Opamp vs. Buffer vs. descreet Class A

[dirkwright]

Wow Jesper, that's an interesting approach. I'll have to experiment with it. Thanks.

 

[audiomixer]

Is this a circuit you have actually build or just simulated? I'd think I could test that, but I would have to swap the transistors to what I have at hand.....

- regards,

Michael

 

[dirkwright]

Is this a circuit you have actually build or just simulated? I'd think I could test that, but I would have to swap the transistors to what I have at hand.....

- regards,

Michael

I've ordered circuit boards from Pad2Pad, so no, I have not actually built it yet. It's only been sim'd to death.

It may not work very well if you swap transistors. The diodes are a special kind, called CRD (or CLD). They cost about $2-4 each. The first transistor has to be a high current jfet, and the second one has to be a medium power npn type.

 

[dirkwright]

Here's the total noise predicted by the software. It's low, as might be expected from a gain of 0dB.

 

[dirkwright]

And here is the S/N ratio predicted as well. I think it's pretty good. The LSK170 is a very low noise jfet.

 

[dirkwright]

Is this a circuit you have actually build or just simulated? I'd think I could test that, but I would have to swap the transistors to what I have at hand.....

- regards,

Michael

I ordered 8 boards. If they are good, I can build one for you, but it's going to cost about $40 stuffed and tested. The boards alone cost me about $17 each. This is what the board looks like in the software. It's 3" x 4" in size.

 

[Harpo]

1st sight, the traces connecting to the 79xx (1-gnd, 2-in, 3-out) seem mixed up.
Why are the separate +/-supplies (2? rectifiers, regulators, ..) needed at all when you could use the supply from the buffer feeding stage ?

 

[dirkwright]

1st sight, the traces connecting to the 79xx (1-gnd, 2-in, 3-out) seem mixed up.
Why are the separate +/-supplies (2? rectifiers, regulators, ..) needed at all when you could use the supply from the buffer feeding stage ?

I don't understand what you mean by "buffer feeding stage"??? This is an independent buffer that I intend to use for hifi.

Oops, I thought the 79XX pinout was the same as the 78XX. Now I've wasted some money!

 

[Harpo]

I don't understand what you mean by "buffer feeding stage"??? This is an independent buffer that I intend to use for hifi.

Just disregard. I thought this circuit only related to the OPs request for a unity gain buffer with an active gain stage in front that would already have a probably fitting supply.
I still don't get your use of two rectifiers (with a more uncommon pinout) when using both positive and negative voltage regulators. A center tapped transformer or using your dual secondaries in a series config only would need one bridge rectifier. Keeping the voltage regulators further apart might save the use of isolation kits if separate maybe needed heatsinks were used for the different potentials at the regulators metal fins.

 

[dirkwright]

I don't understand what you mean by "buffer feeding stage"??? This is an independent buffer that I intend to use for hifi.

Just disregard. I thought this circuit only related to the OPs request for a unity gain buffer with an active gain stage in front that would already have a probably fitting supply.
I still don't get your use of two rectifiers (with a more uncommon pinout) when using both positive and negative voltage regulators. A center tapped transformer or using your dual secondaries in a series config only would need one bridge rectifier. Keeping the voltage regulators further apart might save the use of isolation kits if separate maybe needed heatsinks were used for the different potentials at the regulators metal fins.

All of the transformers I use have dual secondaries. I don't know the exact reason why hifi folks use two rectifiers, but it appears to be a better choice than one. Virtually all of the hifi power supply circuits use two rectifiers.

I doubt any heatsinks on the regulators will be required. The current draw and dropout voltage are low.

So, I contacted the company and it's too late to cancel the order. Thanks for letting me know that I screwed up. I don't know what I can do to fix these now.

 

[JohnRoberts]

Can you SIM the voltage regulators? That might actually be useful.

JR

 

[dirkwright]

Can you SIM the voltage regulators? That might actually be useful.

JR

Yeah, I finally found a spice text model for them. I have to create the macro though. I don't have time right now.

I'm just really bummed out that I screwed up my circuit board!

 

[Andy Peters]

I'm just really bummed out that I screwed up my circuit board!

You can probably do some clever cuts-n-jumpers rework to get you going.

Everyone who does this for a living has made an expensive PCB mistake at some point. The trick is -- can you do enough rework to unfuck yourself?

-a

 

[audiomixer]

I sometimes 'bend and tweak' the components, this could be doable here or you might consider the smaller 79L15 (100ma) I think it has a different pinout or you might check if you can mount it backwards.....

worst case is to build the power supply on some breadboard and connect the board power with some leads....

;-)

- Michael

 

[dirkwright]

Thanks for all of your support and encouragement. Yeah, I just have 2 pins mixed up on the 7915 regulator. I can probably add some wires to fix it.

I had forgotten about the 78LXX and 79LXX series of regulators. Thanks for reminding me.

The stereo circuit draws about 80mA, so I should probably use one set of the smaller regulators for each channel.

 

[Harpo]

something like this little fix without cutting traces or additional wires ..

 

[dirkwright]

something like this little fix without cutting traces or additional wires ..

Wow, thank you!

 

[abbey road d enfer]

I'm too lazy to search out Pass' opinions on this, but what you get from low or no NF is the transfer function of the basic circuit topology without error correction. It is impossible to generalize "no feedback" as having a single sound characteristic. The results will be as variable as number of different topologies available. 

I've been less lazy than you ; I checked Pass's view on NFB. He concludes that he prefers a low THD stage with no NFB than a high THD stage with a lot of NFB. Doh!

 

[dirkwright]

After reading a lot of the old threads on this message board, I can say for certain that I'm at the deep end of the pool on this message board. You folks know far more than I do about electronics and audio. I hope to listen and learn and please be patient with my naive questions.

 

[JohnRoberts]

I'm too lazy to search out Pass' opinions on this, but what you get from low or no NF is the transfer function of the basic circuit topology without error correction. It is impossible to generalize "no feedback" as having a single sound characteristic. The results will be as variable as number of different topologies available. 

I've been less lazy than you ; I checked Pass's view on NFB. He concludes that he prefers a low THD stage with no NFB than a high THD stage with a lot of NFB. Doh!

Perhaps I should have said I wasn't interested enough..  8)

The anti-NFB crowd was one of the sundry alternate religious cults kicking around the fringes of audiophillia over the decades. There wasn't any there-there then, still none now.

If it wasn't for negative feedback (invented by a guy named Black early in the 20th century), they would have never made a workable telephone system and many other technologies we take for granted.

JR