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Old 28th December 2009, 05:20 PM   #1
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Default hammond ao68/7591 question

I have a Hammond ao68 7591 amplifier. It is a well sorted out unit set up for hifi use, it has an rca input and speaker taps. I would love to install a gain/volume control into it. I will power a center channel speaker with it. I have no schematic, despite looking pretty hard. Anybody have a suggestion on how to proceed?
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Old 28th December 2009, 09:19 PM   #2
dhaen is offline dhaen  Europe
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I can't imagine you'd need a schematic to fit a volume control. Wire an (audio-taper) pot of 50k to 100k between the RCA connector and the input stage. Remove the centre connection from the RCA and wire:
CCW>RCA shield (outer)
Wiper>Input stage (the wire you disconnected)
CW>RCA inner (centre pin)
Or you can do the whole thing external to the amp in a little box.

If no one has a schematic, it would be a good exercise to draw one yourself. Download the pinouts of the 7591's and other valves/tubes and have a go. Have a look at a similar amp's schematic for inspiration....
Post the schematic to this thread and we'll tell you if it's more or less correct.
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Old 29th December 2009, 01:36 AM   #3
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John,

It might help if the OP enumerates the tube complement. There may be more gain than necessary for use in a HT rig, which the center channel remark implies. I doubt the OEM envisaged an I/P signal approaching 2 VRMS. It's quite possible that a 51 KOhm padding resistor will be needed in combination with a 50 KOhm control.
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Old 29th December 2009, 09:02 AM   #4
dhaen is offline dhaen  Europe
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Eli,

I think if Pete uses the added control as a preset, he should have sufficient range of adjustment. He will after all have the main volume control to control the overall level. Of course the 51K will be icing on the cake..

What we don't know is the phasing of the amp. Most are 0 degrees but it could be 180 degrees. If the sound image is distorted he will have to reverse the speaker polarity.
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