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Old 19th November 2011, 11:23 PM   #1
Photony is offline Photony  Finland
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Default Preout mod

Hi,
I bought Genelec active speakers and would like to make a preout mod for my old amplifier Technics SA-DA8. The amplifier is a basic 5.1 home theater amplfier.

First of all, I have very little understanding of any aspects of modding electrical devices. So, please be patient with me.

What I have found so far, is that I should take the preamplified signal out from somewhere. Some thread suggested to take the signal after the potentiometer, which makes sense. I suppose there is a line going from the potentiometer to power amplifier for each channel. Right?

If I manage to do this, the next thing I read is that I should match the output impedance of the preamplified section of my amplifier to Genelecs input impedance, which is 10kOhm. It is a still unclear for me, what electrical component should I use to make impedance match. What is considered a good match? What would be a recommended solution? Moreover, I don't know how to measure the output impedance of the amplifier. Is there any easy approximate way?

Is there anything else I should know?

Thank you for any help.


-Tony
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Old 20th November 2011, 03:11 PM   #2
C37 is offline C37  Belgium
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Hi,

Having lived with Genelecs for years, I would recommend you not to and to build either a buffer (there are many excellent ones on this site) or to buy something like SPL Volume 2.
or get a Dac that has volume and balanced out such as tc electronic BMC-2.
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Old 20th November 2011, 05:42 PM   #3
Mooly is online now Mooly  United Kingdom
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Does your Technics even have a potentiometer for volume or is it "electronic" using a rotary encoder with the attenuation being achieved electronically.

It would be very easy to cause damage if you are not experienced. If you have a service manual we would be better placed to advice where to take a feed from if possible.

One possible solution is just to fit a resistive divider across the left and right speaker outputs (done externally to the amp) and to take a feed from there.
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Old 20th November 2011, 07:05 PM   #4
Photony is offline Photony  Finland
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Hey,
Thanks for the answers.

I expect there is an actual potentiometer, but I'll check that out tomorrow.

I found a service manual for the amplifier:
koti.mbnet.fi/photony/technics_sa-da8.pdf
I also found a more extensive manual for Technics SA-DA10, which is essentially the same amplifier
koti.mbnet.fi/photony/technics_sa-da10.pdf

I gave a thought for making a buffer to use with the Genelecs. However, if this preout mod works, I have one needless amplifier less in the circuit. Plus, this is a nice and hopefully fun my first diy audio project =)

-Tony
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Old 20th November 2011, 08:52 PM   #5
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I was trying to repair a bad volume pot on a transistor radio yesterday and discovered it has a single resistor volume pot for a "stereo" radio. This is a pretty good tip-off that the pot is providing an input to the IC, and the attenuation is being done in the IC on the two channels. Real volume pots have as many resistors as the device has output channels.
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Old 21st November 2011, 06:21 AM   #6
Mooly is online now Mooly  United Kingdom
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Can't get the links to work...

Have you tried putting the manual in a zipped folder and posting that here.
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Old 21st November 2011, 07:17 AM   #7
Photony is offline Photony  Finland
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Hi,
Sorry about the links. I should have put http:// in front of the addresses. Here are working ones. Those are quite big files, so it might take a while for the download to finish.
SA-DA8 (3mb)
http://koti.mbnet.fi/photony/technics_sa-da8.pdf
SA-DA10 (7mb)
http://koti.mbnet.fi/photony/technics_sa-da10.pdf

I found from the schematics that there are 3, not 5 or 5.1, electronic volume adjusters in the circuit. Maybe there is some similar circuit to Indianajo's radio explaining the number of the adjusters. Not sure if they are the master volumes though. I think I also found a potentiometers sign, with all channels connected in the circuit. Again, not sure if it's the master volume or not.

I'll open the amplifier today, and try to learn something.


-Tony

Last edited by Photony; 21st November 2011 at 07:38 AM.
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Old 21st November 2011, 06:15 PM   #8
Mooly is online now Mooly  United Kingdom
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Talk about minimalist audiophile approach

It does appear to have a "normal" (if you call 6 gang normal) control.

L and R audio should be available where I marked on the diagram. It's from a low impedance opamp source so no problems driving other loads and it appears to be a separate PCB too. If you do tap into this then I would feed into a 100 ohm resistor and 10uf series cap terminated to ground via a 100K.
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Old 21st November 2011, 11:23 PM   #9
C37 is offline C37  Belgium
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Hello Photony,

Are you confident doing this? I think this is the real question- You sounded a bit hesitating in your first post. If you are by any means try Mooly's suggestion. It will get you going. It will not , i suspect, get the most of your Genelecs. I don't know which model you have, but 1031A 's were *very* responsive to preamp changes.

Have fun experimenting,
T.
PS, make sure you have a good adaptor cable (RCA>XLR) and keep it short, mains cable proved to be sensitive too, but the latter is fine tuning. However, when all else is sorted out, it makes for a surprising difference for the better - just DIY!
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Old 22nd November 2011, 11:06 AM   #10
Photony is offline Photony  Finland
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Hi there,
Thank you for the replies. Especially thanks to Mooly for pointing out the spots in the circuit. I opened the amplifier yesterday and had the opportunity to do some measurements with the guidance of my friend.

First of all I found the spots Mooly pointed out. I also did voltage measurement with signal generator and oscilloscope from these spots. The result was:
Input: 0.5V peak to peak
Output: 4V peak to peak @ 50% of max volume

We were in a little hurry and I forgot to make the impedance measurement. I think it should have been also good to measure the maximum output voltage. Had a slight problem with that measurement since it burned the resistor connected to the normal speaker output terminals. Didn't have a better resistor available. The resistor was needed because otherwise there was no signal coming through. The potentiometer seemed to be logarithmic so I would suspect turning the volume to max giving me 16 volts though.

I still have some questions for you if you don't mind =)

1. Do you think I should make the impedance and max output measurement?

2. Mooly, is the measured output something you anticipated? If not, does the circuit you suggested need some modifications?

3. My friend also suggested to make a voltage follower for each channel by using operational amplifiers. The reason was to avoid causing interference with the amplifier itself. Do you think this is necessary?

This has been fun so far. Learned a lot of things. I don't expect this to be a optimal solution for my Genelecs, but currently the Genes are connected straight to my TV. It's hard not to improve from that =)


-Tony
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