JVC SEA-M9 service manual

SEA-M9B01I’ve put off scanning the service manual for my JVC SEA-M9 for too long. Here it is.

JVC SEA-M9B service manual download
Mirror (MEGA)

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Yamaha P-2200 measurements

Yamaha P-2200 uncased

Yamaha P-2200 PA power amplifier

Measurement results for Yamaha P-2200

Measurement results for Yamaha P-2200

PDF version

After many hours of work, I finally got my Yamaha P-2200 into a state worthy of being tested. The left channel is 100 % original, save for new electrolytic capacitors. It should be fairly representative of a completely original unit. The right channel has received major repairs. The high vol = FS noise floor on the right channel is not due to the repairs done to the amplifier, but rather the factory wiring of the amplifier; it’s mostly made up of 50 Hz hum. There’s probably a signal wire going too close to the transformer, somewhere.

All in all, the Yamaha P-2200 is a very well-performing amplifier, especially for its age and the fact that it’s Yamaha’s first ever dive into the world of PA amps. With plenty of passively-cooled power, tank-like build quality, an impressive noise floor and a good damping factor to boot, Yamaha made sure to make their PA power amp début one for the ages!

Just for fun, four audio gear gifs!

These files are huge (about 3 MB each), so give it some time to load!

JVC SEA-M9 in spectrum analyser mode

JVC SEA-M9 in spectrum analyser mode

The JVC SEA-M9 in spectrum analyser mode, filmed during an earthquake.

The JVC SEA-M9 in spectrum analyser mode, filmed during an earthquake.

The dancing meters of the Yamaha P-2200

The dancing meters of the Yamaha P-2200

Freshly refoamed Acoustic Research AR-7 woofer doing its thing.

Freshly re-foamed Acoustic Research AR-7 woofer doing its thing.

Substituting the SFC6120 double transistor in the Tandberg TR-1055

Tandberg TR-1055

Tandberg TR-1055

This nice-looking unit came into the shop with a noisy left channel as well as an intermittent DC offset. After seemingly repairing it by replacing the output transistors, the customer returned it complaining about the same issue arising after a few hours of use. After some further troubleshooting, the problem was found to be a Motorola-branded double transistor in the power amplifier.

The SFX6120 double transistor

The SFC6120 double transistor in the negative feedback circuit

Neither the part nor a datasheet for it was anywhere to be found, so a substitute had to be manufactured. I settled for a matched pair of the common KSC1845 to do the job. Gain matching is important, as an unmatched pair will result in a DC offset on the output of the amplifier.

Installing the transistors is easy, as the pin-out for the SFC6120 is printed on the circuit board.

Transistor 1 installed

Transistor 1 installed

Both replacement transistors installed

Both replacement transistors installed

However, my KSC1845s had roughly twice the gain of the SFC6120 (380 vs. 160), which resulted in a considerable increase of the amplifier’s gain. To counteract this, feedback resistor R712 was decreased from 10 kOhm down to 3,6 kOhm.

R712, the blue resistor, determines the gain of the amplifier module

R712, the blue resistor, determines the gain of the amplifier module

Since the modification altered the gain of the amplifier, I decided to perform it on both channels to ensure proper matching and guard against future SFC6120 failures. It is important to ensure thermal coupling between the two transistors, in order to guard against DC offset when the amplifier warms up. That’s probably why Tandberg decided to use a double transistor in the first place.

Somewhat unexpectedly, the THD+N of the amplifier decreased from 0,08 % at rated output into 4 Ohm, to a mere 0,033 % after the modification. (Measured with my HP 339A at 1 kHz)

After many hours of heavy load testing into 4 Ohm, I think this unit is ready to go back to the customer again – and hopefully not return!

The finished pair of output modules

The finished pair of output modules. Note the thermal goop on the KSC1845s. The two modules have been (partially) recapped at different occasions.

An untouched Tandberg TR-1055 output module

An untouched Tandberg TR-1055 output module (For reference)

Yamaha AX-590 measurements

 

Yamaha AX-590 integrated amplifier

Yamaha AX-590 integrated amplifier

AX-590_results

For kicks, I thought I’d do a basic THD+N test on my Yamaha AX-590. The results are next to stunning – this is an extremely capable unit. The -88 dBV noise floor (measured with shorted input, CD Direct enabled and the volume at -inf) is simply fantastic for a 100 WPC amplifier, and I had to check that my test load wasn’t broken when measuring the damping factor – almost 1000 at 1 kHz!

What a performer! Given the relatively low (about $600 in 1996) retail price of this unit, I did not expect it to perform anywhere near this well.

Lepai LP-2020A+ mini-review and measurements

Thanks goes out to a friend for donating this amplifier for testing! (You know who you are)

You can hardly enter five forum threads on budget audio without seeing the Lepai LP-2020A+ mentioned. At about 20 U.S. dollars (30 with an AC adapter included), this compact and 20 WPC advertised amplifier looks like a solid deal.

However, despite its popularity, actual information about it seems scarce, aside from the occasional teardown or two. It’s built around a Tripath TA2020-020 class D amplifier chip, which, with its fine specifications and almost 90% efficiency, is what allows for this fantastic power to size ratio.

The specifications printed on the box (that’s apparently used for two models)

However, class D amplifiers require far more attention to detail in the circuit board design and component selection than traditional class AB chip-amps (like the classic LM3886). Did Lepai get it right? Does the LP-2020A+ live up to the specifications quoted on the box, or perhaps even the ones quoted in the TA2020-020 datasheet?

Let’s find out! (The less technically minded may want to skip to the verdict)

The test set-up

The test set-up

The LP-2020A+ was powered by a 5 A industrial switchmode power supply. THD+N measurement was done with an HP 339A, low-passed at 30 kHz. A 4/8 Ohm switchable resistive load was used.

Output impedance/damping factor was calculated by setting a loaded voltage of 5,51 V (8 Ohm) and 5,2 V (4 Ohm) and observing the voltage change when the load was disconnected. A VCC of 13,5 V was used for all tests, 13,4 V was the lowest voltage observed.

The results (updated 9.12.2012):

lepai_results

A-weighted spectrum captured at 5 W into 8 Ohm.

More spectrums (HF noise is included in the unweighted ones):

Sadly, but perhaps not surprisingly, the Lepai did not fulfil the claimed 20 W/ch at <0,05 % THD.

It starts distorting after about 9,6 W into 4 Ohm and 5,6 W into 8 Ohm. It doesn’t show on the THD+N measurements, but the clipping waveform during mild clipping is unorthodox and in my opinion rather intrusive. It causes a popping sound in the tweeters of my test speakers.

It does however far surpass its specification of 80 dB SNR, measuring in at 90,1 dB A-weighted and 87 dB unweighed from 20 Hz to 20 kHz.

Absolute noise measures in at roughly -76 dBV(a). This is 10 dB higher than higher-end amplifiers I’ve measured, and can be audible on many speakers – it is very obvious on mine. If noise above the audible spectrum is included, the numbers falls considerably; if measured unweighted from 10 Hz through 30 kHz on my HP 339A, the noise floor is at roughly -66 dBV. For comparison, that’s nearly two orders of magnitude above the -84 dBV of the Luxman L-120A that resides in my system.

The LP-2020A+ exhibits this behaviour during mild clipping. Picture taken at 0,3 % THD+N.

There is also a constant 200 mV P-P noise at about 160 kHz. Even with the low-pass filters in place, this somehow caused the 339A to go haywire when measuring at very low levels (<250 mW). This noise is probably not an issue in everyday use.

There is a constant noise present at about 160 kHz.

Scope view of a 20 kHz sine wave at 5 W into 8 Ohm. The HF noise can be seen as the abnormal thickness of the trace.

A more noteworthy detail about the LP-2020A+ is that it seems to have a considerable notch in the frequency response above 10 kHz, even with the tone controls disabled. After investigating the issue, I was able to conclude that the notch was not present at the input of the TA2020-020 amplifier chip.

Captured at the output, this curve shows that the frequency response peaks at about +1,6 dB at 30 kHz, and is up by over 1 dB at 20 kHz.

I’m not certain about what causes this anomaly, but I’d wager that it has to do with the way Lepai have designed the output filter. This could very well be an audible problem with this amplifier.

The high-frequency anomaly aside, the LP-2020A+ seems to be flat throughout the frequency spectrum down to about 40 Hz, rolling off by 1 dB at 17 Hz.

Build quality

The LP-2020A+ freed of its case

Build quality wise, you do get what you pay for – the design has a fair few cut corners that deviate from the design guidelines in the TA2020-020 datasheet, in order to reduce component count. The electrolytic capacitors are of varying Chinese brands not renowned for their quality, the potentiometers are pretty nasty and power supply filtering is … minimalistic. All in all, it’s still better than I expected. Soldering quality is decent and the aluminium case is solid.

However, something that struck me about my unit was this:

My LP-2020A+ had very poor heat sink mounting.

Thermal paste was scarce.

Yes, there was an almost millimetre-thick gap between the chip and the heat sink! The over-temperature protection would kick in after a few minutes of heavy 4-Ohm loading. Thanks to the efficiency of the TA2020-020, this would probably not have been noticeable in every-day use. However, one must wonder what it does to the chip lifespan.

Verdict

I entered this with a hunch that the Lepai LP-2020A+ would not deliver its advertised performance. The TA2020-020 chip that it’s built around is rated for 20 W into 4 Ohm at 5 % THD. Despite that, it is still a very decently performing amplifier for the price as long as you stay within its limits. 5 clean Watts per channel into 8 Ohm might not sound like a lot, but for casual near-field listening with small bookshelf speakers it should still be plenty; “normal” background listening level is generally only around 0,1 Watt!

The biggest issue with this unit is the build quality. The casing is good quality extruded aluminium, but the quality control and above all the potentiometers (volume control, bass and treble) are of atrocious quality. The unevenness and scratching of the volume control can be fairly bothersome, especially when close to zero. Since the volume control is connected to adjust the gain of an op-amp rather than simply attenuate the input signal, a bad connection in it can lead to very loud popping and oscillation.

Sound quality wise, there isn’t much to comment on. There is a slight notch in the treble that could colour the sound, but it is a minor issue in this market segment. Other than that LP-2020A+ has a background hiss that’s 10 dB (twice as loud) higher than the considerably more costly amplifier I usually have in my system. Depending on what speakers are used and how sensitive one is to background hiss, this could be an issue, and probably the biggest sound quality concern with the Lepai.

All in all, would I recommend it? The answer is a resounding yes! While it’s hardly “audiophile quality”, you aren’t going to find a better 5-watt-per-channel amplifier for the price – and with proper stereo amplifiers often costing several hundred Dollars, this little Lepai introduces a whole new standard for budget amplification.

Image gallery

Front

Rear

Top

Preamp and tone amp op-amps (JRC 4558)

Output low-pass filter. The blue capacitors are 47 nF.

The Tripath TA2020-020 amplifier chip

Solder side