Methods to reduce the audio output - Page 1
Forum section: electronics / beginners

author: gabric098
author_rank: Newbie
timestamp: February 23, 2017, 10:30:24 am
content: Hi everyone,this is my very first post on this forum, I'm a total newbie so, forgive my silly questions.I'm currently working on a morse code project. I have a paddle morse key which plugs into a keyer circuit (http://www.hamgadgets.com/ULTRA-PICOKEYER) which generates the CW audio tone (around 600Hz). This tone however is full of harmonics and needs to go through another small circuit (it's an audio filter,http://www.4sqrp.com/HiPerMite.php) which will give filter out the harmonics and make the tone pleasant.Now, the issue is that the audio volume coming out from the the keyer circuit is a bit too much for the audio filter, so I'd need to reduce it in some way.A friend suggested to use a step-down audio transformer, he suggested 1000:8 ohm. Are there any other ways to achieve the volume reduction? What are the advantages of using an audio transformer?ThanksGab

author: danadak
author_rank: Super Contributor
timestamp: February 23, 2017, 11:35:06 am
content: W/o keyer schematic a bit of a guess, try a 1K pot at output ofof keyer. That should not shift appreciably the filter characteristics.However a 1 K pot may be too much of a load for keyer, if that's thecase raise it, experiment. Try 5k.Regards, Dana.

author: gabric098
author_rank: Newbie
timestamp: February 23, 2017, 02:09:14 pm
content: Hi Dana,thanks for your reply.I've attached the schematic of the keyer, I'm using the "phone" outputUsing a variable resistor is what I was thinking about too... What's the difference (pro/cons) of using an audio transformer instead?ThanksGab

author: Codebird
author_rank: Regular Contributor
timestamp: February 23, 2017, 04:22:52 pm
content: I really can't work out how that thing works... what is going on with that output jack? It has only two lines going to it, and neither of them can be anything except high-impedance or ground?This is a somewhat strange circuit.

author: ebastler
author_rank: Super Contributor
timestamp: February 23, 2017, 04:31:12 pm
content: Quote from: Codebird on February 23, 2017, 04:22:52 pmProbably the output is meant to key a transmitter, which expects a classical Morse key (i.e. a momentary switch which connects the two contacts). Still this looks strange, and I am not sure why the ground connection?

author: Seekonk
author_rank: Super Contributor
timestamp: February 23, 2017, 05:03:40 pm
content: Put a resistor in series with C3. Never seen a blocking capacitor in the ground side. Try a fixed resistor of 100, 220, or 330 ohms or a 500 ohm pot.

author: gabric098
author_rank: Newbie
timestamp: February 23, 2017, 09:26:15 pm
content: Quote from: ebastler on February 23, 2017, 04:31:12 pmYes, I can confirm that. output is just a way to key a transmitter. However I'm using phone output to get the sidetone audio.Thanks.

author: gabric098
author_rank: Newbie
timestamp: February 23, 2017, 09:28:02 pm
content: Quote from: Seekonk on February 23, 2017, 05:03:40 pmI will give a try. Thanks everybody for the suggestions!Gab

author: orolo
author_rank: Frequent Contributor
timestamp: February 24, 2017, 02:04:07 pm
content: If I understood the situation, the keyer outputs a 3V square wave that goes into the filter. Trying to understand how much do you need to attenuate the signal to accomodate it to the filter, I looked atthe filter schematic. At first sight it's a typical, single supply, three stage low pass Sallen-Key, then one high-pass stage, followed by an LM386 power amplifier. But there is something I don't understand about the schematic, the bias after the capacitor coupled stages (the first and last). Do these stages have the common mode set at the upper rail? There are two components, a resistor and a zener, that seemed to set the common mode at 3.9V, but they were removed. Why not use a couple of resistors?According to the datasheet, the TLC274 the common mode input voltage range doesn't reach the top rail. Is this schematic right?