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Old 9th February 2012, 06:26 PM   #1
dgta is offline dgta  United States
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Default LTP vs single first stage

A driver topology much seen recently is an LTP front end driving another LTP. I am wondering what advantages/disadvantages this offers vs. a single tube first stage driving the LTP second stage.

While both topologies have the same overall gain (everything else being equal), a single first stage would put more of the gain up front, which I consider a slight advantage. Also, a single first stage doesn't require a DC balance pot (when the stages are DC-coupled).

Thoughts?

Edit: I'm excluding drivers requiring a balanced input, that one is pretty clear.
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Old 9th February 2012, 06:54 PM   #2
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LTP stage with CCS has lower distortion than a single end stage with same tube.
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Old 9th February 2012, 09:30 PM   #3
DF96 is offline DF96  England
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LTP with CCS might have lower common-mode distortion if feedback is used. However, first stage signal levels are often low enough that distortion is not a real problem.
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Old 9th February 2012, 10:04 PM   #4
dgta is offline dgta  United States
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Yes, I'm assuming fair comparisons, i.e. a single-tube stage would be loaded with a CCS in the plate and LED bias, vs. an LTP loaded with a CCS in the cathode. I'm not seeing superior distortion except perhaps when the PS is not clean and regulated.

A further disadvantage to the LTP is the necessity of a negative supply for the CCS tail.
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Old 10th February 2012, 04:33 AM   #5
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Quote:
A further disadvantage to the LTP is the necessity of a negative supply for the CCS tail.
This is not necessary. A CCS made with LM317, or with transistors / fets work properly with some 10...20 V voltage drop.

LED bias, which is fixed bias, is not good solution if minimum distortion of the stage is required, since the minimum distortion "sweet spot" can be adjusted especially by fine tuning the cathode resistor.

If you want to have an excellent performing single triode stage, use CCS as anode resistor and adjust the operating point with cathode resistor to minimum distortion.

Last edited by artosalo; 10th February 2012 at 04:46 AM.
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Old 10th February 2012, 07:37 AM   #6
dgta is offline dgta  United States
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Quote:
Originally Posted by artosalo View Post
This is not necessary. A CCS made with LM317, or with transistors / fets work properly with some 10...20 V voltage drop.
Yes, I've used LM334, but in an input stage you only have a few volts on the cathode. Anything more than that you have to AC-couple the input, which I would consider a disadvantage.

So aside from distortion, what other advantages are claimed for the LTP?
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Old 10th February 2012, 11:09 AM   #7
oshifis is offline oshifis  Hungary
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NFB to a high-impedance point (g1) DC coupled, instead of to a low-impedance point (cathode) where some DC exists.
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Old 10th February 2012, 03:57 PM   #8
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I have taken to always using a LTP at the front, but I also drive both inputs via an input transformer. This allows you to step down the input to allow for better loading of the source. Also it allows you to place the grids at any potential you choose because you reference them through the CT of the transformer to the cathode node of the LTP. In this way you can stack the tail CCS at any point you like. I have used this to place the LTP anodes at zero volts direct coupled to the output stage. This effectively allows a design with no capacitors in the signal path at all.
The Input transformers are not especially demanding and I have pressed many different transformers into service at minimal cost.

Shoog
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Old 10th February 2012, 05:10 PM   #9
dgta is offline dgta  United States
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Originally Posted by Shoog View Post
I have used this to place the LTP anodes at zero volts direct coupled to the output stage.
Shoog
Your statement implies that you have only one stage before the output stage, so you have no choice but to use an LTP (or some other PI).

What we're talking about here is a front stage that's followed by another LTP.
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Old 10th February 2012, 06:06 PM   #10
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The same principle applies - its the increased flexibility which is important. You can place your ltp anodes at virtually any potential without having to rc couple the input.

Shoog
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