Replace OP amps. Completely pointless, or not?

[ikd]

I have a different TPA3255 Amp here. It may use a different circuit.

The frontend for the TPA3255 is pretty much "datasheet" and I find that the LM6172 works well there, especially as there is invariably some switching noise flowing into the Op-Amp output.

There is also a Preamp with a volume control, this needs some resistor values changes and then in view works best with a J-Fet (or Fet) Input Op-Amp, I like the OPA2604 with a pull-down CCS, even though it is not the lowest noise or distortion choice. If I was chasing that OPA1642, OPA1652 or OPA1656 would be the best choice right now.

As I have stocks of Cerdip case mil spec OPA2604 and LM6172 I use them where I can.

So my setup is:

(Tone Control - 1/2 OPA2604) ->
Volume Control ->
Preamp w. 6dB Gain - 1/2 OPA2604 ->
Buffer/Phase Inverter - LM6172 ->
TPA3255

DAC is topping D10S with LM6172 in the output stage instead of LME49720.

Thor

What changed in the D10s sound after swapping the Op-amp?

 

[Thorsten Loesch]

What changed in the D10s sound after swapping the Op-amp?

Warmer, more natural, less mechanical/metallic sound.

Thor

 

[solderdude]

Shame on Topping for holding performance back for a measly $2 more in production costs and go from 'typical digital' (mechanical/metalic) sound to 'analog' (warmer more neutral) sound by the touch of a swapable op-amp.
Would have loved to see the measurement differences in response and IM products 19-20kHz to see if electrically things changed in an audible way...

Just teasing you Thorsten.

 

[Thorsten Loesch]

Shame on Topping for holding performance back for a measly $2 more in production costs and go from 'typical digital' (mechanical/metalic) sound to 'analog' (warmer more neutral) sound by the touch of a swapable op-amp.
Would have loved to see the measurement differences in response and IM products 19-20kHz to see if electrically things changed in an audible way...

Just teasing you Thorsten.

I will show measurements when I get my QA-404. Sadly right now my DSO has a FFT that is NG below -90dB but it goes out to several 100MHz... ;-) Measured the same on that, of course.

But the post asked audible changes, so I commented on what I heard.

Some extra notes.

The D10S runs the analog stage on appx. +/-5.6V created from a switcher running at ~ 1MHz with fairly vestigial filtering and switching ripple is observable on the Op-Amp supply lines with a decent fast 'scope.

The LME49720 has no specification of 1MHz PSRR, but at 100kHz it is ~54dB which suggests 34dB PSRR at ~ 1MHz.

The LM6172 has ~ 50dB PSRR at 1MHz.

The LME49720 has ~ 1.3uV output noise in the actual circuit in the D10S, the LM6172 2.1uV output noise, relative to 2V signal levels, so ~124dB to ~120dB SNR.

The LM6172 is not specified for high impedance THD, but being designed to drive 100 Ohm, there is a lot of bias current in the output stage, especially because the "single stage current feedback" nature of the design cannot rely on extra NFB to reduce THD.

The -106dB H3 for a gain of 6dB with 1V PP into 100Ohm rapidly drop into higher impedances. The output stage distortion changes pretty much linear with impedance, so for 2k Loads we can estimate -132dB H3 and -140dB H2. That compares favourably to the LME49720. IME, once the output stage runs into high impedances, the H3 element in distortion drops rapidly, leaving H2 dominant.

To conclude, technically speaking, the LM76172 has slightly higher noise (~ 4dB) than the LME49720, much greater PSRR at the switching frequency of the Power Supply feeding this Op-Amp and comparable to lower Harmonic distortion.

The LM6172 is based on a "high bias, fast silicon" model with a single stage of gain, relatively low levels of loop feedback (< 80dB 10Hz OLG) at lower (audio) frequencies and 100MHz GBWP with 2,500V/uS slew rate.

The LME49720 is based on "low bias, low current noise, slow silicon" model with two stages of gain, very high levels of loop feedback (< 140dB 10Hz OLG) at lower (audio) frequencies and 55MHz GBWP with 20V/uS slew rate.

I think for low noise use with clean, low noise (especially low RF noise) supply rails the LME494720 is a better choice (the discontinued LM6182 would address the noise if it can be used).

For use with high frequency noise on inputs and rails the LM6172 would appear superior, in technical terms. If RF IMD is reduced, this could (usually does) correlate with a sound that has less "grain, grit, edginess, brightness" to use the negative (what it does less wrong) form of statement.

Thor

 

[solderdude]

Yes, the opamps differ in quite a few aspects, and yes some are performing technically better than others in the same circuit.
However, those spec differences does not tell us how different the analog wave forms will actually be.
Would be curious to see comparative plots with the QA. Would be great if you can show audible differences.
Perhaps even do a null with Deltawave ?

That would rock the foundations of ASR if you can show differences that reach audible levels. Would be fun if you could show this.

 

[Thorsten Loesch]

Yes, the opamps differ in quite a few aspects, and yes some are performing technically better than others in the same circuit.

Yup.

However, those spec differences does not tell us how different the analog wave forms will actually be.
Would be curious to see comparative plots with the QA. Would be great if you can show audible differences.
Perhaps even do a null with Deltawave ?

I think it would need a wide bandwidth (> 192kHz) recording to show something. Moreover, we have no agreements on what is audible, so there is no way I can show something that is audible according to any criteria.

I think wideband multitone at -6dBFS might be a way to show something. Will the QA Device show a rise in noisefloor? No idea.

AP2 does. I once convinced a famous designer to throw the NE5532 & TL072's out of his CD-players analogue stage (in favour of LM6182/LM6172) and test with the AP2 multitone test. He found a much reduced noise floor in the Multi-tone FFT. Now that was in late 90's, so basically in antiquity and the years BC/BASR (Before Covid etc.)

If we try to go the "listen route" people will demand an ABX test, which I will decline as a variant of the shell game and we are back at an impasse.

That would rock the foundations of ASR if you can show differences that reach audible levels. Would be fun if you could show this.

Once we have a clear universal and reliable agreement what audibility limits are, we can start putting measurement data into context and show what is audible and to be worrisome and what doesn't matter. Absent such agreements, all I can do ever is show different measurements and proclaim one as better than the other without any reliable evidence that "better measurement = better sound" and "worse measurement = worse sound".

Thor

 

[pma]

LM6172 needs much more careful analog circuit and PCB design compared to NE5532 . Remember, we are at the soil of very essential info here, served to the general public, not much space and not much interest in specialized discussions.

 

[Thorsten Loesch]

LM6172 needs much more careful analog circuit and PCB design compared to NE5532 .

Surprisingly, no. As long as standard rail decoupling and enough supply current is available, the LM6172 is surprisingly docile (again, single-stage low NFB design) next to something like AD797.

Thor

 

[pma]

Surprisingly, no. As long as standard rail decoupling and enough supply current is available, the LM6172 is surprisingly docile (again, single-stage low NFB design) next to something like AD797.

Thor

I have opposite experience with LM6171, re FB stability.

 

[DanielT]

In the world of OP amps, it seems that the NE5532, which came out in 1979, really set a new standard for what you could use OP amps for in audio, HiFi. Regarding good performance that is. But after that, there hasn't been any revolutionary development. More like the type of (small?) improvements regarding OP amps (those that can be used in audio/HiFi), or? Do you think that is true?

NE5532 - Wikipedia

en.wikipedia.org

A before and after, something like this:

 

[Thorsten Loesch]

In the world of OP amps, it seems that the NE5532, which came out in 1979, really set a new standard for what you could use OP amps for in audio, HiFi. Regarding good performance that is.

The 5534/32 (and a few others that have over time faded) were the first Op-Amp's designed explicitly for audio use, with input from genius analogue audio circuit designers, to define requirements. The result is that used well, they are reasonably transparent. It also led to them being applied universally and often unthinkingly applied as "Audio Op-Amp".

But after that, there hasn't been any revolutionary development. More like the type of (small?) improvements regarding OP amps (those that can be used in audio/HiFi), or? Do you think that is true?

It is not true. Recent developments at TI and some others produce Audio-targeted Op-Amp's that are significantly superior.

Discrete circuits can still be made that for specific audio applications that outperform current integrated designs in specific metrics.

Thor

 

[solderdude]

Till the 5532/4 the more common opamps were TL07x.
Both types are found in lots of studio and home audio gear back in the days.
The 5532/4 still stand the test of time in not so critical circuits.

Higher priced OP27 and OP37 along with LM4562 were also quite popular.

 

[fpitas]

Till the 5532/4 the more common opamps were TL07x.
Both types are found in lots of studio and home audio gear back in the days.
The 5532/4 still stand the test of time in not so critical circuits.

Higher priced OP27 and OP37 along with LM4562 were also quite popular.

And the reality is, an awful lot of the music you buy went through lots of NE5532 stages.

NE5532 - Wikipedia

en.wikipedia.org

 

[levimax]

And the reality is, an awful lot of the music you buy went through lots of NE5532 stages.

Or even a lot of TL07x stages

 

[Schlippwhip68]

Well...

View attachment 270549

Hahah, thankyou.

 

[SIY]

I am always amused by the "I peeked and this is what I heard" nonsense from people who clearly know they're lying.

 

[Cbdb2]

Surprisingly, no. As long as standard rail decoupling and enough supply current is available, the LM6172 is surprisingly docile (again, single-stage low NFB design) next to something like AD797.

Thor

Right from the data sheet:

LM6172
LAYOUT CONSIDERATION
PRINTED CIRCUIT BOARDS AND HIGH SPEED OP AMPS
There are many things to consider when designing PC boards for high speed op amps. Without proper caution, it
is very easy to have excessive ringing, oscillation and other degraded AC performance in high speed circuits. As
a rule, the signal traces should be short and wide to provide low inductance and low impedance paths. Any
unused board space needs to be grounded to reduce stray signal pickup. Critical components should also be
grounded at a common point to eliminate voltage drop. Sockets add capacitance to the board and can affect
frequency performance. It is better to solder the amplifier directly into the PC board without using any socket.

COMPONENTS SELECTION AND FEEDBACK RESISTOR
It is important in high speed applications to keep all component leads short because wires are inductive at high
frequency. For discrete components, choose carbon composition-type resistors and mica-type capacitors.
Surface mount components are preferred over discrete components for minimum inductive effect.
Large values of feedback resistors can couple with parasitic capacitance and cause undesirable effects such as
ringing or oscillation in high speed amplifiers. For LM6172, a feedback resistor less than 1kΩ gives optimal
performance.

What I get from this. Rolling a high speed OA into a low speed circuit, (most audio circuits) is asking for overshoot, ringing or outright oscillations.

 

[Thorsten Loesch]

Right from the data sheet:

LM6172
LAYOUT CONSIDERATION

Right from the data sheet:

Essentially the same, except more pithy.

What I get from this. Datasheets contain boilerplate text on layout that are meant as "insurance" against customer complaints. They commonly suggest the same things almost regardless of Op-Amp. Which is not saying that following these is a bad idea.

BUT, if the designer using LME49720 followed recommended layout, LM6172 will work fine and if not, there is a good chance LME49720 will also not work well.

Thor

 

[maty]

NE5532 vs OPA627AU, on an FX-502J PRO. With music that facilitates comparison, with guitar (a lot of harmonics).


The improvement is very obvious.

 

[SIY]

The improvement is very obvious.

Suuuure it is. Suuuuuure.