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Old 28th November 2008, 09:17 AM   #1
cedus is offline cedus  France
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Default 5687 fully balanced line stage

Here is something I drew yesterday.
http://ccedus.googlepages.com/full_b...687_preamp.png
This is my first line stage preamp design and thus I need the help of the ones "who know" to tell me if there is a problem with my design before I start designing the PCB.

My constraints for the project were the following.

- Use the 5687 because I have lot of them
- a fully balanced line stage which could be also used with SE sources.
- use CCS (because I never used them on any of my previous tube amp or preamp).

For info the PS is tube shunt regulated. The cascode CCS with LM317 and DN3545 comes from Walt Jung great article about CCS.

Part 1 :
http://waltjung.org/PDFs/Sources_101_P2.pdf
Part 2 :
http://waltjung.org/PDFs/Sources_101_P2.pdf
Addendum:
http://www.waltjung.org/PDFs/AX_Letters_0907.pdf


Could someone help me to calculate the gain of the whole thing. I don't know how to handle the 500ohms degeneration resistors in my gain calculation as well as how a ccs can affect this gain.

Thanks in advance for your help
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Old 28th November 2008, 01:29 PM   #2
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Default Re: 5687 fully balanced line stage

Quote:
Originally posted by cedus
Could someone help me to calculate the gain of the whole thing. I don't know how to handle the 500ohms degeneration resistors in my gain calculation as well as how a ccs can affect this gain.

Thanks in advance for your help
Spice can be a great help.
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Old 28th November 2008, 01:44 PM   #3
cedus is offline cedus  France
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...especially when everything is drawn with gEDA

But what I was waiting for are the formulas so I can calculate it by myself next time.
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Old 28th November 2008, 10:36 PM   #4
cedus is offline cedus  France
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Here is a new updated schematic.
http://ccedus.googlepages.com/preampv2.png
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Old 29th November 2008, 12:33 AM   #5
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Quote:
Originally posted by cedus
...especially when everything is drawn with gEDA

But what I was waiting for are the formulas so I can calculate it by myself next time.
Try here http://www.aikenamps.com/CommonCathode.htm
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Old 30th November 2008, 12:11 PM   #6
jmbee is offline jmbee  France
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hello,
I think you can use common cathode formulas then half the gain for a differential stage. For 5687, considering Ri= 3,5 K and mu=17, that gives a gain near 14dB
(5) for assymetrical in /out.
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Old 30th November 2008, 12:59 PM   #7
jane is offline jane  Norway
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This spreadsheet will calculate common cathode gain and output resistance. --> http://www.veiset.net/tech/rk.xls

Jan E.
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Life is hard - Then you die.
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Old 30th November 2008, 06:00 PM   #8
46davis is offline 46davis  United States
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Default 5687

I've been working with 5687s for a while and I think what you'll run into is a noticeable amount of distortion from the first (plate follower) stage. Even if the tube is run in a relatively straight part of the transfer curve, drawing at least 10 ma, it generates a measurable amount of second order harmonic distortion. I tried about 5% or about 4dB of local negative feedback to get it under control. Also, this tube is a real current hog making PS design expensive and it runs hotter'n hell. Other than that, great tube.
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Old 23rd July 2009, 10:24 AM   #9
sajti is online now sajti  Hungary
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Location: Vác, Hungary
I plan to build balanced preamplifier. I have 2 topology, and I'm not able to choose, which would be better:

The first:

Click the image to open in full size.

The second:

Click the image to open in full size.

Sajti
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Old 23rd July 2009, 11:16 AM   #10
jmbee is offline jmbee  France
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Hello Sajti,
In my humble opinion , the second topology has no decisive advantage
vs the first; it makes me think to a Audioresearch LS first 2 stages:
http://www.arcdb.ws/LS22/ARC_LS22_schematic1.gif
Unles you need a lot of gain, you might increase R1 and R4 for better linearity.
cordialement
jmb
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