Next project: Adcom GDA-700

[belgianbrain]

About 5 hours on the GDA-700, and so far the GDA-600 smokes it.

It's early. But I'm wondering if the OPA627s are better, or maybe I need to rip out that nasty digital front end.

 

[SoNic67]

You are doing it wrong. The weak links are not the filters as you think... it might be the AD797 not fitting into those spots (can be different reasons). It might be that you like better the NPC5813 digital filter versus PDM100. Hey, I like the sound of early Denon Alpha Processing (SM5848). Who knows? It might be that the 'lytics capacitors need some time.
But hey, enjoy the demolition derby

Sorry, I feel sad when people rip apart perfectly good machines...

 

[belgianbrain]

Yes, granted, it could be many things. I'll give it some more time before messing with it. I do remember being lukewarm to the GDA600 at first too....then after a couple of weeks of television watching, I switched over to audio and my jaw dropped.

We also have PCM1702 vs PCM63. And I did not replace the reconstruction filter caps with Clarity Caps like I did in the 600, but those caps are not in the direct signal path, so I figured the WIMA should be just fine.

The main difference I note right now is more space and air to the music with the 600, i.e., 3D character.

 

[heyraz]

I do remember being lukewarm to the GDA600 at first too....then after a couple of weeks of television watching, I switched over to audio and my jaw dropped.

I had the same experience. I was auditioning a GDA700 I picked up for my brother to make sure it worked before I gave it to him because the last component I gave him (untested) crapped out. Boy, I'm still catching shit from that one.
Initially, I knew the GDA700 was better than what he was using, but after a few hours of listening on my system I decided to keep it for myself and find him another. In it's stock form, I think the GDA700 is quite impressive. I didn't even want to chance getting my brother a GDA600, I waited till another GDA700 showed up.
I found another GDA700 on ebay early last week for $225 and it arrived Friday. Same experience, after a little break in, it sounds wonderful. I love it's neutrality and have no reason to want anything more.

Sorry, I feel sad when people rip apart perfectly good machines...

Unless a vintage component isn't working (at all), I've learned through experience to be conservative and give it a chance, get to know it before changing anything, even capacitors. It's so easy to destroy whatever synergy the original designer used to create the MOJO. I mean...if it didn't have MOJO to begin with, it wouldn't have been successful on the marketplace. As unfathomable as it may seem to most, updating power supply capacitors (especially changing their value) has never been a guarantee of improvement to my ears. In fact, experience has taught me quite the otherwise. I like to listen to a component first, get to really know it and establish a baseline before I consider changing anything. I've got to hear something wrong before I'll consider any kind of modification. And when I do work on a component, I work on one stage at a time. It helps maintain a baseline for reference should things go wrong.

I've destroyed enough MOJO. For me, different does not guarantee better.
That's just my 2 cents. No judgement intended either way.
Enjoy your project. It's a hobby...have some fun.

 

[SoNic67]

I am kind of in the same boat, I just don't call it mojo, I call it "informed engineering decision".
Sometimes I have to stand back and try to understand why the original designer did a certain thing like he did - was it economical, was it parts availability at that time, was it regulations... So yes, sometimes you can improve beyond what was originally there, but you need to know a little more in detail the facts.

One small example of that:
SAA7220 digital filter. Was designed for the TDA1541. At that time, the typical analog filtering for a 4x OS was 3rd order Bessel and had a corner frequency of about 30kHz (-3dB). That translated into an attenuation of -1dB at 20kHz. The digital filter response was adjusted to provide a reverse gain of +1dB at 20kHz (linearly raising from 10kHz mark) for a combined flat response. Now, a lot of people either destroy the original filters (hint, hint, removing RC components) or just design new IV stages with different characteristics (Chebyshev, Elliptic, Butterworth filters). Sometimes, to reduce the parts number, the manufacturers used a simplified Sallen and Key variant of Bessel that uses only one OpAmp instead of two) and that would roll-off more than the 1dB assumed.
That affects the final result and for some, that +1dB sounds better, for others it is not perceived at all (due to the fact that their aged ears don't work anymore past 15kHz).

Sometimes you need to give time for the brain to adjust to the new sound. The brain makes compensations when we listen long term a certain device and when we change something it will re-adjust, but not instantly. Even when going from a bad-sounding device to a better one. Like when it happened the transition from cassette tapes, FM radio or poor quality LP sets (piezo) to CD players - those people where the ones complaining that CD sounds "harsh". I was used more to LP and reel-to-reel played trough good components so the CD never sounded too "bright" for me. My brain was used to that and same songs sounded familiar.

It's all in the eye of the beholder.

 

[belgianbrain]

Interesting points.

Although, in gear made since 1990 which includes virtually all DACS, I would suggest cost is often the reason why certain components were chosen. Why isn't the PS using bigger caps or more expensive caps? Cost, pure and simple. It is always a game of diminishing returns, but where do you want to stop? Adding some capacitance to a PS when reworking a device cost almost nothing. That is my view.

I never believed CDs sounded harsh either. Got my first in the 80s as a kid and thought it was an improvement to my ears...

Now these loudness wars, YUK. I listened to a Luke Bryan CD this weekend and had to shut it off, it sounded so bad.

 

[SoNic67]

Yeah, that messed up big time the music. The kids today don't even know how music is supposed to sound, it's just a loud noise all the time... Details, imagining, separation of instruments? Not a clue!

Example: I have the CD RHCP's Californication (Warner Bross, Germany). Well, it always sounded fatiguing to me. DR on the flac files that I made from it - very small, compressed (Foobar add-on). Recently, I found somewhere a source of flac files that supposedly is created from the pre-mastering material. Higher DR, it sounds so much better! Is that a "legal" copy for me? I consider it is since I paid for a "broken" CD once, this is like a software "patch"

PS: Now that CD's on Amazon costs $5.99, but the mp3's are $9.50. Isn't that geared towards lazy people (that cannot make their own mp3's)?

 

[belgianbrain]

I still buy all my music on CD, simply because most online sales of music files are selling compressed audio. If you could download the FLAC, I'd stop buying CDs.

But, wow, I cannot stop harping on how awful "Luke Bryan - Kill the Lights" sounded on my home stereo. It is a muddled mess with no dynamic range, no space, no nothing. It sounded better on my car radio. Definitely one of the WORST recordings I have heard. Ever.

 

[heyraz]

Why isn't the PS using bigger caps or more expensive caps? Cost, pure and simple. It is always a game of diminishing returns, but where do you want to stop? Adding some capacitance to a PS when reworking a device cost almost nothing. That is my view.

Nope. It's not simply a matter of cost.
Too much capacitance after rectification makes the rectifier work harder, which in turn makes the mains transformer work harder, which produces more heat.
Too much capacitance after a tube rectifier is dangerous for the rectifier should a rapid on-off-on cycle occur.
Power supply component values aren't "pulled out of the air". Power supplies also have impedance to consider. Improperly designed power supplies can "ring".

 

[belgianbrain]

Interesting points. I might take issue with #1. More capacitance after rectification might make the rectifier and transformer work harder for an initial instant while the caps charge up, but assuming the DAC uses the same amount of power, I'd think there would be no difference thereafter.

On another topic, AD797 datasheet:

http://www.analog.com/media/en/technical-documentation/data-sheets/AD797.pdf

The GDA-700 does use some different circuit component values than the ideal ones recommended by the data sheet. There is one thing that stands out: at the top of page 14 it states that a 100 Ohm series resistor is REQUIRED if Cl is greater than 33pF. Such series resistor does not exist on the GDA-700, but could be easily added. Any comments on this?

 

[heyraz]

Google "power supply design too much capacitance".
Regarding your series resistor question...I'm not knowledgeable enough to comment.

 

[SoNic67]

Without a scope to really see if anything bad happens there, you could just try it. But then, you are adding noise to the level of 2.1 nV/√Hz. Hmm.... that's going back to LME47910 (single version of LM4562/LME47920) with 2.5 nV/√Hz and no modes needed...

Also look at fig 36. Look at your bypass caps leads. See something wrong? I personally never go for those hulky caps for bypass because I know the leads will end up too long (5mm). Ceramics and don't look back. They are good for other places like in || with the coupling/output caps.
Oh, I was spoiled to the school of tantalum caps when I started my hobby - basically I had a small source of free tantalums. All my audio projects used them in audio path, in decoupling...
http://www.analog.com/media/en/training-seminars/tutorials/MT-101.pdf

As for "too much filtration capacitance" I say only this: Use faster rectifiers - Schottky. Eventually use small capacitors (47nF) across diodes.
The rest about transformer working too hard... is just bull.

 

[belgianbrain]

Interesting stuff, Sonic. Thanks.

Yes, my leads probably are a bit longer than 5mm. The original bypass cap leads were probably at least 5mm too! They were tiny caps, tantalum I assume, but the holes are spaced quite far apart in the PCB for the size of the original caps, so the leads aren't short, and then there was a bit of a PCB trace after that to arrive at the pin. Did the best I could, but you're right, the MKP are large caps for 0.1uF.

Actually, the original design was odd, I think. It had bypass for the I/V opamps with no power reservoir caps and it had power reservoir caps for the remaining 4 opamps, but they had no bypass caps!

Listened some more tonight. This thing might be improving. I'll give it several more days.

 

[belgianbrain]

GDA-700 is improving, but so far separation of instruments is much better on the GDA-600.

I'm beginning to think either the OPA627 I used in the 600 are better than the AD797, or maybe the 600 just sounds better than the 700.

Since they're cheap, I'm going to order some LME49710 just for kicks to compare.

 

[heyraz]

The rest about transformer working too hard... is just bull.

I'm more of a tube guy with a little solid state experience. Too much capacitance after a valve rectifier can and probably will cause a "flash back" at the rectifier in a rapid on-off-on power situation.

That being said...this is one of the things I read about power supply design and too much capacitance.

There is no one-size fits all answer. But large capacitors can affect the stability of op-amps or switching regulators. And they can give rise to large inrush currents when power is first connected to a circuit. Even if the inrush can be accommodated, it may cause the power source Voltage to droop, and if other circuitry is attached to that power source, it may reset or malfunction due to the droop. Unfortunately, the only real answer to this question is, "You have to know what you are doing."

My statement about the mains working too hard was derived from the voltage droop (implying increasing current) statement quoted above.
Also, even in a linear power supply, a capacitor that's oversized relative to the rectifier (diode) can lead to failure of the rectifier via increased peak [charging] current.
Regardless...I think the practice of increasing capacitance for it's own sake is haphazard and can likely do more harm than good.

Just my 2 cents.

 

[SoNic67]

That's true. You can damage the bridge with too high of charging current - that's why I recommended replacing the diodes with faster ones, Schottky especially. They have higher I2t ratings and being faster they work better, lower ripple, with low loads drawn from big capacitors. It's a long story why...

 

[belgianbrain]

I posted over at DIYAUDIO - chip amplifiers, included a schematic of the GDA-700, and asked about AD797 suitability. There are some very knowledgeable dudes over there.

The consensus seems to be that the topology and circuit values of the GDA-700 just aren't suitable for the AD797. So, the search for something different begins.

They seem to be suggesting that FET based input devioces are best in this circuit, so looks like I may be springing for the OPA627 after all.

 

[SoNic67]

LME47910 man, I told you I have tried a bunch of OpAmps and this was the best "compromise". But sure, you can also splurge for the OPA627.

The other forum: There are some serious guys there, that know what they are talking about, but there are some morons too. I was active over there but some moderator got me upset and I stopped going there. I think ( I forgot details) that I defended some other guy "freedom of speech" in front of a moderator and I got hit with "Posts concerning moderator's directives or decisions"... Like I said - morons. Who cares?

 

[belgianbrain]

I learned that the original Adcom opamps are actually AD711, which are FET based. So, it sounds like I'm best off buying FET based replacements because of the high impedance of the feedback networks.

The sounds of these Adcom DACS when properly massaged is so mind blowing to me that it's worth the extra $100. I'll still only have $600 in this thing, and it beats DACS I've heard at $3,000 if it matches the 600 I have.

Yes, there are some weird forums out there. I used to frequent another called digitalhome.ca, which apparently is heavily sponsored by the big telco companies. Try criticizing those big telco companies in the forums and see how quickly you get cited for non-compliance with the rules by the moderators. I tried to argue my position and got immediately banned!

 

[SoNic67]

Another forum ruled by maniacs and ABX hoards: on HydrogenAudio you can't say that something sounds "better" (like you just said above) because you can't "prove it", so you get warnings and even bans. Now I go there just to download the latest add ons for the Foobar.