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Old 27th September 2015, 04:43 PM   #1
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Default Concerning P3, BA-3

Question after reading this post from Backbones on the BA-3 sticky concerning P3 adjustment observations:

Burning Amp BA-3

I use BA-3 preamp as well as BA-3 Amplifier. Should I leave Preamp P3 alone, and adjust P3 on Amplifier? Or vice versa, or is this tail chasing? I am inclined to adjust power amp, leaving pre alone.

One other question, for reference looking at BA-3 board as in BA-3 build thread: When Backbones says "turn towards R3" or "turn towards R4" I suppose you turn one clockwise and the other counterclockwise? Looking at board, and the way R3 or R4 are laid out, this would be necessary? I may need to PM Backbones on that one...

Thanks in advance for any clarity or suggestion,

Russellc
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Last edited by Russellc; 27th September 2015 at 04:46 PM.
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Old 27th September 2015, 05:22 PM   #2
Hikari1 is offline Hikari1  United States
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I have only used the BA-3 as a pre but I would be inclined to adjust the FE and OS as a group and the preamp by itself using a 47K resistor as load.

And the boards are mirrored which means the pots turn opposite of each other.

I've got my BA-3 FE set up for lowest distortion possible right now, 2nd harmonics are a bit below 3rd. I am not sure it sounds best this way but it is tight and fast and forward.

Next time I adjust P3 (using a FFT analyzer) I will set P3 so the 2nd harmonic is slightly higher than 3rd.

Also, setting P3 at midpoint seems to be completely random and not really an "average" or median setting. I probably depends on your jfet Idss and other components in the circuit. The only way to set P3 so the channels are even is with an analyzer or scope.

The interesting thing to me is that lowest distortion (slightly 3rd dominant) results in a narrower but taller soundstage. And it sounds "harder". 2nd dominant is wider but slower and not as detailed. Also, I have found that the bass is much tighter when the 2nd harmonic is nulled.

I't's probably subjective what someone likes. Depends on the room, speakers, etc.
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Old 27th September 2015, 08:56 PM   #3
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Quote:
Originally Posted by Hikari1 View Post
I have only used the BA-3 as a pre but I would be inclined to adjust the FE and OS as a group and the preamp by itself using a 47K resistor as load.

And the boards are mirrored which means the pots turn opposite of each other.

I've got my BA-3 FE set up for lowest distortion possible right now, 2nd harmonics are a bit below 3rd. I am not sure it sounds best this way but it is tight and fast and forward.

Next time I adjust P3 (using a FFT analyzer) I will set P3 so the 2nd harmonic is slightly higher than 3rd.

Also, setting P3 at midpoint seems to be completely random and not really an "average" or median setting. I probably depends on your jfet Idss and other components in the circuit. The only way to set P3 so the channels are even is with an analyzer or scope.

The interesting thing to me is that lowest distortion (slightly 3rd dominant) results in a narrower but taller soundstage. And it sounds "harder". 2nd dominant is wider but slower and not as detailed. Also, I have found that the bass is much tighter when the 2nd harmonic is nulled.

I't's probably subjective what someone likes. Depends on the room, speakers, etc.
I'm inclined to agree with you. Thanks for clearing up the directional thing, I figured it must work that way. I thought about turning careful counts in the pre first, then going back to original position, and then trying the power amp next, then if any consensous, dialing the pre up again. I also agree it seem to be in a not so much 2nd degree area at the mid point.

I dont have an analyzer, so I am going to just keep careful count of the turns.

All my BA-3 devices, as well as the ones I'm going to build have Toshiba devices on the front end boards and Toshiba J fets around 8 ids. I will wait to see if Backbone responds, then will PM concerning how many turns he was experiencing. Of course if he uses the Fairchild devices or different ids J fets, it may or may not be the same...

Russellc
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Old 27th September 2015, 09:00 PM   #4
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Hikari1 do you have any feel for how many turns you have in from "midpoint"? I am using Bourns 25 turn pots, if you have others this may be apples and oranges.

Russellc
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Old 28th September 2015, 02:35 AM   #5
Hikari1 is offline Hikari1  United States
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I don't know how many turns I have...I didn't really pay attention. I just kept cranking until the 2nd harmonic flattened out and started to go back up. Then I turned it back to lowest point and left it.

But I did notice that the channels were not "even" when set at midpoint. Which leads me to believe that either a jfet mismatch, mosfet mismatch or something else in the circuit causes variation. Also, a few turns(like 10) from midpoint may go toward 2nd harmonic and then back to 3rd. It is not a simple 2nd harmonic = clockwise, 3rd = counterclockwise.

Really, I think you need a analyzer to adjust this thing. Without one you are just chasing your tail. You may be able to hear the extremes of the settings but you won't be able to get identical settings channel to channel...also, it is extremely time consuming to do "by ear". I know before i used the fft analyzer I liked it a few turns from midpoint. Which direction, I am not sure (I am guessing it was towards the 3rd harmonic). The problem is 3 turns from mid might be 2nd harmonic, 6 turns might be 3rd, 10 turns might be 2nd. And the number of turns will vary with each setup and from channel to channel.

I think when talking about p3 settings it would be logical to say I like 2nd harmonic at XXdb and 3rd harmonic at XXdb. Right now I have my 2nd and 3rd about even which is as low as the 2nd harmonic would go.

For what it's worth i am just using a USB audio interface (m-audio) and faber software. I know next to nothing about analog scopes, testing but I figured it out. I don't think you could test an amp with this setup (might be to hot a signal) but a line level preamp, no problem. Literally, I just plugged the RCAs from the preamp into the USB interface and sent a 1kz test tone from the headphone out to the preamp. Not the best setup but it worked.

I had the usb interface from when I used to try to lay down some guitar to cakewalk (god, I am old) or garage band. Similar software to the faber toolkit can be found online for free or cheap. There are even some ipad/iphone apps that can do this. Or, if you are on windows there are some usb scopes/analyzers for about $100. It's pretty nice to send the test tone, monitor and measure all from your computer. I am sure you could put together something tat would be able to measure harmonics for less than $100.
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Old 28th September 2015, 03:14 AM   #6
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Quote:
Originally Posted by Hikari1 View Post
I don't know how many turns I have...I didn't really pay attention. I just kept cranking until the 2nd harmonic flattened out and started to go back up. Then I turned it back to lowest point and left it.

But I did notice that the channels were not "even" when set at midpoint. Which leads me to believe that either a jfet mismatch, mosfet mismatch or something else in the circuit causes variation. Also, a few turns(like 10) from midpoint may go toward 2nd harmonic and then back to 3rd. It is not a simple 2nd harmonic = clockwise, 3rd = counterclockwise.

Really, I think you need a analyzer to adjust this thing. Without one you are just chasing your tail. You may be able to hear the extremes of the settings but you won't be able to get identical settings channel to channel...also, it is extremely time consuming to do "by ear". I know before i used the fft analyzer I liked it a few turns from midpoint. Which direction, I am not sure (I am guessing it was towards the 3rd harmonic). The problem is 3 turns from mid might be 2nd harmonic, 6 turns might be 3rd, 10 turns might be 2nd. And the number of turns will vary with each setup and from channel to channel.

I think when talking about p3 settings it would be logical to say I like 2nd harmonic at XXdb and 3rd harmonic at XXdb. Right now I have my 2nd and 3rd about even which is as low as the 2nd harmonic would go.

For what it's worth i am just using a USB audio interface (m-audio) and faber software. I know next to nothing about analog scopes, testing but I figured it out. I don't think you could test an amp with this setup (might be to hot a signal) but a line level preamp, no problem. Literally, I just plugged the RCAs from the preamp into the USB interface and sent a 1kz test tone from the headphone out to the preamp. Not the best setup but it worked.

I had the usb interface from when I used to try to lay down some guitar to cakewalk (god, I am old) or garage band. Similar software to the faber toolkit can be found online for free or cheap. There are even some ipad/iphone apps that can do this. Or, if you are on windows there are some usb scopes/analyzers for about $100. It's pretty nice to send the test tone, monitor and measure all from your computer. I am sure you could put together something tat would be able to measure harmonics for less than $100.
Thanks for the informative feedback. I get you on the two channels not being the same and all. I guess, as you say, they arent at mid setting either.

This is probably boring the heck out of the more advanced users here (likely most all more advanced than I) so if you wouldnt mind. PM me your setup dumbed down for a moron as myself can understand. I want to set this up! I dont mind buying stuff, many times finding freeware loaded with unwelcome stuff...I just dont know where to begin.

If any other beginners are interested, I will be glad to PM results. If all thinks its of value, post it here, I guess this is my thread, but for the respect of those that find this discussion boring, whatever!

Any help on what I need and how to hook it to amp much appreciated!

Russellc
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Old 28th September 2015, 04:17 AM   #7
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My opinion on the original post after pondering much of Nelson's brain waves is to go for the absolute minimum distortion at all preceeding gain stages and leave harmonic spice additions to the end as much as possible.
That is try and null 2nd harmonic and mimimise distortion wherever possible at the preamp and only add 2nd Harmonic at the end assuming that is what you were chasing.
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Old 28th September 2015, 04:20 AM   #8
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Do you really need 2 BA3 front ends?
Could you just increase the gain of the BA3 power amp front end to eliminate the preamp?
Or do you listen to vinyl?
Just a query.
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Old 28th September 2015, 04:45 AM   #9
Hikari1 is offline Hikari1  United States
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Quote:
Originally Posted by Russellc View Post
Thanks for the informative feedback. I get you on the two channels not being the same and all. I guess, as you say, they arent at mid setting either.

This is probably boring the heck out of the more advanced users here (likely most all more advanced than I) so if you wouldnt mind. PM me your setup dumbed down for a moron as myself can understand. I want to set this up! I dont mind buying stuff, many times finding freeware loaded with unwelcome stuff...I just dont know where to begin.

If any other beginners are interested, I will be glad to PM results. If all thinks its of value, post it here, I guess this is my thread, but for the respect of those that find this discussion boring, whatever!

Any help on what I need and how to hook it to amp much appreciated!

Russellc
I don't think we need to PM, people smarter than us can chime in along the way, no offense taken. I don't know much about this stuff, more knowledgable people can probably give better advice.

Here's the software I used (mac only):

Electroacoustics Toolbox

Here is the USB interface:

M-Audio - M-Track II

The usb interface is good for line level signals (it's meant for microphones and guitars) but not the high level of a power amp. You would probably need a different interface with actual probes for that.

I used the fft analyzer in the software to see the harmonics after feeding the preamp a 1kz signal.

If you are on windows a better option might be picoscope (https://www.picotech.com/oscilloscop...-oscilloscopes).

But it seems alot of the computer based test solutions seem to be focused on the ipad and iphone. Probably because of portability. Something like this looks very interesting:

https://www.oscium.com/oscilloscopes

Honestly, that's about the extent of my knowledge. All the software requires that you know at least something about how a conventional scope or test equipment works. The interfaces seem to be digital versions of analog scopes. I don't know how to use an analog scope so it took me a long time to get what I want. I watched a lot of youtube videos and things. The learning curve was steep. It is similar to photoshop (just trying to find an analogy some of you might know)...the tools and functions of photoshop are kind of digital versions of what you would find in a traditional darkroom but just becasue you know how to print pictures in a real darkroom it doesn't mean you know photoshop. But the learning curve will be easier.
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Old 28th September 2015, 05:08 AM   #10
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Quote:
Originally Posted by Russellc View Post
This is probably boring the heck out of the more advanced users here (likely most all more advanced than I) so if you wouldnt mind.
Nothing boring here in Pass Labs. Plenty of dummies too, no need to feel like you're alone in that department.
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