I need to switch the gain of an audio preamp using a reed relay, Kv =
3 (low), and 10 (high). It's important to avoid audible pops during the switching. The device involved is a lo-noise audio opamp, NE5534AP, using the relay to adjust one of the resistance values by switching a second R in parallel. I'm wondering which configuration would be most likely to be pop-free - inverting/noninverting, which R to change. Or another scheme? Any other hints?
You are always going to risk pops in this configuration. Anytime the instantaenous voltage is different from the zero level, and you switch, there will be a sudden level shift, giving a 'pop'. Best you can do is to switch "slowly", which means not a relay, but something which smoothly changes the resistor value over a few msec. A FET with the gate suitably controlled would be better. Maybe look at some of the eepot type devices. Some are designed to minimise audio zippering effects, which amounts to the same thing. Alternately there are audio opto-couplers using a LED and photo-resistor, which at least gives you electrical isolation from the circuitry.
-- Regards, Adrian Jansen adrianjansen at internode dot on dot net Note reply address is invalid, convert address above to machine form.
If you're game and can deal with the various crackpots there, sci.electronics.design actually has some knowledgeable regulars that could give some good advice...
-- Cheers, chris.
** The NE55534 is a low noise and very low THD op-amp, ideal for low level and line level audio, but it has one problem - high input bias current. The bases of two input transistors require around 1uA of current each and t his has to be supplied via feedback and ground reference resistors. With ty pical values of say 100kohms, a 1uA current produces a voltage drop of arou nd 100mV. Suddenly reducing the value of the feedback resistor by 3 will pr oduce a very audible pop of about 60mV at the output pin.
The simplest solution is to put a film capacitor in series with the switche d resistor to isolate this offset so the gain changes for audio signals but not for DC voltages. This cap must be kept charged to the offset voltage b y a high value resistor of say 1Mohm.
Another issue is with the coil of that reed relay, applying or removing the drive voltage suddenly will induce transient voltages in nearby wiring or PCB traces. Better have that voltage rise and fall over some time period -
50mS at least.Thirdly, the mere act of switching the level of an audio signal creates a f ast transient which will be audible under certain conditions as a distinct click.
The solution that solves *all* the above simultaneously is to use a suitabl e opto-coupler instead of a relay.
A LED driving a small photo resistor ( aka CdS cell)connected in series wit h your switched resistor will do the trick. Typical CdS cells have on resis tances under 1000 ohms and off resistances in the megohms. Switching times are in the order of 100mS. No clicks or pops are possible as it is just lik e turning a potentiometer up and down by hand quickly.
Jaycar and others have suitable CdS cells - also know as LDRs.
You can easily make a opto-coupler by facing the LED against the cell and c overing the pair in black shrink tubing to keep light out.
... Phil
** The entire world of audio seems to be a complete mystery to regular posters on SED - myself being a singular exception.
John Larkin for one is openly hostile to its very existence.
Like "Sheldon Cooper" with his distain for Geologists.
" The only people who are glad when others take their work for granite ... "
.... Phil
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