Audio feedback in modulator

[ka1tdq]

I'm using a modified Heising circuit to modulate my RF deck and I can hear talk-back in the audio output transformer when I modulate.  I'm pretty sure that's what's causing the audio feedback coming back to my audio amplifier, which I'm using to drive the output transformer. 

I'm just taking a stab at this but would adding a 450uF electrolytic in parallel with the secondary of the audio output transformer reduce or eliminate the feedback?  Please see the attached schematic. 

Thanks!

Jon
KA1TDQ

[W2XR]

Jon,

Could you clarify why you think this would reduce the audio talk-back?

Putting such a large value of C across the output of the mod xfmr would effectively shunt all of the audio to ground; clearly, this is something you do not want to do!

A better solution would be to acoustically isolate the talking component(s) from your microphone. Or if you are using EQ, you may be boosting the energy content excessively around the point where the talking component is mechanically resonant. Reducing the boost at that point will reduce the magnitude of the talk-back.

This should be fairly easy to remedy.

73,

Bruce

[W2XR]

Jon,

Could you clarify why you think this would reduce the audio talk-back?

Putting such a large value of C across the output of the mod xfmr would effectively shunt all of the audio to ground; clearly, this is something you do not want to do!

A better solution would be to acoustically isolate the talking component(s) from your microphone. Or if you are using EQ, you may be boosting the energy content excessively around the point where the talking component is mechanically resonant. Reducing the boost at that point will reduce the magnitude of the talk-back.

This should be fairly easy to remedy.

73,

Bruce

[ka1tdq]

Yeah, shunting all the audio would be a bad thing... :-)

Let me just describe the symptoms I'm having and maybe someone could better define the actual problem:

I'm having difficulty modulating over, say, 20%.  Everytime I turn up the volume on my audio amp I'm getting this frying, zorching sound coming from the audio amp and all the lamps dim... bad stuff.  I will then turn the volume down to when all that stops but the person trying to copy me says my audio is p*ss weak. 

At first, I thought it was RF and put filter chokes on both ends and both leads of the audio cable going to and from the audio amp... didn't help. 

I do have the microphone and audio amp in another room from the transmitter so acoustics aren't the problem.

[KL7OF]

The mic and audio amp are in another room?     How much cable do you have going to the transmitter...???What kind of cable are you using??   Could be an issue of too much loss in the cable or a shielding issue with the cable...Good Luck

[ka1tdq]

I'm using about 30 feet of 14 guage shielded cable from the audio amp to the modulator.  It has RF chokes on both ends so I don't think RF is the issue, and the shield is grounded on both ends.

[k4kyv]

Using a Heising reactor shouldn't have anything to  do with unless its insulation is breaking down on voice peaks.  A zorching sound while the lights dim sound like something is seriously shorting out big time.  Hope it's not the modulation transformer or the reactor.

[ka1tdq]

I'm using a 280 watt Hammond audio output transformer as my modulator (plenty scroteful enough and I couldn't imagine that would short out) to drive a 30 watt RF deck.  I shouldn't need to crank the audio up that high in the first place. 

I'm going to experiment with it a little more tonight and use an IFR communications monitor to see exactly what's going on, and be able to hear myself.  It's easier to troubleshoot that way rather than going on the air with a bad signal. 

[ke7trp]

The load presented to the Audio amp is not in fact 8 ohms?

Clark

[WBear2GCR]

Yeah, shunting all the audio would be a bad thing... :-)

Let me just describe the symptoms I'm having and maybe someone could better define the actual problem:

I'm having difficulty modulating over, say, 20%.  Everytime I turn up the volume on my audio amp I'm getting this frying, zorching sound coming from the audio amp and all the lamps dim... bad stuff.  I will then turn the volume down to when all that stops but the person trying to copy me says my audio is p*ss weak. 

At first, I thought it was RF and put filter chokes on both ends and both leads of the audio cable going to and from the audio amp... didn't help. 

I do have the microphone and audio amp in another room from the transmitter so acoustics aren't the problem.

What I would do is to separate the issues of RF from audio by placing a "dummy load" of suitable ohm-age as the secondary load in place of the RF tube's plate Z... then try to drive the pure resistance. IF the amp does not belch, one can then start to look at if something is breaking down with the B+ present, OR if RF is getting into the front end of the power amp.

You can likely get the RF out of the front end of the power amp with a few well placed ceramic caps, even if they are not optimal values. The input of the amp to ground and maybe a small one shunting the output would be a good idea.

IF the primary of the mod iron is floating, not referenced to the RF deck's ground, that also may present a problem.

            _-_-WBear2GCR



PS. what brand and model power amp is being used?

[ka1tdq]

The amp is capable of 200 watts @ 8 ohms so I'm using the 16 ohm tap (100 watts).  I probably should put it over on the 8 ohm tap since it is going to an 8 ohm transformer.

[ka1tdq]

The primary of the mod iron IS floating... how do I get a ground reference in there?  Ground the 4 ohm lead while feeding the 8 and drive more audio?

The audio amp is a Radio Shack special... A PylePro (160 I believe)

[WBear2GCR]

The primary of the mod iron IS floating... how do I get a ground reference in there?  Ground the 4 ohm lead while feeding the 8 and drive more audio?

The audio amp is a Radio Shack special... A PylePro (160 I believe)

No do not ground the "4 ohm lead".

The transformer has a "common" lead, and then taps... ground the "common" lead to the RF deck chassis. It MAY be sufficient (and safer for the poor Rat Shack amp) IF it is merely an RF problem to use a nice largish film cap from the primary to the RF deck ground (grounding only at AC not DC). Something like a 1 ufd or larger at 250wvdc or better sounds like a plan. Still it should be ok to ground the common lead to the RF deck. I'd turn both on, and take a voltmeter and see if there is any real leakage voltage between the power amp ground and the RF deck ground, if it is virtually nil - not like 60volts or 120 volts AC, then it is likely safe to connect the grounds at DC... if you see anything in voltage terms that is suspicious, you might then take an ammeter and see if there is any real current between the two unit's ground terminals. If there is real current, then that is a problem that must be cleared up - it indicates either a fault or a leakage path (bad xfmr in the RF deck? or power xfrmr in the amp?).

How does this (solid state amp) have output Z taps? Or is it an older tube amp?

Assuming it really is a 200 watt amp it ought to weigh about 50lbs or more... if it weighs like 15lbs, then it is a "fake" 200 watt amp and likely is running out of juice. You should be able to test it using a scope and a sig generator and see what the peak voltage is before it clips - then take .707 of 1/2 of the peak to peak voltage and plug that into Ohm's law to see the power: P = E^2 / R where E= that voltage and R is the load resistor... alternately you can plug that into the formula and see what sort of rail voltage is needed to make 200 watts into 8 ohms!  

The transformer's taps are somewhat irrelevant, since the transformer is only a "ratio device" assuming adequate current is always available at the primary side - what matters in practice with this sort of set up is that there is sufficient voltage swing at the secondary to modulate the tube 100% or better, AND that there is sufficient current available to do that. The primary being driven with the power amp, that presumably has sufficient current available. So what matters is the secondary voltage swing - that will change with the "ohm tap" used on the primary. The lower one "4 ohms" providing the highest ratio and step up. The "16 ohm" being the lowest step up ratio.

              _-_-

Edit: the amp might be a more modern Class D or Class H in which case it may be somewhat lighter than a typical class AB power amp - but it also might be more succeptable to RF getting in...

[ke7trp]

We just wasted 2 months attempting to do this at high power.  The trouble is the amplifiers on the market. They are Transformerless and really make about half the power claimed. They are rated in absolute peak Watts and then maybe marketed up from there.

On these PA amps, If you dont present EXACTLY the load the amp is expected on a speaker... It will shut down, make less power, Maybe this is not your problem.. But I am throwing it out there.

C

[WBear2GCR]

Real PA amps - pro ones - are rather robust into their rated loads.
Consumer grade "PA Amps" vary all over the map.

The better grade pro amps will run into 2 ohms all day.

Consider the Crown Macrotech. They can be bridged too for huge power. The small ones bridge for typically over 1/2kW, the big ones tend to get limited by your line current...

There are other pro brands to consider as well. Should modulate the carp out of a normal 100-200watt final... no problemo... ask IFR what he uses...

                        _-_-bear

[N2DTS]

Rf getting into the audio?
The frying lights dimming sounds bad!

I did tests on some pa amps some time ago. i had a 100 watt sae amp that did 160 watts before clipping, the 100 watt radio shack amp did 70.

Brett

[ka1tdq]

Yeah, the modern rat-shack audio amp doesn't weigh very much and they rate it at 400 watts, so I'm guessing it's one of them there fake amps.  I will ground the common lead to the RF chassis though and I have a feeling that will clear things up.  Thanks for that tip!  ...never thought of that.

As for the power out-poot, I literally only need 20-ish watts of audio power to drive this deck so the audio amp I have should still be overkill. 

[Gito]

Hi

What caused all the lamps dim,in my opinion is ,that the audio amplifier is loaded too heavily or load in a dead short load,that makes ,the current it takes is high that makes the power/ac line drops.at the high volume setting  of this amp.

It can be caused by shorted turns in the high impedance part(primary) of the output transformer
It can also caused by a shorted Reacktor (Heizing Re)

if You by pass this primary  with a 450uf ,means that you not only by passed the audio to ground ,but also load the amplifier with a dead load/ shorted load.

 the output transformer "sings",that means its is loaded heavily,only high magnetizing  current makes it "sing".


Gito

[KL7OF]

Recheck your pri/ sec  transformer orientation

[Opcom]

throw a scope on it, see if you have ultrasonic oscillation making that frying sound. Which will blow up the audio amp before too long. some audio amps don't like strange loads.