Acurus A250 and starground

[mondialfan]

Yes a 100W bulb will be fine with the A250. I like to use a 75W for most of the Acurus amps due to the fact that if the bias is set correctly they will run fine with the 75W bulb and it affords slightly more protection for the smaller amps.

 

[KNielsen]

Tonight is happens!!!

Polarity is correct in the PSU, so further testing can begin

 

[KNielsen]

Mondialfan: Is it ok to operate with 93VDC at the rails??

Regards

 

[KNielsen]

WTF just happened..... This thing can play some new tunes!!!!!

All measurements were dead on, I've started out with 5mv at the rails, not even warm!

A quick comparison between this and my other A250, take this amp into a hole new class, on a cold start (5min of music)

No noise at all from the tweeter or trafo.. dead silent
Thanks for all the help Mondialfan

 

[KNielsen]

The rails are 10.4mw right now..
Think this might be the sweetspot with 23°c (~74°f) in our leaving room.

Edit: 3.3mv now, just to compare....

Think I read somewhere that I should try to get the bias between 6-8mw?? Standard is 2.6mw if I remember correctly.

 

[KNielsen]

.

 

[mondialfan]

Mondialfan: Is it ok to operate with 93VDC at the rails??

Yes, I've seen the A250 DC rails that high and even higher.

 

[mondialfan]

The rails are 10.4mw right now..
Think this might be the sweetspot with 23°c (~74°f) in our leaving room.

Edit: 3.3mv now, just to compare....

Think I read somewhere that I should try to get the bias between 6-8mw?? Standard is 2.6mw if I remember correctly.

As a general rule the higher you can operate the DC bias without overwhelming the heatsinks the lower the distortion will be. See this thread where I posted a test I ran on the Audio Precision of an Acurus A150 with various output stage bias levels.

There is no magic number for everyone on this. If you have very efficient 8 ohm loudspeakers then the load on the amp is minimal and somewhat higher bias is possible. If you have a set of Acoustats like me then your bias level is going to be limited because they are 4 ohms and require a lot more power to get normal listening levels out of then say my Vandersteen 2Ci's.

 

[mondialfan]

BTW, 3.3mV is too low for the bias.

Somewhere between 5 - 10mV will generally give better distortion levels and as long as the heatsinks are not getting too hot during normal listening sessions then you should be good to go.

 

[KNielsen]

Thanks.. Since I often listen to music quite loud (when wfife isn't home ) I adjusted to approx ~13.5mV.. it has a nice temperatur in idle and doesn't get to hot when played hard.

My speakers are Dali Grand Diva, 4ohm

 

[KNielsen]

The Acurus has been playing for a couple of days now, and it just keeps sounding better and better.

There are only upsides from stock:
- soundstage massively improved
- overall speakercontrol very impressive, super fast attack in the hole frequency range, and yet silky smooth!
- more details in sound, and vastly improved in both bass and treble reproduction

I could go on, but in the end it luckily still sounds like a "Mondial", but much more high-end..!
Actually I think it sounds better then my friends 8008, and the Aragon 4004's I've heard. It just have something little extra that makes your hair raise instantly, surprised by the accuracy it now pulles of.

In many ways I think the "caps post fuse mod" and the "psudo dual-mono" plays the biggest part according to the above mentioned. But it's just a guess....

In a week ore two I should be able to massure some specs.. I share if I succeed

Regards

 

[KNielsen]

Today's messumeasure:

Pic1:
320watt @ 1KHz, THD+N 0.013% both chanel's driven with a 6.6ohm dummy load.
349watt, one Chanel driven.
Maximum THD+N registered were 0.079%

Pic2:
Noisefloor is superior, and 50Hz hum(60HZ in USA) is completely removed with the new starground.

Pic3:
Imput response: yellow is targetcurve, blue is the amplifier.

Regards

 

[KNielsen]

One problem???

We did an impulse test and it turned out like this:

Right channel


Left channel
Input signal is 1.74v, NOT 17.4V


It should be mentioned that only left channel was driven harder under this test, but is it enough to change the timing on the transistors that much???
Yellow is targetcurve...

I have to do new measurements, but I assume this is an "circuit error" on left channel....??? All resistors are spot on, only C13/14 are now being change, because the didn't measure that well on the left side.. they should be 220pf

But is this scenario normal, more power = longer transistor delay??? Or an error??

Can the MPSA42/92 course this problem?

I do hope it's a common known problem, and someone could point me in a direction for a solution

Thanks // regards

 

[mondialfan]

Do both channels show the same response?

A reference from Elliot Sound for square wave testing.

 

[KNielsen]

Do both channels show the same response?

That we have to measure again. Right now the left channel seams to have some kind of delay, but the THD+N and wattage were exactly the same as left channel.. in a week I should be able to do a complete test, both channels.

- Also, the amplifier clipped at 48Vrms, (320watt @ 6.6ohm). We thought that was a little low since the rails are 90.2Vdc??

 

[mondialfan]

I attached an image of one of the prior tests I performed on an A250 I worked on. This data was collected and input into a spreadsheet manually with an old analog test set I have. I'm now using an Audio Precision system one so I can get much better info. This testing was done with a 30kHz LP filter which shelves the distortion a good bit above 10kHz so to get a comparable test you would need to do the same on the testing system that you're using. If you use 80kHz BW or 20kHz BW (which it looks like you used) then the tests will be skewed from the one I'm displaying. I'll be testing another A250 on the AP S1 in the next month or so and I can post that info when I'm done.

I don't think your numbers are that far out of line with what I've measured in the past. My past tests are a bit higher, but I'm on 60Hz power which will play a role. 48Vrms = 135Vpp and once you factor in power supply drop when the amp is that heavily loaded your right in the region that you should be. Remember that 90.2Vdc is with almost no load on the amplifier. The next time you test the amp clip your DMM onto one of the power supply capacitors and measure the DCV when you do a full power test to see how much the power supply rails are dropping when under test. You also have to factor in how much the main AC line is dropping when the amplifier is drawing this much power from the wall and its certainly dropping several volts when fully loaded.

 

[mondialfan]

BTW I forgot to mention that I wouldn't test the amp at full power with a square wave input. You may damage the amp or the dummy load its connected to. I generally only test at a max of 10W. There's a thread on this topic somewhere I'll try to dig it up and post a link to it.

 

[KNielsen]

BTW I forgot to mention that I wouldn't test the amp at full power with a square wave input. You may damage the amp or the dummy load its connected to. I generally only test at a max of 10W. There's a thread on this topic somewhere I'll try to dig it up and post a link to it.

Thanks alot.. we didn't put at load on the impulse squarewave test, exactly because we didn't want to damage the transistors. But we did apply some voltage..

But can C13/14 mica course the curve to be that much of target when an impulse is send to the amplifier??
I remeasured the micacaps and the are not that good (but the pricetag is! sadly...)
C13 is about 700pf, C14 is approx 400pf.. the should be 220pf according to the micas I pulled from the original circuit and ordered..

Regards and thanks.. your free to post your future findings as this helps alot

 

[KNielsen]

BTW although we did measure some things on left channel that we are now aware of and try to solve, the amplifier can play some nice tunes..

As DIY never stops!, I bypassed C1/C2 as I have no DC comming from my moded L10.. - wanted to hear a "cap free input"
I've also added 2.2uf in parallel to feedback, some Danes had very good experience with the Panasonic EZW(a) 2.2uf cap in their feedbackcurciut (not Acurus) and advised me to try. I had two at home so it's now 220/2.2/0.47uf. There is a big debate on a Danish forum about the feedback and it's funktion, very interesting..!
The sound is now more neutral and straight forward. I think it's a matter of taste (individual as always) whether these mods are positive or should be left original..

Regards

 

[Jdk07]

The schematic you're working with is the worst of the two I've seen floating around on the net. If you'd like a better schematic to work with I recently took the better quality A250 schematic and cleaned it up further and made needed changes to it. I attached a copy. I have a much higher res .bmp of it if you'd like to contact me via PM I can send it to you.