Author: Ale Moglia

"A mistake is always forgivable, rarely excusable and always unacceptable. " (Robert Fripp)

Robustiano (Version 0.4)

Finally back home after a long trip and had the opportunity to put the DN2540 at test and try the topologies discussed for the “Schade” feedback 4P1L SE amplifier. So I re-build my test rig and tried the DN2540 and LND150 at various drain currents. It was clearly to see that in order to keep distortion to a minimum, the VDS needs to be greater than 60V to keep the output capacitance of the FET low. Here are the results of the frequency response at nearly maximum output power (Po=2W):

Robustiano 4P1L VER 0.4 DRIVER TESTSIt is interesting to see that the LND150 which has Coss (max) of 3.5pF doesn’t perform much better than the DN2540 which has Coss (max) of about 30pF. Operating points are different for both FETs but the 4P1L is running about the same operating conditions. What is also interesting to verify with this test is that the higher the drain current, the more capability the FET has to drive the 4P1L input (and Coss) capacitance at higher frequencies as the slew rate of the FET is higher.

ROBUSTIANO 4P1L VER04 THD VS POWER

We can see an interesting improvement from my initial tests at 5mA when drain current was just about 1.5mA. The yellow trace (Id=5mA) shows the best performance of the DN2540. Surely higher drain current will perform better but at a cost as the drain current is part of the OT primary current.

So how do we keep the gain of the FET when increasing the drain current? The natural approach will be to reduce Rf, but this affects the FET gain and the feedback. The alternative is to increase the supply voltage respect to ground. The price we pay here is to increase the cathode resistor and burning the power on it. With -4V as the negative source supply voltage, I had to only reduce RF to 51.5K to set 5mA on the DN2540. The supply power was increased to 350V, the screen (Vg2k) to about 140V (240-98.6V) which is lower than the 150V used before. There is a tad of extra power to extract on the 4P1L but here is close to its maximum dissipation. The Rk is a pair of wirewound 4K7 in parallel.

Robustiano 4P1L SE Schade v01.

 

 

 

 

 

 

 

 

 

 

 

I think it is now time to try the BJT driver. I suspect that it will need at least 5mA of collector current to get on with the task of the input capacitance of the 4P1L when anode to grid feedback is in place.

cheers

Ale

 

 

Robustiano: 4P1L SE Schade Feedback tests Part I and II

Here are some further tests using automated measurements. Firstly I tried same configuration running the 4P1L at closely 38mA and 160V screen voltage. Second test, I dialled up the screen voltage to 171V and tweaked feedback to get current up to 40mA

Interesting to see that the performance is more triode-like and has a higher THD at lower output levels, however at higher levels the THD is lower. Mainly dominated by H3 and significant rise of H5 given grid current I suspect:

Robustiano THD versus power test 1 and 2Here is the harmonic distribution of the first test (more pentode like)

Robustiano test1 harmonic profile

Here is the harmonic distribution of the second test (more triode like):

Robustiano test2 harmonic profileFrom a frequency response perspective, it performs very good with -3dB from 10Hz to up to 35kHz at 1W tested level:

Robustiano FR and THD 1W Test1What surprised me was to find out the increasing distortion at HF. Will this be due to the 4P1L grid capacitance or the DN2540?

 

 

 

 

Starlight Discrete DAC: a learning experience

You may find this post interesting or not. I just learned a lot with these tests and evolution of my design of the Starlight CD discrete DAC. Mostly, I want to thank Tom Browne for his patience and guidance throughout this interesting journey.

The Starlight discrete DAC has a relatively high output resistance: 10KΩ. With TTL-like levels it can deliver 1Vrms maximum. A commonly implementation of this DAC is with an output transformer in step-down mode (4:1) to reduce the output impedance of this DAC. The typical circuit is straight forward with a coupling capacitor between the DAC output stage and the transformer to block the DC current. The value of this capacitor is 2uF or higher. Many have used the Russian PIO with great results. All incarnations of this DAC sounded fantastic in my opinion, so there is plenty room for experimentation around this DAC.

I had a nice pair of LL7903 transformers. They are very nice and perform really well. These can be wired in 8:1 or 4:1. So they looked to be a right fit for this DAC. However, the high output impedance is a warning sign for any transformer as you would look to have a lot of inductance for a good FR. Higher inductance brings with it a higher parasitic capacitance on the winding which impacts the HF response. Achieving a high inductance and low capacitance is a challenge in any transformer design.

To confirm how good this transformer could be for this design, I measured the transformer to obtain its key parameters. To my surprise, it wasn’t that great. transformer model v2

Despite having a nice primary inductance (Lp), the capacitances are big. Looking at the FR response I found this not to be great (about 20KHz). Considering the Starlight CD player has LP digital filter, this will cause a slight loss of treble according to Tom.  At least 30KHz or more is needed to make this not an issue.

Continue reading “Starlight Discrete DAC: a learning experience”

Starlight Discrete DAC – Build Part 1

I don’t like doing the metalwork and perhaps this is the main reason why I ended up delaying the build. Gladly I finish all the drilling and milling required for the DAC. The output transformers were changed to Sowter 9062 (more to follow on this). The outputs have a differential as well as RCA outputs. All good now to do the wiring:

 

6Э6П-ДР driver tests

Here is a quick test I did today that the 6e6p-dr is a promising valve as a driver. I’ve used 6e5p and made several tests with these fantastic valves. Anyway, looking at a single sample from my stock:

6e6p-dr performanceThe 6e6p-dr can do well and provide a nice 0.17% @ 200Vpp. Not many valves can do this in triode-mode.

Now, let’s look at the harmonic composition:

6e6p-dr harmonics

 

Edit (28th July 2014): The harmonics should be read: K1=H2, K2=H3, K3=H4 and so on.

Harmonic level is very low and dominated by H2 with raising H4 above 120-130Vpp. An interesting dip takes place in H3 closer to maximum swing level. This may be due to CCS FET load interaction and harmonic cancellation I guess. Beyond 205Vpp my CCS is limited by its HT supply. It can be boosted, but for a quick test, here is were it ends 🙂

This will be another valve to try as a driver in my next 300B project.