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Old 23rd August 2005, 10:02 AM   #11
rjm is offline rjm  Japan
Richard Murdey
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The only difference is the additional resistance to ground, which does I admit have a the benefit of making the attenuation less dependent on the headphone impedance. Still, in my view -26dB isn't necessary. -6 to -10dB should be enough, taken either at the output, the input, or the feedback loop.

Adding a crossfeed filter somewhere should nicely bring the attenuation down, and be a welcome feature besides.

Ultimately it must be said that I dont much like listening to headphones, even though objectively the HD600s sound very nice. I suspect I suffer more than most people from the disorientation caused by the blobs-in-the-ear effect. So while what I heard through the gainclone was perfectly acceptible, it didnt really have me jumping for joy... and I doubt it was significantly better than a well built ChuMoy.

The only headphone amp I could dig was (my modded version of) the Szekeres SE MOSFET design, both archived at headwize. It had enough treble roll off and added 2nd harmonics to compensate for the in the ear thinness of headphone sound. A decent amount of crosstalk probably helped, too.

I guess what Im trying to say is if heaphone amps float your boat the LM3875 is a hi-fi option, especially for low impedance cans. Hi-fi by itself, however, doesn't do it for me when it comes to headphone listening. Sadly, "Faking it" is probably more important.

-R
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Old 27th February 2010, 12:36 AM   #12
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hi, if i want to make headphone amp..what else do i have to buy beside the LM3875 dual mono kit?

i saw in the kit instruction user manual, each pcb consisting the opamp is connected to L&R RCA. how should i connect if i want to have a 6.3mm headphone out?

and i also want 1 set of L&R RCA input channel. sorry for the noob questions..
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Old 20th February 2011, 12:36 PM   #13
SimontY is offline SimontY  United Kingdom
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Sorry to dredge up an old thread but I thought this more correct than starting a new one...

I spent all of yesterday adding a headphone section to my DIY preamp (which is based on Pedja Rogic buffer circuit). I used a pair of LM3886 on tiny heatsinks running off a nice transformer of 17 - 0 - 17 @1A (26VDC+-). I used non-inverting configuration using 1K to ground with 33uF cap and 10K feedback resistor. I used a 1K input resistor and 20K to ground (for extra protection against the deadly floating input).

At first I noticed 40mVDC on the outputs so I added a DC blocking cap on the input. There's now basically zero DC offset but I *am* a bit concerned that if I get a fault condition my HD600 'phones will be fried instantly. Should I add DC blocking to the output or will it wreck the sound?

The sound? Well I don't own a headphone amp so have nothing valid to compare this to but it sounds great to me running off a test cd player (lightly modified Marantz CD52). After more testing I'll get it in the system running off my FULLY modified Arcam Alpha (TDA1541) player. The sound is I suppose quite dark with great bass and dynamics and is fairly open and harshness-free. You can, of course, just keep going higher with the volume. I did a rough calculation showing max output into these 'phones is probably about 1W if accidentally turned up full. I'm sure it could break them but it's unlikely to happen by mistake.

I don't really like the idea of using output resistors as I expect it would harm the sonics, but is there something real to be gained there?

My test headphones (Creative ear buds) show huge hum and noise which is inaudible through the Sennheisers so I'm not sure what to make of that. The discrepancy is enormous though.

Just wanted to share this little project, which took me about 14hrs non-stop yesterday!

Simon
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Old 20th February 2011, 12:44 PM   #14
SimontY is offline SimontY  United Kingdom
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I wired up the muting circuit using 10K resistors which are joined together and then feed the switch that connects them to -Ve. Tagged off this is a blue LED (via 4 x 1.2K resistors in series [to stay cool]) to indicate the 'phones are unmuted. You toggle the switch to mute and the light goes out, I'm well pleased with that lol.

So far I've not used any bypass caps, which I realise is not really ideal, though the psu leads are short. The PSU is a big packaged bridge, cheap 1800uF Nichicons, bypassed with a 10uF film cap per rail.

The tiny steel heatsinks get very hot (you can just about keep a finger on) but are no hotter after 30mins than 2mins so I think it's perfectly safe. I didn't realise the LM3886 could get so warm at idle - just maybe I have an issue like oscillation - any opinions? Is the gain too low?

Thanks for looking and I'd appreciate any opinions from chip amp aficionados or headphone users, particularly if I've done something stupid.

Thanks,
Simon
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Old 20th February 2011, 02:07 PM   #15
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Here's the circuit, it's only slightly different from the application note.
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Old 20th February 2011, 02:28 PM   #16
AndrewT is offline AndrewT  Scotland
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the gain is almost at the minimum that National recommend.
I think you would see quite a bit of overshoot on faster signals when so much of the signal is fed back.
The reduced feedback when using gains set to between 25 to 30times gives more accurate reproduction of the input signal.
This is why many builders say they prefer a gain of 28times to other gains they have compared.

It is just possible that you may have some oscillation. Have you scoped the output?

How loud is the noise when feeding headphones? I would expect 95dB/mW headphones to show up noise much more than 95dB/W speakers

The 33uF input cap is completely mismatched to the 33uF NFB cap.
3.3uF input and 100uF NFB should sound quite different. You would need to experiment to find which is nicer.
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Old 20th February 2011, 05:20 PM   #17
SimontY is offline SimontY  United Kingdom
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Andrew,

Thanks for your useful input. National say a gain of 20 is normal but I can't see a minimum. I think it's well-known the chip is not supposed to run at unity gain and I sort of guessed ~10 would be a fair compromise between stability and being too loud and noisy (gaining only to attenuate). As this works quite well I could try a slightly higher gain and still have some use of my volume control. To mitigate the volume range issue I could attenuate the input a little more..

I've not scoped the output as I don't have a scope handy, nor do I know how to use one.

The background noise is very low on my Sennheiser HD600s and very loud and clear on cheap nasty headphones. On my Senns I hear just a touch of hum with no music playing and the noise of the source (CDP with Class A FET output stage - quite noisy itself).

If I use an input cap can I disregard/remove the NFB cap? I have some CHEAP 10uF film caps I could use on the input. I may just try this as it may sound better than the Black Gate Nonpolars I'm using.

I've put this amp into my system now and listened to a couple of CDs from my proper CD player. It does indeed sound excellent. I'm quite sure it would disgrace various standalone headphone amps, at least when running these Sennheisers. That's not to say the sound is perfect, it isn't. But then, is headphone listening ever very good??
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