Is the SAA7220P/B really that bad ? - diyAudio
Go Back   Home > Forums > Source & Line > Digital Line Level

Digital Line Level DACs, Digital Crossovers, Equalizers, etc.

Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Reply
 
Thread Tools Search this Thread
Old 7th April 2007, 09:04 PM   #1
diyAudio Member
 
Join Date: Mar 2004
Location: UK
Default Is the SAA7220P/B really that bad ?

Hi,

Well I'm hoping that the title of this thread will stir up some debate !

I am expecting a lot of 'yes it is, it's a bad chip, rip it out and go NOS' but I'd like to understand the reasons behind people thinking NOS is better.

There will be some, I'm sure, who think that interpolation is just wrong and with any oversampling filter you're not listening to the real music. There will be others, who have tried it and prefer the results because it sounds better to them; whether the sound is more 'realistic' or not.

What I am wondering is, why this is? Is it, not because the digital filter is a bad chip, but, actually because it creates a lot of pollution on the power rails which affects the DAC; with the data clocked at 4fs instead of 1fs timing is more critical (jitter) and since everything is running faster more attention needs to be paid to design of the power supply, decoupling, layout, grounding and so on. If all these things are taken care of can the combination of SAA7220P/B + TDA1541A work well ? Is it really that the implementation of the SAA7220 in most players leaves something to be desired ?

Cards on the table. I have an Arcam Alpha 5 (SAA7220P/B + TDA1541A) and I have socketed both chips so I could easily go NOS and I will and compare the result but if I do like it better I'd like to know why, certainly reproducing a 20KHz sinewave with a 44.1KHz sample rate isn't going to sound good !

So, let the comments begin ...

Regards,

Jon
  Reply With Quote
Old 8th April 2007, 08:21 AM   #2
limono is offline limono  United States
diyAudio Member
 
Join Date: Mar 2007
I just yanked out SAA7220 and replaced it with Net-Audio NOS board .Well, it did improve the sound considerably (there were other changes in the player as well ) but judging from the effect of the change it just makes not much sense to me doing clock upgrade if you keep SAA7220 in place. I don't care for NOS or no NOS but clean and properly implemented clock gotta make a difference and what I gather there is nothing you can do to improve on SAA7220. The board isn't cheap at 40 GBP but if I was to make only single mod on my player it would be replacing the filter chip with it. Regards, L
  Reply With Quote
Old 8th April 2007, 10:11 AM   #3
diyAudio Member
 
Join Date: Jul 2003
Location: Bath, UK
Good idea for a thread!
Quote:
Is it, not because the digital filter is a bad chip, but, actually because it creates a lot of pollution on the power rails which affects the DAC; with the data clocked at 4fs instead of 1fs timing is more critical (jitter) and since everything is running faster more attention needs to be paid to design of the power supply, decoupling, layout, grounding and so on.
You've nailed it. The SAA7220 is a noisy beast, and draws quite high currents (c180-200mA). The problem with that is simply the size of the package - you have the digital filter and the clock generator and clock divider all sharing the same power supply pins, which are so far apart that true HF decoupling is difficult to do well (opposite corners of a large DIL package). That gurantees a mess on the jitter front. So there's a two-part fix:

1) Clean-up the PSU, develop decoupling strategies to minimise noise on the supply pin.
2) Simply stop putting the clock signal through the filter. Use a separate 11.288Mhz clock with D-flipflop divider* to feed 5.66Mhz to the 1541, and 11.288Mhz to the 7220 clock input and to the preceeding 7310 decoder. That is - take all responsibility for clock generation and distribution away from the 7220. Problem solved!

Quote:
If all these things are taken care of can the combination of SAA7220P/B + TDA1541A work well ? Is it really that the implementation of the SAA7220 in most players leaves something to be desired ?
Definitely yes to both parts, for the reasons above. Now - is it the best you can do with a 1541? maybe not, but certainly I don't feel I'm missing anything from my current player with this setup.

Do try both methods with you Arcam 5 and see what works for you. I think that the reason NOS remains popular is because it's much easier than 'sorting' the SAA7220 (which is incredibly context sensitive and requires a scope etc). Having tried it both ways, I know that the NOS sound definitely isn't for me.
  Reply With Quote
Old 8th April 2007, 10:33 AM   #4
diyAudio Member
 
Join Date: Jul 2003
Location: Bath, UK
One more little thing - for chips designed to work together - they don't interface very well without a bit of work! Find the long 'TDA1541 thread in this forum of more details, but the key tweak is to use RC decoupling in the signal lines between these two chips to reduce HF noise injection into the dacs substrate (1K/10pF was suggested)*. Your Arcam already has some resistors in place in these lines, so definitely add small caps (10-22pF) to digital ground at the 1541 input pins. It helps a surprising amount.

*this is because the 7220 swings 5v pk-pk on its outputs, and the 1541 only needs a tiny current centred c1.4v to switch - it's a current-routing logic; the excess voltage swing just pushes HF currents into the dac.
  Reply With Quote
Old 8th April 2007, 10:35 AM   #5
ash_dac is offline ash_dac  United Kingdom
diyAudio Member
 
Join Date: Jan 2005
Location: UK
Does anyone know the stopband attenuation of the saa7220 ?

I've assumed it's around saa7323 performance! http://docethifi.com/saa7322-7323.pdf
  Reply With Quote
Old 8th April 2007, 05:37 PM   #6
diyAudio Member
 
Join Date: Mar 2004
Location: UK
Martin,

I like the idea of just using the SAA7220 as a filter and removing the other functions. A seperate clock 'factory' with buffered feeds to the A and B chips and the DAC sounds like a very good idea.

RC in the digital lines is certainly a good idea. You could certainly get away with slowing those edges down a bit, looking at the datasheets I don't think you'd want to go over 22pF though. Reclocking the signal lines should help as well but don't choose logic that is too fast !

Ashley,

I don't know what the attenuation is. The datasheet doesn't go into details about the type of filter; beyond stating that it is a 120 tap FIR no other details are given. I'd guess that with that number of taps it might be anywhere from say 20 to 80dB depending upon the type of filter.

All,

The grounding scheme on the Alpha 5 output board is really nothing to write home about. Doing another board layout would be the best option but I'm not doing that. I did think about fitting a sheet of self adhesive copper underneath the pcb (adhesive backing still in place) and connecting down to that.


Keep the feedback coming !

Jon
  Reply With Quote
Old 9th April 2007, 04:32 AM   #7
Fin is offline Fin
diyAudio Member
 
Fin's Avatar
 
Join Date: Dec 2003
Location: -
Hi Jon and Martin

Have a look at the audioboard in the Alpha 5+. The clock looks a bit better than in the Alpha 5. It is generated outside the SAA7220 and all lines going to the DAC are reclocked. The only downside is that all of the reclocking is done in one quad filp-folp.

A similar approach was taken in the Delta 70.2 but only the bitclock was reclocked in a dual flip-flop (along with the signal for the digital output - which can easily be disconnected to optimise the reclocking of the bitclock).

The ideal situation is obviously to have a clock distribution with 5 outputs (one at half fequency). You could then directly clock the dac with the 1/2 clock, reclock WS and DATA using individual flip-flops and separately clock the SAA7220 filter and SAA7210/7310 decoder.

The closest ready made solution I have found is a Tentlabs XO 2.5. This provides three normal outputs and one 1/2 frequency output. The 1/2 frequency output would no doubt be used to directly clock the dac chip. This then poses the question of which is most improtant to clock/reclock with the remaining three clean clock outputs. Is it better to:

A. Reclock WS and DATA using separate filp-flops and feed the final output to the SAA7220......and let it then feed the decoder.

B. Separately clock the SAA7220 and decoder.....and use the final output to reclock WS or DATA (or both in a a shared package).

C. Separately clock the SAA7220 and decoder.....and not use the final output.
__________________
Regards
Fin
  Reply With Quote
Old 11th April 2007, 09:42 AM   #8
diyAudio Member
 
Join Date: Oct 2004
Hi All,

A good thread indeed.
My only experience is with NOS (I chose it because, at such low frequencies, it should be easier to do as a first DAC), and I want to try oversampling because I want to be able to compare.

For me, I think the biggest problem is finding an SAA7220. Are there any obvious, old donors out there, expecially with a DIL version of the chip?

But, I did always wonder, is the 7220 frowned upon because of its typical electrical environment, or is it because there are perhaps better digital filters out there? For instance, how would a 7220 fare against an AD1896?

Cheers,
Phil
  Reply With Quote
Old 11th April 2007, 10:16 AM   #9
poynton is offline poynton  United Kingdom
diyAudio Member
 
poynton's Avatar
 
Join Date: Mar 2005
Location: A life on the ocean waves when I'm not at home in N. Wales (but I'm not Welsh so no sheep jokes!)
Has anyone thought about producing a daughter board to combine an external clock, divider, SAA7220 etc ?

Andy
__________________
If it ain't broke, break it !! Then fix it again. It's called DIY !
  Reply With Quote
Old 11th April 2007, 10:36 AM   #10
diyAudio Member
 
Join Date: Oct 2001
Location: .
Quote:
Originally posted by philpoole
For instance, how would a 7220 fare against an AD1896?

Cheers,
Phil
It would be an invalid comparison.
  Reply With Quote

Reply


Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
SAA7220P/A vs. SAA7220P/B P-47 Digital Source 14 28th April 2010 11:54 PM
Wanted: SAA7220P/B philpoole Swap Meet 5 31st August 2007 01:05 PM
Wtb: Saa7220p davej Swap Meet 0 9th January 2006 11:42 PM


New To Site? Need Help?

All times are GMT. The time now is 02:03 AM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright 1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2