John Curl's Blowtorch preamplifier part II - Page 701 - diyAudio
Go Back   Home > Forums > Member Areas > The Lounge

The Lounge A place to talk about almost anything but politics and religion.

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 27th November 2010, 02:04 PM   #7001
diyAudio Member
 
Join Date: Sep 2006
Quote:
Originally Posted by john curl View Post
Let me just say that if we take a GENERIC inverting integrator of either RL or RC configuration, the input pulse will be pre-filtered by the passive network, BEFORE it reaches the input stage of the op amp. Therefore, you are not performing the 'miracle' that you think you are doing with your op amp design. Of course, if you only amplified the coil's output, then you would be on to something, and I would thank you for it. However, from my position, your 'real' impulse at the input of any op amp you might have chosen is 2 times slower than my TIM(30,30) pulse although my TIM(30,30) would turn back into a risetime limited square wave, if it went into YOUR inverting integrator before it reached your op amp input, which would be quite easy for you to handle.
Why don't you try to do what you are successful at doing, with a non-inverting quasi integrator, or a typical phono stage like you showed on your graph, and see what happens?
Hi John,

I certainly agree with you that most op amps are happier in many regards when operating in the inverting mode, and that sharp edges in the input signal don't get to the input differential pair as badly, due to the RC filter you describe in the inverting integrator configuration. There is also the matter of little or no high-frequency, sharp-edged common mode applied to the op amp.

This does not, however, make it impossible to get low TIM in an op amp of high quality using feedback in a non-inverting configuration.

BTW, these discussions bring back to mind the use of passive RIAA equalization that has often been preferred as rendering higher quality. In such a case, of course, the fast edges are LPF'd without stress on an amplifier. The use of a passive RC for the high-frequency turnover followed by an integrator-type low-frequency turnover is also a good approach that still keeps fast transients out of the following amplifier.

Of course, there still remains the first buffer stage, which must deal with the fast edges that may come from the cartridge. At least this stage then only has one job to do. It must be a chunk of gain with low noise that has very low distortion, even at high frequencies at fairly high amplitude, and must be very resistant to EMI from all sources. This is particularly important for the first stage in moving coil preamplifiers.

Cheers,
Bob
  Reply With Quote
Old 27th November 2010, 03:41 PM   #7002
PMA is offline PMA  Europe
diyAudio Member
 
PMA's Avatar
 
Join Date: Apr 2002
Location: Prague
Is there any reason why NOT to use the inverting topology for MC phono preamp? The low input resistance for low noise requirement would be fulfilled, as most MC cartridges enable loading like 50 - 100ohm.
__________________
Pavel Macura
http://web.telecom.cz/macura/audiopage.html
  Reply With Quote
Old 27th November 2010, 03:49 PM   #7003
diyAudio Member
 
john curl's Avatar
 
Join Date: Jul 2003
Location: berkeley ca
Of course, an INVERTING input will work. Then, how do you put the optimum resistive loading on the MC phono cartridge? I have generated whole designs around this input summing concept, but they failed to sound 'right'. This is bypassing the problem, but I found 35 years ago, that you could NOT bypass the problem this way, and still get the best sound from the majority of MC phono cartridges. If you don't believe me, try some listening tests yourself, making an A-B switch that will change the input from almost Zero ohms to 100 ohms, for example.
  Reply With Quote
Old 27th November 2010, 03:58 PM   #7004
PMA is offline PMA  Europe
diyAudio Member
 
PMA's Avatar
 
Join Date: Apr 2002
Location: Prague
Thanks for your reply, my experience in MC cartridge field is certainly much lower than yours. Of course that optimal loading would be unachievable.

Then there is a possibility of low noise buffer + inverting RIAA network. Are there disadvantages still.
__________________
Pavel Macura
http://web.telecom.cz/macura/audiopage.html
  Reply With Quote
Old 27th November 2010, 04:06 PM   #7005
diyAudio Member
 
Joshua_G's Avatar
 
Join Date: Nov 2008
Location: Small village, Israel
Optimum load should be set individually for each cartridge.
Of course, this is impossible to do with commercial products.
  Reply With Quote
Old 27th November 2010, 04:12 PM   #7006
PMA is offline PMA  Europe
diyAudio Member
 
PMA's Avatar
 
Join Date: Apr 2002
Location: Prague
This is obvious. But input buffer would enable individual load setting.

BTW, I have sold 2 commercial MC phono preamp designs in last 2 years. The optimum load is set by jumpers, from 5 res. values.
__________________
Pavel Macura
http://web.telecom.cz/macura/audiopage.html
  Reply With Quote
Old 27th November 2010, 04:27 PM   #7007
diyAudio Member
 
Joshua_G's Avatar
 
Join Date: Nov 2008
Location: Small village, Israel
Quote:
Originally Posted by PMA View Post
This is obvious. But input buffer would enable individual load setting.

BTW, I have sold 2 commercial MC phono preamp designs in last 2 years. The optimum load is set by jumpers, from 5 res. values.
Yes, this is a better option.
  Reply With Quote
Old 27th November 2010, 05:57 PM   #7008
diyAudio Member
 
Join Date: Sep 2006
Quote:
Originally Posted by PMA View Post
Is there any reason why NOT to use the inverting topology for MC phono preamp? The low input resistance for low noise requirement would be fulfilled, as most MC cartridges enable loading like 50 - 100ohm.
Hi PMA,

I think that an inverting feedback structure where the load resistor for the MC cartridge goes to the virtual ground of the inverting input of the amplifier would be risky and would provide for less flexibility in loading the cartridge and achieving desired voltage gain. I would worry about having the fairly low-Z inverting amplifier return go all the way back through the moving coil cartridge itself, especially since the impedance of that path will be relatively unknown at high frequencies, is likely inductive, and may have destabilizing resonances.

My vote is for a very good low-noise non-inverting amplifier with moderately high input impedance and whose gain is maybe 20 dB. Cartridge loading should then be independent. The amplifier should be very robust in the presence of EMI and high frequencies.

While low noise is very important, it is not the be-all and end-all for sonic quality. Noise needs to be low enough. Linearity and robustness to high frequencies and EMI should not be compromized in an unnecessary quest for the lowest achievable noise. The use of JFETs should not be ruled out for this reason.

Cheers,
Bob
  Reply With Quote
Old 27th November 2010, 06:00 PM   #7009
PMA is offline PMA  Europe
diyAudio Member
 
PMA's Avatar
 
Join Date: Apr 2002
Location: Prague
Hi Bob,

how about low-noise buffer + inverting topology, that I mentioned in the next post.

Regards,
__________________
Pavel Macura
http://web.telecom.cz/macura/audiopage.html
  Reply With Quote
Old 27th November 2010, 06:46 PM   #7010
diyAudio Member
 
john curl's Avatar
 
Join Date: Jul 2003
Location: berkeley ca
For my MC cartridge loading, I will use a remote controlled 10 turn wirewound triple pot of 5K ohms, that will switch from there. Already designed into my latest preamp. I want resolution to 1 ohm or so. The resistor value will come up on a video screen and can be stored for future setting. We shall see whether we can tell that much difference or not, especially with $5,000 and above phono cartridges, like Joachim uses.
  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



New To Site? Need Help?

All times are GMT. The time now is 11:11 PM.


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