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Acurus A250 and starground

Fsatsil said:
Nice job on that amp KNeilsen it looks very clean. Which power supply capacitors did you use?
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If you mean the picture: Kemet 36.000uf/100vdc :)
Mondialfan brought them to my attention, it's a tight fit, but they will squeeze in with a little help ;)
 
yes I was curious about the caps used in the last picture. Are they the Kemet ALS series? I would absolutely love to build a beast of a power supply like you did for your personal amp but unfortunately that’s way out of my budget for this amp. To many other projects!
 
mondialfan said:
If they are going across the main storage caps to be used as bypass caps then it would be fine to use them, but don't even think about using them for input coupling caps. In that position the large bodied caps can be problematic as Knielsen suggests.

If you didn't install post fuse bypass caps then adding them across the main storage caps is the next best thing. I typically use something physically smaller than the caps you're describing. I like to use 1 - 2 uF polypropylene films like the radial panasonic ECW series.
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I forgot to add the post fuse cap bypass when I had the amp apart but I may do it in the future. I listened with and without those big 8uf film power supply bypass caps the amp came with and could hear no difference so I’m leaving them out. My power supply rails are right around -/+80v at idle with 119VAC at the receptacle is this about right for the 200X3? Thanks
 
Yes, ALS70-series. Be aware of the hight: 105mm
They are 66mm in diameter, and will barely squeeze into the original mounting clip on an A250.
The caps are original 23.000uf, 63.5mm(d) if I remember correctly. Guess it's the same ecaps used in yours.
 
The first model
Fsatsil said:
I forgot to add the post fuse cap bypass when I had the amp apart but I may do it in the future. I listened with and without those big 8uf film power supply bypass caps the amp came with and could hear no difference so I’m leaving them out. My power supply rails are right around -/+80v at idle with 119VAC at the receptacle is this about right for the 200X3? Thanks
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The first model of A200x3 was based on A250. That means if you only drive to channels you actually have the power of an A250. The rails should be 85vdc according to the schematic. I don't know if the extra circuit will course a Vdrop, I haven't measured an X-model yet.. Maby Mondialfan knows??

Regards
 
I believe the 200X3 used a bit taller chassis then the 2channel amps. I just measured and my amp is 6 inches tall
 
Right now I’m trying to figure out how I can use the third channels unused heatsink to help cool the other two channels. Maybe a heat pipe like they use in computer cpu cooling?
 
Fsatsil said:
Right now I’m trying to figure out how I can use the third channels unused heatsink to help cool the other two channels. Maybe a heat pipe like they use in computer cpu cooling?
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You are on your own here... But how much BIAS do you want to pull from the circuit?
The Toshiba power transistors actually perform pretty good even at very low BIAS.
I've been as high as 15mV on the emitter, but found 10-11mV to be more then enough for a good sounding amplifier.
If I remember correctly, Mondial stated 6mV for recommended BIAS, but many I have seen are set even lower ~2.4-3.0mV!!
8mV seams to be sweet spot between "generated heat Vs soundquality" in my opinion for most users.
 
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One thing: the restored I've showed on page#10 has a constant hum from the torodial, also before restore.. Not loud but, if it were my own I wouldn't be satisfied.
First time I've seen this on a Acurus, normally it comes an goes on the amplifiers I've seen.
Is it a bad transformer? All grounds seams in perfect condition, it's a GEN2.
I've tried connecting it with and without GND connected to AC, didn't matter...

EDIT: I could try select 240Vac instead of 230Vac on the AC input, see how it behaves. The rail voltage will be higher though if I do so. At 230Vac I measure 84.9Vdc at the rails.

Regards
 
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Fsatsil said:
Right now I’m trying to figure out how I can use the third channels unused heatsink to help cool the other two channels. Maybe a heat pipe like they use in computer cpu cooling?
Click to expand...

If the reason for using the third channel is to turn the bias up even further then the benefits above 10 - 15mV are minimal. See this thread that I posted with distortion numbers for different bias levels for an Acurus A150.

If you're running low impedance inefficient speakers at high levels and the heatsinks are heating up a lot then a better option may be some low speed fans. As long as the heatsinks are cool enough that you can hold your hand on them for 5 secs or more under full load you're most likely fine. The channels have a thermal breaker on them to protect in the event of a high temp situation.
 
KNielsen said:
One thing: the restored I've showed on page#10 has a constant hum from the torodial, also before restore.. Not loud but, if it were my own I wouldn't be satisfied.
First time I've seen this on a Acurus, normally it comes an goes on the amplifiers I've seen.
Is it a bad transformer? All grounds seams in perfect condition, it's a GEN2.
I've tried connecting it with and without GND connected to AC, didn't matter...

EDIT: I could try select 240Vac instead of 230Vac on the AC input, see how it behaves. The rail voltage will be higher though if I do so. At 230Vac I measure 84.9Vdc at the rails.

Regards
Click to expand...

Do you have a kilawatt meter? If so you can disconnect the bridge rect and measure the power draw from the AC mains with just the transformer in-circuit. The power draw should be very low (close to zero). I would check this, if it measures the same as a quiet A250 transformer then its probably just the windings getting loose from age/heat and there's not a lot you can do for it. One thing you could try to buy some thicker rubber insulation and make a better insulating pad for the transformer to sit on, this may help a bit.

I've found a few transformers over the years that were beginning to break down and had high AC power draw even unloaded. I had a 30VA rated transformer in an Aragon IPS external supply that was pulling 8VA unloaded so had to replace that one.
 
mondialfan said:
Do you have a kilawatt meter? If so you can disconnect the bridge rect and measure the power draw from the AC mains with just the transformer in-circuit. The power draw should be very low (close to zero). I would check this, if it measures the same as a quiet A250 transformer then its probably just the windings getting loose from age/heat and there's not a lot you can do for it. One thing you could try to buy some thicker rubber insulation and make a better insulating pad for the transformer to sit on, this may help a bit.

I've found a few transformers over the years that were beginning to break down and had high AC power draw even unloaded. I had a 30VA rated transformer in an Aragon IPS external supply that was pulling 8VA unloaded so had to replace that one.
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Thanks, very helpful....
84.9Vdc is what all A250 measure in DK if the AC switch is set correctly. So nothing to be afraid of there really. I do have a meter, that should do the job, and since it's quite easy to pull the wires I'll look into it.

Some mentioned in another thread that tighten the bolt on the torodial could help the problem. I tried this also, but as it's not my amp I don't wanna overdue it, ending ud damaging the hole thing. :oops:;)
 
When I set the BIAS I noticed that there is a difference of a couple of mV (max 0.5mV) between the NPN and PNP sitting on the same circuit. Is this correct?
 
KNielsen said:
You are on your own here... But how much BIAS do you want to pull from the circuit?
The Toshiba power transistors actually perform pretty good even at very low BIAS.
I've been as high as 15mV on the emitter, but found 10-11mV to be more then enough for a good sounding amplifier.
If I remember correctly, Mondial stated 6mV for recommended BIAS, but many I have seen are set even lower ~2.4-3.0mV!!
8mV seams to be sweet spot between "generated heat Vs soundquality" in my opinion for most users.
Click to expand...
I’m running the bias at 10mv(20ma) and it’s nice and cool and sounds great. I just figured it’s a shame to have that big heatsink sitting on the back doing nothing!
 
Another thing that can cause a buzzing or humming in power transformers is DC on your AC power lines. I’ve had a few amps over the years that had noisy toroidal transformers and a DC blocker circuit fixed the problem instantly. might be worth looking into.
https://sound-au.com/articles/xfmr-dc.htm
 
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KNielsen said:
When I set the BIAS I noticed that there is a difference of a couple of mV (max 0.5mV) between the NPN and PNP sitting on the same circuit. Is this correct?
Click to expand...
This is normal since the transistors and emitter resistors aren’t all perfectly matched.
 
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