loaded it with an 8 ohm 50 watt resistor it would be "current fed". Since it is voltage fed at a higher impedance, it takes more "volume" from the audio amp to drive it. Also using a 50-100 watt amp gives it some headroom without clipping. 4040 tried using a 35 watts per channel receiver for his amp and the clipping kept blowing the modulator. Prime is using an Art Audio 100 WPC pro audio amp on his and it works great. He just sets his brick wall limiter in Pro-tools at 115-120% mod on the Cobra and calls it good. The brick wall prevents the 100 watt amp from harming the radio in case there is a loud noise in the room.
The problem with using a smaller audio amp is having to use a 25-50 watt, 8 ohm load resistor. That is the advantage of using a voltage fed circuit.

A little more info again thanks to Goldfinger for this info!!!


My question is:

What happen when you use a 8 Ohm Resistor instead of the Teflon Cap for Current Fed ?
Do you still loading the input with the 2 1K Ohm Resistors in Parallel to Ground ?

This is the Resistor that I try to use along with a 30 Watts Audio Amp

Image


Thank you in advance for your opinion and great help

Regards
A365
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Re: 142-2000 Direct Injection

Postby Goldfinger » Wed Aug 16, 2017 3:55 pm

The 8 ohm resistor would replace the paralleled 1K, not the Teflon cap to be current fed. The Teflon cap in the picture is in parallel with the red Tantalum. I did that to lower the ESR of the pass cap for better transmit tone. (A trick carried over from esoteric audio and speaker crossover mods.)
I use the voltage fed version to keep it simple and the load resistor small. Nothing wrong with current fed if you use a resistor with high enough wattage to handle the power. Just make sure it goes to ground on the RCA end of the injection cap, not the base of TR42 end.
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a365
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Re: 142-2000 Direct Injection

Postby a365 » Wed Aug 23, 2017 6:18 pm

Hi Goldfinger:

First left me thank you for taking the time to reply to my questions,kudos to you for sharing your knowledge my hat of to you Sir :D

My understanding is just to substitute the 2 1 K ohms resistors with an 8 ohms high wattage resistor connected as shown in the above picture :?:

The teflon cap along with the tantalum remain the same and also the 220 uF to ground as shown :idea:

Therefore I could use an smaller amp to drive the modulator instead of the 50 to 100 watts amp :mrgreen:

Now let's see if the 2N3055 or Tip41C will not blow up at the first loud noise fed from my small rack gear and amp :roll:


Best Regards
A365
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Re: 142-2000 Direct Injection

Postby Goldfinger » Tue Aug 29, 2017 9:47 am

The C945 usually blows out first (when you get yelled at for not taking out the trash)... ;)

Use a brick-wall limiter in your chain and set the wall at your desired max modulation level. The first 2grand I ever made was for Prime and he learned quickly the first time someone came into his shack yelling. He started using a brick-wall and never had a problem again.
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a365
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Re: 142-2000 Direct Injection

Postby a365 » Tue Aug 29, 2017 1:33 pm

Hello Goldfinger:

Once again thank you for your input in my quest for better audio on my transceiver :D

I been looking at brick wall limiters hardware and they are quite expensive

I guess I will have to keep a good stash of C945 and 2N3055 transistors in the meantime :shock:


Best Regards
A365

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