neve portico discussion and drawn schematics

[ruffrecords]

Interesting they quote distortion at 100Hz which is not surprising because at higher frequencies the flux density will be much lower so there will be much less distortion.

Cheers

Ian

 

[brune94]

koo.corpus.cam.ac.uk/rnd-portico-schematic/

hello, do you have the drawing pertaining to this page? I can not enter because it appears that the domain is nonexistent.

 

[pucho812]

See the way back machine. Use the link from the first post… or check this out. Works on my iPhone.

 

[brune94]

See the way back machine. Use the link from the first post… or check this out. Works on my iPhone.

thank you, that pdf is a very great help. It's exactly what I was looking for!

 

[13engrsapper]

Circuits can't be copyright.

Circuit drawings are always copyright.

A mechanical copy (Xerox) is infringment.

A re-draw is a New Work and has its own copyright, not infringing.

Circuit details may be under Patent. You can re-draw these. You may not assemble the circuit without patent infringement; however broad leeway exists for "study and experimentation", so you typically don't get in trouble until you make a lot for sale. (Maybe if you gave them away in vast quantity.)

I am not a lawyer. Don't trust any legal opinion you haven't paid dearly for, from an expert who has also sized-up the other side. Even if you are right, if the other side is rich, they win.

schematics are copyrighted art - your version is new art - circle C that stuff with your name or groupdiy and the year.

 

[Bertu]

Does anyone have any guesses if the output transformer would be gapped to handle the silk DC offset better? Or is the offset small enough to not cause flux density issues?

 

[abbey road d enfer]

Does anyone have any guesses if the output transformer would be gapped to handle the silk DC offset better? Or is the offset small enough to not cause flux density issues?

Just an edcated guess: if the xfmr was gapped the effect of DC in it would be negligible. Since users want to hear evidence of the effect, I would say it's ungapped.

 

[Bertu]

Another question on this circuit, specifically the input. Does anyone have a guess what the value of the cap that is in series with the 560 ohm resistor across the inputs is? I am assuming this is some kind of high frequency filter for EMI rejection? So something like a couple hundred pF for a cutoff frequency in the 100s of khz ?

Thanks, Bertu

 

[abbey road d enfer]

Does anyone have a guess what the value of the cap that is in series with the 560 ohm resistor across the inputs is? I am assuming this is some kind of high frequency filter for EMI rejection?

Not really. C49 & C54 load the Common-Mode choke, which results in some ultrasonic resonance, that must be tamed with this RC series circuit: it's called a Zobel. The value of these two components is dependant on the characteristics of the CM choke. We don't have enough info to calculate their value.

So something like a couple hundred pF

Probably.

for a cutoff frequency in the 100s of khz ?

Not related to cut-off. Cut-off is more dependant on C48 & C54.

 

[JAY X]

What is the input impedance of this preamp? I only see two 68k resistors in the base to gnd of the transistors...

 

[Khron]

I only see two 68k resistors in the base to gnd of the transistors...

https://www.amazona.de/test-rupert-...511-mikrofonvorverstaerker-im-api-500-format/
According to this test, about 10k, apparently?

 

[abbey road d enfer]

If the schemo published above is complete, the input impedance is defined by the two phantom feed resistors (13.6k) and the 68k's, for an equivalent 12.3kohms.

 

[JAY X]

I have another question about this preamp circuit.

I noticed that the output gain trim is based on a variable gain balanced input amplifier, with the output of the potentiometer/opamp arm, tied back to the NON INVERTING input. I have seen a simmilar circuit in the Douglas self book "Small signal audio design". The difference is that the Self circuit, ties the feedback /potentiometer arm back to the INVERTING input. My question is if there is really a difference, in practice, or both versions behave simmilar?

 

[abbey road d enfer]

I have another question about this preamp circuit.

I noticed that the output gain trim is based on a variable gain balanced input amplifier, with the output of the potentiometer/opamp arm, tied back to the NON INVERTING input. I have seen a simmilar circuit in the Douglas self book "Small signal audio design". The difference is that the Self circuit, ties the feedback /potentiometer arm back to the INVERTING input. My question is if there is really a difference, in practice, or both versions behave simmilar?

That's not what I see here
http://www.douglas-self.com/ampins/balanced/balanced.htm#5Unless you're referring to another circuit than the one on fig. 11.

 

[JAY X]

Hi!

So, the basic functionality of both amplifiers is the same: substracting and give the difference. So i suppose that the Self version would work ok in the same position. But certainly the RND version is very different, and has positive feedback.... I know what positive feedback is and for what it is used (oscillators, filters), but here...I don't understand much..

 

[Khron]

I have another question about this preamp circuit.

I noticed that the output gain trim is based on a variable gain balanced input amplifier, with the output of the potentiometer/opamp arm, tied back to the NON INVERTING input. I have seen a simmilar circuit in the Douglas self book "Small signal audio design". The difference is that the Self circuit, ties the feedback /potentiometer arm back to the INVERTING input. My question is if there is really a difference, in practice, or both versions behave simmilar?

We're working on the assumption that the circuit has indeed been 100% correctly reverse engineered...

 

[abbey road d enfer]

Indeed there's a mistake in the had-drawn schemo.

 

[JAY X]

Hi!

How is it possible for this preamp to cope with line level signals without a 20 db PAD?
Could the diy SSL 9K preamp also manage line level signals without a PAD, without saturating the input?
These questions are scratching my head....

 

[13engrsapper]

It is perfectly legal. The device is on sale so anything in it is public domain unless protected by patent or registered design. A hand drawn schematic derived from the product itself is quite legal.

Cheers

Ian

Late to chime in, but Ian is correct about schematics. The copywrite on a schematic is only good for that "art" You can redraw it, and publish it any way you choose; just not the copywrited version - slap a "circle C' on your art and its relatively protected. "Circle R" is registered trademark - meaning someone spent money to actually register the art.

 

[abbey road d enfer]

Minimum gain on the original ( the one in SSL consoles) is 12dB, so it's capable of handling about +9dBu.
The 9k with a gain switch has a minimum gain of 6dB, which makes it capable og handling about +15..