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Old 15th April 2006, 09:44 PM   #11
kevinkr is offline kevinkr  United States
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Hi Rudiger,
R10 reduces the loopgain to 2 which is why it damps the resonance, if this is a concern a very small amount of resistance in series with the integrator cap(s) will perform the same function and preserve the dc gain required for good servo operation. Squashing down that peak you see by using R10 is actually killing the dc response required for good servo action. Your FFT does not go down to dc, but essentially at 0 Hz an integrator provides gain equivalent to the open loop gain at dc. (Say > 100dB depending on op amp type)

A properly functioning servo loop is going to have what looks like a first or second order step in the response at the very bottom end near dc - this is actually necessary in order to have the high dc gain required for the servo, the integrator topology limits gain above dc as a function of the rc time constant. If you are having audible effects from the servo this means the overall time constant is too short. I think a tc upward of >1 sec is acceptable here.

The integrator time constant should be long enough to keep you out of trouble in the warp frequency range which means it should probably be 1Hz or less.

I understood the purpose of the additional low pass section on the output of the integrator, in this situation I don't believe you really need it, but it can't hurt either. What you want is a low noise op-amp in this location.

An OPA177 might be an ideal device for the servo integrator.
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Old 15th April 2006, 11:27 PM   #12
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Quote:
Originally posted by kevinkr
Hi Rudiger,
R10 reduces the loopgain to 2 which is why it damps the resonance, if this is a concern a very small amount of resistance in series with the integrator cap(s) will perform the same function and preserve the dc gain required for good servo operation. Squashing down that peak you see by using R10 is actually killing the dc response required for good servo action.
You and reality were right...
With R10 present, offset adjust didn't work, I did not think far enough...

But what was the stuff with the mechanical damping?
Rüdiger
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Old 19th April 2006, 02:06 AM   #13
rjm is offline rjm  Japan
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First, big thank you to onvinyl for going the extra mile to check out so many different ideas. Even the frozen cartridge. Wow... somebody actually went and did that! I hope we can now lay that myth to rest.

If the AD797 works for you, that's great. My only concern would be that the opamp is adequately bypassed at high frequencies. As it is a lot faster than the OP27 its probably going to need small ceramics in addition the electrolytics. See the AD797 datasheet for examples. You can also try decompensating it, again as per the datasheet.

The rest - superregs, servo and class A biasing - are all standard upgrades, they can be added on at will since the basic circuit isn't modified. If the sonic benifit is worth the expense and/or complexity, mod on!

Quote:
but the system would almost certainly be highly mechanically overdamped as the coils would be looking into a short circuit
Of the various points Kevinkr raised this is the only one of interest, as electrically the input stage is - or more correctly can be thought of as - an inverting voltage amplifier with the cartridge source impedance as the inverting resistor. It is not - or more correctly should not be analyzed as - an I/V converter. Refer to the Phonoclone homepage for more explanation on this point.

For the record the noise of the RIAA components as configured is too small to contribute anything to the total output noise. All the output noise comes essentially from the voltage noise of the first stage op-amp.

Mechanically, though, I admit that the cartridge is performing under rather different conditions than is usual. Its not a short circuit per se, as the internal impedance of the cartridge coils still act as a load. On average, I suppose its about 3 times smaller than the typical recommended load. (For the DL103, 40 ohms rather than 100+40=140 ohms) Note that in the case of the Denon, 100 ohms is only a minimum recommended value below which the rated voltage output is not obtained, not an absolute dictum from on high.

I have found that reducing the cartridge load used with the DL103 from 47k to 1k to 470 ohms, 100 ohms, and finally zero ohms (phonoclone), generally tended to fill in the bass and soften the treble. So yes, it could be that an ultrasonic peak is increasingly damped. That's complete speculation on my part, though, and to be fair the frequency reponse data that came with the cartridge showed no indication whatsoever of any rise up to 20kHz with a 1kOhm load.

The input configuration of the phonoclone is unconventional, but by removing the voltage divider implicit in the use of a load resistor it does fufill the claim made by the designer of "amplifying all the cartridge output without loss", while offering the lowest possible noise.

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Old 19th April 2006, 07:33 AM   #14
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Quote:
Originally posted by rjm
First, big thank you to onvinyl for going the extra mile to check out so many different ideas. Even the frozen cartridge. Wow... somebody actually went and did that! I hope we can now lay that myth to rest.
Yes, probably, but of course there *might* be other reasons why I did not found it superior after some time, e.g. degeneration of the frozen cart while fiddling with so many possibilities (but it measures still 39Ohms dc-wise, the onyl thing I can actually check), or that mod has only sense with low Z carts (but guess, I will not crack any Lyra or dynavector carts to try )

Quote:
If the AD797 works for you, that's great. My only concern would be that the opamp is adequately bypassed at high frequencies. As it is a lot faster than the OP27 its probably going to need small ceramics in addition the electrolytics. See the AD797 datasheet for examples. You can also try decompensating it, again as per the datasheet.
Yep, I use small additional foilcaps, the sound improves then (with the LT1028, I absolutley needed them, they oscillated without). I hope to get a small working function generation to crosscheck if i have any ringing before I put it all in a nice box...
However, AD811/797 *is* a great match, e.g. all the issues you posted some time ago concerning the little harshness in the heights are completly gone. We have resolution instead

Quote:
The rest - superregs, servo and class A biasing - are all standard upgrades, they can be added on at will since the basic circuit isn't modified. If the sonic benifit is worth the expense and/or complexity, mod on!
Well, beware! Things are not that straight. It all depends on the systems balance. The AD811, for instance, has a comparably poor PSSR-rejection (about 60dB, if I recall correctly). When I first tried it, I still had the batteries, and the sound was not really better as a whole, but mainly different. The big difference emerged when I attached the superreg.

Quote:
It is not - or more correctly should not be analyzed as - an I/V converter. Refer to the Phonoclone homepage for more explanation on this point.
I thought it's mainly that cicken-and-egg issue wether to think of voltage or current but maybe I should check again...
Quote:
For the record the noise of the RIAA components as configured is too small to contribute anything to the total output noise. All the output noise comes essentially from the voltage noise of the first stage op-amp.
I read that you originally had lower resistors in the network and changed it in favour to a lighter load to the first stage. Did you observe any changes in sound back then?

The damping issue: which other carts did you try with the clone? I'm going to upgrade soon and will probably try a dynavector (low Z), a Lyra, a Shelter and a Van den Hul

Thanks again, rjm, for digging me into the grooves of the phonoclone!
Rüdiger
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Old 20th April 2006, 01:19 AM   #15
rjm is offline rjm  Japan
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Quote:
I thought it's mainly that chicken-and-egg issue wether to think of voltage or current but maybe I should check again...
No, that's basically it. An input voltage in series with the source impedance is a better model overall, though, since the source impedance is so low.

To answer your other questions, no, I haven't used any other cartridge that the DL-103, and no I didn't really notice any great change when I changed the RIAA values to higher impedances, though at the time I though it was a slight improvement. I did it mainly to get a more accurate RIAA response, keeping as many of the parts as possible in standard values.


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Old 20th April 2006, 05:04 AM   #16
rjm is offline rjm  Japan
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P.S. on the schematic you posted, are C4 and C5 shown connected correctly? Looks pretty wierd...
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Old 24th April 2006, 04:08 PM   #17
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Some more tests...
Quote:
Originally posted by rjm
P.S. on the schematic you posted, are C4 and C5 shown connected correctly? Looks pretty wierd...
Look Analog's
Application Note 202
Figure 3c, p.3

And yes, the noise of the second stage is so much lower it probably won't make a difference at all.

Regarding the servo: To add the 2nd pole did not make things better in the longer run, the sound got more unstable (don't know how to say it better). But I raised R9 to 1000k, which made the lower regions even more transparent.

Meanwhile I checked the AD844 in the first stage. I couldn't make it work together with the AD797, so I switched back to the OP27 for testing. It's not better than the AD811, even when I tried the non-feedback-tweak with R1 omitted and pulled the TZ-node to ground via a resistor.
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Old 17th May 2006, 08:52 AM   #18
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Nice effort, can you update your schematic now?
Giorgio
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Old 20th May 2006, 09:57 AM   #19
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Giorgio, nothing has changed from the first shown schematic except for R9 that is raised from 100k to 1Meg,
cheerio,
Rüdiger
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Old 26th May 2006, 06:55 AM   #20
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have you made a pcb?
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