Tube-y Noob with SEX 2.1

[dayvo]

First post from a new to tubes guy here.

I received my 2.1 kit on 25 Feb. and have had it up and running for about a week. It sounds great. Thanks for a very cool product, Bottlehead guys. So far I've been listening with my HifiMan HE-400's and Beyerdynamic DT880's (250 ohm). Both sound very nice.

I ordered the kit with both the output impedance switching option and the C4S option. When doing the resistance and voltage checks with the C4S, I was getting the full voltage output (~75 volts, if I remember right) on one channel (A), but only about 50 volts on B. At that point, I pulled the C4S board and installed the baseline resistors instead. That's how I've been listening so far. I'd like to get that C4S working -- it can only make a good thing better, right?

Not sure what I messed up on the C4S. The resistors measure right, the polarities all check out. The only thing that seems wrong in static (unpowered) troubleshooting is the forward V drop on the LED that biases the B channel 2N2907 looks to be .54 volts instead of ~.7 as I would expect. Anybody have any thoughts?

Thanks much.
Dave Olson

[Jim R.]

Dave,

Welcome and congratulations on completion of the s.e.x. kit.  Actually the forward voltage drop of the hlmp-6000 LED is about .54, so that one at least is good.  You didn't sub another kind for the others did you?

If you go through the amp again and post the resistance and voltage measurements that are off, we can help somewhat better.

HTH,

Jim

[dayvo]

Thanks, Jim.

No -- I haven't substituted anything. All the amp resistance and voltage values check out in spec now without the C4S board installed. When I get a chance (maybe this weekend) I'll re-install the C4S and run through the checks again.

[Paul Joppa]

The forward voltage drop on an HLMP running a few mA is about 1.57 volts. If you are measureing much less than that, then the LED is not glowing!

The LEDs on the board are labelled D1 and D2, and there are a pair on each end A and B. From the description, it is not clear to me exactly which LED has a problem.

Another possible cause of the wrong voltage on the driver plate is variations between individual tubes. Check the voltage on pins 5 and 6 of each tube - these are the driver plate (75v) and cathode (2.5v). If they do not match well between channels, swap the tubes to confirm that the tubes are not matched. Post your numbers and we'll determine whether the difference is enough to matter.

If the tubes are close enough together, and if all four LEDs were in fact glowing, then you can re-install the C4S and make the same voltage measurements at pins 5 and 6. This will tell us a lot about whether the board is working correctly, and if not, where the problem might be.

Before you re-install the C4S, look carefully to see that the 2N2907 is installed correctly; this is difficult to see easily and most of us have at one time or another screwed it up!

[dayvo]

Thanks, Paul

I'll check it out this weekend and report back.

[Jim R.]

Oh boy, I'm really batting 1000 here.  No, as PJ points out, it's not .54, but I saw that and was thinking 1.54, which could be in tolerance... probably.  Sorry, didn't mean to mislead you.

Anyway, it will be interesting to see what happens and if that LED is indeed the culprit.

-- Jim

[dayvo]

Apologies for not getting back to you guys sooner. I got side-tracked from trouble shooting the C4S board. Parts arrived for the high(er) efficiency speakers I wanted to build to use with the SEX 2.1. Over the last couple of weekends, I put together a pair similar to the Decware DM945's (same drivers but definitely not the same lovely workmanship on the cabinets). For the the last few days I've been evaluating and enjoying their sound - very nice so far.

Here's an observation that I'd thought I'd mention to see what you guys might think. Driving the amp and speakers from my docked iPad sounds very good. Driving it from my Mac (iTunes, BitPerfect, optical out) through a Headroom Micro DAC sounds even more detailed. Here's the rub - the DAC has a higher voltage analog signal output than the iPad dock (~2 V vs 1.3? - I haven't measured the signals. This is a notional comment). That coupled with the pretty sensitive input on the amp result in kind of strident, compressed sounding highs on some of the "hotter" recordings. If I turn down the volume in the digital domain on the Mac and turn up the gain on the amp for those recordings, good sonic things return.

Does that make sense? Is there a better way to resolve this - I hate losing resolution by mucking with the digital signal.

[dayvo]

I think I have to retract the assertions and questions in my last post. As the speakers break in and open up, that harshness is disappearing.

[manis]

This might not entirely answer your question, but may I suggest trying Audirvana+  to play your audio files on Mac? I find that changing volume digitally in this software does not have an audible effect (to me) in sound quality. FWIW, it uses Mbit+ dithering. It won the Computeraudiophile.com best product of 2012 award.

[Jim R.]

Yes, both audirvana + and PureMusic will give you true dithered digital volume control, which is the best way to go in the digital domain.  I use both myself but am starting to prefer audirvana + slightly -- probably because of the excellent support for the Sonore dac that I have, especially with DSD files -- though this is very preliminary and will take more experimentation and a lot more listening to confirm.

-- Jim