[davidl]

What function do the output buffers perform when driving 300 ohn cans?
In the explanation of the circuit, Tangent notes that the opamp is looking at 200 ohms in the input of the buffer. If the buffer sees 200 ohms in the cans, how does it help? One member got by my screw up in first asking this question and said that the buffer helps with hard to drive cans. But if an opamp can feed a 200 ohm buffer shouldn't it be able to feed a set of 300 ohm cans? I could see it having trouble with low impedance phones.

Trying to educate myself

Dave

 

[Porksoda]

I don't know where you read that, but according to the BUF634 datasheet its input impedance in high bandwidth mode is 8 megaohms.

I don't know the effective impedance of the cascode, but I would bet overall the op-amp sees an output impedance more in the kilo-ohms range.

You could be thinking of the op-amp's open loop output impedance.

 

[davidl]

"R7
Just in case you're familiar with the META42 or PPA, there is no series resistor between the op-amp and the buffer since the BUF634 has 200 ohms of series resistance on its input already. Hence, there is no "R7".

R8"
This is from Tangent's Part Selection guide for the Pimeta. I may be misinterpreting it.

If it's way higher than that, as the data sheet indicates, then the buffer generates current that the op amp may have trouble generating and it makes perfect sense.

Dave

 

[Edwood]

R8 is jumpered unless you hear humming.

-Ed

 

[llmobll]

Quote:

Originally Posted by davidl What function do the output buffers perform when driving 300 ohn cans? In the explanation of the circuit, Tangent notes that the opamp is looking at 200 ohms in the input of the buffer. If the buffer sees 200 ohms in the cans, how does it help? One member got by my screw up in first asking this question and said that the buffer helps with hard to drive cans. But if an opamp can feed a 200 ohm buffer shouldn't it be able to feed a set of 300 ohm cans? I could see it having trouble with low impedance phones. Trying to educate myself Dave

It's not about whether the OPAMP CAN or CANNOT drive the headphones alone, but rather, by taking the pressure away from the OPAMP you get very overall preformance, sound quality, and overall less problems.

 

[looser101]

Tangent means that there is a 200 ohm resistor in series (inline) with the buffer input, internally. That does not mean it has 200 ohm input impedance. The buf634 has a high input impedance and low output impedance (high current).

Cheers

 

[Porksoda]

Quote:

Originally Posted by looser101 Tangent means that there is a 200 ohm resistor in series (inline) with the buffer input, internally. That does not mean it has 200 ohm input impedance. The buf634 has a high input impedance and low output impedance (high current). Cheers

that makes sense

 

[davidl]

Thanks for the help.

Dave

 

[tangent]

Quote:

Tangent means that there is a 200 ohm resistor in series (inline) with the buffer input, internally. That does not mean it has 200 ohm input impedance.

Bingo. The input impedance would be 200 ohms if that were in parallel with the input, not in series, as it is.