Russ,
Two things: 1) I wouldn't advise putting in a compensation cap in the feedback loop; & 2) Don't give up on your circuit yet!
I can't begin to tell you how many iterations I worked through before I achieved stability with my circuit. Nelson kept encouraging me to keep trying, and eventually I got there.
On the feedback compensation cap, when I tried them in my circuit it made the amp even more unstable. That's not to say they aren't worth a try, just preparing you to not be disillusioned if it doesn't work.
The other area I'd return to is the idea of throwing away more gain in the resistor voltage divider between the opamps (R3-6).
Seriously Russ, don't give up! You are probably a lot closer to a working version than you think.
Terry
Two things: 1) I wouldn't advise putting in a compensation cap in the feedback loop; & 2) Don't give up on your circuit yet!
I can't begin to tell you how many iterations I worked through before I achieved stability with my circuit. Nelson kept encouraging me to keep trying, and eventually I got there.
On the feedback compensation cap, when I tried them in my circuit it made the amp even more unstable. That's not to say they aren't worth a try, just preparing you to not be disillusioned if it doesn't work.
The other area I'd return to is the idea of throwing away more gain in the resistor voltage divider between the opamps (R3-6).
Seriously Russ, don't give up! You are probably a lot closer to a working version than you think.
Terry
Thanks Terry, its precisely because of guys like you I iwll not give up, but I may not be able to spend as much time as usual on the hobby for a bit as I sell my house.
I will try a few more changes tomorrow. Today I am too tired to lift the soldering iron.
I just got done pressure washing the entire exterior of my house, plus the patio and then sealed my driveway and porch. Going to sleep well tonight.
The problem with the voltage divider is I can only get the output up to near the THS4131 rail voltage which is 15V, so say 14V. So i don't have much more gain to throw away before I get the THS4131 to clipping.
I wonder if I could put the voltage divider after the Lm3886 output??? Then I could lower the feedback resistor value too.
Thanks to everyone who is particpating in this. The input is very much appreciated.
Cheers!
Russ
I will try a few more changes tomorrow. Today I am too tired to lift the soldering iron.
I just got done pressure washing the entire exterior of my house, plus the patio and then sealed my driveway and porch. Going to sleep well tonight. The problem with the voltage divider is I can only get the output up to near the THS4131 rail voltage which is 15V, so say 14V. So i don't have much more gain to throw away before I get the THS4131 to clipping.
I wonder if I could put the voltage divider after the Lm3886 output??? Then I could lower the feedback resistor value too.
Thanks to everyone who is particpating in this. The input is very much appreciated.
Cheers!
Russ
I have it!!!! Sucess at last!!!!
Ok Guys I have a new schematic a new layout a new outlook! 
THS4131 for SuSy with global feedback works beautifully!!! Here is the trick.
The THS4131 has significantly more bandwidth in the working range we are constrained to (voltage wise) than the LM3886 does. As such the THS4131 is starved of feedback it needs at high frequencies because the LM3886 simply can't keep up in the loop. So the THS4131 essentially has infinite gain at frequencies above which the LM3886 can cope(due to the lack of feedback), thus the circuit become completely unstable and the THS4131 is actually continually clipping.
So, short story, the answer is to add a compensating cap but forming a closed loop only for the THS4131 (not in global feedback, this would actually make things worse). I did this by placing a 200pf silver mica cap between -in and +out as well as +in and -out of the THS4131. The rest of the circuit is unchanged.
The amp is perfectly stable now!!! No sign of oscillation at all!!!!!! Wooooohoooooo!!!! Just great music and nice silent noise floor.
There is a bit of phase shift of course but only at very very high frequncies, well beyond the audio spectrum.
I may try some lower values, but 200pf seems to be working great and sounds excellent.
Thanks for all of the encouragment and ideas! I am one happy fella.
One other thing to try would be the somewhat slower THS4121 which might need slightly less external compensation, and it's a cheaper part.
I will share more details tomorrow after I have slept. What a busy day, I had to completely clean out my garage for my impending home sale. Yikes!!! I am exhausted.
Cheers!
Russ
Sucess at last!!!!Ok Guys I have a new schematic a new layout a new outlook!
THS4131 for SuSy with global feedback works beautifully!!! Here is the trick.
The THS4131 has significantly more bandwidth in the working range we are constrained to (voltage wise) than the LM3886 does. As such the THS4131 is starved of feedback it needs at high frequencies because the LM3886 simply can't keep up in the loop. So the THS4131 essentially has infinite gain at frequencies above which the LM3886 can cope(due to the lack of feedback), thus the circuit become completely unstable and the THS4131 is actually continually clipping.
So, short story, the answer is to add a compensating cap but forming a closed loop only for the THS4131 (not in global feedback, this would actually make things worse). I did this by placing a 200pf silver mica cap between -in and +out as well as +in and -out of the THS4131. The rest of the circuit is unchanged.
The amp is perfectly stable now!!! No sign of oscillation at all!!!!!! Wooooohoooooo!!!! Just great music and nice silent noise floor.
There is a bit of phase shift of course but only at very very high frequncies, well beyond the audio spectrum.
I may try some lower values, but 200pf seems to be working great and sounds excellent.
Thanks for all of the encouragment and ideas! I am one happy fella.
One other thing to try would be the somewhat slower THS4121 which might need slightly less external compensation, and it's a cheaper part.
I will share more details tomorrow after I have slept. What a busy day, I had to completely clean out my garage for my impending home sale. Yikes!!! I am exhausted.
Cheers!
Russ
HBarske said:
I could be wrong, but then so would the THS4131 datasheet, as they do it as I do in their circuit on page 16. Notice cap C1 is directly connected from outputs to opposing(phase) inputs. The short circuit is not a problem until you reach output voltages that would cause excess power disipation. In this case we are filtering out high frequencies before they ever get a chance to reach levels that would cause the THS4131 to work hard.
Cheers!
Russ
The sound is excellent!!
Mind you I am very very biased. But I love it. Bass is rock solid, treble is clear as a bell. Midrange has excellent presence and soundstage is precise.
The gain is a bit lower than Terry's circuit, and I like it that way, it allows me to get better use out of the XBOSOZ and Twisted Sibling preamps.
My Current arrangement is DAC(Cambridge DAC-3) --> Joshua Tree Attenuator(x2) --> Twisted Sibling(x2) at 6db gain --> SuSy.
I have been trying a few speakers. So far Tannoy Reveals, and Cryolites.
I can't wait to try it with my new DAC design.
Cheers!
Russ
Mind you I am very very biased. But I love it.
Bass is rock solid, treble is clear as a bell. Midrange has excellent presence and soundstage is precise. The gain is a bit lower than Terry's circuit, and I like it that way, it allows me to get better use out of the XBOSOZ and Twisted Sibling preamps.
My Current arrangement is DAC(Cambridge DAC-3) --> Joshua Tree Attenuator(x2) --> Twisted Sibling(x2) at 6db gain --> SuSy.
I have been trying a few speakers. So far Tannoy Reveals, and Cryolites.
I can't wait to try it with my new DAC design.
Cheers!
Russ
Moving my design to new thread.
Since my new circuit is really a unique project from what Nelson posted (apart from the obvious result), and since I do not want to dilute a thread about an idea with details on my particular design I have created a new thread here:
http://www.diyaudio.com/forums/showthread.php?postid=940344#post940344
Thanks to everyone here. You have been very kind.
Since my new circuit is really a unique project from what Nelson posted (apart from the obvious result), and since I do not want to dilute a thread about an idea with details on my particular design I have created a new thread here:
http://www.diyaudio.com/forums/showthread.php?postid=940344#post940344
Thanks to everyone here. You have been very kind.
Re: SUSY Comparison
I can't take credit for that amp, its really Maruo Penasa's amp, it is very very good.
Some subjective comparisons:
1) SuSy has deeper contrast, and is more upfront.
2) SuSy has much more "umph" with the additional power.
Other than that I will have to keep on listening. But I can tell you in no way is the SuSy amp inferior to the MyRef Rev C.
Cheers!
Russ
barryblue said:
I can't take credit for that amp, its really Maruo Penasa's amp, it is very very good.
Some subjective comparisons:
1) SuSy has deeper contrast, and is more upfront.
2) SuSy has much more "umph" with the additional power.
Other than that I will have to keep on listening. But I can tell you in no way is the SuSy amp inferior to the MyRef Rev C.
Cheers!
Russ
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.