Hi Phil. I've found the AD797 to be exactly that, a very nice opamp. I've found it has a very similar sound to the AD827, quite warm.
I replaced them with LME49710, simply using one feedback resistor as the I/V stage. The output is discrete, using Ray's design off the CD63 thread. I'd like to build a fully discrete I/V stage, but I'm unsure of which one to use as I need gain. The output stage is a buffer only.
Cheers, Lee.
edit: I'm going to try and use a variation on the Pedja Rogic buffer for output, as this requires no DC blocking caps on the output.
I replaced them with LME49710, simply using one feedback resistor as the I/V stage. The output is discrete, using Ray's design off the CD63 thread. I'd like to build a fully discrete I/V stage, but I'm unsure of which one to use as I need gain. The output stage is a buffer only.
Cheers, Lee.
edit: I'm going to try and use a variation on the Pedja Rogic buffer for output, as this requires no DC blocking caps on the output.
Tubee, I think I'll try it. Do you think sound-wise, it can compare to a fully discrete design?
Also (I'm probably being thick here), in the AD797 I/V stage shown in that opamps datasheet, the gain is shown as increasing with a lower value feedback resistor. But I've found it to work the opposite way round. I used 1k and got a quiet output, but 1k8 is much better. In fact, I'm going to have to try higher values as it's still not at the ~2v output standard.
Lee.
Also (I'm probably being thick here), in the AD797 I/V stage shown in that opamps datasheet, the gain is shown as increasing with a lower value feedback resistor. But I've found it to work the opposite way round. I used 1k and got a quiet output, but 1k8 is much better. In fact, I'm going to have to try higher values as it's still not at the ~2v output standard.
Lee.
Hi Lee,
If it was just a single resistor in the feedback path, the output would be
Vo = -IRf (I think)
So, yes the bigger the resistance, the greater the voltage swing.
I'm interested in that 2000pF cap across the input of the opamp in the datasheet. I suspect that's to supress higher frequencies, and improve stability - like the feedback capacitor.
Mine uses 1.6k I think. Farnell had a sale getting rid of all their non Rohs stuff maybe a year or so ago, and I picked up a small batch on 0.1% 1.6k resistors just for this
Cheers,
Phil
If it was just a single resistor in the feedback path, the output would be
Vo = -IRf (I think)
So, yes the bigger the resistance, the greater the voltage swing.
I'm interested in that 2000pF cap across the input of the opamp in the datasheet. I suspect that's to supress higher frequencies, and improve stability - like the feedback capacitor.
Mine uses 1.6k I think. Farnell had a sale getting rid of all their non Rohs stuff maybe a year or so ago, and I picked up a small batch on 0.1% 1.6k resistors just for this
Cheers,
Phil
I'm just evaluating parts at the moment. I didn't bother with the feedback cap or input cap at the moment, as the filter section of Ray's discrete should be doing that anyway I think.
Any ideas on how to calculate the cap/res to gnd from pin 3 in figure 39? It'd be really handy to remove any dc offset, so I could try the Pedja buffer.
Cheers, Lee.
Any ideas on how to calculate the cap/res to gnd from pin 3 in figure 39? It'd be really handy to remove any dc offset, so I could try the Pedja buffer.
Cheers, Lee.
Fig 39 is about reducing the offset due to input bias current errors; adding Cs and Rs allows the opamp to 'see' the same impedance at both the input pins (which makes the resulting dc offset due to input bias currents common-mode, so it is rejected by ~130dB)
That will not allow you to null-out the 2v-3v of DC offset arising from the 1541s -2mA at digital zero. IOW, you'll still need a coupling cap...
That will not allow you to null-out the 2v-3v of DC offset arising from the 1541s -2mA at digital zero. IOW, you'll still need a coupling cap...
I dropped an Audiocom Super Regulator on the DAC's +5V rail last night. This is fed, for now, from the same PSU as the digital filter.
Huge improvement, nearly as much as a reg on the digital filter. Clearer, more open, harder bass, more dynamic, cleaner mids and top, less harshness/hardness.
We're getting somewhere now...
Simon
Huge improvement, nearly as much as a reg on the digital filter. Clearer, more open, harder bass, more dynamic, cleaner mids and top, less harshness/hardness.
We're getting somewhere now...
Simon
jnewbold said:
The LM is hooked on the raw 11V dc (from filter cap) before the old dig +5V 7805 supply
The LM needs to be cooled, the standard cooler from cdp is enough in my cd880, with a smaller sink like in a CD640/660 (650?) it will get some warmer i bet. A separate sink it should be at least 6cm wide, and 3cm high or so, but only my approximate feeling on that.
When you use the standard cdp sink, the 7805 delivers less heat, and the LM's some extra, so only some small extra heat is added compared to original PS. (shunt vs series)
The BD can run without a sink, but had to mount it in cdp anyway.
And remember: it is worthwhile!
Hi Guys,
Nice thread, I'm in a CDB650 since '86 or there abouts and have traveled many roads similar to most of you.
At the end of the road you get some answers to the quests and one of them is that the I/V should not have feedback and for this you should be looking to some discrete solution on most instances. The other one is to use passive LPF at the output.
I will try to manipulate a pic (to big now) to show how my CDP looks today
Cheers,
Nice thread, I'm in a CDB650 since '86 or there abouts and have traveled many roads similar to most of you.
At the end of the road you get some answers to the quests and one of them is that the I/V should not have feedback and for this you should be looking to some discrete solution on most instances. The other one is to use passive LPF at the output.
I will try to manipulate a pic (to big now) to show how my CDP looks today
Cheers,
poynton said:
Hi Andy,
The Bursons are very complex and sound outstanding, I'd rather spend my time elsewhere than trying to copy them.
philpoole said:
Phil,
Sadly other than a -12V (for output) I have no more negative regulators of the fancy variety. I will have to buy two more S Powers!
apassgear said:
Wonderful, please show us and tell us more.
Simon
The 1541's -15v supply is connected to the DAC's substrate, and making this supply quiet (with good local decoupling to 0v) is pretty important. ON this pin though, is no audio signal, and digital noise. That means that the decoupling quality is actually the key, and the output impedance of the reg is less so. An LM337 can work really well, esp. if the raw supply feeding it is well smoothed (i.e. RF choke, gyrator or similar)
Re: HDAMs (regardless of whomsoevers take on it) - well after the CD63 thread where everyone reported it was worth omitting, it would seem bizarre to start adding the things here
Re: HDAMs (regardless of whomsoevers take on it) - well after the CD63 thread where everyone reported it was worth omitting, it would seem bizarre to start adding the things here