Nakamichi pa-7e + B&W 800 Matrix
Hello, I have a pair of B&W 800 Matrix witch ask a lot of current.
I would like to know if the Nakamichi PA-7E poweramp coud drive well the b&w's.
this amp has been disigned by Nelson Pass and was made during end of the 80's.
The power is always mentioned in 8 Ohms (200W) but never in 4 Ohms.
Does this means that it doesn't handle well 4 Ohms speakers or just that they didn't mentioned the power in 4 Ohms?
Thank you
Bruno
Hello, I have a pair of B&W 800 Matrix witch ask a lot of current.
I would like to know if the Nakamichi PA-7E poweramp coud drive well the b&w's.
this amp has been disigned by Nelson Pass and was made during end of the 80's.
The power is always mentioned in 8 Ohms (200W) but never in 4 Ohms.
Does this means that it doesn't handle well 4 Ohms speakers or just that they didn't mentioned the power in 4 Ohms?
Thank you
Bruno
The recommended power output for the amplifier is between 150w min. and 800w max. into 8 ohm but I found out that the 800 matrix is not at all demanding on amplifiers.
They take about anything, and are very rewarding once you start using better gear.
I'm currently driving them with a pair of AlephX's and I'm very pleased.
/Hugo
They take about anything, and are very rewarding once you start using better gear.
I'm currently driving them with a pair of AlephX's and I'm very pleased.
/Hugo
Nakamichi pa-7e + B&W 800 Matrix
Nakamichi PA-7 amps will run into a 4ohm load. I have a factory piece literature stating that the output is 500 per channel into 4ohms. Here are the factory specs for you to look over.
Dimensions: 17-1/8 (W) X 7-7/8 (H) X 16-9/96 (D) inches Approximate weight: 59 lbs., 8 oz
Continuous Average Power Output (NEW IHF): 200W x 2 (8 ohms, both channels driven, 20-20,000 Hz, 0.1% THD) Dynamic Power per Channel: 300 W (8 ohms) Dynamic Head Room (NEW IHF): 1.7 dB (8 ohms) Power Bandwidth (Half Rated Power, 0.1% Damping Factor: (NEW IHF) 20 Hz/1 kHz/20 kHz: Greater than 60/60/60
Input Sensitivity/Impedance (NEW IHF): 2.0 V/ 75k ohms (rated power) 140 mV (1W output)
Frequency Response: (1 W, NEW IHF): 20-20,000 Hz + 0, -0.5 dB, 7-150,000 Hz +0, -3 dB
Signal to Noise Ratio (Input Shorted, Rated Power, IHF A-WTD) : Better than 120 dB Residual Noise Level (IHF A-WTD): Less than 25 u V/
Total Harmonic Distortion: Less than 0.1%
Intermodulation Distortion: Less than 0.1%
Stereo Separation (Input Shorted): 110/100/80 dB
Output Complement: 16 Transistors Per Channel
Output Current Capability: 18 A continuous, 50 A peak (per channel)
Power Supply: 700 W toroidal transformer, 132, 000 u F total filter capacitance
Power Source: 120, 220, 240 or 110-120/220-240 V AC; 50/60 Hz,
Power Consumption: 700 W max.
Hope this helps, J/S-S1A
Nakamichi PA-7 amps will run into a 4ohm load. I have a factory piece literature stating that the output is 500 per channel into 4ohms. Here are the factory specs for you to look over.
Dimensions: 17-1/8 (W) X 7-7/8 (H) X 16-9/96 (D) inches Approximate weight: 59 lbs., 8 oz
Continuous Average Power Output (NEW IHF): 200W x 2 (8 ohms, both channels driven, 20-20,000 Hz, 0.1% THD) Dynamic Power per Channel: 300 W (8 ohms) Dynamic Head Room (NEW IHF): 1.7 dB (8 ohms) Power Bandwidth (Half Rated Power, 0.1% Damping Factor: (NEW IHF) 20 Hz/1 kHz/20 kHz: Greater than 60/60/60
Input Sensitivity/Impedance (NEW IHF): 2.0 V/ 75k ohms (rated power) 140 mV (1W output)
Frequency Response: (1 W, NEW IHF): 20-20,000 Hz + 0, -0.5 dB, 7-150,000 Hz +0, -3 dB
Signal to Noise Ratio (Input Shorted, Rated Power, IHF A-WTD) : Better than 120 dB Residual Noise Level (IHF A-WTD): Less than 25 u V/
Total Harmonic Distortion: Less than 0.1%
Intermodulation Distortion: Less than 0.1%
Stereo Separation (Input Shorted): 110/100/80 dB
Output Complement: 16 Transistors Per Channel
Output Current Capability: 18 A continuous, 50 A peak (per channel)
Power Supply: 700 W toroidal transformer, 132, 000 u F total filter capacitance
Power Source: 120, 220, 240 or 110-120/220-240 V AC; 50/60 Hz,
Power Consumption: 700 W max.
Hope this helps, J/S-S1A
re: Nakamichi pa-7e + B&W 800 Matrix
FWIW, I happened to visit Nakamichi America in Torrance, CA in the late 80's (dropping off my OMS 7 for repair) and saw an all Nakamichi 7 series (PA 7, CA 7, OMS 7, ST 7) system in the main lobby with B&W 800 Matrix speakers. It was an enviable system then but it was not turned on! I quickly left to find the repair entrance, but I do believe at one point, the PA 7 brochure featured an outline of the B&W 800 Matrix speaker. Not an official endorsement, of course, but more than coincidence I am sure.
twolf5
FWIW, I happened to visit Nakamichi America in Torrance, CA in the late 80's (dropping off my OMS 7 for repair) and saw an all Nakamichi 7 series (PA 7, CA 7, OMS 7, ST 7) system in the main lobby with B&W 800 Matrix speakers. It was an enviable system then but it was not turned on! I quickly left to find the repair entrance, but I do believe at one point, the PA 7 brochure featured an outline of the B&W 800 Matrix speaker. Not an official endorsement, of course, but more than coincidence I am sure.
twolf5
am i wrong by saying that B&W800 are 8ohm?
They are great!
I believe that they shouldn't be as demaning in current as you think they are (differently from the 805 indeed).
They only ask you for a very good and neutral amplification system...that's all!
You ' ll find them to sound Veeeeery different upon the type of amp they are plugged on.
enjoy your speakers.
They are great!
I believe that they shouldn't be as demaning in current as you think they are (differently from the 805 indeed).
They only ask you for a very good and neutral amplification system...that's all!
You ' ll find them to sound Veeeeery different upon the type of amp they are plugged on.
enjoy your speakers.
Hello,
I have a Nakamichi PA7E II here on my bench. It was faulty, mainly dry joints which were particularly bad under the class-A voltage gain stages and associated current sink, (Qs 110, 111 and 112).
Basically went over the entire board and resoldered everything, Q112 was literally rattling loose, only the collector lead holding it. Presumably because of this, it had gone D.C. and the speaker protector had cut in. The owner seems to be the type whom leaves the unit on almost permanently, estimated by the browning of the phenolic under the class-A voltage gain transistors and the degree of recrystallization of the solder. Presumably after it had gone D.C., he had not realized and the feedback capacitor, C104, a Nichicon "MUSE" 220uF 16v Bipolar electrolytic had been overvoltaged for some considerable duration via the feedback network (R121, 2k49 and R120, 51k1), as there was no D.C. division path. This capacitor had "pop-corked", spilling its electrolyte out through the burst scores at the top of the can,. so this had to be replaced and getting this type of capacitor in Australia is not an easy ask. Both feedback caps were changed and both boards carefully resoldered and inspected for damage.
So, after all this, the unit was powered up (in series with 4 paralelled 200w incandescent lamps just in case it's supply port impedance decided to drop to a very low value). The lamps pulsed in brightness as the power supply resevior caps charged and the circuit came into operation normally, producing a voltage amplified signal into no load with no appreciable D.C. offset. The lamps were observed to glow slightly due to the shoot through current of the output stage class-A to Class-B transition bias. The brightness was observed to increase slightly as the output stage and heatsink came into thermal equalibrium after about 10 minutes operation.
It was deemed that the bias was too high from the brightness of the lamps' glow, so the power supply leads to each channel were disconnected in turn so the bias of each individual channel could be guesstimated. There is a small red pin header on the edge of the board, TP11, which has two pins connected across one of the 1 ohm emitter resistors, presumably for connection of a voltmeter to determine the bias current for that output device. It must be assumed that the bias currents for all the other output device pairs are much the same, as the use of batch matched output devices is recommended. The bias was set for 50mV across this 1 Ohm, (50mA) by Ohm's Law, but this increased to 72mA as the thing came to thermal equalibrium again.
I, personally tend to go for lower bias which runs the output devices cooler and thus increases the longevity of the device overall and the tradeoff being that the device passes into class-B operation ant lower input signal levels with some subsequent degradation of the signal quality during quieter passages.
The question is, is there a recommended bias setting and is it measured initially with a cold heatsink or after thermal equalibrium has been attained? If so, does anyone know its numerical value in mA per output device pair?
I have a Nakamichi PA7E II here on my bench. It was faulty, mainly dry joints which were particularly bad under the class-A voltage gain stages and associated current sink, (Qs 110, 111 and 112).
Basically went over the entire board and resoldered everything, Q112 was literally rattling loose, only the collector lead holding it. Presumably because of this, it had gone D.C. and the speaker protector had cut in. The owner seems to be the type whom leaves the unit on almost permanently, estimated by the browning of the phenolic under the class-A voltage gain transistors and the degree of recrystallization of the solder. Presumably after it had gone D.C., he had not realized and the feedback capacitor, C104, a Nichicon "MUSE" 220uF 16v Bipolar electrolytic had been overvoltaged for some considerable duration via the feedback network (R121, 2k49 and R120, 51k1), as there was no D.C. division path. This capacitor had "pop-corked", spilling its electrolyte out through the burst scores at the top of the can,. so this had to be replaced and getting this type of capacitor in Australia is not an easy ask. Both feedback caps were changed and both boards carefully resoldered and inspected for damage.
So, after all this, the unit was powered up (in series with 4 paralelled 200w incandescent lamps just in case it's supply port impedance decided to drop to a very low value). The lamps pulsed in brightness as the power supply resevior caps charged and the circuit came into operation normally, producing a voltage amplified signal into no load with no appreciable D.C. offset. The lamps were observed to glow slightly due to the shoot through current of the output stage class-A to Class-B transition bias. The brightness was observed to increase slightly as the output stage and heatsink came into thermal equalibrium after about 10 minutes operation.
It was deemed that the bias was too high from the brightness of the lamps' glow, so the power supply leads to each channel were disconnected in turn so the bias of each individual channel could be guesstimated. There is a small red pin header on the edge of the board, TP11, which has two pins connected across one of the 1 ohm emitter resistors, presumably for connection of a voltmeter to determine the bias current for that output device. It must be assumed that the bias currents for all the other output device pairs are much the same, as the use of batch matched output devices is recommended. The bias was set for 50mV across this 1 Ohm, (50mA) by Ohm's Law, but this increased to 72mA as the thing came to thermal equalibrium again.
I, personally tend to go for lower bias which runs the output devices cooler and thus increases the longevity of the device overall and the tradeoff being that the device passes into class-B operation ant lower input signal levels with some subsequent degradation of the signal quality during quieter passages.
The question is, is there a recommended bias setting and is it measured initially with a cold heatsink or after thermal equalibrium has been attained? If so, does anyone know its numerical value in mA per output device pair?
Here's what the Service Manual says:
3.1 Preparation for Adjustment
(1) Insert shorting pins into the Input Jacks.
(2) Remove loads of the speaker terminals.
(3) Before starting adjustment, allow twenty minutes or more
after the power is turned ON.
3.2 DC Balance Adjustment
(1) Connect a DC voltmeter to the L (R) channel speaker terminal.
(2) Adjust VR101 (VR201) to obtain 0 +/- 100 mV on the DC voltmeter.
3.3 Idling Current Adjustment
(1) Connect a DC voltmeter between TP101 and TP102 (TP201 and TP202)
on the Amp. L (R) P.C.B. assembly.
(2) Adjust VR102 (VR202) to obtain 40 mV on the DC
voltmeter (idling current: 40 mA).
(3) Repeat 3.2 and 3.3 one or two times.
(4) Remove shorting pins from the Input Jacks.
3.1 Preparation for Adjustment
(1) Insert shorting pins into the Input Jacks.
(2) Remove loads of the speaker terminals.
(3) Before starting adjustment, allow twenty minutes or more
after the power is turned ON.
3.2 DC Balance Adjustment
(1) Connect a DC voltmeter to the L (R) channel speaker terminal.
(2) Adjust VR101 (VR201) to obtain 0 +/- 100 mV on the DC voltmeter.
3.3 Idling Current Adjustment
(1) Connect a DC voltmeter between TP101 and TP102 (TP201 and TP202)
on the Amp. L (R) P.C.B. assembly.
(2) Adjust VR102 (VR202) to obtain 40 mV on the DC
voltmeter (idling current: 40 mA).
(3) Repeat 3.2 and 3.3 one or two times.
(4) Remove shorting pins from the Input Jacks.
Last edited:
Hello again. Thank you Analog S.A. for the 40mA, it's close to the 50mA I had it at. Thanks James, I'm doing it now....
Shorted Input Jacks
Using Oscilloscope (Tek 7603), since I have no milivolt meter...
Right Channel at speaker side of protection relay across Zobel Network. +11mV D.C, with about 2.5mV R.M.S. broadband A.C. noise with a bandwidth exceeding 100MHz superimposed.
Left Channel at output binding posts, +13.5mV D.C. Offset with about 800uV RMS broadband noise superimposed with a bandwith exceeding 100MHz. With Nacamichi off, broadband noise is still present indicating is is probably being generated in the Oscilloscope's Y amp or being picked up by the probe.
This model, the PA-7E II, is a different circuit from the PA-7E, there is no D.C. offset adjustment trimpot. The front end is a diffirential cascode stage consisting of a 2SK240 dual fet, (two thermally coupled fets in a aluminium can), to the drains of this going up to the emitters of two 2SC2240s in a grounded base arangment with their bases 10.2 volts (zener) above the sources of the 2SK240. The collectors of these 2SC2240s provide the differential drive to the bases of Class-A cascode voltage gain stage, a pair of 2SA970s with a pair of 2SA1370s beneath them in a similar, zener biased grounded base arrangment, another pair of 2SA970s sit above the first grounded base pair of 2SC2240s as a current mirror. A 2SC2705 sits beneath the sources of the 2SK240 as a 4.3 mA current sink and 5th 2SC970 above the bases of the grounded base 2SC2240s and zener to provide a 650uA current source. The current source and sink share an 82K resistor to bias up their diode stacks. The input, 50K down, 220 ohm 330pF R.F. LPF goes into one fet gate and the feedback network into the other. 50K from output shunted by 2k5 in series with 220uF 16v Nichicon "MUSE" bipolar electrolytic. This describes the front end of this unit.
After the time it has taken me to single finger type this, I measured the Right Channel, Thermally equlibriated bias across the 1 ohm emitter resistors of the output stage,....
Positive PNP side, measuring from the end most device inwards, 26mV, 29mV, 30mV, 31mV, 31mV, 31mV, 36mV.
Negative NPN side, from end inwards, 29mV, 27mV, 28mV, 29mV, 31mV, 30mV, 23mV. They don't seem to match up, but they varied by up to +-2mV even during the time it took to measure them. Even a cool draught on the heatsink seemed to make them fall slightly.
The number of 200 watt incandescent lamps (in parallel), in series with the amplifier supply was decreased from four to two, so the glow of the filaments was visible. The bias pot was touched about 5 degrees counter clockwise and the filament glow increased in intensity indicating that this direction was increasing the bias. After this adjustment the other two lamps were switched back in to decrease the source impedance of the supply and the thing left to equibrate for a further 15 minutes or so. Periodically the back of the forfinger was run along the row of output devices to check to see if any were running hotter or cooler. It was noticed that the four driver devices, emitters inwards rather than the collector inward output devices, but of the same type numbers, viz 2SC3856 and 2SA1492, which are positioned in the centre of the row of 18 transistors on the heatsink, were running noticeably warmer.
The second lot of measurements, oddly enough, were almost the same as the first, so the bias pot was touched another 3-5 degrees CCW again...
The oscolloscope was monitoring the output offset the whole time, no appreciable (>+-5mV), change was noticed.
Third Set of measurements..., PNPs 36, 39, 36, 37, 38, 37, 41 (264mA in total)
NPNs 39, 37, 38, 40, 42, 40, 35 (271mA in total)
Some of the devices are starting to creep up to and over the mark, so we will proceed gingerly......just a slight tweak of 1-2 degrees...
Fourth Set.............................., PNPs 41, 45, 44, 45, 46, 46, 51
NPNs 45, 43, 44, 46, 49, 47, 42
The transistor drawing 51mA was noticeably hotter than the others and the drivers were getting hot enough to warrant a second careful feel....
It has been decided to back the bias down a touch so the bias pot was "backed up" 1-2 degrees to where it was before.
Fifth set...............................................Not Much Change
Back it down further and wait....
What is really important when adjusting the bias of any amplifier with bipolar output transistors, is not the actual value of the measurements, but which way they drift with time,
because bipolar transistors possess a negative temperature coefficient. So with a constant base current, there should be a corresponding larger collector current and the relationship between them is called the beta, or current gain of the device. Most small transistors have a beta of around 100, but power devices, like these in this amp, it is lower, between about 30 and 50.
So for 1mA into the base of an NPN device, 30 to 50mA is pulled into the collector regardless of the collector voltage, (as long as the transistor is not saturated and its collector voltage is low).
So the collector voltage multiplied by the collector current is a good indication of the power dissipation of the device. In this particular unit, the rail voltage is +-75v, so that's roughly the collector voltage. At 40mA per device each is dissipating 75 x 0.04 watts or,3 watts, enough to make 'em quite hot if not firmly fastened to a heatsink. Now the problem is that the beta value increases as they get hotter, so for a constant base current, the collector current rises and this heats the device more and the collector current rises....and so on until,....poof something melts, usually part of a device die.
To counter this effect, a transistor, called a vbe multiplier, in the bias network, but is fastened to the heatsink, "feels" the heat and its collector current rises too, but as it does th reduces the bias to the output devices. In some designs this thermal sensing is not enough and a thermistor is placed in the base circuit of the vbe multiplier, this is attached to the heatsink too and increases the retarding of the bias when things get hot. The only drag in all of this is the thermal inertia of the heatsink which slows the communication of heat from a hot output device to the vbe multiplier and thermistor. So if an output device has really started to "run away", the vbe multiplier will not feel it soon enough to stop it, and, again, poof, another stuffed output device. It's sort of loke balancing a ball on a hill, if you notice it start to roll down and stop it, all is O.K., but of it has rolled too far and is going too fast, you will never catch it. As the bias pot of an amp like this is adjusted, at low settings the crossover distortion is bad, but the "hill" has a dent in the top into which the "ball" will always roll, and hot output devices will always tend to cool down. However, as the bias is increased, "the dent in the top of the hill" gets shallower and at some point it is flat. Further increases make the "flat area" into into an "ordinary hill" and the "roll down of the ball" is inevitable. The trick is to retain that "dimple", a state of unconditional stability, and the device never gets the chance to run away.
Here in Tasmania it is winter and the heatsinks of this amp are freezing cold when it is off. In the winter cold, the devices have no trouble getting heat away, but when summer comes and the ambient temperature is high, that's like turning the bias up and removing the "dimple on the hilltop", and things could get nasty, so it's always a good idea the back the bias down by a little safety margin, particularly if adjusting in a cold room in winter. On a very hot summer day, one could tweak the bias right up to the mark, knowing cooler weather would simply back it off a touch. So this is the rationale as to why one is so careful with bias adjustments of equipment of this sort. After you adjust it, you want to see it back off slightly as it comes to thermal equlibrium, not keep gradually advancing with gathering pace into eventual runaway. This is another reason for the incandescent lamps in series with the supply, of the output devices do tip over and run away, the increased collector current will "pull against" the higher impedance of the lamps and the power supply/collector voltage will fall, the lamps will start to glow as a warning and the whole lot will come to a second equlibrium, like a valley around the hill, before any real damage is done.
So, what is it doing now?
Sixth Measurement......................PNPs 43, 46, 44, 43, 44, 43, 47
Won't bother to do the NPNs, this is still a bit high, but its settling into the "dimple in the hill". The really hot device, drawing 51mA before, has backed down to 47mA. So now we need to back off the bias as a safety margin for hot summer days to come....
Seventh Measurement..................Back down around 31-36mA
Tweak up again
Eighth Measurement....................Back up around 34-41mA
Tweak down again
Ninth measurement.....................Back down to 31-38mA Range
Try blasting heatsink with neice's hair dryer to simulate a hot day
Tenth Measurement...................30 down to 24, 37 down to 31, so the vbe multiplier is working!
Tweak it up again
Eleventh measurement...............30-38 while the heatsink is still hot from the hairdryer
Twelfth measurement..................36-46 as it cools (42 at TP)
Thirteenth Measurement..............40-50 as it continues to cool (43 at TP)
Fourteenth measurement..............35-43 after cooling with damp sponge, (37 at TP)
Notice how it peaks in the middle and falls off at either end, this is exactly what is wanted and the range of values obtained over this temperature range, (estimated to be from 45C* down to around 20C*) cluster around 40mA. The "falling off at each end" characteristic is obviously from the vbe multiplier and its associated NTC thermistor between its collector and base, in series with fixed 750 ohms. This is where I will leave it, now I have the other channel.....!
Final warm measurement, 37-42, 39 at the test point. Turn off for a couple of hours...
Stone Cold Heatsink, 31-41 and rising rapidly as dies heat up. (27 at test Point).
After a few minutes before heatsink has had a chance to be warmed by transistor dies the bias current range for the PNP devices from endmost to innermost, next to drivers was
61-69mA, (64mA at test point), at this stage enough current is flowing to cause all four 200w lamps to glow at a low level. It seems to hold at 61-70mA until the vbe multiplier starts to feel the wamth.
After 5 minutes or so, the lamps start to dim slowly and range drops to 56-65mA, (59mA at test point), Aafter about 10 minutes, the glow from the lamps is barely visible in a lit room and the current range is down to the 47-57mA range and 51mA at the test point. After 15 minutes or so, the glow from the lamps has faded from view and the range is 41-51mA and 45mA at the test point. It is expected to take about an hour to stabilize at the 37-42mA range.
Half hour later, 38-48mA, 43mA at test point, so its creeping down...
Hope this is of some use to you all, not only in a specific sense, but a general one too in understanding the nature of the bias in bipolar output stage amplifiers.
Shorted Input Jacks
Using Oscilloscope (Tek 7603), since I have no milivolt meter...
Right Channel at speaker side of protection relay across Zobel Network. +11mV D.C, with about 2.5mV R.M.S. broadband A.C. noise with a bandwidth exceeding 100MHz superimposed.
Left Channel at output binding posts, +13.5mV D.C. Offset with about 800uV RMS broadband noise superimposed with a bandwith exceeding 100MHz. With Nacamichi off, broadband noise is still present indicating is is probably being generated in the Oscilloscope's Y amp or being picked up by the probe.
This model, the PA-7E II, is a different circuit from the PA-7E, there is no D.C. offset adjustment trimpot. The front end is a diffirential cascode stage consisting of a 2SK240 dual fet, (two thermally coupled fets in a aluminium can), to the drains of this going up to the emitters of two 2SC2240s in a grounded base arangment with their bases 10.2 volts (zener) above the sources of the 2SK240. The collectors of these 2SC2240s provide the differential drive to the bases of Class-A cascode voltage gain stage, a pair of 2SA970s with a pair of 2SA1370s beneath them in a similar, zener biased grounded base arrangment, another pair of 2SA970s sit above the first grounded base pair of 2SC2240s as a current mirror. A 2SC2705 sits beneath the sources of the 2SK240 as a 4.3 mA current sink and 5th 2SC970 above the bases of the grounded base 2SC2240s and zener to provide a 650uA current source. The current source and sink share an 82K resistor to bias up their diode stacks. The input, 50K down, 220 ohm 330pF R.F. LPF goes into one fet gate and the feedback network into the other. 50K from output shunted by 2k5 in series with 220uF 16v Nichicon "MUSE" bipolar electrolytic. This describes the front end of this unit.
After the time it has taken me to single finger type this, I measured the Right Channel, Thermally equlibriated bias across the 1 ohm emitter resistors of the output stage,....
Positive PNP side, measuring from the end most device inwards, 26mV, 29mV, 30mV, 31mV, 31mV, 31mV, 36mV.
Negative NPN side, from end inwards, 29mV, 27mV, 28mV, 29mV, 31mV, 30mV, 23mV. They don't seem to match up, but they varied by up to +-2mV even during the time it took to measure them. Even a cool draught on the heatsink seemed to make them fall slightly.
The number of 200 watt incandescent lamps (in parallel), in series with the amplifier supply was decreased from four to two, so the glow of the filaments was visible. The bias pot was touched about 5 degrees counter clockwise and the filament glow increased in intensity indicating that this direction was increasing the bias. After this adjustment the other two lamps were switched back in to decrease the source impedance of the supply and the thing left to equibrate for a further 15 minutes or so. Periodically the back of the forfinger was run along the row of output devices to check to see if any were running hotter or cooler. It was noticed that the four driver devices, emitters inwards rather than the collector inward output devices, but of the same type numbers, viz 2SC3856 and 2SA1492, which are positioned in the centre of the row of 18 transistors on the heatsink, were running noticeably warmer.
The second lot of measurements, oddly enough, were almost the same as the first, so the bias pot was touched another 3-5 degrees CCW again...
The oscolloscope was monitoring the output offset the whole time, no appreciable (>+-5mV), change was noticed.
Third Set of measurements..., PNPs 36, 39, 36, 37, 38, 37, 41 (264mA in total)
NPNs 39, 37, 38, 40, 42, 40, 35 (271mA in total)
Some of the devices are starting to creep up to and over the mark, so we will proceed gingerly......just a slight tweak of 1-2 degrees...
Fourth Set.............................., PNPs 41, 45, 44, 45, 46, 46, 51
NPNs 45, 43, 44, 46, 49, 47, 42
The transistor drawing 51mA was noticeably hotter than the others and the drivers were getting hot enough to warrant a second careful feel....
It has been decided to back the bias down a touch so the bias pot was "backed up" 1-2 degrees to where it was before.
Fifth set...............................................Not Much Change
Back it down further and wait....
What is really important when adjusting the bias of any amplifier with bipolar output transistors, is not the actual value of the measurements, but which way they drift with time,
because bipolar transistors possess a negative temperature coefficient. So with a constant base current, there should be a corresponding larger collector current and the relationship between them is called the beta, or current gain of the device. Most small transistors have a beta of around 100, but power devices, like these in this amp, it is lower, between about 30 and 50.
So for 1mA into the base of an NPN device, 30 to 50mA is pulled into the collector regardless of the collector voltage, (as long as the transistor is not saturated and its collector voltage is low).
So the collector voltage multiplied by the collector current is a good indication of the power dissipation of the device. In this particular unit, the rail voltage is +-75v, so that's roughly the collector voltage. At 40mA per device each is dissipating 75 x 0.04 watts or,3 watts, enough to make 'em quite hot if not firmly fastened to a heatsink. Now the problem is that the beta value increases as they get hotter, so for a constant base current, the collector current rises and this heats the device more and the collector current rises....and so on until,....poof something melts, usually part of a device die.
To counter this effect, a transistor, called a vbe multiplier, in the bias network, but is fastened to the heatsink, "feels" the heat and its collector current rises too, but as it does th reduces the bias to the output devices. In some designs this thermal sensing is not enough and a thermistor is placed in the base circuit of the vbe multiplier, this is attached to the heatsink too and increases the retarding of the bias when things get hot. The only drag in all of this is the thermal inertia of the heatsink which slows the communication of heat from a hot output device to the vbe multiplier and thermistor. So if an output device has really started to "run away", the vbe multiplier will not feel it soon enough to stop it, and, again, poof, another stuffed output device. It's sort of loke balancing a ball on a hill, if you notice it start to roll down and stop it, all is O.K., but of it has rolled too far and is going too fast, you will never catch it. As the bias pot of an amp like this is adjusted, at low settings the crossover distortion is bad, but the "hill" has a dent in the top into which the "ball" will always roll, and hot output devices will always tend to cool down. However, as the bias is increased, "the dent in the top of the hill" gets shallower and at some point it is flat. Further increases make the "flat area" into into an "ordinary hill" and the "roll down of the ball" is inevitable. The trick is to retain that "dimple", a state of unconditional stability, and the device never gets the chance to run away.
Here in Tasmania it is winter and the heatsinks of this amp are freezing cold when it is off. In the winter cold, the devices have no trouble getting heat away, but when summer comes and the ambient temperature is high, that's like turning the bias up and removing the "dimple on the hilltop", and things could get nasty, so it's always a good idea the back the bias down by a little safety margin, particularly if adjusting in a cold room in winter. On a very hot summer day, one could tweak the bias right up to the mark, knowing cooler weather would simply back it off a touch. So this is the rationale as to why one is so careful with bias adjustments of equipment of this sort. After you adjust it, you want to see it back off slightly as it comes to thermal equlibrium, not keep gradually advancing with gathering pace into eventual runaway. This is another reason for the incandescent lamps in series with the supply, of the output devices do tip over and run away, the increased collector current will "pull against" the higher impedance of the lamps and the power supply/collector voltage will fall, the lamps will start to glow as a warning and the whole lot will come to a second equlibrium, like a valley around the hill, before any real damage is done.
So, what is it doing now?
Sixth Measurement......................PNPs 43, 46, 44, 43, 44, 43, 47
Won't bother to do the NPNs, this is still a bit high, but its settling into the "dimple in the hill". The really hot device, drawing 51mA before, has backed down to 47mA. So now we need to back off the bias as a safety margin for hot summer days to come....
Seventh Measurement..................Back down around 31-36mA
Tweak up again
Eighth Measurement....................Back up around 34-41mA
Tweak down again
Ninth measurement.....................Back down to 31-38mA Range
Try blasting heatsink with neice's hair dryer to simulate a hot day
Tenth Measurement...................30 down to 24, 37 down to 31, so the vbe multiplier is working!
Tweak it up again
Eleventh measurement...............30-38 while the heatsink is still hot from the hairdryer
Twelfth measurement..................36-46 as it cools (42 at TP)
Thirteenth Measurement..............40-50 as it continues to cool (43 at TP)
Fourteenth measurement..............35-43 after cooling with damp sponge, (37 at TP)
Notice how it peaks in the middle and falls off at either end, this is exactly what is wanted and the range of values obtained over this temperature range, (estimated to be from 45C* down to around 20C*) cluster around 40mA. The "falling off at each end" characteristic is obviously from the vbe multiplier and its associated NTC thermistor between its collector and base, in series with fixed 750 ohms. This is where I will leave it, now I have the other channel.....!
Final warm measurement, 37-42, 39 at the test point. Turn off for a couple of hours...
Stone Cold Heatsink, 31-41 and rising rapidly as dies heat up. (27 at test Point).
After a few minutes before heatsink has had a chance to be warmed by transistor dies the bias current range for the PNP devices from endmost to innermost, next to drivers was
61-69mA, (64mA at test point), at this stage enough current is flowing to cause all four 200w lamps to glow at a low level. It seems to hold at 61-70mA until the vbe multiplier starts to feel the wamth.
After 5 minutes or so, the lamps start to dim slowly and range drops to 56-65mA, (59mA at test point), Aafter about 10 minutes, the glow from the lamps is barely visible in a lit room and the current range is down to the 47-57mA range and 51mA at the test point. After 15 minutes or so, the glow from the lamps has faded from view and the range is 41-51mA and 45mA at the test point. It is expected to take about an hour to stabilize at the 37-42mA range.
Half hour later, 38-48mA, 43mA at test point, so its creeping down...
Hope this is of some use to you all, not only in a specific sense, but a general one too in understanding the nature of the bias in bipolar output stage amplifiers.
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