You can add resistors to lower the volume of a Zero dac/amp

I think the talk about range i kilo-ohms is only confusing. Rather let us talk about the grade of damping instead. (your volume knob turned all the way down will always give no signal to the phones).

Do like this: Let the signal from the preceding stage pass through a large resistor of for example 50k. After this resistor connect a smaller resistor of for example 5k which goes from here and to signal earth. (from the connection point the lead continues to the input of the amp). These two resistors form a voltage divider, and the signal will be damped by a factor of 50/10 =10, that is 20dB. (adjust any of the resistors to get a suitable level). You can place the resistors anywhere before the pot, also inside a signal cable (just don't confuse the direction of the cable afterwards).Hope this helps. Olaf.

I think he was suggesting adding the resistors earlier in the chain - before the inputs to the amp - in the USB signal.

Quote:

Originally Posted by evraamsdonk 

So my question is: can I add two resistors just before the pot to lower the overal volume?

 

The pot is a linear 100K one (B100K). The useful range for me is currently from 0 to 25% rotation, so if I understand correctly that would be the range of 75 to 100KiloOhm. If I were to add two 50KiloOhm resistors then my range would effectively become 50 to 150KiloOhm instead of the current 0 to 100.

 

You can sort of. 

There are techniques to add a resistor to the potentiometer to change its range (or how it works) but on something on a PCB, they usually require a bit of "creative soldering" to implement.

I suspect more of your problems are from the fact that the gear you have has a LINEAR taper pot, as opposed to an AUDIO taper pot. I would bet that exchanging this part for a drop-in replacement in an AUDIO taper will solve the majority of your problems without much work. 

As a side note:

If you have a linear taper pot there should be very little audible difference between half spin & full spin, only a few db. 

Quote:

Originally Posted by evraamsdonk 

Hi Olaf,

Thanks for the explanation. I've tried to draw a little schematic to see if I understand you correctly:

[signal]----[50K resistor]---+---[5K resistor]---+
                             |                   |
                             |                   |
                        [headphone]              |
                             |                   |
                             |                   |
[ground]---------------------+-------------------+

This is mono, for simplicity. Is this correct?

I also very much like your idea of doing it in the cable and leaving the Zero itself untouched. That seems a lot safer

Cheers,

Ewoud

This cable has a VERY VERY VERY high output impedance. I actually like high output impedance stuff, but I think that this will be way past the point of diminishing returns. 

One can never have too many transformers in their signal path (OK, so you can, but usually one more wont hurt that bad) so maybe a 600:32ohm transformer would be nice. I like transformers :) they are cool. 

If your sticking with resistors, use MUCH smaller resistors. 

Replace the 50K in your schematic with 120ohms and the 5K with ~12ohms. Feel free to experiment with these values, but I think 120&12ohms will be a good starting point. 

 what I had in mind...

yes, shown is only one channel, of course

one can also buy plugs with this kind of damping built in, the principle is the same (but not the price) .

(the small resistor drags current through the big one, and thereby causes the voltage to drop)

..

Aah, I only saw "headphones" in the schematic I quoted. 

Something to note with a resistor attenuator before the pot is that you will need to get access to that. I have found this is sometimes difficult in one-box things, but if you can get access it works out. 

I'd still switch the linear pot for an audio one though. 

Quote:

Originally Posted by evraamsdonk 

Thanks for the advice. The reason I assume it's a linear pot is because it's labeled "B100K", and from the info I found the 'B' means linear. I also found that logarithmic pots (or audio pots) are usually used for audio amplifiers, like you said. (Strange that they would use a linear one for this Zero dac/amp then.)

Should I get a audio pot with the same resistance value of 100K?

 

I would do a couple more checks first. 

Do you have any measurement equipment? You can do this with an SPL meter, or a resistance meter (digital multimeter is perfect). IMO *EVERYONE* interested in the audio hobby should own both. Even non-DIYers should own them, The SPL meter is nice for safety, and the DMM has so many uses. 

If you have a resistance meter figuring this out for sure will be so easy! Take the headphone amp board out of the chassis. Set the volume knob to the exact middle of its travel. Measure the resistance from one end of the pot to the middle, and the other way around. With a linear pot the resistance will be roughly half so 50K. With an audio pot, I'd expect something around 250-1Kohm or ~97Kohms depending which side you measure from. 

With an SPL meter I would do the following: 

Obtain a 1Khz test-tone or white/pink noise and play it - you can download one or just make one in all sorts of computer software. Set the volume to minimum and hook up the DT880. Turn the volume knob up roughly 5min on a clock face. note the SPL. turn it up again, note etc. If the pot is an audio taper one the SPL with increase equally with each equal bump of the knob. If its linear the SPL will increase very rapidly for a given spin of the pot at first, and then not increase so rapidly on the louder end of the pot. 

If you have a linear pot, get that out of there. Linear taper pots are often used in less expensive gear because less expensive samples work better. Every time you throw another step into a process there is another point to screw it up.... being simpler to make linear pots DO work better in some ways.

If you do already have an audio taper pot, life is rough :) Im betting you have a linear pot, but please double check me before you go out and get a new one. 

Whether to get 100K ohms, or something else if you need to change it... This is kind of tricky. The absolute sure-fire answer is to get 100K, same as before. 

The pot in the zero is junk, it is a good plan to replace it even if you weren't having volume taper problems.

20k audio taper is what I put in mine, but mine doesn't have the stock headphone amp installed anymore.

Try something like this: http://www.ebay.com/itm/21-Step-Attenuator-Potentiometer-20K-Stereo-Log-2a3-6l6-/120792888917?pt=LH_DefaultDomain_0&hash=item1c1fd13655

It fits the space and original knob and works well for the price.

The headphone amp in the zero is also trash and there isn't really any effective way to "fix" it.

I replaced the headphone amp board in mine with a "Dynalo in a mint tin" amp, but boards for those are

long, long gone. If you wanted a project, something like the Objective II ought to fit in the

space of the original headphone board and probably would be a huge step up, I haven't heard one though.

I know that the mini Dynalo headamp turns the Zero into a very nice all-in-one box. 

The DAC in the zero is actually pretty nice.

The zero also has nice robust +/-15v regulated power to the existing headphone amp board,

and the headphone board is nicely segregated, so wholesale replacing the headphone amp isn't much of a project. 

See schematic of head-amp - decrease R1 to ... I dunno maybe 10k, maybe 4k7 - you try.

And if you plan to do some modification to your ZERO, try this - it is huge step to better sound especially for dynamic headphones. I upgraded some ZEROs and its owners compares "new" ZERO to NuForce Icon HDP, Yulond D100 or HS DAC. So I think, that this upgrade is quite good regarding its price.

BTW: V3 is input, C5 is usualy omitted and R14 is meant to be headphones. Schematic shows one channel only ;)

Heh, the overall topology looks like the O2, except with a discrete class-AB oputput stage.

I won't wanna mess up the internals if i were you, maybe you could just add an 80ohm adapter or something at the output? Never know when you might get a 'phone that needs that high impedance. 

Cheers,

IMAWolf

nikongod: and O2 is what ? 

IMAWolf: it's really safe to lower R1, because original ZERO amplifier has too much gain. It's practically impossible to use this amp with potentiometer set to 12 o'clock and more because of limitation.

Ideal value is R1=6k8 - output voltage at full volume will be few milivolts under limitation.

EDIT: got it ... O2 is Objective 2 by NwAvGuy ;)   It's similar, but I guess, that unloaded OA works better than OA directly driving low impedance ...