Thoeress Phono Equalizer/Enhancer.

Thoeress Phono Equalizer/Enhancer.

Thoeress' lab evaluation was done via a moving coil input (the one offering a 100 Ohm load). Input sensitivity was about 0.380mVrms for -10dBV output level, a value equivalent to a gain close to 60dB (58.4dB, to be precise). Overload margin is very good, as the output reaches its clipping point, approaching 2.5Vrms, with a 3mVrms signal, a very good performance, considering that we are dealing with a device that connects to a line preamplifier and not directly to a power amp!
Distortion was reasonable for the type of the device with THD+N at 0.371% and THD at 0.063% with a reference output level of -10dBV. The significant difference between the two values, which implies that the noise plays an important role, should not be surprising: A phono preamplifier inherently amplifies low frequencies, it is a non-flat device therefore low-frequency noise could be dominant. The signal to noise ratio is typically good, about -53dBr(A) with a reference output level of -10dBV and the same characterization goes to the crosstalk which was about -52dBr/1kHz).
Output spectrum for a 1kHz signal was mainly characterized by the power supply presence (with 50/100Hz components and their intermodulation products amplified for the reasons explained above), the second harmonic at 2kHz being just discernible at a level below the -70dBr and all other harmonics below -80dBr. The output spectrum for different signals applied to the L/R inputs (1kHz/1.5kHz respectively) does not include any radical changes, beyond the fact that it better displays the signal leakage between the channels (channel crosstalk). This becomes evident at a level slightly lower than the -50dBr confirming the static measurement.

Output spectrum for 1kHz signal. Output reference level: -10dBV.

Output spectrum for 1kHz/1.5kHz signal. Output reference level: -10dBV.

Noise spectrum is quite interesting. We performed two measurements, one with the device fully attached through the MC input (100 Ohm) and one with the input shorted. The results show that the amplifier picks some noise from the environment, while the circuit is quite noise-free, with the hum noise being near -80dBr and the peak in the 30-40kHz range practically non-existent. These findings, from a practical standpoint, mean that some caution should be exercised with respect to the signal cables from the cartridge and, also, during the installation of the preamplifier itself with respect to high-frequency noise sources.
The accuracy of the RIAA de-emphasis proved to be very good, within the limits of 1dB (+0.43dB/3180mS, +0.25dB/318mS, -0.85dB/75mS) and differences between channels at these points, do not exceed 0.15 dB. This performance reflects in the form of the compensated response curve which is practically flat until 20kHz.

Output noise spectrum. Reference level: -10dBV. Standard, 100 Ohm terminated input (including cables, red curve) and with shorted inputs (green curve).

RIAA de-emphasis curve (green and red curve) and compensated response curve (via a reverse RIAA network), (orange and violet curve).

Frequency response diagrams, with different settings, show how Thoeress effects the signal and justifies the impression that the circuit acts on a mild and reasonable manner and will not permit excessive adjustments. For example, the control margin of the very low-frequency range is no more than 5dB and a similar performance observed in the mid/bass range.
In the latter case, however, one can observe that the final choice has an effect both at very low frequencies but also in the high-frequency range, justifying Thoeress' recommendation to start the setting procedure from this time constant.

De-emphasis curves with different settings at low frequency. Curve/Position: green/1, red/2, orange/3, magenta/4, purple/5. All other controls are at the RIAA position.

De-emphasis curves with different settings at medium/low frequency. Curve/Position: green/1, red/2, orange/3, magenta/4, purple/5. All other controls are at the RIAA position.

The settings in the high-frequency range offer a higher variability interval of about 10dB. Additionally, one of the positions offers an almost flat response through 1-10kHz range (quite reasonably, the company calls it "FLAT"), probably an effective choice for records with significant attenuation problems in their high-frequency content.
An interesting question here is "what the result of some real life settings would be in the preamplifier response?". The 442 Setting Response curve shows the response measured with a 4-4-2 setting (referring to the control positions from left to right), chosen "through listening" a particular record (a relevant listening session recording can be found at the end of this review, after the conclusion paragraph). To the eye, the general form of the de-emphasis has not changed significantly, but the numbers tell a different story: The new curve offers a gain of about 3dB at 3180mS (near 50Hz), about 1dB in 318mS (500Hz) and 0.6dB at high frequencies (75mS, 2kHz), compared with the standard values of the RIAA curve.

De-emphasis curves with different settings at high frequency. Curve/Position: green/Flat, red/2, orange/3, magenta/4, purple/5. All other controls are at the RIAA position.

De-emphasis curve with a 4-4-2 correction, referring to the control positions from left to right.

The result of the above is shown more clearly in the Compensated Response diagram. Changes in the high-frequency range are very small (and mainly located above 10kHz), but towards the low and the very low-frequency parts of the spectrum, there is a clear "fill-in" behavior with about 4dB gain in the region of 20Hz. What these mean in practice it is of course a matter of opinion and some critical listening…

Compensated response curve (via a reverse RIAA network) with de-emphasis curve set to 4-4-2.

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