moumousganou
I suppose the best replacement to the pot is the Stepped Attenuator by Hans Polak.
By the way can you post the link of tje project of the Hans Polak project please?
Regards
Ronaldo
I suppose the best replacement to the pot is the Stepped Attenuator by Hans Polak.
By the way can you post the link of tje project of the Hans Polak project please?
Regards
Ronaldo
It's on Jan's Linear audio site.
I have already contacted Hans and asked but he has no PCB's remaining.
I may contact him again to ask if he would be prepared to produce some more, or allow us to do so.
Lets Concentrate on getting the main PCB into production first.
http://linearaudio.net/sites/linearaudio.net/files/hp bruno potmeter 3.pdf
I have already contacted Hans and asked but he has no PCB's remaining.
I may contact him again to ask if he would be prepared to produce some more, or allow us to do so.
Lets Concentrate on getting the main PCB into production first.
http://linearaudio.net/sites/linearaudio.net/files/hp bruno potmeter 3.pdf
When discussing an alternative solution to Bruno's original design, please note that using a digital potentiometer, or a relay-switched resistor ladder, or a PGA, changes the preamp topology in that the original design has a variable-gain last stage, whereas the new one would be based on a constant-gain one with an attenuator in front of it.
The above is a bit complicated way of saying that a digital pot, or a PGA can not be used to control the volume in the way proposed by Bruno; and I'm not aware of a relay-switched resistor ladder that can be used with good results within the negative feedback loop either.
Regards,
Braca
The above is a bit complicated way of saying that a digital pot, or a PGA can not be used to control the volume in the way proposed by Bruno; and I'm not aware of a relay-switched resistor ladder that can be used with good results within the negative feedback loop either.
Regards,
Braca
Seems like you can use a digital pot to replace the normal pot in Bruno's circuit. Use an Arduino or something similar to control the pot, and you have remote controllable volume with this project.
If it was me, I would add isolation on the SPI bus (or whatever interface you use to control the digital pot), to keep the Arduino noise out of your preamp.
Am I missing something?
Randy
I'm actually toying with using Vincent's LDR-based volume controller in conjunction with Bruno's pre-amp. I'm hoping to finish it today.
Assuming I can get it working reliably (I'm tweaking the code a bit to adjust for linear response, modify impedences, etc), it's a great way to add Arduino volume control, remote control, source selection, etc. 🙂
Assuming I can get it working reliably (I'm tweaking the code a bit to adjust for linear response, modify impedences, etc), it's a great way to add Arduino volume control, remote control, source selection, etc. 🙂
Using LED/LDR may make matching of channel to channel almost impossible.
The main advantage of using an inverting opamp with volume controlled by feedback is that a linear potentiometer gives a log law control over much of the rotation range of the conventional vol pot.
If you adopt some other resistance topology for the linear potentiometer, you are throwing away this major advantage.
I stand corrected on this one. Studying the MUSES 72320 data sheet I see that it is possible to insert a digital pot into the feddback path, albeit in a rather complicated way.
My original thinking was that an electronic component in the feedback path of a linear opamp circuit would only be used for obtaining special transfer functions.
This notwithstanding, I would still not recommend this sort of volume control in this particular case. Assuming the MUSES chip as the volume controller and the manufacturer-recommended application circuit for bipolar opamps, the last stage of the preamp would have to be modified in a nontrivial way in order to accomodate this solution; and adding three electrolytic capacitors to the original circuit which previously had only a single linear pot entails IMHO a risk for impairing the sound quality.
This is a purist preamp designed with a view to achieving maximum linearity; and unless someone comes up with an economical solution for a relay-switched resistor ladder in the feedback path, the next best alternative to it still appears to be a relay-switched attenuator between the diff. amp output and the last stage.
Regards,
Braca
Hi Andrew, I get that. Vincent's original design emulates a log response, but that can easily be changed in the code to replicate the behaviour of a linear pot.
Matching LDRs isn't an issue as there's a calibration process to ensure they track each other within a percent or so.
But I digress. I'll see how I get on with it and test hopefully this weekend.
Could some look at the PCB trace layout.
It seems to me that the labelling on the sch does not match.
The left diff opamp is labeled as U2/7A and the right vol pot opamp is labeled U2/7B
The PCB looks like the left diff opamp is U2A/U7B and the right vol pot opamp is U2B/U7A
I need to get this right so that I can locate the gain limiting resistors correctly.
I'm thinking of adding in Ra to the left of R5 and adding in Rb to the right of R5.
My values are likely to be around Ra~2k4 to 5k1 and Rb~51r to 200r
It will be interesting to see how small the traces are and whether the 805 or 603 resistors can be fitted on the PCB.
It seems to me that the labelling on the sch does not match.
The left diff opamp is labeled as U2/7A and the right vol pot opamp is labeled U2/7B
The PCB looks like the left diff opamp is U2A/U7B and the right vol pot opamp is U2B/U7A
I need to get this right so that I can locate the gain limiting resistors correctly.
I'm thinking of adding in Ra to the left of R5 and adding in Rb to the right of R5.
My values are likely to be around Ra~2k4 to 5k1 and Rb~51r to 200r
It will be interesting to see how small the traces are and whether the 805 or 603 resistors can be fitted on the PCB.
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Jan, I suspect that we have a misunderstanding here.
The first statement of mine that you quoted refers to replacing a linear pot in the NFB path (Bruno's solution) with a digital (electronic) pot, such as e.g. the MUSES one. I then proceed to describe why I think the latter is inferior to the original solution.
IMHO the only better solution than the linear pot, at least from the standpoint of the volume control law, would be a linear relay-switched resistor ladder replacing the former in the NFB path. I'm not aware that such one exists in the "open hardware" space, but I suspect that Bruno has implemented something like that in his Makua preamp.
On the second reading, my second statement may be prone to causing misunderstandings. I intended to say that a relay-switched attenuator like the one designed by Hans Polak, connected between the differential and the output stages would be a better choice than an electronic digital pot in the NFB path. As you may recall I built the the Hans' attenuator and tested it subjectively and experimentally as far as my humble equipment allowed. Subjectively, I could not detect any differences comparing it to a Vishay conductive plastics pot in the NFB path, and my measurement results, while being slightly different for the two solutions, could not provide a solid basis for proclaiming the one or the other version as the winner.
Hope this clarifies the intended meaning of my writing.
Regards,
Braca
I think J1/J3 goes to U1A/B goes to 2A then 2B (pot is feedback on 2B)
J7/J9 goes to J6A/B goes to 7B then 7A (pot is feedback on 7A)
This is different from what the schematic says for U7.
Randy
I'll have a look at that. Thanks.
Had a look.
The inputs feed through in the order you have stated. It looks like we both see the same routing.
It's U7A and U7B that have been swapped between sch and PCB.
Had a look.
The inputs feed through in the order you have stated. It looks like we both see the same routing.
It's U7A and U7B that have been swapped between sch and PCB.
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Andrew, I rebuilt the schematic and laid it out again on a new PCB, partly as a training exercise for me with KiCAD, and came across a few quirks where the resulting PCB traces didn't match with the electronic circuit schematic.
I posted my comments earlier in this thread, but essentially, there are a few places where the audio signals are inverted and there's a mismatch between the track layout / Gerbers and the electronic circuit schematic. From memory, I think it was between the first set of LM4562 units and the relays where there was an inversion.
However, following the traces, I realised that the inversion is then re-inverted a second time, so the net result is not going out of phase (pun not intended LOL!).
I posted my comments earlier in this thread, but essentially, there are a few places where the audio signals are inverted and there's a mismatch between the track layout / Gerbers and the electronic circuit schematic. From memory, I think it was between the first set of LM4562 units and the relays where there was an inversion.
However, following the traces, I realised that the inversion is then re-inverted a second time, so the net result is not going out of phase (pun not intended LOL!).
You can buy a remote controlled, motorised pot complete with LED position readout for about €14 - why go to all the complication of Arduino etc when a simple electonics solution exists that can be soldered in place as it if was the original pot?
Arduino is fun and interesting but I do see people using such things in the most complicated way when there's a very cheap and much much easier solution!
An unconventional but one ensuring long life and excellent tracking, would be a top quality motorised fader by Penny and Giles. Sadly they don't do motorised rotary faders (their term for a pot). But I'd guess people wouldn't want a fader instead of pot.
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I hope we have a layout/Gerbers true to Bruno's intention now that we are close to order boards 🙂 ?
Maybe we could do some reviewing here if you post what you intend to send for production?
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