Little Bear P5-1 line stage tube preamp

You've probably seen the very, very cheap Little Bear P5-1 tube line stage preamps on EBay. I saw one that had a starting bid of 99 cents. I managed to pick it up for about $4 plus $16 for shipping. So for about $20 I acquired one of these tube preamps. Pretty cool. I didn't get around to assembling and listening to it until today.

This unit has one set of inputs and outputs. It has a switched volume control and a wall wart power supply. It is extremely easy to assemble. There is no wiring or soldering to be done. Just assemble the case and you're ready to go.

My initial impression is that it sounds pretty sweet. As you might expect it's going to be enjoyable on voices and small ensemble music. The places where it fails are that it doesn't have a lot of gain or low end. It also comes up short on dynamics and detail. But that's in comparison to my VTA SP13 tube preamp, which *should* blow it out of the water.

So how good is the Little Bear? I like it. If you're interested in enjoying that euphonic tube sound for real cheap, this is an OK way to go. A lot has to do with what kind of music you listen to. You're not going to get anything like what's needed for big rock 'n roll, big band jazz, or even large orchestral works. Run the Little Bear for that kind of material and it will be found wanting.

Bob Latino wrote:I hope your Little Bear P5-1 is not the same as the Little Bear P6 fraud that I posted about in October of this year. Read about the Little Bear P6 and watch the included video ...

Little Bear P-6 October 2015 post with video ...

Bob

Y'know Bob, I have no idea. I do know that it doesn't have a lot of gain. If it really is just an attenuation circuit, then that would make sense. I suppose I could do the "Let's see if it works with the tubes pulled out of it" test.

I'll give it a try and will let you know.

And here I thought I was getting something cool for $20. If it sounds too good to be true, it generally is... pbfffftttt...

Russ

I pulled the tubes from the Little Bear P5-1 and it failed to operate.  Putting the tubes back in and it did operate.  

I couldn't tell you a thing about the circuit and what it's doing.  So far as I can tell, I've got a $20 tube preamp.  Worth every penny.

Russ

Sounds like $20 well spent. I've spent more on meals that were terrible. I was curious about this as a little "pre" to put on the desk at work. Thanks for posting!

hogzilla wrote:Sounds like $20 well spent. I've spent more on meals that were terrible. I was curious about this as a little "pre" to put on the desk at work. Thanks for posting!

Two things to keep in mind.  

1. Some of these Little Bear pieces may well be bogus.  Note Bob Latino's experience with the model P6 Little Bear unit.
   
2. This is strictly a line stage preamp.  RCA inputs and RCA outputs.  It is not suitable as a headphone amp.
   

Not to drift too far off topic but why would there be a need for gain in a system with an ST-120. My amp has an attenuator.

I was considering a passive preamp simply so I could access more sources and a friend offered to build a Tube Buffer based on s board from the Glass Ware web sight. The Tube Buffer has an Alps potentiometer so I leave the attenuator on my amp cranked all the way up and use the Tube Buffer's pot to control volume.

Listening with tube buffer in the circuit I don't hear an increase in volume but what I perceive is a more open sound. As if the sound stage has expanded left and right as well as front and back.

Incidentally the design calls for 12AU7 tubes but my friend gave me a set of 12AX7's to try and the difference in sound was even more open. He did say the tubes would probably not last very long but he wanted me to hear the difference.

Are my interpretations of what I'm hearing what I am supposed to be hearing and can anyone offer any other suggestions?

Forgive my hijack.

FWIW, since I had a Little Bear "P5" version lying around, I ran some freq response tests with the soundcard in loop back mode. The little amp performs quite well flatness of freq and phase wise (although the response starts to droop a bit towards the lower and higher end of the frequencies). Also compared it to my Dynaco PAS 3.

The real issue seems to be the distortion (THD) - The Dynaco is definitely a lot lower (~0.01% at 1K) and easily outperforms by 10's of dB in this regard. The little bear version I have seems to have unequal THD - 0.34% THD on the left channel vs 1.4% on the right channel! Improperly biased tubes?

Here are some plots:

1. Freq Response


2. Group Delay


3. Distortion (Left)


4. Distortion (Right)



Compared to Dynaco PAS 3 (with tone controls not bypassed, but adjusted for max flatness (<0.5dB) I could achieve by playing around with tone controls). Phase and magnitude flatness seems pretty good.

1. Freq Response


2. Group Delay


3. Dynaco PAS 3 Distortion (Left)


4. Dynaco PAS 3 Distortion (Right)

pakru wrote:FWIW, since I had a Little Bear "P5" version lying around, I ran some freq response tests with the soundcard in loop back mode. The little amp performs quite well flatness of freq and phase wise (although the response starts to droop a bit towards the lower and higher end of the frequencies). Also compared it to my Dynaco PAS 3.

The real issue seems to be the distortion (THD) - The Dynaco is definitely a lot lower (~0.01% at 1K) and easily outperforms by 10's of dB in this regard. The little bear version I have seems to have unequal THD - 0.34% THD on the left channel vs 1.4% on the right channel! Improperly biased tubes?

Very cool analysis!!  Did you try swapping the tubes?  As you can see from the photos I took, I have two tubes, but mine is the model P5-1.  Does yours have two tubes?  If so, does the performance stay the same on both channels or does it follow the tube?

I don't know if there's a bias adjustment on it.  I'll have to take a closer look.

Thanks. The one I have also has two tubes. I swapped the tubes and the distortion issue seems to follow the tube. Maybe a bad tube? Here are the spectrum plots for a 1K sinewave @-12dBFS. Left and Right do look different. The harmonic weightings are different. On the PAS, the harmonic weightings are more symmetrical between the two channels.

Also, no matter how I control the volume of the input vs the volume of the amplifier (while maintaining the same output level of -12dBFS), I was unable to get the distortion to be better than 0.25% or so. The Dynaco easily outperforms the little bear in this regard. Maybe no feeback in the little bear? There is no schematic for the one I bought...

Left:


Right:

pakru wrote:Thanks. The one I have also has two tubes. I swapped the tubes and the distortion issue seems to follow the tube. Maybe a bad tube? Here are the spectrum plots for a 1K sinewave @-12dBFS. Left and Right do look different. The harmonic weightings are different. On the PAS, the harmonic weightings are more symmetrical between the two channels.

Also, no matter how I control the volume of the input vs the volume of the amplifier (while maintaining the same output level of -12dBFS), I was unable to get the distortion to be better than 0.25% or so. The Dynaco easily outperforms the little bear in this regard. Maybe no feeback in the little bear? There is no schematic for the one I bought...

So the distortion follows the tube.  It's pretty clear that you've got a tube with an excessive amount of "funk".     But seriously, at the price of the Little Bear, you're going to get the cheapest tubes that money can buy.  It might be interesting to see what they would do with different tubes, but then, you're dropping more $$$ on something that really is little more than a curiosity.

Let's face it.  This is as cheap a tube line stage as one can buy.  *IF* it works at all and is something listenable, then success in my book.   I think the real comparison isn't with something as substantial as a Dyna PAS, but rather in comparison with something like a Hafler or Adcom solid state preamp.  Would you rather run a garden variety solid state line stage or the Little Bear?  That's the comparison I'd make.

Last night a buddy came over with some 1940's vintage RCA 6AK5 tubes, which are a cross reference to the supplied 6J1 tubes. Wow... what a difference!  It sounds pretty good.

It doesn't have the clarity, detail and low-end punch my VTA SP13 has of course.  Remember it's power supply is a wall wart.  But it sounds pretty good!

For someone just starting out in audio and on a VERY tight budget, this paired with something like a Hafler solid state power amp would be a very affordable and good sounding combo.