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Old 28th December 2004, 05:55 PM   #31
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The audio part
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Old 28th December 2004, 05:56 PM   #32
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The PS part
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Old 28th December 2004, 06:08 PM   #33
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And this is the power supply I think I'll go with. According to the simulation the peak current is ecceeded for about a third of a second with inrush current (hits a max of around 1.3A peak, but settles down to about 400mA peak), but I guess that this assumes the rectifier is preheated and ready to go, and won't start working gradually, so I think this should be ok. I'm tempted to have a go at a regulated B+ supply (the supply in my mini zen headphone amp is LM317 regulated, low voltage and easy to make and I'm happy with the sound of that) but I think I'll go with this for now. Ripple simulates at 4uV which ought to be ok. I've left plenty of room at the back of the case (on my piece of paper anyway) for a mains filter, some DC heater supplies and possibly a regulated B+ circuit in the future.
Thanks,
Steve.....eagerly awaiting student loan
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Old 29th December 2004, 11:38 AM   #34
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What does the 100 ohm resistor do? The input cap is small enough not to present a hazard to the rectifier, so you don't need a resistor there for that reason.
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Old 29th December 2004, 07:04 PM   #35
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I have a convenient source of cheapish Hammonds for that voltage (275-0-275), and without it B+ ends up a bit high, 322V instead of 297V with it. Is this OK or are there more elegent ways of doing this?
Cheers,
Steve
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Old 30th December 2004, 04:55 PM   #36
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Hi there.......I use Sony MDR750S headphones......with 40ohm Z per capsule that headphone circuit is definitely not for me. The THD would rocket.

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Old 30th December 2004, 06:04 PM   #37
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Hi Rich,
Could you elaborate a bit? Could make the difference between wether I use Grados/Senns/Beyers. I understand itll be low powered into a low impedance load, but lack experience to work out wether this will be a problem.
Cheers,
Steve
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Old 30th December 2004, 09:10 PM   #38
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Yes.....the headphone impedance market is about as non standard as 2nd hand cars are......from 20 Ohms til several Kohms.

I don't have any detail on Grados, but for Beyer,Sennheiser they all vary from model to model...... if you are going to use these with cathode follower o/p then one has to pay attention to the loading Z for min thd as this method doesn't have symmetrical drive.; implying a low Z load will clip the signal assymetrically.
Canford Audio has a pretty fair range of headphones with details; www.canford.co.uk
I use a sim circuit to the one you provided except I use ECC88 at 17mA into cap/parafeed step down tranny 3:1...for my main amp. The matching is better. The preamp is capable of 3V o/p at 0.01% THD @ 600 ohm output. I don't use phones oin this o/p simply because it would create distortion.
Although your subject is phones, I can recommend balanced o/ps for main amp use.
For running headphones with a lower Z ratio tranny say 8 ohms the ratio would be too high then the o/p level is way too low, and to bang it up to a couple of volts would push the cath follower stage into excessive distortion. (then one really use a SE power o/p stage and finito..)
Instead for better performance I use a LM1875 for phones to ignore user-specified phones. With the ultra low o/p Z the op amp isn't fussy what load it sees...Obviously the cathode follower running into Beyers DT109 with 400 ohms would be alot happier than 8 ohms with the DT100. You'll get sound alright but the drive will clip in the "tube off" condition. I'm not trying to pry you away from cathode followers for such apps but to see their limitations.
Beware of the old adage
<<It might sound worse than what you originally went out to acheive >> especially on the THD side.

For near-direct coupling in cathode follower use hi-impedance headphones for best results.

There seems to be an awful number of circuits using the "follower" electroytic cap as the cathode coupler.......not for me..., a startup mute is imperative incase you pull the wrong plug out of the wall and quickly re-insert it.......the plop isn't on for ones lug'oles nor pricy phones.

Only my opin.
rich
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Old 30th December 2004, 10:18 PM   #39
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Cheers for the advice. I think I'll probably go for Beyer DT990, they're 250ohm I think so they should work fine. My plan was to build it like this for now, and then experiment with using output transformers and different topologies later when I have some more money. I already have an amp that's perfectly capable of driving low impedence headphones here and I'm very happy with it. I fancy another project now though, hence this. Thanks again though, I'm sure I will end up transformer coupling it anyway, just not yet.
Cheers,
Steve
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Old 31st December 2004, 07:41 AM   #40
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Transformer coupling is the finesse.......I've been using 'em for decades.......toiling with magnetics is also an interesting black art, but doesn't come cheap. www.sowter.co.uk for hints.
The series cap in a parafeed has to be good quality when feeding into a transformer. Alot of parafeeds have composite cores of mumetal/SiFe, as normal SiFe has poor permeabilty at low signal excitation levels so part Mumetal is used....the drawback is any leakage current from a leaky series cap can start magnetising the core. For those who want to get the best performance a high value low leakage film cap is required and this can be large component.
More info.... www.claritycap.co.uk aren't cheap but the sound is good.

rich
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