High Gain Amplifiers?

[mcstatz5829]

control artifacts of the interconnect cable between the preamp and amp

The last function is poorly understood in home audio (although it is significant as an entire cable industry around this issue demonstrates) although it is well-known in recording and broadcast.

Doesn't a well buffered source cover this problem? Just need the right output impedance? I thought it was pretty standard for a modern DAC to have adequate buffering and possible with the right phono amp.

 

[mcstatz5829]

digital-based volume controls that throw bit perfect out the window.

digital volume that compresses the bit depth is the worst. honestly should be considered a crime against humanity.

 

[atmasphere]

Doesn't a well buffered source cover this problem? Just need the right output impedance? I thought it was pretty standard for a modern DAC to have adequate buffering and possible with the right phono amp.

No, it doesn't all the time, although some are better than others. But when you put a passive device between it and the amps, that quality is lost.

 

[mcstatz5829]

No, it doesn't all the time, although some are better than others. But when you put a passive device between it and the amps, that quality is lost.

Do you mind walking me through this a little more? In my barely-passed-EE-101 brain:
Unity gain buffer on dac -> interconnect -> source switch -> volume control -> interconnect -> power amp
seems functionally equivalent to:
Unity gain buffer on dac -> interconnect -> +8db gain stage -> source switch -> volume control -> interconnect -> power amp
in terms of providing the proper impedance matching for the cabling. If the DAC output is already fine for driving the interconnect, why does simply adding a switch and volume control suddenly cause impedance issues?

Edit: is the difference practical vs theoretical? as in "theoretically you can go straight from DAC to passive preamp but practically speaking finding a standalone DAC that can handle it is not trivial"

 

[W9TR]

The 'variable gain amplifier'... is that a Voltage controlled amplifier chip?

We designed a pretty sophisticated reed relay based control that did the same thing. I think that was about 22 years ago. We could never get it to sound as good as the volume controls we were building from custom built switches. I'm guessing the relay contacts were not as good as the mechanical switch contacts but we really didn't pursue it.

They use the same relay based setup to adjust the gain of a discrete amplifier stage. Not a VCA. I can try and find a schematic of one. McIntosh uses vacuum sealed reed relays with Rubidium contacts.

 

[atmasphere]

Do you mind walking me through this a little more? In my barely-passed-EE-101 brain:
Unity gain buffer on dac -> interconnect -> source switch -> volume control -> interconnect -> power amp
seems functionally equivalent to:
Unity gain buffer on dac -> interconnect -> +8db gain stage -> source switch -> volume control -> interconnect -> power amp
in terms of providing the proper impedance matching for the cabling. If the DAC output is already fine for driving the interconnect, why does simply adding a switch and volume control suddenly cause impedance issues?

Edit: is the difference practical vs theoretical? as in "theoretically you can go straight from DAC to passive preamp but practically speaking finding a standalone DAC that can handle it is not trivial"

Sure. The thing is that most line sections have the volume control before the active circuitry, not after. This prevents the line stage being overloaded.
To control the interconnect cable the impedances required are fairly low. In recording or broadcast, the thing driving the interconnect cable usually has the ability to make some power while doing so. So the output of that device, be it a preamp, mixer, recorder or the like has its output measured in dBm. 0dBm is 1 milliwatt driving a 600 Ohm load.

Most DACs and for that matter, many line sections would fall flat on their faces driving a load like that. But if they can do so, they can help insure the cable has no artifact of its own.

Put a series resistance in the circuit, such as you see in a passive volume control and that ability is lost or curtailed, depending on the source and the value of the control.

That is why a buffer after the control is a thing that can work, assuming the buffer is competent and provides a low source impedance.

 

[mcstatz5829]

Sure. The thing is that most line sections have the volume control before the active circuitry, not after. This prevents the line stage being overloaded.
To control the interconnect cable the impedances required are fairly low. In recording or broadcast, the thing driving the interconnect cable usually has the ability to make some power while doing so. So the output of that device, be it a preamp, mixer, recorder or the like has its output measured in dBm. 0dBm is 1 milliwatt driving a 600 Ohm load.

Most DACs and for that matter, many line sections would fall flat on their faces driving a load like that. But if they can do so, they can help insure the cable has no artifact of its own.

Put a series resistance in the circuit, such as you see in a passive volume control and that ability is lost or curtailed, depending on the source and the value of the control.

That is why a buffer after the control is a thing that can work, assuming the buffer is competent and provides a low source impedance.

Honestly a bit over my head

 

[atmasphere]

Honestly a bit over my head

Think about it this way: if the line stage is prior to the volume control, an incoming signal could overload it even though the volume was down or off. So preamp designers put the volume control before the line stage instead. This way its well buffered from whatever cable and/or amp its driving.

 

[mcstatz5829]

Think about it this way: if the line stage is prior to the volume control, an incoming signal could overload it even though the volume was down or off. So preamp designers put the volume control before the line stage instead. This way its well buffered from whatever cable and/or amp its driving.

That part makes sense. Volume before gain not after.

It kinda sounds that needing an active preamp might be less about deficiencies of the dac buffer and more about matching impedances for the amplifier…?

 

[atmasphere]

That part makes sense. Volume before gain not after.

It kinda sounds that needing an active preamp might be less about deficiencies of the dac buffer and more about matching impedances for the amplifier…?

A bit of both really. I've seen a few DAC outputs that were really sensitive to cables and I've seen them that aren't. This is part of why you see such varied anecdotes regarding the use of preamps vs passive controls.

 

[mcstatz5829]

A bit of both really. I've seen a few DAC outputs that were really sensitive to cables and I've seen them that aren't. This is part of why you see such varied anecdotes regarding the use of preamps vs passive controls.

I went off the deep end and refreshed my self on amps. I had some education on op amps a long time ago but never utilized it. The picture is becoming clearer to me. Thanks for the help