A viewer sent me this made in China "Little bear" phono stage to look at, and out of the box it was nothing special. But with some simple mods, it really sounds quite good.
The main issues IMHO were the coupling caps were the wrong sizes: stage 1-2 was a 0.1uf which is fine, stage 2-3 was a .47uf which only needs to be a 0.1uf, and the output cap was a .47uf which needs to be a 1.0uf. The output cap change in particular resolves the low bass output this pre-amp had. I used Mundorf Alum/oil but if you are on a budget, other types could be used. I have a feeling the supplied "Wima" film caps are fakes and I would replace even the correct size 0.1uf first stage cap while I was in there.
The China 12AX7 tubes it came with were noisy and lifeless. Some new production Mullard 12AX7 gave it some life but were still noisy. Ended up with some EH7025 tubes, which massively reduced the noise yet kept the spunky sound of the Mullard 12AX7 tubes.
They also have 62K resistors for the cart loading/input caps which should be 47K for the vast majority of MM phono carts out there. And finally I swapped the ceramic caps in the feedback loop for high precision mica caps, which made a very audible difference! I also did a bit of lead dress while I had it open.
At the end, this is a nice sounding little pre-amp for a small amount of work. I still think my EAR834 clone sounds better, but if you already own one of these, it's worth doing these upgrades to it.
Here some videos on this phono pre:
The main issues IMHO were the coupling caps were the wrong sizes: stage 1-2 was a 0.1uf which is fine, stage 2-3 was a .47uf which only needs to be a 0.1uf, and the output cap was a .47uf which needs to be a 1.0uf. The output cap change in particular resolves the low bass output this pre-amp had. I used Mundorf Alum/oil but if you are on a budget, other types could be used. I have a feeling the supplied "Wima" film caps are fakes and I would replace even the correct size 0.1uf first stage cap while I was in there.
The China 12AX7 tubes it came with were noisy and lifeless. Some new production Mullard 12AX7 gave it some life but were still noisy. Ended up with some EH7025 tubes, which massively reduced the noise yet kept the spunky sound of the Mullard 12AX7 tubes.
They also have 62K resistors for the cart loading/input caps which should be 47K for the vast majority of MM phono carts out there. And finally I swapped the ceramic caps in the feedback loop for high precision mica caps, which made a very audible difference! I also did a bit of lead dress while I had it open.
At the end, this is a nice sounding little pre-amp for a small amount of work. I still think my EAR834 clone sounds better, but if you already own one of these, it's worth doing these upgrades to it.
Here some videos on this phono pre:
A Tim de Paravinci design in Chinese clothes.
RIP Tim......
RIP Tim......
Hello, I have a Little Bear T11 and have done several mods that you mentioned in your videos. Overall I am pleased with the outcome however the excessive hum is very distracting. This is a problem that I can’t seem to solve. Any advice?
Did you twist the power supply wires? Also I have found some tubes hum in these pre-amps, another tube will be silent.
Yes, all the leads are twisted and shortened as much as possible. I also out-boarded and upgraded the power transformer to a metal enclosure. I have tried several 12ax7s and also several 6m2p-evs. No real difference in his m levels. I found that the metal chassis was not grounded so I fixed that as well. There is a small capacitor between the ground lug and signal ground. Is that necessary?
Is the hum present with the inputs shorted? And I assume this is at normal listening levels and not just the volume knob at 10 with no music playing?
I just shorted the inputs and there was no change. I am using 6n2p-ev tubes. Sorry someone suggested grounding the internal tube shield. That did not help either.
And this is at normal listening levels on the amplifier, not just wide open with no music playing? And I assume this hum has always been there?
These minor mods shouldn't introduce hum. Another suggestion is using EH7025 low noise tubes. That's all I use in tube phono stages like these.
These minor mods shouldn't introduce hum. Another suggestion is using EH7025 low noise tubes. That's all I use in tube phono stages like these.
The highest I’ve turned the gain up is around 6:00. The hum has always been there since the day I received the preamp. I did try a 12at7 on the first tube. It kind of helped.
Since there are switches for each tube to select either 6n2 tubes or 12ax7 tubes can they be mixed with possibly a 7025/12ax7 in the first position and the 6n2 tubes in the second and third position?
I would assume so.
Again you haven't answered the question: Is this hum audible at normal listening levels of gain from the listening position in the room? Like where you would normally listen to your vinyl? I see people turning the volume knob to 10-20db higher than they would ever listen (or putting their ear up to the speaker) and trying to fix noise heard there. This phono stage is never going to be silent.
Again you haven't answered the question: Is this hum audible at normal listening levels of gain from the listening position in the room? Like where you would normally listen to your vinyl? I see people turning the volume knob to 10-20db higher than they would ever listen (or putting their ear up to the speaker) and trying to fix noise heard there. This phono stage is never going to be silent.
My apologies, yes the hum is very audible at normal listening levels. I understand that putting my ear up,to the speaker does not work and I know that tube phono preamps are inherently noisy.
Yeah, it shouldn't be doing that. One more test, do you have another amplifier to connect it to? I had one tube amp that didn't like being connected to one of these type tube pre-amps, just trying to rule that out. And 100% not as a permanent solution, have you tried using a "cheater plug" to lift the earth ground to make sure it's not a ground loop issue?
I’ll have to dig out another amp and give it a try. By the way, I found 4 NOS 7025s and tried them. Actually the 6n2p tubes were more quiet! Then I tried mixing and matching. Still the 6n2p tubes were more quiet. I’ll let you know when I have a chance to try a different amp. Thanks again for your input!
Okay, it took a little bit of digging but I managed to unearth my Onkyo amp. Same results with the same amount of hum.
OK I would next just reflow all the solder joints to eliminate that possibility. It uses DC heaters so that shouldn't have any hum unless one of the filter caps either in the heater or power supply has gone bad/open?