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Old 11th February 2007, 07:45 AM   #1
Stuey is offline Stuey  Australia
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Default Dual emitter resistors - where from?

Hi there,

The Marantz receiver I'm currently resurrecting (I hope) has 0.18 ohm 3W Noble dual emitter resistors. I can't find this value - would this have been a custom non standard value made for Marantz?

Will a 0.22 ohm suffice, and if so, what needs to be changed if anything (other than the bias being reset).

Can anyone name any suppliers other than mcminone.com ?

Cheers and thanks

Stuey
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Old 11th February 2007, 08:32 AM   #2
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From the value I assume that is the emitter resistors in the output stage you mean....

There should be no problem using 0.22 ohm and no other components needs to be replaced - I think the output current limiter (if one) can handle this small change
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Old 11th February 2007, 10:04 AM   #3
AndrewT is offline AndrewT  Scotland
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Hi,
you are likely to find that the voltage across the higher value emitter resistor is the same, or very nearly the same, as the original without making any change in the bias setting.

The crossover distortion should be virtually unchanged if you leave the bias setting as the manufacturer set it.

However, if the original setting was poorly chosen, then an improvement is sound quality may come from a slight adjustment. But, do not risk running the output stage hotter than the manufacturer intended.
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Old 11th February 2007, 10:28 AM   #4
Stuey is offline Stuey  Australia
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I hope my terminology makes sense...I'm just starting out on this stuff.

Yes, the resistors across the output transistors. The output and bias transistors and emitter resistors are open circuit on one channel and are being replaced, so the bias current is necessarily being readjusted via the trimmer resistors (also open and being replaced) anyway. This is what I meant by reset - not a different value to the spec. value.

Thanks for the replies. If anyone else has some suggestions for suppliers, I'd appreciate it, especially any Aussies...

Cheers

Stu
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Old 11th February 2007, 10:54 AM   #5
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Stu,

the 0.18+0.18 ceramic is a common production item, whether Noble or Futuba.
Current inventory items they're not, only the single 0.18 Ohm ceramic resistor is.
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Old 11th February 2007, 12:15 PM   #6
Stuey is offline Stuey  Australia
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Thanks Jacco. I'll see if I can dig up some NOS items somewhere; if not, I'll go for the 0.22 ohms jobs.

Cheers

Stu
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Old 11th February 2007, 01:27 PM   #7
AndrewT is offline AndrewT  Scotland
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Hi Stuey,
you misunderstood my post.

It's not the current that should be set. It's the voltage across the emitter resistor that should be set. This Vre is a constant for all normal emitter resistor values.

If the manufacturer specifies Vre then copy it irrespective of what value of emitter resistor you finally fit.

If they specify a bias current for 0r18 then convert that to an equivalent Vre and use that same voltage across the new emitter resistor.
Yes, I realise that gives a different bias current, but current is apparently not the deciding factor, it is the voltage that is important and for a BJT output stage Vre should be between 15mV and 25mV.
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Old 11th February 2007, 01:51 PM   #8
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Stu,

bare in mind that the accuracy of these resistors is +/- 10% or more.
If you have the chance to do a selection of 0.22 ohm resistors to find some with a value of 10% below their nominal value, they'd be equal to an upper limit 0.18 ohm dually.
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Old 11th February 2007, 07:03 PM   #9
radtech is offline radtech  United States
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You could parallel a 1 ohm resistor across each .22, that would give you .18 ohms. Since the originals were 3W, I think you could use half watters for the 1 ohm pieces.
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Old 12th February 2007, 08:09 AM   #10
Stuey is offline Stuey  Australia
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Thanks, all of you. I appreciate the comments.

Yep, Andrew, I did misunderstand!

That's a good idea Radtech. Simple, too. Failing finding the proper parts, it might be the way to go.

I won't have the opportunity to check samples, unfortunately.

I looked at Noble's US website and they still list the 0.18 (and lower) under their type number RGC33 (the three watters) so I've fired off an email to see if they know of one of the bigger online suppliers that stock them.

They also have a sample program, so I may go down that route, above board, and ask if they'll supply direct (paid for of course).

Thanks again

Stuey
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