Juma's Preamp for F5 -new built-

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GentryBa,
you are aware that some of your JFETs are out of specifications (4.7mA and 13.8mA can't belong to BL grade) which makes their other characteristics (gm, transfer curve) highly suspicious.
This preamp works on presumption that JFETs are a complementary pair without big differences in Idss (few mA is OK). Of course, there are ways to balance the input pair (no matter what you choose to use) but soundwise, it won't be worth the effort.

If I were you, I'd choose a preamp made with current production parts - the one with BF862 I like even better, this one too...
 
hi everybody!

@juma:
jepp, I know that they are massive out of the needed specifications. :(
problem is: I have bought everything I need for building it. (2x r-core transformers, capacitors, etc,etc,etc...)
I don't want to spend more money for new parts like transformer, etc... again.
In the preamp-thread, where you posted your layout, everybody said that it sounds very good- so I will hear it, too... ;)

I think I will have a look at kessler electronic and ebay.
 
hi everybody!

@juma:
jepp, I know that they are massive out of the needed specifications. :(
problem is: I have bought everything I need for building it. (2x r-core transformers, capacitors, etc,etc,etc...)
I don't want to spend more money for new parts like transformer, etc... again.
In the preamp-thread, where you posted your layout, everybody said that it sounds very good- so I will hear it, too... ;)

I think I will have a look at kessler electronic and ebay.
G,
are you having a hizzy fit?

Why start again when all you are missing is a couple of jFETs?
 
hi everbody!

my BL-graded fets from china aren't arrived yet. it's taking 6 weeks now. :mad:
maybe they're hanging in the customs duty? :confused:

is there any possibility to use a pair of matched GR-graded types?
which changes would be necessary then? ;)

thank you!

best regards,
matthias
 
hi everybody!

my FETs arrived and Juma gave me all the needful tips to fire up my preamp.
now it was time to build the micro controlled things in it.

I didn't want to have any current through the relais-coils, so I bought some latching types with a single coil.
The latching relais are controlled by normal relais and a atmega 16.

I have 5 inputs and 2 outputs, you can rename them like you want to.
The 2x16 LCD is showing in/output status and some other things.
Selection is done by a alps encoder, if you turn the encoder, then choosing between in/ouput is possible.
Pressing the button of the encoder switches to the next menu.

When I switch to another in/output, then the normal relais get some power (for about 100ms), to switch the latching ones.
After that everything is "dead", only the audio-signal is floating through the wires... :D

I made a short video, sorry for the music, but the original-audio was much much harder to accept. :rolleyes:

https://www.youtube.com/watch?v=Cb7x2WHxk-A

best regards,
Matthias
 
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hi!

@mr. dB:

oh, I see! thank you! :)

things are going forward on my preamp:
the layout from juma is working fine now, so I was working on the rest of this beast. :D


- 21 full µC-controlled relais to switch in- and ouputs with 7 standard and 14 latching relais.
the 7 takamisawa standard coil relais are powered on for 100ms and setting the 14 latch types to on/off.
the idea is simple: no power on the coils while hearing music! ;)
- you can choose all functions via alps encoder on the preamp or IR-controlled
- motor-controlled volume regulation (it has got four single potentiometers in it, and one I use for the µC to get its exact position)
if you switch from tape (my revox B77 tape recorder has got a very small output signal) to cd, the preamp reduces the volume not to destroy the loudspeakers.
you can set up every input with its own volume, or disable this automatic function.

The µC also controls the AC-supplys (r-core transformers)and the DC-ones via relais, and every other part in this preamp.

It is too much to tell, in the moment I'm waiting for the aluminium stripes to complete the housing and then sort all the wires and make it look cleaner.

Here a few pics from testing:

µC-controlled in/ouput PCB with 21 relais:
Relais PCB for µC controlled preamp - My Photo Gallery

testing on my F5 (in the picture you can see only one F5):
µC controlled preamp for F5 - My Photo Gallery

chaotic, but I wait for the housing:
µC controlled preamp for F5 - My Photo Gallery

best regards,
matthias
 
hello at all, i want also build the juma pre, but i want build the power supply with some "superegs". How can i make the pre power supply which must have 40 volt or so. The 317/337 seemed not practicable. Or can the +-33 volt changed to 27-30 volt or so? +-24 volt for the buffer is no problem. any opinions? i stay tuned.
 
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