No I misunderstood your circuit thinking the chip was used as a direct replacement for the linear potentiometer. I misread the schematics for some reason.
FWIW I think I am right about the caps though. The original was totally coupling cap free but in this case they are necessary input caps to the MUSES IC and can not be omitted. Please see the datasheet page 7. They do lack in the other leg.
Another thought that came up is what the exact use of the DC subtraction circuit is when there is a coupling cap further in the line. Add one in the second leg and the whole situation around the TL072 can be omitted. Given that many sources have output caps or DC servos and this design having coupling cap(s) anyway omitting the subtraction circuit would make the design and PCB simpler and more compact. Just 2 extra caps result in 22 Parts less even. Space that can be used for local onboard ultra low noise LDO regulators close to the ICs 😀
Ok now let's talk about features, displays, ESP32 and programming 🙂
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Yes and no.
Yes, they do cause a phase shift.
No - the inclusion of the coupling capacitor between the difference stage and the input to the MUSES chip creates a high pass filter and phase shift is at very low audio frequency (<10Hz)
V(n004) is pre-capacitor. V(p001) is at input to MUSES72323.
Having added the coupling capacitors into the circuit (as per the MUSES data sheet), the dc servo part is pretty well redundant although it does ensure even clipping if you overdrive the whole circuit (not that we would of course!).
I haven't tried it but, as I said in one of the earlier posts, with the dc servo doing its job we probably don't actually need the coupling capacitors.
With the coupling capacitors in place, we could easily remove the dc servo, save 22 parts and free up a pile of space. As they say, you pays your money and takes your choice. 🙂
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The MUSES seems to have considerable DC offset at its inputs. That is the reason for DC blocking input caps. Never used one as I went to another solution but you could measure it. If so I don’t think correction nor DC coupling will work out OK. If the caps wouldn’t be necessary the manufacturer would not draw them in every schematic.
The few mV DC offset of sources are nicely solved in the DC coupled original but in yours they won’t do any harm. As it is half AC coupled anyway one could make that fully AC coupled without losing a nights sleep and have the mentioned benefits.
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If we were to include coupling capacitors in the other legs (R33, R34) this would remove the required dc reference for the difference amplifier stage (currently a net-tie to 0V). I'm happy for the dc servo to be removed or not included but not the 0V reference.
A non issue easily programmed out with complex devices by Mr. Ohm 🙂 Anyway I have to go now. Good luck and please think about the meanwhile standard local regulation (like in the original) but then straight with good uLDO regs instead of 78xx/79xx cameos.
BTW it is not a DC servo circuit but a subtraction circuit.
BTW it is not a DC servo circuit but a subtraction circuit.
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Any personal preferences on LDOs? 🙂
I'm off out too for a while - got to run energy off the two grandsons and give their mother a break!
True. Bruno Putzeys calls it a dc servo in his article then goes on to describe it as a low pass filter subtracting from the input signals.
From our recent posts, we've deduced that, with the coupling capacitors between the difference stage and the MUSES input, we no longer need the DC servo / subtraction circuit. Also, following a request for larger resistor and capacitor SMD sizes, I've re-laid out the board to use 1206 SMDs in place of the previous 0805s This also means that MMELFs can also be used if you so wish.
As you can see, having removed the 22 components of the servo / subtraction circuit, there's now a large area where onboard ultra low noise LDO regulators could be fitted. Any preferences?
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When I have some time over the next day or two, I'll make some dc measurements around the input pins of the MUSES chip on my built board. Dependent on what I measure, I may bypass the coupling capacitors and check what impact this has on the overall volume control performance. If I bypass the caps, things I'll check for are
- audible deviations from spec'd attenuation of 0.25dB
- perceptible noises during attenuation adjustments (zipper noise)
- anything unexpected that crops up while I'm testing
Hi that is a nice board. I will look into the regulators as I forgot the ones I used recently. LT3045/3094 are good but expensive and hard to solder.
No I did not forget. They are TPS7A3001/4901. 150 mA max. so maybe a pair for the opamps and a pair for the MUSES chip. Will have to check datasheets for power consumption. Onboard rectifiers with CLC and all is good.
I tried others but have to go now.
No I did not forget. They are TPS7A3001/4901. 150 mA max. so maybe a pair for the opamps and a pair for the MUSES chip. Will have to check datasheets for power consumption. Onboard rectifiers with CLC and all is good.
I tried others but have to go now.
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TPS7A39 is a 150mA pos/neg dual, quite cost effective and ensures symmetrical (pop-free) startup... unless the charging AC-coupling cap spoil it, that is.
What I found, examining the data sheet, is that the the chip itself can handle DC signals (and zero-biased AC signals) and does not introduce any DC offset of its own, consisting only of pass gates and resistors in the analog path.
But to avoid "Zipper Noise" and make the Zero Crossing Detection (if enabled) work as advertised, AC coupling is recommended and the chip is prepared for that with 20k internal bias resistors tied to the L/R reference inputs, actually these bias resistor appear to be the control elements proper, the resistor ladders for the "att" sections of the chip.
Audio signals don't need to be DC-coupled anyway, quite the contrary, hence AC coupling is preferred and the chip is ready for simple passive high-pass solution with coupling caps -- like the well-known Nichicon UES-series bipolars, also named Muse (coincidence?).
But I see no reasons why other solutions couldn't be implemented as well, like DC-servoing the preceeding stage.
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Busy but thanks for the good suggestion of TPS7A39. Seems to be the TPS7A4901 and TPS7A3001 in 1 package. Is it solderable by humans?
Yeah whatever the reasons I would keep coupling caps (judged from schematics, result is the same) and implement a muting relay. I think the original fully DC coupled version had it too. You don't want to have any risk of plops at power on or off as that is design poverty.
Back to work again.
Yeah whatever the reasons I would keep coupling caps (judged from schematics, result is the same) and implement a muting relay. I think the original fully DC coupled version had it too. You don't want to have any risk of plops at power on or off as that is design poverty.
Back to work again.
With my previous implementation of the Putzeys circuit which I'd done with an MAS6116 chip doing the same job (and in a very similar approach). I'd included the dc servo but hadn't included coupling capacitors - on switch-off of my pre-amp before switching off the power amp resulted in mild speaker thump.
With my current setup which uses the MUSES chip and has coupling capacitors before the MUSES inputs, there is no speaker thump. If my memory serves me properly, the MUSES chip does this internally. As we've made space by removing the dc servo / subtracting stage, it should be relatively easy to include an output muting relay.
No really DIY-friendly I would say, handling leadless WSON-10 is no fun without hot air or infrared, but not impossible either. However, the PCB pads and routing should be optimized for manual tip soldering, otherwise it really sucks.
Having looked at the SMD packages used for the ultra-low noise LDOs, I'm thinking the thought of soldering these onto a board will seriously put off potential builders who are new to SMD. I'll shortly publish board data (schematic, Gerbers, BOM) for one without the hard-to-solder LDOs. If the PSU in use within a pre-amp setup uses the likes of an LM317 for its regulation, like my current setup uses, there's no audibly perceptible noise on the outputs.
I'm happy to pursue the Rolls-Royce approach if we are to agree on which LDO to use. If that makes sense, I'll move that approach onto a separate discussion.
I'm happy to pursue the Rolls-Royce approach if we are to agree on which LDO to use. If that makes sense, I'll move that approach onto a separate discussion.
Yup, it can be painful even with a magnifier, I'd second a request for hand solder pads. Particularly on those electrolytics.
Many thanks for undertaking all these changes to a design that already works for you.