ECC99/2A3 interstage finished, but have one query

[meshplate]

I’ve just finished my second home-brew amplifier. It’s a simple two-stage circuit using an ECC99 to drive a 2A3 via a Lundahl interstage transformer. The two stages simply have a resistor and bypass capacitor from cathode to ground. The main purpose of this project was to build an amplifier similar to my first but use an IT to couple the two stages rather than a capacitor.

The only unusual thing I notice is a dip in frequency response of about 6dB at 30kHz. It dips fairly steeply from 20-30kHz and then gently rises back up between 30-50kHz after which it is flat to about 80kHz where it then gently rolls off. I have tried various loads on the secondary, both resistor and resistor/capacitor, but the response still follows the same pattern. Is there some inherent aspect of an IT that would cause this response? I cannot detect any audible effect, but I’m curious to know why.

Since I used many parts from the first amplifier to keep the cost down, it’s difficult to reliably compare the two, but I would say a small sacrifice in bass clarity has been compensated by the most lucid mid/top I can ever remember. The power supply is probably over-kill but it does mean absolutely no noise whatsoever, even at full volume position with 105dB speakers.

Anyway, I’d appreciate any feedback on the response anomaly, not with a view to change the circuit, but just to understand why.

Steve

[Zen Mod]

that's probably because of inherent characteristics of Lundahl IT, combining with input capacitance ( Miller ) od 2A3 - in any case - nothing to write home about .

btw. visual design isn't my cup of tea , but execution is simply exemplary .....

[Wavebourn]

It is called ringing, frequency depends on transformers. Don't worry. Your design is exceptional, looks like a 21-st century cosmic ship. Very expensive look!

[meshplate]

Zen Mod, I know what you mean about the visuals, I wasn't too sure myself - it's just a bit in your face. I'm also limited by what I can lay my hands on at the time. For example, the mains transformer is potted into a stainless biscuit tin There are 6 large transformers/chokes inside the chassis and there wasn't enough room to get all the big poly caps in as well. If the caps had gone inside then all the trannies would have been a bit much on top. However, I do like working with stainless steel; nothing chips or flakes off, mistakes can easily be corrected, scratches are easily removed, and any level of polish can be put on the finish.

[Zen Mod]

[aptquark]

wow...

....wow.

[SemperFi]

Mighty splendid, just fab!

How many watts/kilo amplifier?

[el156]

Beautifull looking ! Very nice!!!!!

[meshplate]

SemperFi, if I was a salesman I'd turn that around. You get a whopping 15kg per watt.

The way litigation goes these days, I could probably get sued by a burglar who hurt his back trying to steal it.

[philo3]

Meshplate, IMHO the visual design of your amp is awesome! I don't care what Zen Mod says.