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Old 24th May 2006, 06:53 AM   #51
EUVL is offline EUVL  Europe
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Not meant to distract from Grey's circuit, but only for the sake of discussion; how about this ?

Two stage, Cascoded JFET LTP input, X-feedback. Very simple.
(It is basically the circuit from Circlomanen, except that I toke out the source feedback, and use a LTP at the first stage. Of course one can change the power devices to Cascoded Power JFETs.)

Any drawbacks ?


Patrick
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Old 25th May 2006, 07:01 PM   #52
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I am always bothered by the need to attach the center of
the load to a ground, in this case the 56 ohm 10 watt resistors.
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Old 25th May 2006, 10:21 PM   #53
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> I am always bothered by the need to attach the center of
the load to a ground, in this case the 56 ohm 10 watt resistors.

If I remember correctly, we have something similar in Aleph-X with 33ohm (100ohm) power resistors ?

I think, if I am correct, one cannot avoid that in case of a circlotron. In Grey's power stage schematics, I think he tied each output to ground with 1k. I just put down 56 ohm for no specific reason other than that it is the value I used for Aleph-X with same bias. But I guess we don't really have absolute DC in a circlotron ??


Patrick

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Old 25th May 2006, 11:11 PM   #54
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Actually not, in the sense that I can dispense with them and
have to compensate DC offset some other way. Ground is
still the same thing to everybody in the circuit. Here, you create
a virtual ground by way of dividing the voltage across the load
by 2. I didn't say that it didn't work, I said that I was not
comfortable with it.
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Old 25th May 2006, 11:18 PM   #55
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I have read in other published Circlotron info that the value of these resisters was to be 6-8 X the load value >>> 56 ohm... I am also interested in the important criteria for optimizing this value ???
In simulations of my circuit, values up to 1k ohms or so show slightly improved THD. My board has 2 300 ohm 2 Watts to Gnd across the load. I beleive my posted schematic had 220 ohms to Gnd.
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Old 25th May 2006, 11:27 PM   #56
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O.K. That having been said, You need the Gnd ref to turn on the FET but also, any imbalances, unmatched components, etc. will create currents to Gnd through these Rs. Then you have hotter Rs and DC potential devloping across the load... So, smaller is better??? But it will cut into your output power, so bigger is better ??? Will they effect damping??? Apparent source R??? Something else I should be thinking about???
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Old 25th May 2006, 11:32 PM   #57
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Well, you pin it down - smaller is better, and bigger is better.
56 ohms is probably about as good as anything. Personally
I solved the problem by transformer coupling the inputs, so
that the need for a ground vanishes, but that increases the
expense and you have to put up with tranformer distortion
(although it appears that some people like it that way)

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Old 26th May 2006, 07:42 AM   #58
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>I didn't say that it didn't work, I said that I was not comfortable with it.

Would you be kind enough to give us some hints to the reasons behind ?

> Actually not, in the sense that I can dispense with them and have to compensate DC offset some other way.

Let's suppose we take them out.

The driver stage will still be happy, as the gate is driven by current from the LTP. The speaker does not know what ground is, so still OK. The feedback resistors to the LTP might see a floating common mode voltage, so perhaps creat a virtual ground there ? Would the 100k to ground at the JFET gates be sufficient for that ?

> I solved the problem by transformer coupling the inputs, so that the need for a ground vanishes.

That does not seem to solve the problem with floating common mode voltage across the 100k resistor, or am I on the wrong track ? Or are you suggesting that we have no ground at all in the circuit ?

And I don't like transformers. Otherwise I would have been building tube amps. : )


Patrick
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Old 26th May 2006, 09:28 AM   #59
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Hi Patrick,

During waiting for NP's reply to your questions . . .

I do not know why there is a connection from the mid-point of the two 56R to the ground. For me, the mid-point is already a vertual ground without that connection. Meanwhile, the speaker load has also a vertual ground in the mid-point so that probably the two 56R are no need . . . ?

Couldn't we adjust R21 for DC offset control . . . ?

Regards
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Old 26th May 2006, 10:51 AM   #60
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jh6you,

How could we address you by name ?

> Couldn't we adjust R21 for DC offset control . . . ?

Ideally not, as R21 determines open loop gain.
You can adjust differential DC by TR2 at the source of the diff pair. But that is not the point.

I guess I have actually tried to incorporate a means of controlling absolute DC -- the 2x12.4k resistors connecting both outputs to the diff pair current source output, same as in Aleph-X.

But suppose we do not tie the outputs to ground, and they both float to say +12V wrt Gnd at no signal. Speaker sees nothing, but the JFETs get extra 1mA each, increasing bias at both power FETs through R21/R22, which when perfectly match balance themselves out. And we can get to uncontrolled bias instability.

At hindsight, maybe they should be removed, as the cicuit is not push pull, and there is no need to feedback absolute DC.

I shall try to post an updated schematics later.


Patrick
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