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Old 24th October 2010, 02:21 PM   #21
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This unit has three levels of gain. Have you tried it on the lowest setting, 0db in SE mode. I have owned several ARC preamps that use the 6H30 and they have all been dead quiet. I currently drive an ARC VS110 amp with no noise issues. I respectfully disagree with Analog SA on the ARC pre amps. I think they sound fantastic. But they are not everyone's cup of tea. To each his own.

John
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Old 24th October 2010, 03:03 PM   #22
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yup

- Main Output: Selectable for each input: 18db, 12dB, 6dB Balanced output. (12dB, 6dB, 0dB SE output)

in any case - looking at schematic - this is solution made as pure resistive divider in input ( pre fader) and it will not solve possible noise issue
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Last edited by Zen Mod; 24th October 2010 at 03:06 PM.
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Old 24th October 2010, 04:45 PM   #23
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Quote:
Originally Posted by johnmarkp View Post
This unit has three levels of gain. Have you tried it on the lowest setting, 0db in SE mode. I have owned several ARC preamps that use the 6H30 and they have all been dead quiet. I currently drive an ARC VS110 amp with no noise issues. I respectfully disagree with Analog SA on the ARC pre amps. I think they sound fantastic. But they are not everyone's cup of tea. To each his own.

John
Hi John,

The noise I am experiencing is not dependent on any setting on the LS25 mkII except that it is turned on and the mute is off.

The volume or gain setting or any other switch makes any difference on the noise (hum, hiss, and buzz).

Again, this noise is low level. It is not something I can hear above the music. I need to put my face in the speaker to hear it. However, I hear from owners of this preamp that it is dead quiet. Also, it is much noisier than my solid state preamp.

I feel there is something wrong with this preamp. So I am asking for help troubleshooting from the knowledgeable amp gurus here on DIY Audio.
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Old 24th October 2010, 04:51 PM   #24
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Quote:
Originally Posted by Zen Mod View Post
yup

- Main Output: Selectable for each input: 18db, 12dB, 6dB Balanced output. (12dB, 6dB, 0dB SE output)

in any case - looking at schematic - this is solution made as pure resistive divider in input ( pre fader) and it will not solve possible noise issue
In your opinion, what can be causing the noise? Power filter caps? Can you tell by looking at the schematic what areas I should check?

I have measured voltage points and they are all correct if not exact to the schematic. But my bench is 117 volts instead of 120 volts.

The spec of 14 micro volts hum and noise tells me it should be dead silent for this preamp...
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Old 24th October 2010, 05:11 PM   #25
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well - in first place - you should wrote that fact in first post - that you need to stick your nose deep in cone , to hear noise ........

considering fact that power amp is 1V/full throttle sens , my opinion is that you promptly need to shut off that PC , relax about this "issue" , and enjoy in music .

you can also burn phone No. of ARC service

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Old 24th October 2010, 05:23 PM   #26
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In your opinion, what can be causing the noise? Power filter caps?

Seeing that you also have a problem with hum in your power amp i would suggest that your expectations may be unreasonable. Is there any hiss or hum you can hear a meter away from the speakers in a quiet room? Does it really matter that there is some noise with your ear glued to the tweeter? What are the chances that both units are defective?

As for comparing this preamp to a good solid state unit, it is not so straightforward.

Chances are the SS unit will employ a more sophisticated circuit, probably with some global NFB as well which will result a higher PSRR than a simple common source FET. ARC do not disclose the type of FET but it is the main noise contributor in the circuit, not the tube. Apart from its inherent (and unknown) noise this circuit is also very sensitive to PS noise. Although B+ is regulated, the regulator is not a particularly low noise type.

As far as components go, forget about the caps and try to source a pair of known good 6H30, maybe even the DR version. It is possible you have a dud pair.
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Old 24th October 2010, 05:45 PM   #27
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Originally Posted by analog_sa View Post
Seeing that you also have a problem with hum in your power amp i would suggest that your expectations may be unreasonable. Is there any hiss or hum you can hear a meter away from the speakers in a quiet room? Does it really matter that there is some noise with your ear glued to the tweeter? What are the chances that both units are defective?

As for comparing this preamp to a good solid state unit, it is not so straightforward.

Chances are the SS unit will employ a more sophisticated circuit, probably with some global NFB as well which will result a higher PSRR than a simple common source FET. ARC do not disclose the type of FET but it is the main noise contributor in the circuit, not the tube. Apart from its inherent (and unknown) noise this circuit is also very sensitive to PS noise. Although B+ is regulated, the regulator is not a particularly low noise type.

As far as components go, forget about the caps and try to source a pair of known good 6H30, maybe even the DR version. It is possible you have a dud pair.
In a dead quiet room, I can hear the noise from about 3 meters away. By muting and unmuting the preamp.

I did order a pair of 6h30s. I'll see if that helps. I heard from several other LS25 owners and they tell me it is dead silent in their system.
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Old 24th October 2010, 05:51 PM   #28
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Originally Posted by Zen Mod View Post
well - in first place - you should wrote that fact in first post - that you need to stick your nose deep in cone , to hear noise ........

considering fact that power amp is 1V/full throttle sens , my opinion is that you promptly need to shut off that PC , relax about this "issue" , and enjoy in music .

you can also burn phone No. of ARC service

I didn't mean the noise was THAT low. I can hear it from about 3 meters away at my listening seat. The power amp with the inputs shorted have a very quiet hum in the speaker. That, I can live with as it is very low. But with the preamp in the chain and on, the noise is considerably higher.

Can dirty pcb board do this? I noticed strange powdery dust covering it slightly. I wiped it off the best I could.
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Old 24th October 2010, 06:00 PM   #29
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I did order a pair of 6h30s.


From where? I have stopped trusting most US sources for these tubes. There seem to be too many rejects on the market. Getting tubes from any of the ex - Soviet Union countries seems much safer.
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Old 24th October 2010, 06:03 PM   #30
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Quote:
Originally Posted by GeneJazz View Post
....... I can hear it from about 3 meters away at my listening seat. ........
in THAT case - contact ARC

have you any possibility to directly compare your LS25 with anyone else's , or do you have ARC dealer near you ?
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