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Best Way to Attenuate



 
 
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  #1  
Old June 13th 07, 06:55 AM posted to rec.audio.tubes
west[_4_]
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Posts: 102
Default Best Way to Attenuate

Can we take an off the shelf CD Player and install , let's say a 10K stepped
attenuator or Alps type pot inside the CD player on the output line? Would
this make a cleaner audio input to the tube amp? The 2 alternatives are an
outboard passive "preamp" with let's say 100k attenuator or installing it
inside the amp. I tried the passive preamp route with mixed results but got
straightened out from info in a previous thread by using a much shorter
interlink cable.

Cordially,
west


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  #2  
Old June 13th 07, 08:51 AM posted to rec.audio.tubes
Eeyore
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Posts: 8,474
Default Best Way to Attenuate



west wrote:

> Can we take an off the shelf CD Player and install , let's say a 10K stepped
> attenuator or Alps type pot inside the CD player on the output line? Would
> this make a cleaner audio input to the tube amp?


Cleaner than WHAT ?


> The 2 alternatives are an outboard passive "preamp" with let's say 100k
> attenuator


Bad choice. As previously described, 100k is too high a value if you're
concerned about possible treble loss with longer cables.

Use 10k. No CD player will have any trouble driving it.


> or installing it inside the amp. I tried the passive preamp route with mixed
> results but got straightened out from info in a previous thread by using a
> much shorter
> interlink cable.


What exactly are you concerned about ?

Graham
  #3  
Old June 13th 07, 11:12 AM posted to rec.audio.tubes
Sander deWaal
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Posts: 1,141
Default Best Way to Attenuate

"west" > said:

>Can we take an off the shelf CD Player and install , let's say a 10K stepped
>attenuator or Alps type pot inside the CD player on the output line? Would
>this make a cleaner audio input to the tube amp? The 2 alternatives are an
>outboard passive "preamp" with let's say 100k attenuator or installing it
>inside the amp. I tried the passive preamp route with mixed results but got
>straightened out from info in a previous thread by using a much shorter
>interlink cable.



Yes, it is possible to put a potentiometer *before* the last opamp
stage, so that opamp acts as a buffer.
Be sure to remove the muting transistors at the output, they can be
replaced with a small signal relay with its NC contacts to gnd.
The drive signal for the relay can be obtained from the bases of said
mmuting transistors, with a NPN driver inbetween.


But I prefer my volume pot in the (pre) amp, simply because there are
other sources such as my turntables and Roku soundbridge..

--

- Maggies are an addiction for life. -
  #4  
Old June 13th 07, 11:17 AM posted to rec.audio.tubes
Patrick Turner
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Posts: 3,964
Default Best Way to Attenuate



Eeyore wrote:
>
> west wrote:
>
> > Can we take an off the shelf CD Player and install , let's say a 10K stepped
> > attenuator or Alps type pot inside the CD player on the output line? Would
> > this make a cleaner audio input to the tube amp?

>
> Cleaner than WHAT ?
>
> > The 2 alternatives are an outboard passive "preamp" with let's say 100k
> > attenuator

>
> Bad choice. As previously described, 100k is too high a value if you're
> concerned about possible treble loss with longer cables.
>
> Use 10k. No CD player will have any trouble driving it.
>
> > or installing it inside the amp. I tried the passive preamp route with mixed
> > results but got straightened out from info in a previous thread by using a
> > much shorter
> > interlink cable.

>
> What exactly are you concerned about ?
>
> Graham


I know a guy who never would use a 100k pot or switched R bank as an
attenuator.

He prefers to have 1k from the input terminal to an amp input, and
shunting pot
from input to 0V, and thus the series R from source to amp is never less
than 1k.
he says all the CD players have Rout = approx 500 ohms, and
effortlessly can produce say 3Vrms into 1k, ( 3mA rms current ) at
0.001% N&D,
so having to produce 0.1Vrms for average levels for a power amp isn't a
problem.

Many preamps have an attenuator BEFORE a gain tube, so a 1Vrms signal
arrives
from a CD player and is amplified to 10Vrms, and then attenuated down
-40dB to 0.1Vrms
for feeding into a power amp.

Fortunately, its easy to maintain good enough linearity with a triode to
do all this,
but its better if the triode has a CCS anode load and some shunt NFB to
reduce A from say
16 max to 4, and all aspects of technical operation are vastly improved.

Crown opt for using a pot in a shunt FB path to attenuate the signal
around an opamp
in a preamp.

Using a 100k pot before a gain tube isn't all that bad. Usually SNR is
still good enough, and N&D remain very low because maybe only 0.1Vrms
needs to be
produced by the tube, with the gain pot at the input set at -40dB where
its
resistance in shunt with the grid input is 1k, and HF losses are
entirely negligible.

The highest Rout for a 100k pot fed with a lowZ source is 25k if the pot
is set at -6dB,
which is rarely ever used; its always around the -20dB position, noon
position,
on a log pot. Rout is around 9k.
If the pot feeds a cathode follower output buffer of a preamp, the HF
losses are utterly negligible.

There are many ways for anyone to try arrangeing the source - attenuator
- pre-amp - power amp
line up, but hacking into the guts of a horrible pcb board in most CD
players
with micro circuitry to instal some attenuator is never going to be
something I would do.
Many CD players already have a facility for level control, and come with
a remote
for total control of the player from the lazy chair.

Patrick Turner.
  #5  
Old June 13th 07, 01:33 PM posted to rec.audio.tubes
Arny Krueger
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Posts: 17,262
Default Best Way to Attenuate

"west" > wrote in message
news:[email protected]

> Can we take an off the shelf CD Player and install ,
> let's say a 10K stepped attenuator or Alps type pot
> inside the CD player on the output line? Would this make
> a cleaner audio input to the tube amp?


Cleaner than what? If your baseline is a tubed preamp such as those made by
Conrad Johnson, then a well-designed passive attenuator would provide a
measurably cleaner connection than that provided say, a typical C-J preamp.

> The 2 alternatives
> are an outboard passive "preamp" with let's say 100k
> attenuator or installing it inside the amp.


To review, a 100k attenuator connected to the grid of a tube with typical
and nominal amounts of wire and circuitry is also just fine as far as it
goes.

If the tube is always part of the signal path like the input stage of a
power amp, then it is part of the baseline system and causes no additional
harm. I might add that this kind of system would probably be more
effective if the tubed amp had a bit more gain than is usual.

If the tube was added to the signal path, at least partially to solve the
issues raised by the choice of such a high resistance potentiometer, then
there is some harm that is at least measurable.

Well-designed tubed circuitry generally has more distortion than equivalent
SS, but either can reasonably be expected to be sonically transparent.

> I tried the
> passive preamp route with mixed results but got
> straightened out from info in a previous thread by using
> a much shorter interlink cable.


IOW you made some mistakes with the design and/or implementation of the
passive attenuator, and it came around and bit you, at least in your
perceptions.


  #6  
Old June 14th 07, 08:18 PM posted to rec.audio.tubes
Iain Churches
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Posts: 462
Default Best Way to Attenuate


"west" > wrote in message
news:[email protected]
> Can we take an off the shelf CD Player and install , let's say a 10K
> stepped
> attenuator or Alps type pot inside the CD player on the output line? Would
> this make a cleaner audio input to the tube amp? The 2 alternatives are an
> outboard passive "preamp" with let's say 100k attenuator or installing it
> inside the amp. I tried the passive preamp route with mixed results but
> got
> straightened out from info in a previous thread by using a much shorter
> interlink cable.
>


The answer became clear in the previous attenuator thread, I think.
If you must have an attenuator followed by a long cable then it should
have a cathode follower.

I have settled on a cathode follower signal selector/preamp
(just less than unity gain) with a balanced line output so that I
can run a cable from the top of the house down to my music room
on the lower ground floor when required with no losses.

I lent this same preamp (FR 10Hz to 100kHz +/- 0.5dB) to a friend
who had been using a passive controller with a long cable. He
thought the CF was "bright" ! Draw your own conclusions:-))

Normally, if I had a single signal source, say a CD player
I would forget about the preamp altogether, and have a stepped
attenuator (100k) on the grid of the first stage of the preamp and
built in to the same chassis (leads as short as possible) I did
this on my 25W amp.

I am still interested to try the transformer attenuator, but I guess
this will have to wait until after the summer now.

Regards to all
Iain



  #7  
Old June 15th 07, 02:05 PM posted to rec.audio.tubes
Arny Krueger
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Posts: 17,262
Default Best Way to Attenuate

"Iain Churches" > wrote in message
news
> "west" > wrote in message
> news:[email protected]
>> Can we take an off the shelf CD Player and install ,
>> let's say a 10K stepped
>> attenuator or Alps type pot inside the CD player on the
>> output line? Would this make a cleaner audio input to
>> the tube amp? The 2 alternatives are an outboard passive
>> "preamp" with let's say 100k attenuator or installing it
>> inside the amp. I tried the passive preamp route with
>> mixed results but got straightened out from info in a previous thread by
>> using
>> a much shorter interlink cable.
>>

>
> The answer became clear in the previous attenuator
> thread, I think. If you must have an attenuator followed
> by a long cable then it should have a cathode follower.


I would think that a low-gain buffer would be more to the point.




  #8  
Old June 17th 07, 03:32 AM posted to rec.audio.tubes
west[_4_]
external usenet poster
 
Posts: 102
Default Best Way to Attenuate


"Iain Churches" > wrote in message
news
>
> "west" > wrote in message
> news:[email protected]
> > Can we take an off the shelf CD Player and install , let's say a 10K
> > stepped
> > attenuator or Alps type pot inside the CD player on the output line?

Would
> > this make a cleaner audio input to the tube amp? The 2 alternatives are

an
> > outboard passive "preamp" with let's say 100k attenuator or installing

it
> > inside the amp. I tried the passive preamp route with mixed results but
> > got
> > straightened out from info in a previous thread by using a much shorter
> > interlink cable.
> >

>
> The answer became clear in the previous attenuator thread, I think.
> If you must have an attenuator followed by a long cable then it should
> have a cathode follower.
>
> I have settled on a cathode follower signal selector/preamp
> (just less than unity gain) with a balanced line output so that I
> can run a cable from the top of the house down to my music room
> on the lower ground floor when required with no losses.
>
> I lent this same preamp (FR 10Hz to 100kHz +/- 0.5dB) to a friend
> who had been using a passive controller with a long cable. He
> thought the CF was "bright" ! Draw your own conclusions:-))
>
> Normally, if I had a single signal source, say a CD player
> I would forget about the preamp altogether, and have a stepped
> attenuator (100k) on the grid of the first stage of the preamp and
> built in to the same chassis (leads as short as possible) I did
> this on my 25W amp.


Iain, do you mean on the first stage of your amp?

west
>
> I am still interested to try the transformer attenuator, but I guess
> this will have to wait until after the summer now.
>
> Regards to all
> Iain
>
>
>



  #9  
Old June 17th 07, 05:47 PM posted to rec.audio.tubes
Iain Churches
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Posts: 462
Default Best Way to Attenuate


"west" > wrote in message news:R21di.221$%[email protected]
>
> "Iain Churches" > wrote in message
> news
>>


>> Normally, if I had a single signal source, say a CD player
>> I would forget about the preamp altogether, and have a stepped
>> attenuator (100k) on the grid of the first stage of the preamp and
>> built in to the same chassis (leads as short as possible) I did
>> this on my 25W amp.

>
> Iain, do you mean on the first stage of your amp?


Yes. On the grid of the first stage in the power amplifier. The
wiring is about 12cms:-)

I used a 100k DACT stepped attenuator, one for each channel.
People have told me that these tend to get noisy after a couple of
years. But Sander, I think it was, recommended the use of a
470k grid resistor in addition. This seems to work well.

Regards to all
Iain



 




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