Magnavox S-8765 2-Way Horn speakers

Upgrading Classic Magnavox Speakers

DT667 had acquired a set of Magnavox S-8765 2-way horn speakers off of a local EBAY seller almost a decade ago for $50.

These speakers feature a 15" 8 ohm CTS Alnico woofer and 10" x 4" 16 ohm Jensen midrange horn . The horn is an Alnico magnet type with phenolic diaphragm.

The crossover network was what appeared to be a 1st order with a 15 ohm resistor and 50 ohm rheostat for horn volume attenuation network.

To get the speakers going again, DT667 replaced both the 8uf high pass crossover caps and 50 ohm rheostats - they were blown.

The 16 ohm Jensen horns were replaced by 8 ohm Heppner versions of the same horn as one of the Jensen horns had a bad diaphragm.

After using these speakers for in a secondary system for years, DT667 wanted to upgrade the internal wiring , binding posts and crossovers to hear if subjective improvements could be realized with a modest investment of time and materials.

DT667 removed the woofers, horns, internal wires and crossover networks from both cabinets.

2 sets of Nakamichi style binding posts were installed on the back of each speaker.

Internal wiring was made directly from woofer and horn to binding posts so an external crossover could be connected. KnuKonceptz Kord 10AWG OFC stranded wire was used from this purpose.

Audiopipe 2-way passive 2nd order 1.2KHz crossovers with 8 ohm 15 watt L-Pads for horn attenuation replace the original Magnavox 1st order crossover network.

Subjectively, the upgraded wiring and crossover parts made a substantial difference.

The vintage Magnavox speaker is efficient and smooth sounding with selected high resolution Qobuz audio streams.

DT667 has ordered some parts to build an upgraded passive 1st order crossover similar to the original Magnavox design. More on that project at a later time.

DT667 8/7/2021

Magnavox s-8765 crossover schematic

Original Magnavox S-8765 passive 1st order crossover schematic.

The horn section features an 8uF high pass filter and a 15 ohm resistor in series with the + terminal of the horn.

There is a 50 ohm rheostat in parallel with the +/- terminal of the horn that provides volume attenuation.

When the rheostat is rotated fully CW, a 50 ohm load is in parallel with the horn for NO attenuation/FULL volume from the horn.

When the rheostat is fully CCW, the horn is shorted out of the circuit for full attenuation/NO volume from the horn.

The woofer section features a 1.14mH inductor as a low pass filter on the + terminal of the woofer.

The crossover Hz for the horn is 1240Hz.

The crossover Hz for the woofer is 1120Hz.

NOTE: If you are using an 8 ohm horn the following changes to the high pass filter values should be made.

The 8uF high pass filter capacitor should be changed to 16uF.

The 15 ohm resistor in series with high pass filter capacitor should be changed to 8 ohms.

DT 667 8/8/2021

Dayton Audio Horns & compression drivers

Replacing Heppner 10" x 4" Alnico Horns

Having Magnavox S-8765 speakers wired up with dual binding posts for an external crossover makes experimenting with different horn lens and compression drivers easier.

Heppner alnico horns were removed from the speaker circuit and replaced with Dayton Audio H812 1" exponential horn lens and D250P-8 1" Polyimide compression drivers.

The Dayton Audio horns are just as smooth as the Heppner alnico horns but are more efficient and have an enhanced top end Hz response.

The Dayton Audio horns have to sit on top of the cabinets making placement of the external crossovers an issue due to space considerations in my secondary system listening environment.

Cabinet modification could be possible in the future but DT667 would prefer not to have to cut into the cabinets as the S-8765 have something of a cult following in their original condition.

DT667 8/8/2021

Foster 025N03 Horns

3-Way Magnavox Horns

DT667 removed the Dayton Audio horns from the S-8765 speaker circuit and reinstalled Hepper Alnico horns for another listen tonight.

The lack of top end that the Dayton Horn / compression driver combo provided was missed but perhaps another horn added to the Heppner horns as a "treble helper" might also work but be more appropriate than the more modern Dayton Audio products.

Enter the Foster 025N03 8 ohm Alnico horns. These are a smaller version of the Jensen or Heppner horns that were installed in the Magnavox S-8765 cabinets.

Adding a 25 ohm rheostat with 8 ohm series resistor that connects to 1.75uF 1st order high pass cap on the Foster horns for volume attenuation. This circuit gets connected to the input of the crossover. Experimenting with different cap values to get the correct blend of top end.

The arrangement of crossover parts and horns on top of the cabinets is a bit untidy but will suffice until a final format / layout can be determined.

Added were some crudely fashioned wooden stands to keep the Foster horns off the crossovers while perched on top of cabinets.

Subjective evaluations with streaming High Resolution audio from Qobuz will follow to determine if this crossover circuit will stay or be replaced by another.

DT 667 8/13/2021

Return of Dayton Audio Horns

1st Order High Pass Filter with Rheostat Attenuator

DT667 wanted to return to the most basic 2-way setup for the Magnavox S-8765 horn speaker experiment.

Out were the 3-way setups with Heppner and Foster alnico horns.

Dayton Audio products have returned : D250P-8 1" Polyimide Compression Driver and H812 HF Exponential horns.

The horn crossover is a simple 1st order high pass filter with rheostat + series resistor network for attenuation.

The attenuation network uses a 25 ohm RS potentiometer connected as a rheostat and (2) Dale 15 ohm 25W resistors connected in parallel. These two components are connected in series after the capacitor to the + terminal of the horn.

The main cap is an Audiophiler 8uf 400V bypassed with other oil capacitors to get the needed uF value.

DT667 is experimenting with the crossover frequency to determine the best blend with woofers. Somewhere between 2K - 2.5KHz determined by subjective listening for now.

CTS 15" alnico woofers run full range with no energy robbing inductors for improved "transfer efficiency" to the voice coil of the speaker per legendary audiophile Joe Roberts.

The Dayton horns and crossover fit neatly on a wooden platform for a clean simple aesthetic. Not shown are base extensions with risers that support D250P-8 compression drivers.

DT667 8/20/2021

Magnavox s-8765 : 3-way with KLH tweeter

Adding Top End Hz to Heppner Horns

Dayton Audio 2-way horns and compression drivers were removed - the top end was a bit too hot for DT667's ears for long term listening.

Magnavox S-8765's were returned to a 2-way 1st order crossover close to the original circuit values adapted for use with 8 ohm Heppner horns.

Ferrofluid tweeter from KLH 71 speakers were added to the crossover , a Westcap 1.5uF oil cap on the + terminal of the tweeter . This 1st order slope comes in at about 13KHz.

The efficiency of the KLH71 tweeter was low enough not to require any sort of attenuation circuit for level matching.

DT667 has been listening to this setup for a few weeks and likes it so far. There is just enough top end but not too much to cause listener fatigue during extended use.

DT667 9/5/2021

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