Ampex-SA-10-Schematic 电路图 维修手册.pdf

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2、I ,., ow )llXtf CE) -?.:; I lTMlll.l Ujl(r 1 rl(TUII., $1e-) 1-C_l_oo-,.A/V-.- f R32 R2 TREBLE BOOST BASS FLAT) Cl Rl RS BASS CUT (TREBLE FLAT) C2 0-0 -1) 1-C_l 6 _ / Rl R3 RJ2 BASS BOOST (TREBLE FLAT C2 I Tone Swi.tching Combinations 7.2 RS 89-0101 89-0140 Issue A AMPLIFIER CIRCUITS The signal lo b

3、e amplified enters the unit at JI across pins 2 and 3 ( pin 1 is ground). Under unbalanced line operation thG dummy plug at J2 _jumpers the input at pin 2 to ground, and the input at pin 3 to the high side of the signal circuit. lJnder balanced line conditions the plug-in input transformer at J2 con

4、nects the signal al pins 2 and 3 across the primary of tl1e transformer; one end of the secondary vvinding is _ _ grounded whik thG other is con- nected to the high side of lhe signal circuit. In the latter case the shield of the input cable is connected to pin 1 of JI. Vhen the cquip- mGnt is to be

5、 used directly with a rnicrophone, the p1imary of the plug-in input transformer at J2 is connected across pins 2 and 3 of the input connector. ( See schema tic diagram, mi- crophone preamp), The signal next encounters the tone control circuits. Each tone control Sitch (S1 for 10 kc, S2 for 100 cps)

6、has three positions -+4 db, 0, and - 4 db -which corresponds to a frequency response boost, a flat response, or a frequency response attenuation respectively. In the following discussion remember that 89-0101 89-0110 Issue A . - i - ) -,.,. these are relative tcm1s, for example boosting the high fre

7、quencies means that their strength is increas)d in relation to other frequencies. When both switches are in the O (flat) po, sition, RI and R4 constitute a 6 db attenuating circuit for all frequencies. Tone conti:o.l is achieved by switching other cc.m1porn)nts in and out of the circuit. It i casie1

8、: to understand the principles of this circuit if we consider one svitch at a time and assume that the other switch is in tl1e O position ( designated 2 on the; schematic diagr,m1). Starting with the 10 kc, switch, SI: When it is in +4 db position, ( designated 3 cm the schematic diagram) the 6 db a

9、ttenuation nel- work of Rl and R4 i till in the circuit for low frequencies. For high freguencies, however, capacitor Cl now bypasses RI, so normal high frequency attenuation is reduced. Vhcn Sl is placed in -4 db position ( designated 1 on the schematic diagram) JU and R4 still affecl the normal at

10、tcnuati:m of low frequencies. Cl, however, now affords high frequencies a path to ground in parallel witli R4, and those frequen- cies are reduced in strength. If C 1 wGrc in the 7-1 - circuit by itself, ils impedance would progres- sively decrease as the frequency increased, and we would h,ive a .:

11、oncinuous roll off condition. To prevent this, and provide a -6 db high fre- quency shelf, R2 is placed in series with Cl to ground so that the impedance of the Cl-R2 cir- cuit never drops bdow the value of R2. Thus al 10 kc and above, all frequencies are attenuated by an equal amount with no roll o

12、ff. Now for the 100 cps Sitch, In the +4 db position, C2 is in series with the Hl-H4 normal circuit. This capacitor has little effect at high frequencies, so they arc practically attenuated br the normal 6 db. However, G2 adds a quite considerable impedance at low frequencies, increasing the shunt p

13、ath to ground and de- creasing the normal attenuation. In the -4 db position, all frequencies must proceed through c2. adding considerable impedance in the low frequency path and increasing the normal at- tenuation. If C2 were alone .in the circuit, its impedance would increase ;is frequency de- cre

14、ased, and we would have a continuous roll off condition at low frequencies. Resistor R3 is lherc,forc paullcled with C2 to provide a limit to the total impedance. A -6 db shelf thus takes place below approximately 100 cycles. After passing the tone contr()l circuits, the signal is impressed on the g

15、rid of VI through VOLU.v1E control H5. This is a conventional audio amplifier circuit, with C5 and RIO acting as a decoupling nGtwork. Negative :feedback is obtained by returning voltage from the sec- ondary of the output transformer back to the cathode of the :first voltage amplifier stage. The sig

16、nal is directly coupled from the plate of VJ to one control grid of V2. One half of V2 :ices as a conventional audio mnplifier, with the other half providing a differ- ential (grounded grid) phase inversion circuit. Note that the two cathodes are tied together, with R14 as a common unbypassed cathod

17、e resistor, connected to ground through the in- ternal shield connections of Vl ( to prevent cfamage of V2 if V1 is removed). When t.he directly coupkd signal is impressed on the i,rrid of V2a, the potential at both cathodes will vary as a function of the signal voltage. T nsofar as the signal is co

18、ncerned, the grid of the phase inverter stage is grounded through C7. (R11 establishe;s the same de voltage on the grid of V2b as that on the grid of V2a.) Thus in this 89-0140 Issue A type of phase inverter circuit the cathode po- tential varies witl1 the signal, while the grid potential remains co

19、nstant. The two signals, 180 out of phase, from V2 are coupled to the grids of c .he push-pull output stage V3-V4. Fixed bias on V3 and V4 is derived from the voltage divider circuit comprised of potenti- ometer RJ.6 and resistors R19 and R20. Con- twl R.16 adjusts the balance on the two tubes to ac

20、hieve equal cathode cunents, which can be measured across TPI-TP2 and TP3-TP2. Resist.ors R18 and R21 ace as parasitic suppres- sors. The push-pull t11bes operate class B, to achieve relatively high efficiency as compared to class A operation. The places of V3 and V4 are connected in push-pull to th

21、e primary of output transfonner Tl. The high side of the secondary is connected directly to terminal strips which feed the in- temal or external loudspeaker assembly. Con- nected across this line to ground is an output network consisting of R33, R34, R37, Cl7 and C18. Of these components, R34 and CJ

22、8 form the enclosure compensation circuit for the in- ternal speaker; it is shorted out by one pole of S3 when that switch is in the extcrn;il speaker position. The other components in this net- work supply frequency stabilization and nega- tive feedback. The ground side of the output line pr()- cee

23、ds through the contacts of phonr. jack J5 and tl1e second pole of S3 ( which selects the speaker circuit that will be completed to ground). Note that inserting i headset plug into J5 will disconnecl the oulput to both ter- minal strips, with resistor H40 providing proper termination. CAUTION N 11ver

24、 connect head sets to .15 with out firt turning volume control R.5 full counterclochwiJ. Th.i.s control can then be re:adjusted to a comfort- able listening level, but the cruailablc output /(me/ of the amplifier can easily damage smne h.eadsf!ts. POWER SU P PL Y CIRCUIT The power line is connected

25、at J3, with capacitors ClO and Cl J supplying r-f filtering. Convenience outlet J4 is prol-idecl preceding 7-3 the overload fuse Fl. The a-c power is then connected, through power ON-OFF switch 84, across the primary of power transformer T2. There are two secondary windings on T2, one high voltage a

26、nd one low voltage. The lat- ter provides a-c heater power to all tubes in the assembly, and to the Nuvistor in the optional plug-in preamplifier. Rectifiers CRJ, CR2, CR3, and CR4 are connected in a conventional full-wave center- tap rectifier circuit. Ripple filtering is provided by Cl2 and C13, i

27、th R25 and R26 ensuring an equal voltage distribution across these two capacitors. Half-wave rectification and filtering is supplied by CR5 and CJ4 to provide a nega tive bias vollage. Voltage regulation is achieved by an un usual circuit which employs a current sampling procedure to effect compensa

28、tion for chang- ing conditions. Perhaps a general discussion of the engineering objectives of this circuit will .;1id in understanding its operation. As the power output of the amplifier increases from zero output to full output, push-pull tubes V3 and V4 (operating class B,) will draw more and more

29、 plate and screen-grid current causing the plate supply voltage to decrease. If ive either connect the screens co the plate supply or still worse used the usual fixed screen 1,1Tid resistor, this increased current flow would result in de- creasing the screen voltage. Tn turn, the lower screen grid v

30、oltage would cause the operation of these tubes to move from class B, further and further into class C with the result of clip- ping the center section out of the waveform. But if a screen resistor that could be made to decrease autom,ttically as current increased ( and vice versa) were used between

31、 the plate supply and the screen grid circuit, the screen voltage, hence the class of opertttion, could be maintained throughout the power output range. This type of regulator would also allow the maintenance of the designed class of operation under ci.rcumstances where voltage fluctua- tions occurr

32、ed in the power line supplying the equipment if it also adjusted the screen volt age as a function of the bias voltage on the control grids of V3 and V4. Actually, the reg- ulated line also supplies the high voltage to tubes VJ and V2; and to the plug-in micro- phone preamplifier when it is used. Th

33、is is done primarily to take advantage of the filter 7.4 ing action of the regulator and thus avoid heavy and intricate filters in the power supply. With this understanding of the basic ob- .iectives, the operation of the circuit can best be explained by first pointing out the major components. Stag

34、e V5b is the series regulator, the variable resistance mentioned in the pre- ceding paragraph. This stage is similar to a cathode follower, connected in seties between the regulated bus and the high voltage line. Stage V5a is the shunt regulator which sets the operating conditions of V5b. Resistor R

35、27 is the current sampling resistor which controls the operation of the regulator. Under no signal conditions, the current through R27 is very small and establishes only a slight negative potential at the center top of the power trans- former. The current through R27 increases as the amplifier tubes

36、 conduct more heavily with signal applied, since the total de current of the amplifier flows through R27. This current flow establishes the potential ,tt the center tap ol the power transformer (T2) which ,ill of course, be more negative with respect to ground than during the no signal condition. Th

37、e po tential will vary as the total current in the amplifying tubes vaties -becoming more neg- ative as current increases, less negative as current decreases. Resistors R30, R29, and R28 are connected in a voltage divider ckcuit from the negative bias bus to the center tap of the power trans former,

38、 with the con(rol gtid of V5a,connected at the junction of R30 and R29. Resistor R28 is ad.justed to establish the initial negative bias on the gtid of V5; . As the voHage drop changes with signal load, the bias on V5a will change - going more negative as the center tap potential goes negative, and

39、less negative as the center tap potential goes less negative. Thus the volt- age at lhe plme of V5a will vaq inversely as a function of the current through R27. The plate of V5a is directly coupled to the control grid of V5b, and the plate-to-cathode resistance of V5b will therefore vary inversely a

40、s a function of lhe current through R27 -de- creasing as the current through R27 increases and increasing as the current through R27 de- creases. The objective of maintaining a con- stant voltage despite variations in current is 890140 Issue A thus achieved. Capacitor C15 furnishes a nega- tive feed

41、back arrangement, from the ca thde of V5b to the grid f V5a, which acts to remove any ripple component that might be present in the regulated line. There are two main advantages t this voltage regulator. First, it rcli:Jbly maintains 89-0140 Issue A the operation of the push-pull output stage in cla

42、ss B, despite widely varying cnditions of power line voltage and output level. S(!cmd, the amplifier draws more power from the pwer line nly when required to deliver more power to the load -for example, at zero signal input it draws only 50 watts, when delivering full rated power it draws 116 watts.