Audeze LCD 4 - Which amp?

  • Thread starter Thread starter Deke609
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In my post, I mentioned exploring the damping issues - one of them is the effect of changing the load on the amp. It's rather involved, and as I said often a small enough effect to be ignored - or postponed. In any case, a different thing from the power/distortion balance question.
 
Thanks Paul J. I think I understand now: the L-pad is to take the load issue out of the equation so that power/distortion can be cleanly experimented with.  Once that is done, I can return to the load issue if I want. 
 
I would hope that if you are going to explore these small issues you plan to have some equipment besides your ears to measure the results. I would suggest getting some measurement software that will work with a sound card so you can quantify frequency response and distortion. That will help a lot towards understanding what is really going on when you change the loading.
 
Ha! Thanks Doc.  I hadn't thought past my ears, but that makes obvious sense (obvious, now that you've pointed it out to me).  Any recommendations?
 
OK. I've made some small progress in preparing for the load and power/distortion experimentation. Following Doc's suggestion that I get some test equipment, I've ordered a USB oscilloscope (Pico 2204A), together with banana plug terminated connectors, and a basic audio signal generator (B&K Precision 3001), for a total cost of about $250 excl. shipping. I hope this will be adequate for my limited testing purposes. (Reason for the B&K: a reviewer of the scope indicated that the built-in audio generator isn't so good for hifi amplifier testing and suggested getting an external one).

I've also looked into L-pad attenuators and have some questions.

(1) Which one(s) to get?

Parts Express sells 8 ohm speaker L-pads in either mono or stereo, and with 15W, 50W or 100W ratings.  Regarding mono vs. stereo: I assume that a stereo L-pad will allow me to connect both right and left speaker outputs to it - so that the single pot will attenuate both right and left earphones simultaneously and more-or-less evenly. Is this correct?  In which case I need only 1 L-pad?

Regarding wattage rating: Which is likely to be sonically better, if any? The price difference between them is small, so I'll get the 100W if that is likely to sound/work best.

Link to the Parts Express L-pad page: https://www.parts-express.com/cat/speaker-l-pads/306

(2) Implementation: 8 ohm L-pad(s) PLUS 8 ohm power resistor(s), or just 8 ohm L-pad(s)?

I am confused by Paul J's suggestion, quoted below, which I took to mean that I need BOTH an 8 ohm L-pad and an 8 ohm power resistor (or possibly two power resistors - one for each pair of speaker outputs?):

Paul Joppa said:
You can get adjustable L-pads for loudspeaker crossovers. They are not expensive, handle plenty of power, and have wirewound resistance elements (the good kind!). If you load the output of an 8-ohm L-pad with an 8-ohm power resistor (also widely available and inexpensive), it will present an 8-ohm load to the power amp no matter the setting, and still provide an adjustable signal to the headphones (which are in parallel with the 8-ohm load resistor).
Click to expand...

I had assumed that the 8 ohm L-pad would take care of presenting the amp with a constant 8 ohm load and that nothing else would be required to do so.  I'm not questioning the suggestion - I just want to make sure that I've interpreting it correctly and order the right stuff. So I need BOTH the L-pad and an 8 ohm power resistor?

So my questions w/r/t L-pads: How many L-pads (1 or 2), mono or stereo, and at what wattage rating?  How many power resistors, if any, (1 or 2)?

Testing -- Just to be sure I'm understanding this right, once I find the sweet spot on the L-pad pot for the power/distortion issue, I can determine the corresponding impedance value of the L-pad by (a) cutting the power, and (b) leaving the pot turned to the sweet spot, measuring the resistance between two terminals of the L-pad pot?  I.e., the L-pad would have two resistors values - one fixed (8 ohms) and the other adjustable by turning the knob (X) - and I find X by following the above procedure.


Many thanks in advance,

Derek

[edited to add missing word]
 
It's pretty easy and inexpensive to just use ARTA and a soundcard (the soundcard will act as both the generator and the signal measuring device) for your distortion measurements, but it looks like the Picoscope has an FFT.  For years I got away with using a $40 M-audio Fast Track soundcard, but I recently splurged on an Mtrack Plus MKII to get some extra bandwidth.

Yes to the stereo L-pad, that is the way to go.

I would not pay the extra money for an L-pad rated for higher power.

Yes, you need an 8 ohm load.  The L-pad will present an 8 ohm load to your amplifier provided the L-pad itself is loaded with 8 ohms.

As far as finding the sweet spot, if you do your measurements at 100Hz, you can easily measure the AC voltage coming out of the amp and coming out of the L-pad with your DVM, then you would have a good idea of the fixed resistor values you might want to buy.
 
Awesome.  Many thanks Paul B.

Just a quick follow-up question: do I need (a) 1 or 2 L-pads? and (b) 1 or 2 8 ohm power resistors?  E.g., do i need one L-pad/power-resistor combo for the right channel and a second for the left (2x2)?
 
I just got the my SII up and running in stock configuration. That was a bit more involved than putting together the Crack! Only screwed up one thing (installed a 5W 3K instead of a 5W 2K -- but caught that mistake and fixed it). Passed all resistance and voltage checks. Fired her up and no smoke. Woohoo!

Following previous advice, I plan to compare the amp configured at 2 and then 8 ohms with my Audeze LCD 4s plugged directly into the speaker outs. It is currently configured for 2 ohms.

Question: should I install the 3 upgrades before comparing 2 and 8 ohm configurations, or compare 2 and 8 ohms first?

Observations and first impressions with zero burn-in and cold amp configured for 2 ohms (Roon -> Yggdrasil DAC -> SII -> LCD4):

(1) Slight hum when no music is playing - but mourquiet upgrade should minimize that and perhaps even eliminate it. 

(2) With the volume pot ranging from 7 (no sound) to 5 (max) o'clock, my normal listening level is between 2 and 3. I expect this will change when I try the amp configured for 8 ohms -- I assume I'll get a big jump in gain from this.

(3) Overall sound presentation is remarkable for an amp with no burn in or warm-up - Marvin Gaye's album, What's Going On, sounds wonderful: "human" and spacious.

(4) Bass is muffled -- like someone strapped a pillow to the bass drum: "whumphh".  Rush's "Tom Sawyer" lacks punch.

(5) Details are there if I listen for them, but a bit veiled -- opening guitar on "Roundabout" from Yes' Fragile Album missing the micro details and clarity that make you believe you're in the recording studio with the band.

I expect installing the upgrades and switching to 8 ohms (or possibly 4 if 8 goes too far in the other direction) will take care of (4) and (5) above.  That, and letting the amp warm up and burn-in  ;D

If the 3 upgrades are even half as good as others have described them on this forum, the SII and LCD 4s are going to be a helluva match!

Liking what I'm hearing so far!

Derek






 
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
 
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  :D

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




 
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.
 
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 mVrms to 2.0 mVrms.
 
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. 


 
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