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Acurus A250 and starground

@mondialfan
After reading Rod Elliott's page about servos I'm unsure, and this made me skip the DC-servo in favour of a normal feedback.
What's your opinion?
I've never listened to a 8008, so I don't know what to expect, besides from a 0.0mV offset when my servo did work! :)
My feedback now is more like the 4004, so lower f3 compared to A250

Regards
 
I've seen AD711 used in some mainstream amp servos in a nearly identical servo circuit. 1M resistors in those locations and 33k on the Pin 6 output to feedback.
 
You can always put a manually adjusted DC offset pot in similar to the 4004. That's very simple. The nice thing is that the diff transistors don't have to be as well matched when you can adjust out the offset.
 
mondialfan said:
You can always put a manually adjusted DC offset pot in similar to the 4004. That's very simple. The nice thing is that the diff transistors don't have to be as well matched when you can adjust out the offset.
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Matching isn't a problem. I'll try one more time with the servo later this week with new values. The amp is gonna be with speaker protection so all good, and a softstart with Schmitt trigger for a ultrafast relay action (my own drawings).
 
Matching isn't an issue for small scale work, but if you start building a lot more you might wish to use more loosely matched pairs so you don't have to measure and match as many.

Also, Matching can also be an issue if you're using more obscure parts, like say if you want to experiment with a set of older Toshiba JFETs as your input diff and you have a limited selection. Then it's nice to be able to adjust out maybe 30 - 40mV of DC offset.
 
Fsatsil said:
Just wondering what do you guys use when replacing the low ohm emitter resistors in amps you rebuild? I’ve really never had a reason to replace them until now. Do you like the 2-3w metal oxide type resistors Acurus used or do you prefer the low inductance plate resistors that many Japanese amps use
https://www.mouser.com/new/koa-speer/koa-speer-bpr-resistors/
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Wirewound is what I'm looking for, normally in 3watt. The non-magnetic ect ect has become quite expensive, an more theoretic then I've been able to measure.
BTW Panasonic ERG was my choice by far for most 3watt replacements, but obsolete by now. I'm still looking for an affordable good quality power-resistor that'll have the values I use and need in future, but not easy... Through hole is a dying breed... Sadly

That said, Mills MRA05 wirewound is creazy good 5watt that'll fit Acurus perfectly. But expensive. I use them in my own

EDIT: normally it's not needed to change the emitters or zobel-res in A250, but more of a feel-good, and I fully understand

Best regards
 
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KNielsen said:
Wirewound is what I'm looking for, normally in 3watt. The non-magnetic ect ect has become quite expensive, an more theoretic then I've been able to measure.
BTW Panasonic ERG was my choice by far for most 3watt replacements, but obsolete by now. I'm still looking for an affordable good quality power-resistor that'll have the values I use and need in future, but not easy... Through hole is a dying breed... Sadly

That said, Mills MRA05 wirewound is creazy good 5watt that'll fit Acurus perfectly. But expensive. I use them in my own

EDIT: normally it's not needed to change the emitters or zobel-res in A250, but more of a feel-good, and I fully understand

Best regards
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Thanks KNielsen you pretty much confirmed what I suspected. I have a full set of matched output fets for the amp so I’d like to closely match the resistors as well. I have a bunch of these .5ohm 2w 2% wirewound resistors that I was planning to use https://www.mouser.com/datasheet/2/414/AS-1528308.pdf. My other option was these metal oxide types from Koa https://www.mouser.com/ProductDetail/660-MOSX3CT631RR33G that I’ve used before in place of the old Panasonic Erg.
 
Hello , I have solved my issue . Thank you for your advice. It was comming from a ground loop from my defective microwave in my kitchen . I replace It ,and the buzz sound has gone.
If you have some times to reply . Is It possible to bridge output channels in order to have more power for my B&W Nautilus ?

mondialfan said:
The 125X5 uses 5 discrete output boards, one for each channel and a single shared power supply. So if the noise is in all 5 channels then your issues are not related to the channel boards due to the fact all 5 of them haven't failed at the same time. The power supply in this amp is a basic x-frmr/bridge rectifier/pair of e-caps with no regulated stages. This also does not sound like a power supply problem that I have ever experienced. I would look more closely at the preamp/processor or source components as a possible source of the issues.
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Jeandelyon said:
If you have some times to reply . Is It possible to bridge output channels in order to have more power for my B&W Nautilus ?
Click to expand...

I'm glad that you found the source of your noise.
No, its not possible to bridge the channels in the 125X5. This amp only uses 4 output devices per channel and doesn't have enough output current capacity to safely be drive speaker loads in BTL mode. You should consider replacing this amplifier if more power is needed. If you want to keep using an Acurus amp then look at the A200 model. The A200 will give you 250+wpc into 8ohms.
 
mondialfan said:
Je suis content que vous ayez trouvé la source de votre bruit.
Non, il n'est pas possible de ponter les canaux dans le 125X5. Cet ampli n'utilise que 4 périphériques de sortie par canal et n'a pas une capacité de courant de sortie suffisante pour piloter en toute sécurité les charges des haut-parleurs en mode BTL. Vous devriez envisager de remplacer cet amplificateur si vous avez besoin de plus de puissance. Si vous souhaitez continuer à utiliser un ampli Acurus, regardez le modèle A200. L'A200 vous donnera 250 + wpc en 8 ohms.
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Merci beaucoup pour votre réponse rapide , I will Try to find one
 
Finished these two gems. This evening was only small minor adjustments, but always leaving some of the chassis unmounted if bigger changes occur and I'll have to take apart again, but wasn't necessary after a final listening session this evening, so on with the lid and assembly....
Now ready for pickup by owner, who's invited for coffee and some music in my livingroom this weekend. One of the things I like the most after a build :)
IMG20230131223004_copy_1005x756.jpg
Best regards
 
They look beautiful, and so I'm assuming they sound as good as they look. Would you say that the TTA006B/TTC011B pre-drivers sound significantly better than the original transistors? Worth making the change, even if the originals are in good condition?

Thanks! - Six.
 
Six said:
They look beautiful, and so I'm assuming they sound as good as they look. Would you say that the TTA006B/TTC011B pre-drivers sound significantly better than the original transistors? Worth making the change, even if the originals are in good condition?

Thanks! - Six.
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I think they improve the soundstage. It's hard to describe with my relative simple English, but better separation between instruments would be my best words here
 
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