Subscribed...I’d like to get a better understanding of attenuating the output level of digital/coaxial signals on CD players/transports.
I built a Shigaclone transport based on a JVC RC-EZ51H boom box not that long ago. Although I followed the main build guide to install resistors on the output signal and ground as close as possible with what I had on hand (330R&110R vs 300R&100R) my player puts out a signal that is too high for my system to adjust properly.
I use a DCB1 based preamp with about 6dB of additional gain provided by Jensen transformers on the output. It works very well with all my other equipment.
Recommendations for the signal and shunt resistors seem to be all over the place. One of the original guides recommended 75R&75R.
Further research seems to point toward reducing this signal/shunt network resistance as a way of reducing output.
I’d rather not be playing guessing games and pulling the player apart several times to try different values.
Does anyone know of a way to calculate the values?
I only have a pocket scope on hand that was given to me and honestly haven’t figured out how to use it to check the output level.
Here’s links to the original build recommendations of the Shigaclone:
The CD Transport
http://www.tim.cheapo.cc/images/CD%20player%20mods.pdf
I built a Shigaclone transport based on a JVC RC-EZ51H boom box not that long ago. Although I followed the main build guide to install resistors on the output signal and ground as close as possible with what I had on hand (330R&110R vs 300R&100R) my player puts out a signal that is too high for my system to adjust properly.
I use a DCB1 based preamp with about 6dB of additional gain provided by Jensen transformers on the output. It works very well with all my other equipment.
Recommendations for the signal and shunt resistors seem to be all over the place. One of the original guides recommended 75R&75R.
Further research seems to point toward reducing this signal/shunt network resistance as a way of reducing output.
I’d rather not be playing guessing games and pulling the player apart several times to try different values.
Does anyone know of a way to calculate the values?
I only have a pocket scope on hand that was given to me and honestly haven’t figured out how to use it to check the output level.
Here’s links to the original build recommendations of the Shigaclone:
The CD Transport
http://www.tim.cheapo.cc/images/CD%20player%20mods.pdf
I don’t claim to understand anything regarding the application of resistor networks to a digital output.
I was in fact under the impression that it would regulate the digital signal’s voltage level at the output and in turn affect the gain through my system.
Perhaps I have made a mistake in confusing this resistor network used at the Shigaclone output with other statements I have read regarding using a IV resistor of higher or lower value to control the output level on a DAC?... or perhaps I am incorrect about that as well.
As I said I’m here to try and learn about how to solve this issue of having a very high output level from a CD player or a DAC.
I played around with the pocket scope a bit earlier. There is a pic of how I connected it to the end of my digital coax cable. It is a DSO Nano.
I noticed that I can toggle through fields at the top of the display that give me measurements I have seen discussed regarding digital audio signals.
The measurements given in the field all the way toward the top right were:
V p-p : 888.0mV
DC . V : 576mV
V avg : 576.0mV
V rms : 208.0 mV
DUTY : 67.25%
CYCLE : 49.11uS
FREQN : 20.29KC
I don’t know which, if any, of these measurements is the best indicator of the output signal level on the Shigaclone.
I have seen people mention the point to point voltage measurement before...it appears that mine is below 1V.
I was under the impression this value indicated the output signal level and that a common industry level was 2V.
I thought there was mention in the Shigaclone related threads that it’s output level was considered high at about 4V p-p and that the output resistor network was used to help reduce this.
If you can educate or help me further understand this problem I’d be grateful.
Also...should I be concerned about 576mV DC voltage measurement in the output signal? Perhaps I am not measuring properly?
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I would like to learn about addressing output levels of both.
The Shigaclone is functioning fine, but has a very high output level via the digital coax output.
I am also currently building a TDA1541A DAC base on Ryan’s D3 group buy board. It has the option of using just a IV resistor at its output for people that might have enough gain in there system, or it can be used with a variety of other IV stages.
I haven’t completed assembly of this DAC, but I will be in a similar situation where I will need to figure out how it’s signal output level will work with my system and possibly increase or reduce it.
The Shigaclone is functioning fine, but has a very high output level via the digital coax output.
I am also currently building a TDA1541A DAC base on Ryan’s D3 group buy board. It has the option of using just a IV resistor at its output for people that might have enough gain in there system, or it can be used with a variety of other IV stages.
I haven’t completed assembly of this DAC, but I will be in a similar situation where I will need to figure out how it’s signal output level will work with my system and possibly increase or reduce it.
If the audio level is to high then the best way to reduce it is via a simple attenuator using two resistors (per channel). This is done on the analogue output of the DAC or player.
If you want to try that then we can advise on the configuration and typical values.
The digital levels used between players (transports) and DAC's tend to fall into one of two levels, typically around 1 volt peak to peak unloaded or around 0.5 volts peak to peak when loaded by 75 ohm. Less common are 5 volt peak to peak levels. In both cases they are not absolutes and the specification allows for quite large variations before problems arise.
Attenuating these does nothing until you reach 'the digital cliff edge' where suddenly the DAC will not lock to the signal. It either works or it doesn't.
Because the data is a fast moving signal you need something much much better than a pocket scope to make any kind of realistic measurements, however as a general rule if data is present (and the thing is working) then it is going to be correct.
The DC shifts and apparent anomalies are down to the signal being non repetitive and the scope not being able to cope with the frequency of information it sees.
In the digital stream, the level of the audio is encoded as a digital number. It is the series of ones and zeros in the digital 'word' that determines the signal.
So changing the level of all those bits does not change the number encoded in their ones and zeros, so does not change the signal level.
Jan
@Mooly & jan.didden
Thank you for the information and help in understanding.
I think my misinterpretation of what was going on with the signal and its level was due to assumptions I made when I connected the Shigaclone transport to the DAC I am currently using after having used it with other sources like my computers.
The DAC I currently have is a used SMSL M8 I picked up cheap in the swap meet.
When I used this DAC with either my Windows laptop or my current Mac Mini based music server via USB or toslink I had no problems with signal levels in my system. I was able to stream from iTunes, Foobar or VLC and the signal level was comparable to using any other CD player or analog source I have had in my system.
The pots on my DCB1 preamp (with the additional 6dB of gain via the Jensen iron on the output) are pretty much always kept at the 9 o’clock position...which is a good moderate listening level from any other source I have. I did also have the option to adjust the volume via a software control in these different players, but even if maxed out the level was still what I would consider “normal”.
When I connected the Shigaclone transport via the digital coax output to the SMSL and tried playing it in my system the level was completely off the chart. It was so high that in order to play it at what I would consider a “normal” level I had to turn the pots on the preamp down to the point where if they went any lower the signal would be lost.
My assumption was that this was due to the signal level coming directly out of the Shigaclone being too high.
The SMSL does not have any sort of hardware based gain control as far as I know. I have tried researching the unit and it is difficult to find much information.
I’ve attached a screen shot pic of some basic specifications of the M8 from a vendor’s site that used to sell it.
I hope this is still on topic in that I am trying to figure out why I would experience this very high output level from the transport via the M8 DAC.
Thank you for the information and help in understanding.
I think my misinterpretation of what was going on with the signal and its level was due to assumptions I made when I connected the Shigaclone transport to the DAC I am currently using after having used it with other sources like my computers.
The DAC I currently have is a used SMSL M8 I picked up cheap in the swap meet.
When I used this DAC with either my Windows laptop or my current Mac Mini based music server via USB or toslink I had no problems with signal levels in my system. I was able to stream from iTunes, Foobar or VLC and the signal level was comparable to using any other CD player or analog source I have had in my system.
The pots on my DCB1 preamp (with the additional 6dB of gain via the Jensen iron on the output) are pretty much always kept at the 9 o’clock position...which is a good moderate listening level from any other source I have. I did also have the option to adjust the volume via a software control in these different players, but even if maxed out the level was still what I would consider “normal”.
When I connected the Shigaclone transport via the digital coax output to the SMSL and tried playing it in my system the level was completely off the chart. It was so high that in order to play it at what I would consider a “normal” level I had to turn the pots on the preamp down to the point where if they went any lower the signal would be lost.
My assumption was that this was due to the signal level coming directly out of the Shigaclone being too high.
The SMSL does not have any sort of hardware based gain control as far as I know. I have tried researching the unit and it is difficult to find much information.
I’ve attached a screen shot pic of some basic specifications of the M8 from a vendor’s site that used to sell it.
I hope this is still on topic in that I am trying to figure out why I would experience this very high output level from the transport via the M8 DAC.
Attachments
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I think you have pretty much grasped what is happening...
The data fed to the DAC is different depending on which source you use whether that be PC, transport etc.
The PC and software sources are able to attenuate the level in the digital domain (nothing to do with actual level of the data signal you see on the scope).
The Shigaclone will be sending raw data to the DAC. The only way you can attenuate the audio now is by use of a resistive attenuator on the audio output. If you try this then keep the values low(ish) and try for example a 4k7 and a 2k2. The values are all open to experimentation and you can't damage anything. If you need it quieter still then either raise the 4k7 a little and/or reduce the 2k2.
The data fed to the DAC is different depending on which source you use whether that be PC, transport etc.
The PC and software sources are able to attenuate the level in the digital domain (nothing to do with actual level of the data signal you see on the scope).
The Shigaclone will be sending raw data to the DAC. The only way you can attenuate the audio now is by use of a resistive attenuator on the audio output. If you try this then keep the values low(ish) and try for example a 4k7 and a 2k2. The values are all open to experimentation and you can't damage anything. If you need it quieter still then either raise the 4k7 a little and/or reduce the 2k2.
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@Mooly
OK...thanks for helping me sort it brought the problem. It is disappointing that I am stuck having to use some kind of additional attenuator.
It’s also a bit baffling when I look at the output signal specifications for the M8 and see that it is just over 2V.
I know I have used other equipment with similar specifications in my system without this problem.
I have another question still itching my brain. What purpose does the resistor network on the digital output of the Shigaclone serve?
Is it simply a means of creating a certain load at the output in order to optimize signal transfer to a DAC?
OK...thanks for helping me sort it brought the problem. It is disappointing that I am stuck having to use some kind of additional attenuator.
It’s also a bit baffling when I look at the output signal specifications for the M8 and see that it is just over 2V.
I know I have used other equipment with similar specifications in my system without this problem.
I have another question still itching my brain. What purpose does the resistor network on the digital output of the Shigaclone serve?
Is it simply a means of creating a certain load at the output in order to optimize signal transfer to a DAC?
Yes, its most probable an impedance matching and/or digital signal level adjustment circuit. And, as you now know, has nothing to do with the sound pressure coming out of your speakers.
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A similar problem cropped up in the early days of the forum. A Behringer EQ was being over driven. It was eventually solved by a simple digital attenuator between the SPDIF receiver and the EQ circuitry.
Chromenuts,
Why don't you use your scope to measure the audio level coming out of your dac at this point when the new CD transport is the SPDIF source? It shouldn't be more than a few volts peak to peak.
The thing is, when you use digital sources such as computers with a dac, its usually best to leave the volume controls on the computer almost all the way turned up. I only ever turn down the digital gain in my computer very slightly, maybe -3dB below maximum. That's just to minimize distortion in the dac, otherwise I would leave it turned up all the way.
The best place to turn down the volume is after the dac and before the power amp, that is, if the system is all working properly. If the signal out of your dac is too strong for the line level inputs on your preamp, something is probably wrong. Could be you are using a phono input, or some other issue. So, use the scope to measure the voltage coming out of the dac and going into the preamp to see what the problem is. If need be you can always make a simple analog attenuator before the preamp, but a better solution might be to stop using the preamp at all if the only source you use with it is the dac. For dacs that have a volume control, they can often drive a power amp directly. Maybe no need for a preamp in that case.
Why don't you use your scope to measure the audio level coming out of your dac at this point when the new CD transport is the SPDIF source? It shouldn't be more than a few volts peak to peak.
The thing is, when you use digital sources such as computers with a dac, its usually best to leave the volume controls on the computer almost all the way turned up. I only ever turn down the digital gain in my computer very slightly, maybe -3dB below maximum. That's just to minimize distortion in the dac, otherwise I would leave it turned up all the way.
The best place to turn down the volume is after the dac and before the power amp, that is, if the system is all working properly. If the signal out of your dac is too strong for the line level inputs on your preamp, something is probably wrong. Could be you are using a phono input, or some other issue. So, use the scope to measure the voltage coming out of the dac and going into the preamp to see what the problem is. If need be you can always make a simple analog attenuator before the preamp, but a better solution might be to stop using the preamp at all if the only source you use with it is the dac. For dacs that have a volume control, they can often drive a power amp directly. Maybe no need for a preamp in that case.
It isn't a line level issue or a dac issue. The issue is a source with the outgoing digital level too high.
The dac can handle it. It is just that relative to other sources the analogue output is then so high the volume pot barely needs to move.
I haven’t had a chance to take measurements at the output of the DAC...was busy with other things today.
@rfbrw...Are you making these statements about the digital signal level based on the measurements I attempted with the pocket scope?...or just general observations based on my description of the problem?
Do you have a specific recommendation for reducing the digital signal level?
I’m just a bit confused as others have already posted that the digital signal cannot be attenuated...albeit with a resistor network.
@rfbrw...Are you making these statements about the digital signal level based on the measurements I attempted with the pocket scope?...or just general observations based on my description of the problem?
Do you have a specific recommendation for reducing the digital signal level?
I’m just a bit confused as others have already posted that the digital signal cannot be attenuated...albeit with a resistor network.
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I was a bit confused as well as these posts say opposite things to me 🙂
It is a line level issue. The level is to high for the partnering equipment.
As we have said before, attenuating the digital data stream has no effect provided the levels are within the range the DAC can work with. It is all or nothing here.
It is a line level issue. The level is to high for the partnering equipment.
As we have said before, attenuating the digital data stream has no effect provided the levels are within the range the DAC can work with. It is all or nothing here.
I agree with Mooly. The problem is the all the other digital sources used with the dac are turned down too much, something that the CD transport cannot do. The solution is to turn them all up to match the level of the CD transport, then attenuate the line level signal coming out of the dac (that is, if the dac is to be used with an overly sensitive preamp).
I did Google “digital signal attenuator“.
Most everything that comes up is some form of RF attenuator, either digital or otherwise.
One particular product said it was used with microphone signals to help prevent them from over-driving equipment.
Until I can find some information or examples of someone using them for the same situation I am in I doubt I will try it.
As I mentioned previously, I am trying to finish putting together a TDA1541A DAC based on Ryan’s D3 board. I will be curious to see what happens with it as there is a IV resistor only option onboard for the output. Any further amplification would have to be added whether it be a transformer, CEN/SEN JFET based IV stage or otherwise.
I am wondering if the transport will overdrive the D3 or if it will turn out that I can run the D3 with my system without any IV stage amplification. It may be that my other digital sources will struggle with that gain situation...who knows?
Most everything that comes up is some form of RF attenuator, either digital or otherwise.
One particular product said it was used with microphone signals to help prevent them from over-driving equipment.
Until I can find some information or examples of someone using them for the same situation I am in I doubt I will try it.
As I mentioned previously, I am trying to finish putting together a TDA1541A DAC based on Ryan’s D3 board. I will be curious to see what happens with it as there is a IV resistor only option onboard for the output. Any further amplification would have to be added whether it be a transformer, CEN/SEN JFET based IV stage or otherwise.
I am wondering if the transport will overdrive the D3 or if it will turn out that I can run the D3 with my system without any IV stage amplification. It may be that my other digital sources will struggle with that gain situation...who knows?