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External Transformer Recommendations for 4ohm speakers driven by 70.7v mono amp.

Discussion in 'Solid State' started by CDELON, Nov 6, 2018.

  1. CDELON

    CDELON Active Member

    Messages:
    119
    Location:
    PA
    Hello,

    If someone can be found that would have the answer. It would be here on AK.

    I have two 70.7v 350watt mono amps. I would like to use them for two 4ohm speakers rated for up to 250 watts. Another thread has been helpful with giving me some direction.
    I know I will need a transformer. I would like to have a external one. One I can keep and mount by the amp.One that would have a 4 or 8 ohm option would be ideal. Then I can run the speaker wires to the amp. I do not want to alter the speakers at all. This is experimental. I don't want to break the bank putting this together.
    I would appreciate any suggestions and recommendations.
    I do not have foundational knowledge of the sciences of electronics. So please understand that in communications. I will not have the vocabulary.

    Ty
     

     

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  2. c_dk

    c_dk Addicted Member

    Messages:
    6,245
    Location:
    West Michigan
    Certainly a dead end to achieve the highest audio quality but Atlas Soundolier has a number of matching transformers.

    The HT-600 has the wattage options you need.
     
    I LIKE MUSIC likes this.
  3. CDELON

    CDELON Active Member

    Messages:
    119
    Location:
    PA
  4. CDELON

    CDELON Active Member

    Messages:
    119
    Location:
    PA
    Would I have better results if I used the 1.5 ohm connection? Put a 4 to 8 ohm transformer from the 1.5 ohm?
     
  5. I LIKE MUSIC

    I LIKE MUSIC Super Member

    I took a quick look at the specks for the transformer and it has an insertion loss (reduction of available power) of about 2 dB.

    upload_2018-11-9_22-10-26.png

    If you connect a 4 Ohm rated speaker directly to the 1.5 Ohm (direct) output of your amplifier, the output power will be within about 1 dB or so of the power available using an external transformer (this is really close in terms of power and sound volume).

    There may be some that say that loosing 1 dB of volume is a bad thing, but then again the amplifiers are what they are and elimination of two audio power transformers (the internal transformer in the amplifier and your proposed external transformer) is not necessarily a bad thing in terms of sound quality.

    As I mentioned in your other thread, the 1.5 Ohm direct output is the minimum impedance that can be connected to that output tap, it does not mean that you can not connect a 4 Ohm speaker to that tap.

    If you have checked the DC offset voltage on the direct output of each amplifier (it should be less than 25 millivolts maximum to be on the safe side, although smaller is better), I would hook up the amplifiers and give them a listen. If you are not sure about the functionality of your amplifiers, I would use some sacrificial speaker to test the amplifiers. If you do not have any speakers, check your local thrift stores and or Craig's list.

    Using the 1.5 Ohm output on the amplifier will eliminate both the internal transformer and your suggested external transformer.

    In terms of sound quality this would not necessarily be a bad thing.
     
    merlynski likes this.
  6. Pio1980

    Pio1980 AK Member Subscriber

    Messages:
    26,516
    Location:
    Angel Station, Alabama
    Really depends on the native sound quality of the amps for direct connection whether better unfiltered by redundant output transformers.
    Early Altec solid state reputedly not great for instance, early solid state pa in general.
     

     

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  7. CDELON

    CDELON Active Member

    Messages:
    119
    Location:
    PA
    Thank you very much everyone. I do have a set of speakers that I can try it with. I believe they are 6 ohm. I will order the RCA connectors with the wire terminals on the back end today. Interestingly, I am little excited and nervious at the same time with this experiment . I keep visualizing smoke.
     
  8. I LIKE MUSIC

    I LIKE MUSIC Super Member

    According to the spec sheet, the high frequencies are somewhat rolled off when going through the internal transformer.

    upload_2018-11-9_23-36-9.png


    Again, it is true that these amplifiers are designed for professional PA/multiple speaker service, but they are far from the bottom of the barrel in terms quality in this application. They have extensive self protection capability.

    Back in the day, McMartin was a well known manufacturer of quality products for the broadcast radio industry (AM and FM transmitters, station accessories, SCA generators, FM exciters, stereo generators and so on) and audio amplifiers for paging and sound reinforcement and so on. In general they were at least as good as most other brands, it not better.

    Remember that the OP stated in his other thread that the speakers that he would like to use with these amplifiers are known to dip below 2 Ohms impedance.

    To keep the reflected impedance in line, he would likely want to use the 150 watt tap on the external transformer (somewhat limiting the power output) to keep the impedance as seen by the 70.7 volt output on the amplifier above its minimum. On this amplifier it is about 14 Ohms as I posted in the the other thread.
     
  9. I LIKE MUSIC

    I LIKE MUSIC Super Member

    To skip my long posts above, after checking the DC offset if you have not already done so, just connect your speakers directly to the 1.5 Ohm connection (no transformer needed) and see how everything sounds.
     
    Pio1980 likes this.
  10. CDELON

    CDELON Active Member

    Messages:
    119
    Location:
    PA
    I'm now optimistic. I made a base for each of the amps and mounted them. I will find a wire mesh that will be visually appealing to wrap the outer perameter of the amps at some point. When used, I plan on them being on the floor.
     

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  11. CDELON

    CDELON Active Member

    Messages:
    119
    Location:
    PA
    What's the first thing you do? Test! In my vision and excitement, what's the last thing I did? Test!
    Plugged them in. No light on one. 2nd amp, the light came on. But faded away. I checked the slo-blow 10A fuses. They are both still all white. I don't see a on/off switch on these amps. So I am assuming they were intended to connect to a primary source.

    Frustrated with myself at this moment.
     

     

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