UGS adventures

UGS Frequency Response

Hi,

here is a Audio Precision Frequency Response measurement of my UGS all-inclusive build (all-inclusive means PCB set including volume control, input selector, CPU and display boards from french HCFR forum).

The measurement was provided as a free-of-charge service by Mr. Fuchs from Volpe-Audio Electronics - electronics development at the Analogue Audio Association event in Krefeld/Germany. Thanks a lot for the nice talk and measurement session !

Measurements revealed that the frequency response is flat between 10hz and 80khz (-3dB point with 2V input and 2 Volt output)
Frequency response of UGS Preamp shows dependency on the volume setting.
Cause is probably parasitic capacitance (here: Vishay Dale CMF-55) of the resistor network - maybe to some extend also switching relais or PCB circuits.

Red Line: Volume attentuator set to 0dB - no attentuation.
-3dB point is beyond 200khz.

Other lines: Frequency response at different volume levels. -3dB point varies with volume level with minimum at 80khz.

Based on a lot of listening sessions, my impression is that the preamp sounds very neutral - the difference between the amp and amp-bypass at same loudness is minimal. But I also find that when bypassing the amp - or when setting volume attentuation to 0db (none) - the sound is a bit better.

Not sure where to look for, but the volume attentuator resistor ladder and relais circuits could be an option........
 

Attachments

  • frequency_response_vs_level_s.jpg
    frequency_response_vs_level_s.jpg
    87.6 KB · Views: 1,356
UGS THD+N

Hi,

here is a Audio Precision THD+N over frequency measurement.

THD with 500khz Bandwidth is ~ 0.004%.
As this distortion curve is independent from frequency, it seems to be dominated by background noise.

Limiting the bandwith and excluding the high-frequency portion of the noise result is:
THD with 22kHZ Bandwidth is ~ 0.001%

I am not an expert in THD figures of preamps, but I'd say it looks very well...

[Measurement kindly done by Mr. Fuchs from www.volpe.at]
 

Attachments

  • THD+N over Frequency.jpg
    THD+N over Frequency.jpg
    140.7 KB · Views: 1,306
UGS CCMR and FFT measurement

Last but not least the CCMR and FFT measurements:

Common Mode Rejection

Common Mode Rejection on the symmetrical input is around -60dB.
Not too bad, but maybe some even better matching of transistors and some resistors is required here to lower CMRR ?

FFT with suppressed ground/base wave

Residual 50hz AC-line-noise is at approx. -100db - looks fine.

FFT shows some -115dB noise at 5.9khz and it's K3 at 17.7 khz.
There is a small noise-generator hidden somewhere in my circuit layout.
Measurements with different inputs and separate ground-reference of the audio-precision
indicate that there is a grounding problem in my layout.
However at -115db level of this noise - it is probably not really audible on the speakers ?
 

Attachments

  • CCMR symmetrical input_s.jpg
    CCMR symmetrical input_s.jpg
    82.8 KB · Views: 1,250
  • FFT  Preamp_s.jpg
    FFT Preamp_s.jpg
    84.4 KB · Views: 1,246
Ouahouh!!!!

Many thanks Dsdjoy for these nice measures!
Cmrr is finaly not so bad!!!

I am getting convinced too, that this preamp measures (and it also sounds) better than many (and most) commercial preamps. Thanks a lot here to CheffDeGaar, 'Flat' from HCFR forum and last but not least Nelson Pass for this great preamp !

BTW: Now my speakers are the limiting factor....more DIY things to do....
 
Last edited:
Probably the fugliest UGS preamp so far...
Here is my version of this fine preamp. Cheff DeGaar himself helped me a lot with hints and good advice. Thanks again for helping and for making this great design available to the community!
In case you want to see "the making of…" look here
 

Attachments

  • UGS Forum 12.jpg
    UGS Forum 12.jpg
    133.1 KB · Views: 1,113
  • UGS Forum 13.jpg
    UGS Forum 13.jpg
    193.2 KB · Views: 1,013
  • UGS Forum 11.jpg
    UGS Forum 11.jpg
    174.6 KB · Views: 1,015
  • UGS Forum 8.jpg
    UGS Forum 8.jpg
    53.5 KB · Views: 964