[sts9fan]

I am having a strange issue with my Twisted Pear TXO-2 amplifier.  I finished last week and have been using it with my GR Research AV1+ 4ohm MTM speakers.  It has worked wonderfully and is very quiet without music playing.  Everything is great.  Today I switched back in my main speakers which are Zu Driuds which are 12ohm widebanders an a super tweeter with a 12k high pass on it. 
The issue is when I turn on the amp attached to the druids a massive amount of static distortion comes through.  It ramps up over 2-3 seconds and almost sound like a fan winding up.  This will happen with or without my ARC LS3 pre turned on.  I have switched back in the GR Research speakers without switching the amp connections and they are working great.
Any Suggestions on what the issue may be?
Thanks
Kris

[JoshK]

The druids are immensely more sensitive, so if you have ANY noise issue they will show it whereas the low eff speakers won't necessarily.  It could be a number of things.  Do you have a scope?  You could probe for noise. 

I'd take a WAG that it might be a cold solder joint or a bad resistor. 

[sts9fan]

The AV1+ is 90+db sensitive.  This noise is extreme and dramatic.  There are five pcbs in this amp and the issue is symetrical.  Can PS resistor do this?

[JoshK]

Maybe the amp is oscillating?  Again, you'd need a scope to see that.  For grins, you could take an AM radio and turn it on and hold it near the amp with the cover off the amp.  See if you can't get the radio to amplify the noise.  If it does, your amp is oscillating, if it doesn't, doesn't mean the amp isn't.   

TPA would probably be better at helping you out.  I don't know really how to solve such an issue, especially virtually, as I haven't had such a problem before.  I was just giving you a few guesses to help play detective.

FWIW, I think 90db 4 ohm is borderline low efficiency.  I have a pair of MTMs of similar rating.  But we each have our own definitions. 

[sts9fan]

cool I will investgate. I do have scope but do not really know how to use it. I agree that 90 is boderline but I was just saying that the issue is more dramatic then the difference. The speaker has to play a role here.

[AmpDesigner333]

cool I will investgate. I do have scope but do not really know how to use it. I agree that 90 is boderline but I was just saying that the issue is more dramatic then the difference. The speaker has to play a role here.

Does your scope have an "auto set" button?  If so, hook up the amp output to channel 1 and hit the auto set button.  If you see a repeating waveform, you have oscillation.  Zoom out over time by decreasing the horizontal sweep speed (time/div) by a few clicks if the wave you see is only a cycle or two.  Also, try adding an 8 ohm resistor across the amp output to see if the noise goes away.  Try leaving the 8 ohm load there and removing the speaker load too.  Good luck.

[sts9fan]

Thanks Ampdesigner.
 I will see if I can get this thing to work.  Any ideas about what about one speaker vs another could cause this?  Inductance?  I can add a Zobel to the amp boards.  Maybe that will help.
Kris

[randytsuch]

Thanks Ampdesigner.
 I will see if I can get this thing to work.  Any ideas about what about one speaker vs another could cause this?  Inductance?  I can add a Zobel to the amp boards.  Maybe that will help.
Kris

I would guess that you need a Zobel.  Maybe the Druids are a harder to drive load, which causes your amp problems until it fully turns on.  Another option might be to add a switch, that would let you disconnect the speakers for a minute, until the amp is stable.

Have you asked at the twisted pair forum?  The tp guys are very helpful.

Randy

[mgalusha]

The TXO-2 will oscillate if things are not exactly right and can easily kill tweeters. Trust me on this.

I have one sitting on a shelf that was trouble free when I built it and I then tried to convert it to a TXO-4 but it oscillated regardless of the presence of a Zobel. Russ White was very helpful and we tried all sorts of things and could not get it to behave. I measured each module individually and all were prefect, it just wouldn't play nice as a TXO-4 and TPA has stopped offering them this way. When I changed it back to a TXO-2 it was fine on the bench and I had no problems. However when I put it in the system it was OK for a while and then a nasty burst of static. That static turned out to be the sound of $800 worth of tweeters being cooked. I have spent entirely too much time reworking the amp trying to get it stable, going so far as increasing the gain on the LM3886 and decreasing it on the TXD boards. So far no luck and it's not going anywhere near a speaker until I am 100% satisfied it's stable.

You might contact Russ or Brian at TPA, they were very helpful and went above and beyond with assistance. In my case I'm going to try some power supply changes as I've been over all the modules repeatedly and can't find anything wrong and they all measure exactly as they should.

You might check your tweeters, hopefully they are unharmed.

mike

[sts9fan]

I have been in contact with Russ and he thinks the Zobel may help.  The super tweets seem to be ok.  Everything sounds fine with my Trends right now.  This is a big bummer but I guess that's why they discontinued the kit.
Randytsuch:
I think I may try the switch idea.  Its just strange to me that it can act so much different with one set of speakers vs another easier to drive 12ohm jobbies.

[AmpDesigner333]

Thanks Ampdesigner.
 I will see if I can get this thing to work.  Any ideas about what about one speaker vs another could cause this?  Inductance?  I can add a Zobel to the amp boards.  Maybe that will help.
Kris

I still need to read a few more posts on your thread, but for one, impedance is complex (real and imaginary values for a given frequency).  What sometimes drives amps crazy is a capacitive load.  This means the amp is trying to drive hard to compensate for the slowed (by the capacitance) output.  Essentially, it overreacts.  Did you manage to get the scope working?

Many amps use an inductor in parallel with a resistor on the output to make it more load tolerant.  A zobel can help sometimes, but figuring out the right values is not as simple as it seems.  It doesn't always help either since the idea behind that is to provide a decreasing impedance at high frequencies, which can actually make things worse if the phase shift eats up the margin and turns your negative feedback into positive feedback!

Also, can you provide a schematic?

[sbrtoy]

First off let me say I think the Twisted Pear stuff is great, a lot of fun to play with and some great ideas.  The TXO however was their Ford Pinto in my opinion. 

I ordered up a TXO-4 and like others found it impossible to get working properly.  After a month or so pulling my hair out I sent it back to Brian who could not get it to work either, it was just never stable.  He admitted they had never had a working prototype of the TXO-4 and that they never realized the issues it would have.  You have surely noticed how fast the TXO vanished from their site, it just wasn't fully thought through.  They now have the Sympatico which is a one board solution that seems to be the TXO replacement. In my opinion your best option to prevent possible tweeter toasting is just to run each of the modules individually making it a TXO-1....basically a normal chip-amp.