Audiophonics PRE-TC10 preamp with tone controls - review and measurements
I have decided to buy and measure this preamplifier with tone controls, supplied by Audiophonics. It can be ordered here:
and it costs 49.92 EUR without VAT and 59.90 EUR with VAT (depending on country). I paid 72.10 EUR total together with the DPD shipment. The preamp has arrived in less than one week from placing the order.
Photos of the product and first impressions
First I made several photos of the preamp and also some photos with removed top cover.
On the front panel, we can see (left to right) bass control pot, mid control pot, treble control pot and volume pot. If the treble control pot is set full left, there is a switch that bypasses the tone controls. This is very useful.
On the rear panel we can see input and output RCA connectors, AC mains receptacle and the power switch.
We can see that the EURO 230V socket is connected only to L and N, though it is a 3-prong socket. This should not be so, strictly from view of valid standards. On the other hand, PE ground loop issues of this SE signal device are avoided. The manufacturer should have use a 2-prong EURO socket, however.
Measurements
After a visual inspection I have connected the preamp to the 230V ac mains net. The preamp started to work asexpected and as it should, so I started to make measurements. My system uses Topping D10s as a signal sourceand E1DA Cosmos ADC as a measuring instrument, used in stereo mode, Left channel, 4.5V input range, with 830 ohm input impedance.
Basic parameters of PRE-TC10
Input impedance: 30 - 100 kohm, depending on volume pot position
Output impedance: 100 ohm
Max. output voltage: 9.1 Vrms
Crosstalk at 1kHz: -80dB
Crosstalk at 10kHz: -60dB
Gain: 14.80 dB with tone controls bypassed
Frequency responses
It is a tone control circuit so the frequency responses are very important. First, let's see actions of bass and treblecontrols and of a bypass switch.
The middle frequency of the controls is centered around 700Hz, which is lower than usual 1kHz.
Now, with addition of action of the mid controls:
Mid controls center is about 1kHz.
Noise and distortion
This is the noise measured at preamp output, with controls in the middle position and minimum volume.
Output noise: 8.95uV/BW45kHz, 6.84uV/BW22kHz, 5.31uVA.
THD and THD+N at 1kHz/2V with tone controls bypassed
THD and THD+N at 1kHz/2V with tone controls in the middle
We can see rise of the distortion when tone controls are activated.
THD vs. frequency at 2V with tone controls activated and bypassed
THD and THD+N vs. output level at 1kHz, controls bypassed
THD is very low with the controls bypassed and goes quite higher when controls are activated, especially below1kHz.The rise of distortion with tone controls activated is a result of designers/manufacturers choice of capacitors in tone controls they use MLCC ceramic capacitors that are known for their non-linearity. They save space at the expense of higher distortion.
Sound
It definitely helps to get more optimal frequency balance from older vinyl records and from cassettes. It also helps in new recordings, where the mastering engineer did not do a good job, like too emphasized bass.
Conclusion
Audiophonics PRE-TC10 may be a useful product for someone who needs tone controls preamp at affordable price. Built quality is good, with exception of the control knobs that collide a bit with the front panel when turned. Hopefully, it can be fixed by pulling the knobs a bit, as they are not fixed by screws.
----------------------------------------------------------------------------------------------------------------------------------------------------------------
Some measurements with oscilloscope
1) Maximum output voltage before clipping is 9Vrms
2) 10kHz square response
3) Slew rate - measured as 21V/us, defined by LME49720 used
These measurements were done with tone controls bypassed (treble knob full left, controls switched off).
The preamp would be able to drive power amps that have lower gain, due to its 9Vrms clean output.
The preamp transfers DC in full amplitude, that means that it works from 0Hz without LF attenuation and with no phase shift at low frequencies.
-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------
Preamp LM337 failure
Unfortunately, I have found a flaw in the circuit soon, though the frequency responses and distortions measured "normally". Supply voltage at LME49720 pins is +14V and -23.13V. This makes 37.13V total between +Vs and -Vs and it exceeds datasheet absolute maximum rating, which is 36V.
The
opamps works yet, but the conditions are dangerous and may result
in premature failure. I need to checkthe LM337 stabilizer circuit,
which seems not to be working.
The no-name "xifi" LM337 used by the manufacturer was
not working. It had internal short. What a shame, Iexpect the
product control does not check opamp supply voltages. I found it
when measuring the output swingwith the oscilloscope, negative
half worked down to more than -20Vp ...., without clipping.
This is the wrong one:
Fixed by the old made in DDR B3370V (LM337 equivalent)
Fixed, LME49720 have survived supply overvoltage ....
After the repair I have re-measure noise and THD spectrum, the noise and THD/THD+N at 1kHz remained similar, THD and THD+N at high frequencies have improved after repair:
THD is vanishing low. Noise of this preamp is a bit higher, due to 14.80 dB circuit gain.
@Pavel Macura, May 2023