Analog output stage design

Analog stage, outputs and amplification

Answers on this topic

• Should I use balanced or single-ended headphone drive, and why are Chord headphone outputs single-ended?
• Why does Rob use a digital DC servo instead of coupling capacitors, and is there anything worth modding?

Everything he said on this topic (177)

One-line summaries in our words, oldest first. Each link opens the original.

• 2014-04-15Watts rejects calling Hugo a DAC with a good headphone amp, defines transparency against live music, recounts Hugo's development, and explains its single-stage output, 0.075 ohm impedance and tap length. Head-Fi, Apr 2014
• 2014-05-21Watts contrasts four active stages in a conventional DAC with Hugo's single I/V stage with discrete output, and four passive parts in the signal path, enabled by 2048x oversampling. Head-Fi, May 2014
• 2014-07-26Watts says Hugo outputs up to 5V RMS with a 3V preset, advises dropping the pre-amp, and describes damping-factor tests and Hugo's 0.075 ohm output impedance. Head-Fi, Jul 2014
• 2014-08-20Watts explains chip DACs are differential for substrate and reference noise reasons, while Hugo is fully single-ended with one output stage, so balanced adds complexity. Head-Fi, Aug 2014
• 2014-09-08Watts says 6 ohm 86dB speakers are too much for Hugo's 0.5A, recommends Hugo over Qute for line use, and describes its Class A discrete hybrid output stage. Head-Fi, Sep 2014
• 2014-10-20Watts says Hugo's outputs share one 1W power amp, its only active stage, and that a more powerful 20W-into-8-ohm version of Hugo, from a ten-year project, is being prototyped. Head-Fi, Oct 2014
• 2014-11-13Watts says Hugo's outputs are connected internally to one stage combining I-to-V and headphone drive, unlike a conventional DAC's several stages, so there is no fixed DAC output to buffer. Head-Fi, Nov 2014
• 2015-01-28Watts says the impedance may vary by a few milliohms with PCB tracking, measured by loading and unloading the headphone output while reading voltage at the RCA. needs context Head-Fi, Jan 2015
• 2015-02-16Watts contrasts conventional DACs' four active stages with Hugo's single feedback stage using a fast Class A discrete output, leaving two resistors and two capacitors in the signal path. Head-Fi, Feb 2015
• 2015-04-20Watts explains Hugo's discrete Class A output stage, his path from 1989 PDM DACs to the 16-element pulse array, its jitter immunity, time-domain reconstruction, and R2R's flaws. Head-Fi, Apr 2015
• 2015-05-18Watts says DAVE's output stage is a second-order analog noise shaper with negligible distortion change into 33 ohms and no rising HF distortion, keeping one global feedback path. Head-Fi, May 2015
• 2015-06-06Watts says the unit is not class D: like Hugo, a single-stage amplifier drives line and headphone outputs, giving 6v RMS with the same 0.5A RMS current as Hugo. needs context Head-Fi, Jun 2015
• 2015-07-19Watts explains why balanced amplifiers suffer common-mode non-linearity and RF decoupling problems, says balanced mode flattened depth on his amp, but suggests trying balanced if SE is noisy. Head-Fi, Jul 2015
• 2015-12-30Watts says Dave's output stage is a second-order analog noise shaper, so distortion barely changes with current; he suspects it helped HF linearity; output is 6.8 V into 33 and 300 ohms. Head-Fi, Dec 2015
• 2016-01-29Watts explains that Hugo's sound hardened into low-impedance headphones, and that treating the output stage as a second-order analog noise shaper with nested feedback removed the distortion change at 32 ohms. article, Jan 2016
• 2016-01-29Watts explains that the nested loop sits inside the gain-setting global loop, giving local feedback that reduces output-stage distortion before global feedback acts. article, Jan 2016
• 2016-05-24Watts says all his DACs replace an op-amp's output stage with a discrete Class A stage inside one feedback loop, and DAVE adds a second-order analog noise shaper. Head-Fi, May 2016
• 2016-06-17Watts says DAVE is fully DC coupled with no coupling capacitors, using a digitally filtered DC servo, with only small polypropylene filter capacitors in the signal path. Head-Fi, Jun 2016
• 2016-06-17Watts compares electrolytic, tantalum and film coupling capacitors and describes DAVE's digital DC servo, which stores its trim in flash and causes a 20-second turn-on delay. Head-Fi, Jun 2016
• 2016-06-28Watts says DAVE's output stage and second-order noise shaper came from his digital power amp, a more powerful DAVE whose output is linear, not Class D switching. Head-Fi, Jun 2016
• 2016-07-24Watts says Hugo's sound hardened slightly into very low impedance headphones, with higher-order harmonics measurable; since an analog amplifier is a first-order noise shaper, he took a second-order approach. Hifistatement 0
• 2016-07-24Watts says Cordell's 1980s nested feedback loops failed on stability because they were treated as amplifiers; treating the stage as a second-order noise shaper let him solve stability easily. Hifistatement 43
• 2016-07-24Watts says the analog noise shaper output stage removed the distortion problem: into 33 ohms only second harmonic rises slightly, with one global feedback path and minimal analog filtering. Hifistatement 1:30
• 2016-07-24Watts says this means very simple analog circuitry. needs context Hifistatement 2:17
• 2016-07-24Watts explains that a local feedback loop corrects output-stage distortion inside one global feedback loop from the DAC's current output, which sets the gain, so the stage behaves as distortion-free. Hifistatement 2:27
• 2016-07-24Watts likens the output stage to a noise shaper with one global feedback path, and says the music envelope modulates the output flip-flops' reference power, changing the gain. Hifistatement 3:20
• 2016-07-24Watts says reference modulation is not measurable but very audible; his reference has 0.5 of a milliohm impedance to each flip-flop; a digital DC servo removes coupling capacitors and offset. Hifistatement 4:13
• 2016-09-01Watts explains DAVE's output amplifier is a second-order analog noise shaper, so loading barely changes distortion and HF distortion stays low (1 GHz gain-bandwidth), citing 19k/20k test results. Head-Fi, Sep 2016
• 2016-09-07Watts says there are no plans for the change asked about, since savings would be tiny and the output stage needs its current performance even with easy loads. needs context Head-Fi, Sep 2016
• 2016-09-10Watts argues negative feedback is not harmful, attributing low-feedback amps' smoother sound to local feedback, higher bias and wider open-loop response cutting RF, HF distortion and signal-dependent delay. Head-Fi, Sep 2016
• 2016-10-21Watts says DAVE risks headphone clipping only at +5 dB volume, and a 33-ohm load does not harden its sound, owing to the second-order noise shaper and discrete output stage. Head-Fi, Oct 2016
• 2016-10-21Watts explains DAVE's 20-element pulse array feeds current into a single I-to-V converter rather than using voltage mode, and why keeping the node and reference voltage noise-free was hard. Head-Fi, Oct 2016
• 2016-12-11Watts says a 33-ohm load barely changes DAVE's THD+N (-127 to -124 dB), and argues a wider soundstage usually signals lost depth and transparency, except where recordings encode out-of-the-box cues. Head-Fi, Dec 2016
• 2016-12-12Watts says DAVE drives the 46-ohm Abyss linearly up to +4 dB, explains how its 0.055-ohm output impedance tightens bass and cuts load distortion, and suggests EQ rather than distortion. Head-Fi, Dec 2016
• 2017-02-20Watts explains how rail currents through PSU impedance and limited PSRR cause distortion and bass smear, why DAVE uses milliohm supplies and high PSRR, and why big supplies miss this. Head-Fi, Feb 2017
• 2017-03-15Watts says Hugo 2 noise is mostly resistor thermal noise, so units match within 0.2 dB; halving it would raise distortion 16 times, and he hears no warm-up or break-in. Head-Fi, Mar 2017
• 2017-03-26Watts says Hugo 2 needs no burn-in beyond its first full charge, brain break-in takes about a month, and it lacks the caps, RF and bias effects behind break-in. Head-Fi, Mar 2017
• 2017-06-18Watts says Hugo 2's digital DC servo keeps bass flat with crossfeed off, and APx555 peak measurements show 20 Hz to 1 kHz flat within 0.001 dB; DAVE is identical. Head-Fi, Jun 2017
• 2017-07-10Watts says open loop output impedance governs load response, now +/- 0.1 dB at 8 ohms and +/- 0.05 dB unloaded; measuring must allow for 34 mS WTA latency. Head-Fi, Jul 2017
• 2017-07-19Watts explains how coupling capacitors, reference decoupling and LF distortion add bloom, why Hugo 2 avoids them by tackling noise floor modulation, and why its warmer filter options exist. Head-Fi, Jul 2017
• 2017-07-31Watts agrees, saying the output stage current limiters are set below the batteries' own current limiters, which also have built-in current and thermal protection. needs context Head-Fi, Jul 2017
• 2017-09-06Watts says a summing Y connector made of a resistor pair mixing stereo to mono would be fine, because Hugo 2 and DAVE have very low output impedance. Head-Fi, Sep 2017
• 2017-10-23Watts says Hugo 2's battery is not bypassed; a regulated boost PSU replaced Hugo's super caps, cutting PSU distortion 10,000 times, and a linear PSU is inaudible with headphones. Head-Fi, Oct 2017
• 2017-12-05Watts says the analog stage matters as much as the digital, and R2R and DSD DACs need a lot of post filtering. RMAF 45:30
• 2017-12-05Watts says each added resistor degrades depth and detail; pulse array needs two resistors, two capacitors and one op amp, versus four op-amps and twenty or thirty parts normally. RMAF 45:46
• 2017-12-05Watts says pulse array's lower noise avoids input stage overload and NFM, Hugo 2 and DAVE use an analog noise shaper amplifier, and audiophile design swung from measurement to listening. RMAF 46:31
• 2017-12-05Watts shows a jitter test with no visible jitter, says Mojo nearly matches DAVE, and credits DAVE's low 19/20 kHz two-tone distortion to its second-order analog noise shaper amplifier. RMAF 50:44
• 2017-12-05Watts says amplifiers lack high-frequency feedback but his system gives intermodulation below minus 150 dB, versus minus 110 dB in some DACs that reviewers still praise for brand reasons. RMAF 51:32
• 2018-01-10Watts explains the boost-voltage and linear-regulator supply that isolates the amplifier gain section from the output stage in Hugo 2 and Qutest, and the digital DC servo replacing coupling capacitors. Head-Fi, Jan 2018
• 2018-01-10Watts says 2Qute's discrete stage was never meant for low impedances; Qutest uses the Mojo output stage with more drive but is a DAC only, its impedance unmeasured. Head-Fi, Jan 2018
• 2018-01-13Watts says super caps would not help Dave, defends Hugo 2's output stage, and says Dave below +4 and Hugo 2 below white never clip with headphones. Head-Fi, Jan 2018
• 2018-01-19Watts reports measuring Qutest's output impedance at 0.042 ohms at the phono sockets at 1kHz and 20Hz, with no coupling capacitor thanks to the digital DC servo. Head-Fi, Jan 2018
• 2018-01-30Watts lists ways amps add bass bloom (2nd harmonic, low damping, coupling-cap dielectric absorption), says Hugo 2 avoids all three, and advises EQ or better headphones instead. Head-Fi, Jan 2018
• 2018-02-17Watts shares Hugo 2 measurement slides from his CanJam NYC talk, explaining low output impedance, load-independent distortion, no measurable noise floor modulation or jitter, and APx555 methods. Head-Fi, Feb 2018
• 2018-02-17Watts says testing would need a headphone with amplitude-dependent impedance, and would only confirm that lower amplifier output impedance directly produces lower load-induced distortion. Head-Fi, Feb 2018
• 2018-02-17Watts says Hugo 2's output impedance is 0.025 ohms, and load-induced distortion depends on load impedance change against amp output impedance, zero for a zero-ohm amp. Head-Fi, Feb 2018
• 2018-02-19Watts explains his discrete single-ended DAC combines I-to-V, filtering and headphone drive in one amplifier with two resistors and two capacitors; DAVE's balanced outputs add an inverting amplifier. Head-Fi, Feb 2018
• 2018-03-13Watts says he rejected auto switchover because it lost nearly 20 seconds of music while the DC servo reset DC to zero; a 100 mS target failed. needs context Head-Fi, Mar 2018
• 2018-03-22Watts explains the 2.75dB WTA overload margin, delays audio in JRiver to keep 1M taps with video, describes visible noise floor modulation, and gives DAVE and Hugo 2 level limits. Head-Fi, Mar 2018
• 2018-03-31Watts says his DACs use a single-ended output of one amplifier and current-to-voltage converter driving headphones directly, simple because no complex filtering is needed, hence transparent. article, Mar 2018
• 2018-04-01Watts says capacitive loading is fine and will not cause instability, as a large loading capacitor is already part of the RF filter structure. needs context Head-Fi, Apr 2018
• 2018-05-11Watts says TT2 measures like Hugo 2 in DAC mode, gives 18W balanced and 7W SE without degradation, describes its supercap PSU, and denies working on a Dave 2. Head-Fi, May 2018
• 2018-05-12Watts lists TT2 output options (DAC mode 2.5V RCA/5V XLR, low gain 3V, volume at -3dB) and recounts how the DX amp's output stage was adopted in TT2. Head-Fi, May 2018
• 2018-05-13Watts posts his Munich TT2 presentation: larger FPGA, a noise shaper 100 times better than Hugo 2's, supercap PSU, output stage from the power pulse array project, measurements. Head-Fi, May 2018
• 2018-05-14Watts says the supercaps give the output stage ample current; it was upgraded after the TT was found used with efficient loudspeakers, so TT2 can drive them directly. HIFICLUBE.NET JVH 50
• 2018-05-16Watts says TT2's balanced negative output uses the same amplifier as its single-ended output, so the balanced output is low distortion and simple, unlike Dave's. Head-Fi, May 2018
• 2018-05-17Watts says he finished TT2's output stage optimization for production, measuring 0.000195% THD at 20Hz unchanged with an 8 ohm load at 12W. Head-Fi, May 2018
• 2018-05-17Watts says all mains-powered DAC/amps have isolation, USB sounds very close to optical, he is amazed 2Qute can drive a loudspeaker, and he explains his single-stage output amplifier. Head-Fi, May 2018
• 2018-05-18Watts says his DACs have no internal pre-amp but a single amplification stage, and removing a pre-amp always improves transparency, though pre-amps add bass softness and bloom some prefer. Head-Fi, May 2018
• 2018-05-20Watts explains TT2's 18V RMS balanced output, estimates power into 6 ohms, describes the supercaps and second-order analog noise shaper benefits, and says the output is protected. Head-Fi, May 2018
• 2018-05-22Watts says Dave's negative output uses an op-amp for power amps only, while TT2's uses its high-power discrete stage for speakers and headphones, and suggests XLR-terminated headphone cables. Head-Fi, May 2018
• 2018-05-22Watts explains TT2's digital DC servo with no coupling caps, confirms separate crossfeed for headphone and amp/DAC modes, and says -3dB volume equals 3V RMS as on Dave. Head-Fi, May 2018
• 2018-05-24Watts says volume-matched TT2 beats Hugo 2 in depth, detail and separation, he uses only SE outputs on his 88dB speakers, and 20 Hz distortion ignores load. Head-Fi, May 2018
• 2018-05-25Watts describes his power pulse array amplifiers in development (200 watts to a kilowatt, single global feedback) and says TT2 uses their output stage, delivering five amps into headphones. Darko Audio 2:10
• 2018-08-12Watts says the M scaler makes compression losses audible, audiophile BNC cables sound same or worse, TT2 measures identically in Class B, and the units stack. Head-Fi, Aug 2018
• 2018-08-27Watts explains he avoids voltage mode DACs because output voltage modulates switching delays, giving about 0.03% THD and HF noise; his single stage combines I to V and drive. Head-Fi, Aug 2018
• 2018-08-28Watts explains he uses no coupling caps but a digital DC servo trimming the noise shaper, which takes 16 seconds to stabilize, hence the rainbow start-up and Dave's turn-on delay. Head-Fi, Aug 2018
• 2018-09-03Watts says TT2 outperforms Class AB, showing no crossover distortion at 12W into 8 ohms thanks to its second-order analog noise shaper, and likens Chord's Etude feedforward amp. Head-Fi, Sep 2018
• 2018-10-03Watts explains TT2's single-stage SE output amplifier, the relay-switched resistor that sets low gain, and the inverted stage that creates the balanced negative output. Head-Fi, Oct 2018
• 2018-11-11Watts says Qutest uses the same output stage as Mojo, so shorting it is a bad idea, and its powerful discrete stage gives lower distortion even into amplifier inputs. Head-Fi, Nov 2018
• 2018-11-11Watts says Hugo 2 is designed for negligible distortion change into low impedances, while Qutest driving amp inputs stays in Class A and needs no second-order analog noise shaper. Head-Fi, Nov 2018
• 2018-11-30Watts says there is no maximum XLR cable length since TT2's output impedance is so low, but any extra interconnect length degrades transparency. Head-Fi, Nov 2018
• 2018-12-12Watts says simpler analog improves depth and detail; the pulse array's 104 MHz rate gives noise shaper resolution and allows an output of one amplifier, two resistors and two capacitors. Pursuit Perfect System 30:58
• 2018-12-12Watts says chip DACs need differential I-to-V stages to cancel substrate noise, four or five amplifiers in all, while his FPGA and discrete parts let him run single-ended. Pursuit Perfect System 31:42
• 2018-12-12Watts says a conventional DAC uses four amplifiers versus his one, says TT2 has a 10-element pulse array, and explains elements as ten offset 1-bit streams whose clock edges cancel. Pursuit Perfect System 32:30
• 2018-12-12Watts says that if a DAC's output drive is poor, the DAC will be poor. Pursuit Perfect System 36:33
• 2018-12-12Watts says with a 20 hertz signal, higher-order harmonics vanish by 3 kHz on DAVE and his other DACs, at 12 W into 8 ohms balanced, unlike other power amplifiers. Pursuit Perfect System 42:02
• 2018-12-12Watts says Class B crossover distortion creates audible, brightening high-frequency harmonics; his second-order analog noise shaper output stage removes it, a topology used in the new Etude and reference amplifiers. Pursuit Perfect System 42:45
• 2018-12-12Watts says the topology uses a secondary feedback loop that linearizes the output stage and that the measurement proves the concept. Pursuit Perfect System 43:29
• 2018-12-16Watts explains output impedance matters for frequency response and back-EMF damping, recounts damping experiments from 35 years ago, and says damping still matters with planar headphones. Head-Fi, Dec 2018
• 2019-01-23Watts says TT2's amp mode and headphone mode use identical hardware apart from connectors and FPGA configuration, so output is the same with identical settings. Head-Fi, Jan 2019
• 2019-02-05Watts says the output is a discrete class A stage with one feedback loop; the OP07Cs only integrate the digital DC servo, which takes 16 seconds during the rainbow display. Head-Fi, Feb 2019
• 2019-02-05Watts says Qutest has no coupling capacitors; it is fully DC coupled with a digital DC servo, and the capacitors are 200 V C0G parts for the analog filter. Head-Fi, Feb 2019
• 2019-03-13Watts lists TT2's advantages over Qutest: better noise shapers, DAVE reference circuitry, a second-order analog noise shaper, and a far simpler path than Qutest plus headphone amp. Head-Fi, Mar 2019
• 2019-03-17Watts cites a simple but poor TI class D amp and Einstein on simplicity; his DACs use one amp and four passive parts, as simple as possible without degrading performance. Head-Fi, Mar 2019
• 2019-03-18Watts explains TT2's output amplifier as a second-order analog noise shaper replacing a nested feedback loop, giving huge HF gain and no distortion above 3 kHz. Head-Fi, Mar 2019
• 2019-06-11Watts says TT2 follows the same analog philosophy as his other DACs: one global feedback linear amplifier with two resistors and two capacitors in the signal path. Head-Fi, Jun 2019
• 2019-06-14Watts says TT2 can output 10 A in SPICE modeling for short bursts, but peak current is meaningless; current linearity, with distortion stable into low loads, matters. Head-Fi, Jun 2019
• 2019-07-03Watts says TT2's negative output uses the same output stage design as the positive or SE outputs, so the 5 A peak applies to both. Head-Fi, Jul 2019
• 2019-09-20Watts says Qutest uses Mojo's Class A stage for easy loads, while Hugo 2's noise shaper output is crossover-distortion free into 33 ohms, with 120 dB feedback at 20 kHz. Head-Fi, Sep 2019
• 2019-10-03Watts explains clipping versus distortion rising with current, says TT2 avoids crossover distortion and current limiters, drives his 803 D3, and display noise is LED multiplexing. Head-Fi, Oct 2019
• 2019-10-07Watts says noise shaper activity must be signal independent so pulse array jitter noise stays fixed, and that his 2nd-order analog noise shaper amplifier eliminates crossover distortion in Class B. Head-Fi, Oct 2019
• 2019-10-09Watts says DAC mode is identical to amp mode at -5L, one amplifier does I-to-V, filtering and buffering, and gain changes mute 16 seconds while the DC servo settles. Head-Fi, Oct 2019
• 2019-10-14Watts confirms Mojo uses 100uF tantalum coupling caps while his other DACs use a digital DC servo, says tantalums add softness and bloom, and lists three analog design rules. Head-Fi, Oct 2019
• 2019-10-16Watts says ground-plane currents cause noise floor modulation, battery Hugo 2 into headphones showed no optical-USB difference, and his M scaler's dual galvanic isolation makes source noise a non-issue. Head-Fi, Oct 2019
• 2019-11-18Watts says Hugo 2 gives 1050 mW into 8 ohms against about 750mW for the original, and stays more linear into 33 ohms, sounding smoother and warmer. Head-Fi, Nov 2019
• 2019-12-06Watts says TID and SID are non-issues with digital sources, and explains the analog noise shaper amp eliminates crossover distortion, citing TT2 and DAVE plots and gain bandwidth figures. Head-Fi, Dec 2019
• 2020-01-06Watts says Qutest drives any amp input above 100 ohms, but an unpowered connected amp's clamp diodes may degrade linearity, so compare on, off and disconnected. Head-Fi, Jan 2020
• 2020-01-18Watts says Hugo 2 is Class A into 300 ohms at 5v RMS, AB into lower loads, and explains DSD volume needs another noise shaper because DSD is 1-bit PCM. Head-Fi, Jan 2020
• 2020-01-20Watts explains isolating output-stage supply currents, shown in TT2's distortion plot, says delay follows tap length and oversampling, describes the M scaler's asymmetric video filter, and heard no USB-optical difference. Head-Fi, Jan 2020
• 2020-04-11Watts says Hugo 2 is class A into 300 ohms, TT2 is biased an order higher with 30nS transistors, and TT2 shows no crossover or HF distortion at 12W. Head-Fi, Apr 2020
• 2020-04-12Watts says TT2 is Class A into 30 ohms, Class AB by 8 ohms, explains why bias matters despite the analog noise shaper, and tunes it for 4 ohms up. Head-Fi, Apr 2020
• 2020-06-25Watts says Class B crossover distortion yields very audible high-order harmonics, but a Class AB stage can eliminate it, as his second-order amplification does on Hugo 2, TT2 and DAVE. Part-Time Audiophile 19:19
• 2020-06-25Watts says an inner amplifier linearizes the output stage inside one global feedback loop; TT2 plots show no distortion above 3 kHz for 20 Hz, 12 watts into 8 ohms. Part-Time Audiophile 20:05
• 2020-06-25Watts cites no harmonic distortion with noise at -180 dB, and says audiophiles think simplistically about Class A versus B when what is needed is a linear output stage. Part-Time Audiophile 20:57
• 2020-07-23Watts says TT2 is not Class D, which would destroy transient timing; its single amplifier has one global feedback path, a second-order analog noise shaper and fast discrete transistors. Head-Fi, Jul 2020
• 2020-08-17Watts says Hugo 2 has no signal-path capacitors to upgrade thanks to the DC servo, and its noise shaper output and separate rails give 214 dB PSU rejection in SPICE. Head-Fi, Aug 2020
• 2020-09-28Watts says Hugo 2's feedback reference is near the headphone outputs, so the RCA output has 3 milliohms extra, unlikely audible with headphones; connectors are a likelier cause. Head-Fi, Sep 2020
• 2020-11-23Watts compares Hugo 2's out-of-band noise with the APx555's 192kHz DAC, explains the 104MHz noise shaper hump, and why gentle analog filtering needs only 4 passive components. Head-Fi, Nov 2020
• 2021-03-05Watts explains TT2 draws only DC from the brick because a large inductor isolates the supercaps, keeping audio currents and PSRR distortion out; big toroids are not inherently better. Head-Fi, Mar 2021
• 2021-06-04Watts says metal-to-metal interfaces attenuate small signals, his analog noise shaper gives TT2 20GHz and Dave 0.5THz gain bandwidth, and the pulse array creates no noise floor modulation. Head-Fi, Jun 2021
• 2021-08-26Watts says TT2 in high or low gain has no significant DC output thanks to the digital DC servo, so it is safe with a DC-coupled power amp. Head-Fi, Aug 2021
• 2021-09-28Watts says Qutest shares Mojo's discrete output stage, pure Class A into 300 ohms or more; on Hugo 2, TT2 and DAVE a second-order analog noise shaper eliminates crossover distortion. Head-Fi, Sep 2021
• 2021-09-28Watts says the linearizing amp sits inside the global feedback loop, so for transparency all his DACs' direct signal path is equivalent to two resistors and two capacitors. Head-Fi, Sep 2021
• 2021-09-28Watts says loads below 300 ohms give Qutest some small measurable crossover distortion; Qutest was designed to drive amplifiers, so use Hugo 2 for headphones. Head-Fi, Sep 2021
• 2021-10-22Watts says supplied BNC cables are starters; 2GHz ferrites, Wave cables or battery power cure ground currents; Qutest uses Mojo's output stage; M Scaler was developed with TT2 and DAVE. Head-Fi, Oct 2021
• 2022-01-31Watts says the analog stage is nearly unchanged; the supply changed so Mojo 2 runs without the battery, which original Mojo needed to keep supply noise from the amplifier. Darko.Audio 13:48
• 2022-01-31Watts says Mojo 2 replaces the coupling capacitor with Hugo 2's DC servo, losing Mojo's slightly soft, warm sound but becoming fundamentally more transparent. Darko.Audio 34:59
• 2022-01-31Watts says the extra 200 milliwatts funded the digital DC servo, a better WTA filter and noise shaper, and the EQ DSP core; the noise shaper improves depth and detail. Darko.Audio 38:46
• 2022-01-31Watts presents Mojo 2: coupling capacitor removed, new noise shaper, and a UHD DSP EQ he says passes -301dB amplitude, phase and NFM transparency tests that 64-bit DSP fails. Head-Fi, Jan 2022
• 2022-01-31Watts says Mojo 2 auto-offs after 10 minutes without inputs, stays in desktop mode while powered, trails Hugo 2, uses a digital DC servo, and improved 16FS-to-2048FS filtering. Head-Fi, Jan 2022
• 2022-02-02Watts explains Mojo 2's FPGA charger, 0.5A to 2A charging, 99.3dB SPL with the HEKV2, why coupling caps soften bass, why Mojo lacked a DC servo, and USB-C for Poly. Head-Fi, Feb 2022
• 2022-02-12Watts says Mojo and Mojo 2 measure identically, but Mojo's warmth came from coupling-capacitor distortion removed by the DC servo; a bass shelf boost restores it. Head-Fi, Feb 2022
• 2022-02-12Watts explains why his digital DC servo avoids analog servos' capacitor sound, says switching it off on DAVE changed nothing, and that Poly is not his project. Head-Fi, Feb 2022
• 2022-03-19Watts says discrete conversion allows single-ended operation, his noise shapers resolve small signals a billion times better than chips, and running at 104 MHz permits minimal analog filtering. MFG Audio Reviews 14:03
• 2022-03-19Watts says the analog section has one amplifier and four components, ten times simpler than a conventional DAC, giving better transparency, and calls transient timing the top problem. MFG Audio Reviews 14:49
• 2022-03-19Watts says Chord wanted Mojo's coupling capacitor replaced by a DC servo as on Hugo 2, but the capacitor gave Mojo its warm, soft sound, so warmth needed restoring. MFG Audio Reviews 26:14
• 2022-03-19Watts says Chord insisted only on the same size and Poly compatibility; Mojo 2 is DC coupled via a digital DC servo, and the original Mojo had 38,000 taps. MFG Audio Reviews 27:42
• 2022-03-29Watts explains why valve output or input stages would worsen noise floor modulation and RF decoupling, recounts a zero ohm resistor problem, and attributes valve warmth to distortions. Head-Fi, Mar 2022
• 2022-04-04Watts says high current and low output impedance let two headphones share the output, Mojo 2 uses 98% of the FPGA versus 33%, and he avoids trivial updates. Head-Fi, Apr 2022
• 2022-04-26Watts says he and John Franks wanted to add crossfeed, which he always uses with headphones, and remove the single coupling capacitor after the pulse array, which adds warmth. Audio Force 5:22
• 2022-05-18Watts says Mojo 2 has no separate output stage: the discrete output stage sits inside a single amplifier with one feedback path, so nothing is bypassed. Head-Fi, May 2022
• 2022-08-26Watts explains TT2's balanced output (gain 5 and -1 amps), says single-ended is more transparent owing to fewer feedback parts, shows measurements, and says the pulse array is his IP. Head-Fi, Aug 2022
• 2022-09-18Watts says the Mojo 2 project started 5 years ago, and John Franks wanted to get rid of the coupling capacitor. The Source AV 33:15
• 2022-09-18Watts says removing the coupling capacitor makes bass tighter, more extended and deeper, but the capacitor gave Mojo 1 its warmth. The Source AV 33:30
• 2022-11-25Watts says Mojo 2 has the digital DC servo introduced with DAVE in 2015 plus DC protection on its relays, calls the reported fault unique, and wants the unit. Head-Fi, Nov 2022
• 2023-01-03Watts says TT2's XLR outputs trip thermally into 8 ohm speakers, leaving the 7W single-ended outputs, and Hugo 2's second-order noise-shaper amp eliminates crossover distortion. Head-Fi, Jan 2023
• 2023-01-04Watts says the headphone and RCA outputs come from the same amp via different relays, with no separate headphone amp; RCA and XLR are disabled in headphone mode. needs context Head-Fi, Jan 2023
• 2023-02-04Watts says his DACs keep analog simple and digital complex for accuracy: one amplifier, two resistors and two capacitors feed the headphones directly. Dusty Audio 1:00:42
• 2023-02-06Watts says SPICE is accurate only with good parameter data, unlike 100% accurate Verilog, and parasitics in high gain-bandwidth discrete circuits make op-amps the safer choice. Head-Fi, Feb 2023
• 2023-02-26Watts says more voltage comes from amp gain, more elements raise I/V current if the PSU and output stage allow, and Stealth gets ample volume from DAVE or Hugo 2. Head-Fi, Feb 2023
• 2023-03-28Watts says pulse array permits a very simple analog section; he pairs complex digital with simple analog because each metal-to-metal contact degrades small signals and depth. The Headphone Show 48:23
• 2023-03-28Watts says his signal path has two resistors and two capacitors thanks to low noise at 1 MHz, and is single-ended with one amplifier driving headphones, unlike differential DACs. The Headphone Show 49:35
• 2023-03-28Watts says his output amp, a second-order analog noise shaper, removes high-frequency and crossover distortion; loading a TT2 from 300 to 8 ohms at 9 volts changes only second harmonic. The Headphone Show 50:19
• 2023-04-03Watts says improving Mojo meant adding crossfeed, essential for headphones, and moving toward Hugo 2's sound by going DC coupled, dropping the coupling capacitor that gave Mojo its warmth. Amplify Audio 10:23
• 2023-06-19Watts says the 16-second start-up rainbow lets the digital DC servo trim DC to microvolts before the output relays close, and also shows the last settings. Head-Fi, Jun 2023
• 2023-06-21Watts breaks down background noise sources and Mojo 2's trade-offs (2.2 nV/rtHz, 5.2 V RMS), warns attenuators harm sound, and cites 4.4 mm connector cost. Head-Fi, Jun 2023
• 2023-08-06Watts says Class AB is fine with his second-order analog noise shaper output, and compares Class G and H with Class D, which creates transient timing errors. Head-Fi, Aug 2023
• 2023-08-23Watts says the output comes from one amplifier that filters, performs the DAC's I to V conversion, and drives all headphones and efficient loudspeakers. Head-Fi, Aug 2023
• 2023-10-01Watts says output-stage bias must not drift with heat, notes he began as an analog designer, self-taught in digital, and says the DACs sound the same at switch-on and later. The Source AV 47:35
• 2024-02-05Watts warns that replacing op-amps risks instability in the second-order analog noise shaper, which made two op-amps sound identical by raising gain-bandwidth from 40MHz to 80GHz. Head-Fi, Feb 2024
• 2024-03-20Watts says all coupling capacitors and analog DC servos affect sound, and his digital DC servo instead removes DC offsets digitally, though the trim takes a long time. MFG Audio Reviews 52:00
• 2024-03-20Watts says Mojo 2 takes 16 seconds after power-on to null its DC trim, and ferrite-core inductors are nasty because inductance and phase vary with signal amplitude. MFG Audio Reviews 52:45
• 2024-08-01Watts says differential chip DACs need complex analog stages, whereas in his designs the FPGA only makes data and discrete parts on a separate supply do conversion, avoiding digital noise. Audiophilesdoteu 2:13
• 2024-08-01Watts says his analog section is one single-ended amplifier doing I to V and output, and his 104 MHz noise shaper, versus 3 MHz delta-sigma, eases filtering. Audiophilesdoteu 3:07
• 2024-11-30Watts says substrate noise matters less in op-amps than DAC chips, and explains noise floor modulation as RF intermodulation in the differential input stage, reduced by local decoupling. Head-Fi, Nov 2024
• 2024-12-01Watts says op-amps change sound through distortion and gain bandwidth, not substrate noise; he uses discrete output stages, and his second-order analog noise shaper made two op-amps sound identical. Head-Fi, Dec 2024
• 2025-01-13Watts compares feed-forward correction (about 20dB) with his second-order analog noise shaper (90dB), showing TT2 into 8 ohms, and links crossover distortion to amplitude-dependent transient delay. Head-Fi, Jan 2025
• 2025-01-13Watts lists the reference supply, pulse array flip-flops, gain stage and output stage as distortion sources, and says a 20Hz 12W TT2 test shows crossover distortion eliminated. Head-Fi, Jan 2025
• 2025-03-27Watts says TT2's low-gain resistor and relay sit in parallel with the feedback resistor, because series contacts and solder joints degrade depth, detail and transparency while parallel ones do not. Head-Fi, Mar 2025
• 2026-10-02Watts says balanced cancels second harmonics, but his very linear output stages have none, so balanced gives only losses; headphones cannot tell balanced from single-ended. HiFi Buys 1:09:22
• 2026-10-02Watts says single-ended gives better transparency if common-mode noise and output-stage distortion are low, but DAC topology matters more: a DAC modulates a reference voltage. HiFi Buys 1:10:07
• 2026-10-02Watts says the pulse array resolves timing to 9.6 nanoseconds, and analog filtering in the amplifier and I/V decoupling, outside the feedback loop, turns this into an unsampled linear signal. HiFi Buys 1:47:22
• 2026-10-02Watts says analog filtering from 10 MHz to hundreds of megahertz recovers the signal. needs context HiFi Buys 1:48:11
• 2026-10-11Watts adds the output stages (transcribed 'animal', likely analog) to what he designed. needs context Distorsiuni Interioare 25:50
• 2026-10-14Watts says talk of negative feedback is not entirely accurate; all his DACs except Mojo 2 use second order noise shapers giving 120 dB of feedback at 20 kHz. Part-Time Audiophile 1:00:59
• 2026-10-14Watts says this removes crossover and HF distortion; whether feedback is bad depends on open loop performance and feedback factor, so he pairs excellent open loop with high HF feedback. Part-Time Audiophile 1:01:45