Passive Preamp for S.E.X. (and Crack) // S.E.X. (and Crack) as a pure power amp

jufelling · 391

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Offline jufelling

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Hey guys,

i have the following situation:
I've got both the S.E.X. amp and a Crack amp but only one source. I always have to switch the interconnect from one of the amp to the other.

Therefore my question:
Would it be possible to take out the volume control on both of these amps and built a single passive pre-amp with two outputs and a switch?
If yes, can I just leave out the volume controls in both amps or do I have to make adjustments to the curcuit?

Julian



Offline Paul Joppa

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That is certainly possible.  But the two amps can also be paralled, with the source driving both inputs. A pair of splitter cables will do the job. Here is the first example that showed up when I googled "rca splitter cables"

https://www.monoprice.com/product?p_id=663&utm_term=&utm_campaign=PMax:+Smart+Shopping+-+Monoprice+-+Cables&utm_source=google&utm_medium=cpc&hsa_acc=6614305189&hsa_cam=17490724715&hsa_grp=&hsa_ad=&hsa_src=x&hsa_tgt=&hsa_kw=&hsa_mt=&hsa_net=adwords&hsa_ver=3&gad_source=1&gclid=Cj0KCQjw7ZO0BhDYARIsAFttkChRmzF0Z4F7knG3oEi8XyiICB7VE5h7rzK6zlcPXKl8bSNpLG1fGx8aAtGKEALw_wcB

These seem affordable  :^)  but there are many other examples readily available.


Paul Joppa


Offline jufelling

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Thanks for your response and suggestion.
But using a splitter cable is not an option for me.

If I want to do it the way I described above, what do I have to do in the circuit of the amps in order for them
to function as a power amp?



Offline Paul Birkeland

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Paul "PB" Birkeland

Bottlehead Grunt & The Repro Man


Offline jufelling

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There are two reasons I don’t want to use a splitter:

1) I want to use my current rca interconnect cable and not something else.

2) I built a very high quality stepped attenuator into the SEX and would love to use it for both amps.

Therefore my idea was to build a passive preamp with this attenuator + an output switch and adjust the amps to be poweramps only.

Hope that clarifies it up for you.



Offline Paul Birkeland

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Well, you could use the SEX as a preamp and feed it into the Crack!

-PB

Paul "PB" Birkeland

Bottlehead Grunt & The Repro Man


Offline jufelling

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Again, thanks for all the suggestions.

But I would love you to just answer my questions of the first post.

i definitely want to built a passive pre-amp and use both amps without attenuator.

Thank you :)



Offline pboser

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You could remove the volume controls and simply substitute a resistor of the value of the volume control. But the usual advice  is - leave the (stock) volume controls in the units and turn them all the way up, effectively taking them out of the circuit. Could you replace the original volume control in your S.E.X. amp? Then make a passive “preamp” with the stepped attenuator and maybe a switch.

Peter Boser


Offline Paul Birkeland

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Would it be possible to take out the volume control on both of these amps and built a single passive pre-amp with two outputs and a switch?
If yes, can I just leave out the volume controls in both amps or do I have to make adjustments to the curcuit?
If you leave the volume controls out, you need to add resistors from input to ground to provide a DC ground reference for the first stage grids.  You will also be amplifying 100% of any noise that's picked up on the interconnects between passive and amplifier, so you may find that you need to put the level controls back in place.  Depending on the impedance of the passive control you purchased, the added cable capacitance of the interconnects going to the amplifiers in conjunction with the miller capacitance of the amps themselves may lead to suboptimal high frequency response that varies rather substantially as you rotate the volume control. Decreasing the impedance of the passive control to compensate for this may overburden your sources and lead to other issues.

If you have a passive control with two outputs, then you will have to have three interconnect cables rather than the two that you'd require with splitters.

Paul "PB" Birkeland

Bottlehead Grunt & The Repro Man