[budx3385]

I'm modding a PCM63 based Rotel RCD-971 cd player. It came stock with OPA2604 dual opamps on the output of each channel. I've rolled several dual OPs so far (LM6172, LM4562, AD826, AD8066). Next up is a homebrew 2xAD825 adapter.

I'm wondering about splitting the I/V and buffer sections with a Brown Dog adapter, but I don't know how to select the "best" I/V OP to go with different buffer OPs. Like, would an AD825 or an AD8031 I/V stage work better with an OPA627 buffer?

Is this worthwhile?

suggestions???

 

[tangent]

If the two op-amps are performing different functions in a single channel, this idea should work. The only reason you don't see it done more often is because usually the two channels in a dual op-amp in an audio system are used for the same thing, one for left, one for right; you wouldn't want to have the left channel sound different from the right.

As to your specific part numbers, the only comment I have is that I wouldn't use the 627 as a buffer. It's better used in the delicate amplification stages than the output stage. For the output side, anything with high current will probably serve better. I might start with an LM6171, since it's relatively uncolored, and its bipolar inputs shouldn't be a problem in a buffer role.

 

[budx3385]

thanks, tangent - nice to hear from you. I enjoy 627s in my RS XP-7 and 8065s in the Rotel 965 I modded - ya know, new clock and PSU, new caps, new RCAs, muting xstrs x'ed. I just installed the dual Burson OPs in the 971, and now I need to listen for awhile - much cleaner, but so up close it may be fatiguing. Yes, the dual OPs in each channel in the 971 serve two purposes, AFAIK - first the I/V conversion of the PCM63 current output, and then the buffer as load driver. I have the Brown Dogs for the 2x825s, and so playing mix n match would be straighforward - but really tedious, unless I know what direction to go.
best

 

[budx3385]

so, would a current-feedback opamp be more suitable for the I/V conversion stage, and then a voltage-feedback opamp for the gain stage?

 

[Pars]

From a theory standpoint, yes, CFB opamps are more suitable. Most don't like how they sound however. Check this thread for a number of ideas.

You might want to try THS4032 (good results reported... I haven't tried these).

 

[Filburt]

THS4032 might be tough to get stable. They don't like being run in unity gain, though it can be done with a few bits of preparation I suppose.

CFB op-amps sound alright as I/V converters. Theoretically they should be well suited since what you need is transimpedance performance, but a lot of CFB amps have less favourable settling characteristics, from what I've seen, and the output stages are frequently not all that hot for whatever reason. There are better amps that run in the 12V range for I/V but that would involve changing regulators in you power supply.

 

[budx3385]

hello pars, and thank you again. I have not read before of the THS4032, but Carlos' tips in that thread sound very good - that's also what was needed to make the Bursons sound right in the 965. BTW, the Burson duals are sounding AOK in the 971 - they definitely needed burn-in, and they are rather amazing now. But I like to experiment, so I will still try the 2x825s.

The specs on the 4032 are very reminiscent of the AD8066, which I like very much in the 971. Did you mean to use the THS4032 as a direct replacement for the OPA2604 duals? or to use one 4031 as I/V and then something else (627?) for gain?

Filburt, what would recommend for 12v?

best
Bud

 

[budx3385]

and oh BTW, removing the muting xstr's in the 965 was a huge improvement, pars - thank you! The more advanced 971 has a relay, which I have left alone.

 

[Pars]

Quote:

Originally Posted by budx3385 /img/forum/go_quote.gif <snip> Did you mean to use the THS4032 as a direct replacement for the OPA2604 duals?

Yes, that is what I meant. On my 855, I haven't sat down and tried to analyze exactly how and what they are doing. I do know that the second half of the dual opamp is configured as a buffer, but it may also be doing some active filtering. From memory, I don't think it is wrapped inside the feedback loop of the first half, but it may be.

Your idea of different opamp types singles) might work OK, but opens up a whole can of worms regarding opamp (or buffer) selection. And the buffer on these only needs to drive an IC... not headphones, so current requirements probably aren't that demanding.

 

[Filburt]

Well, using two different chips would play heterogenous loads on the local supply that feeds the adaptor. I don't know what effect that would have, but I could think of a few things I guess.

The 4032 is pretty dissimilar from the 8066. 8066 has less open loop bandwidth, more open loop gain, and is JFET input. In my experience, anyhow, they don't sound the same, although the 4032 shares some audible characteristics I suppose. At 12V, there is a variety of high performance chips available. You could use the AD9631, ADA4899-1, OPA690 (or 2690), AD8056, or AD8058. The latter three are duals, so may be easier to implement. You could also try the OPA656.

Pars - half I/V, half LPF.

 

[majkel]

IMHO the CD player modification should start from rebuilding the power supply of the audio section. Op-amps capable of higher output current draw more current from the power supply which working closer to its limits will be more susceptible for ripple. The ripple will decrease the actual bit depth of the output signal from the DAC and add some static to it. You won't hear the static but the SQ will get worse.
Next thing - I wouldn't use OPA627 instead of AD825, it's very expensive and producing "dead", too laid back sound for my ears. For output buffers you can actually use AD825 or LM6171. Both have higher than average output current, AD825 is also very stable and tolerates capacitive loads which is actually important when we consider the capacitances of the interconnect. On the other hand, you don't need huge current efficiency on the output because amplifiers have relatively high input impedance which doesn't load the CD player outputs at all.
OPA2604 is a quite nice op-amp. If you don't want to change the sound signature much, try OPA2107, it's actually the same difet technology as used in OPA627/OPA637/OPA602 op-amps, but a bit different circuit. I'm not a big fan neither of OPA2604 nor OPA2107, but you can give'em a try. Another interesting could be AD8620 or AD8066 but the latter gets unstable sometimes when used as an output buffer. Probably because it doesn't tolerate capacitive loads (cables) without appropriate circuit corrections. Actually the most dedicated ADI op-amp for audio has been till now OP275. It's not very expensive and demanding extraordinary circuit considerations.

 

[Filburt]

I doubt that the power supply is going to have much trouble providing the current. The modification needed is more in the area of bypassing or decoupling than needing more current. Unless it has regulators that are, relatively, very low output, it shouldn't be running near its limits. As you noted, it will be feeding a high impedance load and will not be outputting much current. Thus, the quiescent draw is really what's at issue, and I doubt any of the chips suggested here (even the AD9631) is going to break it.

 

[budx3385]

well, I am still using the dual Burson's - I swapped the output cap last night and enjoyed stunningly dramatic realism and MUSIC - wow. But I will try the 2x825s, and I will definitely order the THS4032s (after I verify my recollection that V+ is 14.8v here).

I am a little confused by your unusual recommendations, Filburt - the ones I checked (eg 9631, 8058) were not described by AD as useful for audio, and most had Ib's that are very high.

majkel, thanks for sharing your thoughts. I like the 8066 a lot, and I will get around to the 825s. I certainly agree on PS upgrades, but I'm really a neophyte at that. I can't tell which cap's and regulators to swap and what to use. (If you have a good schematic for an isolated 15v ps, please send it.) Compliance is the issue, I've read, rather than current capacity. I've not heard of the 275 - I will definitely check that out.

Bud

 

[Filburt]

Ib shouldn't be a problem if the bias currents are balanced, which *should* be standard procedure although perhaps it was neglected. Considering you had NE5532s in there before (which have an Ib of around 500-1000nA, typically), I'd hope it was done.

An op-amp doesn't have to have 'for audio' in the datasheet for it to be useful in that task

 

[Pars]

double post