One more last thing on devinyliser, having audio in mono is one thing but having inverse signal on both channels will cancel speakers operation and cut audio finally, creating dead or damped audio area. It has nothing to do with turntable!
I joined PDF of RIAA response curve with gain on red line. It's as expected!
I joined PDF of RIAA response curve with gain on red line. It's as expected!
Maybe Carl Huff, Doug Self or another knowledgeable individual can help here.
I’m wondering if there is a way to alter the curve law for the volume potentiometer. I spend most of my time below -40dB on other preamps and, if possible, would benefit from this lower volume range expanded over a wider volume knob angle. Based on the Article, volume at top-dead-center (0 degrees) is -4dB. It would be nice to see this moved clock-wise 20 or more degrees to allow for fine low-volume adjustments.
Thank you in advance.
I’m wondering if there is a way to alter the curve law for the volume potentiometer. I spend most of my time below -40dB on other preamps and, if possible, would benefit from this lower volume range expanded over a wider volume knob angle. Based on the Article, volume at top-dead-center (0 degrees) is -4dB. It would be nice to see this moved clock-wise 20 or more degrees to allow for fine low-volume adjustments.
Thank you in advance.
Hi Carl,
Thanks for the prompt replies.
Just peeking at the original article, the resistors that set the final gain were R41, R42, R44 and R45. These are in the feedback of the final stage. I’m not sitting with the actual PCB build schematic at the moment though.
According the graphed gain level, -20dB is at the volume potentiometer’s first 10% of travel, ie the Mk 1 position. -30dB is at the 5% position. Full attenuation to minimum seems to occur very quickly, a very sensitive touch of the volume control.
I’m not sure our PCBs follow the same volume curve law or not, but if so, that seems sensitive. I don’t know if reducing gain by 10dB will be enough. I’m wondering if this is actually how it behaves, and if so, what might I do to ease that a bit without losing gain. No rush here.
Thanks for the prompt replies.
Just peeking at the original article, the resistors that set the final gain were R41, R42, R44 and R45. These are in the feedback of the final stage. I’m not sitting with the actual PCB build schematic at the moment though.
According the graphed gain level, -20dB is at the volume potentiometer’s first 10% of travel, ie the Mk 1 position. -30dB is at the 5% position. Full attenuation to minimum seems to occur very quickly, a very sensitive touch of the volume control.
I’m not sure our PCBs follow the same volume curve law or not, but if so, that seems sensitive. I don’t know if reducing gain by 10dB will be enough. I’m wondering if this is actually how it behaves, and if so, what might I do to ease that a bit without losing gain. No rush here.
I think the best approach might just be to try reducing the gain to see if that will make the lower end of the volume control more useful to you. In my application of this circuit, I did not implement the active volume control stage (only the tone control circuits) as I intended to use a TDK logarithmic taper motorized pot (on a separate PCB). I did include a buffer stage as a driver for the pot and initially used the resistor values for 3X gain. It was way too much gain for the sensitivity of my power amplifier and I believe I experienced exactly the problem you have: too much signal way too early in the pot rotation. I needed to reduce the gain of the buffer stage to 0.3 (yes, less than unity) before the lower range of pot rotation became user friendly.
Reducing the gain of the final stage of this preamp brings up an interesting question. More specifically: reducing the gain to less than unity. I am only a student when it comes to audio circuits so it is time to open a text book.
The “text book” approach would be to use a resistor divider to set the gain and follow that with a unity gain buffer. Unfortunately, for most of us here, the PCB does not have a location for the lower resistor of a divider and adding it would require rework to the PCB. The PCB is laid out with an input resistor and a feedback resistor so that is what we have to work with. Going back to the text book, the circuit looks like a summing amplifier with an equation Vout = -Vin x Rfb/Rin. As long as Rin isn’t zero it doesn’t blow up so we should be good with a gain of less than 1.
The text book summing amplifier also shows an additional input resistor on the positive input going to ground. Its function is to balance the input bias currents. The LM4562 has a typical input bias current of 10nA. Across the 1K input resistor this is 10uV so with luck we can live with not having the third resistor.
The “text book” approach would be to use a resistor divider to set the gain and follow that with a unity gain buffer. Unfortunately, for most of us here, the PCB does not have a location for the lower resistor of a divider and adding it would require rework to the PCB. The PCB is laid out with an input resistor and a feedback resistor so that is what we have to work with. Going back to the text book, the circuit looks like a summing amplifier with an equation Vout = -Vin x Rfb/Rin. As long as Rin isn’t zero it doesn’t blow up so we should be good with a gain of less than 1.
The text book summing amplifier also shows an additional input resistor on the positive input going to ground. Its function is to balance the input bias currents. The LM4562 has a typical input bias current of 10nA. Across the 1K input resistor this is 10uV so with luck we can live with not having the third resistor.
Thanks for your replies.
I've been looking at the original article and have attached the plotted output levels for volume pot position and feedback gain settings. Reducing the feedback gain from the standard +10dB to +0dB (unity) seems to offer only 5dB difference at the volume control's lowest settings. If possible, I'd like to avoid reducing the preamp's maximum gain below unity. Some audio sources have a low output level and it robs the ability to drive some amplifiers.
Above: The graphed plot from the article shows to the range of output versus the volume pot position.
I am a bit confused why it stops at -30dB, as I assume the preamp can be taken to -40, -50, -60, -70 like a conventional preamp. I can't recall anyone commenting on this and that question needs to be answered to move to the next step.
I've been looking at the original article and have attached the plotted output levels for volume pot position and feedback gain settings. Reducing the feedback gain from the standard +10dB to +0dB (unity) seems to offer only 5dB difference at the volume control's lowest settings. If possible, I'd like to avoid reducing the preamp's maximum gain below unity. Some audio sources have a low output level and it robs the ability to drive some amplifiers.
Above: The graphed plot from the article shows to the range of output versus the volume pot position.
I am a bit confused why it stops at -30dB, as I assume the preamp can be taken to -40, -50, -60, -70 like a conventional preamp. I can't recall anyone commenting on this and that question needs to be answered to move to the next step.
Not sure that the graph was intended to convey that the circuit performance stops at -30dB as the maximum attenuation. Note that data with the pot position actually at zero is not shown.
I would think that having a volume control stage that follows a logarithmic curve as closely as possible will give the best user experience as that curve is the best match for human hearing. If the output level of an input device or the sensitivity of the power amplifier and speakers results in using too much or too little rotation of the volume control for a good user experience, then changing gain is still the answer. Note that it may be necessary to change the gain associated with a particular input source if is significantly different than the other sources, and also necessary to change the gain of the preamp itself to best match the power amplifier and speakers
I believe selectable gain is not unheard of in preamps. Not sure if any preamps have offered a way to match input device levels.
I would think that having a volume control stage that follows a logarithmic curve as closely as possible will give the best user experience as that curve is the best match for human hearing. If the output level of an input device or the sensitivity of the power amplifier and speakers results in using too much or too little rotation of the volume control for a good user experience, then changing gain is still the answer. Note that it may be necessary to change the gain associated with a particular input source if is significantly different than the other sources, and also necessary to change the gain of the preamp itself to best match the power amplifier and speakers
I believe selectable gain is not unheard of in preamps. Not sure if any preamps have offered a way to match input device levels.
Hello everyone, my name is Alex.
I'm building the RIAA unblananced preamp from The Signal Transfer Company.
My question concerns some of the components close to the input. They are to be adjusted on test.
How do you do that?
I'm hoping some of you might have built this preamp and are capable of guiding me in the right direction.
Thanks everyone.
I'm building the RIAA unblananced preamp from The Signal Transfer Company.
My question concerns some of the components close to the input. They are to be adjusted on test.
How do you do that?
I'm hoping some of you might have built this preamp and are capable of guiding me in the right direction.
Thanks everyone.
Just a quick point of clarification for those who have recently joined this thread or haven't really been paying attention.
This thread is NOT about Doug Self's 2012 preamp published in Elektor magazine, rather this thread is all about the Doug Self circuit fragments published in Linear Audio #5 in 2013. A couple of years later I strung together Doug's ideas into the project that this thread is about.
Once again I want to thank both Jan Didden and Doug Self for all of their help and support during the long run of this thread.
This thread is NOT about Doug Self's 2012 preamp published in Elektor magazine, rather this thread is all about the Doug Self circuit fragments published in Linear Audio #5 in 2013. A couple of years later I strung together Doug's ideas into the project that this thread is about.
Once again I want to thank both Jan Didden and Doug Self for all of their help and support during the long run of this thread.
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Thanks for the reply back guys.
I did search for some answers on his books, but admittedly I fall short when it comes to understanding all the inns and outs of the books.
I have a mechanical eng background with basic analog electronics knowledge.
I asked in this forum because I saw some photos showing exactly the preamp that I'm building - which is just the phono preamp Mr Self designed.
I did search for some answers on his books, but admittedly I fall short when it comes to understanding all the inns and outs of the books.
I have a mechanical eng background with basic analog electronics knowledge.
I asked in this forum because I saw some photos showing exactly the preamp that I'm building - which is just the phono preamp Mr Self designed.
Hi Carl, I am interested in the boards for preamp. Are they still available? If yes, is there any alternative possibility to contact you?
I am here from 2004 year and this fact is not enough for this nice site to avoid me to contact you because I am "new" here (yes, 19 years only) and I am "under moderation".
I am here from 2004 year and this fact is not enough for this nice site to avoid me to contact you because I am "new" here (yes, 19 years only) and I am "under moderation".
Not a problem Narcissus. Please send this same message to my personal email at 'carl.huff@yahoo.com' and I will respond with the details.