OP-6 from scratch

[DaveP]

"PG 3 of this thread
looking back on it it was S/N not EIN, but
EIN was -108db; Is there any way of raising this significantly? Isn't this quite noisy?"

Bearing in mind that this was designed in the 30's as a portable outside broadcast pre-amp -108dB is not bad. From the Telefunken data, a V72 varies from -111dB to -114dB and that was 20 years later. I don't imagine a studio using one of these in a quiet environment, but for drums its noise would not be noticeable.

 

[C12VR]

"PG 3 of this thread
looking back on it it was S/N not EIN, but
EIN was -108db; Is there any way of raising this significantly? Isn't this quite noisy?"

Bearing in mind that this was designed in the 30's as a portable outside broadcast pre-amp -108dB is not bad. From the Telefunken data, a V72 varies from -111dB to -114dB and that was 20 years later. I don't imagine a studio using one of these in a quiet environment, but for drums its noise would not be noticeable.

what's the limiting factor here, and could it be removed or mitigated?

 

[Stevie342000]

what's the limiting factor here, and could it be removed or mitigated?

Personally I think it is a non-issue given the specs and the age of the design. My recall was bad it was not noise that the VU meter introduces it is distortion which I suppose is a form of noise.

 

[DaveP]

Going back to the page three thread. Ian calculated the noise from the 47k resistor as follows:-

"That looks right to me. 20KHz bandwidth noise in 150R is -131dBu, so in 1500R it is -121dBu so in 15K it is -111dBu. 47K is just over 3 times that which is another 5dB giving a final value of -106dBu. 10KHz bandwidth will be 3dB less or or -109dBu."

So the cause of the noise is the 47k feedback resistor. If you reduce this it will change the NFB and change the gain structure and it will no longer be an OP-6 but something else. I am not saying don't try it, but I was asked for an OP-6 and that is what I delivered as far as possible.

It took a long time to figure out the feedback chain to match the gain, by all means work out your own feedback with a smaller input resistor if you think its worth it. Maybe it would be productive to search the internet to see what studios use the OP-6 for? Then you can decide whether it is worthwhile changing the original design or not?

If you are building your own OP-6 then you could put in a meter switch to avoid your original problem with meter noise. Get the level in the right ballpark, then switch it out?

Best
DaveP

 

[emrr]

The quality of the input tube is the most significant noise factor, followed by the proximity of the power transformer and then the carbon resistors in an original. Then the vagaries of the wiring loom. After reasonable rebuilds I have heard some very quiet originals, and some pretty noisy. Some that were 12dB quieter with an outboard supply (don’t remove the 12 pin power connector!) and some where it made no difference. An original did not come with a VU meter, it was an option. Which includes a variable meter pad, set for highest level reference I think should be quieter than a lower setting. Yes, VU spec is something like 3% distortion if hung directly on the line, the resistive isolation reduces that if I recall correctly.