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iPod Classic... DAC and output stage info from Vinnie of RWA - Page 10

post #136 of 222
9/11/07 at 9:41pm
Quote:
Originally Posted by Vinnie R. View Post
Hi manaox2,

I need to get my hands on a 160GB drive and try it in my 5.5G iMod.

I read a pretty cool idea here (I forgot who posted this) that one can probably buy a 160GB iPod Classic for just a little more than buying the 160GB drive by itself. So just trade the 80 and 160GB drives between the 5.5G and the 6G. Then I guess you can sell the 80G 6G and that makes the price of the 160GB drive seem like a whole lot less!

Of course, I am only guessing that they will work. I still need to try it

I'll keep you all posted,

Vinnie
Vin Why did you get an 80GN 6G? I would have thought trying the swap would have been a no brainer. See why you need me!! LOL!!!
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post #137 of 222
9/11/07 at 9:47pm
Vinnie, why not make a breakout box that does SPDIF and/or optical?
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post #138 of 222
9/11/07 at 10:55pm
Quote:
Originally Posted by Vinnie R. View Post
The digital feeding the dac is I2S (three separate lines), so that won't do you any good. There is no SPDIF to be found anywhere in ANY of the iPods to date (believe me, I've looked very hard for it!).

This company actually mods the 5G/5.5G by grabbing the I2S before the dac, sending it out of the unit and into a dock that has a boad on it that converts the IS2 to SPDIF (optical, coax) and AES/EBU. However, its $2000 (http://www.msbtech.com/products/iLink.php)

Ouch but thanks for the reply Vinnie. Twisted Pear's DAC module takes I2S input to a Wolfson--perhaps they might be interesed in partnering with you to convert I2S to a SPDIF/Coax/Optical output as a small external unit.
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post #139 of 222
9/11/07 at 10:58pm
Vinnie do you think you could do something similar and offer it for a more reasonable price or is that reasonable?
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post #140 of 222
9/12/07 at 3:58am
This sounds like it uses something like Maxim's directdrive chips. It'd be nice to be able to get some sort of quantitative specs on this chip to see what the overall performance is. I'd say there is at least a reasonable chance the new design could have higher performance than the old design, as coupling caps introduce distortion, and electrolytics tend to introduce significant distortion. Additionally, there is room for improvement over the performance of the d/a section of the Wolfson low-power codecs. However, both this codec and the Wolfson chips are probably buffered by a CMOS op-amp, and it's likely a lot of the sound associated with each is going to come from the topology of that portion of the system. There have been some recent advances in the performance of CMOS chips, so if Apple took advantage of some of that, it could turn out well. That said, the added distortion of coupling caps may not make for worse sound; sometimes it can sound better if it ends up masking unpleasant products of the source.

I think it's worth reserving judgment and giving the 6G a fair shot before writing it off as inferior. It's interesting, and pretty neat, that Apple went this route in the 6G design. Maybe someone should do a test where they use the 6G for a while before trying the iMod again, and see what results.
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post #141 of 222
9/12/07 at 4:52am
Quote:
Originally Posted by Vinnie R. View Post
I can see the push-pull output stage as being a good thing for the headphone output (no bass rolloff issues, better drive with low-impedance headphones, etc.) but IMHO, this is not ideal for the line-out.

As I mentioned eariler, the 6G iPod Classic is a very nice solution for those who are looking to connect directly to headphones, or even for those with a headphone amp.
Vinnie
Vinnie,

Do you expect the iPod classic to sound better unamped than through a portable headphone amp?
Can the classic drive the Ety ER4S (100 Ohm) with authority from the headphone out?
Are the line-out and the headphone-out signals identical in the 6G (except for the headphone signal being volume controlled)?
Do you consider the difference in SQ between the 6G and the unmodded 5G when used with an external amp to be significant?
Thank you.
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post #142 of 222
9/12/07 at 3:24pm
Thanx Vinnie for your clarification so far. That was what I was waiting for to hear. Kind of dissapointing to hear modding proabably will be kinda like impossible as for sq. Good to hear the sq stock is not that bad.
I will jump the wagon for a 160gb soon. In this matter I care more for the extra space I can carry around with me. I waited for that a long time. Afterall it remains a portable and will never beat a decent home rig.
But you deliver a wonderful service, Vinnie and I really thought I could make use of that now... I appreciate your open comments about this matter. I hope you'll be in more luck with the nanos and touch if you consider modding the in the future!
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post #143 of 222
9/12/07 at 6:26pm
Vinnie,
If the 6G iPod Classic is driving each line out with a push-pull NPN/PNP complimentary pair operating in class AB, you could attach a current source (e.g., FET diode) at the amp output to B- and force the amp into class A mode using exclusively the (superior) NPN transistor. You could then run the Cardas line directly to the line out pin or just leave the iPod's resistor/inductor line un-bypassed. The line outs would now be class A with no need for coupling caps.
...Jim
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post #144 of 222
9/12/07 at 6:49pm
Yeah......what he said
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post #145 of 222
9/12/07 at 6:49pm
Quote:
Originally Posted by Jim Hamley View Post
Vinnie,
If the 6G iPod Classic is driving each line out with a push-pull NPN/PNP complimentary pair operating in class AB, you could attach a current source (e.g., FET diode) at the amp output to B- and force the amp into class A mode using exclusively the (superior) NPN transistor. You could then run the Cardas line directly to the line out pin or just leave the iPod's resistor/inductor line un-bypassed. The line outs would now be class A with no need for coupling caps.
...Jim
um... wow. That sounds like it would even be almost easy (compared to the old mod). Wonder what that would do for the sound.
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post #146 of 222
9/12/07 at 6:56pm
All that could be deduced from the lack of output coupling caps in the 6G, is that it has a dual-rail power supply which allows the output DC offset to be at (or near) zero. Without a schematic diagram of the internals, one cannot conclude that its output stage is push-pull, complementary, or class AB. Adding a current sink to the output may actually accomplish no benefit except to increase battery draw.
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post #147 of 222
9/12/07 at 8:36pm
Well, he did say 'if'.
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post #148 of 222
9/12/07 at 9:23pm
Thread Starter 
Quote:
Originally Posted by amb View Post
All that could be deduced from the lack of output coupling caps in the 6G, is that it has a dual-rail power supply which allows the output DC offset to be at (or near) zero. Without a schematic diagram of the internals, one cannot conclude that its output stage is push-pull, complementary, or class AB. Adding a current sink to the output may actually accomplish no benefit except to increase battery draw.
Exactly, but I do appreciate Jim's post. This needs to be investigated.

Quote:
Send me a classic and I'll try it with my vCap dock.

Im quite sure if it could or should be done Vinnie would have tried it in his V-dock. I quite sure this is not recommended.
I already did try it and it does not improve the sound. The V-caps are very transparent and don't seem to color the sound, as far as I can tell, but they don't seem to "warm things up" either.

Quote:
Originally Posted by OverlordXenu
Vinnie, why not make a breakout box that does SPDIF and/or optical?
I would need to tap the I2S, feed it out of the iPod and then make a board that converts to SPDIF to output in coax or optical (TOSLINK). This needs to be investigated as well

Quote:
Originally Posted by trose49
Vinnie do you think you could do something similar and offer it for a more reasonable price or is that reasonable?
Yes, it is possible. I just need time to research it. I have a few new products (home audio amplifiers and a preamp) coming out soon and I have been CRAZY busy these days getting ready for production of these, so time is a little limited these days but I am known to burn the midnight oil

Quote:
Originally Posted by brospin
-- Do you expect the iPod classic to sound better unamped than through a portable headphone amp?

-- Can the classic drive the Ety ER4S (100 Ohm) with authority from the headphone out?

-- Are the line-out and the headphone-out signals identical in the 6G (except for the headphone signal being volume controlled)?

--Do you consider the difference in SQ between the 6G and the unmodded 5G when used with an external amp to be significant?
1) probably depends on the headphones and your listening preferences.
2) not sure... never tested it
3) as far as I see, YES
4) not sure about this. it really depends on what amp is being used, what headphones, etc.

Quote:
Originally Posted by Contrastique
But you deliver a wonderful service, Vinnie and I really thought I could make use of that now... I appreciate your open comments about this matter. I hope you'll be in more luck with the nanos and touch if you consider modding the in the future!
Thanks, Contrastique! I'm glad this has been helpful!


Thanks for all your posts,

Vinnie
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post #149 of 222
9/12/07 at 10:12pm
Just thinking out loud here, but perhaps it's possible that output coupling caps are present on the 6G, on-chip? Normally on standalone, general purpose DAC chips, this is not done because capacitors take up a lot of die space, and they want to keep it external to give designers maximum flexibility. However, in this highly application-specific, Apple-branded chip, it may be feasible...
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post #150 of 222
9/13/07 at 1:23am
Quote:
Originally Posted by Vinnie R. View Post
Hi manaox2,

I need to get my hands on a 160GB drive and try it in my 5.5G iMod.

I read a pretty cool idea here (I forgot who posted this) that one can probably buy a 160GB iPod Classic for just a little more than buying the 160GB drive by itself. So just trade the 80 and 160GB drives between the 5.5G and the 6G. Then I guess you can sell the 80G 6G and that makes the price of the 160GB drive seem like a whole lot less!

Of course, I am only guessing that they will work. I still need to try it

I'll keep you all posted,

Vinnie
Not exactly what you were talking about but I came across this news update: 100gb hard drive for iPod 5.5 ~$300 USD)

At this point for the cost it makes sense to go with a iPod 6 and try swapping the hard drive but at least it is feasible to swap hard drives-- just need to know if the iPod 6 would work
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