lampizator srpp help

[chokesrule]

Hi, I recently built the lampizator srpp output stage for my voltage out DAC. It is great! It runs of a 110v 30 ma secondary from a toroid.Then I added a 40H 30ma 700ohm choke, WOW! Hence the user name. I am about to build a markII. This time I want to bias the anode with 200v dc. I have a toroid with two 160volt 30ma secondary windings. I am wanting to give each anode its own winding. I have a few questions I am hoping some one could take pity on me and help me out.

1) The anode should recieve 10ma. But what in the circuit, determines the current the anode receives?Is it the anode itself? May be the 200ohm resistor between the upper cathode and lower anode of the double triode?The 200ohm resistor linking the lower cathode to ground?I ask because I am worried if I up the bias voltage i might up the current?

2) I have calculated if I use a 160 volt ac secondary, rectified to 223 volt dc and have a clclc filter to the anode. A further 1000ohms must be added to bring the voltage down to 200 volts AS there are 1400 ohms of resistance in the two chokes of the clclc filter. Is this right???? and where is the best place to add the resistor ???

3) I dont undersand why the lampizator uses only 0.25watt resistors in the anode circuit. Obviously it works :I am listening to music right now from them.If he wants to use 10ma and uses a 150 volts dc, then surely the resistors should be 150 multiplied by 0.01 which equals 1.5 watt??In a similar vein if I up the voltage to 200, i will need 2 watt, i.e 3 watt (to be safe) resistors?I have obviously mis understood something.

I realise there are some very clever people out there and I apologise if my questions are inane, I am just learning.

[Richard]

Hi Chokes rule,

Don’t worry I’m still learning as well

1) It’s the cathode resistor that determines the plate current. With SRPP the voltage is divided between both tubes, i.e. if you have 200V on the top tube’s anode you will have 100V on the bottom tube. To calculate your cathode resistor value, just choose your operating point using the curves and divide the grid voltage by the current at that point. The current runs through both tubes. At 10mA sounds like you’re using the 6n6p.

2) I would actually download power supply designer from the Duncan’s amps website and use it to model your power supply. Use a ccs as the tube load and set it to 10mA
Software

you can add an RC section to the CLCL if needed, so CLCLRC

3) You can also calculate power dissipation by P = (I^2) R. Current squared x resistance. So for 10ma (0.01 x 0.01) x 200 ohms = 0.02W.

I’m using the 6n2p at the moment. Using an EI core transformer acts as a bit of a filter, since they have a lower frequency bandwidth compared to a torroid, but that is just my preference. Also I like valve rectifiers, you may like to give a low current one a try. The 6BW4 is cheap and easy to implement with a 6.3V heater.
The lampizator circuit is a good start but there are better sounding valve output stages out there.


Rich

[ChrisMmm]

Hi Richard,

Any chance you could point me at better valve circuits than the Lampizator ones? I have built a couple of his for current DACs and been very happy. Other circuits I have seen to date are so similar you would only call them variants of the same SRPP e.g. Thosten Loesch. Tho I would say that the Lamizator circuits have the simplest power supply design.

Cheers

[SY]

It helps to start with, what do you want the circuit to do? As a line-level amplifier, an SRPP is not a particularly good topology. It's more suited for driving highly capacitive loads to large voltage swings and trades off linearity to do so. And this circuit isn't even a good SRPP.

So, what are you trying to do? What's the desired swing and gain? What's the load impedance to be driven? Do you want low distortion?

[ChrisMmm]

First please excuse my poor knowledge of the workings of tube circuits.

I am thinking specifically as the output stage of a DAC such as the TDA1541 or AD1865 current type DACS. Tho I am also interested in the same for a voltage DAC like the Wolfson.

[SY]

OK, so what you really want is an I-to-V converter. An SRPP is definitely out.

For most linear and high dynamic range operation, we're talking about a high bandwidth feedback circuit, with the input taken to a virtual ground. There are probably a few circuits out there, findable with a bit of searching.

Another possibility is to terminate the DAC output with a small (say, 10R) resistor, then build a very quiet 40dB voltage amp and a buffer. That won't be as good as using a proper virtual ground, but it will be a hell of a lot easier to build! I'd avoid trick circuits and use a plain vanilla common-cathode voltage amplifier, coupled to a cathode follower output.

If you absolutely need to have something more complex (more tubes! more tubes!), the cathode follower could be traded in for a White follower or one of the Tektronix-style buffer circuits successfully used by Allen Wright in his preamps (Vacuum State - High End Hifi Equipment).

For a voltage DAC, all you need is a cathode follower.

[chokesrule]

Hi, Richard, NOW!!! I get why he only uses .25 wat resistors lol!!!However I think I have been making the wrong assumptions about this circuit.

1) when we say "we are running the anode at 200v" do we mean upper or lower? If it is lower, my transformer is not man enough and I have made an expensive boo boo. However, this srpp is so dynamic at 110volts secondary I am sure 400 volts to the upper anode would kill my speakers!!!!!

2) In a similar vein, when you say cathode resistor , do you mean the one linking the top cathode to the lower anode OR!!! the resistor linking the lower cathode to ground. I am asssuming that they are always the same value anyway, but I need to know for details. (and this might now be a wrong assumption).

3) Grid voltage.....The dac I am playing with is a voltage out CS4398. When we say grid bias voltage. Is this determined by the output voltage of the dac chip??? I down loaded the 4398 datasheet and tried to find a value, hoping it was -2V . However I can see no value.....If this is not the case , how do we set the grid voltage, so it works with the curve chart i have down loaded for the valve????

Yes you are right 6n6p....and I am begging you all not to mention any other circuits as you can surely see I am maxed out trying to get this working properly. If it is not the best, it is certainly the best by leagues that I have ever heard. I am hoping what I can learn from all of this I can go on to other circuits later but I am surely overloaded now and talk of other circuits will give give me a stroke. Thanks for your time

[chokesrule]

Sorry, I forgot to mention. My transformer has the secondaries as 0 then 160v. I again understand nothing of valve rectification, but would love to try...but I have just been reading up, would I not have to have a transformer with a 160-0-160 output , the 0 going to ground and the two 160s going to the anodes???? I might just get another transformer built as I really want to make a good go of this the first lampi srpp has really got me hooked and I want to make a real proper job of my mark 2.

[Allen Wright]

Originally Posted by SY

Another possibility is to terminate the DAC output with a small (say, 10R) resistor, then build a very quiet 40dB voltage amp and a buffer. That won't be as good as using a proper virtual ground,

Not in my experience, SY. The low value R followed by a fast quiet gain stage sounds WAY WAY better than any high NFB I/V converter. Have you really tried this?

Regards, Allen

[ChrisMmm]

I would be greatly appreciative if somebody could point me at a "fast quiet gain stage"! Would this fill the bill - the output stage taht is?
DAC End 2 - the AD1865N-K with single ended vacuum output stage