KT88 review in HiFi World

[Stever]

Sorry,

Couldn't get near a computer yesterday.

NK states that the amp +4dB subsonic bass lift which he says promotes subsonic core saturation in the output transformers. Again, as he says it should be very easy to fix (just not for me). Which is why I sought guidance from those much more competant.

On another note Mathew the wiring inside the amplifier photographed looks superb. I wish mine looked like that! It's more like spaghetti junction with wires seemingly taking random paths to their destinations.

Steve.

[Matthew Snell]

Thanks for that - but I only build like that because I'm borderline OCD!

It does help with "readability" though - which is important for the manuals. I've half an idea that true "rat's nest" is preferable, but I think it's not a big deal.

Maybe a subject for a new thread?

Cheers, Matthew

[John Caswell]

Matthew,
You must be borderline OCD as you have the letters in the wrong order. A true OCD would have them thus CDO
I also think the levels would have to be quite high to promote core saturation at 20Hz.

John

[Matthew Snell]

I was thinking just that John - but didn't dare say so in case I was displaying profound ignorance.

DOC (Matthew)

[Richard]

Well I mentioned checking down to 20Hz but I'm not sure that f was mentioned as the problem in the review.

"+4dB subsonic bass lift" suggests to me a rise from flat (or at least from the roll off it was on) at subsonic f, which I'd read as below 20Hz. So would this be a LF instability or PS resonance do you think?

Stever, was there a response graph which showed it? The amp sounds great but someone really should have a look at the mag I suppose.

Confused of Nottingham

[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

[bikerhifinut]

Theres an old saying................

"if it ain't broke, don't fix it"

Ask yourself, did you have an issue with this hitherto unperceived problem?

Then go and put some music on and stop fretting about it if the answers no.

I am not sure whether the graph showed the lift as an instability problem, I assume that alludes to the issue of "Motorboating". I may be wrong but I am sure that issue was addressed in the design of the amp in the coupling between V1 and the phase splitter where theres a network "slugging the dominant pole".

If its a rise in the frequency response then if you think your source may have substantial subsonic content would a simple High pass filter set to roll off below 20 Hz across the input work?

Excuse my ignorance you educated chaps out there if I have got the wrong end of the stick here.

Andy.

[Stever]

Well I certainly did have a problem which is why I sent my amp to be checked.

The subsonic lift (mine occurs from 7hz), was but one of the problems found. The others have been cured. I had mentioned this in a prevous posting (around May this year), but, with no one else seemingly aware of it I had to ignore it. Now it's been measured and reported elsewhere I thought perhaps the good folk here would know how to fix it.

I guess the problem I have is that I just don't think my amp sounds the same as the demo amp I heard at Chris's and because it's had all these faults with it I'm probably a bit paranoid. To my mind, mine seems either bass heavy or lacking in top end. Either way the balance doesn't seem as I remember it and certainly not the same as when the amp was first delivered to me! Although it was definately faulty then because as I said, it kept blowing valves!

I suppose what I should ask is if there is anyone who owns a WDKT88 who lives near me (london essex borders) who would mind me coming along with my amp to compare the two. If it turns out they sound pretty much the same then I can forget all about the measurements and my memory of the sound I heard before, in the knowledge that the amp is doing what it is supposed to do.

Is anyone available for that?

Thanks
Steve.

[bikerhifinut]

Sorry Steve,
I didnt mean to infer that there was nothing wrong with your particular amp.
As you said, yours came with some "pre-installed" faults, by the sound of things caused by a sloppy build.
This subsonic issue, I suspect will affect all the amps in the range.
Looking at the response curve again in HFW I suspect its a frequency response issue, but as I said earlier the educated boffins on here may know differently.
When i built mine, I distinctly remember it seeming to have an accentuated emphasis at the LF end of life. I put it down to interaction with my Monitor Audio floorstanders and my smaller new listening room. certainly with the Usher S520's i used in the small room, and now My Kudos Cardea1 I dont find it an issue.
I also changed the gridleak resistors on V1 for 1meg instead of the 100k which I think gives a better impedance match to a 50k to 100k pot when fed from a passive or as an integrated its own volume control. (Mine has the Volume pot removed, unnecessary in a power amp role.).
I wonder if trying a filter network across the input would give an idea if theres an issue with the sound? You could at a pinch just stick a 10nF capacitor in series with the input as a rather crude High pass filter and see what that does? Thats what most of the rumble filters in days of yore were on preamps such as the Leak "point1" and varislope range.
Once again, I apologise if I gave offence Steve.

Andy

[Stever]

Andy,

Not to worry, No offence taken. To be honest if I hadn't had all the problems I have I would probably be of the same opinon as you. Having said that, although it's a great amp, if for the sake of a couple of caps we can make it even greater, then why not? I mean that's the reason everybody is continually tinkering. To try and get the last Nth degree of performance. With different caps, wires and so on.

In terms of the LF problem. It was described to me by my brother in law after measurement as Low frequency instability with a large overshoot from 7hz producing primarily (I think) 2nd order distortion. I didn't understand a word so just tried to look more intelligent than I felt and nodded wisely.

Cheers
Steve.