Audiophonics PRE-TC10 preamp with tone controls - review and measurements

[pma]

I have decided to buy and measure this preamplifier with tone controls, supplied by Audiophonics. It can be ordered here:

AUDIOPHONICS PRE-TC10 Preamplifier with Tone Control 2x AOP LM49720NA - Audiophonics

The Audiphonics PRE-TC10 is a preamplifier offering of course volume control, but especially a triple tone control.

www.audiophonics.fr

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 replaced 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 as expected and as it should, so I started to make measurements. My system uses Topping D10s as a signal source and E1DA Cosmos ADC as a measuring instrument, used in stereo mode, Left channel, 4.5V input range, with 830 ohm input impedance.

Frequency responses

It is a tone control circuit so the frequency responses are very important. First, let's see actions of bass and treble controls 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 is 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 is very low with the controls bypassed and goes quite higher when controls are activated, especially below 1kHz. 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.

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.

 

[pma]

Some more info:

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

THD and THD+N vs. output level at 1kHz, controls bypassed

Sound

It definitely helps to get more optimal frequency balance from older vinyl records and from cassettes. It also help in new recordings, where the mastering engineer did not do a good job, like too emphasized bass.

 

[pma]

It seems that the preamp circuit could be something like this:

The plots in this post are simulated.

 

[holla]

Is it legal to have a mains-powered device like this with no earth connection? I am only dimly aware of the legislation but I thought non-earthed gear had to be "double insulated".

 

[maty]

The question is whether the tone control can be easily bypassed if you only want the gain < 13 dB.

Yes!

 

[Talisman]

Thanks PMA, it is very useful to have measurements even of cheap but potentially well measuring equipment, as in this case.
With the controls bypassed the performances are transparent, with the controls inserted it is possible to correct gross errors of FR. An economic but functional product.

 

[pma]

Is it legal to have a mains-powered device like this with no earth connection? I am only dimly aware of the legislation but I thought non-earthed gear had to be "double insulated".

It is "legal" (in conformance with safety standards) as long as it is a class II device with 2-prong socket or fixed cord. Here, the transformer looks OK and wiring as well. But the 3-prong socket is against the safety standards for the class II device, it should have been a 2-prong socket, like the one below.

 

[pma]

The question is whether the tone control can be easily bypassed if you only want the gain < 13 dB.

Yes!

View attachment 287534

Please see post #1,

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.

BTW, I measured gain 14.80dB with controls bypassed.

 

[holla]

It is "legal" (in conformance with safety standards) as long as it is a class II device with 2-prong socket or fixed cord. Here, the transformer looks OK and wiring as well. But the 3-prong socket is against the safety standards for the class II device, it should have been a 2-prong socket, like the one below.

View attachment 287535

Thanks for this clarification. As I said, I do not quite know what counts as Class II double-insulation. I can't see much that would protect the user if e.g. the live connection failed and contacted the chassis. But I do not mean to derail the thread so I shall shut up!

 

[maty]

Pavel, now you can do a new test of op-amps like years ago in diyaudio. I have a quieter amp and have further optimized my second system so it is supposed to be even easier for me than then.

What do you think of the OPA828 as an alternative?

My "audiophile" LM3886 approach

I can confirm that it's more than 1,5V. The exact value I don't remember, too... My assumption is that here Mauro had tried to optimize between a still practically acceptable level and a safe dynamic headroom so as not to intervene too fast with real music. I can confirm that it works quite well...

www.diyaudio.com

 

[VintageFlanker]

Thanks for the review, @pma. Great job !

 

[pma]

Pavel, now you can do a new test of op-amps like years ago in diyaudio. I have a quieter amp and have further optimized my second system so it is supposed to be even easier for me than then.

What do you think of the OPA828 as an alternative?

Yes, OPA2828 (dual version of 828) should definitely work well in this preamp, with a socket adapter

It has low noise, high GBW, high slew rate and is unity gain stable. A good candidate for those who want to play, definitely.

 

[pma]

Thanks for this clarification. As I said, I do not quite know what counts as Class II double-insulation. I can't see much that would protect the user if e.g. the live connection failed and contacted the chassis. But I do not mean to derail the thread so I shall shut up!

It is OK, as long as the class II device is designed and built properly. Class I for a device like this, which has both single-ended input and output, is always source of troubles, because then groundloops, when connected with another class I device are inevitable and will degrade S/N. I can measure an insulation resistance of L and N pins vs. chassis with a special insulation tester, if you like.

Edit: I did the 500Vdc insulation test, there was no measurable leakage current.

 

[Vini darko]

Seems a decent way to get tone controls for cheap. Thanks for the review.

 

[Matias]

Great review, thanks!

 

[pma]

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.

 

[pma]

Warning!

Unfortunately, I found a flaw in the circuit. 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 check the LM337 stabilizer circuit, which seems not to be working.

 

[antcollinet]

It is OK, as long as the class II device is designed and built properly. Class I for a device like this, which has both single-ended input and output, is always source of troubles, because then groundloops, when connected with another class I device are inevitable and will degrade S/N. I can measure an insulation resistance of L and N pins vs. chassis with a special insulation tester, if you like.

Edit: I did the 500Vdc insulation test, there was no measurable leakage current.

However - If I were inspecting this device I would deem it non compliant, not only for the 3 pin connector (which is more technical than actual safety). There is nothing stopping the mains input wiring contacting the case (or moving to within 5.5mm of it), at which point there is only the single insulation of the wire protecting the user.

It's difficult to see from the photos, but one of the wires might violate the 5.5mm clearance by design due to its position on the PCB.

Another possible area for concern would be the clearance between the protruding pins on the underside of the PCB and the bottom surface of the enclosure.


I have, however voted the unit "fine" for performance.

 

[pma]

Warning!

Unfortunately, I found a flaw in the circuit. 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 check the LM337 stabilizer circuit, which seems not to be working.

!! Fixed !!

The no-name "xifi" LM337 used by the manufacturer was not working. It had internal short. What a shame, I expect the product control does not check opamp supply voltages. I found it when measuring the output swing with the oscilloscope, negative half worked down to more than -20Vp ...., without clipping.

This is the wrong one

Fixed by the old DDR B3370V (LM337 equivalent), Thorsten Loesch would be happy)

Fixed, LME49720 have survived supply overvoltage ....

Now I will to have re-measure noise and THD spectrum, I expect even lower level of mains multiples residuals due to negative voltage regulator working as it should!
This is the way to do technical reviews, check everything and never give up!

 

[pma]

Now I will to have re-measure noise and THD spectrum, I expect even lower level of mains multiples residuals due to negative voltage regulator working as it should!

Done, the noise and THD/THD+N at 1kHz remained similar, THD and THD+N at high frequencies has improved after repair. THD is vanishingly low, noise is a bit higher, due to 14.80 dB circuit gain.