GroupDIY

General Discussions => Drawing Board => Topic started by: Dualflip on January 10, 2018, 10:56:28 PM

Title: What is the purpose of this resistor in a preamp?
Post by: Dualflip 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

(http://image.ibb.co/nqmBK6/One_stage_pre.jpg)

Title: Re: What is the purpose of this resistor in a preamp?
Post by: efinque 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.
Title: Re: What is the purpose of this resistor in a preamp?
Post by: Dualflip 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?
Title: Re: What is the purpose of this resistor in a preamp?
Post by: benb 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.
Title: Re: What is the purpose of this resistor in a preamp?
Post by: benb 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.
Title: Re: What is the purpose of this resistor in a preamp?
Post by: Dualflip 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.
Title: Re: What is the purpose of this resistor in a preamp?
Post by: Dualflip 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?
Title: Re: What is the purpose of this resistor in a preamp?
Post by: gridcurrent 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).




Title: Re: What is the purpose of this resistor in a preamp?
Post by: abbey road d enfer 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.
Title: Re: What is the purpose of this resistor in a preamp?
Post by: Dualflip 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.,?
Title: Re: What is the purpose of this resistor in a preamp?
Post by: abbey road d enfer 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.
Title: Re: What is the purpose of this resistor in a preamp?
Post by: efinque 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.
Title: Re: What is the purpose of this resistor in a preamp?
Post by: Dualflip 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?
Title: Re: What is the purpose of this resistor in a preamp?
Post by: JohnRoberts 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

(http://image.ibb.co/nqmBK6/One_stage_pre.jpg)
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 
Title: Re: What is the purpose of this resistor in a preamp?
Post by: Newmarket 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
Title: Re: What is the purpose of this resistor in a preamp?
Post by: Newmarket 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)
Title: Re: What is the purpose of this resistor in a preamp?
Post by: JohnRoberts 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
Title: Re: What is the purpose of this resistor in a preamp?
Post by: Newmarket 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  :)
Title: Re: What is the purpose of this resistor in a preamp?
Post by: JohnRoberts 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 
Title: Re: What is the purpose of this resistor in a preamp?
Post by: Dualflip 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
Title: Re: What is the purpose of this resistor in a preamp?
Post by: efinque on January 11, 2018, 02:27:11 PM
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

If by Studer you mean their tape recorders; I think you'll be lucky to find one in a studio these days.

I have a Philips EL3556A (mono) tape recorder which I serviced and it seems to work but I have no reels or tape for it. Fancy machine as it uses a tube. However, most professional applications would require a faster machine (this one goes 19/7,5ips)
Title: Re: What is the purpose of this resistor in a preamp?
Post by: JohnRoberts on January 11, 2018, 02:46:42 PM
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
I used 301s back in the early 70's when is competitive technology. It facilitated a unique feedforward compensation (only worked with inverting topology) that delivered something like 10V/uSec (fast for the early 70s). 

They were mooted by early Bifet and 553x technology delivering similar slew rates without the requirement for trick compensation. (The feedforward cap bypassed around the slow input stage for high edge rates.)

JR
Title: Re: What is the purpose of this resistor in a preamp?
Post by: JohnRoberts on January 11, 2018, 03:04:10 PM
I'll bite, as I used to sit under Deane's preaching.
inverting amplifiers were sinful,
Sins could be assigned to non-inverting op amps too.   ::)
Quote
but non-inverting amps especially those lacking emitter resistors in the input pair were more prone to stability issues.
common knowledge to IC designers. Degenerating the input LTP (long tail pair) with emitter degeneration resistors, reduces that input stage transconductance that can be manipulated for improved slew rate all else equal. The reason FET input op amps were naturally faster was because of the lower inherent transconductance of FETs vs bipolar devices.

There were old bipolar ICs with LTP emitter degeneration resistors integrated into the silicon, but in the classic "no free lunch" tradeoff there were downsides to these added resistors degrading DC performance, and adding noise (thermal noise of resistors effectively in series with input), etc.

Deane cleverly (he was a very clever guy) figured out he could substitute inductors for the LTP degeneration resistors to keep the good DC and LF noise performance, while enjoying improved slew rate and stability margin for HF performance.

Unfortunately we can not easily integrate inductors into op amp ICs.  :o  But by the mid-70s off the shelf op amps were more than fast enough for the relatively slow moving audio signals.

JR 
Quote

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).
Title: Re: What is the purpose of this resistor in a preamp?
Post by: Dualflip on January 11, 2018, 03:30:37 PM


Deane cleverly (he was a very clever guy) figured out he could substitute inductors for the LTP degeneration resistors to keep the good DC and LF noise performance, while enjoying improved slew rate and stability margin for HF performance.


Apparently he wasnt the first to use that trick http://www.proaudiodesignforum.com/forum/php/viewtopic.php?f=12&t=675 (http://www.proaudiodesignforum.com/forum/php/viewtopic.php?f=12&t=675)

If by Studer you mean their tape recorders; I think you'll be lucky to find one in a studio these days.


I meant the consoles, specifically the 169 used this.
Title: Re: What is the purpose of this resistor in a preamp?
Post by: JohnRoberts on January 11, 2018, 04:03:58 PM
Apparently he wasnt the first to use that trick http://www.proaudiodesignforum.com/forum/php/viewtopic.php?f=12&t=675 (http://www.proaudiodesignforum.com/forum/php/viewtopic.php?f=12&t=675)

The ancients are always stealing our best ideas.  ;D ;D ;D

I recall Dick Burwens work in single ended NR, also well ahead of his time in the early '70s.

JR
Title: Re: What is the purpose of this resistor in a preamp?
Post by: 57sputnik on January 11, 2018, 04:18:57 PM
In answer to the OP, please refer to Jensen Application Note AN001. It requires a login to download.
Title: Re: What is the purpose of this resistor in a preamp?
Post by: Dualflip on January 11, 2018, 05:17:57 PM
In answer to the OP, please refer to Jensen Application Note AN001. It requires a login to download.

Wow, that was your first post in 8 years!, I will check the application note you mentioned, its settled, it has to do with stabilizing the opamp then.
Title: Re: What is the purpose of this resistor in a preamp?
Post by: abbey road d enfer on January 12, 2018, 04:32:35 AM
its settled, it has to do with stabilizing the opamp then.
Indeed, increasing the source impedance helps with stability, but actually the secondary impedance is high enough (even with the primary shorted) to make the additional 1k resistor unnecessary.
Title: Re: What is the purpose of this resistor in a preamp?
Post by: Newmarket on January 12, 2018, 04:39:33 AM
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

Yes. Similar to TL07x as you say.  Nowhere near the Bipolar type NE553x characteristics but was available and relatively affordable.
Title: Re: What is the purpose of this resistor in a preamp?
Post by: JohnRoberts on January 12, 2018, 10:41:28 AM
Yes. Similar to TL07x as you say.  Nowhere near the Bipolar type NE553x characteristics but was available and relatively affordable.
Over the decades I used truckloads of both Bifet and 553x bipolars. They were both optimal for different tasks.

For audio they were fast enough (and quiet enough at modest gains). Modern uber op amps are better on paper, but not significant for many real world applications.

JR
Title: Re: What is the purpose of this resistor in a preamp?
Post by: mike-wsm on January 13, 2018, 07:16:53 AM
The resistor is to keep the input impedance positive, ie not negative. Even the humble emitter follower can boast a negative input impedance, usually defeated by a few hundred ohms in series. In this case it is a non-invert opamp input; rigorous analysis will show negative input impedance which at some frequencies will combine with the 'nasty' transformer impedance characteristic to produce oscillation. Back when I was in time measurement we used an emitter follower to make a crystal oscillate.
Title: Re: What is the purpose of this resistor in a preamp?
Post by: Newmarket on January 15, 2018, 07:15:11 AM
Over the decades I used truckloads of both Bifet and 553x bipolars. They were both optimal for different tasks.

For audio they were fast enough (and quiet enough at modest gains). Modern uber op amps are better on paper, but not significant for many real world applications.

JR

Yes - when I first got a look at mixing desk circuits - DDA as it happens as involved in development work for automation etc on some of their desks - it was generally along the lines of 5534 where it matters (eg after the front end mic pre transistors) and TL07x elsewhere.
Both cost and current demand reasons ( 553x currents can add up in a large desk !) apart from circuit / impedance considerations.
I guess I'd be looking for something quieter now. For my DIY I have OPA2134.
In fact I recall I had some of the first OPA134 on my desk at Penny & Giles as we were in discussion with the TI guy about op amp options when they were released (derived from the more DC accurate OPA132).




Title: Re: What is the purpose of this resistor in a preamp?
Post by: abbey road d enfer on January 15, 2018, 11:07:53 AM
I guess I'd be looking for something quieter now. For my DIY I have OPA2134.
In most cases (Zsource<50k), OPA2134 will not be quieter, with 8nV/sqrtHz compared to 3.5. The only justification I found for 2134 is for instrumentation, where the level of THD and the HF extension is an order of magnitude higher. Even for DI boxes, the difference with a pedestrian TL072 is marginal, and the price about 20dB more.
Title: Re: What is the purpose of this resistor in a preamp?
Post by: ruffrecords on January 15, 2018, 12:16:20 PM
, and the price about 20dB more.

I really love using electrical terms to describe everyday things. Back in the 70s when I was at Neve, my boss had a pretty loud voice. One day I brought in some sticky chewy toffees. I offered one to my boss and said " Here's a 20dB pad".

Cheers

Ian
Title: Re: What is the purpose of this resistor in a preamp?
Post by: abbey road d enfer on January 15, 2018, 12:37:11 PM
I really love using electrical terms to describe everyday things. Back in the 70s when I was at Neve, my boss had a pretty loud voice. One day I brought in some sticky chewy toffees. I offered one to my boss and said " Here's a 20dB pad".

Cheers

Ian
Many people do not realize how much life is related to exponentials/logs. Life itself is like a capacitor charging then discharging.
Title: Re: What is the purpose of this resistor in a preamp?
Post by: JohnRoberts on January 15, 2018, 01:11:33 PM
Many people do not realize how much life is related to exponentials/logs. Life itself is like a capacitor charging then discharging.

I learned to hate programming 8 bit processors because life has more than 46dB of dynamic range.

Earthquakes and tornadoes are common examples of log/exponential metrics.

JR
Title: Re: What is the purpose of this resistor in a preamp?
Post by: Newmarket on January 16, 2018, 05:04:28 AM
In most cases (Zsource<50k), OPA2134 will not be quieter, with 8nV/sqrtHz compared to 3.5. The only justification I found for 2134 is for instrumentation, where the level of THD and the HF extension is an order of magnitude higher. Even for DI boxes, the difference with a pedestrian TL072 is marginal, and the price about 20dB more.

Yeah - I think my post read ambiguously. I meant quieter wrt TL072 - not NE5534 (or more directly with NE5532 @ 5nV...)
For info - I used them (OPA134) in the front end of an ADC module using a Crystal Semiconductor device.
The equivalent Crystal circuit uses OPA627 but they were ruled out on the basis of cost as you might expect.
Measured performance of the circuit was a tiny (repeatable) bit (<1dB) below using OPA627 so with 3 opamps per channel = quite a saving in commercial terms.
IIRC discussion with TI technical advised that this was likely due to output Z remaining low at high frequencies thus better driving the ADC front end.
Other opamps tested were significantly worse in terms of measurement.