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Old 22nd February 2012, 10:15 AM   #1
azvrt is offline azvrt  Netherlands
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Default Rubicon Picasso questions

Starting a new thread for DJNUBZ here, instead of continuing in an other thread.

He has a Rubicon Picasso and contacted me with question about how to check and set the bias. He also asked me if there were any worthwile modifications to be done on this amp. I told him I believe he should leave it as it is, unless it is malfunctioning. I did a huge op-amp test on a Ref Picasso (very similar to this amp) amongst others, and while at first I thought I heard differences it proved to be my imagination, I could not pick apart the different op-amps.

Anyway, checking and setting (if necessary) the bias is never a bad thing, I believe, so I sent him an instructional pic of where to measure and where to adjust the bias on this particular amp.

The bias is more often than not way off on high bias amps that I buy, and apparently his is not different, at 40 mvdc instead of the supposed 20 mvdc.
The amplifier does not heat up as with the years passing Soundstream decreased the bias and idle current on their high bias amps, due to heat issues.

The Class A 3.0 and 6.0 had very high bias and idle current, they already lowered it in later revisions of these models, so a '95 3.0 / 6.0 might idle lower than a '94 version of these models.
The bias for a 6.0 was between 80 mvdc and 120 mvdc approximately (would have to check).

For the '96 / '97 Ref Picasso, they lowered it to around 20 mvdc and an idle current of about 1.3 ampere (instead of over 2 ampere and over 3 ampere for 3.0 and 6.0, which have smaller heatsinks).

So in my experience, contrary to what people say, Picasso's don't heat up, contrary to 3.0 and 6.0
I haven't tested my 10.0 boards yet, which are of the same model year as Re Picasso, I am guessing they still idle pretty high like 3.0 / 6.0 but it's just a guess.

For the next years, '98 and '99, the Rubicon Class A's idled even lower.
0.9 ampere for a Renoir (2 channel version of your Rubicon Picasso) only !
2 ampere for a 10.2, and somewhere in between for a 5.2

Anyway, unless driven into very low impedances (which cannot be done with Renoirs and Picasso) these amps won't heat up, unless the conditions in your trunk and region where you live make any amp heat up.

I was told by two experts that contrary to other brands of high bias amps, it is not safe on Soundstreams to raise the bias. Apparently, there is a margin, or else at 40 mvdc you would have had a problem.

I would like to know what supply voltage you measure at the power terminals of your amplifier when it idles off the battery you said you would use to power this amplifier. Is it 11.5, 12.0, 12.5 ? This can affect bias and idle current, though maybe not (so much) on Rubicons, contrary to References.

Also I would like you to measure (idle) current draw, but you may not have the equipment for that ?

Personally I would have dropped the bias to where it is supposed to be, but I do raise it on most of my high bias amps except for Soundstreams, so I can imagine you doing the same, especially since it was already high without any problems. Maybe drop it down to 30 mvdc, just to be sure / safe ? Did you set all channels equally, by the way ?

Some people say higher bias results in higher sound quality, others say it doesn't. I like to imagine it does, so except on Soundstreams, I usually set it a little higher myself. If you believe your amps sounds better due to this, you will enjoy your amp more. Simple as that.

Don't get me wrong, I am a firm believer that amplifiers sound very different. However, to me personally, the op-amp test I did makes me think op-amps don't make (much) difference, but after putting in sockets in different amps and performing the test with no real results (detectable by me), I did not put the original op-amps back in but higher spec op-amps, so that I did not go through all the trouble for nothing. But can I hear the difference, probably not. Is there less noise / distortion with a higher spec part, probably. The fact that my amplifier has been 'improved' makes me enjoy my amp 1% more, but would I recommend another person doing the same modification, probably not.

So my opinion is you should leave the amplifier like it is, though if someone has other ideas I am always interested to know.
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Old 24th February 2012, 06:31 AM   #2
JZatopa is offline JZatopa  United States
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Im surprised no one has responded to this yet.

Could someone check my math on the bias?

.020v drop over a pair of 1ohm resistors is .01 amps? From looking at the schematic for this amp, this appears to be what is happening. If so then this amp isn't anywhere near 25w of class a amplification.

Does anyone see a problem with me bringing up the bias aside from heat and possibly reaching the current limits of the PS? If my math is right, even at 1v I would biased at .5 amps. Has anyone else tried higher bias settings on these amps?
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Old 24th February 2012, 09:36 AM   #3
azvrt is offline azvrt  Netherlands
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The Class A thing was pure marketing. These are just high bias A/B amps, and the bias wasn't very high. Fact.

You are worrying way too much and want the amp to be something that it is not and will never be.

What it IS, is a damn good sounding amp which you should enjoy.
Use it on tweeters and mids in an active 3-way, or bridge it for a passive 2 or 3 way.

Set the bias the way it should be. If you want a real Class A car amp, I am sure someone can give you pointers or schematics of how to build one. You could build a 2 x 15 watt pure Class A amplifier for your tweeters, for example. This amplifier would probably need to have a pretty huge heatsink and be fan cooled.

Regarding the bias, I was warned against two people against raising it too much on these amplifiers. One was Soundstream's main engineer, the other the designer of Genesis car amps who knows a lot about Soundstreams since he has been servicing them for years in Britain as the Soundstream importer didn't arrange a service for amps sold over there.
This has to do with the design, I'd have to dig up some very old mails which will take me half a day so I won't.

Just enjoy your amp, set the bias right, don't modify it and build your own Class A amp.

That's just my opinion, hopefully someone can give his oer her opinion as well.
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Old 24th February 2012, 04:02 PM   #4
JZatopa is offline JZatopa  United States
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Class A may have been a marketing gimick but that is what I am looking for. I have plenty of good sounding pieces, if I just wanted to buy amps and enjoy them I would. Although my plan is to build a car version of the F5 in the near future, I want to play with this one first, modifying amps is part of the fun of being into DIYaudio.

As it sits, this amp isn't even a heavily biased class A/B. It's a very low biased A/B. With such a large heatsink I can't see how heat would be a problem so the real question is if this amp suffers from thermal runaway. If so then the bias may need some sort of thermal compensation to lower the bias as the amp heats up.
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Old 24th February 2012, 04:19 PM   #5
azvrt is offline azvrt  Netherlands
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I like doing simple modifications too but I don't see the point in doing so on this particular amp, as not much can be done to enhance its sound quality and since this amp is not meant for midbass I don't see the need to increase its power as it has enough for mids and tweeters.

The heatsink is very efficient so you could easily turn the bias (and idle current) way up without the amp overheating. But I have been told it is unwise to do so for some technical reason, due to these amps designs.

Maybe someone here knows that reason. I do have it somewhere in old mails, but my mailbox is a terrible, terrible mess. I wish I could tell you.

If you want to take the risk, raise the bias to 100 mvdc and see what happens. Maybe nothing will happen and you'll have a higher idling amp. Whether that will improve sound quality is the question. If the amp does damage itself, maybe the damage will be minor, but maybe it won't. Hopefully someone can offer some information, here.
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