Power amp modifications

Folk

I could do with 2 pairs of inputs on my power amps (Rega Maias), obviously one coming from the pre amp, the other coming from a surround processor.

In my crazy idea, I plan to take the cover off the power amp and solder in another pair phono plugs, which will link in to the gubbins through the existing ones, and then hopefully hey presto ! 2 sets of inputs !

I assume as long as I only have one source on at a time I should be fine.

Please would some tell not to do this if they think I might blow myself, the amp or anything else up !

Thanx

 

In theory it should work,but you lay yourself open to hum loops,interference,and of course,the possibility that one day you may have both sources on at the same time,with potentially nasty results.

It should be a lot easier and safer to make up a switching box to do the same job,and this sort of thing may be useful if you want one ready made ....
http://www.qed.co.uk/acc/ix20.html

This avoids invalidating any warranty you may have as well.

 

Eh? I was pretty sure the response would be don't do it, you'll likely kill one or both of the pre-amp sections? Surely this arrangement will lead to an unusually large current flowing through the pre-amp section in use to the output impedence of the other pre-amp section (I don't think this will change when the unit's switched off will it?). The point being the output impedence of the second pre-amp section will be much lower than the input impedence of the power amp.

 

I like the in theory bit !

My only major concern is that when the signal from the pre amp is coming to the power amp, will the signal leak back out and up through the other set of interconnect to the processor ?

I understand your school of thought in regard to the switch box, but you have to have even more wires, cables shelves and I would have thought a loss of signal.

 

I think my response indicated the better of the available routes,on a number of grounds,and made it clear that not doing it would be advisable,but as to impedances,when the other preamp is turned off,it's effective resistance to signal passage should be very high,so in theory(and that was the operative word there)it should work....but it doesn't in any way discount my advice that a switching box,either ready made or DIY,would be a lot easier and safer all round.

 

That's the critical factor, which I said I was unsure of, and my response was on the assumption this is not the case. Is this always true? I appreciate you'd advised a better course of action, but as it appeared more of a definite no to me I felt it worth querying. No offence meant.

 

None taken!....I wouldn't say anything is invariably true,but when an active device is not powered then it's impedance should be very high.

 

I was thinking there might be a circuit path via passive components too, say via a feedback loop or part of the circuit used to give a well defined output impedence. As I'm sure is patently obvious my electronics knowledge is very poor so I'm guessing here!

 

I was actually having a bit of a think along those lines myself,i.e. resistors/caps etc,although I still think that at the first unpowered active component it should meet a very high resistance,but my original intention,and yours also,I know,was to give Levi_501 the impression that any such tinkering with the inside of his amp,to create an unswitched second input was a bad thing.

He really should be looking to make an external switch box or buy one ready made...simpler,quicker and much safer.

 

Levi_501 - While I think of it, there is an alternative to the switching box idea, which might be worth considering if stereo is very much a priority over AV. You could feed the outputs of the AV amp for L and R fronts to an unused input on your stereo pre-amp, and then the signal goes to your power amp as before. You set volume on your pre-amp to some easily repeatable level, say 12 o'clock, and adjust the levels on the AV amp outputs accordingly to match with other AV speakers. Then the AV amps volume control can be used to change listening levels across all channels. The assumption in this sort of arrangement is that the pre-amp section in the AV amp is likely to be worse than in your stereo pre-amp, so any signal degradation by this approach will be minimal. For stereo music listening your system is not affected at all.

 

If I find time I think I might do a spot of measuring on a couple of my pieces of kit out of curiousity...

 

MartinC

That is how my system is set up at the moment, processor though the pre amp. However, there is some time difference between sounds coming from centre to the fronts, most noticable during concert DVDs. In Terminator the bullets just go flying always !

 

Isn't there usually a time delay adjustment between channels via the AV amp, or is the difference just two big to correct?

 

I know you adjust the distance from speaker cone to ear, but I was not aware you could alter distance from processor to speaker cone.

I shall refer to the manual yet again, it is well thumbed by now !

 

Pretty much all AV processors have compensation for the distances between the speakers and your ears as a part of the setup menus,as well as relative volumes from each channel(which ideally should be setup using an SPL meter).

There may also be an adjustment for lip-sync which can be an issue when using with plasmas which employ a lot of video processing...you shouldn't need to adjust that!

 

lip-sync What the hell is that ??? I have not come across that, is it worth it as I have a plasma, ah hang on my processor does not have video switching stuff, just sound.

 

The manual probably won't talk about it, because normally the processor - speaker time differences are inconsequential, but it's all the same thing that's being adjusted, simply adding in a time delay. If the setup is in terms of distances, you'll just have to make up some distances to get everything in sync. It should be do-able though.

 

Lip-sync is to take into account the time differences incurred when a plasma's video processing etc causes a slight but noticeable delay between the sound from your processor/amps,and the displayed picture...it's usually very obvious,and as I said,doesnt sound likely in your system,since you describe a delay between the centre and front speakers....it also has nothing to do with whether or not your processor incorporates video switching.

Thats most likely down to incorrect distance settings in the processor's setup for the speakers....measure as closely as possible,the distance from your listening postion to the speakers,and enter this in,then the problem should go!...any old tape measure will suffice.

 

Out of curiousity last night I measured the DC output impedence of the line out on an old Sony CD player of mine. This was ~100 Ohms irrespective of whether the unit was turned on or not.