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Old 13th May 2003, 07:52 AM   #1
Prune is offline Prune  Canada
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Question Morgan Jones tube amp and equalization

I built the optimized Morgan Jones headphone tube amp with feedback from the Headwize projects (figure 10 here: http://headwize2.powerpill.org/proje...=cmoy5_prj.htm).

I already asked these questions at Headwize but nobody answered, so I'll try here:

How do I choose the optimum values for R8 and R9 to adjust feedback for my 48 Ohm headphones (the ones in the figure are optimized for 32 Ohm cans)?

Also, I want to add a passive equalizer. I downloaded the Tone Stack Calculator (http://www.duncanamps.com/tsc/index.html) but I have no idea how to choose values for the parts and input and output impedance; and even where to place the circuit, before or after the amp (I'm not using a separate preamp, but driving the Morgan Jones directly from the line out of my CD player). I also want to minimize the overall loss in decibels, even if that decreases the amound of variation possible for the frequency curve. So how do I calculate what values to put in the software? Thanks in advance for any help.
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Old 13th May 2003, 08:58 AM   #2
dhaen is offline dhaen  Europe
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Prune,

1. The consequence of adding feedback to this design has been to reduce the overall gain, and reduce the output impedance. A quote from the article says:
to select values that reduced the output impedance down substantially below 32 ohms and reduced distortion, while still leaving enough gain
He doesn't say wahat the output impedance actually is, and it's largely academic. What we are aiming at is a low source (hpamp) to a high load (headphones).
By using higher impedance headphones, the performance should be improved.

The bottom line is: Adjust them only if you want more or less gain.

2. Passive equalisation is very lossy. You will loose slightly more than the maximum "boost" obtainable by the EQ. How much boost will you go for? 10dB was used on some old valve kit, but that "feels" quite gutless compared with the tone control range on equipment with such.
Whether it will work for you will depend on whether you want a gentle tweak, or a conventional control range.
Of course you can do either if you have sufficient gain available, but the tone of your question suggested that you want to implement it with existing kit.
If I had to do it with what you have available, I would put the equaliser between the CD and the amp, at the amp end of the cable. Guestimate the CD output impedance at 1K and the amp input impedance at 100k. With those figures it will work.
You will need all the gain you can get, and I would suggest that you might revert to the "non-optimised" Morgan Jones HP amp" I think he thought he'd optimised it when he published it.

Cheers,
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Old 13th May 2003, 12:07 PM   #3
EC8010 is offline EC8010  United Kingdom
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Quote:
Originally posted by dhaen
I think he thought he'd optimised it when he published it.
He probably did, but the optimised design was backed up with careful before and after measurements that showed a clear improvement. There's no optimum amount of feedback to match to a particular load, the values used and tested for a 32 ohm load will work just as well with your 48 ohm load.
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Old 13th May 2003, 12:35 PM   #4
Prune is offline Prune  Canada
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Perhaps I won't need an equalizer. I'm about to build this DAC:
Cheap 24/96 DAC, Revision B.
and hook it up directly to the MJ. I'll have to figure out how to measure the frequency response of the setup, so I can check if an equalizer is necessary. Is there any easy way to do this? Somehow I doubt that feeding the amp output directly into my soundcard is a good idea...
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Old 13th May 2003, 08:00 PM   #5
dhaen is offline dhaen  Europe
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Prune,

I don't know anything of that DAC so can't comment.

With regard to frequency response measurement, your sound card might be useful, but it will have limitations.
1 It is easily blown-up. You'll need to attenuate the input signal.
2. The upper frequency is limited by Nyquist, and even with a sampling frequency of 48KHz, you still may not get above 20KHz because of the card's pre-filtering. Depends on how good the card is.

There is various free software available for download. Here are some:
http://www.dazyweblabs.com/shannonsoft/page3.html

Cheers,
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