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In Reply to: RE: Need a pdf document with detailed instructions, diagram and pictures posted by cics on June 16, 2010 at 09:32:36
I hope you will like it and I am happy of contribute and bring us to a better enjoyment!Please let me know if there is any correction.
GOOD LUCK!
Edits: 06/19/10Follow Ups:
I done some calculation of the circuit according the tube data and textbook reference:6n14P data: http://frank.pocnet.net/sheets/113/6/6N14P.pdf
mu=25 S=6.8 Ra = mu/S = 3.67K
Output impedance of a cathode followerRout = Ra / mu+1
= 3.67K / 26 = 141 ohm
Voltage gain of a cathode follower
Where Rk= (75+75)=150 ohm
V= (mu * Rk) / (Ra + (mu+1)*Rk) = 3750/7570 = 0.495
With a 3.3V input, this output will be about 0.4V PP.
I built the circuit to test its performance. Since I don't have any 6n14P, I used an equivalant tube, ECC84, instead.
Below pix showed the signal at the input (upper trace) and the tube output
(lower trace). The singal was drawn directly from the FPGA and hex inverter(74hcxx)was removed. No loading was applied to the tube output.
Below pix showed the signal at the input (upper trace) and the tube output
(lower trace) wuth a 75 ohm resistive load applied.
Great! This tube buffer can handle a 75 ohm load, albeit at a reduced output(about 0.32V PP). Not bad for such a simple buffer circuit.By the way, LGA 775, there are some mistakes on the power supply circuit.
The bridge rectifier was not correctly connected, and AC is applied directly to the caps.
I do not know the application of the voltage stablizer in your circuit as it was placed in series with the B+ rail. It is supposed to be connected in parellel with the PS so as to stabilize voltage. It acts like a shunt regulator and a resisitor needs to be put in series with it whereas the cap in parellel should suitably reduced. More, if it is intended to be used as a shunt reg. the rating of 75V does not seem sufficient for this circuit. 125V would be better. please clarify.
Finally, the filament needs a voltage reference. In this case, reference to the ground.
Edits: 06/21/10 06/21/10 06/21/10 06/21/10 06/21/10 06/21/10 06/21/10 06/21/10
Did tried with Resistor as CRCLC, very bad ripple filtering capability.
Sound is not good. when u use metal film will sound brighter and sharp.
Using carbon will sound not transparent.
although B+ has got 75V, but still better than previous circuit.
Higher capability of filtering ripple and more clean sound.
Edits: 06/23/10
.
hi there.. i was quite shocked when i look at ur measured scope that there is some fluctuating. hmmm just wanna report that yes the documentation has a mistake at the AC-DC rectification. After the 0C2 tube there is another shunt 0.1uf bypass capacitor.
Also i measured my B+ i got 115V nominal with +/-1% voltage ripple. Absolute max voltage swing was +/-2V. Inital startup B+ was about 131-132V.
The tests continue. The below pix shows a stock Musiland 01 USD with its BNC output connected to a 75 ohm (3-feet) cable, terminated at the other end with a two 150 ohm resistors in parellel. Not bad for this cheapee! Although noise is a bit high.
Next, a waveform test for symmetrical output. A saw wave is used. Upper trace 3.3V PP to the input of the tube buffer, the lower trace about 0.4V PP take at the output. See the voltage difference between the positive and negative swings of the lower trace.
The below pix shows the output swing at about 1V PP, the diffference is more profound. The negative swing is clipping. The buffer distorted. If you look carefully at the last pix on my previous post, you will notice the same.
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....
Edits: 06/22/10 06/22/10 06/22/10 06/22/10 06/22/10 06/22/10 06/22/10
> 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
Hi,
Looking at your 'scope traces and noting your comment on the SPDIF transformer I cannot but conclude that you are doing a lot of things rather wrong.
Above I posted a trace from my implementation, with ECC88, follower and SPDIF transformer. That is what it looks like if it is implemented well:
Ciao T
Sometimes I'd like to be the water
sometimes shallow, sometimes wild.
Born high in the mountains,
even the seas would be mine.
(Translated from the song "Aus der ferne" by City)
hi bro, good to hear that u have done the comparison of using different tubes with different circuit, may b u can also try 6n14p it is relatively cheap to try out.
Hmmm. did you hear any improvement of sound when u switch from CF to WCF?
Hi Jack
Great follow-up on the design by lga775. Hope your input will result in a better design for the tube buffer.
> The negative swing is clipping.The buffer distorted.
What is the significance of this distortion? Will it manifest in incoherence in the music?
> 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?
Best Regards
Hi Jack! Could you show the oscillogram of standard spdif output without the tube buffer (stock version of juli, musiland etc.) to see advantages of the tube buffer.
Download finnaly successful. Thank you for unselfishly sharing your hardwork.
The file is successfully downloaded. Thanks!
Me cannot download. Can anyone email the file instead at smicyta@singnet.com.sg?
tia
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