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RE: PDF DOCUMENTATION OF DIGITAL TUBE STAGE!

>The negative swing is clipping.The buffer distorted. [What is the significance of this distortion? Will it manifest in incoherence in the music?]

I havn't conducted a listen test on the distorted signal, but I guess I would not like it.

>What next? As a loading of 150 ohm is extremely low for the tube which has a RP of 3670 ohm, the cathode resistor value should be increased....

[Any suggestion to address this shortcoming?]


The tests continue. The ECC84 did not give a good voltage swing and its output impedance was high. Therefore, I switched to ECC88, a more popular tube with a lower Rp. Used as a cathode follower, it has an output impedance of 78 ohm (almost one half of 6n14P) under the given configuration and a better output swing. The ECC 88 is suitable for low voltage applications (in the old days I've built an ECC88 pre-preamp using 24V B+ for my mc cartridges), the value of cathode resistor is relatively low, therefore, 150R of cathode resistor is very suitable for the application and resulted in a better headroom.


Below two pix shows a ECC88 buffer @90V with a cathode resistor of 75R + 75R. The output was taken from the lower 75R where it was loaded with a 1.5M video cable terminated with a 75R resistive load on the other end. I've tried increasing the cathode resistor value using a negative supply rail which resulted in better voltage swing but not lower output impedance or improved waveform. I've also tried using a digital trans. but the result was not satisfatory.

First one, SPDIF signal locked




Second one, SPDIF working with 1Khz sine wave signal. This one looks very much like the "AFTER" pix in the PDF posted by lga775, albeit more symetrical.







In my last post the output waveform of the ECC 84, with no load applied, was respectable in terms of square wave performance, but not its symetrical swing. This was improved by the use of ECC88. However, when loaded with a 1.5M cable and terminated with a 75R resistor on the other end, the output of ECC88 ran out of gas and the square wave was rounder than it should be. My attenpt to put a digital trans. at the O/P was not satisfactory too. The reason for this, I believe, was because the output impedance of the tube was not low enough to drive the load. Although the ECC88 cathode follower has a respectablely low output impedance, it was using its maximum strength to drive a 75 ohm cable terminated with 75R resistor (a capacitive load (of the cable) + resistive one of the R), not to mention adding an inductive load (digi-trans.). Consider this situation, would someone be satisfied with the performance of a set up with an amplifier of 8 ohm output impedance driving a 8 ohm speaker? No. I would not. A much lower output impedance would be required to drive a 8 ohm speaker to satisfaction.

What next? I need a tube buffer with a super low output impedance. The White Cathode Follower (WCF) immediately comes to mind. See this link:
http://www.tubecad.com/2006/10/09/cathode%20followers.png


So I built a WCF circuit using an ECC88. It has a gain of 0.97 and an output impedance of around 10 ohm. You can Google for info on WCF. The inherent uses of WCF were video buffer in the old days and headphone amp. Since it has a much higher voltage swing, a resistive network was used to bring the signal down to SPDIF level. The resistor network was connected to a digital trans. before output to the 1.5m video cable(terminated with 75R resistor).

Below pix : WCF o/p SPDIF signal loack @96K





Below pix : WCF o/p working with 1Khz of sine wave.




Not bad in terms of square wave perforance, although rise time is marginally slower than the original signal. There was slight undershoots which may be improved upon refinement of the circuit.

After some burn in, I will talk about the sound quality of the three configurations, i.e. Two tube configs (CF & WCF) and the original 74HCxxx output.


Edits: 06/26/10 06/26/10 06/26/10 06/26/10 06/26/10 06/26/10 06/26/10 06/26/10 06/26/10 06/26/10 06/26/10 06/26/10 06/26/10 06/26/10 06/26/10 06/26/10 06/26/10 06/26/10 06/26/10 06/26/10 06/26/10
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