Little Bear Tube Phono Pre??

rothwellaudio said:
BTW, in my opinion power supplies should be in ugly boxes. Why? Because ugly boxes get put on the floor behind the rack and their even uglier magnetic fields are kept away from sensitive audio circuits. Beautiful power supplies get put next to beautiful amplifiers and can cause problems. Putting the power supply in the same box as the audio circuit can work out fine but bear in mind that phono pre-amps have a lot of gain at low frequencies (20dB more than the midrange) and transformers can cause serious hum. Be prepared to experiment and find the best layout before committing your self by drilling lots of holes etc.
Click to expand...

That's where I'm at @rothwellaudio as I'm pretty sure the toroidal transformer is the thing that's causing the hum in my T10.

My plan was to run the power main into the Amp still (so I use the on/off switch on the amp) but extend the wires on the toroidal transformer and mount it 1m away in its own enclosure (with aviation connectors for easy separation).

Plan was to twist all wires and also shield them in the T10 too as a extra precaution.

Would that work?

Bought heavy gauge 7.5A wire to complete the job which was the only concern I could immediately think of. Plan on testing the concept tonight by wiring up the transformer with 1m extra leads and putting it a way away from the amp to see if that cures my hum issue.
 
here you can see two analysis with audition, before (V2) and after (V3) my last modification of the little bear with reduced cathode resistor on the cathode follower (tube 3). The first two diagrams show the stereo-balance of a short gong sound (150ms) and the last two show the phase correlation for the same situation. V2 is the old little bear and V3 the actual version and as control the CD Version upsampled to LP recording sample rate (24bit/192kHz):
Vergleich BV2-BV3.png
http://www.imagenetz.de/ffc7be1c2/Vergleich-BV2-BV3.png.html
As you can see in the diagrams this modification brings signal curves under better control and backround lines are clearer. Especially the phase behavior is more straight and concentrated.
The changes aren't a really big difference, but keep in mind that I only changed one single resistor of the whole circuit.
I hope that these diagrams explain more than some circuit measurements.
So please have a lot of fun with your LB and enjoy your music.
Michael
 
These diagrams are special presentations or views in Audition. Stereo-balance shows the distribution and progress of the music signals between the two channels. Left channel is the top line and the right channel is the bottom line. So mono signals are a horizontal line in the middle. The Brightness is correlated to signal level.
Second the phase correlation shows the phase variations with the two channels up to +180/-180 degrees.
 
I think simple measurements of gain, anode current, maximum output voltage swing etc. would tell you (us) more about what effect that change of resistor has had. Why change a 62k resistor to a 17k resistor anyway? Was it just a guess? Was it based on a calculation of something? Or was it just a random swap and 17k was what you had lying around?
 
vinylkid58 said:
Didn't you build a 6DJ8 based phono? What happened to it?

I'll bet real money the Little Bear is based on the old RCA circuit, with a CF to get the output impedance down to an acceptable value. I don't see anything on that PCB to indicate active regulation or anything else for that matter.

jeff
Click to expand...

Hey Jeff- Is this a good one to use to boost up a low wattage SS Rotel receiver to get more out of my vinyl? I am also upgrading my cartridge to a Grado Black. Thanks!

https://www.amazon.com/gp/aw/d/B01M...1_3?colid=24P2G3HQHZZLJ&coliid=I1CRR0EQHIIJ4K
 
Just adding to this thread. I bought one of the new T11's. I just gave it a quick test run and so far I am pretty happy with how it sounds. All metal case, switches next to each socket on top to switch from 6N1's to 12AX7's. I didn't use the Chinese tubes. I used a pair of "blue top" 12AX7 military tubes for the input and gain stage. For the output I have a JJ ECC83S for now. There is now a gain control on the front instead of the hi-low switch used on the older models. There is a very light hum at high volume, but not noticable with music playing. At lower volume its quiet. I haven't tried the metal shields yet, just the acrylic ones. It is definitely a little more detailed than the VTA PH-10 phono board I was using that sports a par of 12ay7's. A couple pictures right after installing the better tubes.
20171110_125439.jpg 20171110_125447.jpg
I will be getting a socket adaptor for the rectifier and use a 6X4 instad of the 6Z4.
 
knockbill said:
That's kinda cute! whats the thought behind the plastic shields? seems metal would help kill the hum...
Click to expand...
Just eye candy. Once I settle on the tubes the metal shields will be installed. I was just curious about how much hum there would be using the plastic tubes. My line stage just has too much gain. Using the passive pre with selector is working well with line level inputs. So I added the phono pre. It all takes up less space too. I just have too much going on to build a new phono pre right now. The T11 does sound pretty darn good so far!
 
Looks like a 'real ' one now. Much nicer. Variable gain too. Cable inputs on the front make untidy look.
If I didn't already get a hand made unit. ........
 
Lavane said:
Jacks on the front aren't optimal I agree.
Click to expand...

Totally agree. That's why I moved them to the back for my modified T10.

Updates on my T10, my hum has almost completely disappeared, thinking of putting the preamp in low gain because of this to see what difference that makes. Looks like the issues were driven by some turntable wire issues in my latest version. I highly recommend these preamps, look great, work great and very competitively priced!
 
DSCF0238.JPG DSCF0241.JPG DSCF0242.JPG Good evening all. I have continued completion of Little Bear. I changed nothing in the scheme, except polarity of heat of lamps. I have added EQ curves select switches (Bass-Turnover and Treble-Rolloff) for switching of standards of record. And the switch for change of capacity of loading of a cartridge. The equalizer was very useful when listening some records.
 
^ That’s impressive!

Quick question for those with Little Bears (I have a modified T10). Is there a way to play around with the cartridge loading? I’m pretty up to speed with the capacitance and how that works and how I can refine it a bit by getting low capacitance cables and short runs, but I’m looking into an Ortofon 2M Bronze and also a Denon DL110 as a entry in to MC and wondering if I can tweak the capacitance and more importantly the resistance of the cart loading?

Little Bear is set up at 220 and 47k I think and not sure how these are calculated or done. Could I solder in new parts to adjust these manually? Or should I leave it alone?
 
Hi Neevo. I will describe only the experience for a cartridge of Shure M75 type D. For him the recommended loading 47k and 400-500pF. The sum of capacities of a cable and LB is obviously not sufficient for the correct alignment. I have decided to add new parts to adjust capacity. By means of the switch of 2x6 positions. 0-20-50-100-220-330pF. Many famous companies use it. But I haven't noticed an essential difference in a sound when switching. To me seems there is small rise from above 10kHz for 330pF. I didn't do measurement. I think that it was possible to leave it as was.
 
Just took delivery of my T11 and am now on my 6th album. I didn't even bother with the stock tubes and put in some new production Mullard's and a Mullard CV4024 in V1 that I had on hand. I am really loving this thing. It's my first outboard phono pre, I've always used vintage integrated amps, Sansui mostly. But, this is a step up for sure. I was bracing myself for the hum I've read a lot about... but, none. Zero hum with no shields... why hide tubes!

So, I am really interested in getting a Denon DL-110. But, I see that there was some question as to the compatibility of the DL-110 and the T11 per Neevo's post above? Is this really an issue, or is anyone having any luck with this cartridge and the T11? I'm currently using a Nagaoka MP-110, but want to move up to the Denon. If this is not advisable, then it will be the Nag MP-150.
 
My DL-110 works fine @loaded, it’s just quieter given it’s a HOMC. If you have zero hum you’d be fine, mine was only borderline because my Little Bear isn’t completely quiet,

I tend to not run my DL much these days as I find my Ortofon 2M Bronze so much better.
 
loaded said:
Just took delivery of my T11 and am now on my 6th album. I didn't even bother with the stock tubes and put in some new production Mullard's and a Mullard CV4024 in V1 that I had on hand. I am really loving this thing. It's my first outboard phono pre, I've always used vintage integrated amps, Sansui mostly. But, this is a step up for sure. I was bracing myself for the hum I've read a lot about... but, none. Zero hum with no shields... why hide tubes!

So, I am really interested in getting a Denon DL-110. But, I see that there was some question as to the compatibility of the DL-110 and the T11 per Neevo's post above? Is this really an issue, or is anyone having any luck with this cartridge and the T11? I'm currently using a Nagaoka MP-110, but want to move up to the Denon. If this is not advisable, then it will be the Nag MP-150.
Click to expand...
I have a T11 as well. It's a pretty nice phono pre for the money. How many hours do you have on the MP-110? It takes about 50 or so hours for it to settle in. Over time it will open up and sound better. The highs especially.
I'm very happy with that cartridge now that it is broken in. Happy enough that I'm seriously considering a MP-150.
 
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