Anybody know the input impedance of Samson amps?

Discussion in 'Archived Threads 2001-2004' started by Cliff, Mar 7, 2002.

  1. Cliff

    Cliff Stunt Coordinator

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    Recently there have been some threads in which people complained that the outputs from their Samson/SVS units are not loud enough. I have noticed these from owners of both Onkyo and Yamaha receivers, and the responses have included advice that there is a voltage or impedance mismatch between sub-out from the receiver and the amp.
    In the past I have learned the rule of thumb that the input impedance of a component should be at least 10 times the output impedance of the signal source. I have a Yamaha RX-V1 with a sub-out impedance of 1.2 KOhms. That means the input impedance of a connected amp should be at least 12 KOhms.
    My question is: what is is the input impedance of a Samson amp? I have the S1000 (and 2 SVS CS-Ultras) and cannot find this spec in the online or hard copy documentation for the amp.
    I might add that I have a Symetrix 551E connected between my RX-V1 and the S1000. The 551E has an input impedance (for unbalanced connections) of 10 KOhms, which is almost 10x the output impedance of the RX-V1. Furthermore, the output impedance (unbalanced) of the 551E is 150 ohms, which means that, for my system, the S1000 should have an input impedance of at least 1.5 KOhms.
    One more thing - I have no complaints about the volume output of my SVSs. I have calibrated with both Avia and VE and have plenty of headroom to spare. So, perhaps my 551E has saved me from the problems that others have reported. Using the rule of thumb I cited, I would need an input impedance in the Samson of 12 KOhms without the 551E, and 1.5 KOhms with the 551E. If the Samson has an input impedance less than 12 KOhms but greater than (or equal to) 1.5 KOhms, then this might explain why it works for me but not for others who have the Samson connected directly to the sub-outs of their receivers.
    Does anyone know the spec for the input impedance (unbalanced) of the Samson S1000 or S700?
    Thanks,
    Cliff
     
  2. Tom Vodhanel

    Tom Vodhanel Cinematographer

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    I think Samson techsupport said 110k when I spoke with them a couple of weeks ago.

    TV
     
  3. Cliff

    Cliff Stunt Coordinator

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    Thanks, Tom. That exceeds the input impedance of any of my consumer-grade amps, which range from 20-47 KOhms. So I guess the input impedance of the Samson amps does not explain the problem some people have had with volume output from their Samson/SVS units.

    Cliff
     
  4. Tom Vodhanel

    Tom Vodhanel Cinematographer

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    My bad,

    it's 10-11k

    here's the email samson sent me a couple of months ago.

    """Dear Sir, according to the tech I spoke to the impedance is about 10K. If

    you have any other questions feel free to call

    the service line at 800-372-6766. THANK YOU."""

    TV
     
  5. Cliff

    Cliff Stunt Coordinator

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    Thanks once again, Tom. That information changes things. At least, if my supposition about matching output-input impedances is correct, then the fact that the Samson input impedance is so low may indeed explain the problem others have had. But I'm no expert on this. If anyone can illuminate this issue, or confirm my information above about impedance matching, please join in.

    Thanks,

    Cliff
     
  6. jeff lam

    jeff lam Screenwriter

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    In my recent weeks of researching pro amps I have found that most of them have an input impedance of 10K ohms unbalanced(1/4") and 20K ohms balanced(XLR). My guess is that your Symetrix 551E has such a low impedance output that it doesn't have any problems driving the samson.

    What are your settings at on your sub level and samson?
     
  7. Cliff

    Cliff Stunt Coordinator

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  8. jeff lam

    jeff lam Screenwriter

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    I'd say you got plenty of headroom to spare.

    Quite possible that the eq helps a lot.
     
  9. Doug BW

    Doug BW Stunt Coordinator

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    Cliff, it sounds like you might be on to something useful.

    If you're so inclined, you might want to try to experiment and see what happens if you bypass your Symetrix. If wiring directly from your RX-V1 to the S1000 gives makes it difficult or impossible to calibrate, you've demonstrated that your theory is likely correct. This might create a viable alternative for those suffering from this problem.

    Although the Symetrix is far more expensive than the Radio Shack amp ($65??) that people are getting to address the problem, the popular BFD can be had for around $135. The BFD has (unbalanced) input impedance of 30 kOhms and output impedance of 30 ohms.

    So someone who found they had the impedance mis-match problem could (potentially...if your theory is correct) buy a single box that not only solves the problem but has the extra utility of a parametric EQ to boot.....which they might need anyway.

    Just a thought.
     
  10. jeff lam

    jeff lam Screenwriter

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    I have the BFD and it didn't help. It doesn't have a level output like most EQ's do. I think the Art has a level output that you can boost the signal. The BFD does not drive the line level higher. You will still need a line driver. Too bad too because that would be awesome to have an all in one device.
     
  11. brucek

    brucek Second Unit

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    Cliff.

    I'll give you my take on input and output impedance's.

    The typical interface between a preamp and power amp is a line level, high impedance connection, where the output impedance of the preamp is usually in the order of about 50 ohms to 150 ohms and the input impedance of a power amp is in the order of 10Kohms to 50Kohms.

    This is known as a voltage bridge connection where the preamp is acting as a voltage source and almost no current is drawn. In fact, a perfect voltage source would have an output impedance of zero ohms at all frequencies. This would result in all the output voltage from the preamp being dropped across its load (the power amp) with no voltage being lost to the output impedance of the pre amplifier.

    Output impedance is basically the internal resistance of the amplifier seen at its output. The value can be slightly frequency dependant because of reactance caused by inductance and capacitance, but either way, a low output impedance is desirable.

    In its simplest terms you can view the internal resistance of a preamplifier and its load (the power amp) as a voltage divider. The higher the output impedance of a preamplifier, the more voltage will drop across it instead of its load (the power amp). This in effect means less voltage will be received at the power amp.

    If the input impedance of a load device is not significantly higher than the sources impedance, the signal will be reduced or "loaded down" and its signal to noise ratio and frequency response will suffer. Certainly the load can become too much for the source to supply adequately.

    Generally, a high output impedance requires close attention to cable lengths. The concern is that the high reactance (frequency dependant resistance caused by capacitance) of the longer cable, combined with a high output impedance of the preamp, creates a low pass filter which adversely affects bandwidth. This distortion of the higher frequencies increases with higher output impedance's.

    So, here's the rub. Unlike speaker cables where we don't give a darn about capacitance, with an interconnect it becomes important. Capacitance results in a reactance (frequency dependant resistance) that will roll off the high frequencies. It becomes more and more a factor, the higher the output impedance becomes.

    This reasoning is why they insist when you use a "passive" preamp you use super short, super low capacitance cables, because they have fairly high output impedance's. This impedance usually varies too, because the output is connected directly to a potentiometer that alters the output impedance as the dial is turned - pretty undesireble. People weigh this off against the benefit of low noise of the passive preamp.

    Tubes preamps also have horribly high output frequency dependant impedance. This is what gives them that "warm" sound you hear about, actually it's distortion - oh well, each to their own. Cables are very important to valve lovers for a reason.......

    Anyway, there's an old rule of thumb as you have indicated that says, the input vs output impedance of a voltage bridge interface should be a minimum of 10:1. That's minimum. It should be greater in a high end system to ensure high frequency preservation. So, if I fed a power amp with 10Kohm input impedance I would "just" be OK with 1000 ohm output impedance, but I certainly wouldn't want to split that signal.

    But, when you are feeding a sub amplifier you get off the hook a bit, because it's a low frequency interface, so the whole issue of capacitance of your cables is bit moot because the parallel reactance at these frequencies is insignificant. This means that generally the 10:1 voltage division rule stands.

    Personally, I feel the output impedance of your receiver's sub output at 1.2K is unnecessarily high, but as long as the output has sufficient voltage level to drive your Samson amp, you're fine...

    More often than a impedance mismatch, I think the problem has to do with the voltage mismatch between consumer and professional amps. I see the Samson 700 amp doesn't specify its input level, but the PDF picture of the amp from their site shows that the input connector has balanced 0dBm written on it.

    Well, this is a professional amp level designating the standard reference level of 0dBm of 0.775voltsRMS for a 600 ohm load. This can generally be equated to 0dBu fairly reliably and so the spec is specifying an input level of 0.775RMS volts. This shouldn't be too much of a problem for most receivers to drive to full power.

    Certainly adding a BFD in between the receiver and Samson amp won't do anything for your level. Especially since the BFD has unity gain. If you have a problem with a voltage level from your receiver to your sub amp, then you simply need to boost the level. I have often championed the Paradigm X-30 as a great line driver. It provides about 13dB gain with appropriate input and output impedances and gives you the added bonus of placing it directly beside your receiver with a convenient volume and phase control.

    Anyway, just some thoughts on it..

    brucek
     
  12. Cliff

    Cliff Stunt Coordinator

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    That was an extraordinary lesson you presented, brucek. Thanks so much - I think I get the gist of what you're saying, but I am going to have to study your essay in order to really capture the details. Much appreciated.

    Cliff
     

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