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Old 7th April 2011, 04:06 PM   #1431
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Quote:
Originally Posted by AndrewT View Post
What is the sensitivity of your speaker?
What are the DC & AC measurements of output noise, when volume is at minimum and maximum (with the input shorted)?
Andrew,
I attached the jblJ2080 catalog for your reference. It's 89dB.

No input:
-16mV DC on left and -24mV DC on right @ minimum
-57mV DC on left and -65mV DC on right @ maximum

input shorted: i don't get how to do that. You mean to short ground with signal?

HT
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File Type: pdf J2080 ts.pdf (148.3 KB, 30 views)
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Old 7th April 2011, 04:08 PM   #1432
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Yes, short ground with signal
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Old 7th April 2011, 04:18 PM   #1433
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Quote:
Originally Posted by Peter Daniel View Post
Tweeters always produce some hiss, and depending on listeners perception it may be more or less of a noise. But if it's not heard from your normal listening position I wouldn't be loosing sleep about it.
Peter,
It's wise advice. I cannot wake the neighbors up at midnight. I'd better sleep now and will listen tomorrow. Normal listening position... OK, I do remember. Will report then.

Have a nice day!
HT
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Old 7th April 2011, 04:25 PM   #1434
AndrewT is offline AndrewT  Scotland
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89dB/W @ 1m is not particularly onerous.
The sound outputs from the drivers with the power amp input shorted, should appear silent at 50mm from the treble dome and from the bass cone.
Try turning everything else off in the room and wait till the traffic outside has gone away, and the wind has stopped blowing.
1mVac into an 8ohm speaker is ~1uW (-69dB ref 1W)
An 89dB/W speaker will reproduce that as a 20dB SPL @ 1m.
at 50mm the SPL will be ~ 46dB or about 20dB above the threshold of hearing (in the middle range).
0.1mVac will be ~@ the threshold of hearing @ 50mm
That is approximately where a chipamp should be. I have most of mine below 0.05mVac, but this is with a DMM that does not have the correct 20kHz/100kHz filter. It reads low at higher frequencies.

Use an RCA plug to short the RCA input socket.
remove the back of the plug and solder a short from RCA ground/return to RCA pin/hot.
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Old 7th April 2011, 04:44 PM   #1435
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Here the measurement with short input:
-16mV DC on left and -24mV Dc on right @ minimum
-15mV DC on left and -23mV DC on right @ maximum

Andrew, I heard hum at about 1 finger distance, 50mm approximately.
Cannot hear hum from normal listening position with volume set at low now (midnight). But will try louder tomorrow. Also will try with no device on but I don't think I will be so serious about it.

More information for your comments: I used CAT5 UTP cable (which has been used for computer network for >2 years) for all signal connection in AMP. Peter suggested CAT5 is fine. I used wire with its plastic shield.
For grounding, I used the core from 75ohm coaxial. Star ground. In fact, I did not make it star at the middle point between two amp board, but from the one end of the ground copper wire.
Power wire: normal wire, newly bought with 3 wires N-L-E, fuse integrated on power plug (Singapore standard)

I think it OK, right?
HT
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Old 7th April 2011, 05:03 PM   #1436
AndrewT is offline AndrewT  Scotland
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I like CAT5 as well. That and 0.6 diameter insulated solid core is what I use for most of my wiring. I do have a lot of other bigger solid core and much bigger stranded, but I rarely use them until I reach Mains voltages.
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Old 7th April 2011, 05:07 PM   #1437
udailey is offline udailey  United States
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There is definitely a difference between humm and hiss. You will get hiss at that distance, no doubt. If you are a meter away and can not hear it then its nothing to worry about. Your DC offset is good. Not a problem.
If you have humm then you have a grounding problem somewhere.
Uriah
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Old 2nd May 2011, 10:21 AM   #1438
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Hi everyone, i need a little advice concerning my dual mono gainclones. I have had my amps up and running for about 6 weeks now and all was fine, i now have given a small tweak replacing the rca input and binding posts + wires to one amp. When connecting to mp3 player for tests the music can be heard very very faintly at full volume, have i blown something here or could it be a mismatch of components maybe. Any help would be much appreciated.

regarts Lee
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Old 3rd May 2011, 01:23 AM   #1439
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Mostly likely something wrong with wiring. What's the DC offset on the "quiet" one?
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Old 3rd May 2011, 11:12 AM   #1440
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Hi Peter, thanks for speedy reply. I have changed signal wires back to origanal and it now seems to be working again. Being a noob not 100% sure on where to use multy meter, but with probes on binding posts i am getting a reading of - 0.32 on first amp and - 0.29 on the one ive just tweaked, im sure they were both - 0.32 before mod. Hope these readings make sense, can you think of any reason for variation in readings or if it will make much of a difference to sound.

regards lee
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