jc
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Post by jc on Aug 25, 2014 17:13:33 GMT
Hmmmm, I do have a 18-0-18 80va lurking in a box that I could squeeze 20vdc out of...
I've yet to hear a pair of K340s, always been curious about those.
Any particular reason I should source a LME49860 west of the pond? (RS have them for £3.21)
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Deleted
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Post by Deleted on Aug 25, 2014 21:25:45 GMT
Hmmmm, I do have a 18-0-18 80va lurking in a box that I could squeeze 20vdc out of... I've yet to hear a pair of K340s, always been curious about those. Any particular reason I should source a LME49860 west of the pond? (RS have them for £3.21) Hi Chris Check out ebay too. They are quite reasonably priced for the genuine article. How much P&P for a small order from RS ? If you simply regulated the I.C. voltage for the LME49720HA, I wonder how much of the JLH advantage in SQ would be lost ? Kind Regards Alex
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jc
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Post by jc on Aug 25, 2014 23:13:48 GMT
Hi Chris If you simply regulated the I.C. voltage for the LME49720HA, I wonder how much of the JLH advantage in SQ would be lost ? Kind Regards Alex
Exactly, hence my first comments. For that method a second JLH may be required. |
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Post by Deleted on Aug 26, 2014 23:47:57 GMT
Hi Chris If you simply regulated the I.C. voltage for the LME49720HA, I wonder how much of the JLH advantage in SQ would be lost ? Kind Regards Alex
Exactly, hence my first comments. For that method a second JLH may be required. Hi Chris Seeing you have the 18-0-18 transformer laying around, why not try it at + and -20V with a LME49860 plugged in, but with a DIP8 heatsink attached to it ? I suspect that the main advantage of the LME49720HA may have been due to it's superior thermal management.
Regards Alex |
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jc
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Post by jc on Aug 27, 2014 16:10:49 GMT
I just might give it a whirl...
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jc
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Post by jc on Oct 7, 2014 15:57:58 GMT
I just might give it a whirl... ....And I did! Out with the original 15-0-15 30va TX and in with a new 18-0-18 80va lump. Swapped out the electrolytic caps on the PSU to cover the higher rating and recalculated R1 & R2 for the regs, adding extra heat sinking to them whilst there. The wick was turned to +/- 20.1vdc. The JLH, by chance, already had sufficient size caps so I only added a couple of heatsinks to the first pair of Tosh transistors. Then finally the LME49860 complete with its heat sink was popped into the main board in place of the LM4562HA. After all testing was done and the amp reassembled I left it to burn in the new components over night before starting any listening. Was the modification worthwhile? Damn right it was! The first thing that hits you is the greater authority, accuracy and tunefulness of the bass registers, it's suddenly there in spades, dynamics are far more impressive. The ever so slightly wayward treble is under more control and there seems a much easier flow to the music, instruments and events have more space. I started to listen particularly in the midrange (the main strength of this amp) to see if anything had been lost. Really nothing that I can tell but it is sweeter toned, voices in particular sound more realistic and intimate. I'd say this modification has taken the SCHA a large step in the direction of the Aclass pre/headamp' (the other more ambitious DIY headphone amplifier featured elsewhere on this forum), showing to a lesser degree many of the same attributes. Even the rather amp-fussy K701s sound great on it now. Another plus I noticed was the massive increase in effect cast over the resistance matching switch. Before this was quite subtle but now it's very obvious in effect, just like on the Aclass. I've been throwing all my favourite testing music at it today and all has been handled with great aplomb. I have not yet found any ifs, buts, maybes or any negative point after the change so far. Not a tweak bringing a subtle gain, more like a blunderbuss upgrade.  Alex, if you are still in contact with Robert, tell him he was right about the voltage rail upgrade all along! We were all just too chicken to try  |
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Post by Deleted on Oct 7, 2014 20:40:37 GMT
Hi Chris
Very pleasing results. Unfortunately, I no longer have Robert Dwyer's email address.
Kind Regards
Alex
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Post by videoguy on Oct 8, 2014 15:05:48 GMT
Hi Chris: Glad you decided to take the plunge & bump up to +- 20vdc. I have not used a heat sink on my LME49860 as it does not run that warm, in my amp. I am very encouraged with the sonic improvement you are hearing. When I brought my SCHA on line I was blown away by the sonic results. Robert Dwyers method involved installing 15 volt regulators in the supply lines to the LM4562HA; a bit of work on the PCB.
Very glad to have my mods improvement confirmed by one who has both amplifiers for comparison. With regard to the AKG K340's; they are highly modified & have the drivers with the stronger low end. Listening to an unmodified K340 is not that impressive.
Best Regards
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Post by Deleted on Oct 8, 2014 20:27:56 GMT
Hi Chris: Glad you decided to take the plunge & bump up to +- 20vdc. I have not used a heat sink on my LME49860 as it does not run that warm, in my amp. I am very encouraged with the sonic improvement you are hearing. When I brought my SCHA on line I was blown away by the sonic results. Robert Dwyers method involved installing 15 volt regulators in the supply lines to the LM4562HA; a bit of work on the PCB. Very glad to have my mods improvement confirmed by one who has both amplifiers for comparison. With regard to the AKG K340's; they are highly modified & have the drivers with the stronger low end. Listening to an unmodified K340 is not that impressive. Best Regards Hi Bill Did you use the metal can LM4562HA originally ? We found that it ran quite warm, and the use of a heatsink on it seemed to keep the I.C. in more of a "sweet spot". Given that the DIP8 version was identical internally, it suggests that "wearing an overcoat" may have helped to prevent it from really shining . Perhaps just better thermal management with the HA version? I doubt that the use of + and -15V regulators for the I.C. only would achieve the same results as the JLH directly to the chip, which is possible with this higher voltage rated chip.. I wonder if Miguel would be interested in upgrading his ? He would need assistance though. Kind Regards Alex
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Post by videoguy on Oct 9, 2014 15:57:25 GMT
Hi Sandy: I have let Miguel know about the sonic improvement the mods have effected & offered to update his amp., if he so desires. No I did not start off with the metal can HA version first; have to be careful with my expenditures at present. When I was service mgr. for the Nashville MCI (recording equipment) dealer, they were using metal can IC's from Harris Corp. They operated them near the upper limit of the specified voltage range
& experienced thermal shutdown; this required using heat sinks to cure the problem. My next mod. will be lowering the value of the emitter resistors on the output pair to 3.9 ohms. I will test this out & see if there is anything to gain by it; if not it will be discarded.
Best Rgards
Bill W.
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Post by videoguy on Dec 27, 2014 20:20:02 GMT
I have been running the SCHA wth 3.9 ohm 1/2 watt emitter resistors in place of the 4.7 ohm units originally specified on the output transistors with no problems, for the last 3 months. This seems to make the output slightly more robust.
See my new post of the SCHA mods in this part of the forum.
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robertkd
Been here a while! 
Electronics Engineer from sunny Queensland
Posts: 111
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Post by robertkd on Jul 12, 2016 11:34:18 GMT
boo
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Post by PinkFloyd on Jul 12, 2016 12:30:46 GMT
boo
I nearly jumped out of my skin there! Good to see you Robert  |
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robertkd
Been here a while!
Electronics Engineer from sunny Queensland
Posts: 111
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Post by robertkd on Jul 12, 2016 13:03:25 GMT
Been a while got a email and thought feck it, I see there has been some changes ;0)
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Post by PinkFloyd on Jul 12, 2016 15:35:10 GMT
Been a while got a email and thought feck it, I see there has been some changes ;0) Unfortunately so Robert and not changes for the better either  How are you keeping mate? It's been ages since you were last here, I wish you'd turn up more often Brian The forum hasn't been the same without you Sid. Your barbecues on a Wednesday evening were to die for John and the spit roasts (when you were at work) were absolutely mind blowing...... Linda was always great when she had her apron on All the best, Mike. |
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