I have now found my old dt880s and a 12v 1.5A PSU from an old router and decided to fire it up to see if it works.
I am working at my desk with no music so I dug out my old iriver H140 and used that as a source - success but it sounds crap. Loud but crap.Unlistenable to crap. Muffled, flat, no soundstage etc.
I opened her up to see if there are any obvious bulging Caps etc but no. Tried another basic psu, still no change.
Hmm. So I re-read the thread and saw that Canjunkie used the "record from tape" connection not the "playback from tape" connection I was using.
I have no idea what that means but I Assume I have bypassed something or other but what a change no transformation - a heck of an improvement. The 600 ohm Beyers are actually making music! Simple Rock e.g. Chris Rea thumping out quite nicely but classical music is not doing well. Currently listening to the Hallelujah Chorus and it is not bad but I am only really getting an over view rather than the detail I know is there. I must see if I can find a nice piece of Bach Organ music and see how that copes...
I am really looking forward to watching your new upgrades for these Mike and giving it a go.
This is the QED headpohone amp from 1988.... it was simply known as the "QED Stereo Headphone Amplifier" and was superseded by the QED MB-45 in 1992. It is essentially the same amp as the MB-45, both utilised the LM1877 Dual Audio power amplifier.... the only real difference is the layout is slightly better in the MB-45. The MB-45 resides in a cheap plastic enclosure whereas the QED Stereo Headphone Amplifier has a nice metal enclosure..... if anything, the MB-45 would have been a downgrade considering it is essentially identical to the QED Stereo Headphone Amplifier apart from residing in a cheap plastic enclosure. I think the MB-45 was touted (at the time) as being an upgrade over the QED Stereo Headphone amp but it was pretty much business as usual but in a cheaper enclosure.
You are pretty much limited as to what you can do (tweak wise) with these QED amps as the LM1877 is what it is and you can't go under the bonnet and mess about with it Having said that, the SQ really does benefit if you have a tweak of the capacitors and here is what I have done.
Before the tweaks:
After the tweaks:
I got rid of the 470uF axial capacitor and replaced it with a 1000uF radial type...... simple to do, I simply drilled a 1mm hole and soldered the negative leg of the cap to ground..... no need for that long axial 470uF cap with it's long leads (think inductance!) much better to have a radial cap of 1000uF in this position.
The output coupling caps were 100uF electrolytic (directional) caps..... there is zero voltage across these caps so I replaced them with 470uF Nichicon MUSE non polar types. The 470uF caps will let the bass dig deeper (especially with low impedance 'phones) the 100uF directional caps were not doing the corner frequency any favours and quite frankly sounded like shite in the signal path With the 100uF caps a 32 ohm load was rolling off @ 50hZ....... with the 470uF caps a 32 ohm load was rolling off @ 10hZ so 100uF was totally inappropriate for a 32 ohm load. Perfectly OK for a 300 ohm load but pretty naff if driving a 32 ohm load..... the 470uF caps also help higher impedance 'phones dig deeper (as low as they can go) so it's a win win situation.
I replaced the 1uF electrolytic caps with 1uF Film caps...... who on earth designed that board?....... the pitch for the radial electrolytic caps was WAY too wide! I decided to use some of the established (hard wired) lead from the original 'lytics to mount my 1uF film caps on... seemed the most logical way to do it.
Replaced the 2 x 10uF no name caps with ELNA Silmic ll (Japanese silk fiber) caps for no other reason than I know they do the job (superbly) in this position...... they really do.
I whacked a Vishay class X2 suppression cap across the input (the blue one).... again, a 1mm drill job.... works wonders.
Removed the stock ELGEN (?) 2200uF / 25V cap and fitted a Panasonic NHG 3300uF / 35V (105C) in its place.
That's pretty much it..... an hours work and a totally different amp sonically..... different in a VERY positive way...... sounds amazing!
Post by freddypipsqueek on Jul 7, 2017 13:28:29 GMT
Super job Mike - I'll keep my eye on ebay for one.
Black Box 2, Benchmark DAC2D, Arcam irDAC, SoTM SMS-100, Sony CDP227ES, QA550, Systemdek IIX & mods, Chiara & Andante, WNA MK3 & Cascode, Music First Phones, Audiophileo, Singxer F1, SoTM USB cards, Mission Cyrus One & 751's, Headroom The Little, HD580, HD590, HD600, HD555, HD700 K612. Too much wire !! - Kimber, DIY silver stuff and Stereovox cables - AND a bottle of wine & a lot of Mods.