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
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
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,
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,
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
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
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.
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.
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
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
Rarkov said:
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,
You can make a good Volume control and switch box with that.Rarkov said:
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
"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.
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.
Shoog
I built a passive preamp and got very favourable results with just a 7way switch and a 100k pot. I was using it with a valve amp which was fairly forgiving. I used "Shark Cable" which is availble from Maplin-I highly recommend this for all internal wiring, and its really cheap. I did find that the log pot that I used tended to have very poor adjustment with the useful range been bunched at one end. I ended up adding faily high resistances infront of the pot. The reason I could get away with been so lax with the technical issues was because i had a Musical Fidelity X10D line conditioner between the pre and power amp, which acted as a buffer stage. If you have little joy with getting things working properly a simple op-amp buffer stage might been a cheap and easy solution.
If you wanted to splash out, a stepped attenuator presents a stable load to the sources whatever the volume setting and eliminates at least one of the problems- those who are serious about their passive preamps always use them.
Whats all this crap about "Whats a Passive Preamp ?", pedantry is at the root of most of the worlds evils. if it works (and it does) then use it.
Shoog
I built a passive preamp and got very favourable results with just a 7way switch and a 100k pot. I was using it with a valve amp which was fairly forgiving. I used "Shark Cable" which is availble from Maplin-I highly recommend this for all internal wiring, and its really cheap. I did find that the log pot that I used tended to have very poor adjustment with the useful range been bunched at one end. I ended up adding faily high resistances infront of the pot. The reason I could get away with been so lax with the technical issues was because i had a Musical Fidelity X10D line conditioner between the pre and power amp, which acted as a buffer stage. If you have little joy with getting things working properly a simple op-amp buffer stage might been a cheap and easy solution.
If you wanted to splash out, a stepped attenuator presents a stable load to the sources whatever the volume setting and eliminates at least one of the problems- those who are serious about their passive preamps always use them.
Whats all this crap about "Whats a Passive Preamp ?", pedantry is at the root of most of the worlds evils. if it works (and it does) then use it.
Shoog
Sch3mat1c said:
Impressive command of the English language, Tim, but not much value in a Forum like this which is about DIY Audio.
I am 'too long in the tooth' to fall for that one, anyway!
The originator of the thread has explained why he needed this arrangement, which is always referred to as a "passive preamp" even if to pedantics this may appear to be a contradiction in terms.
A question I have for you is, are you interested in DIY audio, or what?
Regards,
Hiya,
Sorry to confuse anyone...Here is my:
1) Passive Pre-processor
2) Passive Pre-Volume, Balance box
3) Passive Pre (meaning 'before') Power Amplifier (AKA Preamp)
No offence...Hadn't even thought of it as a condradiction until you mentioned it.
Anyway...Here is a design. I have the parts - just have to build it. I'm not going to add the balance just yet...Oh, and I forgot the DC blocking caps (You'll have to imagine them! )
Red lines denote a 'ganged' component...My CD player does have a volume control, but these are usually to get all of your source components at the same level. Using it as a full scale volume introduces a very loud hiss at low volumes. It sounds awful. I always have the CD player on "full volume" for it to sound at it's best. I wish I had a Krell player...but I'm afraid it's just an old kenwood! On the other hand my Tuner, Modded Xbox, TV Preout, Free2View box all come without a means of volume control. And Finally, as for my Amps, since they are monoblock Power Amplifiers, I didn't want to put volume controls on them...I think they are too difficult to keep at the same levels to be convienient.
Hope you like the Schematics...
P.S. Isn't Oxymoron a type of anti-spot wash?
Sorry to confuse anyone...Here is my:
1) Passive Pre-processor
2) Passive Pre-Volume, Balance box
3) Passive Pre (meaning 'before') Power Amplifier (AKA Preamp)
No offence...Hadn't even thought of it as a condradiction until you mentioned it. Anyway...Here is a design. I have the parts - just have to build it. I'm not going to add the balance just yet...Oh, and I forgot the DC blocking caps (You'll have to imagine them!
)Red lines denote a 'ganged' component...My CD player does have a volume control, but these are usually to get all of your source components at the same level. Using it as a full scale volume introduces a very loud hiss at low volumes. It sounds awful. I always have the CD player on "full volume" for it to sound at it's best. I wish I had a Krell player...but I'm afraid it's just an old kenwood!
On the other hand my Tuner, Modded Xbox, TV Preout, Free2View box all come without a means of volume control. And Finally, as for my Amps, since they are monoblock Power Amplifiers, I didn't want to put volume controls on them...I think they are too difficult to keep at the same levels to be convienient.Hope you like the Schematics...
P.S. Isn't Oxymoron a type of anti-spot wash?
Attachments
Ah, ok.
(I have a problem with the words "passive preamp". )
The design looks fine to me, but if you have marginal signal levels now (like, listening levels just fall within control levels), this will attenuate the signal a lot, meaning you won't even get to listening level before your controls hit the wall.
But I take it you still have the headroom on your monoblocks' controls?
I'm not sure of the balance...don't one of the sides have to be antilog for that to work?
Tim
(I have a problem with the words "passive preamp".
)The design looks fine to me, but if you have marginal signal levels now (like, listening levels just fall within control levels), this will attenuate the signal a lot, meaning you won't even get to listening level before your controls hit the wall.
But I take it you still have the headroom on your monoblocks' controls?
I'm not sure of the balance...don't one of the sides have to be antilog for that to work?
Tim
Sch3mat1c said:
(I have a problem with the words "passive preamp".
It is illogical in the sense that it is an amplifier that doesn't
amplify, but it is logical in the sense that it is used in the
same way and in the same place in the signal chain as
a preamplifier. Either way, the term has been around for
over ten years, I think. They started to appear when many
people had switched over entirely to CD and didn't need
a phono amp anymore.
Hi,
Thanks for all the helpful replies! As I said, the balance is totally optional, so I'll leave it out.
When my sources are fed directly into the amps, I can't listen to them. In fact, I have my setup in a typically "american setup" (don't flame me - I read it in a magazine!). That is where you have your source and amps etc. between your speakers. I couldn't get close enough to turn it all off without seriously hurting my ears - Thank god for remotes!
My monoblocks have absolutly no controls on them whatsoever (apart from a power switch.) Here is a link to some pics of them...
PICS
(They're from about half way down the page)
Thanks,
Gaz
Thanks for all the helpful replies! As I said, the balance is totally optional, so I'll leave it out.
When my sources are fed directly into the amps, I can't listen to them. In fact, I have my setup in a typically "american setup" (don't flame me - I read it in a magazine!). That is where you have your source and amps etc. between your speakers. I couldn't get close enough to turn it all off without seriously hurting my ears - Thank god for remotes!
My monoblocks have absolutly no controls on them whatsoever (apart from a power switch.) Here is a link to some pics of them...
PICS
(They're from about half way down the page)
Thanks,
Gaz
Hi Rarkov,
I am sorry to see that you appear to be disregarding some of the advice which has been given to you in this thread, and fear that you may end up with an arrangement which is as unsatisfactory in use, as you say your first effort was.
If what I, and several others, have said is ignored you will almost certainly end up with something which is definitely *not* Hi-Fi, but if that is OK with you, as I also said before, almost *anything* will do.
One cannot simply overcome the 'physical laws' associated with impedances and capacitances, and with your present proposal, you will inevitable end up with a severely curtailed HF response.
As Traderbam suggests, you need to keep the pot's value low, and even the 10K which he (and I) have suggested is not ideal here, although it is probably about the best compromise, especially for SS gear.
The output impedance of the 'passive' will depend entirely on the output impedance of the various bits of preceeding gear you use as your source, together with the value of the pot chosen, and you do need to take these into account.
If, for example, the output impedance of the passive ends up at perhaps 50K at full rotation (quite possible with say a 100K pot) and the capacitance of the following cable is 300pF (again, quite likely), the 3dB time constant (i.e. where the power is *halved*!!) will occur at just over 10KHz.
This is well within the human hearing range, especially for someone quite young, like yourself, whose 'upper limit' will be very much higher than an old guy like me!
As the reduction in power *commences* at a decade *lower* than the 3dB point, the signal will start to be reduced from just over 1KHz! Is that what you want in use?
Under these circumstances, you will find that the volume control will act more like a tone control, unfortunately!
As I said earlier, I don't wish to deter you in any way, but with a little effort in research, you can overcome (most of) the problems, but the result will inevitably always be a compromise, based on all of the parameters of your existing equipment and the chosen cables etc., and only you can make the right choice here.
However, you do also need to understand the other requirements over impedance 'matching' which I, and some others have referred to, as otherwise, you may end up with very little signal being transferred between the source and the amp, even though the signal might not be reduced severely at HF.
Incidentally, I would certainly dispense with the proposed balance control, as someone else has said, as this will also tend to have a deleterious affect on the sound.
Regards,
I am sorry to see that you appear to be disregarding some of the advice which has been given to you in this thread, and fear that you may end up with an arrangement which is as unsatisfactory in use, as you say your first effort was.
If what I, and several others, have said is ignored you will almost certainly end up with something which is definitely *not* Hi-Fi, but if that is OK with you, as I also said before, almost *anything* will do.
One cannot simply overcome the 'physical laws' associated with impedances and capacitances, and with your present proposal, you will inevitable end up with a severely curtailed HF response.
As Traderbam suggests, you need to keep the pot's value low, and even the 10K which he (and I) have suggested is not ideal here, although it is probably about the best compromise, especially for SS gear.
The output impedance of the 'passive' will depend entirely on the output impedance of the various bits of preceeding gear you use as your source, together with the value of the pot chosen, and you do need to take these into account.
If, for example, the output impedance of the passive ends up at perhaps 50K at full rotation (quite possible with say a 100K pot) and the capacitance of the following cable is 300pF (again, quite likely), the 3dB time constant (i.e. where the power is *halved*!!) will occur at just over 10KHz.
This is well within the human hearing range, especially for someone quite young, like yourself, whose 'upper limit' will be very much higher than an old guy like me!
As the reduction in power *commences* at a decade *lower* than the 3dB point, the signal will start to be reduced from just over 1KHz!
Is that what you want in use?Under these circumstances, you will find that the volume control will act more like a tone control, unfortunately!
As I said earlier, I don't wish to deter you in any way, but with a little effort in research, you can overcome (most of) the problems, but the result will inevitably always be a compromise, based on all of the parameters of your existing equipment and the chosen cables etc., and only you can make the right choice here.
However, you do also need to understand the other requirements over impedance 'matching' which I, and some others have referred to, as otherwise, you may end up with very little signal being transferred between the source and the amp, even though the signal might not be reduced severely at HF.
Incidentally, I would certainly dispense with the proposed balance control, as someone else has said, as this will also tend to have a deleterious affect on the sound.
Regards,
It will work - without the balance
Rarkov,
Apart from the balance circuit your schematic will work.
If you knew the recomended minimum load impedance
of all your source components, and it was below 10k
(I would bet they are), I would choose a 10k log pot rather
than the 100k shown.
As Bobken pointed out the effective output impedance will
interact with the cable capasitance (and amp input
capasitance) to form a lowpass filter - the 10k pot would
improve the situation and with short / low capasitance cable
the effect would be negligable.
If you must stick with 100k vol pot you should think about
going the active-passive-pre-amplifier route
with a
unity gain buffer.
Even with half-reasonable source components you won't
need balance - I don't and find it a blessing as there is one
less thing to fiddle with.
BTW I don't think you will need a blocking capasitor, just put
1M to ground on each input and on the output. This will pull
any source component coupling capasitors to ground.
If you get a thump when you switch you will know I was wrong
.
Maybe you could add the coupling caps only to those
inputs that need them, with a 1M to ground.
Dave
Rarkov,
Apart from the balance circuit your schematic will work.
If you knew the recomended minimum load impedance
of all your source components, and it was below 10k
(I would bet they are), I would choose a 10k log pot rather
than the 100k shown.
As Bobken pointed out the effective output impedance will
interact with the cable capasitance (and amp input
capasitance) to form a lowpass filter - the 10k pot would
improve the situation and with short / low capasitance cable
the effect would be negligable.
If you must stick with 100k vol pot you should think about
going the active-passive-pre-amplifier route
with aunity gain buffer.
Even with half-reasonable source components you won't
need balance - I don't and find it a blessing as there is one
less thing to fiddle with.
BTW I don't think you will need a blocking capasitor, just put
1M to ground on each input and on the output. This will pull
any source component coupling capasitors to ground.
If you get a thump when you switch you will know I was wrong
. Maybe you could add the coupling caps only to thoseinputs that need them, with a 1M to ground.
Dave
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