Tritschler Precision Audio Devices TPAD 1000 Phonostage

Modern Twist on a Classic Circuit

Equipment report
Categories:
Phonostages
Tritschler Precision Audio Devices TPAD 1000 Phonostage

You would think that RIAA de-emphasis circuits have been refined to death over the past 50 years, and you would be right. After all, the task shouldn’t be that difficult. Basically, the phono signal needs to be boosted gradually below 1kHz to a total of about 20dB at 20Hz and reduced gradually by about 20dB between 1kHz and 20kHz with a knee in the inverse RIAA curve between 500Hz and 2kHz. In the 1960s the favored approach was to use a feedback loop around a cascaded pair of high-gain triodes, usually 12AX7s, to tailor the inverse RIAA curve. In the 1970s a passive approach became popular, mainly due to the writings of Walt Jung and Stanley Lipshitz, in which a passive network is sandwiched between two active stages. In 1985 Erno Borbely published an article in Audio Amateur in which he sought to combine the best features of these two approaches with a half passive/half active de-emphasis circuit. Erno is well known to us old-timers. He was hired by David Hafler in 1969 and contributed several designs to the Dynaco product portfolio, most notably the Dynaco Stereo 400 amplifier in collaboration with Jim Bongiorno. The Hafler DH200 power amp is also his doing. The original Borbely phono circuit was transistorized, but while still a senior at Wright State University, Joe Tritschler designed a tube version of the circuit using high-transconductance frame-grid tubes. Joe described his original design and its subsequent revisions in two audioXpress articles. Dr. Tritschler goes by the self-ascribed moniker Dr. “Crazy Joe,” but don’t let that fool you, he’s a sane and capable designer.

Tritschler Precision Audio Devices was formed in 2003 to commercialize Dr. Tritschler’s phonostage. It is currently being sold direct, which partly accounts for its astoundingly affordable asking price, another factor  is its circuit simplicity. A single 6922 dual triode is used per channel. The triode sections are cascaded with the passive section of the RIAA section sandwiched in between. Instead of using a cathode resistor, cathode bias is established using red LEDs, a biasing technique Joe picked up from Morgan Jones’ Valve Amplifiers book. Being a constant-voltage device, the LED drops a fixed voltage over a wide range of quiescent current. According to Joe, replacing the cathode resistor and its bypass cap with the LED significantly improves low-level resolution.

The latest version of the design dispenses with the response shelf at 50kHz, which is not actually called out in the RIAA standard, and instead continues to roll the treble off into the ultrasonic range. In his experience, the shelf increases the annoyance factor of record surface noise and pops. I agree with Joe that there’s nothing good to be had from opening the door to ultrasonic noise. Overall gain is rather low, even for a typical mm cartridge, so plan on providing 10dB or more of line amplification depending on the sensitivity of the power amp and speakers.

The stock tubes are a pair of Russian new-production Electro Harmonix 6922s selected for low noise and low microphonics. Right out of the box, the presentation was closed-in and lacking spatial incisiveness. I suspected that much of the responsibility for this lay with the Russian 6922. And so, I rolled in a couple of vintage GE 6DJ8s. Yes, the stock tube was quiet to be sure, but in hindsight rather musically insipid. The NOS 6DJ8 opened up the treble and expanded the boundaries of the soundstage.

After my first listening session, I was ready to declare the midrange a low-distortion zone. There were no response nasties in evidence. In particular, the lack of gratuitous brightness was very much appreciated. It lent the presentation a relaxed character that greatly enhanced long-term listenability. Low-level detail resolution and transient response were surprisingly good, especially considering the price point. In these respects, the TPAD 1000 exceeded the performance of the Conrad-Johnson PV8 that I keep around as a vintage 1980s phono reference. And that’s saying a lot, since the PV8 in its day was a reviewer’s favorite.

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