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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…..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.
‭ ‬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 ‬5687‭ ‬biased at‭ ‬12mA per triode,‭ ‬210‭ ‬V to anode.‭ The input transformer T1 is the excellent Lundahl LL1676 in 1:1 configuration. Multiturn trimmer on tube bias ( TB ) allows to perfect balance the stage. For GS gate stopper resistors use carbon composite type: any value from 220 and 1kohm is OK.
‬In the output stage‭ ‬I choose the Exicon‭ ‬ECX10N20R N channel lateral mosfet,‭ ‬high performance device designed specifically for linear audio amplifier. ‬It‭’‬s biased at‭ ‬1A of current and‭ ‬38‭ ‬V drain-source voltage.‭ ‬Two device per amplifier are needed and they must be well matched for Vgs.‭ ‬This reduce the output offset and distortion.
‭ ‬For the power supply‭ ‬in the stereo amplifier‭ ‬are needed four‭ ‬independent‭ ‬( floating‭) ‬units‭ ‬able to give‭ ‬38‭ ‬V.‭ ‬I use two toroidal power transformer‭ ( ‬one per channel‭) ‬with‭ ‬two‭ ‬independent‭ ‬secondary‭ ‬at‭ ‬30‭ ‬V,‭ ‬112,5‭ ‬VA,‭ ‬Schottky‭ ‬diode as rectifier‭ ‬and‭ ‬excellent‭ ‬Siemens Sikorel 3.300 uF/63V capacitors‭ ( ‬36‭ ‬total capacitors in the stereo amplifier‭ ‬).‭
In the PS tube stage I use an R-Core power transformer with‭ ‬260+260V at‭ ‬100mA‭ ‬ for‭ ‬5687‭ ‬anode voltage‭ ‬,‭ ‬6,3‭ ‬V at‭ ‬2‭ ‬A‭ ‬for‭ ‬5687‭ ‬filament‭ ‬,‭ ‬5‭ ‬V at‭ ‬3‭ ‬A‭ ‬for GZ34‭ ‬rectifier,‭ ‬6,3‭ ‬V at‭ ‬0,1‭ ‬A‭ ‬for bias power mosfet‭ ( MB ),‭ ‬12‭ ‬V‭ ‬at‭ ‬0,1‭ ‬A for‭ ‬negative‭ ‬bias‭ ‬of‭ ‬5687 ( TB ).‭ ‬5687‭ ‬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.
This amplifier run in class A. Put a DC voltmeter across 1 ohm/20W resistor and rotate the multiturn trimmer P1 for 1V reading ( 1A current ). Adjust P2 multiturn trimmer for minimum VDC output terminal blocks.
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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