KT88 review in HiFi World

[Stever]

Hi,

Did anybody see the review of the KT88?

I was interested to read about the +4dB subsonic lift, as mine seems to do the same under measurement. NK implies this is readily solvable by re-proportioning some coupling caps. I have no idea which caps to change and to what to achieve the desired result!

does anyone have any ideas?

Thanks
Steve.

[John Caswell]

Hi Steve,
Well the first thing to ask is did you notice it before NK brought it to our attention, and did it affect your listening, or did you decide to measure it after the comments and does it affect you now? (Medical dictionary syndrome)
Initially I have no idea which capacitors he is talking about but reckon it can only be the coupling capacitors. I will investigate further.

John

[Richard]

Yes I read this last night and was going to reply as Confused of Nottingham

Is this WD88? I had one and it tested fine down to 20Hz, at which it was definitely rolling off.

Did the review actually say 4dB subsonic LIFT? That suggests an increase over level which doesn't sound right.

I wonder if Noel found extended LF response and feels it should be rolled off more quickly, perhaps to avoid OPTX saturation, as he was with the original 1993 HFW 5-20 amp where he fitted a 0.1uF/220K 7Hz filter to the front end?

It would be good to clarify before tinkering. The LF roll offs are cumulative and I can see at least 5 components which could be changed to influence it.

[bikerhifinut]

I find it a bit strange that this was never commented on the first time round when the amp was launched and subjected to rather a lot of coverage and description by both the designer, and NK.

Am I going to worry about it and fiddle with my WD88?

errrr.............. no!

Andy.

[Stever]

Thanks for the replies.

I purchased my amp ready built (lack of any technical knowledge). It sounded great but kept blowing valves. Sent it back to Chris who said that a mistake had been made with the wiring but all was now OK. It didn't blow valves anymore, but didn't sound as good either. I tried to contact WD but they had gone under so I just lived with it for a while. at the begining of this year I gave it to my brother in law to have a look at and he found ( and fixed) these problems : -

Dry Joint on Star Ground Connection from Left Channel Input Board.

Input Connector Screens connected at both ends of shielded cables.

Screen wire not connected at either end of shielded cable from the volume
control to the Input Board on the Left Channel.

C3 not connected to R11 on Right Channel

R26 not soldered to ground (Full HT to C16 and Heaters of valves U1 and U2 on both channels. C16 damaged as a result.

C14 loose

Volume control casing making poor electrical contact with chassis.

He then measured the amplifier and it was flat from 20-20 but, a strong overshoot was measured at frequencies from 7hz and below. Now these will be inaudible in themselves but as Richard and NK say this saturates the core. NK said this would be easy to fix in a kit amp by just re proportioning (whatever that means) a couple of capacitors. I just wonder if this is causing the lack of sparkle that I feel the amp has.

I dont have the Magazine on me at the moment but I will post exactly what he said later on this evening. The increase was clearly visible on the graph printed.

Thanks
Steve.

[Matthew Snell]

That sounds like rather too many mistakes on one build! I would like to think that this wouldn't happen under my watch. Sorry - this isn't quite "on-topic" but I felt it should be commented on. We might not have so many new projects on the go these days but I'm getting the basics right!

More "on-topic": I'd love to get to the bottom of this problem(?) and look forward to all input: keep posting!

Matthew

[Greg.]

Which edition of HFW is this review in, please?

[justin brown]

It is this months' (November).

[Greg.]

Thanks Justin.

[Richard]

Hi Greg, all,

I've moved your last post about getting HFW online to the Coffee Bar to start a new topic

http://www.world-designs.co.uk/forum...ead.php?t=6648