The new DIY preamp designed during pandemic

[pma]

Same capacitors, THD and THD+N vs. voltage, at 50Hz. Should go to another thread, IMO.

New thread on capacitor distortion:

Capacitor distortion

Though I posted this as a reply to @syn08 in another thread (preamp ULTIM2), I think it deserves its own thread. As an example, 2 electrolytic capacitors were measured: 1) Aluminium 10uF/100V 2) Tantalum 4.7uF/6.3V they were put in the loop between Topping D10s and E1DA Cosmos ADC. Load...

www.audiosciencereview.com

This thread is about the preamplifier.

 

[syn08]

I would never use and electrolyte as a coupling cap.

I am not sure that I spoke about any beliefs.

Neither did I. But you still missed the question: why $0.05 electrolytics are good enough for the ADC input coupling (and coincidentally with the same order of magnitude load as in your test) while they are not good enough for audio stages coupling.

The fact that you found some pathological cap samples is totally irrelevant in this general question context. The only conclusion from your test is: use good quality capacitors.

 

[Patty]

Great work @pma! I’m just getting into pre-amp design for my university’s senior design project and I’m having trouble determining THD. How would you recommend measuring THD and what equipment is necessary? I’d appreciate any help!

 

[LTig]

A good USB sound interface and REW is more or less all you need. If you want to measure THD below -100 dB it gets expensive as few sound interfaces have DACS and ADCs of sufficient quality (e.g. RME ADI-2 PRO fs).

 

[Patty]

Would a Focusrite Scarlett be of sufficient quality for measuring DHT? Also, I’m assuming this method only works from input to output with the pre-amp acting as a “black box” (pun intended ). Would there be a way to measure the THD of individual components within the pre-amp to determine a main source of the distortion? Like a jerry-rigged quarter inch cable or something?

 

[LTig]

Would a Focusrite Scarlett be of sufficient quality for measuring DHT?

If you already own one it's a good start. You can run a loopback test (connect its input with its output) tofind out how much THD it shows. Then you can measure the preamp, and as long as those THD values are 10 dB higher you're good. If not the USB interface is not good enough.

Also, I’m assuming this method only works from input to output with the pre-amp acting as a “black box” (pun intended ). Would there be a way to measure the THD of individual components within the pre-amp to determine a main source of the distortion? Like a jerry-rigged quarter inch cable or something?

If the preamp consistes of several stages you can measure the output of each stage. If THD goes up significantly after one specific stage it is the culprit.

 

[Patty]

Sounds good, thank you so much for the help!

 

[pma]

For more new measurements of this preamp please visit my web page

ULTIM2 preamplifier

I bought the E1DA Cosmos ADC (which I use for measurements now as a rule) later than this thread was created. The preamp has been still my main preamp.

 

[egellings]

The new DIY 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.

View attachment 94528

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

View attachment 94529

View attachment 94530

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.

Photos of preamp construction

View attachment 94531
This is the photo of partially assembled main board under square wave test

View attachment 94532
The main board inside the box. The box was bought at https://modushop.biz/site/, together with panel drilling and engraving.

View attachment 94533
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.

View attachment 94534
This photo shows the completed preamp.

Technical parameters

3 single ende inputs, SE1 – SE3
2 balanced inputs, BAL1 and BAL2
1 balanced output, usable as single ended as well
Input impedance SE 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
118 dBA / 4 Vrms volume at 11:00 am
S/N BAL in to BAL out 106 dBA / 4 Vrms at full volume
119 dBA / full output
118 dBA / 4 Vrms volume at 11:00 am
THD I am not able to measure it as my system limit is about 0.0005%

Some measurements

View attachment 94535
Step response for 20Vp-p, SE input. Blue = input, Red = output.

View attachment 94536
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.

View attachment 94537

However, I do not consider distortion measurement as important if it is lower than 0.01%. There is no evidence that distortion below 0.1% would be audible, if it contains only low order harmonics. So it makes no sense to discuss about distortions that are less than 0.001% and to consider them as a measure of quality.

The new DIY 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.

View attachment 94528

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

View attachment 94529

View attachment 94530

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.

Photos of preamp construction

View attachment 94531
This is the photo of partially assembled main board under square wave test

View attachment 94532
The main board inside the box. The box was bought at https://modushop.biz/site/, together with panel drilling and engraving.

View attachment 94533
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.

View attachment 94534
This photo shows the completed preamp.

Technical parameters

3 single ende inputs, SE1 – SE3
2 balanced inputs, BAL1 and BAL2
1 balanced output, usable as single ended as well
Input impedance SE 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
118 dBA / 4 Vrms volume at 11:00 am
S/N BAL in to BAL out 106 dBA / 4 Vrms at full volume
119 dBA / full output
118 dBA / 4 Vrms volume at 11:00 am
THD I am not able to measure it as my system limit is about 0.0005%

Some measurements

View attachment 94535
Step response for 20Vp-p, SE input. Blue = input, Red = output.

View attachment 94536
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.

View attachment 94537

However, I do not consider distortion measurement as important if it is lower than 0.01%. There is no evidence that distortion below 0.1% would be audible, if it contains only low order harmonics. So it makes no sense to discuss about distortions that are less than 0.001% and to consider them as a measure of quality.

15V/uSec is more than adequate for home audio use.