simple passive preamp

[merlin el mago]

Mooly inputs and outputs?

 

[scottjoplin]

just found this picture - i guess that's what's pretty much what you guys decribed yesterday.

Err, not really what I described, I can't see any coax.....or selector switch....or pot 😉🙂

 

[Mixi]

yeah, you're right! i guess i didn't look close enough.

here's the page i took it from but that's not really what i want to build:

Glasshouse TVC Passive Pre-amplifier - Description | Hifi Collective

i'm more aiming for something like this:

Glasshouse Passive Pre-amplifier No.1 Kit | Hifi Collective

 

[scottjoplin]

That looks to be the same chassis. You could probably get away with doing it like that in a well screened chassis

 

[PRR]

That found picture is odd one way: all jacks plastic-insulated from panel, all jumpered together, and then bonded to that big panel screw.

Me, I'd save the penny on insulators and the 15 minutes of wire bonding. Assuming the panel is bare metal (or can be quickly cut to bare metal). On a BIG system (my 13 output hi-gain DA) you could have reasons for isolated grounds. But here in a passive switch/pot box, you really "can't"(?) have ground problems with the handful of jacks on one metal panel.

This also points out "what do you do about switch and ground?". Nothing. Ground is not switched. The only part of the switch which might be grounded is the bushing/frame (so it won't carry external crap into the box). Assuming an all-metal box and no huge crap (welder), this happens naturally.

 

[merlin el mago]

There are no hard and fast rules, you have to build the preamp to suit your requirements 🙂

Typically something in the region of 4.7 to 10 ohm would be used. That ties the case to audio ground for shielding purposes. If you built a dual mono construction then tying both channels this way gives continued shielding even if only one channel is used, and the resistors still allow enough isolation to prevent any issues due to circulating currents.

Mooly inputs and outputs?

Mooly?

 

[Mooly]

I'm not sure what you are asking 🙂

 

[merlin el mago]

In all preamps there are inputs & outputs, my question is where have I to place the 4,7R to 10R resistors between the audio ground & the case: only the inputs, only the outputs or both?

 

[Mooly]

The resistor only loosely ties the case to ground to aid in shielding, and so tying to the output socket ground would be reasonable since that is the final point all the grounds meet up.

There is no signal flow involved, you are just stopping a metal case from floating and being prone to pick up noise.

 

[merlin el mago]

Thank you.

 

[Big Cheese]

One source, multiple Power Amplifiers

I have been reading this thread with great interest.

My project is the opposite (or inverse (?)) of the OP. I want to hook one source (DAC) up to multiple Power Amps.

My amp and speaker collection is steadily growing. Rather than swapping amps in and out, I want to keep all amps in the rack and hooked up to one single DAC. Depending on mood / album I want to select an amp-speaker combo by means of a selector switch. Ideally I would also include a volume control in the same enclosure.

Q1. would the below concept work?
DAC > RCAs > Potentiometer > Selector switch > 3 x RCAs > AMP & Speaker 1, AMP & Speaker 2, AMP & Speaker 3

Q2. In the above concept, would make-before-break or break-before-make be preferable?

Your input would be much appreciated.

Best regards,

Laurens

 

[Mooly]

Yes it would work. Switch type should make little difference, the make before break simply allowing two amps to receive signal for a split second.

 

[Big Cheese]

Thanks Mooly!

 

[scottjoplin]

If your amps have widely varying impedances it may be worth having a buffer after the pot

 

[Big Cheese]

Thanks Scott,

The Amps will be
- Folsom DIY7297,
- Aleph Mini (Work In Progress), and
- Baby Huey EL34 (still deciding if I am courageous enough to build it).

How would I go about determining each Amps impedance?

And, if the impedances do vary widely, should the buffer be built to put out the best compromise between the extremes?

Actually, come to think of it, would any pre-amp work if you connect a source to the *single* output and the amplifiers to the *multiple* inputs? In other words, is anything in a amp directional?

 

[PRR]

Thinking differently....

The power amps can probably be driven from a 10K pot without problem.

The DAC can probably drive three 10K pots.

So you have three volume knobs Amp1 Amp2 and Amp3. Turn-up the one you want to hear. (When bored, turn up more than one.)

 

[Big Cheese]

Well PRR, you have put me on to an even simpler idea.

I can wire one set of RCAs to the potentiometer "IN" and three sets of RCAs to the "OUT". This way I can skip the selector altogether.

As long as only one Power Amp is switched on at any given time, we are basically back to the OPs design.

Thanks!

 

[cacao ambiance]

ive been wanting to switch between: a audio isolator, tone board or direct to amp input options. how do i avoid the "pop" when switching between these? ’Im planning to use a toggle switch to switch between two. hmm maybe a on-off-on switch and a extra RCA socket for direct to amp input?
What resistors/caps??? do i need to use to stop the annoying loud pop sound when switching?
Please?
Thanks, really apprenticed. i really don't know what to do. tried hours searching...

 

[AndrewT]

The DC blocking capacitor can hold charge on it's disconnected side. This is avoided by using a high value resistor to shunt that charge to audio return/ground/cold. This also works if the capacitor leaks a little. Typically Builders use anywhere from 100k to 2M2.
Each open connection needs it's own grounding resistor.

If you use a make before break switch, then you should have pop/click free switch-over.

 

[cacao ambiance]

Thanks AndrewT.

Didn’t know about make before break switches! arr 🙂

Hints for how to work out value of grounding resistor?
I use class d amps like TPA3116 and tone boards like NE5532 and OPA2604.