little bear P8 info thread

[AudioCats]

Posting some LB P8 related info here. (tube headphone amp; my understanding is that this is the toroid transformer version of little bear P7)

the build quality is not as solid as the darkvoice 336se. feels a lot more flimsy.

the volume pot is small, probably of the carbon-film type. 100ka (audio taper). knurled shaft.
volume knob is very well made, solid aluminum, has a set-screw (so it is intended for a round shaft pot). This knob fits an Alps blue pot.

P8 has a black PCB making it easier to mod. markings on PCB: "little bear P8".

The transformer was mounted atop the case in the very back, right on the edge. The whole setup looks like a joke.
Transformer is 3.1"OD x 1.8" height; rated 50va per the label.
There is a "removable" stamped-steel cover for the transformer, can be "installed" using double-sided sticky pads......inside diameter 3.5", 2.2"h

Voltages:
with no tube installed:
HV caps at 277v DC
"0/170v" terminal at 198v AC
"6.3v/1A" terminal at 7.88v AC (supplys input tubes' filaments)
"3.15/0/3.15v" terminal at 8.5v AC (power tube filament supply; the "0" is connected to ground line)

with stock tubes installed:
HV caps 229v/217v/205v/192v DC
"6.3v/1A" terminal: 7.2v AC (7.14v measured at the tube)
"3.15/0/3.15v" terminal: 6.9v AC
power consumption, read using a kill-a-watt: 64w/71va (AC power input at 122v)
====> the transformer is too small (rated 50va). The easiest way to go around this problem is by adding a separate filament supply for the output tube, this will reduce the transformers load by about 15w, maybe 20va. The transformer will then be (barely) adequate.


115/117v DC across the 2k cathode resistor of power tube.

sound impression, no mods, stock everything. phones are DIY with Beyer T1 drivers
--no hum;
--warm sound, no harshness at all, a little soft; Very easy listening.
--not very detailed, kind of a "wall of sound", good for pop music.

 

[AudioCats]

PCBA, top side

PCBA, underside

main HV reservoir caps are 150uf/450v.

available mod space:
if components are transfered to the top side of the PCBA (max compnent height 10mm), then the area under the PCB can be used for mod parts. ~35mm max height
the space in the rear of the case is mostly empty, probably can do 60mmx120mmWx50mmH max. Maybe a small transformer here for the output tube filament supply.

 

[AudioCats]

moved the transformer forward a little (there is already a slot cut in the top cover).

now it looks a little more presentable.

cut some holes in a bamboo coaster pad and "installed" using double sided tape (like how the transformer cover is "installed"), to hide the tube sockets.

 

[AudioCats]

different tubes

Tried a matched pair of RCA 5670 (from partsconnexion)
==> a little clearer sound, but with a sharp/bright top. Not as easy-listening as the stock 6n3p tubes.

I prefer the stock 6n3p.

 

[AudioCats]

the space under the PCB accommodates four ASC polyprop (20uf/400v, $2 each, allelectronics.com ) as output caps ==> 40uf/ch, large enough for 600 ohm phones.

 

[AudioCats]

* added a 20w EI core transformer to the back of case to handle the filament supply duty (output tube only). Now the toroid on top draws 50va, the EI inside draws 25va.

* Rectified the input tube filament supply (200v 2A bridge) and added 3900uf/16v FM filter cap; added two 0.47 ohm resistor to drop some voltage ==> 6.45v DC at the input tubes.


* added a Kobiconn 3.5mm jack in the face plate for signal input. (The RCA jacks in the rear were removed to make room for the 20w EI core transformer).

* moved the three 100r power resistors in the HV supply section to the top side of PCB and added thermal pads so heat can transfer to the top case.

* removed green 2k on-board cathode resistors; added wires and bolted MP930 power resistors (two 1k in series, per channel) to the underside of top plate, so the heat can be dissipated directly to the whole aluminum case. This will help keeping the capacitor temperature down since most of the heat will go directly to the aluminum case.

* added Nichicon KX, two 330uf/450v and one 220uf/450v as HV reservoir.

The sound is very listenable as background music, especially for pop and trance. Zero hum, even with the more sensitive "Beyer T1 driver" phones. The sound gets a little better after a few hours.

After running for 3hr: (read using thermal-couple; room temp 17C)
top case gets very warm, the hottest spots (where the cathode power resistors are bolted underneath) at 52C, the area above EI core transformer at 50C;
top disc of toroid at 50C (transformer cover not installed)
faceplate at 40C

Left runing overnight:
top case temps remain the same
transformer top plate 54C

the effect of putting on the transformer cover:
put on the cover and let it run for about 1hr, top surface of the cover at 40C, barely warm; removed the cover and measure toroid top plate temp right away, 59C, very hot.
so there is a ~19C differential between the temperature at the cover and the real toroid temp.
(and toroid top plate temp went up by 9C with the cover in place)


lessons learn:
--- power resistors must be able to pipe the heat directly to the aluminum case (instead of baking everything inside first)
--- the main reason of going through all the trouble to add a filament transformer (transformer mounting, wiring, and adding the input jack to the face plate), instead of just use a 100va toroid on the top, was so I can still use the transformer cover. Now it looks like using the cover will increase toroid temp to probably 65C, too hot. probably should not be used ==> the 100va top mount toroid route would have been an easier solution.
Antek 160v 100va transformer is 4" OD x 2"h, around $50.

 

[HiGHFLYiN9]

Interesting project. You could drill some vent holes in the chassis to reduce the heat a bit. It will reduce the life of the electrolytics if used continuously.