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Old 9th August 2012, 08:23 AM   #1
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Default Juma's Preamp for F5 -new built-

hi everybody!

my F5 mono's are working fine, so I thought I have to build a suitable preamp for them.

I use jumas layout for it.
Preamp ideas for F5

This time I'm building it dual-mono, both preamps in one enclosure with own r-core-transformers, supplys, relays, etc...

Controlled by an Atmega16 with 16x2 lines display. The menus are controlled by an alps encoder with switch.
Relays (and everything else like the supplys, etc...) will be switched by the controller, too... The software is ready and tested, now it's time to get all the parts together.
The controller gets his own power supply, which also supports the relais, the leds, etc... (31 relais, about 40 orange led's)
It will have 5 inputs and 2 outputs.

enclosure-dimensions: 430x280x50mm
bottom 3mm aluminium, top 6mm security-glass brown tinted
sides and front/back 10mm aluminium with all the holes for needed parts (display, volume + preamp control)

Here a few pics from the parts and first measurements on a demo-pcb:


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more pics will follow, forgot to make some of the display and the black laquered PCB's...

regards,
matthias
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Old 14th September 2012, 08:27 PM   #2
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Hello Matthias, any progress on that ?
I am building an F5 and looking forward i will need a preamp for it. RGDS
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Old 7th November 2012, 07:25 PM   #3
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hi everybody!

long time since the last update, not much time to work on it.

small progress with a big problem:
I build the power supplys the last 2 days:

12V AC r-core transformer with 12000F and bridge rectifier, gives about 16V DC.
I use 7812 for +12V, and 7912 for the negative (-12V) lines.

I bought 10 devices of every type, and ALL of them have about 400mV difference in ouput voltage?!
the 7812 types always have about 11.88V while the 7912 have -12,35V.

Are there any ideas of getting the correct voltage?

best regards,
matthias

Last edited by GentryBa; 7th November 2012 at 07:30 PM.
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Old 7th November 2012, 07:54 PM   #4
juma is offline juma  Germany
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Quote:
Originally Posted by GentryBa View Post
...small progress with a big problem:
... the 7812 types always have about 11.88V while the 7912 have -12,35V...
Not a problem at all. It won't influence your preamp except for slight asymmetry in clipping behavior which is absolutely unimportant...
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Old 7th November 2012, 08:09 PM   #5
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hey juma!

hmm, I can't get the DC-offset under -310mV in my circuit.
when I turn the pot from lowest to highest, the difference on the ouput is +-8mV.
The parts arent matched, but could that bring such an offset?

best regards,
matthias
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Old 7th November 2012, 08:12 PM   #6
juma is offline juma  Germany
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What's the Idss of your JFETs ?
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Old 7th November 2012, 08:18 PM   #7
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maybe a very silly question, but how can I get the Idss from them?

I'm not a bloody beginner, but not far from that.
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Old 7th November 2012, 08:52 PM   #8
juma is offline juma  Germany
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Idss is Id at Vgs=0V (Vds=10V)
Please read this (figure 5):
http://ken-gilbert.com/images/pdf/er...y_on_jfets.pdf
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Old 7th November 2012, 09:01 PM   #9
tinitus is offline tinitus  Europe
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found this a while ago and thought I would save it
dont know if you can use it
Attached Images
File Type: jpg jfet matching.jpg (50.3 KB, 916 views)
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Old 7th November 2012, 09:09 PM   #10
juma is offline juma  Germany
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Quote:
Originally Posted by tinitus View Post
found this a while ago and thought I would save it
dont know if you can use it
It won't tell you what's the Idss
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