best transistors to simulate a relais?

[.rhavin]

If this was already answered elsewhere i beg pardon, i wasnt able to find something useful.

As this is my first post: cheers to everyone;-)

Here it comes...

Im trying to understand the pros and cons of transistor vs. relais. Basically i dont want to amplify a signal, i want to switch it, so that signal "source" goes to either OutA or OutB. Principially, that could be done by a relais or by two transistors. As this is for Studioequippment, the relais may be a little bit to loud clicking but it would effectively cut off OutB if OutA was selected.

On the other hand, a transistor is silent but may add noise and perhaps the (admission/passage, ?... dont know the correct word...) would be >0.

So what Transistor would you suggest if the following is required:
Noise: *very low*
Maximum V: professional line level with some tolerance, lets say 3V.
Maximum A: do you think 0.5A is OK (too much? too low?)
C to E if B is set: as 100% as possible.
C to E if B is unset: as 0% as possible.
Affordable in europe;-)


TIA, .rhavin

[Mooly]

Hello,
Is it an audio signal you want to switch ? If it is, then it's a bit more complicated than you seem to be saying. A transistor on it's own won't do this. You would get lots of distortion and a problem with the DC voltage on the base breaking through into the audio signal. You can use FET's more easily, see below with very good results.
Small signal relays or glass reed types are near silent and might be your best option.

[h_a]

Some manufacturers even offer "low noise" relays, like Omron.

Have fun, Hannes

[Calvin]

Hi,

even so You won´t get the output really noise free.
The advantage of teh FET is that there is no bouncing and You can steer it up smoothly.

jauu
Calvin

[peranders]

When you talk about noise, is it 10 uV, 1 uV ior even nV?

1 uV compared to 3 V is > 130 dB

It's very easy to use relays. Using analog switches requires more skills from you.

[AndrewT]

Hi Mooly,
you specify 2sk152.
Which parameter/s is critical to the operation of the pass/mute ability?
Are they different for the two locations?
Which grade Idss is acceptable?

[Mooly]

Hi Andrew,
Not critical at all, as they are either fully on or fully cut off. I didn't select them at all. The 2sk152 was the best out of the small signal FET's I tried in terms of the lowest on resistance and as a bonus it was the cheapest.
Even a series FET works well on it's own, with just a little breakthrough at HF due to junction and circuit layout capacitance coupling effects -- no worse than many a switch. The addition of the shunt element totally kills this. I can't measure the isolation, it's too high. I can say it totally blocks (audibly) any signal applied -- even something ridiculous like an 8 volt peak to peak squarewave. Feed the output straight into a power amp and there is zero audible breakthrough.
The 2sk152 got down to around 30 ohms at 0.3 vgs and around 70 ohms at 0.0 vgs.