KT88 review in HiFi World

[rickychance]

Hi all, I also read the review & was quite surprised that 3rd order harmonics dominated the distortion graph rather than 2nd. Is this down to phase splitter choice/implementation? Anybody know how to tweak it so 2nd order would dominate?

[John Caswell]

Why would you want to do that? I doubt very much that you would hear any difference
All of these KT88 amps that I have built and measured have had such low THD that it was difficult to decide whether it was 2nd 3rd 4th 5th or nth.

John

[Richard]

Originally Posted by Stanton

Hi All
I've just been reading the comments on the review. The frequency graph was pretty much flat to 20Hz then rose with a peek at +4db around 6Hz. Being an absolute beginner I eagerly await any suggestions to which capacitors and what values to try.

The main thing about the review I think, is how well the amps were received. Lots of good things were pointed out about the sound: openness, lucid, detailed, fast, rhythmic etc.. Even that the XL was '...something a little special.' All of which we already knew of course but nice to see in print all the same.

Frank

Hi all,

Well this sounds like motorboating / LF modulation, and Stever too confirms 7Hz. It's probably at a low and inaudible level in real terms and you could take a look at your woofer cones to see if they are flapping! My amp was quite honestly the quietest valve amp I've owned. Worth mentioning too that it was XL spec and imho quite sparky sounding, tight bass and not shy in the treble. Perhaps the new owner will let us know how he's getting on with it and what he reckons to the overall sound balance.

Most likely the designer will have seen it and deemed it reduced to insignificant level though it must in theory cause some saturation of the OPTXs.

Easiest thing to try and reduce it would be to lower the gain at those low frequencies by raising the LF roll off point of the coupling at caps C5/C6. This shouldn't affect the rest of the amp at all and is quite safe to try.

Unfortunately I no longer have mine, and my test gear stops at 10Hz anyway so I can't do this, but just swap out the 0.47uF's for 0.22uF's and test and have a listen. You could even try 0.1uF's it won't do any harm but at some point it will start to measure and sound bass light.

The roll off of the coupling involved is between C5/C6 with gridleaks R12/R13. As built this is 1.5Hz. Going to 0.22uF will make it 3.3Hz and 0.1uF would make 7.2Hz. Bear in mind the roll off is cumulative with other roll offs through the amp so that won't be the overall LF response and empirical testing would be the way to confirm.

Maybe Stever could swap them and see if his Bro would test before and after?

[John Caswell]

Hi all,
I disagree slightly with Richard in that none of the amps I have had/built have shown any signs of LF instability or motorboating and nothing was at all obvious on connecting them to my old transmission line L/S.
My test gear like Richard's stops at 10Hz (metering/analyser) and 3 Hz (generators), so I am a bit stuffed there.
Quite frankly I would have thought that if there is/was core saturation it would tend to show up in sine/square wave testing and the level would have to be fairly high, and not so much, if at all, on real music. This of course assumes there is anything at below 10Hz in the signal or the preamp passes it.
I regret I don't have an amp to try different coupling C's on, but if anyone lives near me with one I would be happy to experiment.

John

[Richard]

Originally Posted by John Caswell

Hi all,
I disagree slightly with Richard in that none of the amps I have had/built have shown any signs of LF instability or motorboating and nothing was at all obvious on connecting them to my old transmission line L/S.
My test gear like Richard's stops at 10Hz (metering/analyser) and 3 Hz (generators), so I am a bit stuffed there.

John

Hi John,

No disagreement, I didn't hear or find anything with my amp either . However, if Noel and Stever found it we must accept that and try to assess the significance.

If it's there it must (in our experience of the amp) be at a very low level but it will do no harm to try and lessen it for anyone who fancies a go. It would be interesting to hear how it goes

[vinylspinner]

Hi Guys,

I am the new owner of Richard's kt88 power amp and can only say that I am very happy with the overall sound from this amp, plenty of bass and nice clean treble, it is the best amp that I have used to date.

John, I can loan you my spare one if you wish to have a play with the caps etc, just pm me and I will drop it off at your place.

Nigel

[John Caswell]

Hi all,
Well I have been through the article and can find no reference to LF instability or motorboating, and as I wrote before none of the amps i have handled have exhibited any of these problems, so i can only assume that Stever's amp was a one off oddball occurence.
Vinylspinner (Nigel) has kindly offered the loan of a KT88 to experiment on, so I will take him up on this, all I ask is don't expect instantaneous results, as thes things take time.

John

[Richard]

Hi John,

That's good (borrowing Nigel's amp) and it will be interesting to see what you make of it.

I'm hoping to get the mag today when out shopping. Does it attribute the lift to anything at all then? What do you think it would be other than LF modulation? Extended LF response down to a lower frequency than might be needed I could understand but it is "lift" that I do not

[Richard]

Hi Nigel,

Glad you like it, I was wondering which one you'd end up using

[Richard]

No joy getting the mag yet, not in local Smith's and I won't be let out again til Friday, but will look again then