AO-29 EM To PM Conversion

?EricSimball has contributed these notes on converting older, ElectroMagneticSpeaker, versions of the AO-29 to newer ?PermanentMagnetSpeaker specs.

(Also see SpinetEMToPMConversion for more notes on the subject.)

EM to PM M3 Amp Conversion (NOT CORRECT: DO NOT USE)

• WARNING, WARNING*:

  MikeFulk and others have warned that these instructions are not correct or complete.
  
 *Do not use these instructions without understanding completely what it is that you are doing!*

• WARNING, WARNING* Indeed:

  MikeFulk is correct. This is missing two important steps:

1. Replace the power transformer with one from a PM AO-29
2. Replace the output transformer with one from a PM AO-29

The field-coil version of the AO-29 uses a 380V power transformer, whereas the permanent-magnet version uses a 320V power transformer. If you follow the steps below, all the voltages throughout the amplifier will be too high, and something will fry.

First, get a copy of the M3 ServiceManual. The parts diagram is essential (unless you are really good at reading schematics). You are basically converting schematic 5 in the manual to schematic 5B. (See below for links to scan of the schematics and parts diagram.)

I'm assuming you are looking at the parts diagram and amp from the underside, with power transformer (T7) to the left and the two other transformers to the right.

1. Add new resistor R92 (500 ohms, (ww 5 watts) between lower end of R30 and upper end of R95.
2. Insert Condenser C62A between junction of R30 and upper end of R95
3. Remove R102
4. Desolder and lift (only the) UPPER end of R103 from its terminal and swing it down to replace old R102.
5. Install new 56K resistor in previous location old R103.
6. Remove R101 and replace with a short length of wire.
7. Replace the field coil in the circuit with a 180 ohm 5 watt resistor.
8. Lift lower end of R93, swing this to the upper terminal where old R101 was before you removed it, which now has the "short length of wire" on it.

former lower terminal of R93 and

former lower terminal of R101. (This is the lower end of the "short length of wire" --- therefore electrically R92 and R93 are tied together.)

You are done! This makes more sense if you are looking at the diagram.

Remember, if you are not going to replace the speaker with a PM speaker, you will need an 8 ohm 25 watt resistor to act as a load resistor for the amp in place of the ?VoiceCoil.

147-type (6W) Leslies can use the speaker level signal. (Differential input --- 122-type or "6H" --- Leslies will need further circuitry, or a LeslieHookupKit of some sort.)

Line out may be taken with an inexpensive passive DI box if you have it, or build the simple circuit listed on LineOutFromASpinet.

References

?PDFs of the AO-29 schematics and parts diagram can be found at~:

 * http://www.hammond-leslie.info/schematics/AO-29-fc.pdf
 * http://www.hammond-leslie.info/schematics/AO-29-pm.pdf
 * http://www.hammond-leslie.info/schematics/AO-29_wiring.pdf

TopicModification