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Old 23rd September 2009, 02:08 AM   #41
Bigun is offline Bigun  Canada
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Aspen Headphone Amp
Default pcb and enclosure size

Is it a good idea to aim for a single pcb, keep it simple and reliable ?

If so, we'd want to pick a low profile pcb mount transformer and allow this to set the maximum height. Choice of capacitors and small heatsinks have to fit within this limit.

Can we fit this into standard 1U case height ?

e.g. Digikey pc-mount Toroid, 15VA is 26.3mm above the pcb surface.
TE70053-ND
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Old 23rd September 2009, 02:11 AM   #42
ostripper is offline ostripper  United States
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The heck with the regulators , go battery. 2 small 13.6v gel-cells with associated trickle chargers would address this issue completely .

I have a home-made gel-cell lm317 charger in my workshop and it has been with me for 4 years , very reliable. I have seen this done with pre-amps (this is what I might do with it - this circuit)

OS
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Old 23rd September 2009, 02:45 AM   #43
MJL21193 is offline MJL21193  Canada
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Quote:
Originally Posted by ostripper View Post
The heck with the regulators , go battery.

I have lots of these:
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Old 23rd September 2009, 02:58 AM   #44
ostripper is offline ostripper  United States
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Quote:
By aksa - The trick with loop gain is useful, thanks to GK and Andy_C, very useful way to figure stability.
Here is another one . It does work with the "older" models (2sc/a) , works even better with fairchild models (ksa/c ksp ). You drive a 2uf - 10uf cap at the OP with a "fast" square wave at 10k, also add a second -2.3v voltage source and short the input cap (attachment 1 is self explanatory). The first output plot is driving the standard resistive load. The second is with a 2uF OP cap and 10pf lead comp. (slight overshoot) , The third is with the original 5pf lead comp. (1.1mhz UGP) , here is where it really gets "ringing' but still does not break out in sustained oscillation. This little amp is REAL stable .. but this is just a single stage OP. If you do this blasphemy on a triple (with more complex models) the "cause and effect" of total loop gain /UGP AND the parasitics/higher gain in the more complex current stage (EF2 or triple) is quite evident.

One can also step (.step command) through different Cdom's , biases and do this with different device models to see the effects on distortion harmonics and stability.
I feel quite confident because when scoped in real life , you see nearly the same responses.
(attachment 5 is the fft , nearly similar to the 55 !! )
OS
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File Type: gif aksaHFSWnormalload.gif (13.9 KB, 475 views)
File Type: gif aksaHFSW2UF.gif (16.1 KB, 456 views)
File Type: gif aksaHFSW10UF.gif (14.9 KB, 445 views)
File Type: gif aksaHFFFT.gif (22.0 KB, 87 views)

Last edited by ostripper; 23rd September 2009 at 03:04 AM.
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Old 23rd September 2009, 03:42 AM   #45
AKSA is offline AKSA  Australia
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IKO,
Thanks for the good explanation, I can see your point, particularly with the sensitivity to noise. Further, the very low energy levels of headphones would clearly mean transient response is not the first priority, too.
I agree about shunt regulated power supplies. I have done a few myself, including one using a 5881 pentode with a TL431 in the cathode circuit which sounded wonderful. If we use 40mA quiescent, same for the shunt element, I'd reckon since that will give just shy of 120mW (5.5Vpp) into 32R cans. Might even be possible to reduce rails to 12V.

We should try to ensure at least 15Vpp output for 2K cans, too, although that's only 14mW......

OS,

Thanks a lot for your stability input. This is certainly very stable, nice to see the sims bear it out. Can I talk privately with you on this?

Hugh
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Old 23rd September 2009, 03:51 AM   #46
ostripper is offline ostripper  United States
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Quote:
Can I talk privately with you on this
But of course !
(PM on ostripper "home")

John , those 18v makita's would be good for 24 hours of headphone bliss , that would look cool .. to retrofit an old dual charger into a headphone amp !!

Last edited by ostripper; 23rd September 2009 at 04:08 AM.
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Old 23rd September 2009, 01:00 PM   #47
forr is offline forr  France
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Hi Hugh and Bigun,

It's also a didactic projet, easily analysed and where effect of some very basic modifications could be easily experienced.

First, I would try it in the inverting mode : it mainly diminishes H2 but not very significantly harmonics higher than H3, it should be subjectively detectable.

Back to the non-inverting mode, I think of a curious way to achieve the current source for the LTP : it uses a bootstrap circuit coming from a divider of the output voltage to the splitted resistor of the tail, as it was shown some time ago for a power amplifier by a transistors manufacturer, but I do not remember the reference.

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Old 24th September 2009, 12:01 AM   #48
AKSA is offline AKSA  Australia
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Thanks Forr!

This same circuit is analysed in an electronics text, 'The Art of Electronics', Horowitz/Hill, published by CUP, 2nd Edition 1989, p. 236-238. I claim no originality at all; this is a stock, easy circuit, with very good performance, something unexpected in this complicated world. The secret with most of these simple circuits is good compensation techniques.

Any circuit with lower levels of H2 is interesting, but not mandatory. H2 in a feedback amp is very low, and not strictly audible until at least 1%. A good way to do this, however, is to use a single transistor input stage, with feedback taken to the emitter.

Cheers,

Hugh
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Old 24th September 2009, 01:46 PM   #49
Bigun is offline Bigun  Canada
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Aspen Headphone Amp
Guys,

We have the basic amp but it doesn't feel as if a decision has been made on the psu. So we're in a holding pattern...

Hugh - what does the 'boss' say here ?
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Old 24th September 2009, 07:04 PM   #50
tessier is offline tessier  Canada
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Mr Dean

Great headphone amp.

Can we use a suck-up capacitor across R11 and R12 to lower the output transistors switching noise ?

Thanx

Paul

Last edited by tessier; 24th September 2009 at 07:21 PM.
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