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Xiansheng 708B headphone tube amp - Page 28

post #406 of 413
11/6/07 at 8:36am
Well very interesting ....

I had my 703b for about a year but was not really that taken with the sound so the unit just sat on one of my shelves.....I run the dt990 dt911's...

early on I swapped the nasty vol pot for a 50k alps blue which sounded great over the mains hum!

but the other week I was asked about the 703b ...which then peaked my interest. I found a pair of matched 6922 spares for my opera cyber800's..... so dropped those in ..wow the chinese valves supplied must be rather poor... I then tried to find a 6n3 !

In the end I bought 10 of fleabay from albainia or somewhere...I was not expecting much but when they arrived in less than a week I dropped one in .....

I still suffer from ac hum but where that was intruding on the music at anything up 3/4 volume this has been reduced to a small ammount on the right channel.

So i've now got a headphone amp thats actually nice to listen to !

With that in mind and reading this thread I had a sort through my spares box and found a 100k stepped attenuator [dale vishay resistors] ....I had to increase the size of the hole again .... but once in and fitted using the original cables for the time being ...the sound has opened up considerably ....

it always suprises me how much the vol pots actually take away from the music signal. I then fitted a new knob turned alu rather than the nasty plastic one that was fitted from new.

So rather than wade through ....any quick and simple ac hum fixes ?
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post #407 of 413
11/6/07 at 10:00am
Quote:
Originally Posted by zanash View Post
early on I swapped the nasty vol pot for a 50k alps blue which sounded great over the mains hum!
...
With that in mind and reading this thread I had a sort through my spares box and found a 100k stepped attenuator [dale vishay resistors] ....I had to increase the size of the hole again .... but once in and fitted using the original cables for the time being ...the sound has opened up considerably ....
...
So rather than wade through ....any quick and simple ac hum fixes ?
The stock volume pot is where the chassis is tied to signal ground. Since you've replaced it you'll need to reestablish that ground connection and doing so should get rid of the hum.
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post #408 of 413
11/7/07 at 5:20am
Actually the replacment has dropped the hum from intrusive to only heard between track and only on the right channel... if that makes any sense. But yes will get my test bits out and give it a careful prod .....I'll implament the chasis earth too which seem like a good idea....

do you recomend floating the signal grounds to the earth ?
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post #409 of 413
12/3/07 at 2:04am
Quote:
Originally Posted by nc60865 View Post
I am using DT990 250ohm version, and listening to ape file, barely notice any difference, so desperate!
may be your soundcard. Augidy is really not the greatest as it is resampling everything to 48Khz. the quality of the HW resampling is not very good.

my XS is being fed by a Bluegears B-enspirer card. i've rolled in Amperex 6DJ8 Orange Globe Halo Getters as well as a late 80's GE Jan 5670W (1960 Sylvania JAN 2c51 on the way).

i definitely notice a differences compared to my solid state amp. while the XS is not as clean/tight, I find it to be more enjoyable/musical and open. this is my first tube amp though, so it may just be that i like the tube sound.

with the stock Chinese tubes, i found everything harsh. even after rolling in the new tubes, things were a bit harsh and bass got quite distorted if turned up too loud. After about 20 hrs usage, i find the sound to be smoothing out for the most part.

all of this is with a pair of Denon D2000's.
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post #410 of 413
1/10/08 at 9:25am
hey,

just got my amp, and am starting to mod it, alps blue velvet is on plus some kiwame resistors replacing the stock ones. However i am trying to get a 0.22uf cap, and I am having trouble finding one that i like. I see you mentioned the mundorf ZN caps, but i can only find them in 250V dc, anywhere i can get them at higher voltage? or would you put two of them together? I know where i can get some auricaps but if you think that ZN caps would be better. What about stock mundorf caps? there are some 450V parts in 0.22uf that seem to fit perfectly.

ALso is it worth replacing the electrolytic caps? and what would you recommend replacing them with?

I have the 240V uk version and the plug they ship was damn right cheap, replaced it with an EMI shileded plug and replaced the socket by an anti-emi filter socket :
Audiophonics - Filtre secteur IEC anti-parasites-EMI 230V 10A

Also i have taken the decorative tube out mostly for heat removal, and i want to rotate it so the glass panel can show a usefull tube.

Anyways it'd be great if you could get more some capacitor recomendations.

Cheers,
brendan
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post #411 of 413
1/28/08 at 12:02am
Could somebody tell if it is possible to remount the original knob on alps blue velvet with some tweaking (I am aware that it doesn't fit)?
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post #412 of 413
1/28/08 at 4:42am
like i said yes, either drill the knob or get a screwdriver and enlarge it. read my post
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post #413 of 413
7/29/08 at 7:15am
Hello everyone Iam a newbie.
Just Bought a 708B secondhand.
It has got some hiss coming from the left channel
i think it is a dodgy Valve
Looking forward to doing some of the mods
Starting with the pot and input output resistors
Best Wishes Martin
Thanks for putting the mods up on here
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