2. amps-preamps
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Bypassing A Preamp With Volume Pot In Amp

I see preamps as a convenient tool to control the volume of multiple sources. It only adds another link in the chain. It also adds to the cost of your system because you will need other pairs of interconnects and a power cable.

I was thinking about completely bypassing a preamp by adding a volume pot or a resistor based stepped attenuator into the amp, greatly reducing the signal path from the source to the amp.

The only detriment I can think of, is switching the interconnects from each source which is as troublesome as switching out a disk from the CD player.

Please chime in with your opinions and especially your experiences with this "issue".

The volume control I have in mind is the Khozmo stepped attenuator using 2 Vishay TX2575 resistors in a hybrid tubed/ss amp.
c_avila1
georgehifi's avatar
georgehifi
4,966 posts
 
C_avila1: Great idea, if your source has equal or higher output voltage than the input sensitivity of your amp great, you are almost there.

If your source has a nice low ouput impedance say below 100ohms, this will happily drive down to 1kohm (1:10 ratio).

EG: The combined load of say a 10kohm passive and a 47kohm input impedance (industrie standard of most amps)amp is still 8.2kohm, the 100ohm source will happily drive this (1:82 ratio), then this will be the most transparent/dynamic sound you will get.

Cheers George
ptmconsulting
1,000 posts
 
Sounds like a DIY passive preamp. Why not get a selector switch and a few more RCA's and avoid the plugging and unplugging of the interconnects?
georgehifi's avatar
georgehifi
4,966 posts
 


You'll find a lot of so called intergrated amps these days are exactly just that, a power amp with a passive and input switching, many of these are coming from EU and Asia and some are "Hi-End" tube mostly but now some solid state as well.
Cheers George
c_avila1 OP
71 posts
 
@Ptmconsulting, @Atmasphere: RE: DIY passive preamp - That's pretty much what I am aiming for but having it built into the amp with only one pair of single ended inputs. I just can't use another interconnect if its not as good as the pair of Hidiamond D9 interconnects that I already have. It would really hurt my wallet to purchase another pair.

@Georgelofi: Someone recommended to use a 50kohm attenuator. Would that possibly work better than the 10kohm passive that you mentioned in your example?

georgehifi's avatar
georgehifi
4,966 posts
 
C_avila1
You haven't mentioned the input impedance of your poweramp.

I say 10K as a general even when direct connected, as most amps are 47kohms (industry standard) input impedance.

If you have say a Rogue tube poweramp which are 1mohm input impedance then yes a 50kohm pot will be more than fine.

Just remember it's easier to manufacture a 10kohm stereo log pot so that both channels track each other within a db or so than it is to manufacture a 50kohm stereo log pot to track that well.

Cheers George
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