Shane Hanify Posted July 26, 2010 Posted July 26, 2010 Gidday folks, being the inquisitive 'hey if it blows up you probably weren't meant to do that' type, I was eyeing up my passive preamp as a headphone amp. I reckoned that as long as you didn't wind the volume up too high (CPD - 2.5V output!) then it would be fine. Dug out some patch leads (you can never have too many adapters) and gingerly put it to the test. I'm er, well blown away by the SQ. It is utterly utterly open. Somewhat bass shy (passive remember?) but phenomenally good to listen to. It is jaw droppingly good. Unfortunately it makes my main 2ch system sound like it should be in the dark ages by comparison. Ah well, swings and roundabouts. Someone please pipe up if I'm putting my beloved phones in any sort of danger. They certainly don't sound stressed but I've not exactly got them cranked. If there's no danger of making things go POP then I'm seriously considering making a dedicated passive HP preamp (with a limiter). PS the vol pots on the passive pre are 50 ohm. The headphones are 55. Cheers, Shane.
Ernie1553552694 Posted July 26, 2010 Posted July 26, 2010 You wild and crazy man Shane. Seems like you did the maths, took the risk and it paid off. Not something you hear about on Head-fi. You might start a craze if you posted there.
Shane Hanify Posted July 26, 2010 Author Posted July 26, 2010 Haha Ernie, I'm already a member of far too many forums that I'm not active on. Right now I should be sleeping but I'm enjoying the music too much. Cheers, Shane.
Owen Y Posted July 27, 2010 Posted July 27, 2010 Intrepid Shane ... There should be no reason, if matched well, why your passive should be 'bass shy'. What phones? You're using a 100k pot IIRC? Cheers, Owen
Shane Hanify Posted July 27, 2010 Author Posted July 27, 2010 Gidday Owen, It's not so much bass shy, as drier than an active giving you the illusion. All of my active amps have more weight. A well designed passive with good components has none of these issues, but I'm running carbon pots (vol) and a bog stock plastic input switcher with some entry level panel mount RCA's all point to point wired with some old copper 14g speaker cable I had lying around. My initial one was 100k pots but I got more headroom with the 50 ohm ones. At least I think they're 50. One ea for L + R channel. Phones are AKG k171s. Cable is my own. Cheers, Shane.
Owen Y Posted July 27, 2010 Posted July 27, 2010 Hi Shane, Some notable audio types favour only carbon film pots. Do you mean 50 ohm pots or 50k ohms?
Shane Hanify Posted July 27, 2010 Author Posted July 27, 2010 Gidday Owen, My bad. I meant 50k. But reading the earlier post I made on the passive I see they're actually 25k. Of course, not having any gain means that you can only listen to high output (eg CDP) sources through it. Last night I gently increased the volume bit by bit and ran out of travel. Still wasn't particularly loud but the disc I was listening to was mastered at a lower volume than modern pop ones. It was still at a fairly decent volume. So, the question is, (and if I do my maths correctly) does the CDP see a 25k + 55ohm load? and does it really care? Next step : build a tiny but fairly transparent amp. Handily enough I have the little firestone I can plug into it, but that's hardly the last word in see through. Cheers, Shane.
Owen Y Posted July 27, 2010 Posted July 27, 2010 Hmmm...(excuse the long rave, it is worth studying as passives can be impressive if everything is set up right, but the sweet spot is often VERY narrow). At full vol, the CDP is looking at 25k + 55R in parallel = ~55 ohms. Not so good. At half vol the CDP is looking at >10k probably (log pot)...OK. From the CDP point of view, it's probably best to have at least 500ohms load, ie. not at max vol. (Assuming the CDP has output imped 50-100ohms??) However as far as the phones are concerned, they are low imped (55ohms) so should be fed from a v low imped source, eg 10 ohms or less. You probably won't get that with the 25k pots + CDP. A lower imped pot may help (say 500 ohm) but depends on the CDP output imped... ~50 ohms?? So... driving h'phones (even higher imped ones) is a tall ask through a passive setup. However, it sounds like the transparency though the phones is stunning
Shane Hanify Posted July 27, 2010 Author Posted July 27, 2010 Thanks Owen, that's exactly the sort of thing I was after. I see I'll need to buy a few bits and bobs and cobble together a testbed, a couple of 1/2 k pots may be the go. CD specs : Output impedance 56 ohm : Output level at 0dB 2.2 Vrms This is going to be fun (assuming of course I don't brick anything). Cheers, Shane.
Owen Y Posted July 27, 2010 Posted July 27, 2010 Worth a try. At max vol, the pots should make no difference (as far as h'ph source imped is concerned)... but an interesting exercise nevertheless. You could go as low as 250k pot even. Downside is that there may be some loss of volume.
Shane Hanify Posted July 27, 2010 Author Posted July 27, 2010 Gidday Owen, don't you mean 250 ohm ? Cheers, Shane.
Shane Hanify Posted July 28, 2010 Author Posted July 28, 2010 Drat. The smallest log pot I can find is a 1k. I guess linear would work but I'm not overly happy with that. Schematic is easy though. RCA (input) > pot > 1/4" jack. Done. Cheers, Shane.
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