2A3 Build

[Alonzo]

I think they are 93db rated.  Just going to have to learn to live with the hiss or see if the Smash is the component injecting the hiss vs the amps.

[Paul Joppa]

OK, so the resistors gave a squeal which was cured by restoring the 431. But we are using resistors in Stereomour II with no problems, leading me to suspect the wire loop even though most of it is twisted.

It would be valuable to know whether the 431 is actually the source of the hiss. The resistor experiment was intended to answer that. The impedance of the 431 is so low that bypassing it with a capacitor won't have much effect. An alternative is to remove the preamp (use shorting plugs) from the input of the Stereomour. Perhaps you have already done this - I didn't see it in a quick re-read of the thread.

If the hiss is audible on 93dB speakers, it might be measurable as an output voltage on the speaker terminals. That would make comparisons between configurations more accurate and useful.

[2wo]

You should be able to beat this. I have ~95db speakers and have no notable hiss with 45, 2A3 or 300B. a little hum, but no hiss...John

[Alonzo]

Paul,
More sample points.  The original LM431's that came with the kit had hiss, I don't know the kit purchase date, I bought the kit from Jim Redman (I'll search my bin for those and get you the type).  I tried the following ones, 2 tests from each type;
Fairchild LM431ACZ, STMicro TL431ACZ, all 5 tries hissed but the STMicro's were the best tried in the amp.  I have TL431AILPG from Digikey that are ON Semiconductor but manufactured in that country that copies things a lot.  I will give these a try the next time I'm under the hood
I'll give it a check with shorting plugs and also measure the output volts with the current 431's and with the next pair.

John, I know its there, eventually I'll get it figured out.  I may have to accept it though, I've added a lot of iron and caps, the baseline noise floor may be higher than I expect.
Thanks,
Alonzo

[clowkoy]

I've had this same problem about a month ago. My set-up is also a Smash and Stereomour 45. It turned out it came from the Smash. I changed the Smash tubes and the "squeal" went away. The next day, I put the old tubes back and it played fine again. After about an hour, the squeal came back and this time I held the tubes with my hands and the squeal slowly went away. Didn't have same  problem ever since.

[Alonzo]

Will be rebuilding the C4S board this weekend, messed up something while changing 431's and one channel of LED's blew.  It was an awesome pop, but didn't blow the fuse.  So somewhere I have a problem that caused too high a voltage maybe.  Going to the ON Semiconductor 431's and will post the board voltages and the output voltages.

[Alonzo]

An update in the search.
Both Stereomours now have STMicro TL431ACZ's and both still have a hiss that is apparent with no source playing and in the quiet passages or single instrument music from about 5 feet from the speakers.  Each amp measures less than 0.020V.  It can be adjusted to 0.000 for about half a turn of the hum pot (10 turn pots) then jumps back up to 0.020.
Next steps;
Made up some shorting plugs so will test/measure with them later.
Tube roll the 12AT7's
Swap out the 431's in the Smash for STMicro's
purchase some extra boards so all can be changed at once
Try the TL431AILPG in pre and amps

[mcandmar]

Just had a read of my own thread to remind myself, http://bottlehead.com/smf/index.php?topic=5316.msg76160#msg76160 in my application i found the On Semi TL431AILPG was the quietest of the lot (tried half a dozen), but ultimately none of them were quiet enough.  Keep us updated as i'm curious to know where you get with this...

[Alonzo]

Yep, will do.  Trying the On Semi's next but my boards are crappy now from all the desoldering and installs.  Sent Eileen an email to see if I can buy a few more.  In the mean time I can tube roll. 
With all this, the amps do sound wonderful.  It's just that nag that now that I know it's there I have to at least try to fix

[Alonzo]

Is the shunt regulator/active load printed circuit board for the Stereomour available to purchase?  Haven't heard a response from Eileen via email, thought I'd post the question and call tomorrow.  Is the 4.4 version for the SEX the same or does it have extra traces?

[fullheadofnothing]

Eileen is out of the office. Send another e-mail later in the week after she has had a chance to go through her correspondence.

That is the correct board.

[Alonzo]

Ok thanks!

[Alonzo]

Just to finish this out.  Currently all the TL431's in my system have been changed out.  Received new SmashUp boards today and put them in with ON Semiconductor 431's.  While assembling I ran both amps into my horns to test, only a very low hiss could be heard at about the 1" range from the speaker, with no music, drowned out with music playing.  Finished the SmashUp and the hiss is back but not equal, left side is louder, and really obvious in the tweeters (duh).  Threshold moved about a foot out from the speakers, with no music and with music you can hear it during quiet passages.
So it seems the Smash is amplifying the background hash I have.  I'm about done with chasing it in the pre and amps, I'm going to work with maybe adapting one of my Fix's to filter it next, maybe pad down the tweeters more.

[Alonzo]

Finally fixed!!!!  Replaced the Smash w/Smash-Up with a passive preamp.  No haze, no hash, no hiss.  Loving Mike's B7 Mexico series input transformers.
The ugly wiring job shown...