Iain Churches wrote:
>
> "TT" <TTencerNoSpAm@[EMAIL PROTECTED]
> wrote in message
> news:13ti8c1lmhp1i0b@[EMAIL PROTECTED]
> >
> > "Mark" <mark334@[EMAIL PROTECTED]
> wrote in message
> > news:BT6Cj.25151$421.14494@[EMAIL PROTECTED]
> >> "Iain Churches" <IainNG@[EMAIL PROTECTED]
> wrote in message
> >> news:4dCAj.307829$By6.57209@[EMAIL PROTECTED]
> >>>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!
> >>
> >>> Iain
> >>>
> >>>
> >>>
> >>>
> >>
> >> 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.
>
> Iain
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.
Patrick Turner.
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