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Old 18th November 2009, 09:01 PM   #981
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you could replace the CCS with a resistor, as you did with the earlier SE CFP, and not see much quality degradation.
I did this, using a 91 ohm resistor and saw a huge increase in THD in the simulation over the CCS. Why is that?

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Old 18th November 2009, 09:27 PM   #982
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If we want to get exotic, we could go with a CCS diode...

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Old 18th November 2009, 09:48 PM   #983
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John,

A huge increase. Be more specific. What harmonics in particular?

Hugh
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Old 18th November 2009, 10:05 PM   #984
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John,

A huge increase. Be more specific. What harmonics in particular?
Everything.
Resistor:

117.PNG

CCS:

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Old 18th November 2009, 10:36 PM   #985
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John,

You are talking of gross clipping, at around 6Vp due to current limiting!! This is an SE amp, of course this is a problem - Andrew points this out earlier. It is, like all topology, a compromise..... but most people will not be listening at 3.07Vp on a 24R can. High Z cans are no problem to about 10Vp, in fact.

My comment about distortion obviously relates to the rated 1Vrms, the 0dB standard, where you get a distortion profile hardly any different to the CCS loaded one. Obviously the CCS confers flexibility for all sorts of cans, and that is the purpose. I have attached the distortion profile for a 91R resistively loaded output at 20KHz signal into 24 + 10 ohm load at 1Vrms, which is 1.414Vp.

I thought you said you were leaving?


Hugh

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Old 18th November 2009, 10:52 PM   #986
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This is Hotel California - nobody can leave. And a good thing too.

The resistor CCS is the simple option, allows swings closer to the upper rail. But if we were to go passive, why not an Inductor - it's the most efficient option ? Or is the dc bias current going to make the Inductor an expensive monster ?
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Old 18th November 2009, 11:00 PM   #987
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About that inductor - perhaps we could make a "plasma" switch, which sends it into oscillation, effectively becoming a tesla coil at several MHz, burning out all that plastic garbage from the headphones and turning them into full-blown plasma transducers...?

Sounds feasible to me...

Anyways, it would be interesting to use something other than a real CCS, just to make the circuit a bit exotic, but... (BTW I'm all for the inductor if it proves feasible; it's more linear than a CCS at least)

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Old 18th November 2009, 11:41 PM   #988
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Originally Posted by AKSA View Post
John,

You are talking of gross clipping, at around 6Vp due to current limiting!!

I thought you said you were leaving?
I come and I go,
I'm pleasant and I'm nasty,
And apparently I'm smart and I'm stupid...

You are right, of course, I didn't even bother to check if it was clipping - just assumed that it wasn't and ran the FFT.

With that said, input sensitivity is very low, less than 1Vpp to before clipping might be too low, unless you have something attenuating at the front that I don't know about?
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Old 19th November 2009, 02:08 AM   #989
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Good point, Gareth.

John,

The pleasant and smart we can handle. The stupid is forgiveable, the nasty is challenging. OTOH, we can all ignore you if you turn into Freddy Kruger.....

It would be nice to have an amp which sounds 'nice' when it's clipping. That would be quite a coup. Any ideas? We could start with tubes!

Yes, in front of the amp is volume, balance, tone and crossfeed. Tone has gain of 6dB, all the others severely attenuate and this is the reason for the highish gain and resulting low overload margin.

Getting all circuit blocks together is the next step in the design process.

Nico, can you advise please on a crossfeed circuit at the front of the amp, rather than at the end?

Kean,

We would need at least an inductive reactance of 34R at 10Hz, yielding a 6dB point at this frequency and 125mA. This equates to L = Xl/2piF, which is 540mH, with a DCR giving say 2V drop at 125mA corresponding to 16R. It would need to be gapped, increasing core size, to prevent saturation. This would be large, perhaps an EI cube around 3" or more on a side. Cost would be on the order of $US80 minimum. It is not practical, unfortunately. Furthermore, it would have capacitive losses unless very carefully wound, leading to loss of HF response.


Cheers,

Hugh
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Old 19th November 2009, 02:17 AM   #990
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It would be nice to have an amp which sounds 'nice' when it's clipping. That would be quite a coup. Any ideas?
I remember Wavebourn had some ideas on this, use of diodes in some strategic locations if I remember.
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