Sandy,Don't forget that these home brew IDC cables can also cause all kinds of funny things to happen with DACs too.
Regards
Alex
Is there a potential problem in using standard ribbon cable for this pre-amp solution? It is being suggested by Elektor or Doug Self. Or, should I attempt to weave 10 individual wires from the pins of the sockets. The former suggestion seems to be the easiest, but maybe not the best in terms of sonics? What issues should I be concerned with when using ribbon cable with a DAC?
Which of the acoustic pianos sound the most like a real piano? If measured against the first piano ever made then all successive pianos are distorted.Me too, the acoustic ones make a much nicer sound even though the electric ones are probably considered blameless now
Thanks Mooly, I'm 100% with you on all that
Me too, the acoustic ones make a much nicer sound even though the electric ones are probably considered blameless now
Nice one
It is indeed
Ignore that redjr, Alex was having a dig about another unlrelated project I was discussing with him offline
The ribbon cable in the 2012 preamp is only used for control signals to switch the relays so can't affect SQ - you'll know if there's a problem if your input switching starts going haywire.
For the signal connections in your preamp between the boards I would suggest using sheilded audio cable to minimise induced hum/noise from the power transformer and surrounding environment. Don't use ribbon cable!
OK. I haven't decided for sure what type of cable I'm going to use for the internal signal interconnects. I'm just trying to sort through the various 2, 3 and 4-pin headers on all the boards and how the puzzle fits together. I think I will draw up a block diagram of this beast since there are so many boards, and how they all connect was not laid out all that well in the article. Also it seems Elector chose to use 4-pin header on several of the boards when 3 pin headers would have sufficed. The center two pins seem to always be tied together as a ground. So the the extra center pin is redundant and is not required.Ignore that redjr, Alex was having a dig about another unlrelated project I was discussing with him offline
The ribbon cable in the 2012 preamp is only used for control signals to switch the relays so can't affect SQ - you'll know if there's a problem if your input switching starts going haywire.
For the signal connections in your preamp between the boards I would suggest using sheilded audio cable to minimise induced hum/noise from the power transformer and surrounding environment. Don't use ribbon cable!
Where is the KISS approach here!It's late here so I'm going to bed.
Somehow I don't think the KISS principle is what this design is all about
I know it seems redundant but there may have been a reason for using a 4 pin header when a 3 pin would do: once you start trying to solder or crimp shielded cable braid into those pins for the 0.1" header sockets you'll see that soldering two braids onto one earth pin can easily make it too fat to fit into the connector housing in which the socket pins are housed. I'd rather have the 4 pins, one for each wire.
I know it seems redundant but there may have been a reason for using a 4 pin header when a 3 pin would do: once you start trying to solder or crimp shielded cable braid into those pins for the 0.1" header sockets you'll see that soldering two braids onto one earth pin can easily make it too fat to fit into the connector housing in which the socket pins are housed. I'd rather have the 4 pins, one for each wire.
It may run out of class A in certain conditions but the high feedback will erode most of it so measurements will still be clean.
Some time ago i measured how much class A certain Opamps have. You can put a low value resistor, say 50 Ohm, from the output to ground. Then you put say 10 Ohm resistors into the voltage feed to Pin 4 and Pin 7. You then increase the input voltage of a 1kHz signal ( can be 10kHz or higher ) until the current goes up that flows into the Opamp from the PSU. The 10 Ohm resistors function as sensors by measuring the voltage over them. I would say that the LME49710 has around 2mA class A. When you drive a 600 Ohm load it will leave class A at 1,2V.
Some time ago i measured how much class A certain Opamps have. You can put a low value resistor, say 50 Ohm, from the output to ground. Then you put say 10 Ohm resistors into the voltage feed to Pin 4 and Pin 7. You then increase the input voltage of a 1kHz signal ( can be 10kHz or higher ) until the current goes up that flows into the Opamp from the PSU. The 10 Ohm resistors function as sensors by measuring the voltage over them. I would say that the LME49710 has around 2mA class A. When you drive a 600 Ohm load it will leave class A at 1,2V.
Try and visualise how the active gain stage works. It doesn't see a constant load that it can swing a voltage across because the load depends on the setting of the volume control.
Taking a simple inverting amp and with say a 1k input resistor and a 10 ohm feedback resistor and applying +10 volts pk as an input then the output has to feed -10 milliamps "back" into the virtual ground point at the inverting amp input in order to maintain equilibrium. As a gain stage it has a gain of 10/1000 which is 0.01 So the output voltage from the opamp is 10*0.01 which is 0.1 volts yet the opamp is working really hard delivering that small voltage because of the current it has to feed into the 10 ohm. As the volume is increased (we make the feedback resistor higher) then the current becomes less and the voltage swing greater.
Here's a (maybe poor) analogy.
Imagine for normal low level listening your power amp only needs put out a few 10's of millivolts of audio across the speaker. Now imagine adding a really low value resistor across the speaker (say 0.1ohm) such that the current demands become relatively high and the amp now shows crossover distortion. That is similar to what is happening with the active gain stage. As the attenuation increases the opamp delivers lower and lower output voltages but at ever increasing currents.
Hi mrlincoln61,
That is indeed a beautiful plot, but that measurement may mask what we are talking about, since the operating voltage level is fairly high where the distortion is extraordinaily low. Try this. Build a Baxandall volume control and connect its signal input to ground. Then back-feed a test signal into it through 600 ohms. Drive the 600 ohms with 5V rms from another LME49710. Look at the output of the Baxandall volume control op amp, after amplifying it with sufficient gain to get it up to a decent signal level. Measure the THD of that resulting signal. Do this at both 1kHz and 20kHz. With this technique, you will be able to see whatever crossover distortion there is without it being swamped out by a large signal.
Cheers,
Bob
I thought I had demonstrated in Post #401 that nothing bad happens in the active gain control at very low volume settings.
Hi Joachim,
My bad. I should have stated it more clearly. Backfeed the test signal into the output of the Baxandall volume control. That node, although of quite low impedance, is not a short.
Cheers,
Bob
we do have Cherry/Cambrell's Interface Modulation papers results to help - "output injected" current gives the same or less distortion/IMD V at the output than the amp gives operating in the "forward" direction into the same impedance load at signal levels giving the same ouptut device current
Hi jcx,
Also, don't forget my AES paper on IIM in 1982, "Open Loop Output Impedance and Interface Intermodulation Distortion in Audio Power Amplifiers". The preprint is available on my website at CordellAudio.com - Home.
I don't think the IIM measurement and theory results apply directly to the situation here. You don't need to set up an intermodulation scenario to deal with questions about crossover distortion magnification concerns in a Baxandall volume control. In rough terms, I believe that if you have 0.0001% crossover distortion at full output with a Baxandall volume control, that number will be possibly be multiplied by 100 (in terms of percentage) when the volume control is set to 40dB of attenuation if the primary load on the output stage is the feedback network (i.e., if the output current being driven is approximately the same in both cases).
Cheers,
Bob
Cheers,
Bob
I've not looked hard at the numbers for this application - just pointing out that we do know some helpful relations to calculate with
Groner's op amp distortion paper shows the 5534 output linearity much worse than the LME part he measured, but +20 dBu, 600 Ohms is a pretty heavy load
another consideration is the Class AB stges will give half wave rectified currents in each PS rail - there can be coupling from that too - though Self is quite aware of it, listing it in his numbered amplifier distortions
Groner's op amp distortion paper shows the 5534 output linearity much worse than the LME part he measured, but +20 dBu, 600 Ohms is a pretty heavy load
another consideration is the Class AB stges will give half wave rectified currents in each PS rail - there can be coupling from that too - though Self is quite aware of it, listing it in his numbered amplifier distortions
Apologies for interrupting an interesting technical discussion that I hope continues. I've been doing some blind testing of my (D.S. '96) preamp against a couple of others and wanted to provide an update. I decided to do this first before I start trying to work out whether the Baxandall gain control is influencing the sound by bypassing it etc per previous discussion.
I first set the volume control on the '96 preamp to a convenient level (about 1 o'clock using a normal line input, so not anywhere near the highest attenuation setting) and then matched the volume setting on the other preamp to it using a 400Hz sine from a test CD and and a 5 digit DMM. I then got SWMBO to connect up the two preamps under test via two randomly chosen inputs of another remote controlled preamp and took the output to the power amp via its tape output. This allowed me to switch between and compare the two preamps while listening. I didn't know which inputs she used so the test was "blind".
After the familiar initial shock at how small the difference in SQ was, I still thought I knew within a few seconds of the first track I listened to which one was the '96 preamp. With some recordings the difference was marginal at best, but with others it was quite obvious (to me). After listening to about 20 different tracks I checked the connections and found that I was correct. I then got SWMBO to reconnect the two preamps to a different set of randomly chosen inputs. She went off to do more interesting things after doing this and was not involved in the listening test so had no influence other than assigning the connections. I repeated the testing with the same result.
This may not be a scientifically valid test, but I think it was good enough to prove (to me anyway) that the difference in sound quality I was hearing earlier is real. The reason I could pick the '96 preamp was usually because it sounds slightly "fat" compared to two others I tested, ie there is a very mild emphasis on the lower mids and upper bass. Or to put it another way, the other preamps might be slightly "thin". I find this very interesting, since the frequency response of all units must be effectively flat. I wonder if it comes down the the harmonic content of what small distortion there is. I do also think the '96 preamp sounds very slightly "forced" or "hard", which was evident with some recordings. I think these two factors might be why my impression was of a slightly compressed sound earlier.
Having said all this, I preferred the sound of the '96 preamp with about 60% of the tracks I played, which surprised me and shows the importance of blind testing. With some recordings when I closed my eyes I found that it provided a slightly more believeable illusion of a real group of musicians playing, while with others it seemed slightly more artificial. Whatever small SQ characteristics it has (if any) work very well with many recordings. It may well be that it measures better than any of my other preamps so I am hearing a more accurate representation of what's coming from the source and the fact is this isn't always more pleasing to the ear
So in summary I would emphasise that it is a superb preamp, as you would expect with an intelligent design that measures impeccably from a master of the subject. Whatever slight SQ characteristics it posesses, if any, are hard to pick and may well be to the listeners liking. Or it may just be that it is more accurate. I suppose there's no way of knowing without having a real group of musicians playing to order in the same room behind a curtain
I'm still curious and planning to try taking the output from before the Baxandall gain control though. I'll report on my findings in due course.
I first set the volume control on the '96 preamp to a convenient level (about 1 o'clock using a normal line input, so not anywhere near the highest attenuation setting) and then matched the volume setting on the other preamp to it using a 400Hz sine from a test CD and and a 5 digit DMM. I then got SWMBO to connect up the two preamps under test via two randomly chosen inputs of another remote controlled preamp and took the output to the power amp via its tape output. This allowed me to switch between and compare the two preamps while listening. I didn't know which inputs she used so the test was "blind".
After the familiar initial shock at how small the difference in SQ was, I still thought I knew within a few seconds of the first track I listened to which one was the '96 preamp. With some recordings the difference was marginal at best, but with others it was quite obvious (to me). After listening to about 20 different tracks I checked the connections and found that I was correct. I then got SWMBO to reconnect the two preamps to a different set of randomly chosen inputs. She went off to do more interesting things after doing this and was not involved in the listening test so had no influence other than assigning the connections. I repeated the testing with the same result.
This may not be a scientifically valid test, but I think it was good enough to prove (to me anyway) that the difference in sound quality I was hearing earlier is real. The reason I could pick the '96 preamp was usually because it sounds slightly "fat" compared to two others I tested, ie there is a very mild emphasis on the lower mids and upper bass. Or to put it another way, the other preamps might be slightly "thin"
. I find this very interesting, since the frequency response of all units must be effectively flat. I wonder if it comes down the the harmonic content of what small distortion there is. I do also think the '96 preamp sounds very slightly "forced" or "hard", which was evident with some recordings. I think these two factors might be why my impression was of a slightly compressed sound earlier.Having said all this, I preferred the sound of the '96 preamp with about 60% of the tracks I played, which surprised me and shows the importance of blind testing. With some recordings when I closed my eyes I found that it provided a slightly more believeable illusion of a real group of musicians playing, while with others it seemed slightly more artificial. Whatever small SQ characteristics it has (if any) work very well with many recordings. It may well be that it measures better than any of my other preamps so I am hearing a more accurate representation of what's coming from the source and the fact is this isn't always more pleasing to the ear
So in summary I would emphasise that it is a superb preamp, as you would expect with an intelligent design that measures impeccably from a master of the subject. Whatever slight SQ characteristics it posesses, if any, are hard to pick and may well be to the listeners liking. Or it may just be that it is more accurate. I suppose there's no way of knowing without having a real group of musicians playing to order in the same room behind a curtain
I'm still curious and planning to try taking the output from before the Baxandall gain control though. I'll report on my findings in due course.