OTL Class A Valve amplifier

Build yourself project, Class A, valve amplifier for headphones.

Background

Modern integrated amplifiers do not have an output for headphones. Small valve amplifiers are easy and nice to build yourself.

There has been a debate for the last 40 or more wheter valves sound better than transistors. An article by James Moir that appeared in "Wireless World" July 1978 pp. 55 shows that an audience of "experts" reported no significant difference in sound. However all the tested amplifiers were produced by a single company, this implying that design and engeneering have a greater effect on sound than a change from valves to transistors.

Construction

I found the circut on internett at a site for headphones, Headwize. The used construction was presented by Kurt Strain. I've done some modifications, my version runs on 330 V. I also included a regulated filament supply. See Strain.

The box was drilled and painted by Tor Gaare at Grorud Jernvarefabrikk a large manufacturing industry in Oslo.

The Amplifier

The finished Amplifier

Closeup of the valves 12AU7 and 6GC7

Internal

Schematics

The amplifier, one channel

Power supply

Drawings for drilling and building

Holes to be drilled

Placement of components

Parts list

I bought all the parts at Elfa in Sandakerveien close to my residence.

Komponent Artikkelnr. Stk.

ELFA

Forsterker:

Rør V1 12AU7 / ECC82 74-220-33 2

Rør V2 6922 / 6CG7 74-224-39 2

Rørholder Chassis 48-087-21 4

Kond. C1 10 uF 200V 65-471-29 2

Kond. C2 1 uF 200V 65-470-04 2

Mots. R1 90.9 ohm ¼W 60-710-96 2

Mots. R2 5.6 k 2W 60-795-86 2

Mots. R3 22 k ¼W 60-737-87 2

Mots. R4 1 M ¼W 60-757-09 2

Mots R5,6 100 k Log Pot. 64-145-36 1

Mots R7 51.1 ohm ¼W 60-708-17 2

Mots. R8,9 100 ohm ¼W 60-711-46 4

Mikrofonabel Audio 55-815-09 1

Kabel 55-817-07 1

Kopplingstråd svart 55-228-00 2

Kopplingstråd rød 55-228-26 2

Kopplingstråd grønn 55-228-59 2

Kopplingstråd gul 55-228-42 2

Krympslang 55-091-20 1

Loddeører 48-930-04 1

Gummiføtter 50-570-13 6

Power:

Transformator 235V/6,3V 56-006-06 1

Likeretter 1.2A 500 V 70-074-53 1

Likeretter 3A 280 V 70-080-06 1

Regulator +6V 1.5A 73-091-56 2

Kond. C1,C2,C3 220 uF 400 V 67-075-66 3

Kond. 2200 uF 16 V 67-011-89 1

Mots. R1,R2 220 ohm 1W 60-784-14 2

Kond. 220 uF 16 V 67-011-06 2

Kasse 50-025-06 1

Phonoplugg hvit 42-263-04 1

Phonoplugg rød 42-263-12 1

Jackuttak 42-768-04 1

Knapp volumkontr. 38-163-94 1

Bryter 35-038-02 1

Sikringsholder 33-152-07 1

Sikring 500 mA pk. á 10 stk. 33-105-21 1

Komponent	Artikkelnr.	Stk.
	ELFA

Forsterker:
Rør V1 12AU7 / ECC82	74-220-33	2
Rør V2 6922 / 6CG7	74-224-39	2
Rørholder Chassis	48-087-21	4
Kond. C1 10 uF 200V	65-471-29	2
Kond. C2 1 uF 200V	65-470-04	2
Mots. R1 90.9 ohm ¼W	60-710-96	2
Mots. R2 5.6 k 2W	60-795-86	2
Mots. R3 22 k ¼W	60-737-87	2
Mots. R4 1 M ¼W	60-757-09	2
Mots R5,6 100 k Log Pot.	64-145-36	1
Mots R7 51.1 ohm ¼W	60-708-17	2
Mots. R8,9 100 ohm ¼W	60-711-46	4
Mikrofonabel Audio	55-815-09	1
Kabel	55-817-07	1
Kopplingstråd svart	55-228-00	2
Kopplingstråd rød	55-228-26	2
Kopplingstråd grønn	55-228-59	2
Kopplingstråd gul	55-228-42	2
Krympslang	55-091-20	1
Loddeører	48-930-04	1
Gummiføtter	50-570-13	6

Power:
Transformator 235V/6,3V	56-006-06	1
Likeretter 1.2A 500 V	70-074-53	1
Likeretter 3A 280 V	70-080-06	1
Regulator +6V 1.5A	73-091-56	2
Kond. C1,C2,C3 220 uF 400 V	67-075-66	3
Kond. 2200 uF 16 V	67-011-89	1
Mots. R1,R2 220 ohm 1W	60-784-14	2
Kond. 220 uF 16 V	67-011-06	2

Kasse	50-025-06	1
Phonoplugg hvit	42-263-04	1
Phonoplugg rød	42-263-12	1
Jackuttak	42-768-04	1
Knapp volumkontr.	38-163-94	1
Bryter	35-038-02	1
Sikringsholder	33-152-07	1
Sikring 500 mA pk. á 10 stk.	33-105-21	1