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PostPosted: Thu Dec 14, 2017 1:10 am 
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Sheep

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Location: Australia
hanski wrote:
As an improvement I tried to cascode the cascode. The sim said beautiful numbers.
But in reality I had to add a snubber in the cascode transistors midpoint to make it stable.

With this snubber it wasn't any better than the single transistor cascode.

What to try next?
I'm looking for about 10MOhm @ 10kHz impedance.
Attachment:
CCS_2.PNG


Try gate stoppers on ALL FETs. Make them at least a few hundred ohms. A 10 ohm
gate stopper will do next to nothing.

T


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PostPosted: Sun Dec 31, 2017 2:36 pm 
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Sheep

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Kuei Yang Wang wrote:
...Long standing personal fav line stage, with the actual attenuator AFTER this stage...Ciao T


Thorsten, what's your experience in locating the volume attenuator at the output of a linestage, rather than at the input? I presume that you use the lowest possible attenuator value so as to limit the output impedance to some reasonable range. But, of course, that goal is opposed by the increasing THD due the load represented a lower value attenuator. Since the peak output resistance of an attnuator is 1/4 it's full resistance (@ the -6dB point) what attnuator value do you fnd to work well in your linestage designs? The lowest audio quality commercial attenuator value I've seen is 5k.


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PostPosted: Wed Jan 03, 2018 8:15 am 
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Muriel
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Hi,

knewton wrote:
Thorsten, what's your experience in locating the volume attenuator at the output of a linestage, rather than at the input? I presume that you use the lowest possible attenuator value so as to limit the output impedance to some reasonable range. But, of course, that goal is opposed by the increasing THD due the load represented a lower value attenuator. Since the peak output resistance of an attnuator is 1/4 it's full resistance (@ the -6dB point) what attnuator value do you fnd to work well in your linestage designs? The lowest audio quality commercial attenuator value I've seen is 5k.


Some things to mention, all my equipment has tube inputs. This means high impedance (100kOhm), low input capacitance (<< 100pF) and I use short, low capacitance interconnects.

So I have been using 10k Attenuators and/or TVC/AVC.

It is important to get the system gain right, I usually shoot for around 6-10dB gain reserve, so normally -6...-10dB volume control setting corresponds to the maximum SPL with "normally loud" recordings, modern heavily compressed recordings will normally have lower settings.

In the end what I describe as "Volume Control after Gainstage" must be designed into the system for best results, though at least one preamp is being sold (expensively) for more general use:

https://www.firstsoundaudio.com/

Abundant photos show the details here:

http://www.6moons.com/audioreviews/firstsound/1.html

If you really need "drive" you can always add a cathode follower after the volume control, in which case you can use a 100K pot and much lower coupling cap values in the gain stage, or (shock, horror) use a solid state buffer with the option of one buffered and one unbuffered output.

Ciao T

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PostPosted: Sun Jan 07, 2018 11:29 pm 
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Sheep

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Thorsten,

Thanks, for your response regarding my attenuator query.

On another subject, I note that your description of the FET circuit open-loop distortion spectra - "For 1kHz 4V PP into 3.4kOhm the simmed THD (and I have measured similar circuits on AP2 and know this is not far off) is 0.2%, practically all H2 at -54dB with H3 at -100dB and nothing else of consequence" - is very similar to that which you show for your favorite 5687 based linestage.

Setting aside for the moment the excessive gain of open-loop operation of the FET stage for use as a line amp, I wonder whether the FEt and tube stages would sound very similar. With 56dB of open-loop gain, perhaps, your FET based stage would make an interesting sounding phono amplification stage.


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PostPosted: Mon Jan 08, 2018 12:26 pm 
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Muriel
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Hi,

knewton wrote:
On another subject, I note that your description of the FET circuit open-loop distortion spectra - "For 1kHz 4V PP into 3.4kOhm the simmed THD (and I have measured similar circuits on AP2 and know this is not far off) is 0.2%, practically all H2 at -54dB with H3 at -100dB and nothing else of consequence" - is very similar to that which you show for your favorite 5687 based linestage.


So it is.

knewton wrote:
Setting aside for the moment the excessive gain of open-loop operation of the FET stage for use as a line amp, I wonder whether the FEt and tube stages would sound very similar. With 56dB of open-loop gain, perhaps, your FET based stage would make an interesting sounding phono amplification stage.


Similar to a degree, yes. But real tubes sound different from Fet's.

In some parameters though (especially this "warmth" thing) it is possible to make FET circuits that are more "tubey" in the classic sense of the word than tubes.

Ciao T

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PostPosted: Tue Jan 09, 2018 5:00 pm 
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How low of a closed-loop gain is the circuit stable down to?


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PostPosted: Wed Jan 10, 2018 2:42 am 
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Muriel
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Hi,

knewton wrote:
How low of a closed-loop gain is the circuit stable down to?


True single stage circuits should be unconditionally stable, in theory, all the usual caveats apply.

Inverting single stage circuits can easily have negative gain.

Ciao T

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PostPosted: Thu Jan 11, 2018 12:12 am 
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Sheep

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Kuei Yang Wang wrote:
Hi,

knewton wrote:
How low of a closed-loop gain is the circuit stable down to?


True single stage circuits should be unconditionally stable, in theory, all the usual caveats apply.

Inverting single stage circuits can easily have negative gain.

Ciao T


T, I had phrased my question poorly. I'm wondering if your sims predict stability down to unity gain. Yes, the low-ish open-loop gain should bode well for closed-loop stability, but the operative word is 'should'. I'd sim it myself except that I'm away from home (and from my sim s/w) for an extended period and find your design intriguing.


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PostPosted: Fri Jan 12, 2018 2:55 am 
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Muriel
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Hi,

knewton wrote:
T, I had phrased my question poorly. I'm wondering if your sims predict stability down to unity gain. Yes, the low-ish open-loop gain should bode well for closed-loop stability, but the operative word is 'should'. I'd sim it myself except that I'm away from home (and from my sim s/w) for an extended period and find your design intriguing.


First, the low OLG is not the issue. Low OLG circuits oscillate happily, as long as there is enough phase shift between input and output to make the feedback signal in phase.

With only a single stage (or a gain-stage + buffer topology where the buffer is fast enough that it has a flat phase response to past the frequency where the gain-stage OLG drops below unity) the system requires significant effort to create an additional 180 degrees phase shift. A crystal or LC series circuit usually does it, which is why you see single BJT/FET oscillators.

Using an essentially phase linear feedback network should guarantee stability with a single stage at all gains.

And yes, it is possible to use a shunt-feedback Volume control with this sort of circuit. I have reasons why I usually do not use it.

You can see an example of this sort of thing here:

http://www.renardson-audio.com/hp2-amp.html

Image

It is actually a pretty nifty and clever design.

I'd probably swap the FET output for a J-BI-Sziklai with N-Channel J-Fet and nice fast power PNP transistor, which should give better results than the Mosfet.

For fun, one could make the output into a white follower which lower output impedance and doubles current capacity into lower impedances...

Ciao T

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