Little Tube

Big Fun with Little Tubes

Part 1: Started November 2006

Little Dot II++

This is the version of Little Dot that I own myself. The Little Tube 2++ has the following characteristics:

2 * CV131 input tubes (pentode)
2 * 6C19 output tubes (military version)

This is the description found on eBay for the Little Dot 2++ :

The All New Little Dot II++ Headphone Tube Amplifier/Pre-Amp
Mullard EF92 driver tubes
6C19 J (Military) power tubes
Optimized circuit layout for minimal signal paths.
High quality ALPS-16 Potentiometer provides far better tracking between channels, which translates into more precise sound imaging.
Upgraded circuit components include high quality German WIMA capacitors and Vishay-Dale resistors.
Pre-amp functionality via RCA outputs on rear
Handles low impedance headphones as well as low-impedance, low-sensitivity far better than the original.
Includes transient voltage suppression diode to shunt off voltage spikes.
All vacuum tubes are now tested twice for reliability and performance!
The Little Dot II++ improves upon the highly successful Little Dot II+ in a multitude of ways. First and foremost the original 4P1S power tubes have been replaced by the much superior 6C19 power tubes, which translates into deeper bass, and slightly more transparency throughout the frequency range. The Little Dot 2++ should also operate cooler, and with much greater stability and reliability than its predecessor.
Capacitors, resistors, and the potentiometer have all been upgraded to NIPPON, WIMA, Vishay-Dale, and ALPS, respectively, to provide the listener with the best sound quality at this price and even beyond.
Paired with your favorite headphones, the new Little Dot II++ will provide you a thoroughly musical, and never fatiguing experience!
Little Dot II++ Technical Specifications:
Input: Gold-Plated Unbalanced Phono (RCA) Jacks
Output: 1/4" Gold-Plated Stereo Headphone out
Driver Tubes: Mullard EF92
Power Tubes: 6C19 J (Military)
Frequency Response:20HZ - 100KHz (-3dB)
THD+N:Less than 1% (50mW, 300ohm)Less than 3% (100mW, 300 ohm)
Suitable Headphone Impedance: 32 - 600 ohms
Power Output: 300mW into 300 ohms
Measurements:Metric: 230mm (length) by 130mm (width) by 110mm (height)
Weight: 2.6 kg or 5.73 lbs

Main parts of the LD2++ Circuit

I've opened up the Little Dot II++ as it became hot, I mean very hot. I have version 1.0 of the LDII++ board, designed by Sword Yang. I looked at the board and the circuit and found that the Cathode resistor of 330 Ohms and probably 3 Watts (in the circuit below its called R8) is one of the components that need upgrading. BTW, the board has a marking of 220 Ohm/3W for this resistor.

Anyway, the Little Dot II++ circuit looks similar to the circuit below, I did my best to get all components in the circuit on paper, but I did not re- check all of them. After all, it's not my intention to rebuild Sword Yangs design. It is my circuit description which I distilled from my own Little Dot 2++. It is probably not complete but contains the most important elements for you to judge the circuit and the setup of Little Dot 2++ and make modifications.

Let me state again, that the circuit found above is probably not a 100% correct although I've done my very best to make it so. I've simulated the circuit and therefore I'm pretty sure it works.

The feedback loop is an area of further checking. As defined now I'm not sure it adds too much to the design of the amp.

Suggested Modifications

Things to do to improve Little dot:

As R8, the big resistor of 330 Ohm/3W becomes very hot over time (it turns brown in color as well) I would recommend to change these resistors to 5W versions. Be sure to mount/solder the resistors such that about 1cm of space remains between the PCB and the resistors. This will greatly enhance the air ventilation and will prevent the PCB from discoloring and the solder joints from breaking.
Change C1 for .22 uF types. The small blue capacitor on he input is according to my initial simulations (I do not have exact EF92 parameters) not large enough to get the promised frequency response 20-100 kHz at -3dB.
Change C2 to black Gates type. Since these capacitors determine the low-frequency response of the amp (next to C1, C5 and C6,7 of course) it is good to have them changed for better types. Especially since these are electrolytic types of only 16V this change will offer a lot for little money. I need to see whether increasing their value to 220uF would offer additional benefits.
Roll tubes CV4015 for the EF92. Though it's not at all sure that this will be an improvement.

Upgrading internal components is relatively easy, as there is enough headroom for components as long as they are smaller than the large filter caps in the PSU.

Other Little Dot Amplifiers

Little Dot MK III

This is the latest version of the Little Dot amplifier (I used pictures put on ebay from the USA distributor).

According to the eBay website there are substantial improvements over the previous model (LD2++). But looking at the output tubes used, the 6N6P, output impedance is proably higher than with the provious model. Also, the power rating of the 6N6P is lower than the 6S19P-B.

And this is how the board looks, it is similar in lay-out to the LD2++ board I know well, but as you can see it uses other/better components and of-course it's circuit is tuned to other tubes.

I must say, although this is not a beauty contest, the new amps doesn't look bad.

Technical Specifications:

Frequency response: 12 hz~100 Khz (-1dB)
THD+N: 0.15% (80 mW @300 ohm)
Power Output:
350 mW @ 300/600 ohm
300 mW @ 120 oh
100 mW @ 32 ohm
Variable Gain: 3,4,5, or 10x

Recommended Load Impedance: 32 ohm~600 ohm
Pre-Amplifier Output Impedance: 10K ohm

Driver Tubes: 2x GE5654, tubes rolling possible with:
EF92, EF91, CV138, CV131, WE301A/B, GE5654, M8100, CV4010, EF95, 6JI
Power Tubes: 2x Soviet 6H6PI

High quality aluminum chassis
Dimensions (Metric): 270mm deep by 175mm wide by 123mm high
Dimensions (American): 10.6 inches deep by 6.9 inches wide by 4.8 inches
Weight: 3 Kg or 6.6 lbs

2 Year Warranty

Datasheets:

datasheet for the 6N6P

Little Dot 2+

The Little Dot II+ is a dual purpose tube amplifier/pre-amp which can be used to drive both headphones. The Little Dot II+ uses two Mullard EF91 tubes for driver output, and two 4P1S as the power tubes in a Class-A biased amplification circuit for maximum sound quality.

Tube used: Mullard EF91 *2, 4P1S*2
Output terminal: 6.5mm Mic Jack
Frenquency response: 10HZ—35KHZ (-3db)
Distortion: <=1% (50mW, 300ohm) , <=3% (92mW, 300 ohm)
Power handling capacity: 35W (under alternating current situation)
Input terminal: RCA Jack
Color: silver, black
Can drive headphones with 64 ohm to 600 ohm
Power supply – 220/240V
Case dimension: 200(L) *120(W) *105(D) mm (before tube installed)
Weight: 2.5k

The LD2+ is/was available in silver or black.

Little Dot 2

This is the initial version of the Little Dot 2 (2005). As you can see was the amp available in several colors.

Little Dot

This is the original Little Dot amp. I do not (yet) have more background info on this amplifier.

Link

This is the link to the manufacturer of Little Tube amplifiers: www.little-tube.com

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