Audeze LCD 4 - Which amp?

[Paul Birkeland]

I would recommend installing the DC filament upgrade first, then evaluating the various impedance taps.  There isn't going to be any hum at that point, and you can focus on listening to what you get at the various impedance taps in a more qualitative way rather than focusing on noise floor.

With the reduced noise floor, there may be a bit more bass bloom at higher impedance taps, but this will also depend a little bit on break-in as well.

To complicate matters further, you can technically get deeper bass out of a Stereomour at reduced power levels by converting it to run the #45 tube.  I would go ahead and do all your analysis without considering that, then maybe explore that more at a later date.

-PB

[Deke609]

Many thanks again, Paul.  I'll install the Mourquiet DC Filament next as you suggest.

Regarding the bass: based on reviews, it sounds like the Shunt Regulation upgrade will give me more slam and oomph as well.  But I'll wait to install it and the DC Filament Mourquiet upgrade until after I'm done playing around with output impedances.

And after all of that, I will build the L-pads and try to figure how my new oscilloscope works. Some suggest reading the manual first - I may try that this time 

[edited in line with correction from JH and PB below]

[fullheadofnothing]

Why is the attenuator upgrade being perceived as reducing hum, rather than the DC filament upgrade, which measurably reduces hum?

[Paul Birkeland]

Oops, yes, that's a typo.  Do the DC filament upgrade first, not the attenuator.

[Deke609]

Many thanks guys - I was just about to start on the Mourquiet when I saw this.  So I will do the DC Filament instead.  I suppose there'd be no harm in doing the MQ too.  But I'll wait on that as suggested.

[fullheadofnothing]

I was about to post about the hum balance, and then I saw that the instructions are missing from the manual. I'm getting that fixed in the manual, and I will post a sticky thread to the forum, but the basic process is:

( ) Connect the black lead of the meter to the black binding post of the right channel. Connect the red lead of the meter to the red binding post of the right channel.
To find the lowest possible position on the hum pot, you will also need to connect a speaker to the binding posts

Set the meter to read AC volts. If you meter does not autorange, set it to the lowest AC millivolt (mV) scale. Turn the trim pot all the way down to short the input. This will help keep outside electrical interference from affecting the reading on your meter.

( ) Install all tubes into their respective sockets.

( ) Turn the amplifier on let it warm up for a minute or two. Then slowly adjust the hum balance potentiometer behind to the four pin tube socket to get the lowest reading possible on the meter. This should be 0.8 - 2 mV depending on the tube used.

( ) Repeat this process for the left channel tube.

[Deke609]

The DC Filament is in and the SII is DEAD SILENT through the headphones. Wow.

Joshua: thanks for the hum pot instructions - On initial setup I simply set the pots to approximate mid-point on the dial. I hear no hum now with the DCF upgrade, but that may change when I switch to 8 ohms. In any event, I will follow the instructions you provided, if only "just to see".  Is it possible to use an o-scope in place of a speaker? I don't have any speakers - My house is "semi-detached", meaning I share a wall with my neighbor and so speakers are a no go since I prefer to listen at loud-ish levels - that's the reason for the LCD 4s and building a system around them - for headphones, they are very "speaker-like".  But I can probably borrow a speaker if necessary.

Listening impressions with the DCF installed are essentially the same as before, minus the hum with maybe a bit more detail. Based on my initial impressions I figured that my SII, as currently configured, would excel with classical -- strings in particular.  So I am listening to Yo Yo Ma's The Cello Suites. Wonderful.

If the Shunt Regulation and Mourquiet upgrades give me some more detail and bass, I will be in heaven.  But even as is, my SII with DCF upgrade is great with the LCD 4s.

I am now going to rewire the OTs for 8 ohms to do a quick comparison. Here's hoping I don't have to undo too much the DCF install I just completed!

[fullheadofnothing]

Once you get the DC upgrade in, the hum pots aren't going to do much. With AC filaments, tiny movements of the pot can cause shifts of several mV, so it's worth carefully checking. It was your post that made me realize it was missing from this manual, so thanks for making me go looking for it...

The reason for attaching a speaker is the loading will affect the performance, so you want to set the hum balance with something besides the meter loading the amp. The load doesn't have to be a speaker, it could be a dummy load (a high power rated 8Ω resistor (ask a smarter person than me how high rated it needs to be.))

Also, more than throwing upgrades at it, give it some hours and see how the sound opens-up with break in.

[Doc B.]

To be more specific about the loading, the reason for it is to establish a point of reference for the suggested level of 0.8 to 2mV of hum. If there is no load on the amp that number will be higher and thus not relative to the ~8 ohm loading method we used to establish the 0.8 to 2.0 mV reference. And by the way that is specifically 0.8 mV_rms to 2.0 mV_rms.

[Deke609]

Yup. Sound advice. Couldn't pull myself from listening - the sound is opening up quickly and the bass has gotten MUCH tighter - the amp was just cold.

But I'm going to need more gain - I can max out the vol pot on Rush's Tom Sawyer and some other "rockin'" tracks.  In fact, the last bit of dial doesn't seem to do anything, so I wonder whether I've hit the max that the amp can put out at 2 ohms and am experiencing "soft" clipping?  In any event, I don't have much headroom on the 2 ohm setting and am going to try a higher setting.  I'd like to find an ohm setting where I'm listening around 11-12 o'clock on the dial.

Question: Does you think 4 ohms will be enough of a jump in gain, or should I try 8 next?


I may still go ahead with the other upgrades this wknd - only because I have the time now and won't again for a while.  While this will deprive me of experiencing the impact of the upgrades on a fully broken-in SII, I can live with that.  In the end, I won't care whether the amazing sound is attributable 70/30, 60/40 or some other ratio of stock to upgrades.  I just want the amazing sound!

Re hum pot setting: I have a 8 ohm dummy load!  I think it's 100W or 200W - can't remember. Picked it up earlier in the summer as parts for playing around with L-pads for the SII. 

[Deke609]

Well, curiosity got the better of me and I installed the Shunt Regulation upgrade tonight.  IT IS AMAZING. This $89 upgrade just gave me sound quality improvements that I would expect to pay AT LEAST another $1K to $2K for were I buying retail and looking to upgrade models.  WOW.  The sound is superb: detail, coherence, punchier bass, "realness", "spaciousness" -- it's all here.  Unbelievable.  This is not a subtle "tweak" - it's a night-and-day obvious improvement.  The amp I am listening to now is not the same amp that I was listening to 5 hours ago.  I am floored.  And I haven't even installed the Mourquiet yet!

Many thanks to all of you at Bottlehead for making such an incredible piece of audio equipment available,

Derek

[Doc B.]

The amp I am listening to now is not the same amp that I was listening to 5 hours ago.

Just keep running it in. It will get even better.

[Deke609]

A question about voltage output and testing

The amp is still in the early stage of burning in, but already sounds stellar with the LCD4's.

I want to start playing around with my o-scope.  I have a set of BNC probes that will allow me to connect both the scope and the headphones to the speaker posts.  My thinking is, if and to the extent that the load the headphones put on the amp affects frequency response, distortion, etc., it make sense to measure the amp under the load of the headphones.

The scope has an input sensitivity of +/- 20V (with overage protection at +/- 100V (DC and AC peak)).  My DAC puts out 2V rms from the rca outs.  I have the amp configured for 2 ohms. I don't want to exceed the input sensitivity of the scope, but I haven't gotten to the point in my self-directed learning about amp electronics to make sense of the voltage gain formulas or their relationship to dBs (all I remember is that it's logarithmic).   

Is there an ideal and scope-safe volume setting, in dB, that I should use for playing around with my scope?  I am assuming that an increase in dBs translates to an increase in output voltage.

Many thanks in advance,

Derek

[Paul Birkeland]

Most scopes will handle a considerable amount of AC voltage, generally far more than you're going to see on the speaker jacks of a single ended tube amp.

For measuring distortion, a scope isn't going to tell you all that much unless it has FFT functionality built in.  A $50 soundcard and some free software will though!

[Tom-s]

Derek, keep up the good work.
Keep posting about the upgrades and how they affect the Stereomour II with your headphones.

If your still planning on a 45 conversion; be sure to let us know how it worked out!

And could you perhaps test/compare with other headphones?

I'm very happy to learn that there's more after Mainline for headphones (and speakers ).