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Old 7th February 2003, 06:59 AM   #1
Rarkov is offline Rarkov  England
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Default Passive Preamp

Hi,
I've ordered a dual 100k and 50k log potentiometer and a 2 pole 6way rotary switch. I want to make a passive preamp using these components. I was thinking, 5 pairs of inputs (1 is off) into the switch and the ouput from the switch goes to one side of the pot. I'd put the other side to ground and use the centre tap as my signal ouput.

So, here are the questions:
Will this work with only two components?
Should I use the 50k or 100k pot for volume?
If I wanted to add balance (same but with input signals to pot reversed) should I use the 50k or 100k
Will the volume at 50k/100k resistance be (virtually) nill?

Thanks for any help!!!
Gaz
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Old 7th February 2003, 11:48 AM   #2
Bobken is offline Bobken  United Kingdom
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Hi,

From what you say, I don't wish to discourage you nor appear to be negative, but I would suggest that you do some research into these things before you start, as it doesn't seem that you understand how pots are conected up for attenuators.

I mean no disrespect in saying this, but for example, *any* value of pot if used in the correct intended manner, will effectively kill all of the signal. The reason being that with the output at 0 (fully anti-clockwise) the wiper of the pot, which is connected to the succeeding stage, will be shorted out to ground and there will no signal there to be passed on to this next stage. It doesn't matter if the pot is 10 Ohms, or 1Meg Ohms.

As a general rule , most SS gear will use a lower value pot with 10K being perhaps the most common, but valve gear seems to use much higher values like 100K, I believe, although I am not a valve man!

What you need to be aware of is the effect of the pot's impedances associated with the capacitances of the cables etc. (which feed into, and more importantly out of the passive attenuator) as where the pot's impedances are higher, the capacitance of the cable will act like an HF attenuator. In short, the higher the impedance, the lower the cable capacitance must be to avoid any curtailing of the HF response.

There are other factors like the output impedance of the preceeding gear and the input impedance of the succeeding equipment to take into account as well.

There must be a lot of info and guidance on this on the 'net, and I would encourage you look at this to grasp the important issues, as otherwise you will probably be dissatisfied with the results, which would be a shame.

Don't be deterred by anything I have said, as properly executed passives can and do sound very good, but you do need to pay attention to the points I mentioned.

Regards,
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Old 7th February 2003, 03:10 PM   #3
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What's a passive preamp? (Be aware I already know the colloquial answer.)

TIm
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Old 7th February 2003, 03:23 PM   #4
Rarkov is offline Rarkov  England
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Hi,
Bobken: thanks for all your advice...

Maybe I should shed a bit more light on why I'm doing this. I've built a pair of Randy Slone OptiMOS monoblocks (with a slightly raised rail voltage). I do not have a preamp, so I modified a cheap second hand integrated amp to bypass the amps and run as a preamp. It sounds just awful! I am planning to get a Marantz 4300 or 5300 (can't remember which) HC amp. This outputs preouts but for music, I want a really simple preamp. In my "junk box" I have an Alps 100K dual log pot and a 2 pole 6way switch. These are fixed (preferrably).

I suppose what I am asking is: Can I make a good/great sounding passive preamp from these components? I have a virtually unlimited supply of resistors and caps if I need any in the circuit too.

Thanks,
Gaz
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Old 7th February 2003, 03:26 PM   #5
Bobken is offline Bobken  United Kingdom
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Quote:
Originally posted by Sch3mat1c
What's a passive preamp? (Be aware I already know the colloquial answer.)

TIm
Don't understand the question, Tim, if you already know the answer.



Regards,
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Old 7th February 2003, 03:33 PM   #6
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Don't forget to check the tech. specs. of the CDp and other
sources you'll be using. For instance, my Sony CDP should
have a load of at least 50kOhm according to the specs. I
don't know how important this is, though. Maybe it just means
that it won't give the specified output voltage if going below
50kOhm. Anyway, you should keep this in mind and be aware
of it.

Also remember that a passive pre will have an input impedance
that varies when you turn the pot. Well, there might be some
clever ways to avoid this, perhaps, but not with any of the
straightforwards solutions.
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Old 7th February 2003, 03:36 PM   #7
halojoy is offline halojoy  Sweden
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Thumbs up Yes you can make a good Input Signal Control Device

Yes, I think that it just the components I would buy.
If you should use 50k or 100k depends on the situation.
I am in favor of low inputimpedance.

From where comes the signal. The source.
And to where goes the signal - input impedance of real Amplifier.
Best is to have the Volume Control near to the Input of a power amp.

Incorporation of balance can be done in a number of ways.
-----------------------------

Rarkov,
draw us a suggestion.
Then post a preliminary schematic here.
Then we can say more precise.
It is always better to start out with something
and then alter,
than to think on the best way for a long time, without doing anything.

/halo - likes the stuff Rarkov will use
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Old 7th February 2003, 03:59 PM   #8
Bobken is offline Bobken  United Kingdom
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Quote:
Originally posted by Rarkov
Hi,
Bobken: thanks for all your advice...

Maybe I should shed a bit more light on why I'm doing this. I've built a pair of Randy Slone OptiMOS monoblocks (with a slightly raised rail voltage). I do not have a preamp, so I modified a cheap second hand integrated amp to bypass the amps and run as a preamp. It sounds just awful! I am planning to get a Marantz 4300 or 5300 (can't remember which) HC amp. This outputs preouts but for music, I want a really simple preamp. In my "junk box" I have an Alps 100K dual log pot and a 2 pole 6way switch. These are fixed (preferrably).

I suppose what I am asking is: Can I make a good/great sounding passive preamp from these components? I have a virtually unlimited supply of resistors and caps if I need any in the circuit too.

Thanks,
Gaz
As far as the first part of your query goes, you will need to know the various impedances of your existing gear (output of 'send' & input of 'receive') and associated capacitances/ foot and the lengths of cable used etc., to get this right, as I said earlier.

In this connection, it is best to look up some more comprehensive info on the net, as it is too involved for me to explain. Simply put, if you don't 'match' (i.e. get the ratios correct) the input to output impedances very well of any interconnecting equipment, the signal will not 'transfer' properly from the 'sending' gear to the 'receiving' gear.

Unfortunately, with a passive arrangement, the impedances of the pot are not even constant, as they vary with rotation of the pot, which complicates the issue and it has to be a well-chosen compromise, still bearing in mind the overall capacitances of your cables.

It is possible (and it is frequently done) to modify the 'value' of a pot by using what is known as a 'law-faking' resistor, and in your case the result could be something satisfactory, as in such cases is it usual to end up with a 'final' pot value of about one tenth of the original pot value. So you could end up with a pot of about 10K which would probably be OK with most SS gear.

However, it is also usual to use a *linear* pot in these cases, because the addition of this resistor affects the 'taper' of the pot in action, and you say yours is a *log* pot.

In case you don't know the difference, with a linear pot, half the resistance occurs at half rotation, two thirds at two thirds rotation etc. etc., but with a log pot this not so, and the 'taper' can be many different shapes.

Almost anything is possible, but I think if you try with a log pot, you will end up with 'all of the action' so to speak, squeezed up at one end of the pot's rotation, so it will not be very useful to you.

As I said before, you would do well to research the matter before you start, as, otherwise there are too many variables to get wrong, and I am sure with a bit of effort you can find many references to both Passives and law-faking resistors, which should give you what you need to know.


Regards,
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Bob
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Old 7th February 2003, 04:25 PM   #9
halojoy is offline halojoy  Sweden
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Quote:
Originally posted by Rarkov
Maybe I should shed a bit more light on why I'm doing this. I've built a pair of Randy Slone OptiMOS monoblocks).

I suppose what I am asking is: Can I make a good/great sounding passive preamp from these components?
Thanks,
Gaz
You can make a good Volume control and switch box with that.
In fact it will be better for the sound, hifi,
than to have active preamplifiers.

So, if only the impedances match in a fairly good way
and you check up on capcitors,
the variations of the volume will not make any real harm
to the signal.
The outputcap from CD etc. form a RC filter
together with the volumepot.

/halo - waits for the first version drawing
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Old 7th February 2003, 05:09 PM   #10
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"Can I make a good/great sounding passive preamp from these components?"

In my opinion: YES
As you say you want to keep it simple. For good volume range use a LOG pot. Most sources have an output Z < 100-ohms and will happily drive loads in the low K-ohms. A Krell CD will probably drive a power drill directly.

There are a couple of things to bear in mind:

1) When you use a pot the source resistance seen by your amp will vary from zero (when volume is off) to Nohms (on full volume) where N is the CD output R in parallel with the pot R. So typically, the maximum R seen by the amp is the pot R divided by 2. The only problem here is that this R is in series with the amp input capacitance and can cause a low pass filtering effect. You need to do a calculation to ensure this has negligible effect within the audio band. I imagine pots of 50k or less will be ok; I'd go for 10k myself but use what you've got.

2) Make sure you block any dc from the switch and the pot. If there is any dc (even small) flowing through the pot you can get a nasty sound when turning it and any dc on the switch may cause loud clicks when selecting inputs. Use dc blocking caps and 1M-ohm bleed resistors liberally.

Have a go. It is definitely worth putting your concept into action.
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