What is the purpose of this resistor in a preamp?
« on: January 10, 2018, 10:56:28 PM »
Hi everyone, I downloaded this schematic from Jensen's website

I was wondering, what is the purpose of R1?

My best guess is that its used to rise the impedance presented to the opamp, or to help stabilize it somehow.

P.S. there is a note on the schematic that says that R1 should be placed as close to the A1 to prevent spurious oscillations

So maybe its to stabilize the opamp?

Thanks



« Last Edit: January 10, 2018, 11:55:54 PM by Dualflip »


Re: What is the purpose of this resistor in a preamp?
« Reply #1 on: January 10, 2018, 11:16:21 PM »
I guess it's a filter of some sort or then it has something to do with matching the signal with the opamp input.

Re: What is the purpose of this resistor in a preamp?
« Reply #2 on: January 10, 2018, 11:23:05 PM »
I guess it's a filter of some sort or then it has something to do with matching the signal with the opamp input.

The optimum source impedance of a 5534 is around 7k, and the transformer in that schematic is 1:5, so an impedance of 150 ohm on the primary would be reflected as a 3.75k to the secondary, my best guess is that the 1K resistor is used to add to the 3.75k impedance and get closer to the optimum source impedance?

benb

Re: What is the purpose of this resistor in a preamp?
« Reply #3 on: January 10, 2018, 11:31:16 PM »
It's an attempt to balance the voltage offset caused by input currents. This is sometimes done with bipolar-input op-amps to balance the voltage drops from the (nearly equal) bias currents of the inputs.

Check out input current here:
https://en.wikipedia.org/wiki/Operational_amplifier

Here's a 'real' explanation, and despite the title it's not by Bob Pease:
http://blog.parts.io/whats-all-this-input-bias-current-anyhow/

Here's another explanation:
http://www.ecircuitcenter.com/Circuits/op_ibias/op_ibias.htm

Looking at the whole circuit, the only DC path to the - input is the 10k feedback resistor R2, so R1 should actually be 10k (R1 doesn't really go to ground, it goes through the transformer secondary, which probably has at most a few hundred ohms resistance, but I think it's a bad idea to measure the DC resistance of a signal transformer with a regular DMM). Also, the DC gain is 1, so the output voltage will probably be insignificant.

Input bias current worst case (TI datasheet) is 2000nA, or 2uA. That times the 10k resistor is 20mV offset, that would be fixed by  R1 being 10k. There's also input offset current (the max the two input currents would differ by) of 400nA, which tells you how bad the offset could be even with these resistors being the right value.

Or maybe it's there to  make a low-pass filter with the input capacitance to reduce possible RF interference.

benb

Re: What is the purpose of this resistor in a preamp?
« Reply #4 on: January 10, 2018, 11:35:09 PM »
The optimum source impedance of a 5534 is around 7k, and the transformer in that schematic is 1:5, so an impedance of 150 ohm on the primary would be reflected as a 3.75k to the secondary, my best guess is that the 1K resistor is used to add to the 3.75k impedance and get closer to the optimum source impedance?
That + op-amp input in that circuir has an input impedance at audio frequencies in the megohms or maybe tens of megohms, so it makes no difference to the signal or impedance matching. It's RN and RL that do "matching" and loading for the transformer.

Re: What is the purpose of this resistor in a preamp?
« Reply #5 on: January 10, 2018, 11:40:35 PM »
It's an attempt to balance the voltage offset caused by input currents. This is sometimes done with bipolar-input op-amps to balance the voltage drops from the (nearly equal) bias currents of the inputs.

Check out input current here:
https://en.wikipedia.org/wiki/Operational_amplifier

Here's a 'real' explanation, and despite the title it's not by Bob Pease:
http://blog.parts.io/whats-all-this-input-bias-current-anyhow/

Here's another explanation:
http://www.ecircuitcenter.com/Circuits/op_ibias/op_ibias.htm

Looking at the whole circuit, the only DC path to the - input is the 10k feedback resistor R2, so R1 should actually be 10k (R1 doesn't really go to ground, it goes through the transformer secondary, which probably has at most a few hundred ohms resistance, but I think it's a bad idea to measure the DC resistance of a signal transformer with a regular DMM). Also, the DC gain is 1, so the output voltage will probably be insignificant.

Input bias current worst case (TI datasheet) is 2000nA, or 2uA. That times the 10k resistor is 20mV offset, that would be fixed by  R1 being 10k. There's also input offset current (the max the two input currents would differ by) of 400nA, which tells you how bad the offset could be even with these resistors being the right value.

Or maybe it's there to  make a low-pass filter with the input capacitance to reduce possible RF interference.

I was wondering if it could be a bias offset compensation resistor, but the value is what doesnt ring a bell.

Re: What is the purpose of this resistor in a preamp?
« Reply #6 on: January 10, 2018, 11:42:47 PM »
That + op-amp input in that circuir has an input impedance at audio frequencies in the megohms or maybe tens of megohms, so it makes no difference to the signal or impedance matching. It's RN and RL that do "matching" and loading for the transformer.

I didnt mean that R1 would do the loading of the transformer but rather add to the reflected impedance of the secondary, RN is in series with a low value cap, so its not going to have a huge impact on audio frequencies, I guess RN is to tame transformer ringing.

 According to Self, having a series resistance  of considerable value with the non-inverting input of the opamp when used in series feedback (just like the schematic) will increase THD due to common mode distortion. However the 1K resistance is relatively small and shouldnt cause any problem in that regard.

P.S. there is a note on the schematic that says that R1 should be placed as close to the A1 to prevent spurious oscillations

So maybe its to stabilize the opamp?
« Last Edit: January 10, 2018, 11:55:42 PM by Dualflip »

Re: What is the purpose of this resistor in a preamp?
« Reply #7 on: January 11, 2018, 01:41:10 AM »
I'll bite, as I used to sit under Deane's preaching.
inverting amplifiers were sinful,
but non-inverting amps especially those lacking emitter resistors in the input pair were more prone to stability issues.
you can read Deane's notes in "Recording Engineer Producer", June 1978.
R1 has virtually no effect on the transformer,
neither is such series resistor found in time tested mic preamps of similar design (SSL).





abbey road d enfer

Re: What is the purpose of this resistor in a preamp?
« Reply #8 on: January 11, 2018, 03:07:40 AM »
I was wondering, what is the purpose of R1?
The answer is included in your question.
"there is a note on the schematic that says that R1 should be placed as close to the A1 to prevent spurious oscillations"

Quote
So maybe its to stabilize the opamp?
It acts as a LPF, in conjunction with the input capacitance of the opamp, which is not on the schemo since it's a parasitic element, but quite real.
Who's right or wrong is irrelevant. What matters is what's right or wrong.
Star ground is for electricians.

Re: What is the purpose of this resistor in a preamp?
« Reply #9 on: January 11, 2018, 08:07:51 AM »
The answer is included in your question.
"there is a note on the schematic that says that R1 should be placed as close to the A1 to prevent spurious oscillations"
 It acts as a LPF, in conjunction with the input capacitance of the opamp, which is not on the schemo since it's a parasitic element, but quite real.

The input capacitance spec is not found on the datasheet, how can I find the cut-off freq.,?


abbey road d enfer

Re: What is the purpose of this resistor in a preamp?
« Reply #10 on: January 11, 2018, 09:18:12 AM »
The input capacitance spec is not found on the datasheet, how can I find the cut-off freq.,?
Input capacitance is not a guaranteed spec so you just have to know it exists. Different manufacturers, even different batches would have different values.
BTW, I never had to use a resistor in series with the non-inverting input of a 5534, as long as the connection is kept reasonably short.
Who's right or wrong is irrelevant. What matters is what's right or wrong.
Star ground is for electricians.

Re: What is the purpose of this resistor in a preamp?
« Reply #11 on: January 11, 2018, 09:33:38 AM »
I said filter even though I knew I'd get flak for it.

But I remember seeing a very simple crossover/filter circuit "design" in a theoretical explanation on how they work.

Re: What is the purpose of this resistor in a preamp?
« Reply #12 on: January 11, 2018, 09:44:55 AM »
Input capacitance is not a guaranteed spec so you just have to know it exists. Different manufacturers, even different batches would have different values.
BTW, I never had to use a resistor in series with the non-inverting input of a 5534, as long as the connection is kept reasonably short.

Yes, its the first time I see an NE5534 with an input resistor and I have an MCI JH528 full of 5534's .

Regarding the capacitance, I guess that the 1K value was found experimentally?

JohnRoberts

Re: What is the purpose of this resistor in a preamp?
« Reply #13 on: January 11, 2018, 10:15:36 AM »
Hi everyone, I downloaded this schematic from Jensen's website

I was wondering, what is the purpose of R1?

My best guess is that its used to rise the impedance presented to the opamp, or to help stabilize it somehow.

P.S. there is a note on the schematic that says that R1 should be placed as close to the A1 to prevent spurious oscillations

So maybe its to stabilize the opamp?

Thanks


Lots of interesting speculations. 

I have seen that done, for the reason they describe (to prevent oscillation), but I am not aware of any mechanism that would benefit from that.  There is reportedly some tiny capacitance between inputs (low single digit pF) but generally not a lag (LPF) problem unless using crazy high Z feedback values.  In the extreme an R in series with the + input with a few pF between input pins would roll off NF at very HF (improving stability margin).  I have never looked at this on the bench to determine the actual stability limits, but perhaps in the margin it could make a small difference, while that schematic shows a 22pF compensation capacitance that should be unity gain stable.

The 5534 app note shows a series input R  and they say....
Quote
(4) Excessive current will flow if a differential input voltage in excess of approximately 0.6 V is applied between the inputs, unless some limiting resistance is used.
So the 1k could serve to current limit any voltage transients.

FWIW I never added a R there, but you could do a lot worse than copying a Jensen design literally. His designs are generally conservative (and do not suck).

JR 
Don't only half-ass tune your drums. Visit https://circularscience.com to hear what properly "cleared" drums sound like.

Newmarket

Re: What is the purpose of this resistor in a preamp?
« Reply #14 on: January 11, 2018, 11:22:38 AM »


The 5534 app note shows a series input R  and they say....


(4) Excessive current will flow if a differential input voltage in excess of approximately 0.6 V is applied between the inputs, unless some limiting resistance is used.
So the 1k could serve to current limit any voltage transients.

So the 1k could serve to current limit any voltage transients.

FWIW I never added a R there, but you could do a lot worse than copying a Jensen design literally. His designs are generally conservative (and do not suck).

JR

Yes - those opamps have parallel diodes across the + and - inputs so a problem if you should exceed the forward voltage with enough current on tap to exceed the current rating.
( Don't try and misapply them as comparators  :o

Newmarket

Re: What is the purpose of this resistor in a preamp?
« Reply #15 on: January 11, 2018, 11:26:43 AM »
The optimum source impedance of a 5534 is around 7k, and the transformer in that schematic is 1:5, so an impedance of 150 ohm on the primary would be reflected as a 3.75k to the secondary, my best guess is that the 1K resistor is used to add to the 3.75k impedance and get closer to the optimum source impedance?

You don't gain anything by purposely increasing the impedance seen by the opamp input pin.
(There's no associated signal level increase to outweigh the added noise)

JohnRoberts

Re: What is the purpose of this resistor in a preamp?
« Reply #16 on: January 11, 2018, 11:36:01 AM »
Yes - those opamps have parallel diodes across the + and - inputs so a problem if you should exceed the forward voltage with enough current on tap to exceed the current rating.
( Don't try and misapply them as comparators  :o
Yes it is common practice to add reverse polarity clamp diodes across low noise transistor b-e junctions to prevent them from zenering. If allowed to zener bipolar device junctions will get noisier. While by today's standards the 553x is not very low noise, back when first released they were cutting edge for low noise and high slew rate.

JR
Don't only half-ass tune your drums. Visit https://circularscience.com to hear what properly "cleared" drums sound like.

Newmarket

Re: What is the purpose of this resistor in a preamp?
« Reply #17 on: January 11, 2018, 12:27:38 PM »
Yes it is common practice to add reverse polarity clamp diodes across low noise transistor b-e junctions to prevent them from zenering. If allowed to zener bipolar device junctions will get noisier. While by today's standards the 553x is not very low noise, back when first released they were cutting edge for low noise and high slew rate.

JR

Indeed. I recall NE5534 being sold in Tandy (Radio Shack)in the UK for several £ back in the early 80s. Not that Tandy were known for value though.
I think I started building stuff with LF351 instead  :)

JohnRoberts

Re: What is the purpose of this resistor in a preamp?
« Reply #18 on: January 11, 2018, 01:10:11 PM »
Indeed. I recall NE5534 being sold in Tandy (Radio Shack)in the UK for several £ back in the early 80s. Not that Tandy were known for value though.
I think I started building stuff with LF351 instead  :)
The LF 351 an early Bifet (similar to tl07x) so pretty much a different animal... Way lower input noise current but higher input noise voltage than bipolar input op amps (like 553x). So probably better for high impedance sources.

JR 
Don't only half-ass tune your drums. Visit https://circularscience.com to hear what properly "cleared" drums sound like.

Re: What is the purpose of this resistor in a preamp?
« Reply #19 on: January 11, 2018, 01:57:31 PM »
While by today's standards the 553x is not very low noise, back when first released they were cutting edge for low noise and high slew rate.

JR

I still consider the NE553x to be one of the best options out there both sound and pricewise,  surpased by the LME49720 or the LM4562 but not by much, the AD797 has really low noise but costs much more,  not to mention the prohibitive price on the OPA627. So yeah, like Douglas Self mentions in his book: you really need a very good reason not to use a 553x

Another dirt cheap opamp is the LM301, which  in my opinion is a very interesting part, I´ve seen Studer designs in which they replaced the entire input section of the 301 with discrete circuitry, it seems however that it is mostly frowned upon for modern work, the national semiconductor application handbook is full of 301/101s, it is my favorite opamp to use as a comparator thou, if im not using a lm311 /339
« Last Edit: January 11, 2018, 02:04:55 PM by Dualflip »