Audio processor

[W9BHI]

I have been offered a Symetrix 421M.
Is this a good unit for microphone audio processing on AM?
I would like to try and use it on a T-368.
Thanks,
Don W9BHI

[w4bfs]

hi Don .... I cant speak to the 421 but can speak to the 528e (literally) .... It is an effective unit and recommended

[WA3VJB]

The "M" has a mic pre-amp in it  (basic 421 did not), which means one less box to get to the level the T-3 would want to see, and presuming you want to get around (not through) the stock speech amp.

There is a a little bit of equalization available, brute force, fixed-frequency low and high cut options.

If it's being "offered" to you inexpensively it's worth having and trying out. If it doesn't satisfy you could pass it along for $150 or so, the market seems to show.

I used an audio power transformer to take 600ohm line level up to the impedance of the 4-125As, and fed external audio to a chassis mount connector in the hole on the front panel where a BNC used to be for external excitation.

As with any gear not intended for broadcast or other high-RF environments, you'll need to take a little extra care not to get any RF into the 421 or connecting lines.

[W3GMS]

Hi Don,

I like all the gear Symetrix makes.  I also have a 421 that I plan to use on my repeater for mild level compensation. 

For my home station, I use the Symetrix 528E which I think is a fantastic box. Remember, the audio chain is only as good as the weakest link so all of these boxes will do better if going into a low distortion audio chain that is capable of cleanly modulating 100%.  The 528E has a parametric eq which allows you to move the boost or cut frequency any where you like. 

I would suggest a good EQ to add to the 421 especially if your running a flat response dynamic mike.  Some controlled mid range presence adds a lot to the quality of the signal.

Joe, GMS     

[W9BHI]

I decided to pass on the 421 and picked up a used 528E.
It arrived yesterday.
It has a few scratches but a quick connection to a scope and osc showed it was working ok.
I am waiting for the amp to show up and then I can start playing.

Don W9BHI

[AB2EZ]

Reminder:

Since you are planning to use a modern solid state audio amplifier (a Symetrix 420), and since this amplifier has an output power capability that is far in excess of what is needed in this application (the amplifier is specified as capable of delivering 20 watts per channel into a pair of 4 ohm loads or 40 watts into an 8 ohm load in bridged-mono mode)... it does not matter if the load impedance that the amplifier is looking into is higher (even significantly higher) than the impedance that is specified as the "minimum" load impedance.

From the Symetrix 420 manual:

"For solid-state amplifiers, operating without any load connected or with a load whose impedance is
higher than the amplifier’s rated minimum is harmless. Of course, there is a price, and that price is
diminished power output. As the load impedance doubles, output power halves (this is a best-case
scenario and depends on actual circuit design). Thus, an amplifier having a 50-watt, 8-ohm, power
rating would only deliver 25 watts into a 16-ohm load."

With the 12k ohm 5 watt resistor across the output of the step-up transformer (as discussed in your other post: http://amfone.net/Amforum/index.php?topic=34478.0), and with 192V (peak) across that resistor... the peak power that the amplifier will have to deliver is 192V x 192V / 12000 ohms = 3 watts.

If you use a step up transformer with a turns ratio of 230:12 (or 115:6, depending upon whether you put the windings in series or parallel) = 19.2, the impedance that the amplifier will see is 12000 ohms / [19.2 x 19.2] = 32.7 ohms... which is perfectly fine for this application. With a 40 watt peak power rating in bridged-mono mode, into an 8 ohm load, this amplifier will deliver up to 10 watts peak power into a 32 ohm load. That's 3.3 x the power you will need to drive the grids of your modulator tubes (essentially all of that power going into the 12000 ohm  resistor).

If you use a transformer with a turns ratio of 230:6 = 38.3, the impedance seen by the amplifier will be 12000 ohms / (38.3 x 38.3) = 8.2 ohms. That will also be fine.

In either case: be careful to start with the amplifier's volume control set low so that you don't apply too large a peak voltage to the grids of the modulator tubes. In bridged mono mode, the peak output voltage of the amplifier (regardless of the value of the load), with the amplifier's volume set at maximum, will be: the square root of [40 watts x 8 ohms] = 17.9 volts. With a 19.2:1 step up transformer (i.e. a 230:12 turns ratio) the peak output voltage across the secondary of the transformer (i.e. grid-to-grid of the modulator tubes), with the amplifier's volume set at maximum, will be 17.9V x 19.2 = 343 volts.

Stu

[W9BHI]

I will be using just one channel to experiment with to see how much gain I really need to drive the mod deck.
I might just build a driver amp with an LM-1875 or a TDA2050.
They are rated at 25 watts into 8 ohms @.025% THD.
Running it at a lower voltage should reduce the output to where it needs to be.
I will let you know what happens when the Symetrix amp arrives.

Don W9BHI

 

[W3GMS]

I decided to pass on the 421 and picked up a used 528E.
It arrived yesterday.
It has a few scratches but a quick connection to a scope and osc showed it was working ok.
I am waiting for the amp to show up and then I can start playing.

Don W9BHI

Don,

You will enjoy the 528E and should you ever need parts for it, Symetrix's is a very helpful company to deal with.  I needed a new Mic/Line switch for mine and they shipped it right out and the price was very reasonable.  I also bought mine new 10 or so years ago. 

Joe, W3GMS