Hi, I'm on my way repairing a Cambridge D500SE by modding it to a different DAC (the CS4391 has died and instead of substituting it, I'm going to use a CS4398 on a small diy board and modify and improve some parts of the original board so to connect it all). Now, I've been consulting the datasheets and the CS4398 has balanced outputs so the output stage is different, so I'm going to make an appropriate output stage with high quality opamps, now the question arises: is it best to stick to the design shown in datasheet (and evaluation board, I also had a look) for unbalanced outputs, or shall I consider some of the alternative circuits which can be found on the net? I got myself some service manuals of CD players which use this DAC and besides some discrete circuits, I also found the one I attached, which is from a Mcintosh player. I would only use the three opamp stages as I don't need balanced outputs. The passive components have completely different values than in datasheet. Maybe they are more adequate, as this is in fact a circuit for a CD player? While the datasheet output suggestion is a generic one? In fact having correct line level would be nice. On the other hand, datasheet output stage has been recommended on this forum a lot of times as seems to have correct filtering and is based on only one opamp stage, simplifying the design and which means audio signal will pass through less transistors, which is good.
So, what do you suggest? Unfortunately I'm no opamp design wizard so I can't really decide this alone... Stick with the datasheet standard suggestion or try the implementation from the mcintosh?
Btw, the original output stage in the cambridge player add an additional opamp stage after the output filter stage (which is done exactly like in CS4391 datasheet), I wonder if this is to get correct line level and all? So maybe I'd really need an additional opamp stage for this to get to correct levels, if I use the output stage of the datasheet of CS4398?
Attached goes schematic of mcintosch and original cs4398 datasheet output stage.
Thanks in advance for all suggestions.
So, what do you suggest? Unfortunately I'm no opamp design wizard so I can't really decide this alone... Stick with the datasheet standard suggestion or try the implementation from the mcintosh?
Btw, the original output stage in the cambridge player add an additional opamp stage after the output filter stage (which is done exactly like in CS4391 datasheet), I wonder if this is to get correct line level and all? So maybe I'd really need an additional opamp stage for this to get to correct levels, if I use the output stage of the datasheet of CS4398?
Attached goes schematic of mcintosch and original cs4398 datasheet output stage.
Thanks in advance for all suggestions.
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Osvaldo, I'm asking for opinions on which circuit might be better for the output filter stage for CS4398 - the one from the datasheet or an alternative circuit, as for example the one I attached which is taken from the schematic of a CD player. There are obviously lots of other alternatives, even discrete ones, or even with tubes. What I DON'T want is NO filter stage, as some people do, because this will let the HF noise through and load the output of CS4398 too much, also it would have to be used as balanced signal or half of signal would get lost.
The circuit from the datasheet is a simple filter stage to get rid of the HF noise of the DAC and also converts to unbalanced output, it's as simple as it gets with one opamp and is widely accepted. But I've read one some places that some of the passive components might not have the optimal values, not sure. Also I'm not sure if it will provide enough output level for line level (as this is a CD player).
The other one is a little more complex, with three opamps for each channel (at least) when using unbalanced output and uses passive componentes with quite different values. As it's from the schematic of a good cd player, I guess the component values are optimized for the right frequencies and line level.
Basically, I'm inclined to go with the schematic from the datasheet, as it's really simple and straight forward, but am afraid it might not be optimized for CD player output, while the other one surely is optimized (but more complex, which is a disadvantage for hifi - normally less components in audio path mean less colouring and less probability of reduction in sound quality).
The circuit from the datasheet is a simple filter stage to get rid of the HF noise of the DAC and also converts to unbalanced output, it's as simple as it gets with one opamp and is widely accepted. But I've read one some places that some of the passive components might not have the optimal values, not sure. Also I'm not sure if it will provide enough output level for line level (as this is a CD player).
The other one is a little more complex, with three opamps for each channel (at least) when using unbalanced output and uses passive componentes with quite different values. As it's from the schematic of a good cd player, I guess the component values are optimized for the right frequencies and line level.
Basically, I'm inclined to go with the schematic from the datasheet, as it's really simple and straight forward, but am afraid it might not be optimized for CD player output, while the other one surely is optimized (but more complex, which is a disadvantage for hifi - normally less components in audio path mean less colouring and less probability of reduction in sound quality).
As the preamps and the amps has limited bandwidth and then act as low pass filters, your idea of taking rid of the filter is doable. But there may be a further problem if you link it to a class D amp that can heterodyne with the carrier or harmonics, crating lots of noise and splatters. Also, maintaining a good shielding is important because of the same reason. My doubt is the interference caused into other devices, like AM receivers, and the like.
I think we're not understanding each other well. I'm NOT trying to get rid of the filter. I'm convinced that there's a filter stage necessary after CS4398, just like it says in the datasheet. But as I'm building a new DAC from scratch for this player (with a CS4398), I can choose which output stage I will implement, it can be different from datasheet. I'm just not sure which to use, that's why I'm asking for opinions on which of the two output filter stages shown could be better.
The recommended output filter in the CS4398 datasheet is perfectly adequate. I have a CS4398 DAC (build on an ebay board) and it uses the recommended output filter (but with one dual opamp). Output level is plenty and the sound quality is excellent, IMO. The AD8620 sounds particularly good to me in this board.
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Thank you cogitech, this is exactly the kind of opinion I need, from people who have used it or listened to it in real life
The problem is, I have not heard the McIntosh version so I cannot say which is "better".
Still, I would say you are not going to be disappointed with the recommended circuit.
Ok cogitech. I think I will build the version of the service manual (as the existing dac board will be easily adapted to the circuit of the datasheet, the McIntosh version is different) and then I will test it with signal generator and oscilloscope, to see better how it performs over the audio spectrum, and also I will perform listening tests with the DAC. If I like it, or if I need to modify something, fine. If not, I will try the version of McIntosh.
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