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Falstaff Hybrid Circlotron Power Amplifier

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There has been considerable debate over the characteristics of tubes versus transistors…..
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The goal of this hybrid power amplifier is to take advantage of best performance of both technologies tubes and solid state device.‭ ‬Tube is highly linear without negative feedback,‭ ‬smooth clipping,‭ ‬wider dynamic range,‭ ‬characteristics highly independent of temperature,‭ ‬and so on‭…‬but its disadvantages such high-impedance devices that usually need a matching transformer for low impedance speakers.‭ ‬I prefer to use active devices in places where they operate the most linear.‭ ‬For voltage amplification,‭ ‬I rate tubes highest because they have a lot of headroom‭ (‬if the operating points and bias are correct‭) ‬and a long straight usable working range.‭ ‬In the power and/or current amplification stage,‭ ‬I rate discrete power transistors best.‭With them there is no need for an output transformer and it’s easier to achieve very low output impedance to match complex speaker loads. For a hybrid amplifier, MOSFETs combine well with tubes because they need no gate current. To build this amp I was inspired to a classic Thirties-style, tube push-pull interstage transformer coupling amplifier due to his highly symmetric phase splitting that was invariant with signal level.
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‭ ‬In the output stage I‭ ‬tested various topologies and at last I chose‭ ‬Mosfet Circlotron configuration with a single device per phase biased in class A and without feedback.‭ ‬It‭’‬s preserved the fully symmetrical architecture of the amplifier as a standard push-pull‭ ‬ amps but eschews the output transformer.‭ ‬The output mosfet,‭ ‬biased in class A,‭ ‬never switch off but have a constant well-defined drain-source current,‭ ‬there are no crossover or switching distortions.‭
‬OK let‭’‬s look at first stage:‭ ‬this is a full push-pull tube stage‭ ‬. For the lowest distortion,‭ ‬the drivers need to have essentially horizontal load-lines,‭ ‬i.e.‭ ‬a very high impedance load relative to the plate impedance‭ ‬…‭ ‬preferably more than ten times higher.‭ ‬Transformer bandwidth is improved by having a low impedance on the primary.‭ ‬Tubes with low plate impedance that fit this application are the‭ ‬6h30,‭ ‬5687,‭ ‬7044,‭ ‬7119.‭ ‬After some listening test session‭ ‬I chose‭ ‬the excellent russian 6N6P ( 170 V anode at 13 mA per triode ). Take a look at the excellent anodic curves‭.
The input transformer  ( Lundahl LL1676 ) can be configured for XLR or RCA and a multiturn trimmer between the secondary coils allows to perfect balance the stage.
For the output devices‭ ‬I choose the‬ high performance Exicon‭ ‬ECX10N20R N channel lateral mosfet designed specifically for linear audio amplifier. ‬It‭’‬s biased at‭ ‬1,5 A of current and‭ ‬38‭ ‬V drain-source voltage.‭ ‬Two device per amplifier are needed and they must be well matched for Vgs for reduce the output offset and distortion. Rg gate stopper resistors are carbon composite 330 ohm type welded very close to gate. For the power supply‭ ‬in the stereo amplifier‭ ‬are needed four‭ ‬independent‭ ‬( floating‭) ‬units‭ ‬able to give‭ ‬38‭ ‬V.‭ ‬I use two 225 VA toroidal power transformer‭ ( ‬one per channel‭) ‬with‭ ‬two‭ ‬independent‭ ‬secondary‭ ‬at‭ ‬30‭ ‬V,‭ 10 A Schottky‭ ‬diode as rectifier‭ and a CLC ripple filter with Siemens Sikorel 3.300 uF/63V capacitors‭ and an 9mH-5A choke. In the follow schema each capacitor drawn actually consists of 6 Siemens Sikorel 3.300 uF/63V in parallel.
For high voltage and other stages I use a custom made power transformer with‭ ‬260+260V at‭ ‬100mA‭ ‬ for‭ ‬6N6P‭ ‬anode ‬,‭ ‬6,3‭ ‬V at‭ ‬2‭ ‬A‭ ‬for‭ ‬filaments‭ ‬,‭ ‬5‭ ‬V at‭ ‬2‭ ‬A‭ ‬for 5Y3GT‭ ‬rectifier filament,‭ ‬9 ‬V at‭ ‬0,1‭ ‬A‭ ‬for bias power mosfet‭ ,‭ ‬12‭ ‬V‭ ‬at‭ ‬0,1‭ ‬A for‭ ‬negative‭ ‬bias‭ ‬of‭ ‬6N6p.‭ ‬The ‬anode power supply is a very fine choke input type with split L-C filter after first‭ ‬47uF capacitor for best L-R separation.‭ ‬All capacitor are MKP type bypassed with‭ ‬0,1‭ ‬uF fast type.

Setup the amplifier. Power on ONLY the V2 power supply and turn the 1Kohm trimmer for 6 V output. Turn the P1 trimmer near mosfets for 1,5 V Vgs ( gate- source voltage ). Turn the P2 trimmer for perfect balance: first Mosfet Vgs must be equal to second Mosfet Vgs. At this point connect two resistors 8 ohm/20 W at the speakers terminal, apply a short circuit at the input ( RCA or XLR ), connect provisionally an ammeter in series to one of the 30 V ( Va or Vb ) power supplies and power on the amplifier ( with all power supply connected ). Gentle turn the P1 trimmer for 1,5 A. Check with a voltmeter the output offset ( across the 8 ohm resistor ) and maybe correct it with P2 trimmer to reach almost zero.

Published inAmplifier


  1. nice work, very

    I asking myself, what is the purpose of 0,47uf wodrer caps there…

    after breaking my px25a tube (accidently) im in similar project

    love and peace
    Bojan from Slovenia

    • enzo enzo

      bypass for electrolytic capacitors

  2. Anthony Anthony

    Can you give more detail on the interstage transformer. There is no mention of it in the article. Is it a another Lundahl and if so which model and configuration.

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