"Patrick Turner" <info@[EMAIL PROTECTED]
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"TT" <TTencerNoSpAm@[EMAIL PROTECTED]
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"Mark" <mark334@[EMAIL PROTECTED]
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"Iain Churches" <IainNG@[EMAIL PROTECTED]
> wrote in message
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>> >>>I have updated my page on measuring tube amp noise.
>> >>> Any comments/additions would be appreciated
>> >>>
http://www.kolumbus.fi/iain.churches/ThermionicThoughts/TubeAmpNoise.html
>> >>>
>> >> My tube amp makes a noise why I turn up the volume knob. Is that
what
>> >> you
>> >> mean? ...duh!
>> >> Well, you did say ANY comment would be appreaciated. ;)
>> >>
>> > Here's a comment then, "Don't give up your day job because you will
>> > starve
>> > trying to be a comedian". :-)
>> > BTW for the record this is one of the reasons why I don't have valve
>> > gear,
>> > the hiss, hum and other noises are a put off!
>> It is not too difficult to build a tube powered system
>> which is totally silent with the CD player on pause,
>> even ear-against-the-speaker. "Inky blackness" my
>> pal Richard calls it.
>>
>> My 50W PPP EL34 amp has a SNR of 106dB that's
>> 10dB better than a CD:-)))
> 50 watts into 8 ohms = 20Vrms of signal,
> and if the noise was 106dB below this, unweighted, 20Hz to 20kHz -3dB
> BW, then noise = 20V / 200,000 = 0.2mV.
>
> This is a fair result achieved easily with most tube power amps
> if competently designed and not faulty.
>
> As long as the noise stays low with an increase of signal level then the
> noise won't be heard.
>> http://www.kolumbus.fi/iain.churches/Pics/HomeAudioSystem/C50_002.jpg
>>
>> Tube amps usuallly have fairly simple linear PSUs
>> but they need quite a lot of good iron to perform well,
>> and this is first place where builders try to cut corners.
>> The amp shown above has a C-L-C-L-C-R-C supply
>> chain with each stage fed separately.
> To achieve the above good noise figures, the power supply filtering does
> not have to be so good for PP circuits and a basic CLC for OPT CT supply
> is
> usually very adequate, and Quad-II with their appalling toy like PS
simply
> relied on
> common mode rejection to allow a good noise performance.
>
> But the Quad22 preamp was not so hot.
>> Also special attention needs to be paid to the grounding
>> scheme (there is a good thread on this topic with plenty
>> of info on RAT at the moment) and also to the heater
>> and anode supplies. It can be done. It is well worth
>> the effort - especially if it's a homebrew amp:-)
> All that is basic good practice.
>> Chinese amps are not silent because, to cut costs,
>> they often have AC heaters and very basic supplies.
> Some very cheaply priced Chinese Kracker Amplifiers which go off in
> smoke after a month's use are very quiet, but also very poorly designed
in
> other
> ways, and they go noisy if at first they were not, simply because of the
> initial
> balance of Idc in each half of the OPT and the CMRR in class A PP, like
> Quad-II.
>
> AV heaters do help, but Quad-II never had AC heaters, and hum from
> heaters was low.
> You have referred us to your web page on tube amp noise where you
> introduce the subject with...
>
> """"""Measuring Tube Amplifier Noise.
> Unlike distortion, which is a produced by the amplifier and added
> to the input signal during the amplification process,
> amplifier noise is not signal related. It can sometimes be heard
> during low volume passages of music, and can be measured
> when there is no input signal present. Such measurements are
> usually taken with the input grounded.
>
> In a tube amplifier, the internal noise is caused by the
> components which comprise the circuit, such as tubes, resistors,
> capacitors, gain controls etc. The power supply may add ripple
> voltage, and there also may be magnetic coupling through
> the chassis which will cause hum.
>
> The noise level can be quoted in two ways, in decibels, referred
> to the maximum power output, or as a voltage. The first
> method may be misleading, as the amplifier under test may rarely
> be reproducing music at full power in the normal
> listening environment. """"""""
>
> I have a few comments.
>
> Amplifier noise can be signal related if the noise increases with
> increased signal.
> Class AB amps are subject to this phenomena since the PSU has to provide
> additional Idc to the output stage as the amp crosses over from the
class
> A to AB
> threshold.
>
> Most certainly, the noise of any amp should be measured with its input
> terminal
> grounded. For quietest noise performance, series resistance between the
> input terminal and grid should be minimal, about 3k3 maximum.
>
> Large anode swing signals in class AB are therefore in series with PSU
> ripple at the CT, and not rejected by CMRR, and this rectifier noise is
in
> the output
> unless excellent hum filtering is used along with a large value of C
> between the CT ands 0V.
>
> The noise of a typical 3 stage amplifier is usually determined by the
> FIRST tube in the line up.
>
> Adding NFB does nothing to reduce the noise in the input tube grid
> circuit which is responsible for most noise in most power and preamps.
>
> SO, if we have done our design homework we will have applied DC to the
> heaters of at least V1 of a 3 stage amp.
> We will have chosen the input tube carefully for low grid noise and
> microphony, especially im****tant with a phono or microphone amp.
>
> Suppose we have chosen a quiet 6CG7 for the input tube to a power amp.
>
> The grid noise with dc to the heaters will be perhaps 2uV.
>
> If the gain with NFB of the power amp is say 20x, or 26dB, then the
> noise at the output from grid input noise will be 20uV x 20 = 400uV, or
> 0.4mV.
> Usually hum from other sources within the amp might equal this hiss
> noise from the grid and the final outcome could thus be 1.41 x 0.4mV =
> 0.56mV
> and quite OK.
>
> Phono preamps and mic amps have lots more gain yet the grid noise is
> the same as above with 2uV being typical noise.
> So you need a large input signal to get a 60dB SNR.
>
> The tube preamp will be the amp which determines the noise figure.
>
> I have said a lot more on how to measure preamp tube noise at
> rec.audio.tubes, and how to test each triode for its noise by amplifying
> the anode
> output noise with grid grounded using a typical well measuring opamp
> preamplifier,
> gain = 1,000, and with 20Hz to 20kHz BW. Its noise will uisually be well
> below the
> tube noise which is being tested.
>
> Most ppl wouldn't use a tube to amplify a low level mic signal.
>
> They'd have a fet at the input, or a step up tranny.
>
> Ditto MC phono.
>
> With Phono, the BW of the amp is reduced from 20Kz-20kHz to 20Hz-50Hz
> because of the RIAA filter.
>
> The bandwidth is reduced by a factor of (50 - 20) / (20,000 = 20) =
> approx to 30/20,000 = 0.0015,
> and noise is reduced by the square root of this bandwidth reduction
> factor, ie, by a factor of 0.038
>
> So, if the 20kHz BW noise at the input was 2uV, and gain was say 10,000
> without RIAA then noise at the output = 2uV x 10,000 = 20mV, which is
loud
> as
> bugary!
>
> But with RIAA, noise becomes 20mV x 0.038 = 0.76mV, and mainly all
> rumble at LF and which does not offend the ear as much as it would if
the
> noise was
> unattenuated at say 1 kHz.
>
> SO, if the signal was 0.76Vrms at the output of the phono amp, we'd have
> an SNR = 60dB.
>
> In practice, this would be hopelessly too noisy unless we had a large
> input signal from an MM cart, and so to get a phono amp to be quiet with
> tubes we
> need a step up tranny or use a fet input stage which is tyically 10
times
> quieter than
> any tube, because of the higher transconductance.
>
> More about noise and its causes is in RDH4, and because j-fets were not
> invented in 1953 when RDH4 was being written, I suggest interested ppl
> search on
> the net and read many books to put themselves in touch with what makes a
> quiet
> preamp.
>
> A typical tube phono input preamp using a well chosen 12AX7
> meant for MM will need at least 2mV of average input signal to give a
> good enough SNR for most ppl, but 5mV is better, say from a Shure V15.
>
> If an MC cart with typically 0.4mV of rated input is used you should
> hear the amp noise being louder than the noise from an un-modulated
vinyl
> groove.
>
> Hence the need for the quieter fet input for MC.
>
> Step up trannies with 1:10 voltage ration will lift MC signals from
0.4mV
> to 4mV
> without adding noise.
>
> The typical output resistance of MC = 20ohms, and such a low Z means
> very low noise is made by this resistance.
> When the signal is inceased, the low noise of the 20 ohms is also
> increased, but the SNR is very good at this early point in the circuit,
> and better than
> the following amp.
>
> In the past very few commercial phono amps or phono stages within
> integrated preamps ever had a step up transformer for MC, unless it was
> used for
> professional studios or used in broadcast stations.
>
> I recall that after FM broadcasting was introduced to Oz in the 1970s,
> many vinyl records were used as signal source, and usually MC was used.
>
> MM was a basically a consumer friendly vinyl replayer, because a
> broken/worn stylus was so much cheaper than an MM stylus.
>
> I like MC better though.
>
> Mic amps are more noise prone because they have no filtering of HF.
>
> I have cross posted to rec.audio.tubes where there may be some other ppl
> able to comment and stay on topic, apart from the usual useless crew of
> no hopers
> who warp the topic into an abortion.
A most interesting & informative post Patrick. I hope to add something by
way of a reply tomorrow.
ruff
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