[Vladco]

What do you guys think about this http://www.diyaudio.com/forums/showt...threadid=32095 discrete opamp. Could I use it in Pimeta or PPA?
I think –not really sure- some one post on DIY board what it sound better then opa627.
Vlad

 

[peranders]

I think you should know that a discrete "opamp" has shortcomings and if you except them it's OK.

Not so high PSRR
Not so high gain
Not so low input offset voltage
Gain dependent of load
Limited speed
Not so low distortion
No short circuit protection (may not be important)

This is what I can think of right know.

If you feel it's interesting, just go for it.

 

[Demolition]

I wonder if this opamp PCB will fit on a PIMETA board without impinging on the buffers. It's almost 1.5 in. long by 7/8 in. wide. That's about twice as long and wide as a dual DIP8 Browndog adapter.

D.

 

[peranders]

Quote:

Originally Posted by Demolition I wonder if this opamp PCB will fit on a PIMETA board without impinging on the buffers. It's almost 1.5 in. long by 7/8 in. wide. That's about twice as long and wide as a dual DIP8 Browndog adapter. D.

If you really want to replace the BUF634 and the PPA pcb with my or for that matter Glassman's pcb buffer, you have problems. If you really want the highest performance out of these buffers you must have a decent pcb meaning no "air construction".

 

[Demolition]

Quote:

Originally Posted by peranders If you really want to replace the BUF634 and the PPA pcb with my or for that matter Glassman's pcb buffer, you have problems. If you really want the highest performance out of these buffers you must have a decent pcb meaning no "air construction".

My response above was in answer to Vladco's question about whether these PCB opamps would fit on a PIMETA or PPA board. They look a bit too long to fit.

As for me, if I was going to build one, I'd just use the recommended parts and follow the tried-and-true instructions found at Tangent's site. I don't want to have to deal with some other weird, untested quasi-opamp PCBs.

D.

 

[peranders]

Quote:

Originally Posted by Demolition I don't want to have to deal with some other weird, untested quasi-opamp PCBs.

Why not? The true art of designing is that nothing is 100% right => you can change everything! True artists must explore the options.

 

[Vladco]

Please clarify yours comments because I’m could use some additional explanation
“Not so high PSRR” –what does it mean?
”Not so high gain” If you listen to Dr Gilmore it’s a positive reason
”Not so low input offset voltage” I’m lost on this one.
”Gain dependent of load” I’m lost on this one too.
”Limited speed” It probably bad but how limited and how it influence the sound?
”Not so low distortion” It probably bad but how low is ok and how it influence the sound. Some people swear by tubes with pretty high distortions.
Thank you for the response.
Vladimir.

 

[tangent]

Quote:

“Not so high PSRR” –what does it mean?

Power Supply Rejection Ratio is the ratio of voltage at the output of the amplifier to the voltage of ripple riding on the power rails, when there is no other output signal. Let's say there's 1V of ripple on the rails, and the amp puts out 0.01V. It should put out 0V in an ideal amplifier (infinite PSRR) but this particular amp has 40dB PSRR at the test frequency. If that's power supply ripple, it should be low-frequency. For an IC op-amp, you'd expect ~90dB PSRR at that point, or about 1/300 the output voltage!

Quote:

”Not so high gain” If you listen to Dr Gilmore it’s a positive reasonp

It isn't a yes/no choice. Without some excess gain, you have no feedback, hence it isn't really an op-amp after all. You might as well just play your music through an open-loop buffer. You don't see too many feedback-free headphone amps do you?

Quote:

”Not so low input offset voltage” I’m lost on this one.

That's a measure of the DC offset errors within the op-amp. I'm not familiar with the circuit in question, but it probably results from mismatched input stage transistors. In an IC op-amp, the transistors are inherently matched because they're grown on the same substrate.

This error results in higher output DC offset, which gets worse as you increase the closed-loop gain.

Quote:

”Gain dependent of load” I’m lost on this one too.

Pretty simple: as the load changes, the gain of the circuit changes. This isn't just a worry when switching headphones that the sound will change: since the impedance of headphones changes as frequency changes, you can expect additional changes in distortion vs. frequency over a similar circuit that had a constant gain vs. frequency.

Quote:

”Limited speed” It probably bad but how limited and how it influence the sound?

The speed of an op-amp relates to sound quality in two ways:

1. If it's low enough, the chip simply can't reproduce all audio frequencies accurately.

2. Output current might also be a factor. (Again, I'm not familiar with the circuit, so this one might not apply.) If it's low enough, the capacitance of the headphone cable can cause the speed of the op-amp to drop enough to meet condition 1.

Quote:

Some people swear by tubes with pretty high distortions.

Tubes distort in euphonic ways. Transistors do not. Ergo, you want a solid-state amp to have very low distortion.

 

[jefemeister]

Here's a article/design on DIY opamps from Nelson Pass.

 

[peranders]

I ment by my comments that this circuit is not an opamp as we know it but can work pretty good if you use it with taste and have appropriate power supply.

 

[peranders]

Quote:

Originally Posted by tangent Power Supply Rejection Ratio is the ratio of voltage at the output of the amplifier to the voltage of ripple riding on the power rails, when there is no other output signal. Let's say there's 1V of ripple on the rails, and the amp puts out 0.01V. It should put out 0V in an ideal amplifier (infinite PSRR) but this particular amp has 40dB PSRR at the test frequency. If that's power supply ripple, it should be low-frequency. For an IC op-amp, you'd expect ~90dB PSRR at that point, or about 1/300 the output voltage!

PSRR has a strong influence on distortion also, mainly if you use the amp in non-inverting mode.