KEF Chorale - Rebuild

I would like to share my newest Project ---> KEF Chorale rebuild.

I was (and still am) a happy owner of the old KEF chorale series, but it was time to either buy new speakers or rebuild the old KEF's
The enclosure was severely damaged (not leaking air) and not fit for restoring.

So i devised a new project:

A few initial thoughts where:
1* It had to be active.
2* So an active filter is favorable.
3* Maintain all the retro features.
4* made of solid wood - no pressed / glued boards or anything.

First step was designing the amp:

First i designed a fully complementary dual rail amplifier with a Op-Amp filter and all other bells and whistles. (in simulation) it was working fine.
But then i realized that this is way to modern for a KEF Chorale rebuild so i stepped of this approach.
I went back to the drawing-board and designed a amplifier that had all the seventies characteristics, this included a single rail voltage, singleton current feedback pre-section and a simple bootstrapped VAS.
Though i included "some" features to reduce distortion quite a bit.
One is a fully symmetrical final stage with relatively high β.
the other is a carefully tuned (relatively high current) singleton pre-section with feedback cap MKT bypassing. all the transistors are going to be 70's BC154 and BC2XX gold leads TO-108 types and BD137, BD441 etc.

I personally prefer the softer, warmer slightly more even order distorted sound of this kind of amplifier vs the modern "clean and optimized" amplifiers, provided that the amplifier is well designed and not overcompensated and almost deliberately poor designed as many where in those day's:D.

P.S. all the transistor types in the schematics below are going to be different ones (these work well in simulation and are almost fully compatible)

Second step was to design the filter:

I already decided on using active filtering, but refused to use any integrated circuitry. So i took a dive in my (small) but plentiful part box and found some great BC149C gold lead low noise units. I was going to respect the 3.5Khz cross-over frequency originally implemented by KEF. So in an Linkwitz-Riley filter I came up with a R of 6.8KΩ and a C of 4.7nf. All the unity-gain blocks will become emitter followers. (see the schematic in the pictures below)

Third, the enclosure:

Well this is a different story, i wanted to build a cool 70's look so the final enclose will consist of a lower unit for the B200 woofer and a separate cabinet containing the T27 tweeter and amplifiers for both units.

All the screws still visible in the semi completed enclosure in the pictures below will be made invisible by a wooden finnish "boarding" (don't know if this is the correct word) but a sort of small wooden strip.

I will keep updating this post for whoever is interested -- just sharing my project.

Grtz, Pim


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