Banana's Pre - inelegant way to contrl vol - Page 3 - diyAudio
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Chip Amps Amplifiers based on integrated circuits

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Old 31st August 2007, 03:48 AM   #21
banana is offline banana  Hong Kong
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Page 1 of the discrete ADC schematic.

Total 10 quad comparators. In order to gain some precious PCB real estate, I use the more expensive push-pull comparator (instead of good old LM339). 40 pullup resistors are save.

LVL01 is the maximum attenuation. Below LVL01 is mute state, and opamp output relay will open.

LVL37 is 0dB attenuation, all 8 shunt relays are open.

LVL38~39 are the active gain state, audio signals are rounted to the variable gain opamp, without passing through the shunt relay attenuator. The comparator is connected as non-inverting for these 2 levels.
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File Type: pdf discrete adc vol (pg1).pdf (33.6 KB, 130 views)
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Old 31st August 2007, 05:07 AM   #22
banana is offline banana  Hong Kong
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Page 2, the XNOR chain.

This logic stage is to convert the cummulative output from comparator into decimal (01111 -> 01000).

Diode delay is added to some levels, to create anti-pop "make before break" for the relays. It's only required when relay state carries to the next digit (011 -> 100).
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File Type: pdf discrete adc vol (pg2).pdf (21.0 KB, 78 views)
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Old 31st August 2007, 05:34 AM   #23
banana is offline banana  Hong Kong
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Page 3, the diode mapping network.

36pcs of quad diode, correspond to 36 attenuation levels (LVL01~36). I assigned at most 4 shunt relay to be closed at a time, to save some power dissipation.
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File Type: pdf discrete adc vol (pg3).pdf (25.0 KB, 79 views)
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Old 31st August 2007, 05:37 AM   #24
banana is offline banana  Hong Kong
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The gain table that shows detail of shunt relay assignment.
V0~V7 are the 8 shunt volume control relays.
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File Type: gif gain table.gif (13.4 KB, 689 views)
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Old 31st August 2007, 02:25 PM   #25
digi01 is offline digi01  China
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nice very nice!
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Old 1st November 2007, 06:01 AM   #26
banana is offline banana  Hong Kong
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Now another clumsy logic circuit, a remote control for my preamp.

Elementary FSK, 310Hz for volume down, 435Hz for volume up.
To avoid interference from other remote, IR carrier is 16.384KHz, much lower than normal 38K.

Receiver side is just bandpass pass filter, and a BJT H-bridge to drive the motorised volume pot.

Click the image to open in full size.

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Old 1st November 2007, 06:23 AM   #27
banana is offline banana  Hong Kong
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Schematic for the remote unit.

I didn't know why, but I couldn't get a 32.768KHz tune fork crystal to work... I lost patient and settled to use oscillator module IC, at 10 times the cost
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File Type: pdf remote control sch.pdf (47.0 KB, 49 views)
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Old 6th November 2007, 05:48 AM   #28
banana is offline banana  Hong Kong
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A photo of the IR receiver board, mounted behind the front panel.
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Old 6th November 2007, 10:21 AM   #29
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Hi Banana,

Good job!

How about the sound? do you like it?

Jopie
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Old 6th November 2007, 01:57 PM   #30
banana is offline banana  Hong Kong
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Thanks Jopie.

I was using passive preamp (24 steps grayhill switch) for years, and in fact I couldn't notice substantial difference after switching to the new preamp. The LME49710 is quite neutral and quiet.

The user control is better, there are finer steps in the low volume range, volume knob is much smoother than the stiff old grayhill, and now I have a remote.
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