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Old 9th March 2004, 08:09 PM   #11
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Default 5534 better than told

...I found the data sheets of NE5534.
The current noise is stated around 0.6pA/Hz^0.5!
Also the low frequency current noise is fine with 2.6pA/Hz^0.5!
Much better than I thought.
OK, there seems to be no need to reduce the values of the
resistors. Current noise seems uncritical.
Sorry for causing confusion.

Cheers
Markus
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Old 10th March 2004, 02:49 AM   #12
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Default Eq config.

Hi JCX,

I wrote out a post yesterday about the eq but something happened and the post was deleted even before putting it up. Got quite mad about it and so decided to do rewrite later.

The input stage has the 75uS eq so that the HF overload capability of the second stage is good at HF. If we used the 3180/318 uS eq here it would not have as good an overload figure as compared to the 75uS at HF.

Having written that I think this should not be an issue because the first stage is the one that really determines overload ! That has been taken care of already.

Now we come to the S/N ratio. The max S/N ratio is determined only by the input stage. Subsequent stages will only degrade it (and of course improve a bit due to the eq filters !).
With a relatively high signal at the input of the second stage (75uS eq ) the S/N ratio will be better as compared to the signal with 3180/318uS which would be 20db lower in level.
With opamps like the AD829 with a noise voltage of 2nVrtHz this may not be an issue.

The second stage could possibly have the 3180/318 eq in its feedback loop. The HF will not roll of till unity gain as it will be in the case of the 'all at one go' feedback network.
This would however mean a frequency dependent feedback network which we are trying to avoid.
I am currently working on a board with active/passive eq switchable. That way I can see if there is really an audible difference (with modern opamps).
The other option that I am also working on is an 'all passive at one go' filter after the first stage . I need to work out the details and determine if it will load the first stage. Capacitors will have to be as large as possible to keep impedance down for the next stage and to overcome stray caps.

Has anyone already done this?

This has to be done. Like the difference in sound between the NE5532 and OPA2134 is really heard only by practical implementation, I guess we have to build the circuits to determine their sonic capabilities.

Another doubt. Did the NE5532 sound different from the AD826 or OPA2134 because of the large difference in the input bias current flowing through the cartridge coils ? This must be causing a dc offset in the operating point of the cartridge. The magnetic flux will have an offset operating point.
I have no coupling capacitor at the input. Where ever possible I try to tolerate dc offsets and avoid capacitor coupling.

In another phono stage with 0 volts dc at the input a film type coupling cap very audibly degraded the sound. After that I try to avoid use of such caps as far as possible - and we aren't even talking about electrolytic caps !

Cheers.
Ashok.
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Old 10th March 2004, 06:00 AM   #13
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Quote:
Another doubt. Did the NE5532 sound different from the AD826 or OPA2134 because of the large difference in the input bias current flowing through the cartridge coils ?
No. The input bias current effect on the cart is negligible. As i don't use MM carts there is always an intermidiate stage before the first opamp and they still sound different.
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Old 10th March 2004, 07:54 AM   #14
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Default Some more opamps.

I tried the LM833 and the LF353 yesterday.

The LM833 sounds surprisingly good. Closer to the OPA2134 than the NE5532.
The LF353 sounded light in the bass . It took away the foot tapping quality from the OPA2134. This is more prominent in the AD826. I am waiting for my AD829's.

In order of preferance ( in the RIAA amp) I like the op amps as follows :
AD826
OPA2134
NE5532/LM833
LF353

Wonder if I should check out the TLO72 and TLO82 ??
Cheers.
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Old 10th March 2004, 08:02 PM   #15
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Default AD829

Hi Ashok!
...checked through the data sheet of AD829...
This fancy OP amp might become one of my best friends
when I need much gain for small signals!!!
THX for the hint!

Bye
Markus
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Old 11th March 2004, 07:43 AM   #16
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Default Other configurations.

I have been trying some other configurations ( with R and C in the -ve feedback loop) and find that the small resistor in the -ve input of the opamp ( going to ground or through a capacitor to ground ) affects the accuracy of the equalisation quite a bit. Keeping the eq passive ensures that the number of components that affect the equalisation is kept to a minimum.

Any thoughts on that ?
Cheers.
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Old 11th March 2004, 10:35 AM   #17
JonPike is offline JonPike  United States
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Quote:
Originally posted by analog_sa
Hi Ashok

Apparently there are other opinions as well. If you look at the Hagerman Buggle you may notice the gain structure is x11,x11,x10 and the low-frequency correction comes first.

No idea if this sounds better or not but it's certainly easy to test. What worries me though is that in your curcuit the mid opamp is only there to allow a split RIAA. The question is then if splitting warrants the inclusion of a dummy stage.

regards
I have a Bugle, sounds pretty good. There was a lower end/ kit preamp shootout in AudioXpress mag some months ago, and the Bugle was the best of the bunch.. including that NAD and others that cost up to $250. Some did have some overload while testing on a test record, the Bugle didn't if I recall correctly..

Quote:
In order of preferance ( in the RIAA amp) I like the op amps as follows :
AD826
OPA2134
NE5532/LM833
LF353

Wonder if I should check out the TLO72 and TLO82 ??
Cheers.
Interesting... I was looking over op amps, and came across the AD826, got some samples and tried it.. (instead of the original OPA2134's) We decided there was a definite improvement..

I'd guess the TLO's would be a step down.. but there's one way to be sure..
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Old 11th March 2004, 12:03 PM   #18
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Default AD826

Never tried this one. From OPA627/637, AD8610, AD825, AD845, AD8065 my uncontested favourite is 8610. I use it with a BUF634 and NFB RIAA and there is almost zero DC at output.

Initially i didn't like the sound all that much but with a Lundahl 9206 in front it became much better than i could ever hope for op-amps.
As soon as i get some time i'll rebuild it as an all-out teflon board/bells and whistles phono wonder
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