All about electronic valves and hi-fi
Quick breadboard this evening after watching the world cup match.
Increased screen voltage to 160V and dialed the feedback resistor to achieve minimum distortion without jumping back to 100% pentode mode.
Nice to see 2W at less than 0.5% with a nice harmonic profile:
Don’t you?
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. 
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”
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:
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:
The 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:
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.
Added the set of decoupling SMD low ESR capacitors for each board
I used a aluminium bar structure to hold both LL7903 transformers and the Russian PIO output capacitors. The LL7903 will be wired in 4:1 and the capacitor will prevent the DC current from the DAC to shunt to ground. Also used some silicon rubber to protect the mu-metal can of the LL7903:
Here are the DAC boards mounted on a PTFE board with a set of copper strips that will be used to solder 10 x 100μF SMD low ESR capacitors to improve decoupling of DC rails:
I think all fits in the box I’m planning to use 🙂
After testing the accuracy of the R and 2R network, thanks to Jon managed to complete my discrete DAC boards for the Starlight CD player. Not very keen to solder SMD ICs but couldn’t get away from the tedious task of soldering 10 ICs:
Now need to continue with the DAC (or the transport). Lots of work to do still…
It was time to take the 814 SE Class A2 amplifier measurements. The challenge though, is that the amp is so heavy that I will never take it up to the workshop. Therefore, I decided to take my workshop PC down from the loft this time to see how the 814 really responded.
First test was to do a THD analysis as a function of the total output power. As you don’t want to do this with your speakers, and also the classic wire wound resistors (Alu-clad) are inductive, you want to use non-inductive resistors like this test jig:
I bolted on to a large heat-sink an array of resistors to form 8Ω in value by using a pair of 10Ω in parallel and three 1Ω in series. I added a set of binding posts and connectors for the speaker cables. this way you can easily wire your speakers and connect your audio test set to take the measurements.