Happy Holidays and a Great New Year to All!

JE Labs Loctal Preamp

Motivated by Ding's octal version of the 834P, I quickly got my hands dirty punching out and painting a chassis. 

Loctal tubes

The concept of this project was to build a full function stereo preamp utilizing loctal tubes based on the schematics below.

Preamp circuit

!!!WARNING!!! 

The voltages found in this circuit can be lethal! 
Build at your own risk!!!

Power Supply

5AZ4 rectifier = 5Y3 on a loctal base

Line Stage

Zero feedback single gain stage direct coupled to a cathode follower line stage topology featuring the two medium-mu triodes below

The 7AF7 is very similar to the 7N7 and 6SN7
but has slightly less
transconductance and inter-electrode capacitance

7N7 = loctal based 6SN7

Phono

The stock EAR 834P circuit (above) used all high-mu 12AX7s with the second stage capacitor coupled to the cathode follower stage. While preparing this blog entry, I remembered that my dearly departed friend, mono aficionado Brian monofantastico Clark, grafted his own interpretation of an EAR 834P phono stage to his SE 801 mono integrated amplifier. He was an active participant in this discussion of 834P modifications initiated by Thorsten Loesch.

Ding's octal version of the 834P uses all 6SL7s with the second stage direct coupled to the cathode follower stage. Without ever sounding harsh, Ding's octal 834P sounded brighter and had a refreshing airiness in the high frequencies compared to my darker and more midrange oriented RCA derived phono preamp. Even if I never heard an original EAR 834P, I now have a good idea why it was received with great enthusiasm.  

7F7  = 6SL7 on a loctal base

Although the 7B4 and 7B6 are high-mu loctals, they are single triodes. AFAIK, the 7F7 is the only high-mu twin triode available in loctal and was the obvious choice for the 1st and 2nd stages of the 834P circuit given my chassis configuration.

Tube rolling loctal style

I've been tweaking and listening to this preamp for the past 6 months.

During my Dyna PAS hacking days, the 12AU7 always sounded gutsier in line stage and cathode follower applications than the 12AX7 because it had lower Rp and pulled more current. Thus, there was some room to voice the 834P circuit through the cathode follower section.

Warning: 7F8 is NOT plug-in compatible
with the 7N7 and 7AF7

I initially set up the phono section to use the unique 7F8 loctal in the cathode follower position, hoping that the 7F8's greater transconductance will produce a darker and more robust sound. But ultimately, it was the ubiquitous 7N7 (loctal 6SN7) that gave me the tonal balance I wanted, with the 7AF7 as alternate.

With an amplification factor of 48, the 7F8 may not have enough gain as a phono preamp with the RIAA EQ in the negative feedback loop. However, it was too much for a zero feedback single stage line amp.

Typically, I don't split halves of twin triodes for left and right channels. I made an exception in wiring this line stage so that I could exploit the 7AF7 vs. 7N7 comparisons and usage options.

My ears finally chose the 7AF7 as the voltage amplifier for the line stage. The slightly lower transconductance and amplification factor mimics the exotic tonal qualities of its grandad, the 76. Each half of the 7AF7 is direct coupled to half of a 7N7 configured as the cathode follower. This combination is as close as I could get to my 76/6SN7 line stage.

But don't limit yourself to this report, use it as a starting point for your loctal tube rolling adventure. 😉

Which is better?

or

Both are here to stay. 😎

🎅🎅🎅 Happy holidays and merry listening! 🎄🎄🎄

Line Transformers Redux + Art Dudley + AQ Dragonfly Black

My boring digital life

Tascam DA-P1
courtesy of pgr.tv

Probably the most significant digital audio gear I ever owned was the Tascam DA-P1 DAT recorder. This machine recorded my recitals, chamber music performances and served as a back-up and playback deck for my CD projects from the mid 90s until the early 2000s. It was rendered obsolete when HD based recording became the industry standard.

DAC in the Box
courtesy of Wiki Commons

I never invested in a high-end CD player. The most I ever did for CD playback was get an Audio Alchemy DAC in the Box fed by the digital output of a consumer grade Philips CD player. When the Philips CD player conked out in the early 2000s, it was replaced by a Sony DVD/SACD player.

Line transformers as Digital Sound Processors

Tamura line transformers

Ever since I inserted a pair of Tamura 600:600 line transformers as a digital sound processor (DSP) between the CD player/DAC and my preamp line inputs, I didn't see the necessity of upgrading the Audio Alchemy DitB. I don't know if this holds water but my empirical rational for this is, magnetic coupling filters digital artifacts and RFI noise from the DAC output, which is manifested as harshness to my ears.

Art Dudley Listening

If I don't consider the few issues of Stereo Sound Tube Kingdom and MJ I picked up in Tokyo in 2015, it's been a long while since I subscribed to, let alone bought, a single issue of Stereophile, The Absolute Sound or any audio magazine. After the demise of Sound Practices in the late 90s, the only audio magazine I read was Art Dudley's Listener.

Although I've never had a chance to meet Art personally, I know he is a fine gentleman. I've been following his Stereophile column online and he has given credit to my musings on the open baffle as well as the Altec 755A. He even sent me a private message when my father passed away earlier this year. I was looking forward to meeting him at the Capital Audio Fest 2017 but missed the event because it coincided with the Harrisburg Symphony's November Masterworks.

iPad mini + UTC A20 line transformers

Due to my current location, access to quality FM programming is practically nil. I've relied mainly on digital streaming via iPad mini + DSP to widen my choice of free music. 😜

USB DAC

Audioquest Dragonfly Black V1.5

When I realized that the iPad mini's internal DAC can be by-passed through an Audioquest Dragonfly USB DAC, I searched the archives of Art Dudley's Listening to see if he had anything to say about the AQ Dragonfly USB DACs. After getting Art's stamp of approval, I placed an order for the cheaper Black.

Stereo > Mono

Whether I was listening in mono or stereo, this was the best $100 (ok, + $38 for the Lightning to USB 3 Camera cable adapter) investment I ever made in digital audio. The degree of improvement was at least as significant, if not greater than, when I started playing with transformer DSP(s) between the digital source and my line preamp input(s).

Stereo set up =
AQ Dragonfly Black > a pair of UTC A20 wired 1:1
> preamp line stage

The combination of the iPad mini + AQ Dragonfly Black + DSP = smooth analog-like detail across the audio bandwidth. So I take back my word, I need this USB DAC. 😎

Thank you Art and here's to us meeting some time soon. Cheers! 🍻