Making a Battery Operated EF95/6AK5 Hybrid Tube Amplifier

Experimental tube amplfier project

The completed amplifier

I decided to make this as I could not find any EF95/6AK5 battery operated tube amplifiers. Also I was bored and fancied a challenge, by the end of this project I had learnt a lot of things, I will now be working on new projects. It seems I can't stay idle for too long.

The project started when I purchased a Fosi Audio T20 tube amplifier from Amazon, this was a polished hybrid tube amplifier with a TI power amplifier. I went on AliExpress and decided to purchase a bare PCB tube amplifier with an op-amp used for the headphone amplification. I was intrigued and decided to take it apart. The end result is this improved amplifier that runs on batteries.

The original schematic was based on a standard design and was nothing new, I found multiple examples based on this schematic, I made a few changes like adding a cathode resistor for a better bass response and adjusted the plate voltage and bias voltage.

Just to be clear this amplifier is not a true tube amplifier, the line input is fed into the EF95 tubes and amplified for the next stage, this is an op-amp which amplifies for the headphones. So technically speaking the tubes are adding their effect/distortion and this is then amplified by the op-amp.

It's also not hifi, but based in my testing and my 41 year old hearing it sounds great to me, I'm no audiophile but I have decent speaker/amplifier setups and headphones, I have to say this does sound good. I noticed changing the tubes and op-amp has an effect out the output sound.

The two different tube LEDs can be turned off.

The amplifier consists of a main board that holds the actual tube/op-amp circuit along with a high voltage booster, so this board on its own can be used with all the other modules.
The battery module consists of a large PCB that hold 4x 18650 batteries, this board hold the components for providing a 4.6v supply to the TP4056 battery charging module, and a LDO linear regulator for converting the 16.8v from the 4S ABLIC s-8254 battery protection system to 12v.

Due to the requirements of voltage regulators, a heatsink is requred, this is added to the battery module PCB. I also decided to add a LM3914 battery level indicator, this is useful for keeping an eye on the individual battery voltages.

A bluetooth module, based on the Mh-M18 has been added as an input option.

The amplifier draws about 300mA and has a battery runtime of about eight hours. The amplifier can also be run from the mains, it can charge the batteries whilst you listen as well.
A DC power supply of 12V at 3.5Amp is required.

A total of six relays are used, two of the relays are used to isolate/connect the + power of the batteries, one is used to switch between DC and battery operation of the amplifier, another is used for the auto activation of the battery protection circuit and the two more are used for switching between bluetooth and line in and adding a delay before the headphones are connected to the output.

On the front we have a bluetooth switch with a blue LED, a red power indicator LED, a large logarithmic volume control, a on/off/charge toggle switch and the TP4056 charging module is visible so you can see the battery charging state.
On the left side we have the battery protection system with a power indicator LED.
The panel consists of gold plated line in/pre out RCA connectors, a 2.5mm x 5mm DC in, tube LED switch and three switches, one for running the amp on DC mains, disabling the battery charger and a battery protection reset switch which is not connected.

Note: See the other posts for more info on the modules mentioned below.

Videos of the amplifier.

Part 1

Part 2

Part 3

Links

EasyEDA Projects:

Main Amp:
https://easyeda.com/Ratti3/ef95-6ak5-hybrid-tube-headphone-amplifier

Battery Board:
https://easyeda.com/Ratti3/ef95-6ak5-hybrid-tube-headphone-amplifier-4s-18650-module

S-8254 Battery Protection:
https://easyeda.com/Ratti3/ablic-4s-18650-li-ion-battery-protection-system-with-auto-reset 

TP4056X Charging Module:
https://easyeda.com/Ratti3/tp4056x-18650-li-ion-battery-charger

LM3914 Battery Level Indicator:
https://easyeda.com/Ratti3/lm3914n-li-ion-adjustable-battery-level-meter

MH-M18 Bluetooth Receiver:
https://easyeda.com/Ratti3/mh-m18-bluetooth-audio-receiver-module 

YouTube Videos:

S-8254 Battery Protection:
https://youtu.be/SwKMUWJow2g

TP4056X Charging Module:
https://youtu.be/Aoi0kJcRnSE

LM3914 Battery level Indicator:
https://youtu.be/u_-uY8or-Cc

The schematics

Main amplifier:

All the resistors are SMD, the capacitors chosen are all audio grade. The resistors have been adjusted to provide a plate voltage of 85V and a bias voltage of 1.35V. These values change depending on the EF95 tubes used.

Main amplifier

100V booster:

All components used here are SMD, the schottky barrier diode is rated for 150V max. The circuit is configured to oscillate at 75KHz so it cannot be heard when listening to music. Output has fairly low ripple current. Two capacitors and resistors provide the ripple current filter on the output.

100V booster

Headphone start delay:

A three transistor configuration is used to add a delay (set by the capacitor value), so that the headphones are not connected immediately to the output.

Headphone start delay

Input selector:

A simple relay circuit to control switching between line in and bluetooth.

Input selector

LEDs:

Some cheeky LEDs under the tubes, two switchable colours, they can be turned off completely.

LEDs

4S Battery Module:

This is the battery module PCB, it provides 4.6V for the TP4056 charging module, it converts the 16.8V from the battery protection system to 12V and holds the four 18650 batteries.

Battery module

TP4056X Battery Charger:

This module charges the four 18650 batteries individually, cut-off voltage is 4.18V and charging current is 600mA.

TP4056X battery charger

ABLIC S-8254 Battery Protection:

This module protects the batteries from over-discharging (3V), and shuts down if the batteries are unbalanced but does not balance the batteries.

S-8254 battery protection

Auto Activation for the S-8254

LM3914N Battery Level Indicator:

This module is used to monitor the voltage of the four 18650 batteries individually, it can be calibrated to detect the min and max voltages of the batteries.

LM3914N battery level indicator

MH-M18 Bluetooth Audio Receiver:

This module adds a simple bluetooth receiver to the amplifier, the additional components are there to remove ground loops and ripple current reduction.

MH-M18 bluetooth audio receiver

The calculations

Check the posts on the individual modules to see the fomulas used for the UC3843, TP4056, S-8254 and the start delay circuits.

Thermal calculations for LM2596T:

    V_OUT = 4.63v (the desired output voltage)
    V_IN = 12v (supply voltage)
    I_LOAD = 2.5A (maximun load current)

    I_Q = 7.5mA (Quiescent Current)
    V_SAT = 1.2v (Saturation Voltage)

First we work out the duty cycle:
    d (Duty Cycle) = V_OUT/V_IN
    d  = 4.63/12 = 0.386

Next we work out the power dissipated:
    P_D  =  (V_IN x I_Q) + d x I_LOAD x V_SAT
    P_D  =  (12 x 0.0075) + 0.386 x 2.5 x 1.2 = 1.43W

These are the known and assumed values needed to work out the junction temperature with and without a heatsink.

    T_JMAX = 125°C (max junction temperature of the regulator)
    R_θJA (Thermal Resistance Junction to Air) = 50°C/W
    R_θJC (Thermal Resistance Junction to Case) = 2°C/W
    R_θCS (Thermal Resistance Case to Heatsink) = 0.5°C/W
    R_θSA (Thermal Resistance Heatsink to Ambient) = 7°C/W
    T_A (Room Ambient Temperature) = 40°C <---- Using a higer value for worst case scenario

To work out the Junction Temperature with a 7°C/W heatsink:
    T_J  =  P_D (R_θJC + R_θCS + R_θSA) + T_A
    T_J  =  1.43 x (2 + 0.5 + 7) + 40 = 54°C

To work out the Junction Temperature without a heatsink:
    T_J  =  (R_θJA)(P_D) + T_A
    T_J  =  (50 x 1.43) + 40 = 111.5°C <---- Junction temperature without heatsink, a bit toasty but possible.

Thermal calculations for MIC29300:

    V_OUT = 12v (output voltage)
    V_IN = 16.8v (max inout voltage)
    I_OUT = 500mA (max current used by amplifier)

To work out the power dissipated:
    P_D  =  I_OUT (1.01(V_IN - V_OUT))
    P_D  =  0.5 x (1.01 x (16.8 - 12) = 2.42W

These are the known and assumed values needed to work out the junction temperature with and without a heatsink.
    T_JMAX = 125°C
    R_θJC (Thermal Resistance Junction to Case) = 2°C/W
    R_θCS (Thermal Resistance Case to Heatsink) = 0.5°C/W
    T_A (Room Ambient Temperature) = 40°C

To work out the Thermal Resistance Heatsink to Ambient:
    R_θSA  =  ((T_JMAX - T_A) / P_D) - (R_θJC + R_θCS)
    R_θSA  =  ((125 - 40) / 2.42) - (2 + 0.5) = 33°C/W <---- 33°C/W or lower heatsink required

To work out the Junction Temperature with Heatsink:
    T_J  =  P_D (R_θJC + R_θCS + R_θSA) + T_A
    T_J  =  2.42 x (2 + 0.5 + 7) + 40 = 63°C <---- 63°C Junction temperature with a 7°C/W heatsink

LM2596T adjustable resistor calculations:

    V_REF = 1.23V
    R1 = 470R (MIN 240R : MAX 1.5K)
    R2 = 1.3K
 
    V_OUT  =  V_REF (1 + (R2/R1))
    V_OUT  =  1.23 x (1 + (1300/470)) = 4.63V

These values produce the desired 4.63V needed by the TP4056X charging module.

The PCBs

The below images are the PCB layout, both the top and bottom are in a single image. The 3D models are also shown.

Main amp board

Main amp board 3D model

Battery board

Battery board 3D model

LM3914 battery level indicator

LM3914 battery level indicator 3D model

MH-M18 bluetooth receiver

MH-M18 bluetooth receiver 3D model

Rear panel

S-8254 battery protection

S-8254 battery protection 3D model

TP4056X charging module

TP4056X charging module 3D model

The build

For the enclosure I decided to use Perspex, I like using tinted Perspex for electronics projects as I like to see the insides.

I did not use any special tools to make the case, just a tenon saw, a dremmel and a drill press along with some wet and dry sand paper.

Bill of materials

See the individual module blog posts for those BOMs.

This is the BOM for the main amplifier board:

Name	Designator	Qty	Manufacturer Part	Manufacturer	Supplier	Supplier Part
47K	R13,R14	2	RC1206FR-0747KL	YAGEO	LCSC	C137270
4.7K	R6,R3	2	RC1206FR-074K7L	YAGEO	LCSC	C137262
56K	R33	1	RC0805FR-0756KL	YAGEO	LCSC	C137509
470K	R4,R1	2	RC1206FR-07470KL	YAGEO	LCSC	C137273
100nF	C19	1	CGA4J2X7R2A104KT0Y0U	TDK	LCSC	C342919
S115FA	SD1	1	S115FA	ON Semicon	LCSC	C258208
MHT170UGCT	LED4,LED3	2	MHT170UGCT	MEIHUA	LCSC	C397047
270	R2,R5	2	AC1206FR-07270RL	YAGEO	LCSC	C229488
ZMM22	ZD2,ZD1	2	ZMM22	SEMTECH	LCSC	C8073
2.2	R38,R39,R40,R37	4	RC1206FR-072R2L	YAGEO	LCSC	C137327
220u	C13,C14,C12,C11	4	UFG1E221MPM	Nichicon	Mouser	647-UFG1E221MPM
100u	C6,C5	2	UFG1E101MPM	Nichicon	Mouser	647-UFG1E101MPM
1.5K	R18,R15	2	RC1206FR-071K5L	YAGEO	LCSC	C114929
4.7nF	C17	1	CGA4J3C0G2E472J125AA	TDK	LCSC	C193083
2.2K	R41	1	RC0805FR-072K2L	YAGEO	LCSC	C114561

This is the BOM for the battery board:

Name	Designator	Qty	Manufacturer Part	Manufacturer	Supplier	Supplier Part
LM2596T-ADJ/NOPB	U1.LM2596T-ADJ	1	LM2596T-ADJ/NOPB	TI	LCSC	C21149
ZMM22	ZD3,ZD2,ZD1	3	ZMM22	SEMTECH	LCSC	C8073
MIC29300-12WT	U3.MIC29300-12WT	1	-	Microchip Tech	LCSC	C14399
XH-3A	CN1,BMS_HML,CN2,CN3,CN4	5	XH-3A	BOOMELE	LCSC	C2316
VS-50WQ03FN-M3	SD1	1	VS-50WQ03FN-M3	Vishay Intertech	LCSC	C222476
330uF	C2	1	EEUFC1V331	PANASONIC	RS-Online	315-0732
1N4002W	D3,D1,D2	3	1N4002W	Jingdao	LCSC	C169542
7447471470	L1	1	7447471330	Wurth Elektronik	RS-Online	749-8375
VH-2A	12VDCIN	1	VH-2A	BOOMELE	LCSC	C16728
Relpol DPDT 12V 8A	RL2,RL1,RL3	3	RM84-2012-35-1012	Relpol	RS-Online	123-2079
PH-2AK	CHG_PWR,BAT2,BAT3 BAT4,AMP_SW,BAT1	6	PH-2AK	BOOMELE	LCSC	C8886
22uF	C4,C3	2	EEUFC1J220	Panasonic	RS-Online	315-0940
XH-2A	BMS+,BMS-,12VDC_OUT	3	XH-2A	BOOMELE	LCSC	C20079
1.3K	R2	1	AC0805FR-071K3L	YAGEO	LCSC	C144581
470R	R1	1	AC0805FR-07470RL	YAGEO	LCSC	C144560

Additional components not on above BOM:

Heatsink
Gold Plated RCA Connectors
DC Socket
Headphone Socket
LED Indicator
4PDT Switch
LED Switch
Mini Switch
18650 SMD Battery Holder
Knurled Aluminium Spacers
Tube Sockets
Aluminium Knob

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