Glenn, I'm amazed (and really pleased) to hear that the balanced output genuinely comes from a pair of differential DACs because I will soon be wanting a proper balanced output and had rather assumed that a nasty op-amp implementation would have been used. Good for Cambridge Audio!
I haven't taken mine apart yet, but I assume you've scribbled a bit of a diagram. Would you care to post it?
I haven't taken mine apart yet, but I assume you've scribbled a bit of a diagram. Would you care to post it?
Click on this link to download the service manual for the Cambridge 740C.
Cambridge Audio 740C Service Manual
On page 20, you'll find the DAC section.
The Dacmagic has the same filter circuit (even the same part labels), apart from:
a) the resistor/capacitor values in the filter are slightly different due to the upsampling being 384kHz in the 740C and 192kHz in the DM no doubt,
b) the DM takes the balanced signals from each of the first 4 dual op-amps (NE5532) to the XLR sockets, which the 740C doesn't have, as well as the summing op-amp (OP275), so the stuff at the output with the coupling cap, relay, etc. is present 6 times in the DM instead of twice in the 740C.
The peculiar op-amp arrangement where the o/p is inverted using the other half of the dual op-amp, and fed back to the non-inverting input gives a kind of balanced feedback, which is something they've done in their players for years. (640C, 640CV2, 740C, 750C). It may be so that each DAC o/p is driving a virtual earth instead of the -ve one driving a virtual and the +ve driving a real one. I'm sure it must lower distortion or else they wouldn't persist with it. Whether it sounds any better for it or not I couldn't say.
Oh, and if you're planning on taking the DM apart, you'll need a philips screwdriver with a shaft as long as the DM itself and a torch (flashlight) because there is screw holding the lid to the front panel that can only be reached by taking off the back panel.
Hope that helps!
Glenn
Edit: It's a matter for debate whether or not what is used still classifies as a nasty op-amp implementation anyway. It sounds rather better hooked up to an LC Audio ZAPfilter Mk2, but that's another story altogether.
Cambridge Audio 740C Service Manual
On page 20, you'll find the DAC section.
The Dacmagic has the same filter circuit (even the same part labels), apart from:
a) the resistor/capacitor values in the filter are slightly different due to the upsampling being 384kHz in the 740C and 192kHz in the DM no doubt,
b) the DM takes the balanced signals from each of the first 4 dual op-amps (NE5532) to the XLR sockets, which the 740C doesn't have, as well as the summing op-amp (OP275), so the stuff at the output with the coupling cap, relay, etc. is present 6 times in the DM instead of twice in the 740C.
The peculiar op-amp arrangement where the o/p is inverted using the other half of the dual op-amp, and fed back to the non-inverting input gives a kind of balanced feedback, which is something they've done in their players for years. (640C, 640CV2, 740C, 750C). It may be so that each DAC o/p is driving a virtual earth instead of the -ve one driving a virtual and the +ve driving a real one. I'm sure it must lower distortion or else they wouldn't persist with it. Whether it sounds any better for it or not I couldn't say.
Oh, and if you're planning on taking the DM apart, you'll need a philips screwdriver with a shaft as long as the DM itself and a torch (flashlight) because there is screw holding the lid to the front panel that can only be reached by taking off the back panel.
Hope that helps!
Glenn
Edit: It's a matter for debate whether or not what is used still classifies as a nasty op-amp implementation anyway. It sounds rather better hooked up to an LC Audio ZAPfilter Mk2, but that's another story altogether.
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Well, that's excellent! Many thanks. I see they have a standard circuit similar to the "Superbal" circuit championed by Steve Dove of Alice in the early 80s for making both inputs of an op-amp a virtual earth and allowing balanced feedback. At the very least I'll be making myself some XLR to phono leads for amplifiers with unbalanced inputs, and after that, the long-handled Pozidriv No2 will be out to short out those electrolytics.
As for "nasty op-amp implementation", what I had in mind was a circuit whereby the differential output from the DACs was converted to single-ended, amplified, filtered, then converted back to differential by adding an inverter (possibly with added fiddle circuitry to ensure that the output voltage doubled if one leg was shorted to ground). And if you don't think anyone would be daft enough to do all that, take a look at the Behringer DCX2496 active crossover.
By contrast, the DACmagic is really quite sanitary. Perhaps that's why it sounds nice.
Thanks again...
As for "nasty op-amp implementation", what I had in mind was a circuit whereby the differential output from the DACs was converted to single-ended, amplified, filtered, then converted back to differential by adding an inverter (possibly with added fiddle circuitry to ensure that the output voltage doubled if one leg was shorted to ground). And if you don't think anyone would be daft enough to do all that, take a look at the Behringer DCX2496 active crossover.
By contrast, the DACmagic is really quite sanitary. Perhaps that's why it sounds nice.
Thanks again...
Well, I fought my way in there (Pozidriv No1, not No2), measured the offsets (all < 1mV) and performed an electrolyticectomy. It was easier and neater simply to remove them and replace with wire links. Hard to tell what the difference is, but when my other DACmagic comes back, I'll be able to switch between the two and compare.
One thing that may be of use to somebody is that while it was on the bench I investigated the S/PDIF output. It produces a nice clean signal from a 75 Ohm source impedance and does so even when the input is USB. The significance of that is that you could daisy-chain DACmagics from any source simply by connecting later DACmagics via S/PDIF and allowing the first to be the selector. Why would you do that? Suppose you wanted an active loudspeaker system but with analogue crossovers. You could put delay in the S/PDIF signals to time-align the drivers and analogue crossovers after each DAC. And that's why I have two DACmagics - I intend to use one for the bass and another (delayed) for the remainder.
One thing that may be of use to somebody is that while it was on the bench I investigated the S/PDIF output. It produces a nice clean signal from a 75 Ohm source impedance and does so even when the input is USB. The significance of that is that you could daisy-chain DACmagics from any source simply by connecting later DACmagics via S/PDIF and allowing the first to be the selector. Why would you do that? Suppose you wanted an active loudspeaker system but with analogue crossovers. You could put delay in the S/PDIF signals to time-align the drivers and analogue crossovers after each DAC. And that's why I have two DACmagics - I intend to use one for the bass and another (delayed) for the remainder.
First phase, few days ago: I've shorted all of the output caps and added the Maplin XLR -> RCA leads pictured above. As mentioned before, pin 3 in this ready made lead was going to the RCA ground. For neatness, I know have two wires going to pin 1 (ground).
Second phase, tonight: Added two 5 Volt Audiocom super regs (MK1) that I had in my old CD player. They barely fit. Height is the issue but laying them diagonally works. If I had a second attempt I'm sure I could make them fit slightly better. I think the later Audiocom regs are shorter.
Next phase: Would like to try a Maplin power supply when they're back in stock. Upgrade caps for better quality. All of the caps in the Dacmagic are 105 °C.
I've got some pictures before the regs went in which I'll post soon.
Attachments
I have just bought a Dacmagic and find the work done very interesting. I will shortly open the box.
I want to introduce two downstream analogue inputs and a gain control so I can dispense with my preamp. I need to stay balanced.
Effectively a three crosspoint switch following the DAC section output allowing me to select DAC or FM input or Phono input with a gain control or attenuator. I will build this into another box with a PSU in a similar fashion the the pic posted by Glenn2
Any circuit suggestions info or links relating to the above would be appreciated but I would feel more comfortable if I had a circuit of the Dacmagic.
Have downloaded the 740 manual as suggested
Anyone come across a cct diagram of the current Dacmagic?
Ianmac
I want to introduce two downstream analogue inputs and a gain control so I can dispense with my preamp. I need to stay balanced.
Effectively a three crosspoint switch following the DAC section output allowing me to select DAC or FM input or Phono input with a gain control or attenuator. I will build this into another box with a PSU in a similar fashion the the pic posted by Glenn2
Any circuit suggestions info or links relating to the above would be appreciated but I would feel more comfortable if I had a circuit of the Dacmagic.
Have downloaded the 740 manual as suggested
Anyone come across a cct diagram of the current Dacmagic?
Ianmac
Yes the analogue circuit is identical even in terms of part labels.
The actual values of the capacitors and resistors are different though, as is necessitated by the 192kHz upsampling vs the 384kHz of the 740C.
What Ianmac has planned doesn't make a whole lot of sense to me though, unless I'm missing something. So the phono stage (with RIAA eq) is to be fed into an analogue low-pass filter with balanced inputs? Is the FM source balanced too?
The analogue stage as it is has no gain to speak of too.
Another thing, the SMD components are small and hard to get to.
What you are proposing is something I personally wouldn't attempt even if I thought it was a good idea.
But in the true spirit of DIY, don't let me put you off!
Glenn
The actual values of the capacitors and resistors are different though, as is necessitated by the 192kHz upsampling vs the 384kHz of the 740C.
What Ianmac has planned doesn't make a whole lot of sense to me though, unless I'm missing something. So the phono stage (with RIAA eq) is to be fed into an analogue low-pass filter with balanced inputs? Is the FM source balanced too?
The analogue stage as it is has no gain to speak of too.
Another thing, the SMD components are small and hard to get to.
What you are proposing is something I personally wouldn't attempt even if I thought it was a good idea.
But in the true spirit of DIY, don't let me put you off!
Glenn
Apologies If I have not made myself clear
I am looking for the same functionality from my DAcmagic as one gets from cyrus DACXP or a Benchmark Pre.
The ability to process Spdif and Toslink inputs plus two analog inputs from my FM tuner and Phono amp
I need balanced outputs to drive my Hypex mono amps and a gain/attenuation facility to accomodate the differences in level from all input sources.
The analoge inputs will be unbalanced and need to be electronically converted to balanced. Three crosspoints (dual for balanced) will be needed to allow selection of DAC output or tuner or phono. This could possibly go in between the DAC output and balanced output stage.
Looking at the comments on power supply issues it seemed logical to me to address these at the same time thus my looking for a complete diagram so I get a full picture of the Dacmagic.
Like your avatar hope its real 625 pal
Ianmac
I am looking for the same functionality from my DAcmagic as one gets from cyrus DACXP or a Benchmark Pre.
The ability to process Spdif and Toslink inputs plus two analog inputs from my FM tuner and Phono amp
I need balanced outputs to drive my Hypex mono amps and a gain/attenuation facility to accomodate the differences in level from all input sources.
The analoge inputs will be unbalanced and need to be electronically converted to balanced. Three crosspoints (dual for balanced) will be needed to allow selection of DAC output or tuner or phono. This could possibly go in between the DAC output and balanced output stage.
Looking at the comments on power supply issues it seemed logical to me to address these at the same time thus my looking for a complete diagram so I get a full picture of the Dacmagic.
Like your avatar hope its real 625 pal
Ianmac
OK, I don't really see how it can be done with this circuit, at least to good effect, but happy to be proved wrong!
The circuit takes two pairs of balanced signals (there's a dual differential DAC per channel) to produce one. So each pair of balanced signals is processed by a differential op-amp circuit.
DAC L+ & DAC L- ==> DAC L+
DAC L- & DAC L+ ==> DAC L-
DAC R+ & DAC R- ==> DAC R+
DAC R- & DAC R+ ==> DAC R-
I don't see where/how you can bring in your FM/phono signals.
The circuit takes two pairs of balanced signals (there's a dual differential DAC per channel) to produce one. So each pair of balanced signals is processed by a differential op-amp circuit.
DAC L+ & DAC L- ==> DAC L+
DAC L- & DAC L+ ==> DAC L-
DAC R+ & DAC R- ==> DAC R+
DAC R- & DAC R+ ==> DAC R-
I don't see where/how you can bring in your FM/phono signals.
I think that it is the best way to output the signal from XLR.
http://www.lynxstudio.com/nav/getFile.asp?i=10&t=productfile
http://www.lynxstudio.com/nav/getFile.asp?i=10&t=productfile
Could someone dump for me his dacmagic eprom?
You can't get this part in service because Cambridge don't sell it separately (only complete main PCB) and it is one component which you don't buy in ordinary shop (24lc256 of course, but not programed).
In my dacmagic power supply (stabs on main board) has broke down (probably the reason was voltage surge) and burned few components. I have changed it, but eprom is dead (dacs probably work and receiver too). I don't know if DSP is broken, but without EEPRON I couldn't test it.
This is simple to do and wouldn't take much time. EPROM is easy to remove (socket) and if you want, you can make the programmer by yourself (JDM and serial port in computer). No one of my friends is dacmagic owner and on polish DIY forum I didn't receive any answer. This DIY community is a very famous one, and I hope to find someone to help me. Thank you in advance.
You can't get this part in service because Cambridge don't sell it separately (only complete main PCB) and it is one component which you don't buy in ordinary shop (24lc256 of course, but not programed).
In my dacmagic power supply (stabs on main board) has broke down (probably the reason was voltage surge) and burned few components. I have changed it, but eprom is dead (dacs probably work and receiver too). I don't know if DSP is broken, but without EEPRON I couldn't test it.
This is simple to do and wouldn't take much time. EPROM is easy to remove (socket) and if you want, you can make the programmer by yourself (JDM and serial port in computer). No one of my friends is dacmagic owner and on polish DIY forum I didn't receive any answer. This DIY community is a very famous one, and I hope to find someone to help me. Thank you in advance.
Grateful new arrival seeks service manual
Thanks for sharing this information, I just opened my DACMagic to bypass the caps. You helped me to get an immediate improvement and saved me a lot of experimentation.
I couldn't get the service manual from the link given. Does anyone have a copy they can send me, please?
Thanks for sharing this information, I just opened my DACMagic to bypass the caps. You helped me to get an immediate improvement
and saved me a lot of experimentation.I couldn't get the service manual from the link given. Does anyone have a copy they can send me, please?
The other coupling caps in DACMagic
Has anyone replaced the 10uF NP caps (not the PS filter caps) between the 2 DACs and the filters? I assume this will improve the clarity, but there is limited space so what did you use for this? And is it worth it?
BTW Ian Mac, your postings on VE inspired me to do a lot of beneficial work on a CA 540P and then rebuild a 640P. Thank you very much!
Has anyone replaced the 10uF NP caps (not the PS filter caps) between the 2 DACs and the filters? I assume this will improve the clarity, but there is limited space so what did you use for this? And is it worth it?
BTW Ian Mac, your postings on VE inspired me to do a lot of beneficial work on a CA 540P and then rebuild a 640P.
Thank you very much!