[Otis]

Occasionally I've read vague references to the effect that electronic volume controls are bad, while old style Alps or Noble pots are the best.

Is this true and more importantly, why? You can't beat the convenience of electronic controls,  - so why not use them? Do they damage the tiny signal somehow? 

Just curious...

[jules]

I've read vague references to the effect that electronic volume controls are bad

Something of a generalization. There's first class motorized pots and remote controlled attenuators that will exceed the performance of Alps or Noble. You will pay for quality in a remote controlled attenuator.

The answer to your question also depends on whether you are asking about what you should use for a project or if you just want to assess the quality of a particular piece of equipment.

jules

[richidoo]

All volume controls are electronic, they do something to reduce the voltage of the signal.

There are three popular ways to accomplish this, and within the three groups there are multiple versions.

Resistive attenuators limit the flow of current through resistance and thus reduce the signal voltage - potentiometers, resistor/relay networks, light dependent resistors. There are volume control  integrated circuits which are controlled by a micropocessor or pot, but they are still resistive analog attenuators.

Magnetic attenuators use magnetic inductance to convert signal current to magnetic flux in a magnetic core, then convert that flux back into electric current on a separate output winding. Transformers (TVC) and Autoformers use this method.

Digital volume controls change the voltage of the signal in the digital domain, and can only effect a digital signal. An analog signal must first be converted to digital using a ADC before volume proicessing. Attenuation of digital stream can be done two ways. The first and most popular by far is simple dropping the least significant bits from each digital word before it enters the DAC, done by a simple DSP chip. This lowers the signal voltage coming out of the DAC, but also loses resolution by throwing away data. This is audible on high resolution system with lots of attenuation. YMMV. This is usually called a digital volume control.   Another method of digital attenuation requires huge processing power, and multiplies the value of each digital word by a percentage, thus generating a new low value without throwing away any data. The resolution is retained, but processing demands are greatly increased. This method is used in post processing of digital recordings in mastering with digital audio workstation.  It is not practical for this type of volume control to be used in consumer audio when analog attenuators are just as effective when the signal is already in analog domain anyway. 

Good bad is in the eye of the beholder. Most people find they need an attenuator of some kind, and they are all electronic. So choose your poison. Even pots and resistors ad noise to the signal. Optical reisstors (LDR) are probably the lowest distortion attenuator besides high bit depth digital DSP.

There are DACs available now with internal 32 bit volume control. When playing a 16 bit CD they can attenuate the signal by 96dB with no loss of resolution. It really works! ESS Sabre32 is one, but there will be more coming.

I hope this helps.
Rich

[Kevin Haskins]

All volume controls are electronic, they do something to reduce the voltage of the signal.

There are three popular ways to accomplish this, and within the three groups there are multiple versions.

Resistive attenuators limit the flow of current through resistance and thus reduce the signal voltage - potentiometers, resistor/relay networks, light dependent resistors. There are volume control  integrated circuits which are controlled by a micropocessor or pot, but they are still resistive analog attenuators.

Magnetic attenuators use magnetic inductance to convert signal current to magnetic flux in a magnetic core, then convert that flux back into electric current on a separate output winding. Transformers (TVC) and Autoformers use this method.

Digital volume controls change the voltage of the signal in the digital domain, and can only effect a digital signal. An analog signal must first be converted to digital using a ADC before volume proicessing. Attenuation of digital stream can be done two ways. The first and most popular by far is simple dropping the least significant bits from each digital word before it enters the DAC, done by a simple DSP chip. This lowers the signal voltage coming out of the DAC, but also loses resolution by throwing away data. This is audible on high resolution system with lots of attenuation. YMMV. This is usually called a digital volume control.   Another method of digital attenuation requires huge processing power, and multiplies the value of each digital word by a percentage, thus generating a new low value without throwing away any data. The resolution is retained, but processing demands are greatly increased. This method is used in post processing of digital recordings in mastering with digital audio workstation.  It is not practical for this type of volume control to be used in consumer audio when analog attenuators are just as effective when the signal is already in analog domain anyway. 

Good bad is in the eye of the beholder. Most people find they need an attenuator of some kind, and they are all electronic. So choose your poison. Even pots and resistors ad noise to the signal. Optical reisstors (LDR) are probably the lowest distortion attenuator besides high bit depth digital DSP.

There are DACs available now with internal 32 bit volume control. When playing a 16 bit CD they can attenuate the signal by 96dB with no loss of resolution. It really works! ESS Sabre32 is one, but there will be more coming.

I hope this helps.
Rich

I was going to add digital control of fixed resistors but I see you covered that.   

Something like a TI PGA-2320 is controlled by a PIC microcontroller that switches internally to different fixed resistor values.   It also has an internal opamp for buffer & voltage drive.    Some people poo-poo the performance because they are opamp snobs but the performance is hard to argue with.   

I've seen them used in $10K preamps that are reviewed well (doesn't mean much) and they are essentially a $20 part.   You have to build a power supply, design the board, program the PIC but they do everything you need to do in a volume control and maintain linearity much better than most approaches.   

[bummrush]

 What category do Ayre pre amps come under? 

[richidoo]

Cirrus Logic also makes a nice 8ch VC chip.  These ICs are often used in the pro-sumer audio gear that records most pop and rock music and they are very linear. But audiophile quality recordings are more demanding. You won't see VC chips in sota mic preamps or mixing boards.

The Ayre KR pre varies the voltage gain of the amplifier in a proprietary way. Variable gain is the ideal because it eliminates the noise and distortion of the attenuator and reduces noisefloor too, but in practice it is hard to pull off, there are a lot of issues that arise with variable gain, like variable impedance and variable tone. Especially in a no feedback design like the Ayre. But it has good reviews so far (don't they all?) PS Audio also had such a product a couple years ago.

[mgalusha]

Additionally on the Ayre the use Shallco switches with surface mount resistors to control their variable gain section. The inside of an Ayre preamp is a thing of beauty. The local audio society held a meeting at Ayre last year and the engineers were surprisingly open about how things worked and we spent a considerable amount of time in the assembly area. 

[art]

Something like a TI PGA-2320 is controlled by a PIC microcontroller that switches internally to different fixed resistor values.   It also has an internal opamp for buffer & voltage drive.    Some people poo-poo the performance because they are opamp snobs but the performance is hard to argue with.   

I've seen them used in $10K preamps that are reviewed well (doesn't mean much) and they are essentially a $20 part.   You have to build a power supply, design the board, program the PIC but they do everything you need to do in a volume control and maintain linearity much better than most approaches.

That is a bit of an over-simplification of how they work. They have a very limited attenuation range. To achieve the wide range that they are capable of, they change the gain of the internal op-amp, in very broad steps. This not only affects THD (probably not important), but also SNR and BW.

Measurements aside, we find them to be very disappointing. Yes, I realise that many companies use them (including ones we are "buddies" with), but you can add us to the list of those who find them wanting.

Look at it another way: would you want to have a TL072 in your system? I doubt that it uses anything that good.

Pat