Tag: CCS

More on CCS

As I continue with my design of CCS to be used on my next designs as part of the supply filtering stage, I looked at testing the performance of my latest CCS using the following circuit:

CCS test The limitations I have currently is that my waveform generator can do 7Vrms maximum and in low frequency the existing noise level will be the limitation clearly.  As suggested by Gary Pimm, adding a battery operated differential preamp at the point of test will be a great way of raising the low level signal from the sound card interface noise floor. That, will be for a future day. I just want to see how well the CCS performs.

I set the CCS to 30mA and measured attenuation from 50Hz to 30kHz. The results are quite encouraging despite the lack of pre-amplification:

ccs NOISE TEST

The real life CCS is not that great as in the simulation. There is about 10dB difference with the Spice simulation. You can see that I can measure below -130dB attenuation without a pre-amplifier. Still is quite good, more than what we need for.

The CCS is operating to the level of what I need, so test passed 🙂

 

26 THD analysis

ux226 globe valves

As I’m proud of my 26 DHT pre-amp and also looking to use this valve as the first stage of my 4-65a SE amplifier shortly, I looked at how linear this valve is.

Unfortunately I don’t have an extensive set of valves of same brands, so albeit I have about 40 valves if this type there is a big mix of different brands and many of them are used ones. Having said that, I think this may be the case of many of you out there, so I think that probably the results of this test may be relevant to you (if you are still reading this post).

So in summary, this is the sample set I tested:

  • 39 valves
  • 12 brands
  • ST (34 valves) and Globe (5 valves)
  • Type: 26, 226 and 326
  • State: used (80%) and NOS (20%)

All DUT were tested with the same test set and operating point:

  •  Operating point: Ia=5.5mA and Vg=-10V
  • Anode load: Cascoded DN2540 CCS set to Ia
  • Test signal: 1kHz, Adjusted to obtain Vo=10Vrms (+22.22dBu)
  • THD analysis: Audiotester via Pete Millett’s interface:
    •  32768 sample FFT (2.96Hz resolution)
    • 256 averaged windows
    • Van Hann window
    • THD for H2+H3+H4

So in summary the results showed that you should expect a 26 to have around 0.08% (Std  Dev = 0.00047). Here is an histogram showing a summary of the tests done:

26 THD histogram

Looking at brands, the following average THD ranking was produced:

26 THD by Manufacturer/Brand

Interesting to find National Union leading the chart. I found RCA and Sylvania to be my preferred ones in terms of sound. THD shown above are average of sample sets of 4 valves or more. Although I tested 12 brands in total, some of them were just a pair so they are not a representative subset.

Finally, here is a sample THD of a very nice 26 valve:

26 DUT

 

I wish I could have a bigger collection of 26 to improve the accuracy of this statistical analysis. Either way you can get a view of what you should expect from this great valve…

 

CCS load for THD measurements

Here is a simple point to point soldered cascoded MOSFET CCS using the classic DN2540. A very simple design: carbon grid 1K resistors and two potentiometers I had at hand: 2K (coarse) + 100Ω (fine). I can set the operating point of the valve under test from 3mA to 100mA. The anode output is directly coupled to a BNC connector which is hooked to the Pete Millett’s interface. No capacitor used as the interface has a DC blocking capacitor.

 

I used an old aluminium box and build this takes less than 30min!

 

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Measuring Transconductance (Gm)

GM tester jig circuit

After a bit of work, got the transconductance jig working fine. Made an obvious omission which was not bypassing the CCS. The CCS present a very high impedance in AC to the circuit, therefore not developing the current variation on the measuring resistor. Bypassed by an electrolytic presents a path to ground.