RIAA preamps versus real loads

[merlin el mago]

Yes I know what Impedance is, but I don't understand the reason of your question.
This amp is supposed to drive a high input impedance preamp as from the second LT1792 stage, so output impedance can never be an issue.
You could still insert an additional 100R resistor at the output to match the interconnect's impedance.
But anyhow, here is the output impedance for this Riaa preamp using the LT1792.

Hans

I guess you know, I'm using AVC and don't like high output impedance from source.

Felipe

 

[MarcelvdG]

Right, but that would give 1dB rise at 10Hz.
Not a drama, but this is where you don't want to amplify the rumble.
But as shown, the whole cap can be discarded.

Hans

OK, fair enough. You replace the capacitor with another capacitor, but it is a nice trick to get away with a much lower capacitance.

 

[MarcelvdG]

Assuming an offset voltage of 3mV max, this translates into 300mV max at the output, so when your preamp following this Riaa preamp is DC coupled, you will need an output cap.

Don't you mean roughly 1.5 V? It doesn't change the conclusion anyway.

 

[Hans Polak]

Yes of course, you’re completely right.
I should have known better.

Hans

 

[MarcelvdG]

No worries, the conclusion remains the same.

 

[carlmart]

Good point.
Assuming an offset voltage of 3mV max, this translates into 300mV max at the output, so when your preamp following this Riaa preamp is DC coupled, you will need an output cap.
When also implementing the cooling, you could discard the 4.7uF because less than 0.5uA will flow into the Cart, which IMO is acceptable.
But with two additional resistors and one cap, you can even reduce this to >100nA through the Cart.
My feeling is always that the best cap is no cap, so better to leave this 4.7uF.

Am I wrong or this is a DC servo?

 

[MarcelvdG]

It's a creative way to make a high-pass filter, but I don't see why it would be a servo. I may be wrong, though.

 

[Hans Polak]

Am I wrong or this is a DC servo?

What's in a name, it's a bit of a philosophical issue.

According to Wikipedia it is:
Servo is an automatic device that uses error-sensing negative feedback to correct the action of a mechanism.
In that way it fulfils IMO the definition by automatically doing error sensing of DC output voltage caused by offset and correcting the unwanted DC current through the Cart by negative feedback.

But in Audio a servo is usually referred to as a way to reduce the DC output voltage of a amplifying stage, which is definitely not the case here, so I agree with MvdG.

Hans

 

[merlin el mago]

Hans i guess 50 ohms or less, right?

 

[MarcelvdG]

Less than 3 ohm, see post #75.

 

[merlin el mago]

Wow, thank you.

 

[MarcelvdG]

What's in a name, it's a bit of a philosophical issue.

According to Wikipedia it is:
Servo is an automatic device that uses error-sensing negative feedback to correct the action of a mechanism.
In that way it fulfils IMO the definition by automatically doing error sensing of DC output voltage caused by offset and correcting the unwanted DC current through the Cart by negative feedback.

But in Audio a servo is usually referred to as a way to reduce the DC output voltage of a amplifying stage, which is definitely not the case here, so I agree with MvdG.

Hans

Interesting, the way I looked at it you were eliminating a loop for DC rather than adding a loop, but in the way you describe it, you can also see it as adding a loop.

 

[carlmart]

Is there a way I can model my Grado cartridge without getting the model?

I imagine I can't.

 

[MarcelvdG]

With the right test equipment and an accurate test record, you could measure the impedance and the frequency response, and then a model could be fit to the results of the measurements.

 

[Hans Polak]

Is there a way I can model my Grado cartridge without getting the model?

I imagine I can't.

The alternative is to make some pictures of the Cart and ask Billshurv.
He is the Wiki for Carts, but MvdG's proposal to measure is the better one.

Hans

 

[dennis h]

The alternative is to make some pictures of the Cart and ask Billshurv.
He is the Wiki for Carts, but MvdG's proposal to measure is the better one.

Hans

Could you point us to a detailed explanation how to measure our carts ?
Tia,
Dennis h

 

[Hans Polak]

The simplest way is with an Ohm and an induction measurement if available on your multimeter.

Hans

 

[dennis h]

The simplest way is with an Ohm and an induction measurement if available on your multimeter.

Hans

Thanks
Dennis h

 

[MarcelvdG]

Besides, you will need an accurate test record and something to measure the cartridge's output level.

 

[billshurv]

Or a calibrated test record.. Hoping to clear the backlog on that action.