ULTIM2 - the new preamp designed during pandemic

I needed a preamp with several single ended and balanced inputs, so it was a challenge during this strange pandemic times to design and make a new audio toy that would satisfy my needs. You cans see the result sitting between the Vincent CD player and DacMagicPlus D/A converter, in the next photo.



Design goals

- 3 single ended, 2 balanced inputs
- 1 balanced output usable as a single ended output as well
- very wide bandwidth that would not degrade hi-res sources
- excellent immunity to input HF and EMI
- considerable reduction of HF interference content arriving from digital signal sources
- high output current and ability to drive 600 ohm load impedance
- improvement of S/N and distortion compared to digital volume control

Circuit design

To fulfill the goals stated here above, the following circuit was designed





The inputs are switched by high quality relays. Single ended inputs go, after input RC filtration, directly to input buffer (IC9, IC101). Balanced inputs go, after common mode and differential RC filtration, to instrumentation amplifiers (IC1 – IC4) and then to the input buffer. Volume control (P1, blue Alps 2x10k/log) is connected behind the input buffer and effectively reduces noise of the preceding integrated circuits at lower volume settings. This is important to get the best S/N at low volume settings.
IC7, IC8, IC102 and IC103 are the output stage opamps. All the opamps used have very high BW and slew rate, that is 125V/us and higher. The slowest parts are the instrumentation amplifiers at balanced inputs wit slew rate of 15V/us, which is, to my measures, quite low value. They are protected from very high frequency input signals by input common mode and differential RC filters.
I do emphasize the speed of the opamps for the reason that IME the influence of input EMI is just the reason of different sound of different opamps.

Photo of preamp construction



This is the completely assembled box with wiring. One can see 2 power supplies, one for the electronics and the second one for relays. This reduces interference coupling from relay coils to the signal path. There is a common ground point for these 2 power supplies.



This photo shows the completed preamp.

Technical parameters

- 3 single ended inputs, SE1 – SE3
- 2 balanced inputs, BAL1 and BAL2
- 1 balanced output, usable as single ended as well
- Input impedance single ended.................................. 100 kohm
- Input impedance balanced ...................................... 136 kohm
- Output impedance SE ............................................. 47 ohm
- Output impedance balanced .................................... 94 ohm
- Max. output current ................................................ 67 mAp
- Max. input voltage SE1-3, BAL1 ................................ 9 Vrms
- Max. input voltage BAL2 .......................................... 4.14 Vrms
- Max. output voltage SE ............................................ 9 Vrms
- Max. output voltage balanced ................................... 18 Vrms
- Gain SE1-3, BAL1 inputs .......................................... 0 dB to SE output, +6 dB to balanced output
- Gain BAL2 input ..................................................... +6.62 dB to SE output, +12.62 dB to balanced output
- Frequency range SE inputs ....................................... 1.6 Hz – 1.06 MHz / -3dB
- Frequency range BAL inputs ..................................... 2.3 Hz – 134 kHz / -3dB
- Rise time SE inputs ................................................ 330 ns
- Rise time BAL inputs ............................................... 2.6 us
- S/N SE in to BAL out ............................................... 116 dBA / 4 Vrms at full volume, 129 dBA / full output
- S/N BAL in to BAL out ............................................. 106 dBA / 4 Vrms at full volume, 119 dBA / full output
- THD ...................................................................... I am not able to measure it as my system limit is about 0.0005%

Some measurements



Step response for 20Vp-p, SE input. Blue = input, Red = output.



100 kHz clipping, SE input.

Speaking about distortion, my system limit is about THD = 0.0005% and the preamp measurement is a copy of the soundcard loopback result. So I can only say that the distortion is lower. Next plot shows that there is no usual rise of distortion at higher frequencies, due to fast opamps used.



December 2020

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