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Old 27th November 2003, 12:48 PM   #1
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Default simple line buffer

I am considering building this very simple line pream.
It avoids input capacitor and rejects noise from supply
well.

What do you think about it?

thanks
Federico
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Old 27th November 2003, 01:16 PM   #2
Aiace is offline Aiace  Italy
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Sometime CD players have some bias at the output. If this approached the tube could burn. It' safe use a capacitor at the input.

At the output you just have 2 capacitors: the supply capacitors. You should use 2 fo good quality.

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Aiace
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Old 27th November 2003, 03:54 PM   #3
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Hi,

You have you poor 6DJ8 floating in a 500VDC supply.
It wasn't made for that high supply voltages.

I also don't see a coupling cap on the output.
You can build a CF that can do without it but for a linestage I'd rather use a cap for safety reasons.

Cheers,
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Old 27th November 2003, 04:41 PM   #4
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Excuse me Frank, I have not understood

I have a 250 V supply and the anode to catode voltage
difference is about 125V with about 8/8.5 mA.

Again, the circuit is not DC coupled.
There are the two escape capacitors.

Note that despite their value is 1 uF, they
act as they are about 30 uF (mu times),
and this is one of the reason to prefer this
circuit ( as for me , obviously).

grazie e ciao

Federico
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Old 27th November 2003, 05:15 PM   #5
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Ciao,

Quote:
I have a 250 V supply and the anode to catode voltage
Don't you say your PS is floating? It says 250V, so I think it is +/- 250V, the + goes to the anode of the tube, the - to the cathode?

Quote:
the anode to catode voltage difference is about 125V with about 8/8.5 mA.
That's voltage drop across anode and cathode.

Quote:
Again, the circuit is not DC coupled.
Aren't they referenced to ground?

Quote:
Note that despite their value is 1 uF, they act as they are about 30 uF (mu times), and this is one of the reason to prefer this circuit ( as for me , obviously).
As capacitance multipliers?

I haven't seen anything like it before but it looks like this one:

Maybe it's just me not understanding your circuit, in which case I apologise.

Cheers,
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Old 27th November 2003, 05:38 PM   #6
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I also think that you have dc on the output, which is whatever voltage the valve pulls across the 14k resistor. You must place a cap at the node of this resistor the 300k and the 1 meg before the output if you wish to block dc.
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Old 28th November 2003, 11:12 AM   #7
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No Paul,

I have not DC on the output due to the floating PSU.
It floats in such a way to always provide 0 volts at the output
irrespective of the input, as you can see in the attached image.

Only the current across the tube will change.

Best

Federico
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Old 28th November 2003, 11:50 AM   #8
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No Frank,

the two circuits are different in various ways, for example the different position of the capacitors. Look at frequency response
that follows. The 3db frequencies are very different,
the ratio being about 18 that is mu/2.

With this arrangement we can use smaller caps of high quality.
Otherwise, with equal caps we have more extension at the low freqs.
The point, however, is not the extension of the frequency response, but the output resistance behavior at low freq.
as you can see.

Ciao

federico
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Old 30th November 2003, 10:30 PM   #9
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Hi,

Quote:
the two circuits are different in various ways, for example the different position of the capacitors.
Yes, not that it matters.
Have you actually built it?

How about DC offset at the output?
I usualy don't count on miracles if it's critical.

Quote:
Otherwise, with equal caps we have more extension at the low freqs.
Which caps?
One circuit is DC coupled at first look but isn't really, the other isn't at all and you have 0.200F working into slightly less than 50K. (1M//50K)
Which goes some way to explain the roll-of in the bass department.

Enlighten me because wherever I see differences and vice versa you seem not to.

No offense, just trying to understand what I don't see.

Cheers,
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Old 1st December 2003, 08:22 AM   #10
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Hi Frank,

First of all I have to apologize for my bad knowledge
of English language.

There is no DC offset at the output if there is no
grid current. That is, if the supply provides a
voltage sufficiently high and the tube is properly biased
there are no problems.

However if one, for instance, uses batteries and works
with a voltagesupply smaller than, say, 60 Volts,
grid current is likely to appears since we have small polarization.

In this case we have an offset at the output and
we have to modify the circuit as reported.
That is, we have to put a cap at the input.
Again, with this last arrangement there is
absolutely no DC offset at the output.
(Automatically, no manually adjustment required)

I am experimenting at low voltage (actually
about 37V supply) with ECC88, E88CC, PCC88
but I had the best results with ECC86.

Regarding the caps., I simply want to say that
if I have at my disposal two 0.1uF caps Ill
use them in the proposed circuit rather than
in a cathode follower because of the better
performance, with particular reference to the
output impedance.

Bye

Federico
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