Nakamichi SR-4A Idle Current Procedure - diyAudio
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Old 3rd December 2011, 05:35 PM   #1
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Default Nakamichi SR-4A Idle Current Procedure

I've attached the procedure from the service manual. It states:

"Insert shorting plugs into the CD player input jacks."


This sounds a bit odd to me, what would be accomplished by shorting the CD inputs? I'm assuming they want you to use an rca cable (or the jumpers on the preamp) and connect the positive to the negative terminals?

Unjumpered, I'm getting about 10mv on each side.
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Old 3rd December 2011, 06:01 PM   #2
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As a general rule you should have no speakers connected since any DC offset can affect the current in a DC coupled amp.

As to the shorting plug, it just makes sure there is no audio present... you know yourself if there is any output present during the test.
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Old 3rd December 2011, 06:12 PM   #3
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The inputs are shorted when testing small potentials in audio amplifiers so as not to confuse signal with bias. The "signal" or rather electrical noise is picked up by an amplifier input that is not prevented from it in some way. Even a normal signal source will permit some noise to pass to the amplifier.

Listen to the speaker with no input leads......Turn up the volume until you hear noise, then repeat with a shorted input - It's quieter eh? So a quiet input doesn't mess up the bias voltage measurement so bad. That's the idea, at least.

BTW It's not necessary to use leads for this. If the cover is off, an alligator clip across the input + to- or any well-fitting shunt like a a spare RCA plug with the terminals soldered together is fine so long as you don't leave a long lead that might still act as an aerial and make a large noise if the earth slips off for some reason. Take care.'
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Old 3rd December 2011, 07:11 PM   #4
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So is each phono input (one red, one white) individually shorted to itself so prong contacts the outer shell (as in cutting an rca cable's connector off and soldering together the two wires in it) or is it prong to prong, like the preamp jumper, connecting the two female parts of the red and white jack? Is there a danger in any of these combinations?
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Old 4th December 2011, 03:04 AM   #5
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Quote:
Originally Posted by DreadPirate View Post
So is each phono input (one red, one white) individually shorted to itself so prong contacts the outer shell.... ?
I think I've already stated "+ to - ". You can't confuse that or the meaning of shorting with just an allligator clip and described shorting as soldering across a plug. If that is not clear enough, just think about one amplifier at a time - like the bias setting itself.
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Old 4th December 2011, 06:22 AM   #6
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Quote:
Originally Posted by DreadPirate View Post
So is each phono input (one red, one white) individually shorted to itself so prong contacts the outer shell (as in cutting an rca cable's connector off and soldering together the two wires in it) or is it prong to prong, like the preamp jumper, connecting the two female parts of the red and white jack? Is there a danger in any of these combinations?
You really don't need to do this. Just make sure the volume is on minimum and do the adjustment as per the values in the manual.
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Old 4th December 2011, 08:57 PM   #7
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I'm just being paranoid, want to make sure I've got it right, since I've not seen that step in a procedure before.

Okay, so if the reason to do this is the possibility of signal influencing the bias setting, then why not just remove the preamp jumpers and take that completely out of the picture?

I've measured the bias before adjusting and I'm getting around 10mv, spec says 23mv. Am I correct in assuming these units find it hard to hold their bias over time? I read posts where people are disappointed in the sound, I'm wondering if maybe that is the reason.
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Old 4th December 2011, 11:41 PM   #8
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"why not just remove the preamp jumpers and take that completely out of the picture?"

Why not? Because you are still left with an unconnected input and the same issues, perhaps worse, if you have nothing that turns it down anymore.

Just proceed as Mooly suggests and set to the specified 23mV. Note the procedure of allowing the amplifier to warm up. I think a longer period like 10 minutes would be needed even with the cover on. At this level of bias, it will take some time to even be noticeably above room temp. Recheck some minutes after after setting as it settles to the the new equilibrium.

Bias stability can be a problem which gets worse with age of the amp. and overuse of the adjustment. However, the fact that you seem to have both channels "around 10mV" suggests this is an intentional setting, perhaps by a previous owner?
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Old 5th December 2011, 07:11 AM   #9
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As to sound quality... in all honesty I think you will find that the bias setting makes zero difference. Anything over a couple of milliamps in the output transistors and audible distortion will all but disappear. It won't change the character of sound of the amp.

As to the procedure... prove it to yourself ! Connect the DVM up and monitor the voltage and then DO turn the volume up (with audio and speakers) and see how it affects the reading. It would have to be louder than you think to alter the reading significantly. And turn it right down and see when the sound becomes distorted. What value ??

Set it and forget it...
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