TRC449 LC audio filter

[brandon7861]

I got distracted while looking at the vintage realistic thread. I saw an audio LC filter. Most often I see RC filters for audio, so I thought it would be fun to try putting that into circuitlab. I combined the model with the service manual image of filter response. Thats all, just thought it might interest someone. Sweep is from 100Hz to 10kHz

 

[brandon7861]

I tinkered with the capacitor values shooting for 4.7kHz at -6dB. Not to try, I don't even have a 449, just bored at work.

 

[loosecannon]

hey Brandon,

first, very cool!

i need to get circuit lab!

can you try using a 400Hz tone instead of a 1K tone and show those results?

the reason i ask is because of something i've always noticed about this chassis when doing the alignment.

the PEP output of the radio seems quite a bit higher when saying "ahhh" into the mic as opposed to using a 1K tone, and i always attributed this to that circuit.

many people might not notice this because so many of these radios had that section bypassed. it was a VERY popular mod back in the day.

but if you have one that is stock, i have found that using a lower tone frequency that resembles the male voice will provide a higher output.

anyone else notice this?
LC

 

[brandon7861]

Where it says 1kHz, thats just there for an initial generator value. When I did the frequency sweep, it ignores what that generator is set to and sweeps the selected range. That chart sweeps 100Hz to 10kHz, so it includes a datapoint at 400Hz.

I made a minor change and re-ran the circuit, I added the proper part labels and added C60 and R100 to better match the low end response and a load of 10k to represent the input at TR16. Not much changed besides the low frequencies dropping off a little faster.

I can understand aahhhhh showing more power since it is harmonic rich, but I cannot explain a 400Hz sine wave producing more power than a 1kHz sine wave. It would be fun to run a sweeping tone into the radio and follow it to see where 1kHz drops below 400Hz. Could the mic response be to blame?

 

[brandon7861]

@loosecannon As for getting circuit lab, its just a website. There is nothing to download, you just go to circuitlab.com and make an account. It will let you play around with it for free, save a couple circuits, and run a limited number of sims.

I think I paid 70 something for a year subscription. It comes in handy when you have a kirchhoff-type situation with diode junctions and you are not feeling mathematical.

 

[loosecannon]

thanks Brandon,

not sure how i misunderstood that you swept the freq range LOL!

its my friday and its been a hell of a week, yeah lets go with that.

i'll be sure to check out their site. thank you.
LC