Acurus A250 and starground

[KNielsen]

Sometimes things aren't just that easy
Made the first circuit of an A250 today that I bought a week ago, all good before soldering, measuring like it should and even better, but wanted to make a full restore with decoupling caps and the new drivers.
So started building, went ahead for a pre-test before installing the board in the chassic again: And absolutely nothing happened..! 16Vdc and my workbench powersupply shuts off. I always run low amps at test, as 0.3A should be more then enough to check BIAS ect at a maximum of 64Vdc that my powersupply can handle, with no signal applied that is of course according the 0.3A.

Checked all mods, diodes and resistors (only checked the new driver-transistors), resoldered every joint on the outputstage including those I haven't messed with, checked for shortcuts (several times) and 6 hours later I'm still getting a cutoff a 16Vdc at the rail where the ampers starts claiming like h.........!

Although the amp is in perfect condition both inside and outside, the fuseholders were all rusty inside the clips and all of the 4 fuseclips (for the 2 fuses) broke at one board when I took the amp apart, with the fuses dangling in the chassic afterwards. This leaves me thinking that maby there is a transistor that might have bad legs from poor conditions like a garage ect, although great care is taken not to bend the old transistors when I work on these.... I simply cannot see anything else. They look ok from a visual point of view, and fully working before take apart.. So..........???
-It's late, so no more time for today but...... I really wish that I found the error before the end of the day though

Regards

So far: haven't found anything unusual since my last post, check all transistors and they are OK. But BIAS is 90mV(!) on both NPN and PNP-powertanies at 16VDC. The BIAS-pot is set "neutral" to 0.5kohm... I cannot get pass 16VDC no matter how high I set the amps on the supply, it just run away.... Tried 0.8amp so far, still clipping hard...
If I lower the rail VDC a bit to around 15VDC so the powersupply doesn't cutoff, I can measure increased ampers in the circuit if I touch either of the new driver-transistors or Q1/Q8....

EDIT: the Hfe is measured to around 180 on the drivers. The last amps I did was around 160Hfe
Bypassed all mods so I knew they didn't do harm, except for the 300ohm still in place in this post Vs the stock 330ohm and the new drivers.

Best regards

 

[KNielsen]

12 workhours later on this, and it came to life!
An error so obvious that I turned off all test equipment and left the room for today!!
Even though I disconnected the decoupling caps when the error was first discovered, there were a bridge in the remaining wire on the backside, touching zobel at the resistor side, the hole time since the beginning.......... it wasn't visible by eye, and discovered it as a final approach when I moved the wire while power was applied, and the amp/powersupply returned to 0A consumption.



EDIT: have to say I'm very pleased with my investment in equipment, I don't think a dim-bulp would have saved this one

 

[KNielsen]

Have to reply on this. The schematic floating says R20 is 10K.. This isn't correct. 1K is the one to use.
Couldn't understand why my BIAS was all over after changing all resistors to Dale's according to schematic................

 

[zg3]

Halo, have not been following this thread for quite some time. I’m still collecting the parts for rebuild of my A250. So what is your final preference still A200 over A250? For polypropylens do you prefer Wima or Panasonic? I have bought Wima but MKP10 insted of MKP4 and all 2% (if it matters anyway). Thanks!

 

[KNielsen]

Halo, have not been following this thread for quite some time. I’m still collecting the parts for rebuild of my A250. So what is your final preference still A200 over A250? For polypropylens do you prefer Wima or Panasonic? I have bought Wima but MKP10 insted of MKP4 and all 2% (if it matters anyway). Thanks!

When I restore Acurus, I go with Wima MKP all the way, except for the feedback. Here I use Wima MKS.
I do these rebuilds according to the A200 gen2, using both the 4 and 10th series. If it makes a difference I haven't experimented yet

Best regards

 

[zg3]

Thanks! Do you apply A200 gen2 to the A250 pcbs also? Can you point to the schematics? I have a good A250 schematics. MKP10 are significantly thicker than MKP4 (some say it translates to better sound when it comes to polypropylene caps, also going for the higher voltage does the same).

 

[KNielsen]

Thanks! Do you apply A200 gen2 to the A250 pcbs also? Can you point to the schematics? I have a good A250 schematics. MKP10 are significantly thicker than MKP4 (some say it translates to better sound when it comes to polypropylene caps, also going for the higher voltage does the same).

According to A250 I use:
-all 0.1uf/250v Wima MKP4
-0.22uf/250v Wima MKP10
-0.47uf/250v Wima MKS4

The rest of the A200 is not to apply on A250, this will be the 10ohm resistors.

But stick to the above, pared with Nichicon UES bipolar at input and feedback, and some good Kemet's e.caps at the PSU, and you'll have a great sounding amp.

Also, If the ground from the input goes directly to speaker minus, I highly recommend that you disconnect this and reroute input ground seperat to the common ground at PSU. This enhance the hights and better stereoimage. This "mod" were also performed as standard in the later gen2 A200! -but some late A250's were build with this feature before the A200 arrived, so not necessarily something that you need to do

Best regards

 

[KNielsen]

Halo, have not been following this thread for quite some time. I’m still collecting the parts for rebuild of my A250. So what is your final preference still A200 over A250? For polypropylens do you prefer Wima or Panasonic? I have bought Wima but MKP10 insted of MKP4 and all 2% (if it matters anyway). Thanks!

As a final answer my newly build A250 playes way better then the a modded gen2 A200. But this includes new driver-transistors and mods to some resistors beyond what is mentioned in this thread! FYI this build has replaced my own monster of an A250 shown in the beginning of this thread!
Railvoltage was lowered from 92VDC to 84.9VCD to keep the drivers at a reasonable temperature.
The post-fuse-caps (decoupling) is 47uf/1uf in this build, 2.7uf as bypass at the PSU.

 

[KNielsen]

My first tryout with epoxycoat CW2500 to repair small damages to the PCB...
-it's not that easy to apply even with a toothpick, but the A250 I'm work on right now really sufferes from "almost, to naked" copper traces arround the ZTX-drivers. The copper traces were not slipping the surface though and the copper in good condition, so no retracing was needed....
The CW2500 gives the feel of quality that I like to present, and recommend here.
The rails measured 94.30VDC(!!!!!) when the amp arrived, so not much headroom for the 95VDC E.caps, with the PCB taking alot of heat over the years from the ZTX, that made significant brown marks on this unit

VDC is lowered via the internal jumpers to a more reasonable state

 

[Maxnak4]

Whats up everyone. I've got an A200 and an A250 that I just adjusted the bias on. Both were reading 2mv which is pretty common from what I've heard. I set them to the recommended 7mv but after a couple hours decided the heatsinks could take far more abuse. I eventually settled on 20mv for both amps, but even still the heatsinks only get about as warm as you would expect for a class ab amp. Am I crazy or did Mondial nerf the crap out of the bias on these so they wouldn't encroach on their Aragon line?

Side note: the right channel on my A250 was putting out 9vdc (you read that right, not 9mv) at idle. After a lot of testing I gave up on the channel and removed it, now I use the amp as a monoblock. If anyone has ideas for fixing it I'm all ears.

 

[zg3]

It strongly depends on the speakers. I have Thiels CS6 which are heavy load. I do not remember exactly what is the bias set but I recon it is quite low, 3,5mV or something like that. Even though in hot days when i play music loud heatsinks get crazy hot.

By the way Acurus plays nice with CS6 but it does not drive them well. They open up and get dynamic at minimum 400 watt per channel into 8 ohm and double that at 4 ohm.

 

[Maxnak4]

Ah ok, I drive an LS50 (center channel) with the a250 and two boston acoustic towers that are quite efficient with the a200. I've never so much as felt the amps get warm with the 2mv setting, so I'll keep an eye (or rather hand) on them at 20mv to make sure they're not getting hot.

Edit: misread what you said about driving those CS6s

 

[zg3]

Not Acurus, just another brands I tried, typically top of the line powerfull (and expensive, even today) Krell, Levinson, Classe, Threshold etc... Thiel CS6 are flagship 80kg per speaker monsters. Need real ampers and still are one of the few best speakers ever made (not my opinion). That Acurus plays really nice with them is complement to Acurus, they are not forgiving for poor quality electronics. I’m just saying Acurus does not drive them really well. But it sounds very nice!

 

[Maxnak4]

Gotcha, I bet a set or Aragon Palladiums would match very nicely with those

 

[KNielsen]

Whats up everyone. I've got an A200 and an A250 that I just adjusted the bias on. Both were reading 2mv which is pretty common from what I've heard. I set them to the recommended 7mv but after a couple hours decided the heatsinks could take far more abuse. I eventually settled on 20mv for both amps, but even still the heatsinks only get about as warm as you would expect for a class ab amp. Am I crazy or did Mondial nerf the crap out of the bias on these so they wouldn't encroach on their Aragon line?

Side note: the right channel on my A250 was putting out 9vdc (you read that right, not 9mv) at idle. After a lot of testing I gave up on the channel and removed it, now I use the amp as a monoblock. If anyone has ideas for fixing it I'm all ears.

6mV is the factory recommend.
I strongly advise to lower the BIAS to a max of 13mV... 8-10mV should be the best in between.
There is a possibility that the BIAS hasn't been setup correctly since you end up with 20mV without heatproblems. Do you let the amp heat up and then settle in temp before adjusting the BIAS?

Btw the Toshiba transistors funktion very well even low BIAS'ed, so don't rub your head to much arround this aspect. I'll suspect most people won't hear the difference between 8mV and 15mV

Best regards

 

[Maxnak4]

6mV is the factory recommend.
I strongly advise to lower the BIAS to a max of 13mV... 8-10mV should be the best in between.
There is a possibility that the BIAS hasn't been setup correctly since you end up with 20mV without heatproblems. Do you let the amp heat up and then settle in temp before adjusting the BIAS?

Btw the Toshiba transistors funktion very well even low BIAS'ed, so don't rub your head to much arround this aspect. I'll suspect most people won't hear the difference between 8mV and 15mV

Best regards

The heatsinks do get slightly hot but not enough to cause pain regardless of how long I hold them. Even after leaving them on overnight they don't get any hotter than they do after about an hour of warm up. I assumed this was alright because the heatsinks on my 1st gen Palladiums get hot enough to burn if I leave my hands on them long enough. I wouldn't expect them to ever get hotter than they do at idle since class B operation seems to be very efficient on these amps. That said I'm fairly new to this so you're probably a good bit more knowledgeable than I am when it comes to this stuff. You don't think Mondial set such low bias settings to account for extreme environments and/or to prevent the amps from sounding as good as their Aragon equivalents?

 

[KNielsen]

The heatsinks do get slightly hot but not enough to cause pain regardless of how long I hold them. Even after leaving them on overnight they don't get any hotter than they do after about an hour of warm up. I assumed this was alright because the heatsinks on my 1st gen Palladiums get hot enough to burn if I leave my hands on them long enough. I wouldn't expect them to ever get hotter than they do at idle since class B operation seems to be very efficient on these amps. That said I'm fairly new to this so you're probably a good bit more knowledgeable than I am when it comes to this stuff. You don't think Mondial set such low bias settings to account for extreme environments and/or to prevent the amps from sounding as good as their Aragon equivalents?

I think the recommended BIAS is a good starting point. Maby a bit higher, but the heatsink quickly become undersized in the A250 if the BIAS is raised above 10mV and the amplifier is driven hard or low ohm speakers is connected.

My own A250 is roughly 10mV if I remember correctly, with a 6ohm load speaker.
When playing loud bass heavy music, the amp reach temps at arround 60-65°C on the heatsinks! And probably to much in my opinion.

Railvoltage in my amp is above 90VDC and will also contribute to more heat, over the standard ~85VDC as shown in the schematic.

Lowest voltage I've seen in A250's is 84.9VDC and highest is close to 94VDC! Mostly the variety it's due to a voltage selector that hasn't been setup correctly, if we follow the guidelines in the schematic.

 

[KNielsen]

As a sidenote to the above written:
I do tend to think that an A250 with correct railvoltage at around 85VDC sounds more musical over the higher voltage setting.

I have no measurements supporting this aspect

 

[Maxnak4]

I think the recommended BIAS is a good starting point. Maby a bit higher, but the heatsink quickly become undersized in the A250 if the BIAS is raised above 10mV and the amplifier is driven hard or low ohm speakers is connected.

My own A250 is roughly 10mV if I remember correctly, with a 6ohm load speaker.
When playing loud bass heavy music, the amp reach temps at arround 60-65°C on the heatsinks! And probably to much in my opinion.

Railvoltage in my amp is above 90VDC and will also contribute to more heat, over the standard ~85VDC as shown in the schematic.

Lowest voltage I've seen in A250's is 84.9VDC and highest is close to 94VDC! Mostly the variety it's due to a voltage selector that hasn't been setup correctly, if we follow the guidelines in the schematic.

I could be mistaken but I was under the impression that the amps provided the most class a power at 8 ohms. Wouldn't high bias only alter the idle temps? If the bias were set to say 40wpc I would expect the heatsinks to dissipate 40wpc at all times until the amp switches over to class b mode. Class b mode operation should otherwise be identical either high or low bias, other than the transition point from class a to class b. Or is it not that simple?

In regards to rail voltage I haven't actually checked mine but I should. The specified rail voltage of my Aragons (which are far superior to my Acurus amps) is only 70V, so I don't suspect higher rail voltage would translate to better sound.

 

[KNielsen]

I could be mistaken but I was under the impression that the amps provided the most class a power at 8 ohms. Wouldn't high bias only alter the idle temps? If the bias were set to say 40wpc I would expect the heatsinks to dissipate 40wpc at all times until the amp switches over to class b mode. Class b mode operation should otherwise be identical either high or low bias, other than the transition point from class a to class b. Or is it not that simple?

In regards to rail voltage I haven't actually checked mine but I should. The specified rail voltage of my Aragons (which are far superior to my Acurus amps) is only 70V, so I don't suspect higher rail voltage would translate to better sound.

High correctly adjusted BIAS will give an amplifier that produce more heat, also at higher levels where equally high BIAS follow.
Regarding the voltage, I often chose the higher voltage for more wattage in "the old days". Nevertheless some amps do sound better with lower voltage.
I do not put Acurus next to Aragon as a comparison, since the Aragon has more benefits even when a A250 is fully modded. An A250 can come really close, but often the bass will lack the punch that is suspected from an dual-mono like Aragon. The best straight up match regarding the lower voltage would be the A200 (gen2), witch by far is more musical, but also better designed over the A250

Best regards