Make preouts for a receiver?

Discussion in 'Archived Threads 2001-2004' started by Jeff_Blanch, Aug 13, 2002.

  1. Jeff_Blanch

    Jeff_Blanch Agent

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    Is it at all possible to add preouts to a receiver that does not come equipped with them? Electronically, it would be just finding the location on the internal circuit boards where the appropriate stage is, and wiring in some RCA jacks, right? Or I am completely out in space here? Is there any real reason why this couldn't be done?

    The reason for this is that some receivers come THIS close to meeting what I am looking for, with the exception of the preouts, like the Onkyo 600. I mean, why would I want to pay $250 more for the 700 with its supposedely better amps and power supply just so I can get the option of not using said amps if I plan on getting a separate power amp in the future?

    Jeff
     
  2. Bill_Weinreich

    Bill_Weinreich Second Unit

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    Possible? Maybe. Plausible? I would say no.

    With today's surface mount components and many separate circuits combined into single chips, wiring in pre-outs may prove to be an impossibility. Let alone for the fact that you would definitely void any warranty on a $$$ receiver. If you do not have separate amps now, opt for the model you are looking at and upgrade to separates later.

    Bill
     
  3. Saurav

    Saurav Cinematographer

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    You'd definitely void the warranty. Then, like Bill said, you'd need a fairly qualified tech to be able to find the points to tap off of, assuming that these points are 'findable' (and I think they should be). However, there's another issue - you might not be able to make do with just a simple tap. You might have to put in a buffer stage of some sort, or disconnect the receiver's amps, or something. The thing is, if you tap off a preout and hook it up to an external amp, that results in the preamp section seeing a load that the parallel combination of the external and internal amps. This reduces the impedance that the preamp section needs to drive, which makes it harder to drive, which results in greater distortion. Depending on the circuit parameters, you may end up with a situation where the sound starts to distort as you turn the volume up. To get around it, you'd have to disconnect the internal amps, or feed the preouts through an impedance buffer so that the preamp stage 'sees' only the internal amps, and doesn't see the load of the external amp.
     
  4. Dan Pawlowski

    Dan Pawlowski Stunt Coordinator

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    Possible? Yes. Plausible? I would say yes.

    I think it is easier than what Bill_Weinreich suggests. The problem is you need a competent technician equipped with the correct tools. How much are you willing to pay him for his time and materials to get the job done.
     
  5. Jeff_Blanch

    Jeff_Blanch Agent

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    I appreciate all the responses. I am quite qualified myself at soldering any kind of connection (even fine pitch surface mount components with a scope) but locating the correct spot would be the difficult part for me. I have access to electrical techs at work who would be willing to help me out, so maybe that wouldn't be so difficult.

    The part about the amps running in parallel and the need to somehow turn the internal amps off was the kind of roadblock I was thinking of. Would a rigged switch to cut the signal to the internal amps suffice, or would I need some kind of isolation circuit? What would this circuit look like? Circuit design was definately one of my least favorite parts of college, so I would need some help there.

    Oh yeah, I know it would definately void the warranty, but I thoroughly enjoy voiding the warranty on my computer parts (modifying and overclocking), so that's not really what I am worried about.

    Thanks,

    Jeff
     
  6. Wayne A. Pflughaupt

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    Jeff,

    If you have the necessary skills and access to “back up” technicians, I suggest calling Onkyo’s parts department and ordering a schematic or repair manual. Of course, you would want to wait until the warranty expires, as has already mentioned.

    In the mean time, it is fairly easy to convert speaker level to line level, if you don’t mind having an extra stage in the chain.

    Regards,
    Wayne A. Pflughaupt
     
  7. Bryan Michael

    Bryan Michael Supporting Actor

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    the thing i hate the most when i fix gear is traceing traces when you dont have a scmatic and sime times the trace runs throgh the center of the circuet bord but the easyest thing might be to tap into where the first hat the amp stage there are big transistors that are black with a silver tab tap the center pin on them that might work
     
  8. Mark gas

    Mark gas Second Unit

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    You could just use a LOC (line output converter) people use these car audio when they don't have preouts on a head unit. It hooks up to the speaker wire that would connect to your speakers and turns them into line level outputs.
     
  9. AndersP

    AndersP Stunt Coordinator

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    Hi. You defeat the internal amp by connecting the input to ground. Then it gets no signal. This can be done by a makeshift shorting plug that you easily make for yourself. I have done all the above 10 years ago, but on a 2 ch integrated. The real bitch is not the electronic part. It is the drilling/fitting the new rca plugs on the backplate, finding space for them. I would not try this again to save $200 and most definately not on a 5 - 6 ch reciever.
    wbr
    AndersP
     
  10. Daniel J

    Daniel J Stunt Coordinator

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    I'd go with mark's suggestion, a bunch of LOCs may not be the cheapest way to do this, but you can always sell them later. You can't sell a dead reciever.
    Of course, if you have money that you could use to replace the reciever (just in case...), then modification is definitely the method that's more 'fun'. Just make sure you know exactly what you are tyring to do before you even think about picking up that screwdriver! I'd say that a repair manual is the minimum amout of documentation that you should have- the more you plan this, the better it will work.
    Oh yeah, and lay off the sauce until after you're done! [​IMG]
     
  11. Wes

    Wes Screenwriter

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    I had this same thing done to my older Pioneer VXS9300 DPL receiver a few years ago. I took it to a technician and it cost around $80. He just tapped on to the board just before the surround amps. The Pioneer is still running today but now as a sub amp.
    Most car speaker to line converters only work with small less than 10w (realistically 2w) input from a deck!

    Wes
     
  12. Jeff_Blanch

    Jeff_Blanch Agent

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    Again I appreciate the suggestions. Daniel, thanks for the warning, but I don't drink so I am probably OK there.[​IMG]
    If the LOC's expect a low wattage input, they probably won't handle 80-100W from a speaker output of a receiver.
    This really isn't about the money involved to possibly upgrade the receiver in the future. Will I be able to afford something better in the future? Probably, but if I just wanted to buy stuff, I wouldn't be in the DIY section, would I?[​IMG]
    This is all about the challenge, the fun of it, and making my purchases stretch, while giving them more value, with the "screw the man" factor thrown in. Saving money is just a very attractive fringe benefit. Much like modifying and overclocking computer CPUs and graphics cards.
    This brings up another question. I assume that building receivers is much like building anything else that has increased features for more cost, and that different receviers carry much of the same parts. Let's look at a computer motherboard. There are two versions, one with a whole bunch of integrated hardware like extra IDE ports, SCSI, or sound. They also sell a stripped down version without all this stuff, but the PCB itself has all the places to mount the missing components. With the correct skill and supplies, one could turn the lesser into the greater.
    On the receiver end take a look at the Onkyo 600 and 700 again for example. The 700 does not have very many more features than the 600. The backplates are very similar with the preout area on the 600 blank. Since I don't think preouts would require any major IC or chip or anything to make them work, I would be willing to guess that behind the backplate on the 600 is a small board with empty spots (with solder-filled holes) ready to mount some RCA jacks to match the 700. Anyone have a 600 they want to take a look at? [​IMG] If this isn't the way it works, then we would have to use alternate methods.
    Anders, thanks for the specifics. This shorting plug, it would just connect the input to the internal amps to ground when I would be using the preouts? How about a switch for each amp, that would accomplish the same thing, and give me control over which amp to ground?
    Wes, If you paid $80 to get it done, it must not be all that difficult. Did they work out OK?
    Thanks,
    Jeff
     
  13. Geno

    Geno Supporting Actor

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    Jeff, If you end up doing this, please let me know how it works out. Ive been thinking about doing that for a while but went ahead and built a passive preamp and I am using the decoder within my dvd player. my beef with my setup is that I dont get DPLII or virtuall surround when I play my CD's. anyway, it would be a great DIY project. what receiver are you thinking about converting. I allways have asthetics as a high priority and was looking at an HK receiver.
    pLEASE LET US KNOW HOW IT ALL WORKS
     
  14. Saurav

    Saurav Cinematographer

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    If you're doing it for the fun of it, by all means go ahead [​IMG] I managed to keep the warranty on my current amps valid for... I think a month [​IMG]
    Anyway.... yes, one way would be to rig up a switch to 'disconnect' the internal amps. I would go the other route - hook up the preouts in parallel, but put in an impedance buffer there so that it looks like 'infinite' impedance to the preamp, which effectively 'see's just the internal amps. Such a circuit shouldn't be hard, all you'd need is an op-amp wired up for unity gain (or any level of gain/attenuation you want). I'm a software engineer, so I would recommend you double-check this with someone who's more familiar with digital circuit design [​IMG] But based on some of the circuits I've worked on, this should be simple.
    And definitely order a service manual. You'll get block diagrams, schematics, PCB layout diagrams... it's invaluable, and shouldn't cost more than $30 - $40. Once you have that, you couold easily try upgrading opamps in the preamp stage, for example. I did that in the output stage of my CD player, and the improvements were quite dramatic.
     
  15. Jeff_Blanch

    Jeff_Blanch Agent

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    Saurav, thanks for the info, but you've lost me. Is an op-amp a type of transistor, or is a type of complete circuit? Where exactly would this circuit go? Before the Y-connection, or after the split in either the amp line or the preout line? And wouldn't this be an analog circuit instead of digital, as it is already past the DAC, heading to the amp?

    I'm a manufacturing engineer who had to sit through one class of basic electrical engineering in school so forgive my ignorance. The soldering skills came before school when I did circuit board rework for TI.

    As for receivers to try this on, I was looking at both the Panasonic HE100 and HE200 receivers. They both offer an extraordinary level of processing for the money, and would possibly make good pre/pros for an external amp in the future if only they had preouts. The Onkyo 600 would be another possibility, but I was using it for more of an example than anything. The Panny HE200 is actually interesting because it looks like it has preouts for the front mains, but not the rest of the channels. I don't know for sure but it looks that way from a very grainy shot of the back panel I have seen. It may be possible to duplicate what they have in place for the mains for the other channels.

    Jeff
     
  16. Saurav

    Saurav Cinematographer

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