• The move to the new server is done. There are some software and database maintenance updates in process. This has us passing the hat around to help out. We appreciate any donations. Seriously, even a dollar helps. The payment page may be found here - https://www.audiokarma.org/support.html

OpAmp Phono Preamp Compatibility

TriodeLuvr

TriodeLuvr

So many electrons, so little time!
Subscriber
I recently came across a comment about a phono preamp issue that I've never considered. The poster claimed that the input bias current of opamps can degrade the performance of MM phono cartridges. He went on to say this current can cause long term changes to the magnetic structure. Is this a real phenomenon, or just Internet litter? Do opamp-based preamps need a blocking capacitor at their input?

Jack
 
Register to hide this ad
if its got any DC appearing at the input, probably a good idea to have one. A split rail supply I'd think ought to be running at 0v to the input though.

no idea about permanant changes to the mag structure but I can fully believe coil saturation that causes poor response.
 
Here are the specs for an opamp typically used for this application. The input offset current is in Nanoamps!
1698115278355.png
 
dr*audio said:
I call "bullshit."
Click to expand...
That's what I thought, too. But then I started thinking about opamp specs for this parameter. The NE5532 is rated for 1,000nA max over the full temperature range of the standard part. That's 1mA potentially passing through the cartridge coils. The LM833 is rated for a max of 800nA over the full range, and so is the 4558 (the chart you posted only shows input bias current at 25ºC). Fet-input opamps like the OPA134 are much better, but I have never seen this characteristic noted as a particular reason to use them in this application.

I've also searched typical phono circuits to see if there might be a general consensus among commercial manufacturers regarding the need to block DC. Elliot Sound Products uses a NE5532 with no blocking cap. The Hafler PH-60 uses the LM833 and a 22uF (!) input cap. These are both split supply circuits. I'm stumped.

Jack
 
Well, 1000nA X 47K = 47mV, which is significant for a cartridge with an output of 2mV. So I guess using a coupling cap would be a good idea if you use that opamp, but it's much simpler to select the proper opamp for the application. You don't need high slew rate for a phono preamp, you need low noise and low input offset.
 
dr*audio said:
You don't need high slew rate for a phono preamp, you need low noise and low input offset.
Click to expand...
Why low input offset?

The input bias current has to come from somewhere, and if it's passing through the coils of the cartridge what harm might it do? Well, it would cause the coils to be offset from their normal rest position in the magnetic field. Enough to be significant?
Well, lets consider how much current would flow through the coils in normal operation without any bias current. Say 5mV into a 47k load. That would be about 0.1uA, or 100nA. Now 500nA is starting to look like quite a large amount of current, at least compared to the signal current.

Does it matter? Which is worse - the bias current passing through the coils or the inclusion of an "evil" DC blocking cap? :dunno:
I suppose that's a designer's judgement call.
 
TriodeLuvr said:
I recently came across a comment about a phono preamp issue that I've never considered. The poster claimed that the input bias current of opamps can degrade the performance of MM phono cartridges. He went on to say this current can cause long term changes to the magnetic structure. Is this a real phenomenon, or just Internet litter? Do opamp-based preamps need a blocking capacitor at their input?

Jack
Click to expand...

many seem to to have a blocking cap. If you are going to do an opamp phono stage, why not try the Colorado guy's discrete opamps>

Sorry can't think of the name now.
 
mcgjohn said:
If you are going to do an opamp phono stage, why not try the Colorado guy's discrete opamps>
Click to expand...
That wouldn't address the question of whether or not DC blocking is required.
Personally, I'd question the point of discrete op-amps anyway. If you want discrete circuitry, why not just use discrete circuitry instead of dressing it up as an op-amp?
 
JP said:
1000nA is 0.001mA.
Click to expand...
Thank you.
dr*audio said:
Well, 1000nA X 47K = 47mV, which is significant for a cartridge with an output of 2mV.
Click to expand...
The resistance is closer to 800Ω, which is the approximate DCR of the cartridge. I don't think standing DC current can be correlated with signal levels, because the AC impedance of the cartridge is much higher.

I searched for the thread that mentioned this problem, but can't find it now. There are others, although most are just a one-sentence remark referring to bias current in the coil being undesirable. Nothing that quantifies the issue.
 
TriodeLuvr said:
I don't think standing DC current can be correlated with signal levels, because the AC impedance of the cartridge is much higher.
Click to expand...
I'm not sure I understand what you're saying. If a cartridge is developing a signal voltage across its terminals and the cartridge has a load on it, current will flow. In the case of an MM cartridge with zero source impedance (yes, a practical impossibility, I know) the current would be the signal voltage divided by 47k. In the case of an MM cartridge with source impedance Z, the current would be the signal voltage divided by 47k + Z.
Z will reduce the flow of current, making it even smaller than the bias current.
 
dr*audio said:
Well, 1000nA X 47K = 47mV, which is significant for a cartridge with an output of 2mV.
Click to expand...
With no DC blocking capacitor the 1000nA doesn't all flow the 47k resistor, it flows through the cartridge coils as well, so the DC across the coils is much lower than 47mV.
 
TriodeLuvr said:
I recently came across a comment about a phono preamp issue that I've never considered. The poster claimed that the input bias current of opamps can degrade the performance of MM phono cartridges. He went on to say this current can cause long term changes to the magnetic structure. Is this a real phenomenon, or just Internet litter? Do opamp-based preamps need a blocking capacitor at their input?

Jack
Click to expand...
Those input bias currents of bipolar input op amps in in the low uA range. FET input ones are in the pA range. Both are way too low to have any real effect on any phono cart, MM or MC. If the op amp is run from a single supply, then input and output coupling caps would be mandatory.
 
rothwellaudio said:
With no DC blocking capacitor the 1000nA doesn't all flow the 47k resistor, it flows through the cartridge coils as well, so the DC across the coils is much lower than 47mV.
Click to expand...
Yes, so it's not possible to correlate the AC signal output with the effect of DC current through the coils in any meaningful way. That's what I was saying earlier.
dr*audio said:
Thanks, John. This was my point; select a device more appropriate for the application. LM833 and NE5532 should be used for line level.
Click to expand...
The OPA2604 is not a panacea for compatibility. 15nV/√Hz at 100Hz is not low noise. None of this answers the question though - will input bias current of opamps like the NE5532 and LM833 degrade cartridge performance or create long term effects?

Jack
 
Last edited:
egellings said:
Those input bias currents of bipolar input op amps in in the low uA range. FET input ones are in the pA range. Both are way too low to have any real effect on any phono cart, MM or MC.
Click to expand...
Not saying you're wrong, but how do you know? Just to add another perspective to all this, it's possible to buy a mechanical meter movement with 10uA FS sensitivity. Can we say definitively there's no effect on a cartridge if DC current approaches 1uA?

Jack
 
You must log in or register to reply here.
Back
Top Bottom