That power amp has... Jitter?
 

In my travels I've discovered a source of technical tests right here in the
UK.

It is the Miller Audio Research Avtech web site:
http://www.milleraudioresearch.com/avtech/index.html

Miller Reasearch is well-known for their heavy involvement with testing
audio gear for jitter, but they also do most if not all of the classic
tests.

They do require you to register for a (generic) userid and password, but its
via autoresponder and all they want is some name and an email address.

At any rate I noticed a test that makes me smile: atest of a legacy Leak
20.

http://www.milleraudioresearch.com/d...stereo_20.html

Now we all expect legacy power amps to be a little dirty, but this one has
seems to have an unexpected featu jitter.

Take a look at the FFT plot for "Stereo Continuity" (first FFT plot) and
tell me that you don't see the classic symmetric spurious response pattern
characteristic of modulation distortion around the fundamental and first few
harmonics.

Now, I guess we can't tell if this is AM distortion or FM distortion, but
modulation distortion it is for sure. It appears that the source of the
modulating signal is the power line (50 Hz).

Any speculation about how this came to be? My first 2 guesses being tons of
hum in the power supply and directly heated cathodes don't seem to fit with
other evidence at hand, including a schematic.

That power amp has... Jitter?
 

"Arny Krueger"

In my travels I've discovered a source of technical tests right here in
the UK.

It is the Miller Audio Research Avtech web site:
http://www.milleraudioresearch.com/avtech/index.html


** No " sign in " sites PLEASE !!

Miller Reasearch is well-known for their heavy involvement with testing
audio gear for jitter,

** So they are full-on lunatics from the dark side of audio.


At any rate I noticed a test that makes me smile: atest of a legacy Leak
20.

http://www.milleraudioresearch.com/d...stereo_20.html

Now we all expect legacy power amps to be a little dirty, but this one has
seems to have an unexpected featu jitter.

Take a look at the FFT plot for "Stereo Continuity" (first FFT plot) and
tell me that you don't see the classic symmetric spurious response pattern
characteristic of modulation distortion around the fundamental and first
few harmonics.

Now, I guess we can't tell if this is AM distortion or FM distortion, but
modulation distortion it is for sure. It appears that the source of the
modulating signal is the power line (50 Hz).

Any speculation about how this came to be? My first 2 guesses being tons
of hum in the power supply and directly heated cathodes don't seem to fit
with other evidence at hand, including a schematic.

** See schematic:

http://www.reocities.com/stereo20/dwnloads/stereo20.gif

The LEAK Stereo 20 is an ULTRALINEAR tube/valve power amp.

One thing that all UL output stages are very susceptible to is *supply
ripple voltage* !!

Why? Cos unlike most class A or AB tube PP stages, the screen supply is
subjected to the SAME ripple voltage as the plates are. If there is
significant ripple on the plate supply - it MODULATES the gain of the
output stage.

( Some AM transmitters work exactly this way)

Soooo, if you elect to use a UL output stage - you have just got to filter
the DC supply properly. This pretty much means using an iron cored choke in
the HT filter circuit to reduce the ripple voltage to well under 1 volt p-p.

Harold Leak cheap skated and went for a 100 ohm resistor instead of a choke.

Ergo, the output stage has some residual 100 / 120 Hz amplitude modulation
that the NFB cannot fully correct.


BTW:

The famous Fender " Super Twin" guitar amp has a MASSIVE dose of the same
problem. The output stage uses 6 x 6L6GCs in UL with SFA filtering of the HT
supply - so the ripple is about 40 volts p-p at full power.

The modulation is so bad a deaf person can hear it on a 1kHz sine wave and
it makes the amp sound just awful on guitar chords. Big mistake.

The amp is a notorious lemon as a result.


..... Phil

That power amp has... Jitter?
 

In article , Arny
Krueger
wrote:
In my travels I've discovered a source of technical tests right here in
the UK.

It is the Miller Audio Research Avtech web site:
http://www.milleraudioresearch.com/avtech/index.html

FWIW Paul is also the editor of HFN. (You may have noticed that JA also
uses his test gear for various purposes in Stereophile measurements.)

They do require you to register for a (generic) userid and password, but
its via autoresponder and all they want is some name and an email
address.

At any rate I noticed a test that makes me smile: atest of a legacy
Leak 20.

http://www.milleraudioresearch.com/d...stereo_20.html

Now we all expect legacy power amps to be a little dirty, but this one
has seems to have an unexpected featu jitter.

Take a look at the FFT plot for "Stereo Continuity" (first FFT plot)
and tell me that you don't see the classic symmetric spurious response
pattern characteristic of modulation distortion around the fundamental
and first few harmonics.

Now, I guess we can't tell if this is AM distortion or FM distortion,
but modulation distortion it is for sure. It appears that the source of
the modulating signal is the power line (50 Hz).

Can't say for sure. But to me it looks like gain modulation by the rail
ripple.

The article in the March 2010 issue of HFN from which the measurements stem
says that the Leak 20 tested was refurbished by John Howes. This meant that
all the caps, resistors, and the output (Mullard) EL84s were 'new'. However
I have no idea how well the amp was redone. Like yourself I only have the
results to go on. Paul Miller did the measurements, but the article was by
John Howes and Ken Kessler.

Personally I always thought Leak was a better showman and self-publicist
than a maker. But YMMV. :-)

Slainte,

Jim

--
Please use the address on the audiomisc page if you wish to email me.
Electronics http://www.st-and.ac.uk/~www_pa/Scot...o/electron.htm
Armstrong Audio http://www.audiomisc.co.uk/Armstrong/armstrong.html
Audio Misc http://www.audiomisc.co.uk/index.html

That power amp has... Jitter?
 

A lot of valve amps seemed to have a 50hz 'watermark' effect. If you look at
some older mags I seem to recall they often filtered the 50 hz out to make
the rest of the graphs less wobbly. Its probably been there for ever.
it is hard to completely get rid of hum components which can get in and
intermodulate of course.

Brian

--
Brian Gaff....Note, this account does not accept Bcc: email.
graphics are great, but the blind can't hear them
Email:
__________________________________________________ __________________________________________________ __________


"Arny Krueger" wrote in message
...
In my travels I've discovered a source of technical tests right here in
the UK.

It is the Miller Audio Research Avtech web site:
http://www.milleraudioresearch.com/avtech/index.html

Miller Reasearch is well-known for their heavy involvement with testing
audio gear for jitter, but they also do most if not all of the classic
tests.

They do require you to register for a (generic) userid and password, but
its via autoresponder and all they want is some name and an email address.

At any rate I noticed a test that makes me smile: atest of a legacy Leak
20.

http://www.milleraudioresearch.com/d...stereo_20.html

Now we all expect legacy power amps to be a little dirty, but this one has
seems to have an unexpected featu jitter.

Take a look at the FFT plot for "Stereo Continuity" (first FFT plot) and
tell me that you don't see the classic symmetric spurious response pattern
characteristic of modulation distortion around the fundamental and first
few harmonics.

Now, I guess we can't tell if this is AM distortion or FM distortion, but
modulation distortion it is for sure. It appears that the source of the
modulating signal is the power line (50 Hz).

Any speculation about how this came to be? My first 2 guesses being tons
of hum in the power supply and directly heated cathodes don't seem to fit
with other evidence at hand, including a schematic.

That power amp has... Jitter?
 

Also with older amps, unless great care in checking and replacing components
is taken can have aged capacitors which make this sort of thing far far
worse at all harmonics of the ripple.
Brian

--
Brian Gaff....Note, this account does not accept Bcc: email.
graphics are great, but the blind can't hear them
Email:
__________________________________________________ __________________________________________________ __________


"Phil Allison" wrote in message
...

"Arny Krueger"

In my travels I've discovered a source of technical tests right here in
the UK.

It is the Miller Audio Research Avtech web site:
http://www.milleraudioresearch.com/avtech/index.html


** No " sign in " sites PLEASE !!

Miller Reasearch is well-known for their heavy involvement with testing
audio gear for jitter,

** So they are full-on lunatics from the dark side of audio.


At any rate I noticed a test that makes me smile: atest of a legacy
Leak 20.

http://www.milleraudioresearch.com/d...stereo_20.html

Now we all expect legacy power amps to be a little dirty, but this one
has seems to have an unexpected featu jitter.

Take a look at the FFT plot for "Stereo Continuity" (first FFT plot) and
tell me that you don't see the classic symmetric spurious response
pattern characteristic of modulation distortion around the fundamental
and first few harmonics.

Now, I guess we can't tell if this is AM distortion or FM distortion, but
modulation distortion it is for sure. It appears that the source of the
modulating signal is the power line (50 Hz).

Any speculation about how this came to be? My first 2 guesses being tons
of hum in the power supply and directly heated cathodes don't seem to fit
with other evidence at hand, including a schematic.

** See schematic:

http://www.reocities.com/stereo20/dwnloads/stereo20.gif

The LEAK Stereo 20 is an ULTRALINEAR tube/valve power amp.

One thing that all UL output stages are very susceptible to is *supply
ripple voltage* !!

Why? Cos unlike most class A or AB tube PP stages, the screen supply is
subjected to the SAME ripple voltage as the plates are. If there is
significant ripple on the plate supply - it MODULATES the gain of the
output stage.

( Some AM transmitters work exactly this way)

Soooo, if you elect to use a UL output stage - you have just got to
filter the DC supply properly. This pretty much means using an iron cored
choke in the HT filter circuit to reduce the ripple voltage to well under
1 volt p-p.

Harold Leak cheap skated and went for a 100 ohm resistor instead of a
choke.

Ergo, the output stage has some residual 100 / 120 Hz amplitude modulation
that the NFB cannot fully correct.


BTW:

The famous Fender " Super Twin" guitar amp has a MASSIVE dose of the same
problem. The output stage uses 6 x 6L6GCs in UL with SFA filtering of the
HT supply - so the ripple is about 40 volts p-p at full power.

The modulation is so bad a deaf person can hear it on a 1kHz sine wave and
it makes the amp sound just awful on guitar chords. Big mistake.

The amp is a notorious lemon as a result.


.... Phil

That power amp has... Jitter?
 

In article , Brian Gaff
wrote:
A lot of valve amps seemed to have a 50hz 'watermark' effect. If you
look at some older mags I seem to recall they often filtered the 50 hz
out to make the rest of the graphs less wobbly. Its probably been
there for ever. it is hard to completely get rid of hum components which
can get in and intermodulate of course.

It seems to be one of the "In theory, theory and and practice agree, but in
practice they don't!" issues. :-)

In theory using a pair of output devices 'balanced' via a push-pull output
transformer shoud reject line ripple. But this relies on various
assumptions that don't always work in a real system.

FWIW I couldn't see the graphs very easily so could not tell if the
sidebands were +/- 50 Hz or +/- 100 Hz.

I assumed 100 Hz as that would correspond to full-wave rectified ripple
with a symmetric rectifier system. But if it is 50Hz it may be due to
either imbalances, or maybe gain modulation from the heating ac not being
balanced in its effect on the gain of the valves. So might not be line
ripple but an effect due to the heaters and maybe poor dressing of the
wiring.

The article says all the caps, resistors, etc, were replaced by 'good' new
items. However it is all too easy when modifying or rebuilding an amplifier
not to take into account details like the dressing of the wiring loom or
effects of component interactions via stray fields. Then if you check THD
you miss the tell-tale components as they aren't harmonics of the test
sinewave. :-)

Slainte,

Jim

--
Please use the address on the audiomisc page if you wish to email me.
Electronics http://www.st-and.ac.uk/~www_pa/Scot...o/electron.htm
Armstrong Audio http://www.audiomisc.co.uk/Armstrong/armstrong.html
Audio Misc http://www.audiomisc.co.uk/index.html

That power amp has... Jitter?
 

"Brian Gaff"

** What a good surname ....


A lot of valve amps seemed to have a 50hz 'watermark' effect. If you look
at some older mags I seem to recall they often filtered the 50 hz out to
make the rest of the graphs less wobbly. Its probably been there for
ever.
it is hard to completely get rid of hum components which can get in and
intermodulate of course.


** Simple 50 / 60 Hz ( plus harmonics) injection from internal AC wiring and
magnetic fields will never cause " intermodulation " of the AM or FM sort.
Just a background noise.

The real cause of the so called " jitter " is particular to the amp in
question as is just as I have described and Jim L has confirmed.



.... Phil

That power amp has... Jitter?
 

I'm glad I chanced by and had a look as this has made me giggle..
Arny just discovers PM's jitter measurement suite, which is the
de facto 16 bit jitter test suite and has been for past 20 years
(Though I do remember some threads whereby Jim tried to hint
at something?? I may try and dig them out as it was curious Jim
may have been trying to undermine his boss's jitter testing?)
So amateur Arny was unaware that PM is a long time regular
contributor to Stereophile, that PM and JA are close personel friends
and that for 20+ years it is PM's J.M.Suite used for 16 bit data in
Stereophile, - but JA uses another jitter measurement suite for 24bit.
You must be feeling very silly Arny.
And the strange Phil has resurfaced in the original thread in the
almost dead UK group. It should be good for a larf....


"Arny Krueger" wrote in message
...
In my travels I've discovered a source of technical tests right here in
the UK.

It is the Miller Audio Research Avtech web site:
http://www.milleraudioresearch.com/avtech/index.html

Miller Reasearch is well-known for their heavy involvement with testing
audio gear for jitter, but they also do most if not all of the classic
tests.

They do require you to register for a (generic) userid and password, but
its via autoresponder and all they want is some name and an email address.

At any rate I noticed a test that makes me smile: atest of a legacy Leak
20.

http://www.milleraudioresearch.com/d...stereo_20.html

Now we all expect legacy power amps to be a little dirty, but this one has
seems to have an unexpected featu jitter.

Take a look at the FFT plot for "Stereo Continuity" (first FFT plot) and
tell me that you don't see the classic symmetric spurious response pattern
characteristic of modulation distortion around the fundamental and first
few harmonics.

Now, I guess we can't tell if this is AM distortion or FM distortion, but
modulation distortion it is for sure. It appears that the source of the
modulating signal is the power line (50 Hz).

Any speculation about how this came to be? My first 2 guesses being tons
of hum in the power supply and directly heated cathodes don't seem to fit
with other evidence at hand, including a schematic.

That power amp has... Jitter?
 

"FedUpLurker"

And the strange Phil has resurfaced in the original thread in the
almost dead UK group. It should be good for a larf....


** Any audiophool out there who feels his or her listening pleasure is being
RUINED by that DAMN jitter !!

- just needs to put on some warm clothing or heat the damn room !

Maybe take a sip of gin ....

Puff of weed.

All helps.



.... Phil

That power amp has... Jitter?
 

"FedUpLurker" wrote in message


I'm glad I chanced by and had a look as this has made me
giggle..

As usual lurker, your rant is a self-serving total fabrication of your
overheated mind that is caught up in your usual rush to judgement. If you
were serious you wouldn't hide behind an untracable alias.

Arny just discovers PM's jitter measurement suite, which
is the de facto 16 bit jitter test suite and has been for past
20 years (Though I do remember some threads whereby Jim
tried to hint at something?? I may try and dig them out as it was
curious Jim may have been trying to undermine his boss's jitter
testing?)

I've known about PM's poorly founded voyages into the world of jitter nearly
from their onset. I suspect I could find posts from me saying as much in the
rec.audio.opinion archives.

The audiophile myths that Paul Miller perpetuates (probably unintentionally)
is that the perception of jitter can be characterized by one number, and
that this number can be measured with one or two test signals.

What the jitterbugs need to explain is how we tolerated analog tape all
those years when it has about a million picoseconds of jitter at frequencies
where the ear is highly sensitive to it. You can answer this question at
your leisure! ;-)

Note that in your case, its more like liesure than leisure.