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  #1  
Old 16th January 2020, 10:11 PM
daxtojeiro daxtojeiro is offline
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Default K5881 Mk1 mods

I have a K5581 Mk1
I built this a lot of years ago and have had it as my main amp ever since.
A few years ago I carried out some mods to it and I'm now wondering if I did the right things, so would like your expertise:

1) Linked out the input capacitor - good idea or should I have kept it in place?

2) Fitted a pair of 1uF capacitors in series to replace the 0.47uF coupling capacitors - should I have simply fitted one 1uF, as I see the "Tweaks for Mk2" trials a Wondercap 1uF, or are 2 in series better as that keeps the values similar to the original design?

3) I couldn't get a 310pF feedback capacitor, so fitted a 330pF. Is that acceptable, as I still can't find a 310pF?

4) I used a polystyrene 330pF cap for the feedback, is that an acceptable type for feedback, or are there others I should be looking at?

This week I have ordered some tantalum resistors (the 2 input resistors and the feedback resistor) after reading a lot of info on this forum. Looking forward to having a listen to what difference they make

thanks for any advice
Phil
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  #2  
Old 16th January 2020, 10:28 PM
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Greg. Greg. is offline
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Default Re: K5881 Mk1 mods

I suppose the question is, after you carried out the alterations, how did it sound to you? Did you think your mods made an improvement over standard or otherwise?
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  #3  
Old 16th January 2020, 11:14 PM
daxtojeiro daxtojeiro is offline
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Default Re: K5881 Mk1 mods

Hi Greg,
thanks for the reply.
I moved shortly after doing the mods, so probably didn't get any time for a like for like comparison.

Now I have got rid of those pesky kids I have setup a room to sit in and enjoy my hifi, so figured I would ask this forum for advise on if I did the right mods, or if its worth doing something different
Phil
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  #4  
Old 16th January 2020, 11:22 PM
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Phil Y Phil Y is offline
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Default Re: K5881 Mk1 mods

Hello Phil,

1, The input capacitor is a safety precaution, in most cases sources won't have any DC on the output. It is down to your discretion as to remove it or not.

2, Generally, increasing the size of a coupling cap a bit will do no harm, I would probably
have just used a single cap.

3, Obviously the designer settled on 310pF as the best value but 330pF isn't far off. You
could fit 2 150pF caps in parallel to make 300pF which is closer but really, unless you
have a signal generator and oscilloscope to check the square wave response it may not
be worth worrying about.

4, Polystyrene are a good choice for a fb cap.

The above are general comments, I am not particularly familiar with the K5881.

Let us know how you get on with the Tantalum resistors. I tried them in my Phono stage some years ago, personally, I didn't get on with them. In hifi reviewer speak, I found them to sound too "brightly lit" but many people seem to like them.

Phil.
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  #5  
Old 16th January 2020, 11:58 PM
Richard Richard is offline
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Default Re: K5881 Mk1 mods

Hi Phil and welcome, do you have a Dax Cobra? I digress but mostly agree with Greg and Phil and would say,

1) the input cap will block DC from a source but that's unlikely to be needed as the onus is usually on a source not to have DC on its output. What may have been over-looked (in removing it) is that it forms a low roll off filter, 0.22uF into 100K = -3dB 7Hz, to avoid saturating the OPTXs should there be LF signals from rumble or music.

2) these roll-offs are cumulative as the signal passes through the amp chain. The 0.47uF into 470K is a very low coupling already at -3dB 0.7Hz so will not help in this respect. Instead of reducing it further by trying 1uF I'd go the other way and use 0.22uF into the 470K gridleaks which will give the classic 1.6Hz coupling and help restore the LF filter a little.

3) 310pf or 330pf will be fine, you will not hear the difference as it is only correcting the high frequency harmonics visible on the corner of the SW.

4) polystyrene would be the high quality cap of choice in this position but polypropylene would be fine too.
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Old 17th January 2020, 03:12 PM
daxtojeiro daxtojeiro is offline
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Default Re: K5881 Mk1 mods

Hi all,
thanks for the replies. Looks like I haven't done anything that would effect it too much then. I will continue to enjoy it as it is and trial the new tantalum resistors.

Richard, yes I do have a Cobra replica (Dax), built it a couple of years after doing this amp,

Phil
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  #7  
Old 19th January 2020, 01:13 AM
Richard Richard is offline
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Default Re: K5881 Mk1 mods

Hi, yes if you're not hearing any distortion or getting motorboating all should be well. Great car, I bet that took longer to build than the 5881! Best film I saw last year was Ford vs Ferrari, I imagine you'll have seen it but well worth watching for anyone even vaguely interested
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  #8  
Old 19th January 2020, 11:00 AM
John Caswell John Caswell is offline
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Default Re: K5881 Mk1 mods

Phil wrote
"Richard, yes I do have a Cobra replica (Dax), built it a couple of years after doing this amp,"
I think I may sit down on the kerb and cry, I am extremely envious

John
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  #9  
Old 21st January 2020, 01:25 PM
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Ali Tait Ali Tait is offline
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Default Re: K5881 Mk1 mods

Quote:
Originally Posted by Richard View Post
Hi, yes if you're not hearing any distortion or getting motorboating all should be well. Great car, I bet that took longer to build than the 5881! Best film I saw last year was Ford vs Ferrari, I imagine you'll have seen it but well worth watching for anyone even vaguely interested
Yes, great film, one of the best I've seen in a while.
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  #10  
Old 27th January 2020, 11:18 AM
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Baggy Trousers Baggy Trousers is offline
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Default Re: K5881 Mk1 mods

Quote:
Originally Posted by John Caswell View Post
Phil wrote
"Richard, yes I do have a Cobra replica (Dax), built it a couple of years after doing this amp,"
I think I may sit down on the kerb and cry, I am extremely envious

John
Very many years ago, about the time when the G500 was the cartridge of choice, for a short period I owned an AC "Ace" with a Bristol (pre-War BMW) engine. This was an exceptionally pretty car (like its "Aceca" sister) and so much nicer than the vulgar thing it became with Carroll Shelby's desecration of John Tojeiro's beautifully balanced design. Forgive me if I don't join you on the kerb, John.

Last edited by Baggy Trousers; 27th January 2020 at 11:36 AM.
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