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Old 5th August 2008, 07:38 AM   #11
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Default fundamental design mistake ...

Quote:
Originally posted by mashaffer
...
BTW Here is RJM Phono


Do you know the fundamental design rule for low noise amplifiers?

Try to find an answer in the question
link.

Kind regards,
Darius
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Old 7th August 2008, 08:32 PM   #12
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Quote:
Do you know the fundamental design rule for low noise amplifiers?

Try to find an answer in the question
link.
Sorry that I am not that well versed yet and I was not able to follow the relevance of the link. I could take a few guesses.

Keep resistances as low as reasonably possible, use low noise resistors, use low noise input tubes, don't use a pentode for the first stage, parallel multiple input tubes... did I stumble on it by chance?

Seriously though I have heard many good comments about this phono stage and would appreciate knowing what it is that you dislike about it.
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Old 8th August 2008, 12:18 AM   #13
wa2ise is offline wa2ise  United States
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Quote:
Originally posted by alexg



I find it hard to compute the RIAA, I can't find reliable data on the plate resistance of the 6n1p, Svetlana says 4.4K, some say it is as high as 7K. RJM computed the RIAA using 6dj8 which has a plate resistance of around 2K. I am recomputing the RIAA using 4.4K on the 6n1p and 22K plate resistor.
4.4K plate resistance in parallel with your 22K resistor gives an R1 of 3.6K, and at http://www.kabusa.com/riaa.htm that yields R2=533ohms, C1=0.6uF and C2=0.2uF (to block the plate voltage from getting into the next tube's grid1, you could use a coupling cap and resistor for this grid circuit (which can be high impedance, which won't hardly much load the RIAA circuit) after this RIAA network.

I see your problem for getting a reliable value for the plate resistance, as the older datasheets imply a much different value. And as this value swamps the 22K plate resistor, getting this wrong will mess up the RIAA curve. At the cost of gain, you could insert some "artificial" resistance for R1 by adding a series resistor from the plate to the R2 C2 node. So if you assume the grand total for R1 will be 45K (add a 39K resistor in series) you'd get R2=6.5K, C1=0.047uF and C2=0.016uF and this would cut down the sensitivity of the accuracy of the RIAA curve to varying plate resistance of the tube.

You could use a trim-pot for adjusting R1, but that means you have to measure the response of the RIAA circuit, and that would be a PITA.
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Old 8th August 2008, 01:05 AM   #14
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Quote:
I see your problem for getting a reliable value for the plate resistance, as the older datasheets imply a much different value. And as this value swamps the 22K plate resistor, getting this wrong will mess up the RIAA curve. At the cost of gain, you could insert some "artificial" resistance for R1 by adding a series resistor from the plate to the R2 C2 node. So if you assume the grand total for R1 will be 45K (add a 39K resistor in series) you'd get R2=6.5K, C1=0.047uF and C2=0.016uF and this would cut down the sensitivity of the accuracy of the RIAA curve to varying plate resistance of the tube.
This is kind of what I was thinking. As long as we are using a tube with an rp substantially less than the series resistance the circuit should be relatively immune to tube changes. Of course we could complicate it further by using a choke for the plate load.
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Old 8th August 2008, 07:35 AM   #15
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Default @ mashaffer

Hello mashaffer

Quote:
Originally #12 posted by mashaffer


Sorry that I am not that well versed yet and I was not able to follow the relevance of the link. I could take a few guesses.

Keep resistances as low as reasonably possible, use low noise resistors, use low noise input tubes, don't use a pentode for the first stage, parallel multiple input tubes... did I stumble on it by chance?

Seriously though I have heard many good comments about this phono stage and would appreciate knowing what it is that you dislike about it.
Yes, in your circuit you have to keep the resistances as low as reasonably possible,
use low noise resistors, use low noise tubes at the input and
the second stage must be low noise too.
This is because the topology has a fundamental mistake.

Maybe the attachment in this post
helps you understanding what's wrong.

You will see that lots of phono stages are suffering with these problems.
I think it must be a mixture of ignorance, commercial interest and low technical knowledge
why the "audiofools" still do it this way.
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Old 8th August 2008, 09:40 PM   #16
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Sorry, I am still not following you. The only attenuation is the unavoidable attenuation of the filter network at the output of the first stage.

One could use an active load such as cascode or mu stage to increase the gain of the first stage which would, in theory, reduce the proportion of noise presented to the second stage. Is this the approach that you are thinking of or am I off course here?

Of course then we have to be sure that no negative effects result. Questions such as does the extra active device add its own noise?; Does the more complex circuitry constrict dynamics and openness? come to mind right off the bat.

I am not suggesting what the answers to such questions might be as I have no idea. I am sure this has been addressed somewhere.

It seems worth while for me to build a basic circuit first and then try the fancier approaches and see for myself. It won't happen very quickly because of other irons I have in the fire (and limited resources) but it seems the best way to learn.
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Old 10th August 2008, 09:01 AM   #17
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Default @ mashaffer #16

Quote:
Originally #16 posted by mashaffer
Sorry, I am still not following you. The only attenuation is the unavoidable attenuation of the filter network at the output of the first stage.

One could ... increase the gain of the first stage which would, in theory, reduce the proportion of noise presented to the second stage. Is this the approach that you are thinking of or am I off course here?

...
Hello Mike,
Yes, you are on the right way.
In a well done design, the noise of the input stage is
relevant for the signal to noise ratio of the whole preamp.
You must ensure that the gain of the input stage is
more than the damping of the filter.
In your case the signal damping of the RIAA network is up to
40dB but the gain of your triode is less.
This is a fundamental mistake because the noise of the RIAA components
and the noise of the second stage becomes relevant now.
You will see that lots of passive RIAA pres suffer with this problem.
The attenuation is not unavoidable,
because the RIAA eq. can be done in two steps.
This makes it possible to use the passive triode topology you want.

See attachment, passive RIAA in two stages.

See fig 4.4.1 and Ronald's design
http://Maarten@platenspeler.com/back...kground_4.html

Kind regards,
Darius

http://coupling-triode.blogspot.com/
Attached Images
File Type: jpg riaa_in_two_stages.jpg (55.8 KB, 394 views)
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Old 10th August 2008, 10:46 AM   #18
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Ahh... I had seen that approach I just forgot about it. Thanks.
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Old 12th August 2008, 01:50 PM   #19
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Quote:
Originally posted by mashaffer
Ahh... I had seen that approach I just forgot about it. Thanks.
Maybe you are able to explain this to John B. .

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Old 12th August 2008, 10:14 PM   #20
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Oh, I would never presume to teach JB anything. He has forgotten much more about tubes than I have ever even known.

One of the interesting things about electronics is that there is always another way to do things that can improve some aspect of performance or sound quality. It gives spice and interest to life.

I appreciate you staying with me while we sorted this out.
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