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Acurus A250 and starground

Updated the driver-transistors in the right channel today for a quick test. A topic that came up and mentioned by @mondialfan earlier, but this one is an A200 instead of A250.

-I simply cannot find my small ø3mm screws, so no heatsink attached yet.

Anyway :), the 3Vpp squarewave came out nice.
The output of the amp didn't climb, but only measured one channel, so maby a different result with two channels under load.
The temperature on the transistor was low enough for a listening-test, and it worked really well.
Thanks @mondialfan for the info.

The transistors does need heatsink when playing music. Compared to the ZTX, the TT-series does become quite hot (almost identical temp to ZTX), but with the TT speced at a working temp up to 150°c Vs 200°c on the ZTX I feel that a heatsink would be preferred in the long term.
IMG_20201227_143110.jpg IMG_20201227_145813.jpg
 
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Andersonix said:
Happy New Year, KNielsen and MondialFan especially!

Thanks for sharing all this experience on these amps! It's taken me weeks to finish reading this thread....

About 10 years ago, I put in a big PS (bigger VA, lower voltage transformer and lots of caps) and made a fat heat sink for my Acurus 100X3:
https://www.diyaudio.com/forums/sol...grading-acurus-dia-100-amp-2.html#post1848545

Cheers!
Click to expand...
And you too @Andersonix .... I've read your thread several times before I began on my own yars later...

The heatsink you used: were it custom-order or made by your self?

I've been looking for bigger heatsinks that fit the A250 chassic and has to be customized to fit, but the pricetag on these...!!!:confused::)

Regards
 
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KNielsen said:
Updated the driver-transistors in the right channel today for a quick test. A topic that came up and mentioned by @mondialfan earlier, but this one is an A200 instead of A250.

-I simply cannot find my small ø3mm screws, so no heatsink attached yet.

Anyway :), the 3Vpp squarewave came out nice.
The output of the amp didn't climb, but only measured one channel, so maby a different result with two channels under load.
The temperature on the transistor was low enough for a listening-test, and it worked really well.
Thanks @mondialfan for the info.

The transistors does need heatsink when playing music. Compared to the ZTX, the TT-series does become quite hot (almost identical temp to ZTX), but with the TT speced at a working temp up to 150°c Vs 200°c on the ZTX I feel that a heatsink would be preferred in the long term.
View attachment 2078722 View attachment 2078723
Click to expand...
Didn't see Mondialfans altered schematic at the driver-resistors in parallel witch changes from 82Ω to 41Ω, so corrected this. Maby this will change the output and better squarewave...?? To be continued when finished:)

Btw this amp has been serviced before, look at the brown area around the powertransistors at the top PCB. 4 new powertransistors and some other parts. Alot of heat, even visible at the heatsink
IMG_20210112_225105.jpg
 
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You should have heatsinks on those transistors before you go from 82 to 41ohms and double the current. Failure to heatsink them will result in short transistor life.
 
mondialfan said:
You should have heatsinks on those transistors before you go from 82 to 41ohms and double the current. Failure to heatsink them will result in short transistor life.
Click to expand...
Yes, I'm not gonna power up before this is fixed... Covid is really slowing things here, been waiting for my second packed with Elna/Wima ect and screws for the heatsink for over a week now. Normally 28-48hours from ordering to delivery
 
A200 so far..... The Kemet 36.000uf Ecaps does fit, but the groungbar needs to be reversed and installed on top, as the diodebride is closer to the Ecaps then normal distance in the A250, and doesn't make room for ground. This requires new hookup cable from the PCB's to the PSU also as polarity is reversed in the chassic.. Anyway, here's some pics from today's work, the circuit's aren't done yet, but gives an idea of the progress including a pic of the chassic with the new capsIMG_20210114_180113.jpg IMG_20210114_180047.jpg received_859642414812249.jpg
 
Finishing this A200 today, hopefully do some measurements and maby some music..... Need to fix the wiring to the LED in the front panel.

Had some trouble with one channel, giving no BIAS at all or voltage throughout the PCB, putting out a maximum of 120mVDC-offset at 64VDC at the rails from my powersupply, showing no current flow at the time.... Turned out there was a short between R12/R13, an almost invisible solder "splash". Took me 2 hours before I spotted this, when R12/R13 measured like they were in parallel(!!), it had my attention. Cleaned the PCB one more time and everything worked like a charm.
Anyway I'm really excited about this one and what to expect:)
IMG_20210116_232956.jpg
A250_KC_rev.jpg
IMG_20210117_120726.jpg
 
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The output didn't change after the resistor was altered form 82 to 41Ω at the drivers in the A200. But the squarewave looked much nicer with very sharp edges, no ringing what so ever at all!:) (forgot to take a picture, but it looked really good)

Both channels driven at 8Ωload/1khz is 245watt
Single is 293watts
 
KNielsen said:
The output didn't change after the resistor was altered form 82 to 41Ω at the drivers in the A200. But the squarewave looked much nicer with very sharp edges, no ringing what so ever at all!:) (forgot to take a picture, but it looked really good)

Both channels driven at 8Ωload/1khz is 245watt
Single is 293watts
Click to expand...

The output will not change due to the resistor change but a bias adjustment should be needed.
Sounds like your modded A200 is working well.
 
mondialfan said:
The output will not change due to the resistor change but a bias adjustment should be needed.
Sounds like your modded A200 is working well.
Click to expand...
In "testmode" the BIAS is set at 6.5mV for now. The squarewave is by far one of the best I've seen from an Acurus. The BIAS is raised in a couple of days when I'm sure everything is in good order, for now I'm mostly monitoring the temp at the drivers, so....

I've had a quick listeningsession this evening, and the first thing is the upper midrange and treble. It's so reviling, yet silky soft. It's a matter of taste, but it really sounds good, and in my living room it provide a good stereo image in all frequencys.

Regards
 
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KNielsen said:
In "testmode" the BIAS is set at 6.5mV for now. The squarewave is by far one of the best I've seen from an Acurus. The BIAS is raised in a couple of days when I'm sure everything is in good order, for now I'm mostly monitoring the temp at the drivers, so....
Click to expand...
Couldn't wait, so upped the BIAS to 8mV..... The level of details remain intact, but it definitely do good to this configuration in the A200, and a further improvement in the topend.
I have 3 amps right now, and this is one of them, build only to be sold in a couple of weeks. If I knew where it would be placed in the future (surroundings/air) I wouldn't hesitate to bump up the BIAS a bit more!?:)
 
If you're selling this amp then I'd set the bias for the stock value or maybe 2mV above stock. Going any higher risks over heat in a future situation after the amp changes owners 2 - 3 more times in the future. Mondial set it conservatively for this reason I'm certain.
 
mondialfan said:
If you're selling this amp then I'd set the bias for the stock value or maybe 2mV above stock. Going any higher risks over heat in a future situation after the amp changes owners 2 - 3 more times in the future. Mondial set it conservatively for this reason I'm certain.
Click to expand...
Sadly the heatsink doesn't allow to much fun.... Many of the amps I receive measures around 2.4mV at the emitters! The Toshiba's can be run at quite low BIAS, but is this a factory BIAS setup??
I know they recommend 6mV, but many A250 measures much lower..
I have come to the conclusion that Mondial knew that heat was a problem?

Regards
 
KNielsen said:
Sadly the heatsink doesn't allow to much fun.... Many of the amps I receive measures around 2.4mV at the emitters! The Toshiba's can be run at quite low BIAS, but is this a factory BIAS setup??
I know they recommend 6mV, but many A250 measures much lower..
I have come to the conclusion that Mondial knew that heat was a problem?

Regards
Click to expand...

I think that Mondial also set them a bit on the low side to hinder sound quality in order to keep them from competing with the Aragon's. At 2 - 3mV I've never seen an Acurus very much above room temp even when playing difficult speakers at high levels. At 6mV they will start to heat up a little under the same circumstances. At < 9mV the A200 and lower models should be fine in most circumstances but the A250 with its somewhat higher rails (>90V) will begin to get quite warm into difficult loads.

By difficult load I refer to my Acoustats which are 4ohm and take a good bit of power to get a reasonable volume. An A250 set at 10mV bias will get the heatsinks uncomfortably hot to the touch after about 30 - 40 min of driving my Acoustats at a loud volume. I've never had the temp switch open and have the amp go silent due to the high temps, but the level of heat cannot be good for longevity. I typically set the bias at 6 - 7mV for the amplifiers that I work on for other people and call it good.

There is not a huge difference between distortion at 6mV and distortion at 10mV. However the difference between 2 - 3mV bias and 6 - 7 mV bias is respectable at higher frequencies. See chart here. I think I posted the link to that in this thread before. That test was performed at 10W of output into 8ohms. The top two lines are 1 & 2mv of bias respectively. The red line is 5mV.
 
Been listening to the newly built A200 for some days now, and the treble startede to ignorere me. An awful lot of details, but in many occasions just
to straight forward.
-This post is not something special, but over and over again the small Panasonic's ECWF(a)-series filmcaps seams to surprise me, when used as bypasses... And tonight I tried this trick one more time: 1uf/250v across each E.cap and the amp settles in a way that is worth sharing!
It just a pleasure to listen to!!

Best Regards :)
 
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