[Mullet]

I recently built "The Wire" amplfier. Just finished it last night. Pictures to come in the build thread. Here's my issue.
 
When I turn the volume up it takes a super long time for the volume to get loud. It gets loud enough at around 3 o'clock on the pot. I've read somewhere that the Wire's input impedance is 10Kohms. I'm using an 50K RK27 pot. Help me understand in simplistic terms why this is the case. I want to be able to wrap my head around it. Some plausible solutions are to change the pot or change the input resistance of the amp. Also how does my different headphones at 32 to 250ohms effect this?

 

[NA Blur]

I would definitely check both the resistor values and cap values you have used.  This includes the POT. resistance value.  It sounds like the RC time constant is much too high.  Basically the time it takes to charge / discharge a cap. is approximately the resistance in Ohms times the capacitance in Farads.  Keep in mind that the capacitance is usually in microFarads so the time constant ( RC ) should be pretty small.  Below is a sample calculate for the RC time constant.
 
Resistance of output component:  2 kOhms
Capacitance used to charge the output stage:  300 microFarads
Time Constant = R*C
= 2000 Ohms * 300 *10^-6 Farads
= .6 seconds
 
That means that it will take over .5 seconds to charge and discharge the cap.
 
There are some other factors in there as well like the cutoff frequency, but the RC time constant is probably your culprit.
 
Does your board look like the one on this page:
http://www.diyaudio.com/wiki/The_Wire_Headphone_Amp_Build_Wiki
 
Let us know what you find, but it really sounds like you mixed up a cap. or two.

 

[Mullet]

I think you're misunderstanding me. I turn up the pot and the volume doesn't get loud until 3 o'clock. It's not taking a while for the volume to catch up or get loud in general. But you might be onto something with having a wrong part somewhere. I doubt it though -- I was pretty careful when assembling the amp.

 

[wakibaki]

Pots are available in a number of 'tapers'. Logarithmic (audio), linear and antilogarithmic. I don't know about antilogarithmic, but I think the RK27 series are available in logarithmic and linear. I've googled around, but I can't verify this for sure. There may be a complication in that these parts are sometimes forgeries.
 
You need a logarithmic, using a linear type may give the problem described. Try checking the numbers after the 27 to identify the exact type.
 
An incorrect taper is the most likely cause of your problem.
 
Different headphones have different sensitivities and can sound louder or quieter at the same setting, but 'the wire' is capable of driving most types adequately with a suitable supply voltage, although you may prefer to adjust the amplifiers intrinsic gain if it is quiet with insensitive phones 
 
w

 

[jdkJake]

There are no caps in "The Wire" signal path.

What gain did you settle on. Did you build it unity gain or did you end up bumping it up?

You are probably used to an amp with high levels of gain, hence the very small amount of travel needed to reach loud volume levels. Less gain, more travel on the pot until you get to loud volume levels. Ideally, the gain setting is such that you normally spend most of the time in the most linear, channel matched section of the pot (usually away from the extremes).

The best gain setting is the one that maximizes the amount of signal allowed to pass through the attenuator and still reach acceptable volume levels without clipping or distortion.

Sounds like you have it just about perfect to me.

 

[Mullet]

Ok. This is what I did to remedy the problem and it seems to be a balance of whether to change the input resistors or gain resistors. I changed from a gain of 2 to 4. This helps and I'm able to obtain the loudness I'm looking for at around 1-2 o'clock and perhaps a little higher for LCD-2s.
 
The reason I was thinking it had to do with the input impedance vs. the pot impedance was from the "The Wire" thread on DIYaudio. A fellow mentioned that "The SE-SE has an input impedance of 11k I'd use a 2K or maximum 5k type A taper pot to get a relatively even volume change across the dial range. With a 50k pot, though I've never tried myself, you'll probably get little volume change when you turn it up until you are approaching the maximum end of the travel." -- this is what I thought I was experiencing. He also mentioned that 100k resistors would match up with a 50k pot. I never really understood why. Anyways, I do have the 100k resistors on hand if I felt like changing the input resistors out.
 
Typically, I'm used to the Cavalli CTH and of the many I've built they are always at around 11 to 1 or 2 o'clock on the pot depending on the choice of tube and headphones. Guess that amp has a lot more gain than the Wire.
 
Thanks for the suggestions guys.

 

[stv014]

Quote:

the Wire's input impedance is 10Kohms. I'm using an 50K RK27 pot.

 
If you mean there is a 10 kΩ input impedance after the potentiometer, then, since it has an impedance of 12.5 kΩ at 50% volume, but (almost) zero at 0% and 100%, it changes the taper function so that medium volume settings become quieter by up to 7 dB. If it is additionally a logarithmic potentiometer (like it normally should be for a volume control), then you need to turn it up quite a bit to get high volume.
As a solution, I would recommend using a lower resistance potentiometer, e.g. a 10 kΩ one.
 
Quote:

Also how does my different headphones at 32 to 250ohms effect this?

 
It should not really matter.

 

[qusp]

Quote:

There are no caps in "The Wire" signal path.
What gain did you settle on. Did you build it unity gain or did you end up bumping it up?
You are probably used to an amp with high levels of gain, hence the very small amount of travel needed to reach loud volume levels. Less gain, more travel on the pot until you get to loud volume levels. Ideally, the gain setting is such that you normally spend most of the time in the most linear, channel matched section of the pot (usually away from the extremes).
The best gain setting is the one that maximizes the amount of signal allowed to pass through the attenuator and still reach acceptable volume levels without clipping or distortion.
Sounds like you have it just about perfect to me.

 
this^^ why are you trying to make it so you reach louder levels sooner? the ideal is for it to be your preferred listening level most of the way turned up with a little bit to adjust for different headphones and source material. making it louder sooner means higher noise levels at normal listening; its called setting a suitable gain structure, you never want much more gain than you actually need.
 
RC time constant hehe sorry that was such a wordy answer that completely and utterly missed the point

 

[qusp]

Quote:

 
If you mean there is a 10 kΩ input impedance after the potentiometer, then, since it has an impedance of 12.5 kΩ at 50% volume, but (almost) zero at 0% and 100%, it changes the taper function so that medium volume settings become quieter by up to 7 dB. If it is additionally a logarithmic potentiometer (like it normally should be for a volume control), then you need to turn it up quite a bit to get high volume.
As a solution, I would recommend using a lower resistance potentiometer, e.g. a 10 kΩ one.
 
 
It should not really matter.

yup, it has 10k after the pot, the wire was initially designed to be used with dacs with digital volume control

 

[Mullet]

Quote:

You need a logarithmic, using a linear type may give the problem described. Try checking the numbers after the 27 to identify the exact type.

 
I'm pretty sure it's not a fake. I bought a few from beezar. I trust TomB is selling me a legit item. Here's the part # RK27112A-S25-C0-A503.
 
 
Quote:

 
If you mean there is a 10 kΩ input impedance after the potentiometer, then, since it has an impedance of 12.5 kΩ at 50% volume, but (almost) zero at 0% and 100%, it changes the taper function so that medium volume settings become quieter by up to 7 dB. If it is additionally a logarithmic potentiometer (like it normally should be for a volume control), then you need to turn it up quite a bit to get high volume.
As a solution, I would recommend using a lower resistance potentiometer, e.g. a 10 kΩ one.
 
 
It should not really matter.

So what would creating a 100k ohm input resistance do? It seems to me I'd have to up the gain a bit to compensate for more input resistance. I've already changed the gain to 4 and it's a happy medium. Just curious, how did you get the 12.5k ohm figure? I might try and get a 10k RK27 to see what happens. If I can find one that is.
 
 
Quote:

 
this^^ why are you trying to make it so you reach louder levels sooner? the ideal is for it to be your preferred listening level most of the way turned up with a little bit to adjust for different headphones and source material. making it louder sooner means higher noise levels at normal listening; its called setting a suitable gain structure, you never want much more gain than you actually need.
 
RC time constant hehe sorry that was such a wordy answer that completely and utterly missed the point

I suppose you're right. If I can achieve the proper listening level then why not stick with what I got.
 
So then this brings me to my next question. How does one know what impedance to choose for a pot? Let's say you don't know what your input impedance is or the output impedance of your source.

 

[wakibaki]

I too doubt very much that something sold by tomb is not legit or is seriously inappropriate.
Quote:

How does one know what impedance to choose for a pot? Let's say you don't know what your input impedance is or the output impedance of your source.

 
If you really don't know the source and input impedance then you use a value based on what is typically found in the wild. You have to take into account anyway that somebody may plug in something unknown to you at design time.
 
Most DAPs have 600ohms or less out, often much less. This is because they have an integral amp to drive headphones. Most CDPs have an output impedance in this range too, but probably at the higher end because they don't have to drive phones.You're going to use a value as low as possible because it keeps the Johnson noise down but the lowest value commonly available in dual-gang logarithmic is 10k. You can get lower but obtaining them may be difficult. Also you don't want to load down the source so a value of about x10 the source impedance is usually chosen. The amplifier input impedance is unlikely to be less than 10k. All you're really doing though is following the example of typical solid state equipment. If it was a valve guitar amp you'd probably use 250k or 500k, because guitar pickups would be loaded down by less and valve amps have a very high input impedance.
 
When you actually know the impedances of the driving and driven equipment you can be more precise, but the driving and driven equipment see different values of impedance depending on the actual volume set and on the value of the source impedance when looking back out and on the amplifier impedance when looking in. This is probably why you're seeing the effect on the apparent taper that you are. It hadn't occurred to me that this might be what is going on because a pot of the wrong taper is the most common beginners mistake since the actual values are usually chosen by someone more experienced..
 
This is why the choice of these values is, to an extent, not much more than custom and practice. They're a compromise that has been considered many times previously. You're unlikely to see less than 5k and unlikely to see more than 50k (in solid state equipment) and either will probably work more-or-less OK with whatever SS gear you plug in.