Best Op amps to use in Phase Linear Series 2 preamps and amps?

[George S.]

The 700 Pro chassis didn't come with the switch or hole for it, was wired direct from the factory. Was the coupling switch meant to switch in DC blocking capacitors on the inputs? Are those caps now on the WOAD control board?

Sitting here working with the QA403 and PL2000S2.
Running freq sweeps out to 30 kHz at 6.02 dBv which I understand is 2 vrms output from the preamp.
I'm finally understand how the QA403 input attenuators work.
Working on understanding measuring distortion above 0 dBv.

 

[laatsch55]

Those caps are not on the control board now George.

 

[George S.]

Well, I'm pleased to post the results of many hours of careful testing.
The BrownDogs with 0.1 uF caps on the OPA 2134UA are the clear winners at 10k and 20k for lowest THD and THD+N.
Bare BrownDogs with no caps beat the original opamps. Add the film cap, and THD is further reduced.
Joe said to aim for 2 vrms, so I did so attempting to maintain unity gain on the preamp. Was able to keep close to unity, but not exact due to the stepped volume attenuator.
0 dBv is 1 vrms, so aimed for 3.01 dBv as 2 vrms is 6.02 dBv. I think and hope this is correct. Did some sweeps at higher voltage and really didn't see any difference. Was striving for accuracy.
Used seperate linear and logarithmic scales for all sweeps. Ran the sweeps out to 40 kHz.
Here's the results with linear scaling and a 20 K fundamental for original opamps, bare BrownDogs, and capped BrownDogs (linear and logarithmic).

 

[Gepetto]

Well, I'm pleased to post the results of many hours of careful testing.
The BrownDogs with 0.1 uF caps on the OPA 2134UA are the clear winners at 10k and 20k for lowest THD and THD+N.
Bare BrownDogs with no caps beat the original opamps. Add the film cap, and THD is further reduced.
Joe said to aim for 2 vrms, so I did so attempting to maintain unity gain on the preamp. Was able to keep close to unity, but not exact due to the stepped volume attenuator.
0 dBv is 1 vrms, so aimed for 3.01 dBv as 2 vrms is 6.02 dBv. I think and hope this is correct. Did some sweeps at higher voltage and really didn't see any difference. Was striving for accuracy.
Used seperate linear and logarithmic scales for all sweeps. Ran the sweeps out to 40 kHz.
Here's the results with linear scaling and a 20 K fundamental for original opamps, bare BrownDogs, and capped BrownDogs (linear and logarithmic).

That is a clear difference George. I will have to check how they used the original quad packages as well. I cannot recall whether one quad was used on each channel or if they shared them between channels.

Checked and yes, they did the unthinkable and shared the 4 op amps in the quad package across both channels which compromises channel separation. I will have to research if the brown dog adapters with 2 dual packages unscrambles this or not. I doubt it but worth checking.

 

[George S.]

Thanks Joe! I couldn't have done it without your help. I learned a lot doing this little exercise.
Going to do some reading about the frequency response and IMD tests.
One thing that stuck out was the higher output of the preamp with the OPA2134UA installed. The Fundamentals gain had to be increased with the original opamps or decreased when the BrownDogs were installed to maintain 2 vrms.
Thanks again! Later.

 

[laatsch55]

Coolness George...

 

[marcok]

Nothing to complain , but
can you hear the difference between a TL 072 and an OPA 2134 , for example ?
Ciao
Marco

 

[George S.]

Don't know what a TL 072 is, so I just looked it up, a low noise dual opamp. I have no answer for you.
The original main opamp is a RC4136, still in production by TI, and very inexpensive.
Lots of options here.
https://www.cimarrontechnology.com/

 

[George S.]

That is a clear difference George. I will have to check how they used the original quad packages as well. I cannot recall whether one quad was used on each channel or if they shared them between channels.

Checked and yes, they did the unthinkable and shared the 4 op amps in the quad package across both channels which compromises channel separation. I will have to research if the brown dog adapters with 2 dual packages unscrambles this or not. I doubt it but worth checking.

Thanks Joe, I appreciate it. Perhaps another WOAD product? Easy enough for most builders to solder those pins and packages if the correction can be made on the small board.
Now getting ready to test some 0.1 uF COG MLCC between the supply pins and ground rail.

 

[marcok]

TL 07xx series ( expecially TL 072 ) is probably the most popular op amp for audio applications.
( high slew rate ) Carver C 4000 uses this one for RIAA (MM) , for example .
RC 4136 is a quad uA 741 ( poor slew rate ) and this parameter of course affects the sound .
PL 1000 and PL 2000 use RC 4136 because uA 741 was probably " the best " in the 70s .
For me slew rate is the most important thing to have a good sound ( IMHO)
This is the reason of my question .
Ciao
Marco

 

[George S.]

No luck with the C0G MLCC. Slightly lowers THD but raises noise on bare BrownDogs.
Raises THD and noise on filmed BrownDogs.
Now I know, been wondering about it for a good while.
Time to break from this for a while.

 

[George S.]

TL 07xx series ( expecially TL 072 ) is probably the most popular op amp for audio applications.
( high slew rate ) Carver C 4000 uses this one for RIAA (MM) , for example .
RC 4136 is a quad uA 741 ( poor slew rate ) and this parameter of course affects the sound .
PL 1000 and PL 2000 use RC 4136 because uA 741 was probably " the best " in the 70s .
For me slew rate is the most important thing to have a good sound ( IMHO)
This is the reason of my question .
Ciao
Marco

Was reading about slew rate yesterday, but I really know nothing about it. Thanks, I'll do some reading about it.

 

[Gepetto]

TL 07xx series ( expecially TL 072 ) is probably the most popular op amp for audio applications.
( high slew rate ) Carver C 4000 uses this one for RIAA (MM) , for example .
RC 4136 is a quad uA 741 ( poor slew rate ) and this parameter of course affects the sound .
PL 1000 and PL 2000 use RC 4136 because uA 741 was probably " the best " in the 70s .
For me slew rate is the most important thing to have a good sound ( IMHO)
This is the reason of my question .
Ciao
Marco

It wasn't the best in the 70s, but it was the only quad in the 70s which made it attractive for designing smaller and more dense designs. And it was cheap and easy to employ, still is. For demanding applications in the early 70s, I always employed LM709s which were decompensated LM741 cores or LM733 video amplifiers. You could tailor many of their operating characteristics with several external components. The LM709 and LM733 are not easy to employ in designs, requires knowledge of what you are doing and very careful board layout.

The PL2000 clearly did not need smaller and more dense. It is basically a box full of air. But it did need cheap and easy, and there you go...

 

[Hexis22]

No luck with the C0G MLCC. Slightly lowers THD but raises noise on bare BrownDogs.
Raises THD and noise on filmed BrownDogs.
Now I know, been wondering about it for a good while.
Time to break from this for a while.

Adding additional small value bypass caps can sometimes be a double edged sword, especially if they have long leads like pictured.

The leads can act as antennas and actually inject noise into Vcc if not implemented properly.

 

[Gepetto]

Adding additional small value bypass caps can sometimes be a double edged sword, especially if they have long leads like pictured.

The leads can act as antennas and actually inject noise into Vcc if not implemented properly.

It also depends on the ground trace that you are dumping these decoupling currents into. In general, I always strive to dump the power supply decoupling currents into a separate path to the single point ground. I am not sure what that trace that George tied to is also responsible for. If you are not very careful, the decoupling current can end up being interpreted as a signal in the audio signal path. When you decouple differentially between the V+ and V- as George originally had, you do not run into such problems.