Saturday, October 17, 2015

Zero Negative Feedback Preamplifiers: An Audiophile Must Have?

Given the deleterious effect of negative feedback circuits on sound reproduction, would a working zero negative feedback preamplifier soon become an audiophile must have?

By: Ringo Bones

Either by first-hand listening or by reading them in hi-fi oriented publications, negative feedback circuits “suspected” deleterious effect on music signals – like loss of natural timbre, a duller less expressive performance, increased aural fatigue and missing life and energy in reproduced sound – may be the primary consequences of the application of negative feedback. And even though modern electronic industry and the availability of modern solid-state amplification would be impossible without the application of negative feedback, many seasoned audiophiles have deplored its existence since its widespread use near the end of the 1920s. And some seasoned audiophiles even reasoned that the primary raison d’être of audiophile recordings is making solid-state hi-fi gear sound like they are made of vacuum tubes. 

It is no secret that single-ended triode audio amplifiers with zero negative feed back have that magical quality of making less-than-pristine recordings – especially mainstream rock recordings – sounds as if they are made by boutique audiophile labels, literally!!! And it is also no secret that making zero negative feedback single-ended triode vacuum tube audio amplifiers as user-friendly as a typical packaged Bose Lifestyle Systems also makes them prohibitively expensive for the typical audiophile, never mind a first time one. So what would one do, would zero negative feedback vacuum tube preamplifiers provide a more economically viable option? 

Ever since the Musical Fidelity X-Pre vacuum tube preamplifier during the latter half of the 1990s, many “working-class audiophiles” saw it as a godsend of making their real-world priced audiophile solid state amplifiers  - like the Pioneer A-400 Power Amplifier – sound as if it is a single-ended vacuum tube audio amplifier, at least up to a point. But unknown to most audiophiles at the time, the vacuum tubes used on the 250 US dollar Musical Fidelity X-Pre are the same ones used on the 15,000 US dollar Conrad-Johnson ART preamplifier – the 6922 dual triode – the high-voltage Russian equivalent of the ECC88 / 6DJ8 dual triode preamplifier vacuum tube.  

While the circuit on the Musical Fidelity X-Pre is not a zero feedback preamplifier, the negative feedback used is much lower than that typically used on most mass-market solid-state audio gear that, sound quality wise, the Musical Fidelity X-Pre managed to fly rings around comparably-priced solid-state audio gear in its price range at the time. Not to mention that it also has the ability to improve a bit the duff sound quality of most mainstream pop and rock recordings of the time – albeit just a hint of what a full-blown zero negative feedback vacuum tube gear can do. 

Comprising just one stage: a true zero-feedback common cathode or anode follower preamplifier circuit employing paralleled 6922 / ECC88 / 6DJ8 dual triode vacuum tubes. And even though tube purists might argue that the choice of Conrad-Johnson to use 6922 / ECC88 / 6DJ8 dual triode vacuum tubes on their 15,000 US dollar ART preamplifier isn’t optimal, the non use of negative feedback made the Conrad-Johnson ART sound way better than the 250 US dollar Musical Fidelity X-Pre. But is there a potential to make a zero negative feedback amplifier that’s potentially better sounding that the ART and the X-Pre? 

A few months ago, I managed to DIY a common cathode / anode follower zero feedback preamplifier that can use ECC32 / CV181 or other 6SN7 type dual triode vacuum tube provided the requisite biasing voltage adjustments are made. To make such a preamplifier that uses gigantic pre World War II vacuum tubes made before Dr. Harvey C. Rentschler managed to perfect how to manufacture acorn-sized miniature vacuum tubes like the 12AX7 run as noiselessly as a modern solid-state preamplifier necessitated an über-designed power supply with big choke filters and high-voltage capacitors. And just like the 15,000 US dollar Conrad-Johnson ART, it ended up as a two-box preamplifier. Sound wise, it has the ability to make duff recordings sound more audiophile up to a point and it can be connected to a real-world solid-state amplifier like the Pioneer A400 and make it sound really gorgeous. Though a 6SN7 / ECC32 based zero negative feedback preamplifier can never be manufactured and sold as inexpensively as the 250-US dollar Musical Fidelity X-Pre. Well, at least it sounded like a scaled-down ART. 


Brittany said...

I don't know about a zero negative feedback preamplifier using ECC32 vacuum tubes but there was a commercially made one that uses 6SN7 double triode preamplifier vacuum tubes called the deHavilland UltraVerve Preamplifier which won the 2004 Enjoy The Music Best Of Award. Costs about 2,500 US dollars and I think I saw one being displayed at our local Upscale Audio branch a few weeks ago.

Angel Dove said...

The deHavilland UltraVerve was designed by Kara Chaffee at deHavilland back in 1997 and has since in production. But deHavilland has a zero feedback preamplifier that is probably better-sounding than the 6SN7 vacuum tube based UltraVerve, it is called the Mercury 3 preamplifier and uses the Type 85 six pin vacuum tube which Ringo has still several lying around in his listening room that still registers "OK" when connected to a small-signal vacuum tube checker. Maybe Ringo should DIY a zero negative feedback preamplifier using the Type 85 vacuum tube. The Type 85 vacuum tube is probably a direct descendent of Lee De Forest's Audion triode vacuum tube - the first ever triode vacuum tube capable of signal amplification.