Hi All,

I hope you can help me with this. I have a Select Zen SE84CS which I've been using for about five years.

I've had an issue, though, which has been going on for ages (years) and I think the amp may be at fault, though it's a problem which I have gradually become more aware of over time. Any help would be gratefully received.

Basically the problem is the sound has been "thin" for a long time. I am not even sure if it has always been like this but ever since I made my Parker Audio 98MKII speakers I started noticing that the sound, while beautifully clear, has had a lot of high frequency element - not that it didn't have bass.

(As a tinnitus sufferer it's been a cause of real pain as lots of high frequency noise really hurts!)

So I made the Parker Audio speakers to partner my Zen amp a few years back. Ever since then I've improved other components with a new Origin Live turntable and Silver arm. Then a Vertex AQ Jaya mains filter. The sound improved enormously gaining a sense of space and clarity. But the sound was still painful.

I decided to build the Decware HDT speakers, wanting to build a single driver speaker as they are so renowned for having a fantastic and smooth midband. The speakers are absolutely wonderful, giving superb imaging and even more detail over the Parker Audio speakers. Still the over-bearing top end  remained. The speakers sounded better off-axis as listening directly on-axis was painful.

Just two days ago I took delivery of a new phono stage - a Lehmann Black Box - the remaining link in my vinyl source chain to be upgraded. Wonderful as this new phono stage sounds, I was hoping it would bring back my midband and reign in the high frequencies. But it did not.

So, I thought I'd do a bit of investigation. I got my Hi Fi World test LP and recorded the full frequency sweep from the output of the phono stage into my computer to look at the response. It's pretty much flat, as you'd hope (see below)


Full frequency sweep output from phono stage

Then I recorded into the computer the output from the amp and was horrified to see that instead of presenting a flat response, there's a huge curve starting from roughly 2KHz. Unfortunately I cannot accurately measure the amount the output increases as I have no means of calibration but you can see that it's pretty large compared to the existing waveform (see below).


Full frequency sweep output from amp (speaker terminals)

(I am no expert on amps, especially valve amps, and I am aware that measuring the frequency response from the speaker out may or may not be accurate (because of the output transformers). My dad suggested it might be necessary to measure the output from somewhere further back in the circuit).

So I attached my really awful NAD amp and was not surprised to hear that the tone was much more normal, no overpowering high frequencies (but horribly shut in, grainy sound!)

There definitely is a problem, then, with the amp because I can now hear it (or not hear it, as the case may be, in this old amp).

What could the problem be? Why is the response not flat? I did try replacing the input tube but got the same curve. Would any of the other tubes cause this? I thought probably not, as it's in both channels - so it wouldn't be the power tubes.

Is it a fault or something I can recifty?

Many thanks,

John

Thanks, Steve.

I've sent you an email using the website contact form.

John