rp

Trying to figure out the Urei 1178 preamp
« on: March 11, 2021, 04:28:48 PM »
I've attached a screenshot the 1178 preamp section with some elements removed for legibility.

I'm trying to understand what's going on between the inverting input of the op amp and ground and how that relates to the Fet. Can anyone recommend some resources or search terms that would help me get there? Or if you're up for it, can you explain what's going on here?

Things I don't understand:
1 - What's the point of R9 and R10?
5 - Why isn't C10 just tied to ground?
2 - What's the point of R6/C9 if they are mostly bypassed by R7/R8?
3 - How do I figure out the gain of IC2-B? Is it just 1+((R7+R8) / (R9+R10))?
4 - What's R5 doing?
6 - Why take the distortion-improving dry signal (R15,C12) from the output of the op amp instead of directly from the drain of the fet?

I'm guessing most of these features are related to feedback aimed at lowering distortion?


abbey road d enfer

Re: Trying to figure out the Urei 1178 preamp
« Reply #1 on: March 11, 2021, 06:35:03 PM »
I've attached a screenshot the 1178 preamp section with some elements removed for legibility.

I'm trying to understand what's going on between the inverting input of the op amp and ground and how that relates to the Fet. Can anyone recommend some resources or search terms that would help me get there? Or if you're up for it, can you explain what's going on here?
This has puzzled many over the years.

Quote
1 - What's the point of R9 and R10?
They are part of the distortion cancellation system that puts the gate at half the drain-source AC-wise. I must admit I could never prove it works better than the more usual that just does it on the gate. And I've never seen a study that proves it. I'm open to suggestions... :)

Quote
5 - Why isn't C10 just tied to ground?
Again, this to inject a fraction of the output voltage into the source of the FET.

Quote
2 - What's the point of R6/C9 if they are mostly bypassed by R7/R8?
It's there to balance the offset due to the bias current of the opamp.

Quote
3 - How do I figure out the gain of IC2-B? Is it just 1+((R7+R8) / (R9+R10))?
R5 changes the gain slightly under heavy GR. And don't forget R6 and R11. Their contribution is marginal, though.

Quote
4 - What's R5 doing?
  R5 is what decreass the gain under heavy GR, by feeding some anti-phse.

Quote
6 - Why take the distortion-improving dry signal (R15,C12) from the output of the op amp instead of directly from the drain of the fet?
  because if you do that, the coupling cap and resistor come in parallels with the control voltage, which slows the attack.
Who's right or wrong is irrelevant. What matters is what's right or wrong.
Star ground is for electricians.

squarewave

Re: Trying to figure out the Urei 1178 preamp
« Reply #2 on: March 11, 2021, 06:58:43 PM »
1 - What's the point of R9 and R10?
Yes, this all looks like a scheme to reduce distortion. R15 and R16 mixes the op amp output with the CV. If you mix exactly 1/2 of the drain source voltage with the gate voltage, you get a fairly good variable resistor which is the attenuating element here. However, that assumes the signal level is pretty small. If the drain source voltage is higher, that can also cause distortion so they also inject an AC signal on the source so that the source follows the drain thereby reducing the drain source voltage. The regular 1176 does the same thing. So R9 and R10 also injects 1/2 of the drain source voltage on the source. Note however that there is some gain there. Not sure if that's important to the whole linearization technique though. It might be that the gain improves the effect. Not sure without simulation. But note that the op amp is just going to do whatever it needs to do to make the negative input match whatever is on the positive input. So the top of R9 is going to equal the drain voltage at AC at least. That's how I know R9 and R10 are presenting exactly 1/2 of the drain source voltage at the source.
5 - Why isn't C10 just tied to ground?
Because it's passing that 1/2 drain source voltage to the JFET source as described above.
2 - What's the point of R6/C9 if they are mostly bypassed by R7/R8?
An op amp input must have a DC path. Otherwise it would just drift high / low. So that 100K just keeps the op amp in check at DC. For AC, R7 and R8 dominate.
3 - How do I figure out the gain of IC2-B? Is it just 1+((R7+R8) / (R9+R10))?
That's probably about right but there could be a weird interaction with the AC being fed back through the JFET so I would need to simulate in LTSpice to fully understand what's going on. Pretty much everything I know about electronics I learned from playing around with LTSpice (not a joke).
4 - What's R5 doing?
Not a whole lot actually. Like I said before, the net it's connected to follows the input. So if the voltage on either side of it is the same, that means there's very little current going through it and so at AC it will look like a very high impedance.
6 - Why take the distortion-improving dry signal (R15,C12) from the output of the op amp instead of directly from the drain of the fet?
That might actually work as long as long as it also has C11 in series to take out the DC of the CV. But the op amp output is a well defined low impedance. If the drain signal causes the gain to change, that could cause the impedance of the drain to change and it could create a weird feedback loop.

This is all wild guessing at this point though. If I really wanted to understand this circuit I would model it in LTSpice and really study it. It's complex enough that I'm probably missing important stuff.

rp

Re: Trying to figure out the Urei 1178 preamp
« Reply #3 on: March 11, 2021, 07:29:14 PM »
Thanks abbey, this is so helpful!

This has puzzled many over the years.
Glad to know I'm not alone :)

They are part of the distortion cancellation system that puts the gate at half the drain-source AC-wise. I must admit I could never prove it works better than the more usual that just does it on the gate. And I've never seen a study that proves it. I'm open to suggestions... :)
What is the "more usual" that you are referring to here?

I'm pretty sure I'm following your other answers, but it'll take a little time to process. I appreciate you hitting every question!

rp

Re: Trying to figure out the Urei 1178 preamp
« Reply #4 on: March 11, 2021, 07:37:29 PM »
Thanks squarewave! Also incredibly helpful.

If I really wanted to understand this circuit I would model it in LTSpice and really study it. It's complex enough that I'm probably missing important stuff.
Good to know that this circuit is legitimately tricky. I just assumed I wasn't getting it due to gaps in self-taught knowledge (probably also true). I've not made the leap into using LTSpice, but maybe this would be a good reason to do so.

abbey road d enfer

Re: Trying to figure out the Urei 1178 preamp
« Reply #5 on: March 12, 2021, 04:52:59 AM »
What is the "more usual" that you are referring to here?
See attached.
Who's right or wrong is irrelevant. What matters is what's right or wrong.
Star ground is for electricians.

JohnRoberts

Re: Trying to figure out the Urei 1178 preamp
« Reply #6 on: March 12, 2021, 08:40:31 AM »
See attached.
Yes, old JFET trick that works, superimpose (drain?) audio signal attenuated -6dB on top of the gate control voltage.

I forget where I first saw this but it can make an audible improvement in JFET mutes/gates especially if control voltage is slow moving through on/off transition. 

Sorry I looked at that schematic and it hurt my head....Indeed I will add my vote for distortion reduction.

JR
Cancel the "cancel culture", do not participate in mob hatred.

rp

Re: Trying to figure out the Urei 1178 preamp
« Reply #7 on: March 12, 2021, 12:40:42 PM »
See attached.
Got it. Thanks! I haven't come across this before.

I forget where I first saw this but it can make an audible improvement in JFET mutes/gates especially if control voltage is slow moving through on/off transition. 
This is good to know.

Sorry I looked at that schematic and it hurt my head....
Right there with you!


For others who might be interested in the topic, I found some of it covered briefly in the Art of Electronics in the "Fets as Variable Resistors - linearizing trick" section. I wonder if it would be cool to add a user control to de-linearize the jfet for more distortion...

JohnRoberts

Re: Trying to figure out the Urei 1178 preamp
« Reply #8 on: March 12, 2021, 01:00:35 PM »


For others who might be interested in the topic, I found some of it covered briefly in the Art of Electronics in the "Fets as Variable Resistors - linearizing trick" section. I wonder if it would be cool to add a user control to de-linearize the jfet for more distortion...
Over the decades I designed my share of crude (cheap) JFET noise gates.

Of course I listened to it with and without, it was a very easy call to make it cleaner.

Note: it will never be too clean. The designer made it as clean as (s)he could for good reason.

===

For today's TMI the only time I tried to sell a SKU based on distortion was when I was trying to move out some excess obsolete (Peavey) inventory. In the warehouse we had a pile of these simple vacuum tube mixers designed for managing guitar pedal efx.

I figured that audiophools like distortion, especially tube overload, so this should be easy...   bzzzt fail. I even experimented with juicing up the gain to generate more overload distortion, but still a very hard sell...

Arguably my heart wasn't into making more distortion on purpose, but I was definitely motivated to move out the obsolete inventory (that's life working in the management lane).

JR


 
Cancel the "cancel culture", do not participate in mob hatred.

 

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