KT88 review in HiFi World

[Richard]

Originally Posted by Matthew Snell

Yes, we definitely intend to!

Reluctant to be in touch with Andy Grove, or to challenge Noel's results, until we have managed to replicate them. Or not, as the case may be. As John said previously this may take a little while.

In the meantime it's worth noting that I've had several reports from owners who are unable to replicate and none who haven't. (Apart from Stever's post above, that is.)

I shall start publishing the score soon.

Matthew

Hi Matthew, John, all,

Noel has found something factual he feels worth commenting upon. It's not likely he will be wrong and so any challenge would just be a case of how you see it affecting use in the real world.

John should test the review amp as it may simply have a fault. It may be a common fault due to a wiring method, missed connection, or component such as TX batch, that several builders repeat and so may be in some amps but not others. Is it just on Std or XL build etc.

If you don't find a problem you still can't deny Noel found one so I'd ring him first for clarification of exactly what you're looking for and what he thinks will fix it.

[Matthew Snell]

Richard

I agree with you on every point!

I may have come across as being over-relaxed on this issue: trust me, it hasn't been like that. Thank you for the comments, and expect a report soon.

Cheers, Matthew

[John Caswell]

Hi all,
Today I had a look at the amp that Noel had for review and checked it over.
I concur with what Noel found except the rise in my case was only 2dB, but this is most likely due to test equipment variations as my generator only goes down to 3Hz, and I have not got such fancy expensive test equipment as Noel.
Results were flat to 20Hz then as follows
15Hz + 0.15dB
10Hz + 1.0dB
9Hz + 1.2dB
8Hz + 1.5dB
7Hz + 1.8dB
6Hz + 1.9dB
5Hz + 1-9dB
4Hz + 1.9dB
3Hz + 0.5 dB - probably generator drop off.

I did a bit of juggling with components and passed the findings to Matthew who is going to chat to Noel about it and then post on the forum. Until then be patient!
Personally I think this is making a mountain out of a mole hill as I do not believe that there is much information below 15-20Hz to cause core saturation except perhaps warp from vinyl but I think that Phono II/III roll off a bit higher. I will have a look at my Phono III and comment on here.
Just had a pm from vinylspinner (Nigel) and he is getting his amp to me for the weekend so I will have another to measure.

John

[Richard]

Originally Posted by John Caswell

Hi all,
Today I had a look at the amp that Noel had for review and checked it over.
I concur with what Noel found except the rise in my case was only 2dB, but this is most likely due to test equipment variations as my generator only goes down to 3Hz, and I have not got such fancy expensive test equipment as Noel.
Results were flat to 20Hz then as follows
15Hz + 0.15dB
10Hz + 1.0dB
9Hz + 1.2dB
8Hz + 1.5dB
7Hz + 1.8dB
6Hz + 1.9dB
5Hz + 1-9dB
4Hz + 1.9dB
3Hz + 0.5 dB - probably generator drop off.

I did a bit of juggling with components and passed the findings to Matthew who is going to chat to Noel about it and then post on the forum. Until then be patient!
Personally I think this is making a mountain out of a mole hill as I do not believe that there is much information below 15-20Hz to cause core saturation except perhaps warp from vinyl but I think that Phono II/III roll off a bit higher. I will have a look at my Phono III and comment on here.
Just had a pm from vinylspinner (Nigel) and he is getting his amp to me for the weekend so I will have another to measure.

John

Hi John,

Does the output actually increase (ref 1KHz) below 20Hz rather than rolling off? That must be a fault surely?

I usually do a quick sweep either side of 1KHz at 8V p-p 8R (1W) and response usually falls below 20Hz if not before. The amp I had certainly sounded that way and I feel sure I'd have noticed it on test.

I then check full power into 1KHz, 20Hz and 20KHz and valve amps are always down on power bandwidth as expected

For the low stuff, do you have a copy of HFN 015 test disc or similar? That has 0dB 10 second sine waves at 2, 4, 8, 17 Hz which can just be played through a cdp into the amp, no problem with a generator then, given a flat cdp (which can be checked first into the scope).

[John Caswell]

Hi all,
The figures shown are as measured with response flat (±0.1dB) from about 30KHz down to 20Hz. then the response gradually starts to rise as shown up to a peak of +1.9dB at 6-4Hz.
Doesn't necessarily mean a fault, could be all sorts of things, wrong choice of coupling components, transformer inductance and the like limited by cost possibly.
Having a CD doesn't necessarily mean the the CD player can handle the low frequencies and also give the same output as at 1kHz.
I have the Denon test CD which gives amongst many other tracks a frequency sweep of 5Hz to 22.5kHz (naturally) which I tend to use for all sorts of things.
You need to be very careful measuring full power bandwidth as you can easily run into core saturation at the low end giving erroneous results.
I will wait for Noel's comments if any and also measuring Nigel's amp

John

[Richard]

Hi John,

It would be good if Nigel can let you check the one he got from me if he still has 2 WD88s, the bigger number of samples the better.

I've used HFN 015 test cd many times, only a few days ago in fact to check the CD40 coupling, see other thread The standard Marantz and Pioneer players I've tried it in give a flat output right through 2Hz to 20KHz ref 0dB 1KHz, so no problem using it as a LF signal source. The player can easily be checked first anyway into the scope to be sure.

[Matthew Snell]

Just to report that the LF Lift now has a "fix" thanks to John Caswell.

Do have a look at the sticky in News.

Thanks all,

Matthew