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Old 6th July 2014, 04:27 PM   #1
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Default HELP: tuning for AKG601 + MJ mini tube amp

I built Morgan Jones Mini Tube Amp with feedback exactly according CMoy drawing. I am using it with AKG 240 Monitor 240 Ohm and feel that the amp has difficulties in driving the headphone. It shows distortion on the lows if we set the volume higher and is also suffers for fast jazz music. the situation is better only for source with higher amplitude.

This week I bought AKG 601 and found out that though the impendance of 601 is half of the 240 (120 Ohm) the situation is worse with 601

Does anybody has the same situation like I have now? I think i have two options now:

1. Take off the feedback resistor. But it's not a guarantee that the problem resolved.

2. Change the feedback resistor value. Reducing 100k to 50k might help. Another thinking is to reduce resistor on grid size from 10k to i.g. 5k

Many thanks before fore any advice,
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Old 7th July 2014, 04:27 PM   #2
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K240 Monitor were 600ohms. I'm surprised that the MJ cant drive them cleanly. Exactly how loud do you want them to go?

The MJ clone was never *really* known for its resolution. If you used the funky back to back transformers it is basically a guitar pedal, with B+ sagging on peaks. If you used a more conventional transformer (Mains:heater & B+) the amp holds up much better to loud sections (and recovers MUCH faster if you overload it) but still has a slightly soft resolution.

In the time it took you to write that post you could have removed the feedback resistors... It's not a very labor or time intensive thing to test. Look up Alex Cavalli's articles on the MJ - his schematics show the amp with a pot in the feedback loop so you can play with feedback on the fly.

Morgan Jones with a K601 is a bad match if you listen at higher volume levels. The K601 are not particularly efficient, and will quickly run the MJ clone beyond its abilities. If you used the funky back-to-back transformer setup the amp is truly hopeless with those cans.

That being said, the K601 sound much nicer than people give them credit for*. If you want to stick with tubes, look into transformer coupled designs, with a dedicated 120ohm output winding (150 ohms is close enough). If you want SS, add some output impedance to taste.

*601 is 100 less than 701. The 701 are basically worthless, simple math tells us that the 601 are 100 less.... Totally wrong. The 601 are a hidden gem. Plus its easier to find off-the-shelf transformers that work with ~120ohms than 62.
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In most systems a line stage is not really necessary, and no line stage will almost always be better than a bad line stage.
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Old 7th July 2014, 05:41 PM   #3
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I am with Nikongod on pretty all accounts... the MJ clone is in fact not the most revealing amp... which from my knowledge can mostly be attributed to the fact that it is an OTL amp and that you are passing the audio signal through 'lytic output caps.
I built one many years ago and even converted it to LED bias of the first stage (with a little help of MJ himself). This improved things a bit but I eventually sold the amplifier (and found the beauty of transformer coupled amps ).
Vincent, is your source 2V? I was using Sennheiser HD600 at the time and didn't have a problem to drive them to sufficiently loud levels... but Senn's are more efficient than your AKG's...

When you want to stay with tubes try a single tube transformer coupled amp... else try "The Wire" SE-SE...
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Old 7th July 2014, 06:29 PM   #4
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My last attempt with a MJ style amp had grid bias and plate CCS on the gain tube I also un-balanced the output stage to reduce output impedance (use a larger plate resistor, like 450ohm) at the expense of voltage swing, which was a neat change. They are fun little test-beds, and the price can be very low, but they are easy for a DIYer to beat for a bit more money.

I don't think the problem is in the output caps although I do agree that different caps sound different.

I think the problem is that we are driving these little tubes in ways nobody ever planned for them to be used and they just can't handle it. While we are not technically violating any design limits with the MJ it's just not right. It has been my experience that more powerful/higher voltage (6SN7, 5687, 6H30, etc) tubes do "hold up better" to this abuse, but the overall amp gets much more expensive QUICKLY.

Once we start talking about more money, a simple transformer coupled amp offers some really nice advantages.
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In most systems a line stage is not really necessary, and no line stage will almost always be better than a bad line stage.
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Old 8th July 2014, 01:45 AM   #5
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Well, I think I agree with you guys. I will try to tweak a bit on this amp and better spend more money on transformer coupled amp. Temporarily I can use headphone output from the DAP with some coloration.

I still try to stick on a tube first. The sound MJ mini tube delivers is very sweet and cannot matched non of my solid state amp (people say they are not designed to give good headphone amp quality).

BTW: I try Creek OBH amp with Beyerdynamic T1. It sounds right. I think I need to ask the shop owner to let me try the creek + 601. Maybe I will switch my thinking from the tube.

Thanks,
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Old 11th July 2014, 04:24 PM   #6
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Ordered the wire bal-bal and se-se. Wow the quest of perfect sound starts again.

Aditionally also one AK100 II.

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Old Today, 09:30 AM   #7
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Quote:
Ordered the wire bal-bal and se-se.
Good choice! It doesn't glow and it is tiny but a killer of a head amp... best solid state I owned so far

Good luck with building all your new toys!
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Old Today, 10:20 AM   #8
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The k601 is a very hard beast to drive well, but has so much potential! The best amp I've heard driving them is the Millett Hybrid Minimax....that's quite a loooong build, but sounds phenomenal.
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