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Acurus A250 and starground

zg3 said:
I will also search for power capacitors replacement. Quite tricky if one wants to stay with original clamps as all today same diameter and capacity caps are longer (95mm original, 105 and 120mm today’s).

Kind Regards
Click to expand...
You should be able to squeeze a 105mm into the original clamps. With a diameter up to 66mm if I remember correctly. It will bailey fit, so it need some "love" and a bit patients when mounting with the bigger diameter caps, but eventually sit in place.;)

Regards
 
Halo,

Many thanks for the answer! Well I triple checked after soldering with very strong flashlight from few perspectives also lightning through the pcb you see the traces... but never mind that I did it only to check and I was sure what I was doing.

I know 105mm fits. There are few caps available. Nippon Chemicon are the cheapest, 3-4 others are from 1,5 to 4 times more expensive. I think any new cap will be better than old in case of these large cans. But what is your take as to the brand? For now I would stick to orginal configuration.

A quick questions. I have Black Gate N for input cap. What best do you recommend for non polar C7 220uF?

Second what bias is the proper and which sourse resistor to measure this is best?

I know the answers are somewhere in this thread and I will look for it but maybe someone can make quick answer.

Thanks!
 
zg3 said:
Halo,

Many thanks for the answer! Well I triple checked after soldering with very strong flashlight from few perspectives also lightning through the pcb you see the traces... but never mind that I did it only to check and I was sure what I was doing.

I know 105mm fits. There are few caps available. Nippon Chemicon are the cheapest, 3-4 others are from 1,5 to 4 times more expensive. I think any new cap will be better than old in case of these large cans. But what is your take as to the brand? For now I would stick to orginal configuration.

A quick questions. I have Black Gate N for input cap. What best do you recommend for non polar C7 220uF?

Second what bias is the proper and which sourse resistor to measure this is best?

I know the answers are somewhere in this thread and I will look for it but maybe someone can make quick answer.

Thanks!
Click to expand...
C7 220uf 35v:
https://www.mouser.dk/ProductDetail/Nichicon/UES1V221MHM?qs=sGAEpiMZZMtZ1n0r9vR22e0BBN1kF1oiOeFuT9YxkYI=

You can select a 50v Nichicon as the one above in the feedback, but it doesn't fit as good as the one in the link.

About the big e.caps: it's really a matter of taste. Personally I prefer Kemet at the PSU.

BIAS is measured across the emitters. 6mVdc is factory-standard, although many A250's only measure 2.4mVdc. The Toshiba powertransistors does perform quite well, even when underbiased.
If the amp doesn't get to hot, I would go for 8-10mVdc at emitters. It very much depends on the load from your speakers

Regards
 
KNielsen said:
@mondialfan is the GEN2 duallayer.. I se the emitters are placed different form the A250 circuit without the need of jumpers

Regards
Click to expand...

Yes Gen2 is a dual layer board and due to this a ground jumper is not needed between the diff pairs. The 10ohm emitter degen resistors are placed between the diff pairs on these later boards.
 
mondialfan said:
Yes Gen2 is a dual layer board and due to this a ground jumper is not needed between the diff pairs. The 10ohm emitter degen resistors are placed between the diff pairs on these later boards.
Click to expand...
C16/17: is this the post caps that are not attached. Also C4??? What is this:)
A200_L_Ch.jpg
 
zg3 said:
Halo,

While waiting for my friend with scope to have some time I just fixed my Acurus myself

The problem was the cold solder joints. Both C7 had cold joints and loosen. Funny it happend in both channels at the same time. After resoldering both channels work and play music. If you remove C7 the symptoms are no bass and drive power, simply mid and highs are present. Additionally one channel that was showing 120mV dc offset had loosen joint on C15 and after resoldering that dc offset in this channel shows 1-2mV, the other channel shows 9-10mV

I did resoldering these without taking the pcb off, constructed the special long curverd soldering end.

Now I will take the pcbs off and resolder all the joints. Also as first mods will change C7 and C2 input caps. Will put Black Gate N in the input, it is just about the best electrolityc cap you can get, also I have 10uF ones. I will search my drewers for 220uF same type, maybe I also have them.

I will also search for power capacitors replacement. Quite tricky if one wants to stay with original clamps as all today same diameter and capacity caps are longer (95mm original, 105 and 120mm today’s).

I also have to set the bias. I remember seeing somewhere in this thread the procedure, measuring dc on some source resistors but have to dig which ones (maybe quick advice here). I remember 6-8mV is the proper. I think mine shows 4mV and 5mV but not sure I’m measuring in the right place.

Kind Regards
Click to expand...

C15 being loose and causing elevated DC offset was due to oscillation. The high DC was just a secondary symptom. Lucky you caught this before it destroyed the amp channel or speaker that was connected to it.

You'll measure the bias across one of the large .5r emitter resistors on the output transistors, Pick one it doesn't matter which. 5mV is a good place to set it with the channel fully warmed up. If you have extremely efficient speakers and the amp sits out in the open where it has good air flow then you may be able to push it up to 10 - 12mV but 5mV is a better all around setting.
 
Just some thoughts.... Why is an RL11 so hard to find.......!! Been searching for half a year now.
I've come to one conclusion: I have to accept the 17" front model also if I want to find an item
 
Screenshot1.jpg Screenshot2.jpg
Images as promised. Only thing I moved this evening is the 1uf postfuse-caps just to see the layout in GEN2 spec, or how it could be :)

Edit: its still a prototype draw, and moving parts as I progress. I've got a couple of days off work the next few days and hope to be more dedicated if if time will allow this. Installing all parts before copper layout would be alot easier, so this is the first thing to get done.

Regards
 
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Looking good. What wattage rating are the degen resistors between the diff input transistors? They look overly large. 1/8w is all that is needed.
 
Many thanks for your help! From your expirence are the original mica insulators on transistors reusable? Or I should rather prepare the new ones before disassembling?

My speakers are Thiel CS6, I would rather avoid damaging them :)
 
The original transistor insulators are made from Kapton sheeting. They appear to hold up very well and I have cleaned and reapplied thermal compound to them and reused them in the past. I have also used individual mica insulators which also work very well.

If you do not have an 8 or 4 ohm dummy load then I would always recommend that you use a small cheap set of speakers to fully test your amplifier for a period of time before connecting your more expensive speakers.
 
Thanks! This is what I do :)
In some post earlier you mentioned you have some expirence with Pass amps. I have built a few of them in the past, also had some originals, I know Aleph 4, Aleph 3 and Aleph J. I still use copy of Aleph P preamp. How would you describe Acurus sound compared to Aleph? Since I own Thiel CS6 Alephs are no go, simply can’t drive them. This is why I got Acurus A250. Right now it’s stock with old capacitors. Sound very nice smooth, extended kind of dark. A lot of highs and bass, a bit recessed and not so detailed midrange. On some material it is softened and rounded too much. Piano does not sound very true, trumpet too little sharp etc.. So far I want to stay with Acurus as original as possible so I will not add capacity in PS etc, I do not want to loose resale value of it. I wonder how will it sound with fresh caps. I’m also interested in Aragon 8008BB so if I come across very nice one in top condition and resonable price I may be tempted. I also think it would be a nice project to build in similar way like KNielsen does with A250 here. I’m also temted to try Krell KSA250 but a nice example is beyond my budget at the moment. Thanks for the help!
 
- post deleted.. guess I had to many beers with my neighbor last evening. Headache today:)

@zg3 both the Krell and Aragon are fine units.

I don't think an A250 loses value due to new caps, if this is done properly. I am thinking of selling my own some day, we'll see how the market behave if I do so. But as a sidenote I also think that the market for a good treated Acurus is mush better in the US. In DK the prices for a Acurus in good condition is not near that high, mainly because many people do not now about this product.

Regards
 
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mondialfan said:
Looking good. What wattage rating are the degen resistors between the diff input transistors? They look overly large. 1/8w is all that is needed.
Click to expand...
I measured the original A250 1/4watt resistor-holes at the board and applied this to the new PCB. But I might alter some of the components. It all went a bit fast when I discovered that the ratlines were ok after some trail and error with some footprints. Also the 10ohm at input GND is oversized I see.

Regards
 
Couldn't resist when I got an offer on these two amps in mint condition. A100x3 and a A200 (both Gen2) for a total of 430$. It's COVID-19 days, so it all happened in a parking lot, sadly I've forgot my tools to pop the hood on the amps at scene.
When I got home I were sorry to see that the A200 had 2 probes sitting at the emitters on the right channel, and this channel has been tried fixed, and way to much heat applied on all solderings. Also the fuse is blown. I'll send a picture of the internals later.
Luckily the diff-section seams by view to be ok and not messed with.
The 100x3 is perfect both inside/outside.
IMG_20200410_155611.jpg IMG_20200410_160420.jpg
 
Regarding the parking lot before it all becomes to suspicious. They were moving from one place on DK to another, and this were just in between, and easy for both of us. The amps are ok and not stolen or anything.. he just "forgot!" to mention the burned channel:oops:
 
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KNielsen said:
Regarding the parking lot before it all becomes to suspicious. They were moving from one place on DK to another, and this were just in between, and easy for both of us. The amps are ok and not stolen or anything.. he just "forgot!" to mention the burned channel:oops:
Click to expand...

That sucks on the burned channel. Hopefully its not got shorted output transistors, then it gets expensive to repair.
 
zg3 said:
Thanks! This is what I do :)
In some post earlier you mentioned you have some expirence with Pass amps. I have built a few of them in the past, also had some originals, I know Aleph 4, Aleph 3 and Aleph J. I still use copy of Aleph P preamp. How would you describe Acurus sound compared to Aleph? Since I own Thiel CS6 Alephs are no go, simply can’t drive them. This is why I got Acurus A250. Right now it’s stock with old capacitors. Sound very nice smooth, extended kind of dark. A lot of highs and bass, a bit recessed and not so detailed midrange. On some material it is softened and rounded too much. Piano does not sound very true, trumpet too little sharp etc.. So far I want to stay with Acurus as original as possible so I will not add capacity in PS etc, I do not want to loose resale value of it. I wonder how will it sound with fresh caps. I’m also interested in Aragon 8008BB so if I come across very nice one in top condition and resonable price I may be tempted. I also think it would be a nice project to build in similar way like KNielsen does with A250 here. I’m also temted to try Krell KSA250 but a nice example is beyond my budget at the moment. Thanks for the help!
Click to expand...


The Aleph is a more laid back presentation, but it lacks the bass punch of the A250. However coupled with the right speaker the Aleph is hard to beat. The problem with the Aleph is that its so picky on the speakers due to the fact it likes an easier 8 ohm load. The Acurus is much more forgiving when it comes to which speaker load you pair it with.
 
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