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Old 16th November 2007, 09:34 PM   #1
pchw is offline pchw  United States
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Default How quiet is dead quiet?

I have seen people saying their tube preamps or power amps were dead quiet, but exactly what is considered as dead quiet? I have to admit that my DIY experience is short and lack of any EE background, but I have been able to scratch build from someone else's complete or partial design successful. While I managed to build some that are hum free, none of them is hiss free. Some hiss louder than the others. Due to this inperfection, I haven't built one "dead quiet" tube amp of any kind ....... So, what is your definition of "dead quiet"? Also, it is probably impossible to eliminate hiss, but what are the techniques to reduce them?
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Old 16th November 2007, 09:42 PM   #2
SY is offline SY  United States
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"Quiet" for me means that at 1M or more from my speakers, I cannot discern any noise. "Dead quiet" for me means that with my ears within an inch or two of the speakers, I cannot discern any noise.

There are a number of things that determine noise in a circuit: the thermal noise from the tubes, thermal and excess noise from plate load resistors, and power supply noise and ripple are all very important factors to try to minimize.
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Old 16th November 2007, 09:45 PM   #3
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Quote:
Originally posted by SY
"Quiet" for me means that at 1M or more from my speakers, I cannot discern any noise. "Dead quiet" for me means that with my ears within an inch or two of the speakers, I cannot discern any noise.
I fully agree with Sy's definition. However, keep in my that what may be quiet in one system may be louder in another. This is especially true for those of us who have very high efficiency horns - in my case 106dB efficient!!!

-- josť k.
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Old 16th November 2007, 10:34 PM   #4
pchw is offline pchw  United States
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LOL, my lame way to reduce noise is to use inefficient speakers. The most efficient that I have is at best 89dB :-) Seriously, those were inherited from sand amp era. I am going to build a pair based on FE207E just to get a taste of FR. That's the reason I posted the thread. Relatively quiet from a 89dB pair can be a totally different animal with the 95dB fostex!!

SY, what is the typical technique to deal with the thermal tube and load resistor noise? Better resistors, or choke load?

Thanks!!
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Old 16th November 2007, 11:30 PM   #5
Jeb-D. is offline Jeb-D.  United States
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Your method of noise reduction by using poor efficiency speakers is common for car audio. But they have a noise infested ground to deal with.

Reduce hiss:
-Don't use over sized grid stoppers.
-Done use over sized grid pull-down resistors
-Use the lowest Rp tubes that can still get the job done
-Don't use plate resistors larger than needed to get the job done.
-Use resistors with a power rating quite a bit larger than needed.
-Use Metal Film and Non-inductive wire wound resistors.
-Use higher Gm tubes when the option is available.
-DC heaters and/or referencing heaters above ground potential can help hiss and hum depending upon the circumstance.

Reduce hum:
-Proper grounding techniques (this takes research + trial and error to master).
-Have enough power supply filtering
-Proper physical layout techniques (don't place input stages next to the power transformer, don't route signal next to heater, ect)
- When possible, twist the conductors that have equally opposite current flow.
- When routing/wiring power supply, go from the transformer to the rectifier to the filtering caps to the load. Do not ground or get power for the load at any physical point in between.
-DC heaters and/or referencing heaters above ground potential can help hiss and hum depending upon the circumstance.
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Old 17th November 2007, 01:15 AM   #6
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One more suggestion to avoid hiss: don't use small-signal pentodes as voltage amplifiers. If you need that much gain in a single stage, you could try a cascode,
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Old 17th November 2007, 04:40 AM   #7
SY is offline SY  United States
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pchw, that's a difficult question to answer generally. Much of it has to do with choices in basic circuit design. But... wirewound resistors have the lowest excess noise (though the same Johnson noise) of all resistor types. Tube noise usually goes as the inverse of transconductance. Good power supply rejection minimizes noise from that source. And grounding/layout are critical.

(Broken record time) If you don't have a copy of "Valve Amplifiers," run right out and get it. There is a detailed discussion of noise in tube amps and how to design properly to minimize it.
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Old 17th November 2007, 05:07 AM   #8
pchw is offline pchw  United States
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Yes, I have both MJ's books - hard copies, not printed from Google :-)
They are indeed very informative. The amps that I built after I owned the books definitely have lower noise the the early ones. There is a wealth of advise here. I will reread MJ's books, in particular, those points mentioned here.

Thanks!!
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Old 17th November 2007, 11:40 PM   #9
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Quote:
Originally posted by ray_moth
One more suggestion to avoid hiss: don't use small-signal pentodes as voltage amplifiers. If you need that much gain in a single stage, you could try a cascode
A more general rule is to look at the overall gain of the circuit and use the right amount of gain that will get the job done. It is better to have two gain stages than a single stage operating at max gain with no headroom.

Excessive gain = Excessive noise

For example, when listening to the Cary 805 (6SL7, 300B,845) amp using my 106dB horns, I have to subsitute a 6SN7 to lower the gain and bring down the hiss.

-- josť k.
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Old 18th November 2007, 03:06 AM   #10
jnb is offline jnb  Australia
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Good point korneluk. Using high efficiency speakers with "standard" equipment will simply increase noise (and the signal at the same time), but this is rectified when the gain is corrected for the system as a whole.
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