[Iain Churches]

I have noticed that a number of transformer
manufacturers are offering an audio attenuator
transformer with a 24-tap secondary, for
use as a preferred option to a passive controller.

Sowter says, on their website:
quote

Attenuator transformers used in conjunction with a multi position switch
provide a high performance volume control function. Sowter proprietary
winding technique ensures 100 kHz bandwidth at all switch positions. The
transformer action of these attenuators ensures the loading circuit sees a
low driving impedance regardless of the switch position. This ensures the
maximum bandwidth potential of the whole circuit can be realised at all gain
settings. As the attenuation is related to the number of turns at the
winding tap, the attenuation is exactly matched between stereo channels.

unquote.

Stevens and Billington say, on their website:

Quote:

The primary application is in so-called passive preamplifiers, more
precisely passive control units. As such it allows a number of functions,
the most important being the ability to change the output level of the
passive control in multiple steps to control the volume of a high fidelity
system. Applied in a passive control unit the TX-102 by applying transformer
technology offers a material improvement over all previous such schemes
based around resistive attenuation.

The so-called passive preamplifier appeared on the map of High Performance
Audio with the emergence of the CD Player. Modern Sources, such as
CD-Players, DVD Players, SACD Players and others generally offer output
levels sufficient to drive power amplifiers to full power (usually 2V RMS or
more for digital full scale) and also offer sufficient drive for external
devices and cables. Many CD-Players and similar devices have output
impedances lower than 1kOhm, some are materially lower. Whilst "passive
preamplifiers" initially created notable interest as a sonically extremely
pure method of controlling volume and selecting inputs, they have soon faded
back into obscurity, not surprisingly as most of these devices suffered from
substantial impedance mismatches with either sources or loads.


If for example a 5k Ohm resistive volume control were to be employed in a
passive control unit the source would be required to drive all the time a
quite severe load of 5k Ohm. If combined with a 1k Ohm source impedance the
worst case output impedance of the combination would be 1500 Ohm at -6db
attenuation, while at -20db attenuation the output impedance would still be
around 540 Ohm. If combined with around 1nF load capacitance (easily found
in longer, high capacitance interconnects), this leads to a 0.3db
attenuation at 20kHz for a 20db attenuation setting, practically showing the
absolute permissible limit for load capacitance. The worst-case attenuation
at 20kHz almost reaches 1db!!! If, to provide our source with an easier load
we choose a 50 kOhm resistive volume control we must either accept
drastically higher levels of roll off at 20kHz or we must limit the load
capacitance to less than 100pF. Such a level of capacitance (100pF) can
easily be found with only 1m of high quality interconnect cable and is often
exceeded by the input capacitance of many amplifiers!

Thus the resistive volume control employed in passive control units must
navigate a course between the Scylla of excessively loading the source,
leading to increased distortion and the Charybdis of excessively high output
impedance.

unquote.

Iain asks.
1.Does anyone have experience of such transformers?
2. Are they actually better than a passive stepped attenuator controller?



Regards to all

Iain

[Peter Wieck]

On Jun 6, 3:55 pm, "Iain Churches" wrote:
I have noticed that a number of transformer
manufacturers are offering an audio attenuator
transformer with a 24-tap secondary, for
use as a preferred option to a passive controller.

Sowter says, on their website:
quote

Attenuator transformers used in conjunction with a multi position switch
provide a high performance volume control function. Sowter proprietary
winding technique ensures 100 kHz bandwidth at all switch positions. The
transformer action of these attenuators ensures the loading circuit sees a
low driving impedance regardless of the switch position. This ensures the
maximum bandwidth potential of the whole circuit can be realised at all gain
settings. As the attenuation is related to the number of turns at the
winding tap, the attenuation is exactly matched between stereo channels.

unquote.

Stevens and Billington say, on their website:

Quote:

The primary application is in so-called passive preamplifiers, more
precisely passive control units. As such it allows a number of functions,
the most important being the ability to change the output level of the
passive control in multiple steps to control the volume of a high fidelity
system. Applied in a passive control unit the TX-102 by applying transformer
technology offers a material improvement over all previous such schemes
based around resistive attenuation.

The so-called passive preamplifier appeared on the map of High Performance
Audio with the emergence of the CD Player. Modern Sources, such as
CD-Players, DVD Players, SACD Players and others generally offer output
levels sufficient to drive power amplifiers to full power (usually 2V RMS or
more for digital full scale) and also offer sufficient drive for external
devices and cables. Many CD-Players and similar devices have output
impedances lower than 1kOhm, some are materially lower. Whilst "passive
preamplifiers" initially created notable interest as a sonically extremely
pure method of controlling volume and selecting inputs, they have soon faded
back into obscurity, not surprisingly as most of these devices suffered from
substantial impedance mismatches with either sources or loads.

If for example a 5k Ohm resistive volume control were to be employed in a
passive control unit the source would be required to drive all the time a
quite severe load of 5k Ohm. If combined with a 1k Ohm source impedance the
worst case output impedance of the combination would be 1500 Ohm at -6db
attenuation, while at -20db attenuation the output impedance would still be
around 540 Ohm. If combined with around 1nF load capacitance (easily found
in longer, high capacitance interconnects), this leads to a 0.3db
attenuation at 20kHz for a 20db attenuation setting, practically showing the
absolute permissible limit for load capacitance. The worst-case attenuation
at 20kHz almost reaches 1db!!! If, to provide our source with an easier load
we choose a 50 kOhm resistive volume control we must either accept
drastically higher levels of roll off at 20kHz or we must limit the load
capacitance to less than 100pF. Such a level of capacitance (100pF) can
easily be found with only 1m of high quality interconnect cable and is often
exceeded by the input capacitance of many amplifiers!

Thus the resistive volume control employed in passive control units must
navigate a course between the Scylla of excessively loading the source,
leading to increased distortion and the Charybdis of excessively high output
impedance.

unquote.

Iain asks.
1.Does anyone have experience of such transformers?
2. Are they actually better than a passive stepped attenuator controller?

Regards to all

Iain

This thread... I am watching with great interest.
Thank you, Iain.

Peter Wieck
Wyncote, PA

[Iain Churches]

"Peter Wieck" wrote in message
oups.com...
On Jun 6, 3:55 pm, "Iain Churches" wrote:
I have noticed that a number of transformer
manufacturers are offering an audio attenuator
transformer with a 24-tap secondary, for
use as a preferred option to a passive controller.

Sowter says, on their website:
quote

Attenuator transformers used in conjunction with a multi position switch
provide a high performance volume control function. Sowter proprietary
winding technique ensures 100 kHz bandwidth at all switch positions. The
transformer action of these attenuators ensures the loading circuit sees
a
low driving impedance regardless of the switch position. This ensures
the
maximum bandwidth potential of the whole circuit can be realised at all
gain
settings. As the attenuation is related to the number of turns at the
winding tap, the attenuation is exactly matched between stereo channels.

unquote.

Stevens and Billington say, on their website:

Quote:

The primary application is in so-called passive preamplifiers, more
precisely passive control units. As such it allows a number of functions,
the most important being the ability to change the output level of the
passive control in multiple steps to control the volume of a high
fidelity
system. Applied in a passive control unit the TX-102 by applying
transformer
technology offers a material improvement over all previous such schemes
based around resistive attenuation.

The so-called passive preamplifier appeared on the map of High
Performance
Audio with the emergence of the CD Player. Modern Sources, such as
CD-Players, DVD Players, SACD Players and others generally offer output
levels sufficient to drive power amplifiers to full power (usually 2V RMS
or
more for digital full scale) and also offer sufficient drive for external
devices and cables. Many CD-Players and similar devices have output
impedances lower than 1kOhm, some are materially lower. Whilst "passive
preamplifiers" initially created notable interest as a sonically
extremely
pure method of controlling volume and selecting inputs, they have soon
faded
back into obscurity, not surprisingly as most of these devices suffered
from
substantial impedance mismatches with either sources or loads.

If for example a 5k Ohm resistive volume control were to be employed in a
passive control unit the source would be required to drive all the time a
quite severe load of 5k Ohm. If combined with a 1k Ohm source impedance
the
worst case output impedance of the combination would be 1500 Ohm at -6db
attenuation, while at -20db attenuation the output impedance would still
be
around 540 Ohm. If combined with around 1nF load capacitance (easily
found
in longer, high capacitance interconnects), this leads to a 0.3db
attenuation at 20kHz for a 20db attenuation setting, practically showing
the
absolute permissible limit for load capacitance. The worst-case
attenuation
at 20kHz almost reaches 1db!!! If, to provide our source with an easier
load
we choose a 50 kOhm resistive volume control we must either accept
drastically higher levels of roll off at 20kHz or we must limit the load
capacitance to less than 100pF. Such a level of capacitance (100pF) can
easily be found with only 1m of high quality interconnect cable and is
often
exceeded by the input capacitance of many amplifiers!

Thus the resistive volume control employed in passive control units must
navigate a course between the Scylla of excessively loading the source,
leading to increased distortion and the Charybdis of excessively high
output
impedance.

unquote.

Iain asks.
1.Does anyone have experience of such transformers?
2. Are they actually better than a passive stepped attenuator controller?

Regards to all

Iain

This thread... I am watching with great interest.
Thank you, Iain.

Hi Peter. I forgot to mention that I managed to obtain a pair of
these transformers for evaluation purposes. So far I have
only mounted them on a bulkhead plate which I intend to fit
into my standard preamp chassis for AB testing.

I am interested to know what the technical advantages of
such a topology might be.

Regards to all
Iain

[Arny Krueger]

"Iain Churches" wrote in message
ti.fi

If for example a 5k Ohm resistive volume control were to
be employed in a passive control unit the source would be
required to drive all the time a quite severe load of 5k
Ohm.

5 K ohm load is severe???

If combined with a 1k Ohm source impedance the worst
case output impedance of the combination would be 1500
Ohm at -6db attenuation, while at -20db attenuation the
output impedance would still be around 540 Ohm. If
combined with around 1nF load capacitance (easily found
in longer, high capacitance interconnects), this leads to
a 0.3db attenuation at 20kHz for a 20db attenuation
setting, practically showing the absolute permissible
limit for load capacitance. The worst-case attenuation at
20kHz almost reaches 1db!!!

(a) This is a very atypical cable.
(b) Iain, you shouldn't get your panties in a bunch about it.

If, to provide our source
with an easier load we choose a 50 kOhm resistive volume
control we must either accept drastically higher levels
of roll off at 20kHz or we must limit the load
capacitance to less than 100pF. Such a level of
capacitance (100pF) can easily be found with only 1m of
high quality interconnect cable and is often exceeded by
the input capacitance of many amplifiers!

Using a potentiometer that is over 10 K as a passive controller is unwise.

Thus the resistive volume control employed in passive
control units must navigate a course between the Scylla
of excessively loading the source, leading to increased
distortion and the Charybdis of excessively high output
impedance.

I bet using that reference from greek mythology made you feel really
intellectual, Iain.

Iain asks.
1.Does anyone have experience of such transformers?

No, because potentiometers work so well in real life.

2. Are they actually better than a passive stepped
attenuator controller?

They are clearly better for the retailer that sells them.

Seems like they are right down your alley, Iain.

To make your transformer attenuator complete, be sure to add the $500 wooden
knob. ;-)

[Iain Churches]

"Arny Krueger" wrote in message
...
"Iain Churches" wrote in message
ti.fi

If for example a 5k Ohm resistive volume control were to
be employed in a passive control unit the source would be
required to drive all the time a quite severe load of 5k
Ohm.

5 K ohm load is severe???

Hi Arny. That's a cut and paste from the maker's website, not
my personal statement.

If combined with a 1k Ohm source impedance the worst
case output impedance of the combination would be 1500
Ohm at -6db attenuation, while at -20db attenuation the
output impedance would still be around 540 Ohm. If
combined with around 1nF load capacitance (easily found
in longer, high capacitance interconnects), this leads to
a 0.3db attenuation at 20kHz for a 20db attenuation
setting, practically showing the absolute permissible
limit for load capacitance. The worst-case attenuation at
20kHz almost reaches 1db!!!

(a) This is a very atypical cable.
(b) Iain, you shouldn't get your panties in a bunch about it.

See above. I have no opinion for or against at this juncture.
I can however hear shortcomings in the passive controller
topology, which is usually a 1m screened pair to a box with
a stepped attenuator and a 1m screened pair leading out to
the power amplifier. In addition to the changes in volume there
are changes in timbre at different settings of the attenuator.


If, to provide our source
with an easier load we choose a 50 kOhm resistive volume
control we must either accept drastically higher levels
of roll off at 20kHz or we must limit the load
capacitance to less than 100pF. Such a level of
capacitance (100pF) can easily be found with only 1m of
high quality interconnect cable and is often exceeded by
the input capacitance of many amplifiers!

Using a potentiometer that is over 10 K as a passive controller is unwise.

Most people seem to pick 100k.

Thus the resistive volume control employed in passive
control units must navigate a course between the Scylla
of excessively loading the source, leading to increased
distortion and the Charybdis of excessively high output
impedance.

I bet using that reference from greek mythology made you feel really
intellectual, Iain.

Actually once again that was the manufacturers text not mine,
Arny. I marked it clearly with Quote and Unquote. And please note,
upper case, (capital) "G" for Greek. But yes. I did have the benefit
of a classical education: Greek and Latin, plus French, German,
and Middle English, I have since added Swedish, Finnish, and
some Norwegian and Danish to that list.
Something which I have never had cause to regret.

A classical education is a god insurance against one
ever becoming a compute repair man in Michigan:-)

I delete the rest of your post as unbefitting of a Born Again
Christian:-))

Cordially
Iain

[Ian Iveson]

Iain asks.

1.Does anyone have experience of such transformers?
2. Are they actually better than a passive stepped attenuator
controller?


No. I would be interested to hear from folk who have compared the two
sensibly.

We went through the impedance matching and possible bandwidth limiting
from the combination of impedance and capacitance a while ago, in
response to a question from yourself IIRC.

It is not difficult, given short cables, to match a CD player with a
valve amp via a potentiometer, because there can be a x100 difference
between source impedance and power amp input impedance, thus allowing
for 2 10-to-1 ratios. For example, a 10k pot should be just OK with 1k
source and 100k load. In my view a resistor is preferable to an
inductor if both work OK.

So in most circumstances this is a question about whether a
transformer is better than an active preamp such as your own. In which
case the issue is distortion, presumably. There may be some folk who
would prefer transformer coupling in any case, possibly because they
object to using capacitors in the signal path, but it's a while since
anyone owned up to this apparent fetish, and I forget how they argue
their case.

One thing I wonder about. It is common to use a capacitor in series at
the input of a valve power amp, and this may be used to limit the LF
response. If you use a transformer then arguably you don't need the
series cap for its dc-blocking role. So it may seem that you can rely
on the transformer also for the role of limiting LF. Would that be
wise? I am concerned about the possibility of ensuing distortion.

cheers, Ian

[Phil Allison]

"Iain Cherches Complete ASS "

Stevens and Billington say, on their website:

If for example a 5k Ohm resistive volume control were to be employed in a
passive control unit the source would be required to drive all the time a
quite severe load of 5k Ohm. If combined with a 1k Ohm source impedance
the
worst case output impedance of the combination would be 1500 Ohm at -6db
attenuation, while at -20db attenuation the output impedance would still
be
around 540 Ohm. If combined with around 1nF load capacitance (easily found
in longer, high capacitance interconnects), this leads to a 0.3db
attenuation at 20kHz for a 20db attenuation setting, practically showing
the
absolute permissible limit for load capacitance. The worst-case
attenuation
at 20kHz almost reaches 1db!!!


** The authors have used very funny maths.

With a 1500 ohm source and 1nF load:

- 3dB is at 106 kHz
- 1dB is at 53 kHz
- 0.25dB is at 26 kHz


With a 540 ohm source and 1 nF load:

- 3dB is at 295 kHz
- 1dB is at 147 kHz
- 0.25dB is at 74 kHz


Ordinary co-ax cable ( ie RG59) has only 22 pF per foot - good, low
capacitance RCA leads have similar values.

Takes a monster ** 50 foot long ** lead to create 1 nF.


**Conclusion:

The dB figures are false.

The reasoning is totally false.

What else would you expect from a marketing blurb ?




........ Phil

[west[_4_]]

"Phil Allison" wrote in message
...
"Iain Cherches Complete ASS "

Stevens and Billington say, on their website:

If for example a 5k Ohm resistive volume control were to be employed in
a
passive control unit the source would be required to drive all the time
a
quite severe load of 5k Ohm. If combined with a 1k Ohm source impedance
the
worst case output impedance of the combination would be 1500 Ohm at -6db
attenuation, while at -20db attenuation the output impedance would still
be
around 540 Ohm. If combined with around 1nF load capacitance (easily
found
in longer, high capacitance interconnects), this leads to a 0.3db
attenuation at 20kHz for a 20db attenuation setting, practically showing
the
absolute permissible limit for load capacitance. The worst-case
attenuation
at 20kHz almost reaches 1db!!!


** The authors have used very funny maths.

With a 1500 ohm source and 1nF load:

- 3dB is at 106 kHz
- 1dB is at 53 kHz
- 0.25dB is at 26 kHz


With a 540 ohm source and 1 nF load:

- 3dB is at 295 kHz
- 1dB is at 147 kHz
- 0.25dB is at 74 kHz


Ordinary co-ax cable ( ie RG59) has only 22 pF per foot - good, low
capacitance RCA leads have similar values.

Takes a monster ** 50 foot long ** lead to create 1 nF.


**Conclusion:

The dB figures are false.

The reasoning is totally false.

What else would you expect from a marketing blurb ?




....... Phil

I believe RG6 can also be used. It has even better specs from what I
understand.
BTW Krueger states that any pot over 10K is "unwise." If that's the case,
then why do so many commercial tube amps have a 100K-250K input pot?

[Eeyore]

Iain Churches wrote:

2. Are they actually better than a passive stepped attenuator controller?

Better ?

Explain what better is first !

As I've discoverd, in the tube audio world, "better" has no correlation with
measured technical accuracy / performance is is merely a subjective matter.

I fail to see thefore how "better" can be anything other than a personal
opinion.

Graham

[Eeyore]

Iain Churches wrote:

"Arny Krueger" wrote

Using a potentiometer that is over 10 K as a passive controller is unwise.

Most people seem to pick 100k.

For a passive control unit ? That would be exceptionally silly, but given the
audiophoolery that routinely exists surprises me not one jot.

At its -6dB position, a 100k volume control presnts an output resistance of 25k
to the world. A relatively modest cable capacitance of ~ 300 pF will cause a
-3dB low pass filter effect at ~ 20kHz.

Now that'll be audible.

Graham

[Eeyore]

Ian Iveson wrote:

It is not difficult, given short cables, to match a CD player with a
valve amp via a potentiometer, because there can be a x100 difference
between source impedance and power amp input impedance, thus allowing
for 2 10-to-1 ratios. For example, a 10k pot should be just OK with 1k
source and 100k load. In my view a resistor is preferable to an
inductor if both work OK.

There is NO *MATCHING* involved whatever.

It's purely a voltage transfer process. Voltage transfer works just fine with
*ANY* practical ratio of impedances where the load impedance is typically ~ =
10 times the source impedance for minimal loading ( 1dB ).

Matching refers a situation where 2 equal values of source and load resistance
/ impedance are used to maximise *power* transfer as opposed to voltage
transfer.

Maximum power transfer (impedance matching) is not a requirement for good
performance in modern equipment and would indeed tend to degrade performance in
modern equipment were it practiced.

Please use the correct terms.

Graham

[west[_4_]]

"Eeyore" wrote in message
...


Iain Churches wrote:

2. Are they actually better than a passive stepped attenuator
controller?

Better ?

Explain what better is first !

As I've discoverd, in the tube audio world, "better" has no correlation
with
measured technical accuracy / performance is is merely a subjective
matter.

I fail to see thefore how "better" can be anything other than a personal
opinion.

Graham

Why are you always trying to pick fly **** out of pepper? Maybe you should
frequent alt.lawyers. Let's stop this bs. I know what he means. I'd like to
see you post something in his language. Food for thought.

west

[Eeyore]

west wrote:

BTW Krueger states that any pot over 10K is "unwise." If that's the case,
then why do so many commercial tube amps have a 100K-250K input pot?

They're not connected to a long length of cable are they ?

Have you completely missed the point ?

Graham

[Eeyore]

west wrote:

"Eeyore" wrote
Iain Churches wrote:

2. Are they actually better than a passive stepped attenuator
controller?

Better ?

Explain what better is first !

As I've discoverd, in the tube audio world, "better" has no correlation
with measured technical accuracy / performance is is merely a subjective
matter.

I fail to see thefore how "better" can be anything other than a personal
opinion.


Why are you always trying to pick fly **** out of pepper? Maybe you should
frequent alt.lawyers. Let's stop this bs. I know what he means. I'd like to
see you post something in his language. Food for thought.

"Better" has no definable meaning.

E.g. one often hears here that SET's are better than other amplifiers types yet
they are demonstrably hugely technically inferior in almost every respect. If
this is to determine what 'better' means then better = worse.

How about asking a question that has a possible meaningful answer instead ?

Or alternatively use the word 'nice' so there's no confusion with technical
accuracy. The statement "SET's are nicer than other types of amplifier" would be
a more honest expression of opinion than using the word 'better'.

Graham

[west[_4_]]

"Eeyore" wrote in message
...


west wrote:

BTW Krueger states that any pot over 10K is "unwise." If that's the
case,
then why do so many commercial tube amps have a 100K-250K input pot?

They're not connected to a long length of cable are they ?

Have you completely missed the point ?

Possibly. How much do you consider a cable which is long? I though Phil said
that RG58 (or I mentioned RG6) has negligable capacitance in reasonable
lengths.

west
Graham

[west[_4_]]

"Eeyore" wrote in message
...


west wrote:

"Eeyore" wrote
Iain Churches wrote:

2. Are they actually better than a passive stepped attenuator
controller?

Better ?

Explain what better is first !

As I've discoverd, in the tube audio world, "better" has no
correlation
with measured technical accuracy / performance is is merely a
subjective
matter.

I fail to see thefore how "better" can be anything other than a
personal
opinion.


Why are you always trying to pick fly **** out of pepper? Maybe you
should
frequent alt.lawyers. Let's stop this bs. I know what he means. I'd like
to
see you post something in his language. Food for thought.

"Better" has no definable meaning.

E.g. one often hears here that SET's are better than other amplifiers
types yet
they are demonstrably hugely technically inferior in almost every respect.
If
this is to determine what 'better' means then better = worse.

How about asking a question that has a possible meaningful answer instead
?

Or alternatively use the word 'nice' so there's no confusion with
technical
accuracy. The statement "SET's are nicer than other types of amplifier"
would be
a more honest expression of opinion than using the word 'better'.

Graham

Ok, I'll bite. However, it's better to evaluate the spirit of a post that
the letter of one, especially when the poster is from a non-speaking
country.

west

[Iain Churches]

"Eeyore" wrote in message
...


Iain Churches wrote:

2. Are they actually better than a passive stepped attenuator controller?

Better ?

Explain what better is first !

As I've discoverd, in the tube audio world, "better" has no correlation
with
measured technical accuracy / performance is is merely a subjective
matter.

I fail to see thefore how "better" can be anything other than a personal
opinion.

OK. Just for the sake of clarity, let's use the term
"technically superior" I look forward to your reply.

Regards
Iain

[Eeyore]

west wrote:

"Eeyore" wrote
west wrote:

BTW Krueger states that any pot over 10K is "unwise." If that's the
case, then why do so many commercial tube amps have a 100K-250K input
pot?

They're not connected to a long length of cable are they ?

Have you completely missed the point ?

Possibly. How much do you consider a cable which is long?

A cable inside a piece of equipment like an amplifier is hardly likely to be
longer thana foot or so. There'll never be enough capacitance there to bother a
100-250k pot.


I though Phil said that RG58 (or I mentioned RG6) has negligable capacitance
in reasonable lengths.

Depends what you call 'negligible' and 'reasonable' ! How about some numbers ?
That's all that matters.

Load capacitance is what messes up passive attenuators using pots.

Graham

[Iain Churches]

"Eeyore" wrote in message
...

Load capacitance is what messes up passive attenuators using pots.

OK. Now we are getting to the crux of the matter.
Can you give us a formula and some shirtcuff calculations, Graham?

Best regards
Iain

[Eeyore]

Iain Churches wrote:

"Eeyore" wrote
Iain Churches wrote:

2. Are they actually better than a passive stepped attenuator controller?

Better ?

Explain what better is first !

As I've discoverd, in the tube audio world, "better" has no correlation
with measured technical accuracy / performance is is merely a subjective
matter.

I fail to see thefore how "better" can be anything other than a personal
opinion.

OK. Just for the sake of clarity, let's use the term
"technically superior" I look forward to your reply.

Well.... as discussed, they are virtually free of the capacitive loading
problems that pots suffer from.

In return you have to accept step volume changes and whatever colouration the
transformer introduces. Plus a hefty hit in the wallet department.

Personally I'd not use one.

For a simple source selector and volume control I'd use a pot followed by a
high-performance op-amp buffer of course. Tubist might consider replacing the
op-amp with a cathode follower, which is about as blameless as tube audio gets.

Graham

[west[_4_]]

"Iain Churches" wrote in message
ti.fi...

"Eeyore" wrote in message
...

Load capacitance is what messes up passive attenuators using pots.

OK. Now we are getting to the crux of the matter.
Can you give us a formula and some shirtcuff calculations, Graham?

Best regards
Iain

Iain: I was just about to say the same thing.
Graham: I defer the numbers to you. We are talking about mainly
interconnects. Were you not the one who claimed that most interconnects were
the same? Let's take a very common scenario ... CD direct to tube amp. First
the numbers, then best way to hook up.

west

[Eeyore]

Iain Churches wrote:

"Eeyore" wrote

Load capacitance is what messes up passive attenuators using pots.

OK. Now we are getting to the crux of the matter.
Can you give us a formula and some shirtcuff calculations, Graham?

Assuming the source impedance is lowish compared to the pot value, the maximum
output impedance / resistance of a potentiometer volume control is 1/4 the pots'
value.

For a 10k pot , that's 2.5 kohms.


If we want to be confident that the pot's resistance and the load capacitance of
the output cable's not introducing any colouration we need to choose a 'minus
dB' @ say 20kHz that's small enough to be inaudible.

I've known ppl detect as little as -0.3 dB @ 20kHz so let's say -0.1 dB shall we
?

Doing the sums elsewhere.......

That -0.1dB @ 20kHz will be caused by a tiny *38pF* of load capacitance !

That doesn't buy you much cable length. For very small values of the 'minus dB'
- you can closely approximate other values arithmetically. So 76 pF would be
approx -0.2dB @ 20kHz (worst case) and so on

So, if using a pot, BUFFER IT ! That's my advice.

Graham

[Iain Churches]

"Eeyore" wrote in message
...


Iain Churches wrote:

OK. Just for the sake of clarity, let's use the term
"technically superior" I look forward to your reply.

Well.... as discussed, they are virtually free of the capacitive loading
problems that pots suffer from.

In return you have to accept step volume changes

These transformers are made with 24 or 40 secondary taps, just as
stepped attenuators are also made with 24 or 40 steps.

and whatever colouration the
transformer introduces.

0.05% at +20dBu 50Hz !

Plus a hefty hit in the wallet department.

I got my pair as FOC for evaluation.
Actually, even if you buy from the most expensive maker
in the UK, a pair of transformers cost about the same as the
TKD variable attenuator, which for many is a standard
choice.

For a simple source selector and volume control I'd use a pot followed by
a
high-performance op-amp buffer of course. Tubist might consider replacing
the
op-amp with a cathode follower, which is about as blameless as tube audio
gets.

Yep. That's the concept on which my own preamp is based.

Iain

[Iain Churches]

"west" wrote in message news:zIL9i.212$ng.61@trnddc07...

"Eeyore" wrote in message
...


west wrote:

BTW Krueger states that any pot over 10K is "unwise." If that's the
case,
then why do so many commercial tube amps have a 100K-250K input pot?

They're not connected to a long length of cable are they ?

Have you completely missed the point ?

Possibly. How much do you consider a cable which is long? I though Phil
said
that RG58 (or I mentioned RG6) has negligable capacitance in reasonable
lengths.

I was looking at a typical commercially-built passive controller yesterday.
It had cables of 1m50 on both input and output, and a 100k DACT
stepped attenuator.

Iain

[Iain Churches]

"Ian Iveson" wrote in message
k...
Iain asks.

1.Does anyone have experience of such transformers?
2. Are they actually better than a passive stepped attenuator controller?


No. I would be interested to hear from folk who have compared the two
sensibly.

I am planning to do just that, when time permits.

So in most circumstances this is a question about whether a transformer is
better than an active preamp such as your own. In which case the issue is
distortion, presumably.

I am told that the tracking accuracy between channels is superior,
althought with most stepped attenuators at betetr than 1% I wonder if
the improvement is audible.

I don't think it's a question of distortion either,and besides, the
Sowter has a THD of 0.03% at +20dBu at 50Hz.

There may be some folk who would prefer transformer coupling in any case,
possibly because they object to using capacitors in the signal path, but
it's a while since anyone owned up to this apparent fetish, and I forget
how they argue their case.

One thing I wonder about. It is common to use a capacitor in series at the
input of a valve power amp, and this may be used to limit the LF response.
If you use a transformer then arguably you don't need the series cap for
its dc-blocking role. So it may seem that you can rely on the transformer
also for the role of limiting LF. Would that be wise? I am concerned about
the possibility of ensuing distortion.

Most matchning and input transformers are not designed to have
DC on them.


Regards
Iain

[Iain Churches]

"Eeyore" wrote in message
...


Iain Churches wrote:

"Eeyore" wrote

Load capacitance is what messes up passive attenuators using pots.

OK. Now we are getting to the crux of the matter.
Can you give us a formula and some shirtcuff calculations, Graham?

Assuming the source impedance is lowish compared to the pot value, the
maximum
output impedance / resistance of a potentiometer volume control is 1/4 the
pots'
value.

For a 10k pot , that's 2.5 kohms.


If we want to be confident that the pot's resistance and the load
capacitance of
the output cable's not introducing any colouration we need to choose a
'minus
dB' @ say 20kHz that's small enough to be inaudible.

I've known ppl detect as little as -0.3 dB @ 20kHz so let's say -0.1 dB
shall we
?

Doing the sums elsewhere.......

That -0.1dB @ 20kHz will be caused by a tiny *38pF* of load capacitance !

Typical cable seems to be 10pF/ft, so that represents a four-foot
cable, which on most systems is probably typical.

That doesn't buy you much cable length. For very small values of the
'minus dB'
- you can closely approximate other values arithmetically. So 76 pF would
be
approx -0.2dB @ 20kHz (worst case) and so on

So, if using a pot, BUFFER IT ! That's my advice.

Or use a transformer? :-)

Thanks Graham. I am looking forward to auditioning this pair of
transformers.

Iain

[Eeyore]

west wrote:

"Iain Churches" wrote
"Eeyore" wrote

Load capacitance is what messes up passive attenuators using pots.

OK. Now we are getting to the crux of the matter.
Can you give us a formula and some shirtcuff calculations, Graham?


Iain: I was just about to say the same thing.
Graham: I defer the numbers to you. We are talking about mainly
interconnects. Were you not the one who claimed that most interconnects were
the same? Let's take a very common scenario ... CD direct to tube amp. First
the numbers, then best way to hook up.

With no level control ?

The output Z of a *decent* CD player is so low that you don't really need to
fret over the issue of capacitive loading.

The only problem today would seem to be finding the damn output impedance data
though ! This info on a Marantz suggest 200 ohms in this case.
http://margo.student.utwente.nl/klaas/audio_cd.htm

That would 'tolerate' 475 pF of cable capacitance for our -0.1dB @ 20kHz
criterion.

And this NAD has an output Z of 300 ohms ( only 317 pF of cable capacitance
allowed for him ! )
http://207.228.230.231/info/NAD_C521i.pdf

Using a figure of 30pF/foot for cable C, I was actually slightly surprised to
see that this would be restricted to about 10 feet of cable for our suggsted
-0.1dB point..


The tube amp has a high input Z so the voltage transfer will as close to 100%
(0dB loss) as is practically possible.

Here's what I'd use. I do use in fact. I've bought from this very supplier. It
costs a very reasonable £5 ($10)
http://cgi.ebay.co.uk/HQ-Twin-Phono-...QQcmdZViewItem



And the 'way' to hook up. Errr, plug the connectors into the sockets. There's no
magic here.

Graham

[Eeyore]

Iain Churches wrote:

"Eeyore" wrote
Iain Churches wrote:

OK. Just for the sake of clarity, let's use the term
"technically superior" I look forward to your reply.

Well.... as discussed, they are virtually free of the capacitive loading
problems that pots suffer from.

In return you have to accept step volume changes

These transformers are made with 24 or 40 secondary taps, just as
stepped attenuators are also made with 24 or 40 steps.

and whatever colouration the
transformer introduces.

0.05% at +20dBu 50Hz !

Plus a hefty hit in the wallet department.

I got my pair as FOC for evaluation.
Actually, even if you buy from the most expensive maker
in the UK, a pair of transformers cost about the same as the
TKD variable attenuator, which for many is a standard
choice.

Whose standard ?

There's *no way* a variable R costs as much as tapped transformers in the real
world.


For a simple source selector and volume control I'd use a pot followed by
a high-performance op-amp buffer of course. Tubist might consider replacing
the op-amp with a cathode follower, which is about as blameless as tube
audio
gets.

Yep. That's the concept on which my own preamp is based.

I was toying with the idea of a DC battery powered CF stage actually.
Rechargeable via a wall wart.

Graham

[Phil Allison]

"Eeysore"

For a 10k pot , that's 2.5 kohms.

I've known ppl detect as little as -0.3 dB @ 20kHz so let's say -0.1 dB
shall we
?

That -0.1dB @ 20kHz will be caused by a tiny *38pF* of load capacitance !


** You have shifted a decimal point.

A 2500 ohms source with a 38 pF load gives:

- 3dB at 1.68 Mhz.
- 1dB at 840 kHz
- 0.25dB at 420 kHz
- 0.1 dB at 230 kHz


Using a 10 kohms pot allows up to a 800 pF output cable to be used, with
only 0.25dB loss at 20kHz - worst case.



......... Phil

[Eeyore]

Iain Churches wrote:

"Ian Iveson" wrote

So in most circumstances this is a question about whether a transformer is
better than an active preamp such as your own. In which case the issue is
distortion, presumably.

I am told that the tracking accuracy between channels is superior,
althought with most stepped attenuators at betetr than 1% I wonder if
the improvement is audible.

1% is near enough 0.1dB.

That's close enough for me !


I don't think it's a question of distortion either,and besides, the
Sowter has a THD of 0.03% at +20dBu at 50Hz.

There's more to transformers than just a simple THD figure.

Graham

[Phil Allison]

"Iain Cherchus Asinine ****wit "



Typical cable seems to be 10pF/ft,


** B O L L O C K S !!!!!!!!!!!!!!!!!!!




.......... Phil

[Eeyore]

Iain Churches wrote:

"Eeyore" wrote in message

Doing the sums elsewhere.......

That -0.1dB @ 20kHz will be caused by a tiny *38pF* of load capacitance !

Typical cable seems to be 10pF/ft, so that represents a four-foot
cable, which on most systems is probably typical.

The cable I use isn't 10pF/foot ! More like 3 times that. And I use longer
cables too.

Graham

[Eeyore]

Iain Churches wrote:

"Eeyore" wrote

So, if using a pot, BUFFER IT ! That's my advice.

Or use a transformer? :-)

Thanks Graham. I am looking forward to auditioning this pair of
transformers.

Beware of transformers' sensitivity to *source* impedances. There's usually an
optimal range of source Z.

Graham

[Arny Krueger]

"Iain Churches" wrote in message
ti.fi
"Arny Krueger" wrote in message
...
"Iain Churches" wrote in message
ti.fi

If for example a 5k Ohm resistive volume control were to
be employed in a passive control unit the source would
be required to drive all the time a quite severe load
of 5k Ohm.

5 K ohm load is severe???

Hi Arny. That's a cut and paste from the maker's website,
not my personal statement.

You seem to lack the knowlege and candor it takes to intelligently comment
obvious on weirdness, Iain.

If combined with a 1k Ohm source impedance the worst
case output impedance of the combination would be 1500
Ohm at -6db attenuation, while at -20db attenuation the
output impedance would still be around 540 Ohm. If
combined with around 1nF load capacitance (easily found
in longer, high capacitance interconnects), this leads
to a 0.3db attenuation at 20kHz for a 20db attenuation
setting, practically showing the absolute permissible
limit for load capacitance. The worst-case attenuation
at 20kHz almost reaches 1db!!!

(a) This is a very atypical cable.

(b) Iain, you shouldn't get your panties in a bunch
about it.

See above. I have no opinion for or against at this
juncture.

You posted it without commenting on how strange it was.

I can however hear shortcomings in the passive controller
topology, which is usually a 1m screened pair to a box
with a stepped attenuator and a 1m screened pair leading out to
the power amplifier. In addition to the changes in volume
there are changes in timbre at different settings of the
attenuator.

No doubt a badly designed passive controller.

If, to provide our source
with an easier load we choose a 50 kOhm resistive volume
control we must either accept drastically higher levels
of roll off at 20kHz or we must limit the load
capacitance to less than 100pF. Such a level of
capacitance (100pF) can easily be found with only 1m of
high quality interconnect cable and is often exceeded by
the input capacitance of many amplifiers!

Using a potentiometer that is over 10 K as a passive
controller is unwise.

Most people seem to pick 100k.

Nonsense. 100K is a good value to use in a preamp where the volume control
is followed with some kind of active buffer stage.

Counter-example:

http://www.mhsoft.nl/MySystem/HiFi.asp

"Some important points:

"Use as short interconnect cables as possible
"Use cables with low capacitance
"Use low attenuator resistance values (recommended total resistance:
10kOhms)

[Arny Krueger]

"Iain Churches" wrote in message
ti.fi
"west" wrote in message
news:zIL9i.212$ng.61@trnddc07...

"Eeyore" wrote
in message ...


west wrote:

BTW Krueger states that any pot over 10K is "unwise." If that's the
case, then why do so many commercial
tube amps have a 100K-250K input pot?

They're not connected to a long length of cable are
they ? Have you completely missed the point ?

Possibly. How much do you consider a cable which is
long? I though Phil said
that RG58 (or I mentioned RG6) has negligable
capacitance in reasonable lengths.

I was looking at a typical commercially-built passive
controller yesterday. It had cables of 1m50 on both
input and output, and a 100k DACT stepped attenuator.

There's nothing typical at all about 100K.

I *own* a couple of commercially-built passive controllers, and they use 10K
pots. Balanced in and out.

[Phil Allison]

Iain Cherches Complete ASS "

Stevens and Billington say, on their website:

If for example a 5k Ohm resistive volume control were to be employed in a
passive control unit the source would be required to drive all the time a
quite severe load of 5k Ohm. If combined with a 1k Ohm source impedance
the
worst case output impedance of the combination would be 1500 Ohm at -6db
attenuation, while at -20db attenuation the output impedance would still
be
around 540 Ohm. If combined with around 1nF load capacitance (easily found
in longer, high capacitance interconnects), this leads to a 0.3db
attenuation at 20kHz for a 20db attenuation setting, practically showing
the
absolute permissible limit for load capacitance. The worst-case
attenuation
at 20kHz almost reaches 1db!!!


** The authors have used very funny maths.

With a 1500 ohm source and 1nF load:

- 3dB is at 106 kHz
- 1dB is at 53 kHz
- 0.25dB is at 26 kHz


With a 540 ohm source and 1 nF load:

- 3dB is at 295 kHz
- 1dB is at 147 kHz
- 0.25dB is at 74 kHz


Ordinary co-ax cable ( ie RG59) has only 22 pF per foot - good, low
capacitance RCA leads have similar values.

Takes a monster ** 50 foot long ** lead to create 1 nF.


**Conclusion:

The dB figures are false.

The reasoning is totally false.

What else would you expect from a marketing blurb ?



........ Phil

[Phil Allison]

"Eeysore"

For a 10k pot , that's 2.5 kohms.

I've known ppl detect as little as -0.3 dB @ 20kHz so let's say -0.1 dB
shall we
?

That -0.1dB @ 20kHz will be caused by a tiny *38pF* of load capacitance !


** You have shifted a decimal point.

A 2500 ohms source with a 38 pF load gives:

- 3dB at 1.68 Mhz.
- 1dB at 840 kHz
- 0.25dB at 420 kHz
- 0.1 dB at 230 kHz


Using a 10 kohms pot allows up to a 800 pF output cable to be used, with
only 0.25dB loss at 20kHz - worst case.



......... Phil

[Phil Allison]

"Eeysore ****ing Pommy **** "


For a 10k pot , that's 2.5 kohms.

I've known ppl detect as little as -0.3 dB @ 20kHz so let's say -0.1 dB
shall we
?

That -0.1dB @ 20kHz will be caused by a tiny *38pF* of load capacitance !


** You have shifted a decimal point - ****HEAD !

A 2500 ohms source with a 38 pF load gives:

- 3dB at 1.68 Mhz.
- 1dB at 840 kHz
- 0.25dB at 420 kHz
- 0.1 dB at 230 kHz


Using a 10 kohms pot allows up to a 800 pF output cable to be used, with
only 0.25dB loss at 20kHz - worst case.

****HEAD !!!!!!!!!!!!!!!!!!!!!



........ Phil

[Phil Allison]

Iain Cherches Complete ASS "

Stevens and Billington say, on their website:

If for example a 5k Ohm resistive volume control were to be employed in a
passive control unit the source would be required to drive all the time a
quite severe load of 5k Ohm. If combined with a 1k Ohm source impedance
the
worst case output impedance of the combination would be 1500 Ohm at -6db
attenuation, while at -20db attenuation the output impedance would still
be
around 540 Ohm. If combined with around 1nF load capacitance (easily found
in longer, high capacitance interconnects), this leads to a 0.3db
attenuation at 20kHz for a 20db attenuation setting, practically showing
the
absolute permissible limit for load capacitance. The worst-case
attenuation
at 20kHz almost reaches 1db!!!


** The authors have used very funny maths.

With a 1500 ohm source and 1nF load:

- 3dB is at 106 kHz
- 1dB is at 53 kHz
- 0.25dB is at 26 kHz


With a 540 ohm source and 1 nF load:

- 3dB is at 295 kHz
- 1dB is at 147 kHz
- 0.25dB is at 74 kHz


Ordinary co-ax cable ( ie RG59) has only 22 pF per foot - good, low
capacitance RCA leads have similar values.

Takes a monster ** 50 foot long ** lead to create 1 nF.


**Conclusion:

The dB figures are false.

The reasoning is totally false.

What else would you expect from a marketing blurb ?



........ Phil

[Phil Allison]

Iain Cherches Complete ASS "

Stevens and Billington say, on their website:

If for example a 5k Ohm resistive volume control were to be employed in a
passive control unit the source would be required to drive all the time a
quite severe load of 5k Ohm. If combined with a 1k Ohm source impedance
the
worst case output impedance of the combination would be 1500 Ohm at -6db
attenuation, while at -20db attenuation the output impedance would still
be
around 540 Ohm. If combined with around 1nF load capacitance (easily found
in longer, high capacitance interconnects), this leads to a 0.3db
attenuation at 20kHz for a 20db attenuation setting, practically showing
the
absolute permissible limit for load capacitance. The worst-case
attenuation
at 20kHz almost reaches 1db!!!


** The authors have used very funny maths.

With a 1500 ohm source and 1nF load:

- 3dB is at 106 kHz
- 1dB is at 53 kHz
- 0.25dB is at 26 kHz


With a 540 ohm source and 1 nF load:

- 3dB is at 295 kHz
- 1dB is at 147 kHz
- 0.25dB is at 74 kHz


Ordinary co-ax cable ( ie RG59) has only 22 pF per foot - good, low
capacitance RCA leads have similar values.

Takes a monster ** 50 foot long ** lead to create 1 nF.


**Conclusion:

The dB figures are false.

The reasoning is totally false.

What else would you expect from a marketing blurb ?



........ Phil