[gaboo]

Disclaimer: I seem to never suffer the 'not loud enough' problem with my sources. So...

Question: If the source already has opamps on its output (not "true" line-out?) that provide sufficient voltage gain, is there any advantage to using an amp that contains a voltage gain stage? The alternative would be to use just an output (i.e. current) stage: buffers etc. which seems to follow the KISS principle.

I realize that a multiloop design is better in theory, because you get the extra feedback. So, in an ideal world you'd have no op amps on source outputs. If the source already has amped outputs, what is the best choice? Remove them, and use a multiloop amp? Live with them and use an amp with unity voltage gain stage? Or simply use some buffers?

 

[Edwood]

Hmmmmm.

I can think of a few headphones that would need it.

AKG K1000, K501, Senn HD650, ..............

-Ed

 

[gaboo]

Quote:

Originally Posted by Edwood Hmmmmm. I can think of a few headphones that would need it. AKG K1000, K501, Senn HD650, .............. -Ed

Score -1: off topic.

 

[peranders]

Sometimes you need gain and sometimes not. I depends of signal source, the recording itself and the headphones. My Sennheisser HD545 need gain of 1.5-4 in order to get loud enough from my DAC.

 

[gaboo]

Duh, it seems the question is not clear, so I'll rephrase it:

When you definitely do not need to make your cans "louder", isn't it better to leave it out of the circuit?

 

[peranders]

Yes..... only if your output can drive your headphones with low distortion, not likely with 32 ohms phones from a regular output.

 

[guzzler]

Consider something like a MOSFET follower, ie the Szekeres Class-A amp. This provides plenty of current gain but it just below unity voltage gain. This effectively buffers the output from the source with zero voltage gain.

g

 

[sanaka]

Quote:

Is the voltage gain stage (ever) redundant in a headphone amp?

I think the answer is an unqualified YES. I finally finished my first big amp project, a Pimeta, a few weeks ago. I eagerly hooked it up to my computer/monitoring/main house listening setup, which was to be it's home probably 90% of the time. Guess what? Two volume controls: one on the preamp, one on the headamp. Kind of a Homer Simpson moment.

My preamp will output ~1.5v, so unity gain is fine for my Grados.

In this thread at Headwize, fa-schmidt gives a superb rundown on how to properly figure gain requirements.

I'm now planning to build one of these to up the current. Basically just a buffer like the Szekeres, but appeals to me more.

Peace,
Sanaka

 

[gaboo]

Thank you all for the replies.

In case you are curious about my source & cans: EMU 1212m (can output +4dBV so it's pretty "hot") & ATH-A900 (40ohm). Since the source is balanced, and the cans going to be rewired as dual mono, I'm planning on using 4 buffers (L+,L-,R+,R-).

Still nobody said anything about the benefits of multiloop... I see basically two competing configs here:

• keep the opamps on outputs of 1212m, and use current buffers
• remove the opamps on outputs of 1212m, and use a Jung multiloop amp

The second config appears to have the added benefit of the outter feedback loop. Is the 2nd config worthwhile?

 

[Voodoochile]

Have you ever read Walt Jung's article on this? It's pretty informative, and may answer not only your question, but provide other information as well. It's not that long a read, either.

 

[gaboo]

Well, that article doesn't go into the local vs global feedback issue which is what my dillema is basically about. This issue seems far from decided, e.g. see this recent debate.

Btw, I found this theoretical paper on multistage local feedback optimization. Wait until KG read this.

 

[drewd]

Quote:

Originally Posted by gaboo Well, that article doesn't go into the local vs global feedback issue which is what my dillema is basically about. This issue seems far from decided, e.g. see this recent debate.

The condensed version of local versus global feedback is this: Local feedback is effective at reducing amplifier oscillation and distortion and tends to have little negative sonic impact on the sound, particularly regarding phase problems.

Global feedback is effective at reducing distortion and output impedance, but can introduce phase shifting, causing a loss of accuracy that usually shows up as a smeary sort of sound. It's not all bad, and the degree of negative impact depends on how much feedback is used.

Walt Jung's feedback system uses very high amounts of local feedback and just a bit of global feedback. Like most things, I don't think that you can just flatly say that global feedback is "bad". It does bad things, yes, but it can also correct worse things, although there is certainly a school of thought that says that those "worse things" ought not to be designed into the circuit in the first place!

My feeling (and it's certainly not the gospel...not even close!) is that if you can avoid using global feedback, then don't use it. If you need it, then try Walt Jung's system. The small amount of global feedback is going to have a much smaller impact on the the signal than a single global feedback loop.

And I'm not going to put word's in Kevin's mouth

, but I'll bet that I can guess what he's going to say!

-Drew

 

[gaboo]

Thanks drewd! Very nicely put in layman's terms.

For anyone else looking at this, tangent has more notes on the (now defunct) meta42 page.

 

[sanaka]

Seems to me it's basically a matter of how well you like the out put of your EMU 1212m (not sure what that is exactly). If you totally dig the sound but just need a current boost to drive your cans, then that's the simplest route. Otherwise if you additionally think it's worth it to totally replace that voltage stage, then go for that

I guess listening to something like a Pimeta or PPA will give you some idea whether a multiloop topology would be an improvement versus what you have.

Peace,
Sanaka

 

[rickcr42]

If you like the sonic qualities of your preamp you can add a unity gain high current output buffer to the line out using the gain of the preamp and the muscle of the buffer to drive cans.This "follower" does not even need to have a volume control,it is provided by the preamp.But what you WILL need is some method of switching the preamp output between buffer and speaker amp or you will need to turn of the amp every time you listen to headhones and unplug your headphones when listening to speakers.A simple A/B dpdt switch is fine

if you are using the record out for your headphone amp or you use a passive line stage you are most often better off with a full headphone amp which includes a volume control

But there are no definitive answers that cover all possible systems and combinations so the above is just a general guideline