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post #16 of 125

Brian actually mentioned this to me when I bought my new actives. I haven't tried it yet (mainly because I'm returning them) but I've got my eye on another pair and I think I'll be giving that a shot.

 

Quote:
Originally Posted by runeight View Post

An alternative is to use the #2 jack with a mating cable from phone plug to stereo RCA jacks. This is not a bad solution because a preamp needs some output impedance if it's driving cables. The raw LF output is less than 1ohm. The #2 jack with the resistors would have a 50R output impedance. This is better for a preamp.

 

BTW, you guys may not know that the 50R resistors at the #2 jack are Vishay zero capacitance, zero inductance resistors. They are custom made for each LF production run.

post #17 of 125
Thread Starter 
Quote:
Originally Posted by rgs9200m View Post

Any comments on the TKD 2511 attenuator in the LF? Did your listening tests find that this is a good volume control? Did you consider any others, stepped or non-stepped? Thanks.

 

The performance of the LF with the TKD pot seems to be at a very high level. It is hard to say how much improvement, if any, another type of pot would make.

 

I did consider other attenuators and pots and decided that the 2511 was a very good continous pot, made by a company that pioneered carbon plastic pots and sliders for mixer consoles, and would give the right cost/benefit trade for this amp. It is possible to put other pots into the amp, but I have not heard from anyone who may have done this.

Dr. Cavalli gained notoriety with his first DIY amplifier projects. His success has blossomed into Cavalli Audio, a world leader in amplifier design.
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post #18 of 125
Quote:

Originally Posted by runeight View Post

 

By correctly, I mean with a switchable output for either headphones or preamp.

 

That could be accomplished by the headphone jack itself. The mod would consist of little more than replacing the existing jacks and adding a pair of RCA's on the back.

 

se

post #19 of 125
Thread Starter 

Yes, I forgot about internal jack contacts. Good idea.

Dr. Cavalli gained notoriety with his first DIY amplifier projects. His success has blossomed into Cavalli Audio, a world leader in amplifier design.
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post #20 of 125

Alex, are you able to comment on the cause of the faint high-pitched noise that some of us have experienced?  I know you've said that, in some cases, it was due to faulty tubes.  But where the issue was not caused by tubes, have you been able to discover the source of the issue and what the solution is?

 

I'm on vacation and away from my LF for a few weeks, so I haven't had enough time to see whether the noise appears with both my Siemens and the stock tubes--so far, I've only heard the noise with the Siemens.  The noise generally went away when I unplugged my headphones and plugged them back in (to the same output), at least temporarily.

 

Thank you and thanks to dB for helping me try to find a solution to this problem.  I really love my LF, aside from this issue and maybe one other minor complaint:  The volume is too high with my LCD-3 and PS Audio PWD DAC set anywhere at 60% or higher of its output.  This may be primarily due to my DAC's output being too hot, but I'm rarely able to set the LF's volume at higher than 7 or 8:00 if I'm using the #1 headphone port and have my DAC's output set to 60 or higher (which is what most users of the DAC seem to recommend).

 

Is there a way to add a gain reducer without sacrificing resolution?  I'm able to get a wider volume range with the #2 output, but I slightly prefer the sound of #1 with my LCD-3s.

post #21 of 125

Any chance there will an upgrade path from LF v1 to LF v2?  (Other than buying a whole new amp?  LOL)

post #22 of 125

Maybe I'm missing something, but what's the big deal about the LF (or any headphone amp, for that matter) having preamp outputs? Don't most people with a speaker setup already have a preamp?

post #23 of 125

Many people don't want an extra box, or the ease of just flipping a switch to speakers. Head-fi is also a gateway drug to speakers for a lot of people. 

 

Unfortunately people at the level of the LF are used to great sound, and spending another 2k+ on a pre that would sound just as good isn't something they want to do. 

 

There's a lot of reasons but those are the ones that make most sense to me.

 

Quote:
Originally Posted by mwilson View Post

Maybe I'm missing something, but what's the big deal about the LF (or any headphone amp, for that matter) having preamp outputs? Don't most people with a speaker setup already have a preamp?

post #24 of 125
Thread Starter 
Quote:
Originally Posted by dj nellie View Post

Alex, are you able to comment on the cause of the faint high-pitched noise that some of us have experienced?  I know you've said that, in some cases, it was due to faulty tubes.  But where the issue was not caused by tubes, have you been able to discover the source of the issue and what the solution is?

 

I'm on vacation and away from my LF for a few weeks, so I haven't had enough time to see whether the noise appears with both my Siemens and the stock tubes--so far, I've only heard the noise with the Siemens.  The noise generally went away when I unplugged my headphones and plugged them back in (to the same output), at least temporarily.

 

Thank you and thanks to dB for helping me try to find a solution to this problem.  I really love my LF, aside from this issue and maybe one other minor complaint:  The volume is too high with my LCD-3 and PS Audio PWD DAC set anywhere at 60% or higher of its output.  This may be primarily due to my DAC's output being too hot, but I'm rarely able to set the LF's volume at higher than 7 or 8:00 if I'm using the #1 headphone port and have my DAC's output set to 60 or higher (which is what most users of the DAC seem to recommend).

 

Is there a way to add a gain reducer without sacrificing resolution?  I'm able to get a wider volume range with the #2 output, but I slightly prefer the sound of #1 with my LCD-3s.

 

Yes, I can tell you what I know at this point. This low level, but audible noise seems to be in a small number of LFs. I had never heard it during burn in because it usually happens many hours after that. I use AD2000s to listen to the amps before shipping. They are very sensitive headphones and I can hear everything, including the basic noise floor of the LF from jack #1.

 

Unfortunately for Morbid Toaster (who lives nearby) his LF began to show this noise too. He let me have the amp for a while and I was able to hear, for the first time, the noise signature. I was also able to probe around a bit and do some experimentation.

 

The first thing is, this is a very difficult problem to pin down because it is so intermittent. I had the amp on for days, listening repeatedly. Sometimes it was there, sometimes not. When I heard it, sometimes it would disappear as soon as I started probing with the scope or a DVM. Sometimes I could get it to stop by gently rapping the tubes with my fingernail. Sometimes not. It doesn't happen with all tubes, just a small number of them. But, since it stops when I knock the tubes it is definitely an internal mechanical issue. Possible to stop with tube dampers.

 

So, what I know is that some small number of tubes exhibit this problem. I also believe that it is not solely related to the tube, but is in some interaction with the problem tubes and the circuit. I don't know what it is about the problem tubes that causes this interaction. And, on the face of it, the circuit is behaving properly (and measures ok both with DVM and scope).

 

Because the problem is intermittent (sometimes not showing up for days) and because it often disappears when I start to probe around, it is very hard to identify the exact source or cause. Thus, the only thing to do is to try some reasonable steps and then to listen for a long time.

 

I have purchased some low noise JJs for Morbid Toaster's amp (tested by the seller). I've also added tube damping rings (two per tube) to his amp and returned it to him. He is kind enough to run the amp normally to see if the noise returns. This is the first step. I'll know more after Morbid either says it's back or if it goes for a week or two without the problem.

 

Thanks for your patience on this.

 

There is not an easy way to change the gain of the LF. Initially, I thought about giving you several NFB options which could be used to set the gain. But SQ is really affected by the amount of NFB. The LF has low open loop gain so it has low NFB. Adding more NFB to reduce the gain would probably change the sound signature. So I opted for a single gain. Do you know what the output of the DAC is? The gain of the LF is about 8.5 and its maximum peak output voltage is about 22V. This means that about 2.5V peak or about 1.8V RMS will saturate the amp.

Dr. Cavalli gained notoriety with his first DIY amplifier projects. His success has blossomed into Cavalli Audio, a world leader in amplifier design.
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post #25 of 125
Thread Starter 
Quote:
Originally Posted by LionelH2 View Post

Any chance there will an upgrade path from LF v1 to LF v2?  (Other than buying a whole new amp?  LOL)

 

A couple of LF owners have asked this. I'm thinking on it. ph34r.gif

Dr. Cavalli gained notoriety with his first DIY amplifier projects. His success has blossomed into Cavalli Audio, a world leader in amplifier design.
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post #26 of 125

Aloha to everyone here.  Nice to have the thread!

post #27 of 125

As some of you know, I have been helping Alex in trying to locate this intermittent noise concern. He very adequately summed up the problems with isolating it. I did manage to hear it too, although at some point the noise was coming from cellular interference and it then took me a while to clear out my test bench of most things that could be generating noise. At one time I was fairly convinced that it might be coming from the potentiometer and grounding it seemed to resolve the issue. BUT, this was not the case and it appears that it was just part of the intermittency. I tell you this as there have been a few recommendations to ground the volume control and at this point it is not the final solution so it would be best not to try anything until a true solution has been identified. Changing tubes almost invariably stops the noise, even just changing the tube position ie from 1 to 2 or 3 to 4 and vice versa. I have confirmed repeatedly that the amps perform at spec and that it is not a circuit failure , the power supplies are quiet on the scope and the ground is consistent throughout the amp ( ie no noise and no differential from one point to the other ).   I have also tested the tubes that were noisy on my tube tester and given the limitations of the tester, they all look good. An inexplicaple electro-mechanical interaction does make sense as the noise can be quiesced by taping (gently) a tube without touching any part of the amp with conductive capability. I am hoping that tube dampers are the solution as there are many wonderful flavours to choose from, good old rubber to audiograde ceramic aids for more fun discussion. 

 

..dB

post #28 of 125
Thanks for the helpful reply, Alex. It seems like you are taking all reasonable steps to figure out what's going on with the noise. My experience matches what you described--the noise sometimes takes days to appear and there doesn't seem to be any identifiable cause. When I get back to my LF, I'll see if tapping on the tubes makes the noise go away.

But I'm glad to know you're checking it out and that this seems to only affect a few units. Please let us know if you guys learn anything more.

As for my DAC, its maximum output is 2.8v when the volume is set to 100%. But I can never set it anywhere near that--55-65 is where I have to set it in order to have the LF at 7:00 or higher.
post #29 of 125

popcorn.gif

post #30 of 125
Quote:
Originally Posted by runeight View Post

 

A couple of LF owners have asked this. I'm thinking on it. ph34r.gif

Alex, thanks.  I love my LF, and have no real interest in additional features other than sonic improvements.

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