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Old 7th November 2006, 03:34 AM   #11
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Here's the soft start part. It seems like the BCs with 400-800 hfe ought to be turned on hard enough to operate the relays. However I noticed that P-A darlington connects them. Any recommendations?

The part of this that is mine (and not just a ripoff) is the mute circuit. On power up the mute flip flop is low, connected to the base of Q6, turning it on. When the mute button is depressed, the output goes high, turning off Q6 and removing power from the mute relays.

Would it make more sense to make Q6 a BC546 and connect it between the base of Q5 and ground so that when the Mute flip flop output goes high it shunts the Q5 base current to ground?

Values in the LED circuit are TBD. The idea is that when power is applied to the soft start circuit there is just enough current through R14 to make the LED glow very dimly for a standby indication. When mains power is turned on the Q1 end of R15 goes low enough to allow more current to flow and the LED lights brightly. I found this in a circuit in this forum, but I cannot find it now.

The header allows connection of thermal fuses or temperature switches. I think that fuses should be used in home applications since it forces you to open up the amp and hopefully find out why it overheated. For PA applications it may make sense to use switches.

One fuse should be mounted near the soft start resistor. The others can be mounted on the output heat sinks, mains transformer or wherever you desire. Jumper the pads if not needed. I am undecided if this will be changed to a string of KK connectors for reliability. I guess it depends on the board space available when I lay it out.

Here is this part of the circuit.

EDIT: Yes, I realize that there are no protection diodes on the muting relay circuit. Since those relays will be on a board I plan to incorporate the protection diodes there.

The protection Zeners are backwards in this picture, corrected on the schematic.

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Old 7th November 2006, 04:53 AM   #12
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And finally, here is the audio detect part of the circuit.

It is Rod Elliot's Project 38 with input buffers added to prevent crosstalk, increased gain and provisions for a trimpot to adjust sensitivity. I also use a TO-92 MOSFET to switch the power turn on current instead of the TO-220. I'm thinking I can probably eliminate R27.

Time for some sleep.

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Old 7th November 2006, 03:36 PM   #13
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Slight revision to the soft start circuit.

- changed muting control

- added mute indicator LED circuit

I'm still working on laying this out.

Feedback?

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Old 7th November 2006, 11:19 PM   #14
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Board layout for soft start portion minus ground plane.

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Old 4th February 2007, 10:26 AM   #15
Nixie is offline Nixie  Canada
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My DAC's volume control is a motorized stepped attenuator. The only rotary switch I could get that was both quality yet cheap was a break-before-make type. In its configuration, shunting between inverted and non-inverted balanced signal after series resistance (so only two decks would control volume on a balanced stereo setup), means that when the break occurs, it would go to full volume. The solution was that the infrared monitoring of switch position would activate the mute relays during a transition (and also force the motor h-bridge driver to continue rotation until it clicked into the next position).

I'm having a problem with initial turn on, however, with the same muting relays. There's a very loud pop. Now, the DC offset is about 0.2 mV max, and there are I think 500k resistors to ground. So what gives? I can't understand how 0.2 mV would give such a huge pop. I'm listening to this with an OTL tube headphone amp with an input being a 100k to ground. Could somehow voltage be developing on the tube amp's input, despite the 100k resistor? The input is DC coupled to a grid.
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Old 5th February 2007, 07:34 AM   #16
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Instead of flip flop, for less parts solution refer here

Gajanan Phadte
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