p' ,;; 'd .• ~~"'j.~~ ,

·i

Researoh Department. REPORT No. L.OIO. 16th. May, 1938.

Serial No. 1938L14.

Job Numbers 8.001.10 (8.009.4). Drawing Nos: L.OIO.l to L.01O.4.

Work oarried out by:

A.E.Barrett. H.S.Walker. C.G.Mayo. H.D.Ellis.

THE TECHNICAL DESIGN OF O.B. AMPLIFIER OBA/8.

Summary: This report deals with the oirouit design of the new O.B. amplifier Type OBA/8.

The amplifier consists of two stages employing H.F. pentode valves (AC/SP3), and has a gain of 90 db. The amplifier oan be operated from A.C. mains or L.T. and H.T. batteries.

Introduction: The amplifier described in this report has been designed

to fulfil the teohnical demanocs outlined in Specifioation No'. ED.1405,

Part 2, and any subsequent amendment made thereto.

The ohief requirements are:-

1. Overall voltage gain at 1,000 cyoles 90 db.

2. Total control range ••• o 46 db.

3. Total attenuation in control potentiometer 75 db.

4. Frequenoy oharaoteristio must be ind,ependent of gain oontrol within 0.25 db. for frequencies between 50 and 8,000 cycles/seoond.

BBC R & 0

11111111111111111 11111111111111111111111 300008808 R

____ .-J'

-2-

5. Input impedance 300 ohms + 10% measured at 1,000 cycles.

6. Output impedance 75 ohms! 15% measured at 1,000 cycles.

7. Frequency response characteristic (normal) to be within the limits shown in Figure 1.

Frequency response characteristio with ribbon microphone correction to be within the limits shown in Figure 2.

These frequency response characteristics to be taken with:

(a) souroe impedance 300 ohms non-reactive and balanced. output load impedance 600 ohms non-reactive and balanced.

(b) source impedance any value 150 - 6,006 ohms. output load any value •••• 100 - 2,000 ohms.

8. output power + 14 db. with reference to 1 milliwatt in any non-reactive load between 100 and 600 ohms at any single frequency be~veen 50 and 8,000 p.p.s., the gain control being set at any value between maximum gain and -46 below maximum. The total harmonio oontent under any of these conditions to be less than 3%.

9. The noise level,with amplifier at full gain sr~ll not be greater than -40 with reference to 1 milliwatt.

10. The stability of the amplifier at full gain must be such, that it does not oscillate when a loss of (G + 2) db. is oonnected between the output and input,- G being the maximum .gain of the amplifier with 600 ohms.

11. A peak programme meter to be incorporated.

12. Provision to be made for connecting a loudspeaker amplifier (a) across the line Cb) to a point in the amplifier, so that it is independent of the reflected load of the line 0

13. The amplifier to obtain its source of supply from A.C. mains. Provision to be made for operation from batteries. L.T. 5.5 to 6.2 volts. Current not to exceed 6 amps. H.T. 230 - 260 volts. Current not to exceed 30 mAD

-3-

The "iTari'ous points in the design are dealt 1tvith in the order

in which they were considered.

O.E. AMPLIFIER.

Output Stage. Preliminary tests showed that more than 0.5 watts could be

obtained from a valve of the H.F. pentode class with less than 2% total

harmonic distortion, provided the load was correctly chosen. Owing to

the high impedance of the pentode it must be shunted to obtain the law

output impedance. Hence for 25 mW. (+ 14 db.), the "iTalve is called upon

to supply 50 mW. when the line impedance is 75 ohms and 225 mW. when it is

600 ohms. The H.F. pentode was therefore capable of supplying the output.

In .addition a very high mutual conductance is obtainable (7.5 mA/v.)

giving a high stage gain. This type of "iTalve could also be used for

earlier stages giving an ad"iTantage, in standardisation. The Mazda AC/SP3

was finally chosen in preference to the Mu1lard TSP4 and Marconi-Osram

KTZ 41 on account of: (1) higher mutual conductance for low anode currents:

(2) slightly lower optimum anode current for given load: (3) non-inductive

heater could be fi-1:;ted: (4) the valve was already on the market: (5) low

filament current: and (6) lower screen volts.

To obtain the required output it was essential to pass approx­

imately 12 mA. in the anode, to lead the valve with about 30,000 ohms and

-4-

to maintain the anode potential at about 200 volts. A resistance coupled

output circuit was therefore out of the question with only 250 volts supply,

and it was not possible at that time to make a transformer for suoh a high.

load to carry the D.C. and maintain the frequency oharaoteristic.

A choke coupled output was therefore designed along conventional

lines. It was fotmd, however. that in order to obtain a satisfactory

low frequency cut, referred to later, it was advantageous to reduce the

open circuit inductanoe of the transformer and choke. and to maintain the

law frequency response to 30 cycles by,treating these units and the

coupling condenser as a filter structure with a cut-off at about 25 oycles.

In order to maintain the response of this filter with either 75 ohms or

600 ohms, and also to prevent the impedance thereof as seen by the valve

from rising to very high values, the 30,000 ohm dead resistance necessary

to shunt the valve as mentioned earlier, was divided, one part being

connected on the input and the other on the output side of the filter.

The output impedance as seen by the valve was arranged to be the optimum

required by the valve when the output load was 600 ohms, since the valve

then has to deliver its maximum output (225 mW.). Ls the load impedance

is reduced, the anode load departs from the optimum value, but the valve

is simultaneously required to give a smaller output. In this way the

maximum output power specified is obtained within the maximum limit of

, .

-5-

harmonio for all v~lues of load.

1st. Stage. The AC/SP3 was found to be very suitable for use in the first

stage on ·aocount of the high mutual conductance at low anode currents

(gm = 2.5 mA/v. @ la = I mA.). Greater gain is obtained for a given

volt drop in the anode resistance as the anode load is inoreased and the

anode current reduoed. The limit is reaohed when the input capacity

to the foll~ving stage (30-40 ppF) becomes comparable with the grid leak

and the latter becomes comparable with the anode resistanoe. A gain of

47 db. is obtained from this stage with anode resistance of 150,000 ohms,

anode impedance 94,000 ohms nett, anode volts 65 approximately and anode

current 1 mAD (Under widely varying operating conditions the screen

current of an AC/SP3 is 35% of the anode current, henoe oathode current =

10 35 mA. approximately). Higher gains oould be obtained only at the

expense of maximum output by allowing the anode potential to drop still

lower.

Input Transformer. The impedanoe ratio is 300 to 300,000 and the open

circuit inductance 1~8 Henry. Under correctly matched conditions this

giv~s a loss of 0.3 db. at 50 cycles. The frequency characteristio is

maintained up to 10 1 000 cycles by treating the J.nput capaoity, leakage

inductanoe and output capacity as a low-pass filter section; the input

-6-

capacity is augmented by an additional 0..0.5 JF on the line while the

capacity on the secondary side, including the input capacity to the first

stage, is approximately 50. uuF. " J.I

Gain Control. Consideration was given to the possibility of providing

a gain control on the input of the amplifier, sinoe the maximum voltage

"to be handled by the first valve is then lawer than in the case of a

control following the first valve. This idea was abandoned, sinoe it

necessitates the amplifier being at full gain always, which is undesirable

from considerations of noise. It is also possible that noise may result

from the control potentiometer. It was deoided therefore, to oontrol

the gain after the first valve, and with this arrangement it was found

that when delivering +14 db. to line a maximum attenuation of 24 db. only

was permissible, awing to overloading of the first valve. That is to say "

the maximum output from 1st. stage was about 6.5 volts R.M.S., the limitation

being 2% 2nd. harmonio. It is impossible to design this stage so that the

load is an optimum and still maintain uniformity between valves, beoause

the current is very law and under these conditions there are large

variations in the optimum operating point from valve to valve. ,

A gain reduction of 24 db. was not adequate so additional

attenuation was obtained by applying feed-back to the 1st. stage. This

method has the advantage that not only is the gain reduoed by the fe~d-

-7-

back, but the harmonic content is reduced also so that the valve will

handle a larger nett input without exceeding the distortion limit. In

practioe it is found that the input level may be increased by approximately

twice the number of dbs. applied by the feed-back. In the amplifier under

consideration, 16 db. of feed-back was applied to the 1st. stage in steps

of 2 db. by a potentiometer ganged to the interstage oontrol,in suoh a way

that all the feed-back is applied before the interstage control begins to

take effect. The maximum gain reduction in the amplifier is therefore of

the order of 50 db. while still maL~taining the maximum output capability.

Gain. The overall voltage gain, therefore, is made up as fo11~vs:

Input transformer 300/300,000 ohms •••••• 30 db.

Isto stage (no feed-back) ••••••••••••••• 47 db.

2nd. sta g eo •• 0 0 ••• Q •••• 0. 0 0 0 • 0 •••••••••• ~ 46 db 0

Output transformer 30,000/75 ••••• 00.0 •• 0-26 db.

97 db. ;;=

Since the specification calls for only 90 db. the gain of the output stage

is reduced by feed-back, which also reduces the harmonic oontent. It was

convenient to use the cathode resistanoe for this purpose, and the gain is

reduced by 6 db. by this means, bringing the overall gain to 91 db. and

leaving a small margin for variatlons.

and within i 2 db.

These are expecte~ to be slight

-8-

By suitably shunting the output stage cathode resistance, small

losses of gain in the neighbourhood of 8,000 cycles may be corrected.

~

Harmonic. Measurements on the experimental amplifier indicate that at

full output, 3rd. harmonic will be about 0.75% and 2nd. harmonic 1% or less,

depending on the valve used in the output stage and the accuraoy of soreen

adjustment.

AdjustmerrG of Output Valve. To allow 'for the possibility of wide variations

in the valve characteristics (which cannot yet be assessed), a variable

resistance is included in the screen circuit of the output valve so that

,the anode current may be adjusted to the optimum value. After tests

carried out on a dozen valves, the anode current has been fixed at 12.5 mAD

~ 0.5 mAD when 250 volts H.T. are supplied to the amplifier. The optimum

ourrent is direotly proportional to the H. T. supply volts ..

Frequency Response Correction. (Ribbon Microphones). This consists

essentially of an increase in high frequency response and a sharp

attenuation at frequencies below 50 cycl~s/second. The former was

obtained by a choke suitably shunted in the anode circuit of the first

valve, giving an increase of gain of 3 0 5 db. at 8,000 cycles/second. The

bass cut was obtained by reducing the interstage coupling condenser and the

output coupling condenser. The frequency correction can be included or

-9-

cut-out at will in the experimental amplifier by means of three links.

The gain at 1,000 cycles is unaltered. The design of the output circuit

relies on the load being very nearly resistive at low frequencies; which

is certainly the case.

Peak Programme Meter and Monitor Circuit. This was first designed to

be connected directly across the primary of the output transformer and

was provided with an attenuator which was adjusted, aooording to the

impedance of the line, to read 7 on the scale for a power output level of

+ 14 db. For a given power in the load, however, as the load is varied

from 600 ohms to 75 ohms the anode voltage swing varies by 9 db., whereas

the anode ourrent swing varies by only 3.5 db. By making the reading

, of the programme meter proportional to the anode ourrent, therefore, it

is possible to make the reading correct to within! 2 db. with no adjustment

for load impedance.

By including an impedance of 1,000 ohms approximately in the

return of the anode circuit to earth, a voltage proportional to the anode

current mving is obtained. Moreover, since the pentode has an extremely

high impedance, this voltage is unaffected by spurious disturbanoes in the

load such as may oocur when the load is a transmission line. Such an

impedanoe, therefore, serves as an operating souroe for a peak programme

meter and a loudspeaker amplifier. In the circuit as arranged (see

-10-

Drawing - Fig~3) all the signal current from the anode flows through this

impedance, except for the insignificant amount which passes through the

anode choke and H. T.' source direct to the earth line.

The peak programme meter is arranged to read 5 at a power level

of + 4 db. in a 225 ohms load (i.e. 0.67 mAD in the monitor circuit) so

that 7 corresponds to + 12 db. on 225 ohms, + 13 db s on 75 ohms, and + 9 db o

on 600 ohms. The current in the 1,000 ohms impedance under these conditions

is 1 0 68 mA., and the voltage across it therefore 1.68 volts. The voltage

across a 225 ohm line is 1.9 volts, and the loudspeaker amplifier may

therefore be changed from the monitor circuit to line with but little

ohange of level.

Programme Meter Adjustments. The zero and scale adjustmerrts are made as

in other meters of the new peak type by varying the oathode resistance and

screen potentiometero The sensitivity calibration is carried out by

passing 0",67 mAD from the 50 cycle mains supply through the monitor circuit~

and adjusting the impedance of this until the meter reajs 5. The changes

of impedance which are. found necessary in practice have negligible effec·c

upon the output. level from the loudspeaker amplifier.

~nce anpS-l:;ability of' Amplifier. A balance test for transformers is

described in the O.B. specification, the pass figure mentioned being -60 db,

The input and output transformers specified for the amplifier under

-11-

consideration give a test figure of -94 db. and -84 db o respectively and

thus fulfil the balance test.

The balance figure of -60 is probably inadequate to ensure

stability under all conditions of coupling between input and output leads

which may occur in practice, but the transformers actually proposeds having

the bala~ce figures of 94 and 84, should be adequate ~or all practical

requirements.

It is pointed out that in any transformer output circuit there

is capacity between the two lines and earth. If these capacities are

unequal, there is unbalanceD There is a capacity also between the tvvo

windings of the transformer, due to imperfeot soreening, which may

oontribute to this out of balanoe. A resulting voltage is produoed

effectively aoross the lower arm of a potentiometer, oomposed of the out

of ba1anoe oapacity as the upper arm, and the total capacity to earth as

the lower arm, shunted by any other impedanoe to earth. In thebalanoe

test as speoified, this impedanoe consists of the nominal line impedanoe.

In practioe, this impedanoe is absentp so that the voltage produced by

unbalance depends almost entirely on the "total balanced oapaoity of the

line. The oonditions for stability of the amplifier are thus much more

t/r:ringent than ·chose referred to in the ba1anoe ,,"eest for transformers"

Transformers, therefore, which pass the 60 db. test may fail when the

'HI attenuator stability test is applied, sinoe in this test the resistanoe

-12-

between the centre point and earth is replaced by the capacity (unknown)

of the attenuator to earth.

feature.

It is this capacity which is the ruling

Noise and Hum. Johnsonnoise on a matched 300 ohm input circuit is -134 db.

with a frequency range up to 10,000 cycles. With 90 db. gain, therefore,

the output level is -44 db., and all other sources of interference should

be less than this. With the input of the amplifier shorted ard with DoC.

supplies, the output due to valve noise is of: the order of -65 to -70 db.

When 50, cycle A.C. is supplied to the heater of the 1st. stage, the output

level, using a valve with normal heater construction, is -20 db. approxi­

mately. With a non-inductive heater it is of the order 'of -40' db. Owing

to the frequency characteristic of the ear, it is estimated that the level

of 50 cycle hum may be some 10 db. higher than Johnson noise before it

becomes of equal annoyance. A limit of -36 db. output level of hum when

Johnson noise is -44 db. has been laid dovffi, therefore 1 for valves used in

the 1st. stage. The above figures a'pply when the amplifier is at full

, gain. Any reduction in gain increases the signal-noise ratio by a

corresponding amount.

Some trouble was experienced with 100 cycle hum which occurred

when certain valves were used in the 1st. stage. It was found to be due

to heater-cathode emission or vice-versa, and is cured by biassing

-13-

the heater relative to the cathode. Sufficient biassing potential must

be employea so that at all points on the heater voltage oycle the emission

is either cut off or is saturated, and 10 to 20 volts is satisfactory.

Certain samples of AC/SP3 also produce a 500 oycle buzz which cannot be

measured. The limit for this defect (whioh is being investigated by the

valve manufacturers), must be judged aurally. It was subsequently found,

however, that the cathode-heater biassing described above cured this 500

oycle buzz also, in a large number of instances, though in this case

approximately 100 volts is required. This potential is obtained by

connecting the centre point of the heater circuits to a suitable point on

a potentiometer across the H.T. supply.

Mains interference has been minimised by the use of a shielded

primary mains transformer and spacing of mains and amplifier leads, with

screening where necessary.

The final circuit diagram is shown in Figure 3.

RESULTS OF TESTS ON THE FIRST MODEL AMPLIFIERS. A comp~ehensive set of

tests was carried out on the two first model amplifiers, and the results

of these are considered to be representative of the performance which may

be expected from the production models.

Frequenoy Charact;eristics. Figure 4 shows the variation of frequenoy

-14-

response characteristic taken under the following conditions.

Curve. Source Impedance. Load Impedance. Gain. Ribbon Microphone

---~-.>-.--

_Equaliser.

(a) 300 75 70 ~ Out

(b) 300 75 90 Out

(c) 300 600 70 Out

(d) 300 600 90 Out

(e) 300 75 70 In

(f) 300 75 . 90 In

(g) 300 600 70 In

(h) 300 600 90 In

Curve 5 shows the variation of frequency characteristio with a

change of source impedanoe.

Comparing the ourves of Figure 4 with those of Figures 1 and 2,

it will be seen that the specification is fulfilled in all oases with the

following exception, namely, the response at the lower frequencies with a

load of 600 ohms and with equalisation is l~ db. below the limit at 30

cyc les. This is not considered to be of practical importance.

Referring to the curves of Figure 5, both these are outside the

limits specified, but again this is not considered to be important since

curve (a) represents the oondition for two microphones faded oompleb)ly up,

-15-

and curve Cb) the condition for one microphone faded down on the mixer by

28 db. approximately. The f~equency response is within the limits speci­

fied with one microphone faded up.

Harmonic Content. This under the specified conditions. was less than 1%.

With the gain reduced by 55 db., corresponding to stud 8, the harmonic

increased to 2.2% for the maximum output as shown by a deflection to 7

on the programme meter. These results are considered to be very

satisfactory~

stability and Balance. The amplifiers were stable when subjected to the

'HI attenua~or test, the loss introduced being 92 db. compared with the

amplifier gain of 91 db.

A further test of stability was applied, namely, that of

connecting a 300 ohm centre tapped resistance across the input, a 600 ohm

centre tapped resistance across the output, and connecting the centre

points of these resistances.

conditions.

The amplifier was stable under these

The balance of the input transformers measured by the method

mentioned in the specification; was found to be 105 db. A corresponding

figure for the output transformers was 82 db. The balance of these

transformers is therefore well within the limit of 60 db. which is called

for, and the input transformers give a higher balance figure than obtained

-16-

with the original experimental model.

Noise and Hum. The noise levels with the amplifier set for a gain of 90 db.~

and operating from batteries and A .• C. supply, were -43 db. and -38 db.

respectively. The measurements were made with the amplifiers unequalised.

This is considered satisfactory. The service tests indicated that the

gain normally used was of the order of 70 db. This gives figures for

signal-noise ratio of 75 and 70 db. respectively.

Current Consumpti.on. The current consumption of the amplifier is as

follows:

H.T. 24 mA.

L.T. 3.1 Amps.

Programme Meter. The programme meter was found to satisfy the specified

conditions, but slight difficulty was experienced in adjusting to zero.

This was rectified after slight adjustment of one or two of the components.

AEB/RBI.

· DATA SHEET NO. 16 SPECiAL.

UI CD

,DATA SHEET No, 16 SPs:CiAJ.., L.OGARITHMIC x M/h1 {3 (:Yc!...t:.)

Frequency in Cydes per second.

"

N o Ut

o 0'0 I

o 0 oj oi

I~- ----H ? 38.

I ,. , . i~ I

• I 'I I I I I j

I

I

-.-- ----- --. ---- I - - - -. -- - - - -- - ~- - - - -- - -- -- _ .. - --- - -- -- -- -- --- -

q ---2- 1....01: y' 0 lZ5mA

..t. ... " ... vvvvv

MA.46

O. B.

i 20KJL± ~O% :!: W%

I

I I

AA . A I

FIG,S,

AMPLIFIEJ2 OBAj s.

'ESOv 2:4 m A. .

! I I I !

I ;

I i I I I I I !

I I I i I

~.

~ I NEON.

,

plo I OSRA.M Ir- INDICATOR I LAMP.

I 1

B. B .C ,.., .... == Ar-V"j..J Dr::f-'-1

~

~ ,m' i:?EPORT

A.DDrJ d; 1)L L. ~O' 2.

o· 0(\1

Cl 0 0 i-. to (J! 0 " 0

N e") r.o

'" '0

oJ

:.;)

N o .CJ1

0 U)

r~:::7::~~~ i-~- ."-¥.-r, ;':-~.': ." ,- .-- - ..

I\)

o

Ul

(J)

0>

: '. :~;r.-::;::::-:~':':?>:':;::~;~:~'~~C~-:-::-:~:':--; ! :.: '-:-::~:';~::-7~'.;.: :-:~-~~~m~T:~:!~:;:=~:-- -:-';T?-; . <~-:-T :--;-~l:::-rx-:,~:: :'"7"::-:-, -:::~i-?tJ

... ;L.:.'~ __ :~.··_.o~.a~~ji~.··:,:,:,·-.LJ.E.:.j~ .• ',:.·'·_:'.':~.Q.,::~.e.":-A.,~~/e.Ci,:'" ....... , .. ,:::' .•. L.:.'~ .• ~.:.:.· .. ~,':i: .. ~.: ... :_ .. ~ .• :_·::.-.-:.~._.;, __ .~"_ .. ,::::-.~'-.".:i.:~~_' .. ~~.~.:.:.;-.':-:; ... -.,,_,~,":~,~:~~ "'" . .. ~~.;.::..:.~·-::~:.lJ:::~:~-:~:1~:'-5:-=--'"·--:::-·::..:::. : VAI2IATJOt-J . OF :OV~1<~t-I.;.:FReQUeNCY:E::i;1 ,: 1 . c'''''· - i<::~::;~i

-,e,~ I

i .. -.6.:-=..:'-; .----.----.. '.---.--.-----j::::~-.: r ,- .. ;

o

"

1'---.:; .\\

'r" ,- " • . !"

o c Cl Il} 07 t)

o c o

"

o 0 1"\

o et 0) !:];

Co

'". O· C~. ';'J

.. I

i.J Q :,7'";

o o

~ __ ~~. ~~C"_.~,.. __ ~ __ ........ _ •. _ .~j ... __ -.!.

.-. ".:r-, '.!'.J

·JcJ

REPOI2T:C

L.Ol.O·iST o

(.) ::i U C- l,! (, 1.6 ~

.' ..:'

() 0 Ij <) ,:-'

C c:

--

+4

+c·

o

J • 14; ,-t )

1-6 I

-8

-(4

. , . .. -_ ....... -- -' .. .

"

'::") ;'-,1 'j

':- ;-::~ "-... --:'. -1--: ~.:-.. - .. - !~ .~ .. -: .. :"-:'~':-'~-:-"":""':- ~ - ... - ~.-:- ..... , . .. . .' . '. '. ,:'1'., .

;-'-~-'-~'-40 .. 8.-_AMI;)LIFle~_.OaAIBL .... ;._~1~~] .. ;. OVERALL I=QEQUENCV I<! E=$ D.O t-J se: . i : ' .:' . CHARA.CrSIO?ISTIC·' WITH VAI2IA.Bwe: '., I

.--.. -•.. ' ;SO(JRCE "'IMPEDAl'JCE':"~'" " . . .~.: :~~.~ ... -.---.--.-....... . , ',' , I·" '" . " .. ;

. OUTPUT LOAD' IMI?EOA;NCEi=:600.n::.·.' ,", ._._ .. ~QUTI?UT: :.L~VE~.~;~,: ,~4:~b;.' ... ;:~~~L; ... ~~.+::::::~_':_: ....... ~_._:

.GAIt-.! '" 70 db. '; ., .;'

COI<RECTIOfolJ; 'ItJ: CJ~CUiT; I,

'. ,

~,

.1 ...---- .. ". SOIJRCE

IMPEDANCE.

,~J50.5l.'

Ul ill

.-.-••• -,,-- ••••• ,..~ ... -.-.~' •. ----. <-- ~-- •• '~-~-r'-": ';' --,

, . ~ : '. ". ". '.' . :! :

F.IG ' 5 " --' .. -, ...... ~. ~ •. "- _ .• " ·,-t-: , t'

;, ..

'.} (; 11 ") ,-. 0 0 "_.' '.:../ .~" \ I~ Co D G 0 .. ' t-:- .~ "'

'j ,

r-, ()

til

.;;:.;.

o () L) t:) ClOG"')

l) C'l -0

B. B. C. I2ESEAJ<CH DEPT DJ<N, ~. 'REPORT.

APpD. 4t)?l.-. L...O~O·4~ , .. '\...) ; ) J

, } =:J

t,." C : ; , .. :J '. <.[) ~. ;