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Post by EarFull on Dec 29, 2021 18:46:48 GMT -5
Hi Keith
Bought Mcintosh C30 preamp years ago and have had it tuned up at Audio Classics. Ran it using two Emo monoblocks.
Output to 1978 KHorns which I love.
Due to size and weight of monoblocks, traded in to Emotiva and got XPA DR2 Gen 2 stereo amp.
I know, I know: way too much power as KHorns are so sensitive.
Now, with this newer setup, there is an audible 'idle hiss' in both speakers. It must be coming from C30 Macintosh preamp because it goes away
when it is turned off and only the Emotiva XPA DR2 is on. Again, this hiss isn't heard unless really quiet passages like classic music or piano.
OR, if I put my ears up to the speakers.
Been studying 'gain' and sensitivity and such. The preamp has notched volume attentuator so usually can't go past 1 notch or it is too loud for living room.
Wondering if:
1. Connecting Emotiva CD player ERC4 into variable analog inputs might make a difference but this hiss occurs even without the ERC turned on.
2. Would Rothwell attenuator plugs help in this situation? -10db or -15db ?
3. I'm not 100% sure, but, I do NOT think this hiss was there when the monoblocks were used.
Your thoughts are highly appreciated.
Patrick 'Earfull'
1978 Klipschorns
Emotiva XPA DR2 Gen 2 Amplifier
Emotiva ERC4 CD Player
1986 Mcintosh C30 Preamplifier
Logitech Bluetooth Connector to play from iPhone or iPad
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Post by pallpoul on Dec 29, 2021 23:54:49 GMT -5
Maybe a Ground loop?
I am not Keith (obviously) but I will share my experience with the Khorns.
I have never heard a hiss from my Khorns 70th anniversary edition, now driven by a Cary Audio SL-100 tube amp (KT-150 tubes).
I use the erc-4 as a DAC/CDP. Some time back, I used them with 2 XPA-1 Gen-2 amps, and never had a hiss.
Those speakers are extremely sensitive, but as quite as a church mouse.
IMHO, those speakers will deliver their best with a SE tube amp. My preferred being the 45 SET By Dennis Had (which I am using with my La Scala II's currently).
Let us know when you find the culprit, please.
Good luck.
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Post by Boomzilla on Dec 30, 2021 4:22:11 GMT -5
Thermal hiss from the tubes. Try replacing the tubes with new ones.
That said - If you have to put your ears against the speakers to hear the hiss, is it even an issue? You don't listen at that distance, so you're worrying about it why?
Note that the hiss is probably independent of signal level, so attenuators will ONLY reduce the signal - not the hiss. In effect, the attenuators are decreasing your signal to noise level.
One thing you can try - with NO signal going through the rig, try turning up the volume. Does the hiss increase in volume, or does it remain the same? If the hiss volume increases as you turn up the volume, then the hiss is coming from tubes (or circuitry) upstream of the volume knob. If the hiss remains the same volume despite increases of the volume knob, the hiss is coming from the tubes (or circuitry) downstream of the volume knob.
Boomzilla
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Post by brutiarti on Dec 30, 2021 9:20:41 GMT -5
Thermal hiss from the tubes. Try replacing the tubes with new ones. That said - If you have to put your ears against the speakers to hear the hiss, is it even an issue? You don't listen at that distance, so you're worrying about it why? Note that the hiss is probably independent of signal level, so attenuators will ONLY reduce the signal - not the hiss. In effect, the attenuators are decreasing your signal to noise level. One thing you can try - with NO signal going through the rig, try turning up the volume. Does the hiss increase in volume, or does it remain the same? If the hiss volume increases as you turn up the volume, then the hiss is coming from tubes (or circuitry) upstream of the volume knob. If the hiss remains the same volume despite increases of the volume knob, the hiss is coming from the tubes (or circuitry) downstream of the volume knob. Boomzilla There is no tubes in his system. |
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Post by Boomzilla on Dec 30, 2021 12:40:18 GMT -5
There is no tubes in his system. Thanks, brutiarti - you're right. So it's just background noise of the preamp itself. Live with it or replace the preamp. Only two options. |
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KeithL
Administrator 
Posts: 10,269
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Post by KeithL on Dec 30, 2021 13:13:38 GMT -5
There is one more thing you can try...
Assuming that the hiss remains relatively constant regardless of the setting of the preamp's volume control (at least near the low end of the Volume range)... Then you probably are hearing the background noise of the preamp's output circuitry.... (In most analog preamps the Volume control is at the input and the output of the gain stage is connected directly to the output.)
What you can try is to put a pair of passive attenuators between the outputs of the preamp and the inputs of the amplifier. Simple resistor-type passive attenuators will reduce the level of the ENTIRE audio signal - including the background hiss. You will then be able to turn the level of the audio signal back up again by turning up the preamp's Volume control. (What you're doing is "gain matching" the preamp to the amp by reducing the gain of the preamp.)
Passive resistor attenuators will have no other effect on the audio quality... And they should only cost around $20 for a pair. (They come in different values... I would suggest trying 10 dB...)
You would be looking for something like these: (they also come in balanced) (you want "simple passive line level fixed attenuators)
There is no tubes in his system. Thanks, brutiarti - you're right. So it's just background noise of the preamp itself. Live with it or replace the preamp. Only two options. |
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KeithL
Administrator
Posts: 10,269
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Post by KeithL on Dec 30, 2021 13:15:48 GMT -5
So, yes, odds are that passive attenuators are just what you need. (The fact that you are currently using the Volume control way at the bottom of its range makes this almost certain.)
There is one more thing you can try... Assuming that the hiss remains relatively constant regardless of the setting of the preamp's volume control (at least near the low end of the Volume range)... Then you probably are hearing the background noise of the preamp's output circuitry.... (In most analog preamps the Volume control is at the input and the output of the gain stage is connected directly to the output.)
What you can try is to put a pair of passive attenuators between the outputs of the preamp and the inputs of the amplifier. Simple resistor-type passive attenuators will reduce the level of the ENTIRE audio signal - including the background hiss. You will then be able to turn the level of the audio signal back up again by turning up the preamp's Volume control. (What you're doing is "gain matching" the preamp to the amp by reducing the gain of the preamp.)
Passive resistor attenuators will have no other effect on the audio quality... And they should only cost around $20 for a pair. (They come in different values... I would suggest trying 10 dB...) You would be looking for something like these: (they also come in balanced) (you want "simple passive line level fixed attenuators)
Thanks, brutiarti - you're right. So it's just background noise of the preamp itself. Live with it or replace the preamp. Only two options. |
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Post by pedrocols on Dec 30, 2021 13:28:08 GMT -5
I had some issues as it is well known with the Emotiva USP-1 preamp where the volume will come up very aggressively loud and the passive attenuator alleviate the issue nicely.
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Post by EarFull on Dec 30, 2021 14:42:07 GMT -5
Thank all of you for your replies. Great ideas.
Yes, bet it is the background noise of the older Mcintosh C30 preamp.
Will get some attenuators and let you know.
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Post by Boomzilla on Dec 30, 2021 18:23:09 GMT -5
There is one more thing you can try... Assuming that the hiss remains relatively constant regardless of the setting of the preamp's volume control (at least near the low end of the Volume range)... Then you probably are hearing the background noise of the preamp's output circuitry.... (In most analog preamps the Volume control is at the input and the output of the gain stage is connected directly to the output.)
What you can try is to put a pair of passive attenuators between the outputs of the preamp and the inputs of the amplifier. Simple resistor-type passive attenuators will reduce the level of the ENTIRE audio signal - including the background hiss. You will then be able to turn the level of the audio signal back up again by turning up the preamp's Volume control. (What you're doing is "gain matching" the preamp to the amp by reducing the gain of the preamp.)
Passive resistor attenuators will have no other effect on the audio quality... And they should only cost around $20 for a pair. (They come in different values... I would suggest trying 10 dB...) You would be looking for something like these: (they also come in balanced) (you want "simple passive line level fixed attenuators)
As you point out - this would work ONLY if increasing the volume does NOT also increase the volume of the hiss. But for the price, it's sure worth trying! |
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Post by EarFull on Jan 7, 2022 17:14:25 GMT -5
HISS Followup: It WORKED !!!
Bought two attenuators from Express Parts -12db and now not only no hiss, but volume control on Mcintosh C30 preamp is more 'tame' and I can turn up volume greater than one notch. Thank you so much. No hiss, even when standing w ear at KHorns. Sounds better than ever.
Happy New Y-Ear !!!!
Patrick
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Post by 405x5 on Jan 8, 2022 9:09:34 GMT -5
Hi Keith Bought Mcintosh C30 preamp years ago and have had it tuned up at Audio Classics. Ran it using two Emo monoblocks. Output to 1978 KHorns which I love. Due to size and weight of monoblocks, traded in to Emotiva and got XPA DR2 Gen 2 stereo amp. I know, I know: way too much power as KHorns are so sensitive. Now, with this newer setup, there is an audible 'idle hiss' in both speakers. It must be coming from C30 Macintosh preamp because it goes away when it is turned off and only the Emotiva XPA DR2 is on. Again, this hiss isn't heard unless really quiet passages like classic music or piano. OR, if I put my ears up to the speakers. Been studying 'gain' and sensitivity and such. The preamp has notched volume attentuator so usually can't go past 1 notch or it is too loud for living room. Wondering if: 1. Connecting Emotiva CD player ERC4 into variable analog inputs might make a difference but this hiss occurs even without the ERC turned on. 2. Would Rothwell attenuator plugs help in this situation? -10db or -15db ? 3. I'm not 100% sure, but, I do NOT think this hiss was there when the monoblocks were used. Your thoughts are highly appreciated. Patrick 'Earfull' 1978 Klipschorns Emotiva XPA DR2 Gen 2 Amplifier Emotiva ERC4 CD Player 1986 Mcintosh C30 Preamplifier Logitech Bluetooth Connector to play from iPhone or iPad ….means you’re hearing is still up to snuff! (Forget the hiss and enjoy the music!) |
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