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?
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.
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
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.
The Nakamichi build was pretty beefy in the output
stage and power supply, so I wouldn't have any
doubts about the 4 ohm performance.
4 ohms no problem
My Vienna Beethoven speakers are rated 4 ohms and even dips to 2.5 ohms in the upper bass I think. My PA5EII is very happy driving these and will even melt voice-coils on woofers on occasion so don't worry about the PA7 running on 4 ohms.
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.
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?
PA7 requires 40mV between TP101/102 or 40mA per device. Seems conservative.
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.
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