Well I'm flattered that a few of you appreciate my sharing of insights. I hate to be the whiny critic. I'd rather make people feel better about their projects, not worse. I don't pretend I know it all, and I definitely don't. I try to help other hobbyists with what I've learned, and some of them just get defensive and shut me out. They don't want to hear that much of what they've been led to believe is wrong. I don't blame them for feeling that way. Even when the "purists" are clinging to their poorly based opinions, I still dig their passion.
If I was trying to get maximum quality audio from my computer into my main audio system, if there's any hum I'd look at using isolation transformers to break the ground loop (may or may not help), and I'd definitely put a passive Rf filter at the input of my preamp limiting the bandwidth to maybe 30-40kHZ. A 44 or 96kHZ clock signal, since it's a squarewave not a sinewave, will have significant energy that's way above it's fundamental frequency. Possibly into the megahertz. Opamp circuits (tubes and discrete transistors too) have dominant poles, for the sake of good phase margin, that causes the open loop gain to roll off at a 6dB/oct rate from very low in frequency all the way up. Since the distortion correcting negative feedback of the opamp circuit is dependent on that open loop gain being substantially higher than the desired closed loop gain, what happens is there may not be much distortion correcting going on in the supersonic (above audio) frequencies - especially up in the megahertz range, so Rf clock noise can be detected (demodulated) by the opamp (or tube circuit) becoming non-linear, and spew noise and slew based distortion down into the audio frequency range. A passive Rf filter costs about a buck. It's just an R and a small C. It should be at the input of the preamp, not the output of the computer. Hope this helps.
If I was trying to get maximum quality audio from my computer into my main audio system, if there's any hum I'd look at using isolation transformers to break the ground loop (may or may not help), and I'd definitely put a passive Rf filter at the input of my preamp limiting the bandwidth to maybe 30-40kHZ. A 44 or 96kHZ clock signal, since it's a squarewave not a sinewave, will have significant energy that's way above it's fundamental frequency. Possibly into the megahertz. Opamp circuits (tubes and discrete transistors too) have dominant poles, for the sake of good phase margin, that causes the open loop gain to roll off at a 6dB/oct rate from very low in frequency all the way up. Since the distortion correcting negative feedback of the opamp circuit is dependent on that open loop gain being substantially higher than the desired closed loop gain, what happens is there may not be much distortion correcting going on in the supersonic (above audio) frequencies - especially up in the megahertz range, so Rf clock noise can be detected (demodulated) by the opamp (or tube circuit) becoming non-linear, and spew noise and slew based distortion down into the audio frequency range. A passive Rf filter costs about a buck. It's just an R and a small C. It should be at the input of the preamp, not the output of the computer. Hope this helps.
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Hi, thanks for the reply and nothing at all.
Do you mean i should have something ?
Thanks a lot ! You mean that i should connect the pc to the dac using a toslink cable maybe ?
I could do this quite easily because i have already not used a USB to toslink converter. I will try it immediately when i will be back home.
I am very curious.
Thanks a lot again and best regards, gino
This is great info. I appreciate it very much. I do have one comment/question, however. You say that the Rf filter should be installed at the input of the pre-amp, not the output of the computer. This is true if the computer is doing the DAC stage and it is plugged directly into the pre-amp, but in many cases (including mine) there is a DAC between the computer and the pre-amp. Furthermore, in my case, I have a DIY phono pre-amp between my TT and my main pre-amp. Both the DAC and the DIY phono stage include opamps.
Is it safe to assume that you recommend an Rf filter be installed in these devices, rather than just in the main pre-amp?
Some kind of Rf filter seems wise to me, but you need to integrate it into the circuit without screwing up anything else. The input of a phono preamp needs to have just the right load resistance and capacitance for the particular cartridge you are using, or you get a significantly screwed up frequency response (treble is usually either spiked up or prematurely rolled off). The cables from the turntable make up part of the required (designed for) load capacitance. This will add to the actual capacitor at the input of the phono preamp to get the amount that makes the cartridge happy, and this capacitance forms an Rf filter right there.
All DACs need to have a low pass "reconstruction filter" at their outputs, to filter out the high frequency energy from the stair-step waveform they produce. The typical filters may be good enough for most people, but will not completely eradicate all Rf energy, so more filtering down stream helps further decrease distortions. There's also the chance that interconnect cables that are near any switchmode power supplies or their wires could pick up a little Rf energy on the way to the preamp inputs. Putting passive Rf filters at the input of poweramps is wise too. I do it.
Thanks Bob!
By passive Rf filters, are you referring to those magnet clamp things? If so, yes I use those on most of my interconnects, including just before power amp input.
cogitech said:
If you use ferrites you need to ensure that one of the following is true:Bob Richards said:
1. the go and return audio signal currents match, and both go through the ferrite,
2. the ferrite can cope linearly with the audio signal current.
A simple CR filter avoids this. Some people have a fit of the vapours when they see what it does to a starting transient (such as a gated sine wave) and walk away waving their arms about and muttering 'distortion, smearing'. The rest of us know that music contains no gated sine waves, and a linear CR circuit cannot create distortion (i.e. no new Fourier components are introduced).
I think both of those conditions above would be important, not one or the other. Ferrite beads are a step in the right direction, but are only likely to filter out stuff way high in frequency. I'd rather have a filter that was effective down to maybe 100kHZ, which an R and C to ground can do well. Ferrite beads could actually cause distortion.
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By the way, I have had great success with those things. For example, when I integrated my VSPS phono pre-amp into my main pre-amp, I used CAT5E as internal interconnects. Little did I know that each would act as an antenna. The result was that I could hear every local radio station simultaneously at a very low level, as long as the phono cartridge had no input (no record playing). It didn't bother me that much because it was way down in the noise floor and I couldn't hear it when I listened to records. But then I added one of those ferrite bead clamps to both my TT interconnect and between my pre-amp and amp, and the radio noise is gone.
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