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{Updated Oct. 2017}
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Tube amp output coupling transformer interaction with speaker impedance

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  • Tube amp output coupling transformer interaction with speaker impedance

    Originally posted by artitalia View Post
    Hi guys, i'm really happy owner of a couple of Harbeth m30.1 ae, recently i've had a problem with my previous solid state amplifier and i'm going to buy a integrate tube amp Audio Research Vsi75 .
    Another factor to consider - which has been addressed, before, but is worth reiterating - is the frequency response variations that will occur with an output transformer coupled tube amp. Here is a frequency response plot for the similar Audio Research VSi60 amp:

    The four colored lines show the frequency response into several values of test resistance, while the black line shows the frequency response at the amps output {the speaker command signal} into a typical loudspeaker load with a non-linear impedance.

    Assuming you are acclimated to the sound of your speakers using a solid-state amp, you ought to try the tube amp at home before purchase in order to determine whether or not the change in frequency response will be acceptable.

    If you are amenable to considering another solid-state amplifier, one model to audition would be the Parasound Halo integrated amp:
    This amp has more power than the Audio Research unit - at least 200W into a 6Ω load - along with features such as a DAC for digital sources, a phono preamp (in case you have, or ever want to buy, a turntable) and an output with adjustable crossover to feed a subwoofer.
    FWIW, Harbeth and Parasound share the same importer for Italy:

  • #2
    Correlation between amp output signal variation and speaker frequency response (& perceived balance)

    Originally posted by IMF+TDL View Post
    Another factor to consider - which has been addressed, before, but is worth reiterating - is the frequency response variations that will occur with an output transformer coupled tube amp. Here is a frequency response plot for the similar amp...
    IMF+TDL has on several occasions kindly drawn our attention to the issue of the interaction between tube amp circuitry, and the speaker load that it is driving. As this issue of amp (especially tube amp) euphonics continues to rumble on unabated, it's pretty clear that the wisdom he is trying to pass on is being ignored, or more likely just not deemed relevant. But boy, it really is the heart of the tube amp 'sonic signature' issue.

    Taking the above technical measurement of a tube amp - it could be any tube amp, but this graph is to hand - I've painted out traces that are not relevant, leaving two remaining curves. First to explain the axis: the left vertical scale is in dB, the normal unit of measuring audio levels. The horizontal axis at the bottom is frequency, the entire audio range from the lowest to the very highest notes. So the story this picture gives us, at a glance and unambiguously, is of how closely the output of the amplifier conforms to the signal that arrives at its input terminals, the music signal.

    Click image for larger version

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    Since the blue line represents the 'truth', that is, the music signal at the amp input where all frequencies are treated with equality, we can see that the amp's output signal to the speakers (black trace) is significantly deviant from that input signal across the audio band. The speakers have no means of knowing what the true music signal requires them to generate as sound because the amplifier sits between them and that truth. They have to do what the amplifier commands.

    We can see that at (1) about 65Hz, the amplifier has boosted the bass by about 1.5dB, around 200Hz, (2) the amplifier has reduced the bass by about 0.5dB; across a wide band of frequencies peaking at about 1.5kHz (3) the amplifier has boosted the upper mid/lower presence band by about 1.5dB and at about 4.5kHz (4) has reduced the signal to the speakers by about 1.5dB. So, like it or not, with this particular speaker load connected to the amplifier the signal that you hear over the speakers has been modified by the amplifier to have a +/- 1.5dB difference compared with the source music signal fed into the amplifier. There is nothing at all that the user can do to escape this. In fact, he may really like this modified sound and after acclimatising to it, is likely to reject an amplifier (a SS amp) that is unaffected by load and has a flat input/output response. This would be a very normal human reaction.

    This real-world load-variation resulting in excessive output in some audio frequencies and reduced output at the speakers (which is then turned into sound according to that profile) accounts for most, if not all, of the 'sonic signature' of amplifiers, especially tube ones where the speakers are driven via the amp's output transformers. Conceptually, take a perfectly flat amp - a Bryston for example - and feed the music into it via a graphic equaliser with sliders up/down to mimic the voltage output of a tube amp, and it could be made to sound indistinguishable from a tube amp.

    Just to make this point absolutely clear, let's take an amplifier (most likely a SS amp) that is load invariant (it's output signal is uneffected by the electrical load connected to it) and drive it into a speaker, which for simplicity we assume has a flat frequency response (i.e. unemphasised, equal quantity of sound out for electrical signal in, across the audio band).

    Graph A - We plot the frequency response of an ideal audio amp's output signal to the speakers across the audio band alongside the speaker's frequency responses on log paper, just as above, and we see this:

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    Pix 1. Perfect amp driving flat speaker

    The blue trace represents the perfect amplifier input/output - there is no variation (aside from a gentle roll-off at the extreme top end of the audio band) in output when driving this loudspeaker (green) and together, they extend fidelity to the microphone signal right up to the sound waves generated by the speakers. An ideal situation, and one which is the norm with solid state amplifiers.

    Graph B - Contrast that situation with the real-world reality of tube amplifiers where we take the same speaker, and now drive it from such an amp -

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    Pix 2. Amp sensitive to speaker load, influences speaker frequency response.

    We can summarise thus:

    a) The amp has no knowledge of or interest in the electricity in > sound out frequency response of the loudspeaker it drives
    b) The amp's characteristics will be, to one extent or another, influenced by the electrical load (speakers) that are connected to it
    c) Speakers do not have nice, flat, smooth perfect 8 ohm impedance - their impedance varies wildly with frequency across the entire audio band
    d) That deviation in impedance from flat can cause the amplifier to work harder or less hard than it should at some frequencies than others
    e) The speaker is driven by volts from the amplifier
    f) If the amplifier delivers more volts at some frequencies and less volts at other frequencies than a perfect amp should (i.e. it is influenced by the electrical load) the energy delivered to the speakers will vary across the audio band
    g) As the speaker have no intelligence, they do as they are commanded, frequency by frequency, instant by instant; more volts in at any given frequency = more sound out at that frequency, less volts in, less sound out at that frequency

    and the killer is this .....

    h) Any variation in the amp's output voltage drive level due to variations in the electrical load impedance that it sees across its output terminals connected to the speakers speakers it drives will usually result in a 1:1 change in the frequency response of the loudspeaker's sound output ..... if the amp output level at 1.5kHz increases by +1.5dB, it will mandate that the speaker plays louder by +1.5dB at 1kHz.

    In essence: there is a direct correlation between any dB variation in electrical impedance v. frequency and an equivalent dB change in sound output from the speaker. It should be no surprise at all that tube amps have 'sonic signatures' and those sonic signatures can/will/do alter dramatically depending upon the precise speakers that they are driving.

    The perfect amp should be completely indifferent to the electrical load of the speaker. Such amps are readily available, but as far as I know, none of them have the output transformer that is core to the tube amp.

    So, any electrical measurement of an amplifier - despite extreme protestations that test equipment can be completely discarded and reliance made solely on the ear - is likely to give us tremendous insights into the behaviour of audio electronics. If the frequency response of an amplifier is not flat driving a speaker load, how can anyone seriously expect it to sound flat?
    Alan A. Shaw
    Designer, owner
    Harbeth Audio UK


    • #3
      Which output tap?

      This makes me wonder about the benefits, if any, of using the 4-Ohm output tap instead of the 8-Ohm tap of my tube amp with my P3ESRs (a nominal 6-Ohm design).

      Since my speakers are brand new (and I am a very happy new owner) I haven't had a chance to experiment yet. Spending all my time just listening to music! In theory at least, this so-called "light loading" solution should result in better woofer control and lower distortion, at the expense of a slight loss of power.


      • #4

        Thanks for sharing this information here. I want to know that what are the tubes and the amplifiers are available which must be compatible with each other. Also do you have any specifications for this type of the setup.

        How i can build it myself according to my requirements?

        pcb manufacturer


        • #5

          Originally posted by FrankFinne View Post
          Thanks for sharing this information here. I want to know that what are the tubes and the amplifiers are available which must be compatible with each other. Also do you have any specifications for this type of the setup.

          How i can build it myself according to my requirements?
          I am not sure I understand your questions. My amp is a 40 W/ch (plenty of power for the P3ESR in a tiny room, it seems) push-pull amp using KT88 tubes.


          • #6
            You can see very clearly from this excellent tutorial that the magic "warm sound" may be nothing more than the consequence of boosted bass as delivered to the speaker.

            Scroll down to Valve amplifiers and speaker Q and see for yourself.

            There is no magic at all in this and exactly the same subjective effect and auditory sensation could be achieved with a solid state amp fitted with a bass control and/or a graphic equaliser and/or pre-processing the music with an equaliser in software.
            Alan A. Shaw
            Designer, owner
            Harbeth Audio UK