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Help analysing Hammond organ circuit
Help analysing Hammond organ circuit
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Old 3rd August 2009, 10:11 PM   #1
ChrisA is offline ChrisA  United States
diyAudio Member
Join Date: Jan 2008
Default Help analysing Hammond organ circuit

I could use some help analysing a circuit. Specifically the frequency cut-off between the bass and treble channels. Is it 500Hz, 1KHz or 5Khz. I'm guessing "about a Khz" but I don't know.

I have most of the parts in this circuit and I'm thinking of re-using them to build a three channel keyboard amp. I'd have two "treble" channels, one for left and one right and one "bass" channel. I'd re-use all three OPTs, the PT and choke. It should be a good fit as an electronic keyboard has about the same range of sounds as the Hammond organ this came from. I will use a conventional Hi Fi EL34 based schematic.

When you read the schematic, notice that the input is "balanced", so you do not see a phase inverter. So what we have here are two parallel amps per channel that run in parallel all the way to the OPT but the input signals are 180 degrees out of phase, so it works as push pull. Also for our purposes here, we can ignore the entire "reverb channel". Just assume the switches are "off".

The input is a point G1/G2. The top and bottom channels are tied together because they share G1/G2 in common. The treble channel has a high pass filter built-in while the bass channel has a low pass filter built in. The question is, what is the roll-off frequency. I'm not sure I can figure this out exactly.

In the treble channel I think the cut off is defined by the .0022 uF coupling caps (C4 and C5) between V1 and V2/V3

On the bass channel they use .047 coupling caps between the driver and output tubes and then there is a .0047 uF cap at C21 that "shorts" the highs between the phases.

This was a great design. A Hhammond "Tone Cabinet" was basically a powered speaker. It had multiple speakers and a three channel amp in each cabinet and then you bought as many cabinets as required based on the size of the room. I want to build something like it. I will use a 15" pro audio bass driver. That can go to about 40Hz with about 96dB sensitivity and then some of full range drivers, one left one right. The goal here is to simulate the sound of a grand piano, not to play stereo recordings so I'm Ok with the left and right channels lacking bass.

Here is the schematic:

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