current amplifying phono stage

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Hi all,
Some time ago, there was quite an interesting debate on the asylum concerning current sensing phono stages (for MC, that is).

http://db.audioasylum.com/cgi/m.pl?forum=tweaks&n=72458

It is mainly an I/V Converter with some (remarkable) tweaks.

I understand that some of the protagonists posting here as well.

How might such a design rate against some highly regarded actual projects around here?

I understood, that the current sensing part is discussed in papers like that (Fig. 2(b)):
http://focus.ti.com/lit/an/sboa034/sboa034.pdf

Together with the hints Jon Risch gave in the asylum it should'nt be too hard to track things down

I happen to have two trashed DL103's with working coils, so a balanced version with frozen assembly would be doable.

I would be glad to get some respond,

Rüdiger
 
Hi,

unfortunatley, I did not find any information on the dynavector patent.

I made a quick sketch from jon's announcements, what do you think, might it work?

jonrisch.jpg



Rüdiger
 
OK, since you couldn't find it for yourself.

www.uspto.gov the patent is # 4,644,517

I'll leave it to you to print out.


The circuit you show has too many moving parts. All that
stuff connected to the "+" inputs of the OPA2604's is
probably unnecessary.

Also, to do the frozen coil thing, it looks like you'll
need at least two dead cartridges. I believe tat Risch
said you could get by with just having one of them
in one "side" of the circuit, but maybe more is merrier.
 
Hi,
www.uspto.gov the patent is # 4,644,517
I'll leave it to you to print out.
:blush:
OK, I did not get that the searching machines covered different timeframes... Thanks

The circuit you show has too many moving parts. All that
stuff connected to the "+" inputs of the OPA2604's is
probably unnecessary.

True if offset is no problem.
Also, to do the frozen coil thing, it looks like you'll
need at least two dead cartridges.

Yes, I have two...

Hm, I'm wondering a bit about the low interest this thread provokes. Do you think, that the current sensing approach is crap? Old stuff not worth trying? Anything Else?
Rüdiger
 
Hm, I'm wondering a bit about the low interest this thread provokes. Do you think, that the current sensing approach is crap? Old stuff not worth trying? Anything Else?

I personally think it's great. My only doubts involve the input opamps. IME opamps do not seem to provide the perfect interface for MC carts. No idea why, but transformers or discretes seem to work better at these low levels. Not to mention that good sounding regulators for MC amps seem to exclusively consist of batteries :). Still, the current approach may be different. As this is quite a simple project it's definitely worth building.

As a matter of interest what do you use at present?
 
Hi,
Recently, I finished (?) my version of the la bohème stage
www.klaus-boening.de

It sounds great, and it is more the success that encourages me to try something else/further than an actual discontent.

I'm open minded to tranformer-based solutions, discrete, valve and so on, but Klaus Böning, or Blue Amp or Horch are proving that one can achieve great results with op-amp based/involved solutions as well.

Rüdiger
 
Disabled Account
Joined 2004
Some guys at MIT 20 years ago were convinced that this was the final answer in terms of vinyl reproduction. They used discreet npn's and pnp's (I can't give away their circuit even though they never went anywhere with it). They thought that you got mechanical damping on the cantiliver that greatly reduced mistracking, I have no evidence that this is true. For geometry mistracking it obviously can't be true. BTW some of what Mr. Risch said in that asylum thread is wrong, you do not need low current noise for this circuit. The TI/BB app note is about photo diode amplifiers for CAT scanners and is also not particularly relevant and could lead you astray.
 
Onvinyl:

"if offset is no problem"

OPA2604s are FET-input. You really don't need to match the
impedances to compensate for bias currents as they are
essentially zero. Also, the resistors you need are equal to
roughly half the cartridge coil's resistance which is pretty
small for the MC cartridges. You're not likely to budge any
DC offset by fiddling with pots on the +inputs.

Could the low interest in this thread be because most people
don't "get it"? I see that there are at least two audiophile/engineers
reading and commenting, both of which are involved in high-
performance semiconductors as a profession.

-----

Scott: That's "BB/TI", not "TI/BB" ! ;-)

I agree that the photodiode ap-note is not that relevant
 
Disabled Account
Joined 2004
Brian - If you want a quick try on this, take the lowest _voltage_ noise CFA you can find, use the cartridge resistance as gain resistor and an appropriate resistor as feedback resistance to get the gain. So if you have a cartridge that measures 5 Ohms say use a 500 Ohm feedback resistor. I don't think any offsets get in the way current or voltage (a few microAmps one way or the other flow in the coil). OTOH you might want to select for lowish Vos or tweek it (pot on the rails with a big divider to the +input, for instance).

BrianL said:
Onvinyl:

"if offset is no problem"

OPA2604s are FET-input. You really don't need to match the
impedances to compensate for bias currents as they are
essentially zero. Also, the resistors you need are equal to
roughly half the cartridge coil's resistance which is pretty
small for the MC cartridges. You're not likely to budge any
DC offset by fiddling with pots on the +inputs.

Could the low interest in this thread be because most people
don't "get it"? I see that there are at least two audiophile/engineers
reading and commenting, both of which are involved in high-
performance semiconductors as a profession.

-----

Scott: That's "BB/TI", not "TI/BB" ! ;-)

I agree that the photodiode ap-note is not that relevant
 
peranders said:
I think this is a rather interesting topic. What happens if you take out the current from the pickup with no voltage across it (= zero Ohms inimpedance) compared to "measuring" the voltage (=high inimpedance) and take out no current?

Theoretically not much except the inductance comes into play much earlier.

For my MC15 inductance is not given but I believe its 50uH to 100uH.

Lets assume 100uH (50uH) with specified DCR of 6R.

F3 = 9.6KHz. (19KHz)

Which ties in with recommended loading of > 20R.

F3 = > 40kHz. (80Khz)

:) sreten.
 
Ex-Moderator
Joined 2003
You really don't want to use a FET input op-amp in this application. As a broad generalisation, FET op-amps are dominated by voltage noise, whereas bipolars are dominated by current noise. When you use a moving coil cartridge as the source, its low source resistance means that the current noise hardly develops any voltage, meaning that bipolars are quieter. AD797 might be a good choice.
 
sreten said:


Theoretically not much except the inductance comes into play much earlier.

For my MC15 inductance is not given but I believe its 50uH to 100uH.

Lets assume 100uH (50uH) with specified DCR of 6R.

F3 = 9.6KHz. (19KHz)

Which ties in with recommended loading of > 20R.

F3 = > 40kHz. (80Khz)

Didn't you misunderstood me? What happens if the load is zero ohms, using a true I/V amp?
 
As said, not much happens: the record plays, the music flows ...

I tried Scott's suggestion years ago with AD811 or LM6181, using an Ortofon MC25FL. Worked rather nicely, but I abandoned the scheme because I desire more flexibility/compatibility from an MC-preamp. And anyway, I changed over to a medium-Z medium-output cartridge shortly after.
 
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