Serial No. 1938L14. H.S.Walker. C.G.Mayo. THE TECHNICAL...
Transcript of Serial No. 1938L14. H.S.Walker. C.G.Mayo. THE TECHNICAL...
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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
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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
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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.
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