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Class A bias amp

Discussion in 'Amplifiers' started by dave457, Oct 29, 2011.

  1. dave457

    dave457 Sr. Member

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    Anybody know who makes them or can any "C" be converted


     

  2. GnG8d

    GnG8d Well-Known Member

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    Why would you want class A?
     
  3. dave457

    dave457 Sr. Member

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    Fidelity over efficiency
     
  4. mackmobile43

    mackmobile43 Jock Supporter

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    Yes by adding a regulated voltage supply of at least .6v to the base of the output transistors and to the input transistors if it has a drive stage.
     
  5. ElectronTubesRule

    ElectronTubesRule Active Member

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    The bandwidth you have at your disposal that the radio picks up, modulates and sends out from an audio stand point is very small. Then you have the physical band width allowed on that band that you can use not often ran into or talked about on AM but on SSB it is a limiting factor. Then you have the audio circuit on the receive unit and the speaker. Oh I forgot to mention the mic as well. Point is that nothing you do to that amp will improve the fidelity the bottle necks are many and scattered through out the radio's on both ends on the transmit side and the receive side and the reproduction of the sound. This is why we see so many AB or C type amplifiers in RF but no A's!

    If you want fidelity do not over bias your transistors and chose units that have good inter-mod number's and lots of gain over the highest wattage. High wattage usually means Japanese transistors that are over biased. This is why I for a long time really liked amps with the MRF 455 now that those arenot in production I would favor the MRF421 over the 1446 or 2290 because in a well designed amp operated by someone that knows what their doing they will sound better then someone running a few more watts with say 2SC2789......Especialy on SSB.

    If you want fidelity you need to work on all the bottlenecks inside the radio. In general I think most amateur gear sounds dead as a door nail....Sure nice clean signal, plenty of power, and often they sound artificial and metallic. They have sought efficiency, power, compact design loaded with digital feature to the point that they simply do not sound life like at all. It is kind of like when you look at the spec sheet's of some stereo gear it looks fantastic then you listen to it and sure it is loud and clean but it just does not sound good. Listen to that same record on a two channel amp built around some 2A3 tubes puting out what 1-3 watt's and the music sounds fantastic even though it is only coming out of 2 speakers not 6 counting a sub.!!!

    I can tell you some of the things I did back when CB was popular and family friendly so some time ago.......Depending on the design of the radio I would use two audio ic . I would try to find lower noise higher gain transistors and milspec anytime I could no matter if it was a transistor, diode or resistor etc.....I would play around with component values to see how it would affect my audio trying to make my voice sound like my voice. If you have the time and a schematic for you radio you can really do a lot of little things to improve fidelity especially if your not hunting for every free watt you can get. You mic is a huge limiting factor the better the mic the better your chances of sounding good. You can spend a year playing around with mic elements and audio transformer's trying to get your sound right.

    Sooner latter you will get tired of all the time for such a megar gain and will run a radio mostly stock. You will find a decent mic and maybe a speech processor and call it good.

    I have yet to come across a modern radio that truly sounds great on air. The old tubed radio's with huge bandwidth hoging designs had the best sound but could not make it through type acceptance today with the FCC to save their life. Just like CD's do not sound as good as well cared for vinyl the same thing happens with broadcasting your voice. The more data you want to broadcast the more bandwidth you need. By reducing the amount of data you can reduce the band width and focus your power where it counts the most. If you look at the freq. responce on most radio's and most radio mic's they do not focus on the low freq. or the super high freq. they focus on where most of the audible sound is in a humans voice. This is by design and this is the limiting factor for fiedelity not the amp's biasing not until you start talking about C and SSB. In fact one of the reasons CW can use power so much more efectively as compared to voice is because the amount of data is small you a dit and dash and it is monotone not a lot of data to a dit or a dash......Speech is very wasteful because of all the data in the voice. In fact analog always sounds better if set up right then digital but if I had to talk to someone a long way away on vey little power I would want to use digital burst's with plenty of compression and data checking.....A tube that can only handle 350 watts continuous in modulated traffic like AM voice can often put out 10,000watts in 10 milisecond burst's.

    Really in two way radio's being able to be copied and understood easily is so much more important then sounding like yourself. This is why by the time I hit 20 years old I stopped caring about trying to sound like me and really started to prefer speech processor's. I wanted to be heard more then I wanted to sound like me and anything that made that easier in a pile up seemed like the better way to go.

    If I had all the money I spent on mic safari's trying to find the perfect mic I could probably by a round of beers for the regulars on this site! DSP filtering on your speakers and getting all the white noise out of your rig would be more worth while in terms of bang for your buck!
     
  6. KM3F

    KM3F Active Member

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    Class C is never advised for any AM or SSB operation, only FM.
    The class of operation in usually not understood be the average user.
    For AM use, class AB or AB1 is good enough if the amplifier has a Linear portion to it's operateing dynamic range.
    Class C has the amplifier cut off such that no current flows.
    If you try to drive such a condition with an AM signal, the results it a lot of distortion, harmonics and splatter because the input signal is not reproduced faithfully in the ouput.
    Further, moving the class of bias toward A results in greater standby current flow, heating and less drive ability for output.
    On AM a class AB1 is ok as long as the drive power is kept within the amplifier's linear range.
    You don't know where that is unless you have and know how to interpet the design info for the amplifier.
    Is it any wonder why you hear so much interferance on the 11 meter band by use of these junk illegal amplifiers in the hands of people who have no idea what their doing with them as well as overdriving them to boot?
     
  7. wavrider

    wavrider W9WDX Amateur Radio Club Member

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    Do some research on your biasing.

    There are several threads on this forum that go into great detail on how to bias from Calss C to AB1 or AB2 for solid state amplifiers.
     
  8. AdvancedCommunicationWest

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    Ok first of all there is no "class AB1 or AB2" biasing for a bipolar, solid state amplifier. AB-1 and AB-2 are used to describe the element of a *tube* that is biased with respect to another element in a tube.

    When referring to a solid state amplifier, there are of course, different classes of amplifiers. Class "A" for an RF amplifier for all practical purposes would be overkill for most applications. There are several reasons for this. The first, is that the amplifier would generate a lot of heat. The second would be that careful design techniques would have to be implemented to prevent oscillation. The third, is that the amplifier would only reproduce the signal's waveform that is fed into it. (not the same as the popular "garbage in garbage out theory that circulates on here all too often) Meaning that is a radio or exciter would drive the amplifier with an AB stage, then an AB stage for the final output, or amplifier would be sufficient.

    Class AB would be the best trade-off for linearity and efficiency. In reality, a properly designed class B amplifier would work just fine if it had push pull stages. Even the worst of the CB amplifiers on the market, and don't get me wrong, there are some pretty bad ones out there that people call class "C" aren't really class C by definition due to the fact that they don't have any negative biasing but rather just have no forward biasing and therefore while for all practical purposes are class C, they do not operate "deep" into the class C region. I still disagree with them being used for AM or SSB modes.

    Ok that's it for amplifiers.

    With respect to audio fidelity, there are several issues at hand. The first is the audio cain inside the radio leading up to the balanced modulator. Typically the audio has a high end rolloff, and a low end rolloff sometimes too soon. By changing capacitors in this section one can tailor the TX audio accordingly. In addition some filters can be widened a bit as well. However this does no good if the RX end is very narrow.

    At any rate, just some things to think about.
     
  9. ElectronTubesRule

    ElectronTubesRule Active Member

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    Yes, I know that was my point that until you get to trying to use class C for SSB for instance the amplifiers biasing is not the fidelity issue. I was using an extreme that I thought every body would know not to use as an extreme example of the only type of situation where the amp's biasing is getting in the way of fidelity. I guess I should made that point more clear but I thought it was an obvious point to anyone discussing the different bias setting of amplifiers so I did not state the obvious I am sorry for not making that clear!

    Believe it or not a lot of black market lower power non-ssb compatible linear are biased as class C. Trust me I could not make up stuff this stupid! In the past guys that owned Amateur Tube amps that used them illegally on 11m and only cared about AM would use a 50-100 watt class c modulator type amp so that their single final 3.5-4 watt cb had the required drive to drive the Amateur linear.

    Most of these brands are no extinct mind you!You still see them on ebay and in little CB shops in the middle of no where.

    Is the TS 50 Modulator is that biased for class C?
     

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