ScaleAutoArt
Super Member
Mine just landed in Chicago so a few more days and I will give it a whirl 
Little Bear Tube Phono Pre??
- Thread starter sled108
- Start date
xxmartinxx
New Member
Any updates on this little guy? I'm thinking about picking one up.
TankPopper
Well-Known Member
so my little bear has arrived! first impression out of the box - its much bigger then i anticipated.
second, it sound really good to my ears, but it unfortunatly it has a very distinct hum, which goes down a bit if i touch one of the ground posts with my fingers.. i guess putting it in a metal case and shied it from RF will help alot (or maybe not.. what do you guys think?). ill post some pics so we can start looking into the schematics soon.
second, it sound really good to my ears, but it unfortunatly it has a very distinct hum, which goes down a bit if i touch one of the ground posts with my fingers.. i guess putting it in a metal case and shied it from RF will help alot (or maybe not.. what do you guys think?). ill post some pics so we can start looking into the schematics soon.
NOSValves
Super Member
Put it in the steal box, change out all the WIMA caps, beef up the power supply filtering and you'd have something. Adding tube shields would also help keep the noise from the transformer from entering the tubes.
I believe this even has a cathode follower output although I have no idea how they could achieve 50db of gain with two 6922/6DJ8 voltage amps... I bet it is a feedback type RIAA curve. I'm on my smart phone right now so reading the schematic would take a high powered telescope
I believe this even has a cathode follower output although I have no idea how they could achieve 50db of gain with two 6922/6DJ8 voltage amps... I bet it is a feedback type RIAA curve. I'm on my smart phone right now so reading the schematic would take a high powered telescope
TankPopper
Well-Known Member
i dont have the time to try and trace the Schematics, but ill upload a bunch of good quality pics of it later and hopefully you will be able to figure out what goes were.
btw.. it sound pretty darn good.. if i could rid of the hum ill be one happy costumer.
btw.. it sound pretty darn good.. if i could rid of the hum ill be one happy costumer.
NOSValves
Super Member
On my PC now with my 37" HD monitor I need a clearer scan of the schematic... It is indeed a cathode follower. Your never going to get this thing quiet on a circuit board... lack of signal trace shielding and grounding issues are always the result of circuit board based tube phono preamplifiers.... point to point baby! I'm sure it can be improved though...
It is a cathode follower output which is a good thing. To really improve it someone would have to get the values off the circuit board that relate to the R and C designations.
It is a cathode follower output which is a good thing. To really improve it someone would have to get the values off the circuit board that relate to the R and C designations.
sled108
Super Member
Well then...First off, this should be called the 'Mama Bear'...It's not all that Small. But, it's a Cool Board for DIY...I like the Clear Plastic Covers, and it's a Breeze to Mess with..Just now while listening to it, I decided to change the OutPut Caps...It took all of 5 Mins. to Open it and Swap the Wimas for Russian PIOs...So far, they sound Pretty much the Same. Maybe it'll take Them a bit of Time to Open Up...Or, maybe the Wimas are as Good as a Board like this needs. I've used the MKP10s in other Projects, and I like them. If the PIOs Bloom for me, I'll let you know...
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ScaleAutoArt
Super Member
So adding one of these will help? And just the tube next to the transformer or all of them?
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NOSValves
Super Member
Those will help limit certain electrical noises but usually not hum. Usually hum will be caused by a few things and requires magnetic metal types to shield the circuit from it. Some of the noise you hear could simply be via the circuit traces... this is why I point to point wire everything... The shields in the link stop HF noises for the most part and have to be grounded... Just bolted them in will most likely do nothing and could even make matters worse. You also have to have a way to attach them and with circuit boards that is dicey.... Sockets exist to install in circuit boards with the shields but in most cases would require you to achieve the ground connection. Some unshielded phono section tubes will even pickup radio stations...
vinylkid58
Super Member
The schem. shows the internal shields (pin 9) floating, which may not be a very good idea for a phono stage. Can anyone verify that this is the case?
jeff
jeff
ScaleAutoArt
Super Member
These have the Pin 9 floating. It was very easy to Ground them...
I just got mine today...
I see the 9 pins grounded...what is the red wire for?
This will reduce noise or simply make it better?
Excuse my ignorance
vinylkid58
Super Member
Nice of the guys in ShenZhen to read these boards for customer feedback
We used to call this "field testing" when developing prototypes.:thmbsp:
jeff
