Just finished SEX 2.1, but have odd measurements and audible distortion

[crooner12]

Hi,

So, I finished building the amp and there is very obvious distortion on both treble and most notably the bass when the volume goes beyond 1/3. Otherwise, left and right channel sound equally present and there's no noticeable hiss/hum.

I'm using Audeze LCD-2 headphones and a musicstreamer 2 DAC w/ lossless files out of my computer. These components work just fine out of my other setup, so something is up with my amp.

Note: I wired the OT for 32 ohms, which Audeze recommended, but I have the impedance switch kit (not yet installed), so I could adjust in the future.

Here are some measurements that were somewhat outside of tolerance. Everything else was within an acceptable range.

Resistance
Terminal 15,25 = 673 ohm as measured (should be 890)

Voltage
9,19 = 29,20, respectively as measured (should be close to 0)
A1,B1 = 30,21, respectively as measured (should be close to 0)

Any suggestions as to what's causing this distortion? I'm crossing my fingers that the issue is impedance and nothing else...

Thanks!

[Paul Birkeland]

Yes, something is very wrong with your amplifier.  It isn't going to sound like it should with those voltages being off. There should be a pair of 249K resistors, and one goes from 19 to 17 and one from 9 to 7.  Those resistors should make the voltage at 19/9 0V.  If that voltage isn't 0V, then there is a miswire in the circuit or a short.

[crooner12]

Thanks. I will take another look later tonight. Any recommendations where I should check first?

[crooner12]

I think I found the issue (or at least one major one) after reviewing every single connection and solder.

I reverse wired things between the OT primary wiring and the OT headphone jack wiring. I incorrectly wired the OT terminals 5,10 from left and right channels to the headphone jack instead of 15L and A3. As a result, I also incorrectly wired the wires coming from 7,8 (32 ohm config) for both channels to 15L and A3, 25L and B3.

What a mess. I'm hoping rewiring will give me the correct voltages...

[crooner12]

Fixed! My voltages still read somewhat high (20's), but hooked up it sounds fantastic (i.e., no distortion, dead silent).

Can't imagine how the C4S upgrade will improve upon this current state. I'm planning on keeping it stock for a few weeks and then upgrading.

[mcandmar]

Pics!    Enjoy, the S.E.X. really is a great bit of kit..

[crooner12]

Pics!    Enjoy, the S.E.X. really is a great bit of kit..

Here are some pics. I think my Stereomour needs a 2nd coat of miniwax...

[mcandmar]

Nice, i never realised how much bigger the stereomour was.  I would love to hear your impressions of the two, assuming you have tried your speakers on both?

[crooner12]

@mcandmar I haven't tried hooking up to speakers yet, but just had the Stereomour playing some acoustic this morning and I'm reminded again just how ridiculously good it sounds, even on my modest NHT super zero's.

--

New Problem: I caved and installed the C4S upgrade last night and all voltages checked out fine, but the terminal 10/20 voltage was still closer to 420 and not 405. Also, my 9/19 and A1/B1 is still hovering in the low 20's. I double checked everything and it looks ok.

Fast forward to this morning. After listening to the SEX amp for about 3 hrs last night with no issues (although the power trans gets quite hot - much more than w/ Stereomour) I turned it on this morning and POOF ::lots of smoke::! There goes my 8.2k Ohm resistor on the right channel looking down at the amp. It was measuring fine last night...

No idea what's up...any ideas?

[Paul Joppa]

I speculate that A1/B1 measure 20mV, not 20V. If it were 20 volts, then A3 and B3 would also have unexpected values.

Given that it worked for 3 hours, it seems to me the likeliest cause for the 8.2K resistor to fail would be a blown 22uF capacitor - possibly wired backwards, or faulty. Check resistance across the capacitor; it should settle to something quite large, several hundred K ohms.

[crooner12]

My A3/B3 were 18.8 and 17.4, respectively, so reasonably close to 19v.

I'll have to recheck A1/B1 when I can get a replacement resistor.

I have an auto-range DMM and resistance across the 22uF capacitor (currently in circuit) so it took several minutes for the reading to stop but the 1U/2U capacitor settled on .297M ohms and the 5U/4U capacitor settled on .341M ohms. Both are installed in the correct orientation.