2019/12/08

Nakamichi SR40 STASIS Stereo Reciever

nakamichi_SR40_1.jpeg

Nakamichi SR-40 STASIS Stereo Receiver

An amplifier with a tuner released in 1987. My audiophile friend asked me to this amp if I could repair it.

gSR-40h has a STASIS power amplifier circuit. STASIS is a non-feedback type amplifier developed by Dr. Nelson Pass at Threshold.

Nakamichi received a patent license in 1985, and in the following year it was first commercialized in the power amplifier "PA-70 / 50" series. At that time, Threshold released only super-high-end products and licensed Nakamichi to develop products in the middle range and below.

There was no pre-main amp product at Nakamichi at that time, and the stereo receiver product with STASIS was the SR-40 (SR-4 in overseas), and it was a high-end model with a sales price of 148,000 yen. There seems to be a product called gSR-30 / 20h in the lower model.

Until this "SR-40" was manufactured in Japan, but the models after that seem to have been manufactured overseas, and the product quality seems to have deteriorated considerably. The successor model was renamed gTA-40 / 30 / 20h and was released until around 1990.

SR-4A-brochure.jpeg

At that time, stereo receivers were very popular in North America, and the market size was also large. In the catalog of gSR-40h, gThe Audiophile Receiverh and gRemote Control STASIS Receiverh are displayed and it seems that it was appealing as a stereo receiver for audiophiles.

SR-4A-bro-2.jpeg

The appearance of the gSR-40h is so good that it cannot be seen by an audio product 32 years ago. The owner said that sound was coming out, but sometimes it was also mixed with noise.
However, I was drowned to turn on the power suddenly, so I opened the case and checked it.

nakamichi_SR-40_inner-1.jpeg

The lower board is the main amplifier board, and a fairly large heat sink is placed in the center.
Dust was piled up on the board, but no electronic parts were burned or burned out.
The old famous device Sanken 2SA1491 / 2SC3855 was placed in a 2 parallel configuration in the final stage.

nakamichi_SR40_inner-2.jpeg

Since it is almost composed of discrete parts, the repair is a little easier.
However, this is the first time I have seen an amplifier with so many styrene capacitors. Styrene capacitors are famous parts because of their good sound, but they are very sensitive to heat (heat-resistant temperature is about 85C), so if a damaged part is found, it will be very difficult. Since it is already discontinued as a part, whether there is dead stock somewhere ...

nakamichi_SR40_inner-4.jpeg

Since there is also an AM / FM tuner board on the front panel side, it is a fairly complex and high-density mounting. To improve the sound quality, it may remove the tuner circuit.

nakamichi_SR40_inner-3.jpeg

The power supply has a large toroidal transformer, and the two large electrolytic capacitors were still safe.

I tried to leave it for a while after turning it on, but it did not generate any heat, abnormal vibration, or smoke. Because of the large heat sink, the temperature rise of the transistor is also mild.

When I was fine and put the music signal in the CD input and turned up the volume, the sound came out from the speaker properly. The main volume can be controlled smoothly without any rust, but the volume knob is quite heavy because of the motor control.

I also found that the display brightness of the AM / FM tuner was low, making it very difficult to see. When the FM antenna was connected, the signal meter was also displayed and the FM received sound could be played properly. I was surprised that the reception sensitivity was better than my FM tuner.

After a while, there was a symptom that sometimes the left channel was not output, but I found that the tone control volume seems to be poorly touched.

It seems to be mainly due to poor contact of the signal path, so it seems to be okay to not complete overhaul. For the time being, I'm trying to clean the contacts in questionable parts.

I've never heard of a STASIS amp, but it sounds like a high-end amp rather than a stereo receiver.
It's not particularly powerful, but it's a bit of a beautiful sound!

The reason why it was not popular in Japan was probably due to the product form of the receiver.

According to reviews from overseas users on the Web, the gSR-40 (SR-4)h has the impression that it sounds better than the gPA-50 (PA-5)h power amplifier with a STASIS circuit.

Around 1990, Nakamichi broke down in terms of contract with Threshold, and was unable to sell amplifier products with STASIS circuits.

SR4_ServiceManual-1.jpeg

This gSR-40h is a complex implementation, so it would be troublesome to completely disassemble and clean it.@I got a service manual, so I would like to repair it in the meantime.B

Note (2019/12/11):
In the running test, sometimes the left channel was not output! Not only due to poor contact in the tone control section. I seem to have to overhaul it.
posted by toons at 23:58 | Audio

2019/11/12

OSAKA High-end Audio Show 2019

HighendAudio2019.JPG

I went to Osaka High-end Audio Show 2019 last weekend.

After 16:00 on Friday, each listening room was full and most of them seemed to be in their 50s and 60s.

This year, the venue was only the Hearton Hotel Shinsaibashi, so it may be because people were concentrated.

Courante-3.JPG

https://www.zephyrn.com/bayzaudio/page/courante.html

The Bayz Audio's gCouranteh speaker was very interesting, with a very large and unique pipe shape.

Brumestar1.jpg

The Burmester amplifier, which was placed in a corridor in the hallway, was created a strong presence. (Is the power amplifier's top-plate flat for listening equipment?)

http://www.noahcorporation.com/burmester/

911Power1.jpg

As an audio sound, I was impressed that JBL EVEREST speaker, which was playing in Mark Levinson, was producing a stable good sound.
posted by toons at 16:24 | Audio

2019/09/22

SANSUI AU-907 Limited

a907LTD-front.jpg

SANSUI AU-907Limited

Released in 1994, the third-generation limited products of Sansui 907 integrated amplifier.

Equipped with a HYPER -X balance circuit, it featured a clean sound in the mid-high range of Toshiba's old MOS-FET (2SK405 / 2SJ115). This is still popular as the best sounding device for audio power MOSFETs along with Hitachi 2SJ49 / 2SK134.

Last night, after a long time, I tried to play an analog record to adjust my 2way speaker, but the amplifier was out of order and no sound was produced (CD direct input was sounded).

This morning, I opened the top plate to clean the inside, and after removing dust for the first time in several years, it started to sound normal.

# Inside of amplifier
a907LTD-inner.jpg

# 6mm thick aluminum bonnet with copper powder coating
a907LTD-topplate.jpg

# Left channel power block
a907LTD-left.jpg

# Right channel power block, preamplifier section (black box at right end)
a907LTD-right.jpg

# Power block (large power transformer with a total weight of 13kg)
a907LTD-trans.jpg

It was a total weight of 34kg, so I was concerned that it would hurt my waist when placed on the floor, but there was enough reward to clean it!
posted by toons at 03:15 | Audio

2019/09/20

MarkAudio OM-MF519 vs OM-MF5, Fostex speaker frequency characteristic comparison - extra edition

Extra edition report of my handcrafted speakers characteristic comparison.

I tried to measure the frequency characteristics of MarkAudio OM-MF519, last year's model OM-MF5, and some FOSTEX products under the same conditions as much as possible.

The amplifier volume position was the same, pink noise, and the measurement position was fixed at 1m on the axis, but the height of the unit was adjusted precisely, and there were a difference of several centimeters.

This was measured frequency characteristics with RTA app "Audio Frequency Analyzer".

OM-MF519 double bass reflex vs OM-MF5 bass reflex

# MarkAudio OM-MF519iDuct 15cm + 2.5cmj
OM-MF519.jpg

OM-MF519(3).PNG

# MarkAudio OM-MF5 (Tw is removed.)
OM-MF5.jpg

OM-MF5(3).PNG

As expected, the rich bass of OM-MF519 double bass reflex!
It seemed that the level around 30-50Hz is higher than OM-MF5 bass reflex.
For hearing, the left handed sound of the piano was resonated deeply. The loudness of the contrabass was also rich. And the mid-high range resolution is quite high, and the sound image localization was good due to the narrow baffle.

On the other hand, OM-MF5 bass reflex feeled like being pushed by energy. Since 60-100Hz was the same level as the mid range, the force of the base went up here. Full orchestra was a little difficult, but it was more suitable for Pops and Jazz.

The OM-MF519 double bass reflex was a 12mm front MDF, so I intended to reinforce it, but I found that the top plate was vibrating a little.

Since OM-MF5 bass reflex had 18mm side plate and bottom plate, it may be affected by the difference in rigidity.

Compared to the playback sound of two speakers, I prefered the normal bass reflection.

And measured other speakers under the same conditions!

Fostex FE83n
FE83n.jpg

FE83(1).PNG

Fostex FE83n mini bass reflex, installed in the enclosure in the Ontomo MOOK appendix.
The sound pressure level was the same, and the low frequency response was attenuated gently from 100Hz, making it easy to hear.

Fostex M800
M800.jpg

M800(2).PNG

2016's Fostex M800.
This box was a simple square shape, but the inside is actually elaborate. It had a hard reinforcement and air venting duct port, but was almost a sealed design.

The low frequency response feeled better than FE83, but was less conservative than OM-MF5. Since the sound had a peak in the high range of the unit, there were a feeling that it will be a little audible, such as a scratching sound. The low range was neutral and sound power can be put in as it fully.

Each units and boxes had very different characteristics and sounds. How far I can get to my favorite sound is a long way off.
posted by toons at 02:12 | Audio

2019/09/19

MarkAudio "OM-MF519" Speaker assembly Part 7 - frequency characteristics

OM-MF519_DB_1.jpg

After painting, I played a lot of music with this speaker for sound aging.

My personal impression is that the bass sound is very loud despite the 8cm full range. This is probably due to the bass enhancement effect of the double bass reflex.

# frequency characteristics (distance 1m, Pink noise, measurement by RTA app "Audio Frequency Analyzer"j
OM-MF519_DB_freq.png

However, looking at the frequency characteristics measurement above, it seems that the elasticity and tightness are retreating compared to a general bass reflex type, probably because 125-200Hz is slightly depressed. In a sense, 100Hz or less is too high.

Next, I adjusted the length of the second duct and examined how the frequency characteristics changed.

# Original duct (15cm)
duct-original.png

duct-original.jpg

I prepared a paper pipe with a slightly smaller diameter and measured it in units of 2cm.

The level of 100Hz decreased in the duct +2cm. It may be the effect that the duct area is slightly reduced.

# Duct +2cm
duct-2cm.png

duct-2cm.jpg

After that, the length of the duct was increased by 2cm, but no significant change was observed. It is also possible to pick up ambient noise.

# Duct +4cm
duct-4cm.png

duct-4cm.jpg

# Duct +6cm
duct-6cm.png

duct-6cm.jpg

# Duct +8cm
duct-8cm.png

duct-8cm.jpg

# Duct +10cm
duct-10cm.png

duct-10cm.jpg

# Duct +12cm
duct-12cm.png

duct-12cm.jpg

As a trial, I changed the length of the upper and lower ducts extremely. But, it did not be much different from closing the duct.

# Duct +2cm, +10cm
duct-10cm-2cm.png

duct-10cm-2cm.jpg

These results was not clear, so I have to try several times with different dates. I try listening with a duct length of + 4cm for a while.
posted by toons at 03:20 | Audio

2019/09/18

MarkAudio "OM-MF519" Speaker assembly Part 6 - external painting

DBh(1).jpg

As usual, the back and top plates were painted with black spray. To change the taste a little, I started to paint blue to the side plates, but the gas ran out during spraying on both sides.

DBh(2).jpg

I gave up and overlaid with black from above. After all, it doesn't look good in the surrounding black.

And then I found the remains of the cork sheet!

DBh(3).jpg

If the front baffle is cork-lined, I expect a retro feel. The baffle surface is where the woodworking bond is diluted and bonded, and the surface is coated with a thin aqueous varnish to increase strength.
The baffle edge was not able to be processed well and it floated a little around the middle.

It was made by cutting out round to fit the unit.
The sound absorbing material was inserted with the weight of the left and right channels matched.

DBh(4).jpg

DB`(front).jpg

Finally, the speaker was completed by tightening the duct screws!

DB`(rear).jpg
posted by toons at 00:17 | Audio

2019/09/17

MarkAudio "OM-MF519" Speaker assembly Part 5

MarkAudio OM-MF519 Double bass reflex speaker is almost completed.

OM-MF519-DB`(5).jpg

However, the entire box is still under consideration for external painting.

OM-MF519-DB`(6).jpg

For trial, I painted only the duct part of back side.

OM-MF519-DB`(7).jpg
posted by toons at 01:50 | Audio

2019/09/16

MarkAudio "OM-MF519" Speaker frequency characteristics, part 3

MarkAudio OM-MF519 Double bass reflex speaker is being adjusted.

The characteristic (1) was the result of adjusting the sound absorbing material to match my listening preference.

This was a measurement result at 1m above the unit axis (tilt about 15 degrees to the front of the baffle). The peak at 250Hz dropped, and the characteristics seemed to shift to 200Hz or below.

The characteristics of the second duct seemed to be concentrated around 60-100Hz. Then this area may not change unless the duct length is changed.

(1) Frequency characteristic of adjusting the sound absorbing material
(1)_1ch_z3_4(20190816)-2.PNG

Incidentally, the characteristic (2) was measured on the front axis of the unit. The peak of 12k-16kHz which is the high frequency characteristic of this unit was appeared.

(2) Frequency characteristic on the front axis of the unit
(1)_1ch_z3_4(20190816)-.PNG

The characteristic (3) was the result of distance from the baffle to 2m.
Although a peak was seen at 100Hz, I thought that it was possible to get around 50Hz as a low frequency characteristic.

(3) Frequency characteristic from the baffle to 2m
(1)_1ch_z3_4(20190816)-2m.PNG

As for my impression, I could hear the profound feeling of the bass and strings with the feeling of adding a subwoofer.

However, the feeling of tightness in the low frequency area will be reduced, so the rhythm may be lighter in jazz and pop music. I don't know if this is a characteristic of the box, or if the work accuracy or adjustment is insufficient.

Since the internal adjustment is over, I will consider finishing the box surface.
posted by toons at 02:54 | Audio

2019/09/15

MarkAudio "OM-MF519" Speaker frequency characteristics, part 2

(1)_1ch_z(20190814)-1.PNG

I tried putting the sound absorbing material into the first and second cabinets. It was shifted to the low side, 40/50Hz level up, dipped occurs at 160Hz.

And I reduced them for the first cabinet a little more. Adjustment was troublesome. Although it got more sense of volume in my hearing, the sound response was declining.

I will try to make a sound with this for a while.
posted by toons at 09:29 | Audio

2019/09/13

MarkAudio "OM-MF519" Speaker frequency characteristics, part 1

Attached the MarkAudio "OM-MF519" unit to the double bass reflex speaker box, so I made sound test.

I measured the frequency characteristics at the begining of playback and after aging for 1 hour. It was tested by the RTA app "Audio Frequency Analyzer" with pink noise.

# Begining of playback
(1)_1ch(20190814)-1.PNG

At the start of playback, there was a dip at 125Hz, but the level increased a little in about an hour.

# After aging for 1 hour
(1)_1ch_1ԃG[WO(20190814)-3.PNG

The original high frequency peak of 12-16kHz has disappeared. This is probably because the baffle is slanted and the measurement point is about 15 degrees off the unit front axis.

In addition, when blocking to tape one duct as a test, the 63/80Hz sound level was dropped. The second cabinet duct was seemed to have worked well.

# Blocking one duct
(1)_1ch__Ng1(20190814).PNG

It wasn't the final sound because there was no sound absorbing material yet in the box, but the sound resolution was very good. The bass range did not drop to the lowest sound, but it had enough low-pitched feeling unlike a simple bass reflex.
posted by toons at 00:12 | Audio