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Old 19th November 2003, 09:42 PM   #11
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Richard C, most GOOD cd players and d/a convertors have out put impedances of less than 100ohms, if yours is`nt then do`nt do the passive thing,
for instance my current d/a convertor is 30ohms, my last one msb link dac was 70 ohms which i changed down to 10ohms with a better output opamp, it`s up to you to look at all this.

Yesthereis an active component after the input resistor of poweramps be the input of a bipolar transistor or a fet, the bipolar would be in the order of megs and the fet in the order of gigs! this would dictate the input loading resistor will be the determining factor,and the input of the active deviceswill have little effect.

Once you do all the right things you will get a better more transparent sound, no matter how good a buffer or gain stage is it will sound coloured in comparison to nothing in the signal path, and if you do like the sound of the buffer or gain stage all your doing is bandaid fixing a problem elsewhere with a colouration.

I have proved this countless times with members of our audio society selling their million dollar pre-amps for CORRECTLY impemented passive volume controls

P.S. the input impedance of my amps are 1meg, i have unsheilded interconets. I have no noise problems and no rf break through, even living on a main road with taxis pulling up outside
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Old 19th November 2003, 10:04 PM   #12
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Quote:
Originally posted by georgehifi
Richard C, most GOOD cd players and d/a convertors have out put impedances of less than 100ohms, if yours is`nt then do`nt do the passive thing,
for instance my current d/a convertor is 30ohms, my last one msb link dac was 70 ohms which i changed down to 10ohms with a better output opamp, it`s up to you to look at all this.

Yesthereis an active component after the input resistor of poweramps be the input of a bipolar transistor or a fet, the bipolar would be in the order of megs and the fet in the order of gigs! this would dictate the input loading resistor will be the determining factor,and the input of the active deviceswill have little effect.

Once you do all the right things you will get a better more transparent sound, no matter how good a buffer or gain stage is it will sound coloured in comparison to nothing in the signal path, and if you do like the sound of the buffer or gain stage all your doing is bandaid fixing a problem elsewhere with a colouration.

I have proved this countless times with members of our audio society selling their million dollar pre-amps for CORRECTLY impemented passive volume controls
Totally agree with you. Personal experience with implementing a buffer. It definitely changed the sound in a wrong direction. I personally subscribe also to "less is more" in the way of the signal. Especially adding non linear components like transistors. Than to make it more linear you have to ad current source and then really good and well filtered power supply for that stage and then.......

Properly design passive is better. Improperly implemented one, than who cares, replace it with whatever.

As far as pot value, 100k, if the input impedance of the amp is 33k, the total imp. that the interconect will see is 25K. You can use a lin. pot (better match between L and R ch) that will be close to a log pot (because the 33k in paralel) and your sorce out impedance have to be 300 Ohms. If that's hard to do start asembling transistor preamp. Even if my Source out is 2K I'd still use passive preamp and low cap. interconnects.
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Old 19th November 2003, 10:33 PM   #13
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If the wire from Volume-Pot to power amp is very short, and you do not need gain, there isn't any gread difference. After all, what is the difference between the preamp amplifier input and the power amplifier input? Mostly cable length: 3 feet (or 1 meter) for external power amp, 3 inches (or 0.1 meter) for a preamp circuit mounted right behind the volume pot.

Taking the 10K pot, max output Z about 2.5K, if you hang 10,000pFd on it the response will droop 3dB at about 4KHz, pretty awful. But 10,000pFd is about 300 feet of cable! Use 3 feet, 100pFd, -3dB response is 400KHz, -1dB at 200KHz, -0.5dB at 100KHz, better than many active preamps, not a real issue.

However if you need 30 feet of cable to reach your amplifier, you are -1dB at 20KHz, not great. This will be at worst-case, Volume pot set 6 dB below full-up, and response will be better at other settings. (But with a no-gain "preamp", the -6dB position is likely to be your "pretty loud" position and shouldn't also be your worst treble droop.)

If you need 100K volume pot to avoid loading a source (or to reduce its distortion), then at just 3 feet you are into significant droop (-1dB at 20KHz worst-case) and you should be looking for some shorter low-C cables and jam the amp against the Volume-pot box to reach. (But beware of hum from amplifier inducting into volume-pot!)

The 100X rule is generous. It is always nice to recover 99% of signal voltage, but often not essential (and rarely practical!). A 100K pot with a 25K load (power amp input) will work fine, except if it has dB markings the "-6dB" marking will really be -12dB. The "error" is less as you move away from the "-6dB" mark, reducing to zero error at full-up and full-down. Unless you are a numbers-head who goes by dial-markers instead of by loudness, this is probably no problem at all. (However, in my recording work, I calibrate my knobs because I do think in numbers as a guide to gain structure and to rough-out a mix.)

Since nearly all modern sources (and certainly all home hi-fi CD players) can drive high level into 10K, the decision is fairly simple. If you drive long lines to a power amp, over 20 feet (6 meters), you almost certainly need a buffer at the volume-pot end of the line to push the cable around. If you run very short lines, under 10 feet (3 meters), you can almost certainly be happy with a passive 10K volume pot. If the particular pot you favor only comes in 50K or 100K impedance, you better keep your cables well under 3 feet/1 meter, or use a buffer between the hi-Z pot and the cable.

The above applies about the same to stepped attenuators. The simplest ones are just pots, but with switch contacts instead of a wiper. For double the price you can get a ladder attenuator: the output Z tends to be more constant but not necessarily lower (can be higher). As a rough guide, treat a switched attenuator like a pot but keep the wires a little shorter. The makers of these high-price items should be able to give specific guidelines for how much cable they can drive sweetly.

There are some tapped-transformer attenuators. I think Lundall lists the trannies, not wired to a switch. VERY expensive, and not wide-range at high gain-resolution, but the output Z can be lower than the source impedance at all positions below full-up. Of course even these very good transformers impart a "sound", and possibly more "sound" than a resistive pot/switch attenuator.
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Old 19th November 2003, 11:39 PM   #14
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Originally posted by georgehifi
Richard C, most GOOD cd players and d/a convertors have out put impedances of less than 100ohms


I've got the schematic for a Marantz CD6000 in front of me and that has around 250 Ohms of resistance in series with the output and this is not uncommon.

However, if people are prepared to restrict their choice of source component to only those that work ok with passive attenuators then there isn't a problem.

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Yes there is an active component after the input resistor of poweramps be the input of a bipolar transistor or a fet, the bipolar would be in the order of megs and the fet in the order of gigs! this would dictate the input loading resistor will be the determining factor,and the input of the active deviceswill have little effect.
This depends entirely on the particular amplifier topology, changing resistors will not work on all amplifiers, and who wants to modify their amplifier for the sake of an attenuator.

Quote:
no matter how good a buffer or gain stage is it will sound coloured in comparison to nothing in the signal path.
A passive attenuator is not "nothing".

There seems to be an assumption that just because something is passive it is intrinsically better, quieter, more linear or less coloured than an active device.

Quote:
, and if you do like the sound of the buffer or gain stage all your doing is bandaid fixing a problem elsewhere with a colouration.
A simple emitter follower with a constant current load will be contribute very little to the sound. And yes it is fixing a much bigger problem elsewhere: the problem of output impedance changing by orders of maginitude over the travel of the pot that an un-buffered attenuator suffers from.

Quote:
I have proved this countless times with members of our audio society selling their million dollar pre-amps for CORRECTLY impemented passive volume controls
I don't advocate expensive parts or complexity, just a simple impedance converter.

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P.S. the input impedance of my amps are 1meg, i have unsheilded interconets. I have no noise problems and no rf break through, even living on a main road with taxis pulling up outside
I haven't mentioned rf breakthrough but unless your hearing extends to rf how do you know it isn't there?

Clearly, the biggest problem with a passive attenuator is that it's performance is largely depedent on the characteristics of the source, interconnect and amplifier. That's a lot of variables, variables that can all be vastly reduced by an active buffer.
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Old 20th November 2003, 02:15 AM   #15
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Sorry Richard C, but if you have`nt heard it yet it` looks as though you never will, and as for a cd6000, if it has 250ohms output impedance you really should look at changing the output opamp as it needs to be better if they used 250ohm output resistor.
And Just to let you know the CD6000 standard or limited edition model is a midfi product.
Marantz comissioned Ken Ishiwata to do a CD6000KI model which costs a lot more and sounds much better, and guess what the differences are, yes a completely new output stage to lower the output impedance, so it can drive better.

(the only 2 things a preamp is good for is for analog lp amplification, or a boat anchor if it`s one of the heavy expensive ones)
Cheers George
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Old 20th November 2003, 08:30 AM   #16
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Originally posted by georgehifi
Sorry Richard C, but if you have`nt heard it yet it` looks as though you never will, and as for a cd6000, if it has 250ohms output impedance you really should look at changing the output opamp as it needs to be better if they used 250ohm output resistor.
And Just to let you know the CD6000 standard or limited edition model is a midfi product.
It's not the op-amp that dominates the o/p z of the CD6000 its the series resistance (does the KI model not have this? according to this schematic it does), and I don't own one or believe it to be high-end I merely use it as a popular example.

As for op-amps in the output stage of CDPs: my CDP doesn't use any. As many people on this forum have discovered a simple circuit implemented with a hand-full of transistors makes a much better job of it.


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Old 20th November 2003, 12:30 PM   #17
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Also when you put this in parallel with the input impedance of your power-amp you will find that the characteristic of the pot is far from being the logarithmic ideal.
Well, numbers are typically 2.5K worst-case for the pot and 50K for the input, meaning there's a 5% or < 0.5dB absolute error in the volume - but this is on a control you twiddle by hand anyway. Specifically, this issue doesn't affect frequency response or L/R balance.

I'll agree that 2.5K is too high for driving a substantial length of cable - 1nF cable capacitance has an impedance of 8K at 20KHz. However, 10-20cm cable between the pot and the power amp input will be orders of magnitude below this and (measurably) won't affect the frequency response.

Cheers
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Old 20th November 2003, 06:48 PM   #18
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I`m over this discussion, Fat Marley if you do the right thing by a passive volume control as i explained and use a good cd player with low output impedance (<100ohms) you will never look back.

TRUST YOUR EARS!!!!!!!!!!!

Cheers George
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Old 20th November 2003, 07:02 PM   #19
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I think my Naim cd 3.5 has an output impedance of 10 Ohms, i say think because i can only find the spec's of the previouse model the "cd 3"

My Naim nap 250 power amp has an input impedance of 18K Ohms.

I have a 20K DACT stepped attenuator, would this work?

What spec's should i look for in a cable?

Do i risk blowing my amp up?
I only ask this because Naim amps have to have speaker cables at least 3.5 meters long and using the wrong type can ruin the amp.
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Old 20th November 2003, 07:30 PM   #20
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Fat Marley if your Naim cd has an output impedance of 10 omhs that`s great for use with a 10K passive, but your input impedance of ypur poweramp is too low it needs to be >100k if your not comfortable reading a circuit diagram to see if it`s safe to do then get a qualified technician to do the job. (should only take 15 min or so) and you`ll hear sounds that you never think possible from your Naim cd player.

Cheers George

PS if you must use the 20k dact then the input of the amp should be higher 200k or more.
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