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Crazy idea: add preouts myself?


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#1 of 12 Matthew Ar

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Posted February 07 2003 - 02:42 AM

Does anyone know if it is possible to add preouts to a receiver that does not already have them? I have an older kenwood VR-407 and I want to hook it into my 2-channel system so I can use it for TV watching. However, since it doesn't have pre-outs I was planning on ditching it and buying the cheapest one I could find with preouts.

But then started wondering if I couldn't just take it apart and hijack the wires for the L&R channel before they go into the amp - disconnect them and attach an RCA connector to them.

Any reason this wouldn't work? When listening to music the sound won't go through this receiver, so sound quality is not the highest concern. I just don't want to screw up too badly and cause damage to my external amp or main speakers while watching TV or the occasional movie.

Thanks!
Matt

#2 of 12 Wayne A. Pflughaupt

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Posted February 07 2003 - 04:45 AM

Matthew,

I’m not familiar with your particular Kenwood receiver (you didn’t mention if it’s a stereo or home theater model), but this can be a fairly daunting prospect on late model equipment. It was a fairly easy task with gear from the 70s or early 80s. You could pop the cover and see some wires going from the volume control on the front panel to the amplifier section at the rear of the chassis. That was the main pre-amp signal going to the amplifier (the volume control is the last thing in the pre-amp signal chain.

Everything’s changed with late-model home theater equipment. There probably isn’t a cable going from the volume control to the amps, because now there are crossover circuits between the pre amp and amplifiers. If there is a recognizable cable, it will not be from the volume control – which will make it tricky determine if you have the correct cable. And it will be a tiny ribbon cable carrying the signal to all five amplifier channels. So you have to not only separate out the individual wires, you will have to figure out which ones carry the main L/R signal.

Then you have the prospect of finding a place to mount some retrofitted RCA jacks. Again, “back in the day” this was a minor challenge because there was plenty of vacant rear-panel real estate available. Late-model receiver’s rear panels are farily cluttered with connections for audio, video (both composite and S-video, etc.) and a multitude of speaker connections. Often there is a circuit board across the interior of the rear panel, practically wall-to-wall, that all these jacks are connected to – which means you risk damaging this circuit board if you drill into the rear panel.

Your best bet is to remove the Kenwood’s cover and evaluate what you’re getting into. Any way you cut it, Matthew, it will certainly be a challenge. If you don’t have soldering skills and the ability to do precision, intricate work, you’d better move on.

A much easier alternative, although not as ideal, would be to convert the speaker outputs to a line level signal.

Regards,
Wayne A. Pflughaupt

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#3 of 12 Martin Rendall

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Posted February 07 2003 - 05:06 AM

I think that you will find that even if you could discern the pre-amp section and the amp sections, you would find that the signal between them would have a unusable impedence. That is, to make pre-out jacks which are capable of driving an external amp's impedence (and the cables connecting them) you would have to add a low impedence stage.

A fascinating idea, to be sure... but doubtful unless you have tons of electronics experience.

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#4 of 12 Patrick Sun

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Posted February 07 2003 - 05:17 AM

How about using the Tape loop output?
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#5 of 12 Scott Pagac

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Posted February 07 2003 - 06:17 AM

Patrick:
Using the Tape outputs for anything other than tape decks is a bad idea. I made this mistake once when hooking up a new 2-channel amp to my receiver. Instead of using the pre outs, I plugged into the tape outs (by accident!). Everything came out full blast. The tape outs bypass the volume control, so they send the full signal.

#6 of 12 Matthew Ar

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Posted February 07 2003 - 06:40 AM

Thanks for the replies everyone. I've already decided to give up. I looked inside the receiver and there were no wires anywhere - just circuit boards.

One quick question though. Most of the receivers I'm looking at state that the speaker-level outputs are still active even when an amp is connected to the pre-outs. Is it correct to assume that the amps aren't drawing any power if the no speaker is connected to them?

The reason I'm asking is because I will be using an external 2-channel amp and was assuming that the receiver would distribute it's power among the remaining 3 channels.

Thanks,
Matt

#7 of 12 Justin_Terpstra

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Posted February 07 2003 - 07:19 AM

Dumb ... but could you use the headphone output?

Use a phone plug to RCA cable .... similar to what I do to hook my computer up to my old Pioneer Reciever for some computer tunes ....

#8 of 12 Wayne A. Pflughaupt

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Posted February 07 2003 - 08:38 AM

It will work, but the signal from headphone jacks is much hotter than line level. So you have to keep the volume control down very low to keep from overdriving the inputs of the other component.

Regards,
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#9 of 12 Patrick Sun

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Posted February 07 2003 - 09:07 AM

Oops, didn't realize it was being fed to an amp.

How about buying an inexpensive preamp off Ebay and put that inbetween tape outs and the amp? Or just get a cheap receiver with preouts and be done with it. Posted Image
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#10 of 12 Danny Tse

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Posted February 07 2003 - 11:58 AM

What you need is a Carver Wide Band Z-1 Coupler. It connects between a integrated amp or receiver without preouts via the speaker outputs, and then turning the speaker-level signals to line level signals going into the power amps. There were a stereo version (2 channels) and a 5.1 version. Crutchfield used to carry these, but Carver no longer makes them. Look on Ebay. Otherwise, partsexpress sells something similar.
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#11 of 12 Craig_Kg

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Posted February 09 2003 - 04:42 PM

Quote:
Is it correct to assume that the amps aren't drawing any power if the no speaker is connected to them?

The reason I'm asking is because I will be using an external 2-channel amp and was assuming that the receiver would distribute it's power among the remaining 3 channels.

This is absolutely correct although not in the way you might think. Almost all mass market recivers (save HK, NAD and Rotel) are rated with only one or two channels fully driven and the power supplies cannot maintain full rail voltage when more channels are fully driven. This means that most receivers will have a lower output/channel as more channels are used so by not drawing current from some, you are reducing the power drop in the others.

I made lineouts to do what you wanted. My receiver was rated as 100W/Ch into 8 Ohms so the maximum output voltage would be about 40V. You want about a 2V maximum signal for a source so a 20 to 1 attenuation was about right. I connected a 10kOhm resistor in series with a 470 Ohm resistor across each set of speaker terminals that I wanted to power externally and then took the signal across the 470 Ohm to my 2ch system preamp. It worked fine and the high resistance across the speaker outputs meant that those channels drew very little current (and thus power) from the receiver so my other channels sounded better too. I did encase everything into a little kit box with RCA outs to prevent shortouts Posted Image.

If your receiver is rated very differently, then adjust the resistance ratio to suit.

I did consider making preouts from the amp driver stage but chickened out after trying to trace the circuitry :b.
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#12 of 12 Wayne A. Pflughaupt

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Posted May 07 2003 - 08:56 AM

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