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Old 8th June 2006, 01:36 AM   #31
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Richard Murdey
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The AD811 is a high frequency video op-amp. Your bandwidth in any case is up past 40 Mhz or so. All bets are off as to what is actually going on up there, but its safe to say that treble roll-off isnt going to be a problem.

The AD811 is also a current amplifier, so you can't characterize the bandwidth neatly as a gain-bandwidth product. Although it will never be an issue so far as brute bandwith is concerned, its interesting to note that the output characteristics (including bandwidth but also peaking, overshoot) depend on the absolute value of the feedback resistor (R2) and not just the ratio R2/Zcart. It looks like quite a large difference occures over the range 500 to 1500 ohms (Fig 11, 12). I would have thought that large=better in your case, since only audio frequencies are of interest, but you never know.

-rjm
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Old 8th June 2006, 02:14 PM   #32
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Hi rjm,
late last night I tried a R2 of ~860R, and the sound changed drastically to the bright side! At the same time, it seems quiet independent of injected noise, you hardly hear it when you touch the input with disconnected cart! I tend to agree, that the absolut value of R2 plays a major role, something I did not take into account since now.
It seems to be worth the effort to finetune R2. I seriously doubt now, that you can easily switch cartridges without changing R2 as well.
It would be interesting to see a impedance vs. frequency plot of the 103-R (as you have it for speakers).
Interestingly, the frequency measurement provided with the 103-R is made with an input-R of 47k (instead of 1k with the standard-103), that may be a hint that your mentioned impedance dependency is indeed true.
Rüdiger
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Old 26th March 2007, 07:49 PM   #33
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Default Decompensation cap values

Dear Onvinyl,

You did not specifiy the value you used for the decompensating caps you used with the AD811. What did you use?

I am determined to try the LME49710's but with rjm's concern that it may be too fast I am thinking that decompensation might be the key for the first stage.

Would the scheme you used work with the LM#49710 (Op amps are still mysteries to me, please forgive me)?

Your help is appreciated,
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Old 27th March 2007, 10:40 AM   #34
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Default Re: Decompensation cap values

Quote:
Originally posted by rickmcinnis
Dear Onvinyl,

You did not specifiy the value you used for the decompensating caps you used with the AD811. What did you use?

Are you referring to the decoupling caps?
I use Panasonic FC 100µ here, and paralleled with some foils caps around 100nF. Do you have a scope? You need to check whether parallel caps improve or worsen the op-amps behaviour and if your decoupling works as expected.

Quote:
Originally posted by rickmcinnis

I am determined to try the LME49710's but with rjm's concern that it may be too fast I am thinking that decompensation might be the key for the first stage.

Would the scheme you used work with the LM#49710 (Op amps are still mysteries to me, please forgive me)?

Your help is appreciated,
The key lies in a most optimized physical layout of the circuit. This is in my view the heart of the art of building circuits, and, sadly, I am by no means an expert in this area. I'm just trying things over and over to achieve some decent performance. There is a lot of info and stuff to find on this topic here at the forum. (Tight layout, short signal path, proper grounding, current returns and the like.).

Playing and testing various decoupling schemes comes after the layout, you almost always can't save a 'faulty' layout just by popping in different caps.

After all, I did not succeed in having both the AD797 (radio) and the LT1028 (fancy noises) in the first stage, this is probably because the cartridge is in a way a part of the feedback loop -> byebye short signal path.

The AD811 is suprisingly tame in this position, ask rjm if he tried this with his boards if you use them.
I would guess that your chip might work well in the second stage. I would have tried, but I did not receive my samples.

hope that helps,
Rüdiger
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Old 27th March 2007, 02:11 PM   #35
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Default Another faux pas!

Onvinyl,

I looked at the application note and SHOULD have figured out that these were decoupling caps. That explains why I found it all so confusing. Very embarrassing!

Do you think slowing down the op amp will help? I am planning on having this box within 12 inches of the cartridge. I know that is still a long path, but I wonder if it would help?

I have four of the LME49710's on the way (in TO-99 package).

If you would like to try them I would be glad to donate two to the cause. DIGIKEY is stocking them. It is easy enought to get more if something good happens.

Send me an email of there is any interest.

Thanks for your help and advice,
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