Technical details¶
This page provides some additional technical background. You don't need to read this to be able to use the ADept.
Analog stage¶
Cartridges have a specified output at a given velocity of the stylus tip. Usually, this velocity is 5 cm/second. This velocity is, however, a reference value, not the maximum value. The actual peak velocities on a record can be much higher than this. For example, in the 1970's, Shure measured velocities on LPs exceeding 50 cm/s (ref. 1).
It is commonly understood that non-audio clicks will produce even higher velocities. For example, author Morgan Jones measured transients 22 dB above the nominal 5 cm/s velocity (ref. 2).
A potential problem in phono stages is that the electronics can overload when presented with the high signal voltage that the cartridge generates in response to high stylus velocities. The margin available for overload is commonly called "headroom" and is expressed in dB. Relative to 5 cm/s, a velocity of 50 cm/s would require 20 dB of headroom, for example.
The ADept uses relay switching to change the gain of the analog front end. The analog gains are set so that the ADC will not clip – in the majority of cases – when using a cartridge with output voltage specification as given in Table 1. In that case, the ADept has the following headroom in the analog front-end:
- Moving coil: 26 dB
- Moving magnet: 24.5 dB
Except in extreme cases (such as a deep scratch or embedded grit), the ADept is unlikely to clip the ADC.
Digital stage¶
Here is where it gets tricky. The peaks in the raw audio signal coming from the cartridge tend to contain a lot of high-frequency energy. Non-audio clicks similarly have a lot of high-frequency energy. However, after analog-to-digital conversion, the RIAA EQ reduces the high frequencies:
As a result, we don't need to preserve the full amount of headroom at all frequencies. For example, at 8 kHz, where stylus velocities are the highest, the RIAA curve reduces the output by almost 12 dB. In practice, this means we can reduce the headroom available at lower frequencies without causing clipping in the output waveform of typical musical audio signals.
Therefore, we have added 10 dB of digital gain (at 1 kHz) to the signal flow. This is a compromise between signal level/dynamic range and avoiding clipping. It is still possible that some very "hot" records will clip the digital output even though the ADC doesn't clip. If this occurs, simply reduce the digital volume. (For the full procedure, see the Fine-tuning gain structure page.)
References¶
(1) The Stylus Tip and Record Groove – The First Link in the Playback Chain, by B. W. Jakobs and S. A. Mastricola. Available in the proceedings of a 1978 technical seminar here: High Fidelity Phonograph Cartridge - Technical Seminar.
(2) Valve Amplifiers (book), by Morgan Jones, second edition, page 352.