P3 Adjustment for the average Hobbyist

Member
Joined 2003
Paid Member
After some very recent conversation on the "BA3 as Preamp" thread, several have obtained the 212 interface to use with ARTA or other program is setting of P3.

Some are as lost as I am, others not quite so much. In an effort to enable the average guy to set P3 to one's benefit, (or at least make both channels symmetrical) this thread was created.

Most of those envolved with the discussion either have, or are getting this device:

Focusrite Scarlett 2i2 | Sweetwater

Also, a Tascam unit is listed that would also be appropriate. Some have done this and are going to show how they hooked it up, the rest are flying by the seat of the pants.

This discussion began on the "BA3 as Preamp" thread, post numbers #1298-1313.

Please feel free to join comment inform etc.

Russellc
 
Last edited:
Nice one Russell. I will take some photos when i get to measuring. I should add that just about any home recording interface is usable. I use an mbox3.

I am not sure what voltage input these interfaces tolerate, so it is probably good practice to implement some kind of voltage divider to protect the inputs.
 
Member
Joined 2003
Paid Member
Nice one Russell. I will take some photos when i get to measuring. I should add that just about any home recording interface is usable. I use an mbox3.

I am not sure what voltage input these interfaces tolerate, so it is probably good practice to implement some kind of voltage divider to protect the inputs.

Would be happy to see any and all interfaces, looks like they perform similar job. Me, once I get the device I have no idea other than basic idea, of how to hook it up! Glad to have help from one who is actually experiencing setting of P3 in a meaningful way. Thank you for your input, much appreciated.

Russellc
 
Member
Joined 2000
Paid Member
After some very recent conversation on the "BA3 as Preamp" thread, several have obtained the 212 interface to use with ARTA or other program is setting of P3.

Are trying to build an interface so you can see harmonic distortion on a PC?
Preamp only?

This has been done. You just need a card capable of 24-bit 192khz. There is also a simple interface for power amp measurement. I have the links at home and can post later.

If this is something else or better, then I'd like to know more.
I need to adjust a 085 JFET F6 power amp. I have a PC with a studio Juli@ card.

Amazon.com: ESI Juli@ 4-in/4-out PCI Audio Interface with Swappable I/O Socket: Musical Instruments
 
Member
Joined 2003
Paid Member
Are trying to build an interface so you can see harmonic distortion on a PC?
Preamp only?

This has been done. You just need a card capable of 24-bit 192khz. There is also a simple interface for power amp measurement. I have the links at home and can post later.

If this is something else or better, then I'd like to know more.
I need to adjust a 085 JFET F6 power amp. I have a PC with a studio Juli@ card.

Amazon.com: ESI Juli@ 4-in/4-out PCI Audio Interface with Swappable I/O Socket: Musical Instruments

Thanks for the input. This is exactly why I posted this thread. Would love to have links to anything helpful. Even with the software and whatever device is used as interface, I am at a loss to hook it up and use it! Others who have responded are a little to a lot further up the learning curve.

I just wanted to do something with my units (BA3 Amp and two BA3 preamps) P3 besides leaving them set mid range, which doesn't mean they are symmetrically set, let alone adjusted to "best" distortion flavor.

More advanced uses, such as you suggest are also welcome, its all part of a similar problem. With as many BA3 amps and especially pre amps that have been built here, I cant be the only smuck who's P3 is just setting mid range!

Thanks for your input,

Russellc
 
Member
Joined 2000
Paid Member
Ok, I just have one bit of advise to people starting on using "P3".
Monitor the current draw across whatever resistor you used to adjusted the bias. I had a bad experience and it wasn't my own amp. I was helping someone else. It's real bummer.

Maybe not so much on preamps, but power amp's current setting needs to be watched and your P3 changes should be done slowly. I would still watch preamp current adjustment. Can't hurt.

I have 2 spectrum analyzer applications; one is more complicated than the other. The results are basically the same. The more complicated version allows for more adjustment of measurement, results and views.
 
this rang some bells in my head when I read it.

I'm not sure whether this is relevent or of any use but sometime back(was it really that long ago!) I put together this guide:

http://www.vitalstates.org/diy/amplifiers/simple-thd-measurement.pdf

you can substitute arta for the software mentioned in the doc, indeed there is an example of using ARTA on my write up for the sv572 amp. With ARTA you don't even have to do the thd calculation, it's displayed on the window.
 

PRR

Member
Joined 2003
Paid Member
...I am not sure what voltage input these interfaces tolerate....

"Crazed musicians with no technical sense".

The 2i2 has the extra feature of the knob glowing red when input levels are "too high". So start low and come-up slow. Basic audio common-sense.

You could also read the well-hidden slim specifications:
Downloads | Focusrite -- "User Guide"

"Line Inputs... Maximum Input Level ...+22 dBu"

"dBu" is jargon for "dB re 0.775V". So 9.7V is specified max. Essentially equivalent to a plain opamp fed with +/-15V supplies, still a favored source in semi-pro audio. And well above typical Hi-Fi working levels.

Gain Range is stated 50dB, which would imply signals as small as 31mV could be brought-up to 0dBfs on the digit-stream. (However you don't get 50 smooth dB on a dollar potentiometer, it may be glitchy at the extremes.)
 
"Crazed musicians with no technical sense"

Yeah that’s pretty close to hitting the nail on the head. I don’t come from an engineering background, but have recorded music with my wife (who is a real musician;), so I know the interfaces well enough but have not really needed to know the maximum input voltages. Thanks for showing the way on that PRR.

Yes the behringer will work. It may have a higher noise floor, but it should be low enough for this use.
 
There are also at help menu in the ARTA software. If you click on this there are a lot of topics. One of them is how to perform measurements and how an anp should hook up to a sound card. They also show a schematic of a input protection circuit with a couple of z-diodes etc. It seems the ARTA software has a signal generator so this is used as input via the 2i2 to the preamp and then output from preamp is going back again to 2i2 and then hopefully something will show up in the FFT analyzer window. There are a lot of adjustments possible……..
 
Just another Moderator
Joined 2003
Paid Member
The important thing with the focusrite 2i2 is to ensure that the input is not referenced to ground. fully differential the distortion performance is very good. But if running single ended the distortion goes up significantly.

Using a SE to differential circuit on the input, or even just simply using a TRS plug with only T connected to hot and R connected to earth with no connection to sleeve should improve the results.

I made a simple breadboard differential to SE for the output, thinking the problem was output distortion when in SE mode but that was not enough to fix the problem, the input seems to be key to getting good results.

Tony.
 
Just another Moderator
Joined 2003
Paid Member
Example of the difference between balanced loopback and SE loopback on the focusrite 2i2.

I was going to post results with balanced to SE converter and TR connection comming back but I did not get any better than with just an SE loopback, so not sure what happened there as I was sure I had much better results in the past. I'll have to delve a bit deeper.

Balanced was done with physical volume control at max, arta generator output at -6db and input level pot set to get -9db

SE was done without changing anything except the loopback cable.

Tony.
 

Attachments

  • 2i2_balanced_loopback.png
    2i2_balanced_loopback.png
    19.9 KB · Views: 566
  • 2i2_Bal2_SE_TR.png
    2i2_Bal2_SE_TR.png
    21.2 KB · Views: 573
Member
Joined 2003
Paid Member
There are also at help menu in the ARTA software. If you click on this there are a lot of topics. One of them is how to perform measurements and how an anp should hook up to a sound card. They also show a schematic of a input protection circuit with a couple of z-diodes etc. It seems the ARTA software has a signal generator so this is used as input via the 2i2 to the preamp and then output from preamp is going back again to 2i2 and then hopefully something will show up in the FFT analyzer window. There are a lot of adjustments possible……..

Nice. I will read that over this evening. Thanks everyone who has added to this topic, much appreciated!

Russellc
 
I just received the 2i2 I ordered. Drivers are installed and ARTA software can use it. When I started recording using a sine wave as output I can hear the sine if I plug in a headphone at the monitor jack on the front. ARTA of course only shows noise as nothing is connected to the inputs. Could be interesting to make a loop back and check the quality of the sine. Now there is some work to find connector adapters etc. so everything can be hooked up.
 
Member
Joined 2003
Paid Member
Ok, I just have one bit of advise to people starting on using "P3".
Monitor the current draw across whatever resistor you used to adjusted the bias. I had a bad experience and it wasn't my own amp. I was helping someone else. It's real bummer.

Maybe not so much on preamps, but power amp's current setting needs to be watched and your P3 changes should be done slowly. I would still watch preamp current adjustment. Can't hurt.

I have 2 spectrum analyzer applications; one is more complicated than the other. The results are basically the same. The more complicated version allows for more adjustment of measurement, results and views.

Are you referring to monitoring the FE bias, the output stage bias, or both?

Russellc
 
Member
Joined 2003
Paid Member
I just received the 2i2 I ordered. Drivers are installed and ARTA software can use it. When I started recording using a sine wave as output I can hear the sine if I plug in a headphone at the monitor jack on the front. ARTA of course only shows noise as nothing is connected to the inputs. Could be interesting to make a loop back and check the quality of the sine. Now there is some work to find connector adapters etc. so everything can be hooked up.

Excellent. I am glad you have received your device and have started experimenting. So it begins.

Russellc