Alex,
No problem..... however, ferrites can suck the life out of an amp.
I will hear the problem on Thursday, let you know... and so to bed!
Hugh
No problem..... however, ferrites can suck the life out of an amp.
I will hear the problem on Thursday, let you know... and so to bed!
Hugh
Sc Uld2
"I will hear the problem on Thursday, let you know... and so to bed!"
I was thinking along the same lines. I just hope I can beat my daughter inlaw to the shower !
"I will hear the problem on Thursday, let you know... and so to bed!"
I was thinking along the same lines. I just hope I can beat my daughter inlaw to the shower !
I don't know if this is relevant, but: with the DAC on or off, the DC at the output is the same, at ~4mV in each amp. With the DAC on, but not playing anything, I get 120mV AC at the output - I guess that's the 'hiss' I'm hearing?...
jp_howard said:
Could be
Is there a preamp between the dac and the amp?
First off, plug another piece of equipment in there, does the output dc go up to?
Did you install the 10ohm .6W resistors on the inputs of the amp?
They go from input earth(on the pcb) to output(centre) earth(on the pcb).
Are the input rca plugs earthed?
might be a ground loop.....
Just a thought.
sleep time
No.awpagan said:
Yes - assuming I understand you correctly. There's just the one of them, from signal 'COM' to ground - it's on the SC schematic and on the component diagram. Is that the one you mean?
They're connected to output 0V. The earth from the IEC socket is plugged in to input 0V. Does that count as "earthed"? (Sorry, I find the whole concept of "ground" and "earth" immensely confusing... I've read quite a few articles on the internet which try to explain, but I'm still lost! I've already been warned on another forum that this whole topic is "a world of hurt"!)
Could be - I've always found them a bit confusing... However I have heard a ground loop before, and it was more of a mid-frequency buzz than a high-frequency hiss. Dunno if there's different types with different sounds. The DAC, and both amps, have toroidal transformers, and each has earth from the power socket connected to the PCB's 0V. All 3 devices are connected to the same power strip. Is that a reasonable way to set things up?
no.jp_howard said:
all my amps sound completely silent and measure between 0.0mVac and 0.1mVac before they are cased up.
They are always slightly noisier when in their cases, 0.1mVac to 0.5mVac.
The Aluminium sheet is likely to have less impact than a steel sheet once you have eliminated that noisy source.
Is the DAC sending out HF artifacts that are interfering with the correct operation of the amplifier? Are RF filters fitted between the DAC output and the power amp input?
I don't know - would I have to use an oscilloscope to find that out? (I have one, but I haven't learnt how to use it yet. This could be a good first project for me to learn...)AndrewT said:
I don't think so - that is to say, I haven't added anything myself. I've googled for 'rf filter' and 'rf filter audio' to try to understand what you're asking here, and I assume you're suggesting I need a 'low pass filter' between the DAC and the amp - is that right? I can't find any suggestions online about how to construct such a filter, or what to buy - any suggestions?
I was surprised to discover that if I disconnect the Squeezebox from my DAC (but leave the DAC attached to the amp), the hiss goes away. I'll have to try moving my Transporter from the other room to test - sounds like this hiss is simply something crappy in the Squeezebox.
Even although it appears that the source of the hiss is not the amp, it still seems like a good idea for an amp to deal with such noise as well as possible - e.g. like the headphone amp does, which is whisper quiet even with this noisy source.
Even although it appears that the source of the hiss is not the amp, it still seems like a good idea for an amp to deal with such noise as well as possible - e.g. like the headphone amp does, which is whisper quiet even with this noisy source.
What are the sizes of the component for the input on the HA?
Do you have another source to test with? ie anything with spdif out ie pc/dvd/cdp?
Do you have another source to test with? ie anything with spdif out ie pc/dvd/cdp?
jp_howard said:
Jeremy
The gain of the SC HA as published is only 2 x. The gain of the amplifier is approx. 24.5 times. It is highly likely that if you increased the gain of the SC HA substantially , you would start to hear the hiss.
This reminds me that the noise output of my X-DAC V3 as seen on a C.R.O. after 10 x amplification, is a little higher than I would prefer. I meant to follow that up.
Alex
jp_howard said:
the input of the power amp should have an RF filter (=low pass filter).jp_howard said:
the input of every pre-amp should have an RF filter.
The output of some sources may have an RF filter, but this is much less common.
A low pass filter is a series resistor in the signal line followed by a capacitor to signal ground. The output to the next stage is taken from the junction of the resistor and capacitor.
At first sight it appears your squeezebox is noisy. But it may be sending a pile of vhf along with the audio signal. The DAC and/or the power amp could be misreading the vhf stuff and producing the noise as a distortion artifact.
Check you power amp. Does it have an RF filter at it's input. It might have two. One directly on the RCA input terminal and a second on the pcb directly before the -IN and +IN input pins. This second filter could be a cap across the -IN and +IN pins.
Thanks for that very clear explanation, Andrew. Yes, there is indeed a filter. Here's the relevant part of the circuit:
I've tried connecting my Logitech Transporter to the amp, and there's absolutely no hiss at all. However connecting the Transporter to the external DAC brings back the hiss to the same level as the Duet connected to the DAC.
I've also tried connecting the SB Duet directly (i.e. using it's internal DAC) and the hiss is much worse than the external DAC.
I must say, I'm surprised that a DAC can produce so much noise. Especially an external DAC that seems to be quite well-regarded. I've tried both coax and spdif and it makes no difference. What kinds of things cause this noise in a DAC - especially ones like the Duet and my external DAC which don't even have a display? Are there some simple tricks to reducing the noise by tweaking a DAC?
I know Alex said that wider bandwidth sounds better, but for people that don't have a top-quality source, isn't it a good idea in an amp to provide an option to filter out this HF noise - e.g. provide a switch or dial?
I've tried connecting my Logitech Transporter to the amp, and there's absolutely no hiss at all. However connecting the Transporter to the external DAC brings back the hiss to the same level as the Duet connected to the DAC.
I've also tried connecting the SB Duet directly (i.e. using it's internal DAC) and the hiss is much worse than the external DAC.
I must say, I'm surprised that a DAC can produce so much noise. Especially an external DAC that seems to be quite well-regarded. I've tried both coax and spdif and it makes no difference. What kinds of things cause this noise in a DAC - especially ones like the Duet and my external DAC which don't even have a display? Are there some simple tricks to reducing the noise by tweaking a DAC?
I know Alex said that wider bandwidth sounds better, but for people that don't have a top-quality source, isn't it a good idea in an amp to provide an option to filter out this HF noise - e.g. provide a switch or dial?
And for future reference
Quick calc's for what values for what frequency.
http://www.sengpielaudio.com/calculator-RCpad.htm
Quick calc's for what values for what frequency.
http://www.sengpielaudio.com/calculator-RCpad.htm
Hi,
your existing RF filter is set very high.
Currently the RC = 100 * 820 * 10^-12 = 0.082us.
The has F-3dB @ ~ 1.9MHz.
we usually suggest the RC be between 0.5us and 1.5us.
Try swapping from the 100r to 1k0 and listen (RC=0.82us F-3dB=190kHz).
Has the noise gone or reduced?
Has the treble content reduced?
Report back.
Is this the Power Amp input?
your existing RF filter is set very high.
Currently the RC = 100 * 820 * 10^-12 = 0.082us.
The has F-3dB @ ~ 1.9MHz.
we usually suggest the RC be between 0.5us and 1.5us.
Try swapping from the 100r to 1k0 and listen (RC=0.82us F-3dB=190kHz).
Has the noise gone or reduced?
Has the treble content reduced?
Report back.
Is this the Power Amp input?
Very useful guidance - thanks Andrew. I'm in the middle of Mendelssohn's Violin Concerto right now and don't want to interrupt it (using my Transporter directly for now!); besides which it's getting late. I'll try changing the resistor tomorrow.
PS: Once you explained that the filter can be created using a resistor and capacitor, I googled for 'resistor capacitor loss pass filter', and found lots of great info. In particular, this page at allaboutcircuits.com.
PS: Once you explained that the filter can be created using a resistor and capacitor, I googled for 'resistor capacitor loss pass filter', and found lots of great info. In particular, this page at allaboutcircuits.com.
The transporter is using a 3 pin plug.
The Duet uses a wallwart and 2 pin plug.
The other dac?, Guess a 2 pin?
Any noise will find the least resistance to earth.
With two of them, that sounds like the amp.
The inputs on your sc200, are they separate wires back to each amp pcb? (2 pairs, + and -)
The Duet uses a wallwart and 2 pin plug.
The other dac?, Guess a 2 pin?
Any noise will find the least resistance to earth.
With two of them, that sounds like the amp.
The inputs on your sc200, are they separate wires back to each amp pcb? (2 pairs, + and -)
The other DAC is 3 pin - toroidal transformer, and I've connected IEC socket's earth to PCB's 0V.awpagan said:
Yes, separate pairs.
I tried removing the earth connection from the amp's IEC socket, which didn't make any difference to the noise, FYI. Also: the noise when using the Transporter connected to the DAC (i.e. all 3 components using 3 pin plugs) has the same noise level as the Duet connected to the DAC.
PS: Sorry, I know this has got off-topic from the amp discussion - but it's a really interesting side-bar! 🙂
The filters at the input of your Power Amp are far too wide apart.
We have discussed reducing the low pass by a factor of 10 to see if it affects the noise. It may not.
Similarly the High Pass filter is set far too low.
47uF and 12k is 564ms, F-3dB~0.3Hz.
This can be raised in frequency by about a factor of 5 without affecting the audio signal significantly. 7u5F (4u7F+2u7F both film caps, possibly different type) (90ms, 1.8Hz) is close to what I would recommend. Some builders go much higher in frequency (20ms, 8Hz).
We have discussed reducing the low pass by a factor of 10 to see if it affects the noise. It may not.
Similarly the High Pass filter is set far too low.
47uF and 12k is 564ms, F-3dB~0.3Hz.
This can be raised in frequency by about a factor of 5 without affecting the audio signal significantly. 7u5F (4u7F+2u7F both film caps, possibly different type) (90ms, 1.8Hz) is close to what I would recommend. Some builders go much higher in frequency (20ms, 8Hz).
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