09 December 2008


Energy-Booster Amplifiers

When you do an online search on “power booster amplifier,” many of the search outcomes will discuss with radio transmitters, not audio amplifiers. This is sensible, as few of us would think about cascading energy amplifiers. If we want a extra highly effective amplifier, we purchase a extra highly effective amplifier. Nonetheless, the thought is an intriguing one: take a small, flea-power, single-ended, tube amplifier and an enormous, heavy, solid-state, power-booster amplifier that significantly magnifies the wimpy 2.5W into tons of of watts. The perfect of each applied sciences.

Or wouldn’t it be?

Sure, I do know that the solid-state amplifier is in sequence with the small tube amplifier and that the tube amplifier will solely drive the solid-state amplifier, not the loudspeakers. And I acknowledge that this scheme stinks of the notion that tubes make good distortion, which I most wholeheartedly reject. So why trouble with this method to raised sound? First, few might be shocked by my saying my elementary mental curiosity over any sensible software; that is the Tube CAD Journal, not Sensible-Tube-Hello-Fi.com, in spite of everything. I like tackling heavy concepts the identical approach a weight lifter enjoys lifting metal weights. For instance, the query, How lengthy might a person survive in a vacuum? I discover fairly intriguing, though I’ve no real interest in discovering the reply firsthand. So too, I’m wondering what such a power-booster amplifier would appear to be and whether or not it could operate radically otherwise from a typical solid-state energy amplifier and if the beefy power-booster amplifier want be completely a solid-state affair or can tubes be used as an alternative.

One high-end audio power-booster amplifier already exists within the Musical Constancy 550K power-booster:

“The brand new Musical Constancy 550K Energy Amplifier will increase the ability of any 50 watt plus amplifier to over 550 watts rms.

Most hi-fi amplifiers wouldn’t have almost sufficient energy to ship musical realism. Till now the music lover both needed to put up with inaccurate musical copy or fork out enormous quantities of cash for a bodily large amplifier to get that live performance corridor magic of their lounge.

The brand new Musical Constancy 550K Energy Amplifier is a conceptual breakthrough and has by no means been performed earlier than in hi-fi. The 550K Energy Amplifier is put in between the present amplifier and loudspeakers. Nothing else must be modified or touched. It will increase the ability of the present amplifier from 50 watts to greater than 550 watts which supplies any present system ample energy to provide sensible live performance corridor dynamics within the dwelling.

The 550K Energy Amplifier offers a brand new lease of life to present hi-fi techniques and permits the music lover to rediscover the eagerness and pleasure of music.

Putting in the 550K Energy Amplifier is a doddle. It matches between the present amplifier and the loudspeakers. After it has been linked to the mains nothing else must be performed because the 550K Energy Amplifier has a complicated activate/flip off music sensing circuit. The audible impact of utilizing the Energy Amplifier is dramatic and must be heard to be believed.

The 550K Energy Amplifier is housed in a contemporary modern package deal with a small footprint and may simply be positioned subsequent to or behind the present loudspeaker.”



Fascinating. My favourite bit is using the phrase “doddle,” which as everybody is aware of is: Noun (Brit, Austral & NZ casual) one thing simply achieved: the take a look at turned out to be a doddle [Origin Unknown]. This good-looking 550K power-booster amplifier jogs my memory of my long-held need to construct a energy amplifier qua loudspeaker stand.

I do know little or no in regards to the MF 550K’s circuitry and one of the best description of its functioning I discovered within the following PDF hyperlink. The 550K’s enter impedance is a low 50 ohms (low for an influence amplifier, however excessive for a loudspeaker substitute). It makes use of a differential output stage to maximise the output voltage swing. It presents a voltage step up of about 3.16 (the sq. root of 10). And its output impedance is extraordinarily low. The 550K additionally presents a line-level, high-impedance enter, for many who simply want a strong amplifier.

The primary query that involves thoughts is, Why a 50-ohm load? I imagine it is a nod to all us tube fanciers, as a transformer-coupled amplifier requires a considerably low load impedance, as inductive coupling is the precise reverse of capacitive coupling. For instance, the decrease the load resistance, the much less inductance wanted; the upper the load impedance, the much less the capacitance wanted. In different phrases, if the 550K had offered solely a 47K enter impedance, tube energy amplifiers (the transformer-coupled sort) would expertise gross bass attenuation. Then again, the 50-ohm load impedance is excessive sufficient to maintain each solid-state and tube amplifier working over a wider class-A spread, due to the much less steep loadline. A compromise, in different phrases.

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The subsequent query is Why a achieve of about 3.16? The 550k goals to extend the enter amplifier’s output by tenfold, which implies that the each the output voltage and present have to be elevated by the sq. root of 10, i.e. 3.1627. For instance, 50W into 8 ohms requires 28.28Vpk and 500W into 8 ohms requires 89.44Vpk, which is 3.16 instances larger than 28.28V. This quantity of voltage achieve additionally units the required enter voltage to comprehend the total 500W of output energy, about 28V. In different phrases, in the event you use a tiny 2.5W single-ended amplifier, the 550K will solely put out 25W of energy, though it’s able to twenty instances extra output energy.

Returning to my authentic three questions: What would such a power-booster amplifier appear to be?
Can it operate in a radically completely different vogue from a typical solid-state energy amplifier? And, should the beefy power-booster amplifier be completely a solid-state affair or can tubes be used as an alternative? Let’s begin with the primary query. Nearly all solid-state energy amplifiers encompass three phases, an enter stage, a driver (VAS) stage, and an output stage. Would a power-booster amplifier want an enter stage, as the present energy amplifier will already present little bit of achieve? And what in regards to the driver stage? Can not the present small energy amplifier drive the power-booster amplifier’s output stage instantly?

The above buffer circuit seems promising, however the issue lies in its providing no voltage achieve, so our 2.5W energy amplifier will nonetheless put out 2.5W after passing by way of the power-booster amplifier. That is the issue that Moskido energy amplifier customers are discovering, because the Aikido achieve stage can solely provide a achieve equal to about half of the mu of the enter tube used, which within the case of the best-sounding 6SN7, is simply 20, yielding a closing voltage achieve of solely 10. In different phrases, it seems like at the very least two phases are wanted. Or are they? Let’s do some heavy psychological energy lifting. The 2 energy transistors, within the schematic proven above, use all their obtainable transconductance to ship present achieve, not voltage achieve. (By the way in which, the above circuit may work superbly for somebody who owns a 2.5W flea-power amplifier and 1-ohm ribbon loudspeakers, because the small energy amplifier’s 6Vpk swing right into a 1-ohm load creates 18W of dissipation. Cool. A 20W amplifier’s output voltage would develop 160W!)

What if we use a few of the output machine’s transconductance to supply a little bit of voltage achieve on the expense of shedding a few of the low output impedance? An exquisite trick, however how will we pull it off? Lengthy-time readers of the Tube CAD Journal will bear in mind my early weblog posts on easy energy amplifiers.

Weblog 3 Zen Amplifiers
Weblog 5 Easy SE Amplifiers
Weblog 6 Easy Push-Pull Amplifiers

Within the simple-PP-amplifier submit, I outlined the next easy energy amplifier topology with simply two output gadgets and voltage achieve.

The 150k and 3k resistors outline two suggestions loops, with a summed voltage achieve of fifty and an enter impedance of about 1,500 ohms. (Right here is an instance the place worse MOSFETs may work higher, as an elevated gate-bias [say 5V] would significantly scale back the voltage achieve and, thus, significantly enhance the detrimental suggestions.) The output sits in between the 2 30V batteries, the place we often discover a floor connection. As a substitute, we discover the bottom connection in between the 2 output gadgets, the place we often discover the output. This amplifier is inverted in topology and it additionally inverts the enter sign’s polarity, which permits us to make use of the inverting-amplifier suggestions loops.

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The batteries are little doubt complicated to many, however they serve to indicate how the ability provide have to be free to comply with the output sign. In an precise amplifier, the next setup is extra possible. In different phrases, every channel will need to have its personal energy transformer winding and rectifier circuit.



Sagaris Amplifier
How will we get the voltage achieve right down to a extra useable quantity? The next circuit reveals how a 3rd suggestions loop could be utilized.

Click on on schematic to enlarge

The Sagaris Amplifier has arrived. Sagaris is an Historical Greek phrase for the doubled-headed battle axe utilized by the Historical Persians. (I wished to the decision this topology the Labrys Amplifier, however I didn’t wish to get anymore sexually themed spam than I presently do, though I’m positive that any such new spam would show a welcome reduction from the super-sizing-, super-stiffening-spam I now obtain.)

The Sagaris Amplifier makes use of two DC suggestions loops to maintain the output centered at 0V and an AC suggestions loop to scale back the achieve and decrease the output impedance and distortion. In fact, completely different output gadgets could possibly be used or a number of output gadgets could possibly be positioned in parallel. The 2 potentiometers assist with amplifier housekeeping, corresponding to setting the idle present and the output offset voltage. With the current gadgets and rail voltages, about 50W of output into 8 ohms is feasible.

Right here is another topology, which makes use of two phases.

As configured, such an amplifier couldn’t work in actuality, because the enter stage transistors aren’t appropriately biased, however including two resistors and separating the suggestions loop into two parallel suggestions loops does make one thing nearer to usable.

Truly, the extra I take a look at the schematic, the extra I would favor to make use of MOSFETs as output gadgets. The grounded-base amplifier won’t present any voltage achieve at its collector with the current resistor values, however with a bigger collector resistor, it might.

I might go on with many different attainable solid-state power-booster amplifier designs, however what I wish to finish with is an all-tube power-booster amplifier.


Tube-Primarily based Energy-Booster Amplifier
Tremendous high-wattage tube energy amplifiers could be constructed. We simply want many output tubes and a excessive B+ voltage. However a traditional tube energy amplifier is hardly novel nor would such an amplifier make the most of the smaller energy amplifier’s skill to drive a low impedance.

Click on on schematic to enlarge

The above amplifier makes use of a 5k/10-20W output transformer to behave as a part splitter and to supply an enormous voltage achieve. We want all of the voltage achieve we are able to squeeze out, because the output stage is configured as a push-pull cathode follower. The eight output tubes use each cathode- and grid-bias to set the idle present. The 2 15.6k resistors are mirrored to the first as 50-ohm load, because the transformer’s winding ratio was 1:25, which suggests an impedance ratio of 625 (625 x 8 = 5,000). If EL34s or KT88/6550s are used, a B+ of about 400V to 500V might be wanted. Then again, if a low-voltage/high-current sweep tube, such because the EL509 or 6LF7, is used, the B+ voltage could be lowered to a mere +170V (6AS7s are one other sensible choice).

I’m tempted to cease proper right here, as what follows could possibly be lethally harmful. Since this power-booster amplifier makes use of an enter and output transformer, the energetic circuitry is nested in between, making no DC contact with both the enter or output. In different phrases, this power-booster amplifier is likely to be an ideal candidate for utilizing the in any other case harmful rectified wall voltage trick, whereby no energy transformer is used; as an alternative, the uncooked wall voltage is rectified and used because the B+ voltage. What makes utilizing the wall voltage instantly as an alternative of an influence transformer harmful is the hazard of the enter or output transformers creating a brief between windings. Fuses assist, however I’d nonetheless be nervous, notably if a toroidal transformer had been used, as the first and secondary are in intimate contact. Then again, transformers that use separate wire bobbins, corresponding to many C-core and R-core transformer do, would show a lot safer.

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Even when we forgo a B+ energy transformer, an influence transformer will nonetheless be wanted for the negative-bias voltage energy provide and the warmers. (Until, twenty 6.3V tubes had been used, ten per channel, in order that the 20 heaters components could possibly be positioned in sequence like a Christmas tree gentle string. In fact, in the event you reside in a 230Vac nation, this trick won’t work. In actual fact, I’d be afraid of making an attempt it myself, as I would favor to provide every financial institution of output tubes its personal unbiased heater winding, because the cathodes will transfer up and down tons of volts. By the way in which, if thirty tubes had been used, the amplifier could possibly be named a


One final level to be made is that the output transformer makes use of all the secondary to drive the loudspeaker; we have to use a low-winding-ratio output transformer, due to the low impedance offered by the various parallel output tubes. As well as, the output tubes could possibly be wired as pentodes or triodes and even in an ultra-linear association, as I described in weblog 139.

Subsequent Time
I’ve a number of new PCBs to explain, however they may have wait till subsequent time. Within the meantime, extra energy to the loudspeakers, proper on!


// JRB





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E-mail from GlassWare Prospects

Hello John,

I acquired the Aikido PCB at present – thanks for the primary fee delivery

Wished to let you recognize that that is merely one of the best PCB I’ve had in my fingers, bar none. The standard is fabulous, and your documentation is great. I do know you do that since you love audio, however I feel your worth of $39 is a little bit of a giveaway! I am positive you could possibly cost double and nonetheless have blissful prospects.

Trying ahead to constructing the Aikido, will ship some feedback after I’m performed!

Thanks, regards,


Mr Broskie,

I purchased an Aikido stereo linestage package from you some days in the past, and I acquired it simply this Monday. I’ve a number of issues to say about it. Firstly, I am extraordinarily impressed on the high quality of what I have been despatched. In actual fact, that is the very best high quality package I’ve seen wherever, of something. I do not know the way you managed to suit all these items in underneath what I paid for it. Second, your delivery was lightning-quick. Simply extra satisfaction within the bag, there. I want everybody did enterprise such as you.

Sean H.



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