[till]

I am thinking of using a digital potentiometer in my next amp because the noise, channel imbalance and/or sheer size of most analog pots is getting on my nerves. Most digital pots require a certain voltage to operate and thus need their own electrical system, negating any space saved by ditching an analog pot - well, thinking of this, they allow for a more spread-out design instead of the cubic form factor most quality analog pots have. The only variable and high-voltage model I found is the AD7376 (from +-5V to +-15V), but alas, it is available as a single-channel version only.
These are the ICs I can get over here without more-than-usual difficulty, with some of their characteristics:

1. DS1666: +-5V / +5V, single, log, 10k/50k/100k
2. DS1802: +-5V / +3 to +5V, dual, log, 45k, pushbutton operation
3. DS1267: +-5V / +5V, dual, lin, 10k/50k/100k
4. DS1867: non-volatile version of DS1267
5. DS1808: +-12V, dual, log, 45k
6. AD7376: +-5V to +-15V, single, 10k/50k/100k/1M
7. CAT523: +2,7 to +5,5V, dual, 28k, non-volatile
8. TDA1524: +3 to +18V, dual, not a true digital potentiometer?

As is my understanding, the DS1802 is the only pot on the list that does not require a controller and hence the one to go with if one appreciates a simple and space-efficient implementation. Furthermore, it has two channels and features logarithmic characteristics. The major drawback besides its demands on the power supply is that the wiper position is not remembered between power-off and power-on, but I could live with that. I think I read that this is the IC Dr. Xin uses in his amps, there must be a good reason.
An alternative might be the TDA1524. It needs some extra components such as an analog pot, but it accepts a wide range of voltage and is very cheap. It can also modulate bass, treble and balance.
Here is a link to the datasheet, which is quite hard to find:
http://www.produktinfo.conrad.com/da...-tda_1524a.pdf
Here is a circuit diagram for using it as a volume control (sorry, in German only):
http://mitglied.lycos.de/hobbyelec/a...regelstufe.zip
However, the TDA1524 seems to be quite old-fashioned. Its need for an analog pot adds to space requirements, defeating the purpose of saving space with a digital pot. OTOH, you can use a nice knob instead of pushbuttons, without an analog pot´s scratchy noise

I did not find much information about digital pots. Any recommendations? On Headwize, Apheared wrote that the DS1802 works well for him, but in another posting someone else reported about clipping at high volumes.
Please tell me what you think and also if I got something wrong or missed some important points (which I´m sure of

). Also, I´m just getting started with this and electronics in general and do not get the whole picture, i.e. how hard it might be to provide a steady voltage that most of the aforementioned pots require. Using a microcontroller is beyond my grasp at this time, but I might give it a try later.

Or would I be better off skipping this idea and using an expensive (and big!) analog Alps pot?



EDIT: typo and CAT523 voltage.

EDIT 2: I just found Mark Hennessy's homepage, which has a section that discusses several digital potentiometers:
http://www.mhennessy.f9.co.uk/preamp...e_research.htm

 

[morsel]

The DS1802 draws 20-40µA in standby mode. Perhaps you could hook it up such that the battery is never disconnected from it when the power is shut off, only from the rest of the amp, so it remembers the volume level except when you change batteries. Acceptable for rechargables, possibly a liability for disposable batteries if the amp is not used often.

 

[digi01]

I think BB PGA2310 is a goog chip,it is designed for professional and high¨Cend consumer audio systems.
because of it is a pga ic, it some difficult in mic programing and digital-analog set...

 

[till]

morsel, that is a great idea. Actually, I could try to add an independent power supply, i.e. a lithium knob battery. The Sony CR2450, for example, is cheap, has a voltage of 3 V and gives 600 mAh. So, the DS1802 would run for 15000 h in standby mode (40 µA) and for 300 h in active mode (2 mA) with this battery. I´ll have to see how long it can supply 3 V in real life.

digi01, I read about the PGA2310, it has great potential, but I´m not ready for microcontrollers yet.

I think I´ll give the DS1802 a try and see if it works out for me.

 

[kwhead]

I have no familiarity with those parts specifically, but do remember that digital pots can be canidates for adding more distortion than passive resistors. That said, they are used in a lot of audio equipment these days that have wonderful distortion measurements, so clearly there are good ones and good ways to use them. I'd look for any specs related to linearity or distortion on the datasheet and also see if you can find any app notes on the "right" way to use them (i.e. best place to put them in the circuit). Maybe that was obvious advice...

 

[tangent]

Regarding your power supply worries:

1. You could put a 78L05 next to the chip to drop your amp's supply voltage down to 5V. You wouldn't need a heat sink, for a 2mA draw.

2. If you go the battery route, you could use a diode OR bridge to power the digital pot from the wall supply when it is plugged in, and let the battery run the pot in shutdown mode when the wall power is removed.

3. The main disadvantage of #2 is that 3V probably isn't enough due to the diode's forward voltage drop. To work around that problem, you could use a multipole power switch to jigger all the various power sources so only one is running the chip at a time.

I'd definitely stick to dual log pots. Let everything else be ruled by that constraint.

 

[morsel]

Is this a battery or AC powered amp?

How about a resistor from power source to digital pot before the power switch?

I would stick with the Alps Blue.

 

[till]

kwhead, no, that was not really obvious, at least not to me. I'll keep the possible distortion in mind for future experiments, have to learn to interpret the graphs until then

tangent, thank you for hinting at the 78L05 and a diode bridge. I am just learning about components and their possibilities. Also, I'll see if I can find a nice multipole switch.

morsel, it is a battery-powered CHA47 I am thinking of, but it will definitely have an AC plug. A resistor for diminishing voltage to the digital pot would cause problems because of the battery's constantly dropping voltage, right?
The Alps blue would cost me about twice as much as a DS1802 (including additional parts) and limit the choice of enclosures pretty much, that's the problem. Currently a 80 x 61 x 23 mm case is my favorite. The Alps is 27 x 25.3 x 24.5 mm, so I want to resort to that only if my experiments with digital alternatives turn out to be disappointing. And of course it is fun to try new things

 

[morsel]

If you are concerned you can use a FET current source, but I suspect you can get away with using a resistor.

 

[Dreamslacker]

I reckon you can use a 5v supercap to keep the controller on all the time. Each time you switch on the amp, the cap's recharged again.

 

[till]

morsel, I guess I'll do some tests with a FET and a resistor. Which approach will be more lossy power-wise?

Dreamslacker, if I use a 1 F goldcap and the DS1802 draws 40 µA in quiescent state and it will operate until the voltage drops from 5 V to 3 V, the maximum time of operation would be
t = 1 As / 1 V * 2 V / 40 µA = 50,000 s = 14 h ,
am I right on this? I took my last physics class five years ago... 14 h would not be sufficient, then it would be better to power the DS1802 directly from the battery, bypassing the power switch.

 

[Nisbeth]

Is it necessary to keep the DS1802 powered at all time ? It starts up in mute-mode so each time you turn it on you have to readjust the volume, but I don't feel that this would be a major inconvenience. It might even be an advantage because there's never a risk that the volume is way too loud at startup.

Anyway, just a thought

/U.

 

[morsel]

Nisbeth, your mileage may vary, but I find digital volume control to be a nuisance if it does not remember the volume level. I also do not like up/down volume buttons, I prefer a rotary knob. Too bad rotary digital encoders are a pain to implement.

 

[till]

Nisbeth, I missed that, great news. I thought that the DS1802 starts at medium volume on power-on, but that must have been a different IC I mixed it up with. Anyway, do you know how long it takes the DS1802 to reach full volume when you hold the "up" button? If we're talking about a mere two seconds or something like that, I wouldn't mind since I listen at a low volume.
But a goldcap won't hurt either if I have the space left (20 x 8 mm for 1 F for the part I will order), I think it is quite relaxing just to have to flip the power switch and nothing else

morsel, the knob issue is a point. I do not like pushbuttons, too. It has been a real pain to find a decent (and cheap) car stereo with good looks and a knob. The one I got actually has a rotary encoder.
But with portable devices (and only with portable devices IMO), buttons have some advantages -- they cannot be accidentally operated as easily when the amp is in a bag or backpack. I am even planning to build them recessed into the case to prevent these issues.

 

[Nisbeth]

till, I do not remember the time exactly, but it's in the datasheet somewhere

/U.

EDIT: Found it. 7.3 seconds for full scale travel. (p5-6 in the datasheet)
Sounds pretty much ideal for portable devices. Also, I'm quite sure many portable devices do not have a full 2Vrms output, meaning that clipping shouldn't be a problem. Add an 78L05 for the supply voltage and off you go