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Old 9th February 2007, 08:52 AM   #1
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Default Aragon question...

Hi!

After having sold off my Yammy M-80 quite a while back, and then an episode of lost funds with a previous diyA purchase, was finally able to get a reasonable deal on an Aragon 8002. Big difference in design/layout that I like; super clean. Nice noticeable improvement in sound quality.

Anyway, noticed one channel was running a touch warmer than the other, so I dived in and did the "check the bias thing). One channel read 18mv over the resistor, the other was ~30mv.

I re-set them to 10mv just as a starting base point (seems to be the magic number for a lot of amps. Runs cold and still sounds great. I know that this amp supposedly is supposed to run warm~hot, so the question is...

What is the ideal bias for the 8002?

Thanks.
bluesmoke

SMALL EXTRA: One odd thing (lucky me; like always with amps), I've noticed that when I've listened to the amp at low levels from a cold start, about 1 hour in the left channel seems to break up. As soon as I turn up the volume and run it for a while, things are fine when back down to quiet listening levels. Weird... It FEELS like a relay problem (me and relays... starting to hate them), but both channels seem to be making solid contact. Could still be a relay problem (oxide from arcing~ I can't see), but just wondering if there might be any alternative suggestions. Can live with the weirdness for now, but this is just shouting "fix me!".
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Old 9th February 2007, 12:50 PM   #2
lineup is offline lineup  Sweden
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Default Re: Aragon question...

Quote:
Originally posted by bluesmoke
Hi!

What is the ideal bias for the 8002?

Thanks.
bluesmoke

We aim for some target CURRENT.
now:
Volt = Resistance x Current
( U = R x I )

so,
1. you should try to measure or see what resistance Those emitter resistors have.
for example 0.22 Ohm

2. you decide what current you want to bias to
for example 60mA = 0.060 Ampere

3. You adjust until you get a voltage across this resistor
that corresponds to 60 mA :
for example:
Volt = Ohm x Ampere
Volt = 0.22 x 0.060
Volt = 0.0132 Volt .... ~13 mV

====================


How much bias current for this Aragon 8002amplifier?
We need to know 2 things:
a) What power transistors? What is the text on those?
b) How many of them?
Only 2 per channel? Or maybe 4?

Generally, we can say, that Bipolar power amplifier normal Class AB
would want to have an idle current of something like
50-90 mA in each output transistor.


Can vary a little bit between different devices.
But I am sure someone at this forum has used EXACTLY the same output Transistors
that are used in Aragon 8002.
And so would be able to give you a good advice.


Regards, lineup
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Old 9th February 2007, 01:26 PM   #3
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The Aragon is basically a Krell.
The 8002 is a successor of the 2004, sort off a class AB version of the KSA50, 4 TO3/Ch output devices.
First series 2004 used Krell type Motorolas, MKII had Japanese metal can output devices.
The 8002 had 4 2SA1302/2SC3281 devices per channel, also designed by another guy instead of DanniO.
The 8002 sounds a whole lot better with the bias bumped up a bit, at the factory setting it already does a few watts in A. Best for that would be to have the heatsinks breath open air, instead of covered.
To keep the precious Toshibas in the 2004 alive, better to have them cool instead of wearing them down hot. You're not going to find replacements. (unless you feel the urge to pay my bill for a fancy dinner)
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Old 9th February 2007, 05:35 PM   #4
AndrewT is offline AndrewT  Scotland
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Hi,
that 10mV across a resistor. Is it one of the emitter resistors that you are measuring? There should be one power resistor for each output transistor.

Try re-setting the Vre =20mV and listen for a while. If the heatsink is still cool try 25mV.
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Old 9th February 2007, 11:50 PM   #5
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Thanks all for the info!

Will crack it open over the weekend and apply all advice. Report to follow soon after. It been a cold week, so I won't mind the added heat...
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Old 11th February 2007, 12:48 PM   #6
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Just contact Mondial(now owned by Klipsch). I contacted them a few months ago because I was interested in the schematics for the Aragon 4004. I also recontacted them last week to get an owners manual for the Aragon Soundstage processor I recently purchased. They sent everything right over plus the bias setup procedure for the 4004 mkII.
Here is the email address, Support.Mondial@klipsch.com. I talked with a guy named Steve Philips.

I would do this even if your going to just set it up by temp. You never know when you may need a copy of the schematic in the future and who knows how long Klipsch will continue to keep these materials around now that Aragon/Acurus is a thing of the past...
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Old 12th February 2007, 02:46 AM   #7
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the other way to set the bias is to operate the amp at one watt with a 20khz sine wave, and while monitoring the output with a distortion analyzer, and viewing the analyzer residual on an o-scope, adjust the bias untill the crossover notches just barely disappear. this does not always give the same bias even with two of the same model amp, or even both channels of the same amp, but many manufacturers use this method.

as far as the distortion problem you describe, you could have an intermittant emitter resistor or driver, or output transistor.
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Old 12th February 2007, 09:04 AM   #8
lineup is offline lineup  Sweden
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Quote:
Originally posted by unclejed613

Set the bias is to operate the amp:
- at 1 Watt, one watt
- with a 20khz sine wave,
- adjust the bias untill the crossover notches just barely disappear
.
thank you very much, unlclejed613

I have been wondering what level/frequency for adjust bias.
I was thinking that maybe higher freq, like 20kHz, could be used.
Because,
Douglas Self findings was that 'bad biasing'
had the most clear effects at higher frequencies.
Distortion in Power Amplifiers - by D. Self

Regards
lineup
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Old 13th February 2007, 09:57 AM   #9
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Was a pretty easy tweak, except for lifting the thing... yeesh!

Emitter resistors read 0.5ohms (tested a few), putting it to the enjoyably easy math...

= A setting range of 25~45mV for the 50~90mA desired current range.

I set it to 30mA (in matching with the 60ma original calculation), get's lightly warm to the touch (enough for me; want to keep it alive for a loooong time). After re-connecting it to my system I was surprised, there was a quite noticeable audible difference, for the better.

Hardest part for me was keeping the room at the same temperature when setting bias. Had a drafty window nearby.

Hadn't noticed the channel breakup in the last recent plays, hoping it stays that way, as it would be a real pain to source down something so rarely intermittent.

Had avoided contacting Mondial, after looking at the aged forums it looked like they didn't want to reveal any technical details to those who inquired. Will give them a shout now, seeing that they may be more giving.

Wish I had a scope, and a dist-analyzer, and a lot more goodies. Gotta brush up on my electronics understanding more before I invest in anything serious like that though.

Nothing much left to fiddle with now... Sadly it was the only thing keeping me from doing real work.

Maybe I should vacuum...

Cheers, and thanks all!!!
bluesmoke
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Old 13th February 2007, 10:41 AM   #10
lineup is offline lineup  Sweden
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Quote:
Originally posted by bluesmoke

Was a pretty easy tweak, except for lifting the thing... yeesh!

Emitter resistors read 0.5ohms (tested a few), putting it to the enjoyably easy math...
= A setting range of 25~45mV for the 50~90mA desired current range.
Quote:
Originally posted by jacco vermeulen
The Aragon is basically a Krell.
----------
The 8002 sounds a whole lot better with the bias bumped up a bit,
at the factory setting it already does a few watts in A.

Best for that would be to have the heatsinks breath open air, instead of covered.
.

Great!
'And easy tweak'
Nothing is really difficult, when we know what we do & how to do it.


I would ALSO try jacco_vermeulen nice information:
The 8002 sounds a whole lot better with the bias bumped up a bit,
at the factory setting it already does a few watts in A.


It might very well be, that Aragon 8004 is so much alike Aragon 8002
that it would benefit with bit higher than normal bias.

normal: 50-90 mA
a bit higher bias: 150-300 mA

... a few watts in Class A
Compare with Nelson Pass www.fristwatt.com
where he tries the concept of the first 1 or 2 watts = low level
are the most important watts of an amplifier

==========================

Technical.
Info from lineup.

To bias Aragon 8004 for Class A operation into 8 Ohm speakers
0-1 Watt: set bias 275mA ...... ( actually 250mA + 10% )
0-2 Watt: set bias 390mA

275mA bias = 1 Watt Class A into 8 Ohm.
If resistor is 0.5 Ohm, this will mean 0.137 Volt across 0.5 Ohm.
But if you have 4 BIG Transistors per channel (=total 8 for stereo)
You make it 50% .... 68mV per Resistor 0.5 Ohm.

--------
Final Comment and something to think about, in life:
This is of course because they do 50%-50% of the job, compared to a Single Transistor.
When two persons do a job, they only have to make half of it.
This is why CO-operation is so good, to do
Instead of making things more difficult for others
we should try to support and help eachother!


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