Re: MC PHONO PREAMP



Ian Bell wrote:

Patrick Turner wrote:

Ian Bell wrote:
Patrick Turner wrote:
Ian Bell wrote:
Patrick Turner wrote:
Ian Bell wrote:
Patrick Turner wrote:

snip

I don't see where RDH4 says the noise depends on frequency, or that the
EINR was calculated based on measurements at RF.


It doesn't AFAIK. However, the title of the Harris paper that derived
the 2.5/gm formula makes it clear he is talking about high frequencies.
Chech out the reference at the end of that chapter in RDH4.

There's a reference to Harris, page 946, item B27, but no reference to
RF.

On page 937, there's a table attributable to Harris with formulas for
noise and the table has 4 sections, triode amps, pentode amps, triode
mixers, pentode mixer, multigrid mixer.

The only simple formula for noise resistance is the first one for triode
noise resistance, 2.5/gm., and there's no mention of RF specificness, so
I think that 2.5/gm is the official triode EINR formula regardless of F.

All the others look difficult to work out, but none have reactive
components in the formula.

If you use a 6BE6 as a mixer for two inputs to make one output then
it'll be noiser than if you have two triodes with a common anode load.

All that's important is that triodes are quieter than all other types of
tubes and that the higher the gm the better, and then all one needs to
consider is to have Ia high, and Ea low right at the input of a phono
stage, and hope it works out.

Usually it does for MM. And when Denon invented an MC with less than 20
ohms Rout, and with very low Vout, a transformer with 1:10 ratio was the
normal way to up the Vo and yet keep noise lower than with MM. The ZR of
100 meant that 20 ohms became only 2,000 ohms from grid to 0V of a tube
input stage. If EINR of 2,000 ohms gives 2uV of noise, then 20 ohms
makes about 0.2uV, and when transformed up this noise becomes 2uV, and
when added to the series 2uV of the tube you get 2.8uV. The cart output
of 0.3mV has become 3mV, and snr = -60dB, and thus just OK considering
vinyl noise will be higher.

Now if you have a 2sk369 j-fet with 0.14uV input noise ( typical ) and
0.4mV of rated MC cart Vo, then snr = -69dB, and there's less LF noise
so less grumble in the signal.

It was j-fets that allowed a whole new generation of bugs to be
developed for spying because it was far easier to conceal a little
battery and a fet than have to wire up a triode. AND the fet picked up
more clean sound without the noise of itself ruining the time spent by
CIA operatives in vans outside the russian embassy.



The rules about noise are valid right down to dc once you have said that
the noise is due to a resistance.

So where you have bandwidth of dc to 20kHz, and say you have an R giving
2uV of noise, then if R is reduced by 10 then noise reduces by 1/3.16 or
about a third. And if the bandwidth is effectively reduced from 20khz to
50 as it is with an RIAA filter then noise is reduced the square root of
the amount of bandwidth reduction, ie, sq.rt ( 50 / 20,000 ) = 1 / 20,
or -26dB.


Where did that factor of 50 come from. According to Morgan Jones it is
nearer 1.

50 is the new bandwidth after RIAA is added to the circuit because the
response is dc to -3db at 50Hz. BW was 20kHz before RIAA.
Therefore BW reduction factor = 50Hz / 20,000Hz = 1/400. The noise
reduction factor is the square root of bandwidth reduction = sq.rt of
1/400 = 1/20 = -26dB.

Now in vinyl recording they boost the hell out of treble above the low
bass reference, +40dB at 22 kHz compared to 10Hz.
So the RIAA filter flattens the music response and disposes of most of
the noise. The ears of most people don't notice LF noise as much as
noise above say 300Hz. Now because tubes make a fair amount of LF noise,
the theoretical -26dB noise reduction doesn't get achieved.
There will still be some noise. But on good vinyl its often tape hiss or
venue noise which is the highest noise of the recording. In FM radio HF
is boosted or "emphasised" and after detection the signal is
de-emphasized. Exit much noise.

The reference signal to make a final perception of all this is of course
at 1kHz at which most signal Vout figures for carts are quoted.
So if you have 3mV from an MM cart, there's only 0.3mV at 10hz and 30mV
at 22Khz An MC bass reference is 0.03mV.

I may not have all this 100% correct but Allen Wright's preamp cookbook
has a lot more.

Everything is relative though, and vinyl is very listenable when its
good and turn tables keep being designed and sold at very high prices in
hi-fi shops. Its nice to hear music straight from a black groove. But
TTs are also designed and sold in less salubrious emporioums where the
poor buy plastic contraptions for $100 to move their LP music to CD.
This is silly of course, but people like to feel they have saved the
past even though the vast majority of pop music has no relevance to life
being lived well now and never did 40 years ago.

So, at the end of the day if we want to use MC and get lowest amp noise
we may choose between a fet, a step up tranny, or carefully selected
triodes in about that order. Some say transformers veil the sound and so
forth. Vinyl is the land of claims.

Patrick Turner
.

Relevant Pages

  • Re: 6SN7 Mu Follower Distortion Paper
    ... Its not clear how you would go about biasing the bottom triode with Eg = ... swing is from 137V down about 85pkV before grid current so the max Vrms ... For measuring noise in such a circuit, the bottom grid should be taken ...
    (rec.audio.tubes)
  • Re: 6SN7 Mu Follower Distortion Paper
    ... mu follower and posted it at my website. ... This way the working Ea for top and bottom ... load R to replace the top triode acting as a follower and buffer between ... For measuring noise in such a circuit, the bottom grid should be taken ...
    (rec.audio.tubes)
  • Re: MC PHONO PREAMP
    ... 'Fluctuations in Space Charge Limited Currents at Moderately High Frequencies, Part V - Fluctuations in Vacuum Tube Amplifiers and Input Systems' ... noise and the table has 4 sections, triode amps, pentode amps, triode ... The only simple formula for noise resistance is the first one for triode ... Now in vinyl recording they boost the hell out of treble above the low ...
    (rec.audio.tubes)
  • Re: 6SN7 Mu Follower Distortion Paper
    ... mu follower and posted it at my website. ... Its not clear how you would go about biasing the bottom triode with Eg = ... For measuring noise in such a circuit, the bottom grid should be taken ...
    (rec.audio.tubes)
  • Re: 6SN7 Mu Follower Distortion Paper
    ... This way the working Ea for top and bottom ... load line for 200k. ... load R to replace the top triode acting as a follower and buffer between ... For measuring noise in such a circuit, the bottom grid should be taken ...
    (rec.audio.tubes)