Quote:
Originally Posted by Icarium  My question is... Phil said that the 3 channel topology gives all/most of the benefits of balanced sound without the negatives. What are the negatives?
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From other of the long Uncle Phil's posts in ipodstudio forum long time ago:
(Q) From various sources, the advantages of having a balanced amp with a differential output seem to be:
1.double slew rate due to nature the inverting and non-inverting signal being of opposite phase
(A) The Reason the slew rate is twice that of a non-balanced output is because slew rate defines how many volts in one time period the amplifier can slew or retrace from the positive peak of the waveform to the negative peak. In order for the slewing rate to be improved the Amplifier must be able to slew more voltage in the same amount of time. This is done ether by making the waveform shorter in Duration (Higher in frequency) or larger in amplitude (More output Voltage) the slew rating of an amplifier is mealy the measure of how high in frequency a given Amplifier can maintain a specified output voltage, If the available output voltage remains unchanged then slew rate improvements must come from making the amplifier faster. If the amplifier cannot be made faster the slew improvements can come from making the available output voltage the amplifier can produce greater. By ether doubling the high frequency performance of the amplifier or doubling the available output voltage the slew rate will be twice that it was before. It is interesting to note that since improving the available output voltage and high frequency performance both result in higher slew rate specs that by improving the available output voltages by twice in addition to improving the high frequency performance by twice with give 4 times the slew rate as ether voltage output or speed improvements alone achieve. The answer to this question is that by virtue of balanced output providing twice the output voltage for a given Amplifier configuration than a similarly configured non-balanced amplifier the slew rate of the balanced amplifier is now twice due to twice the available output voltage.
(Q) 2.(in theory) four times the power due to half impedance seen by each amp (per channel) leading to twice the current and twice the voltage due to the signals being of opposite phase
(A) this is actually one of the disadvantages of balanced operation is that since the amp is seeing half the load impedance that a non balanced amp would for the same transducer the lower impedance limit the amplifier can safely drive is now also twice what is is in the unbalanced configuration and thus the load driving ability of the Amp is now reduced by half. It is a misconception to believe balanced amplifiers have twice the output current capability because reality is that the current demands placed on the amp are greater. This causes current limiting and compression in the amp because its power supply and output current capabilities remain unchanged where as the demands placed upon it have just got worse by twice. This is why a typical loudspeaker amp will have a low impedance load tolerance of twice what it would operated unbalanced, thus Amplifiers able to be used with Loudspeakers of 4 ohms minimum are now restricted to loudspeakers with twice that impedance or 8 ohms.
(Q) 3.push-pull output supposedly has more authority on the headphone driver membranes
(A) this is because of more available output voltage results in more power and thus more power adds authority and dynamics however for the headphone amplifier obtaining any required amount of power without resorting to balanced output is obtainable with higher voltage power supply Operating voltages. The Headcode operates on supply voltages that are twice that of the highest voltages used in op-Amp based Designs where as the op-amp is limited to supply voltages of 24-36 volts rail to rail or +/- 12 to +/- 18 Volts the Headcode operates on +/- 36 volts (72 volts rail to rail) and as such will provide twice the output voltages of an Op-Amp based Design operating on its highest safe voltages of +/- 18 Volts and are typically operated on +/- 12 to 15 volts.
(Q) 4.no common ground for better common-mode-rejection and less channel crosstalk (although I have read that the PPA counteracts this with an active ground)
(A) This is one advantage of balanced operation that has some merit and to address this I introduced the ground channel in the PPA. Under development is the next generation of this same technology first introduced to the High End Headphone Community in the PPA, LaRocco Audio’s TRIAD topology continues with improvements to the TRI-Mode of operation of which was first introduced in the PPA. With the addition of one complete additional power supply including power transformer and one complete amplifier channel known previously as the ground channel in such amps as the PPA, the Headcode Dual mono can become the Headcode TRIAD. Once the TRIAD topology is refined to the High standards set by the Headcode Larry at LaRocco Audio will offer TRIAD upgrades. The Headcode Dual mono however is unchained from most of the Limitations imposed upon the PPA such as a single power supply for the entire Amplifier to accommodate portability as the PPA is a portable amp. It was primarily the desire to have an Amplifier that will tolerate poor quality wall adapters that resulted in the Technology used in the PPA. The Headcode by virtue of two completely separate power supplies including the power transformer result in elimination of any inter-channel reactions.
(Q) 5.6dB more dynamical range (picked that up from Rane.com, I’ll have to check on the exact reason but I suppose it has to do with the increased voltage)
(A) This is true since more voltage is available the dynamic range is also improved. However obtaining more voltage dose not requires a balanced output but is obtained in a more linear fashion with more power supply voltage.
(Q) [Just a quick check whether I got the terminologies right: Differential simply means that there a two signals being opposite of phase (+2V and –2V for instance). But then again, the “differential amp” is defined as a amp that takes a negative and positive input signal and one output that responds to the difference between these signal. So does that mean that “differential” only refers to the input? Bridge-tie load means that the speakers/headphones are driven from two outputs via push-pull and with a inverting and non-inverting signal (e.g. +2V and –2V). And balanced is just a general term applying to both driving issues as well as balanced lines.]
(A) Yes just as the term is used to describe the differential of your automobile. If your vehicle is raised upon a lift while in a service station you cold rotate one rear tire and watch the other one turn the opposite direction on rear wheel drive vehicle.
(Q) Now, I understand that these kinds of short lists only cover the theory but not the actual implementation. Still, balanced mode seems to be everybody’s darling at the moment especially because most audiophiles understand enough electronics to grasp the basic concepts like “quadruple power”, “twice slew rate” and “no common ground”, and to be honest, they do sound quite convincing in theory, especially the slew rate part. I wonder whether truly balanced operation of headphone amps also has its disadvantages seeing that many state of the art amps like the RP010 or Angstrom amps are not balanced either. And finally, why did you decide to use a non-balanced single-ended output for the Headcode?
(A) If you consider that each channel of the Headcode is comprised of true fully symmetrical complementary Circuits from the input to the output and are only tied together at the input and output and otherwise contain completely separate however symmetrical circuitry. In Actual operation by virtue of a fully complementary PNP NPN relationship formed between semiconductor devices of each half of the internally balanced circuit the Headcode has symmetrical rising and falling slewing rates and similar transfer function of both the positive and negative half of the circuit. In conventional balanced output configurations the operation of two independent amplifiers in both the inverting mode and the other in the non-inverting mode results in each half of the waveform being processed by Amplifiers with completely different slew rates and frequency response. Even ideal matched dual IC op amps containing two perfectly matched Amplifiers on one chip are subject to this if one Amplifier of the dual is operated in the inverting mode and the other in the non-inverting mode the one operated in the Inverting mode will typically have higher slew rates and bandwidth in addition the spectral content of the distortion spectrum will also be different than the one operating in the non inverting mode. This is thus processing each half of the waveform differently in the ultra critical time domain.
