can I bypass my receiver's input selector and still use volume pot? - diyAudio
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Old 19th February 2008, 08:03 PM   #1
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Default can I bypass my receiver's input selector and still use volume pot?

I have a Proton D940 receiver and love the power amp section. Years ago the tuner stopped working and then the input selector went bad. I had Proton repair both (they say) way back when they were still in biz. perhaps 15 years ago. It didn't last a year if that. Anyway, there are pre outs and ins so I continue to use the poweramp section hooked up to my computer. Problem is some stuff is just so high level that I have to adjust windows' mixer down to barely any volume. Is there a way to wire the soundcard output directly to the D940 volume control and bypass its input selector switch?
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Old 19th February 2008, 08:10 PM   #2
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Hello bonjonno, It is definitely possible to do what you would like. If you could find a schematic of the receiver and post a copy of it, or a link to the schematic, it would make it easier for people to give you advice as to how to go about this modification.

Peace,

Dave
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Old 19th February 2008, 08:20 PM   #3
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Thanks Dave. I figured someone would want a schematic. I have the original documentation, but of course, no schem is included there. This receiver is over 20 years old so I doubt there's one online, but I'll keep looking.
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Old 19th February 2008, 08:27 PM   #4
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all I can find online is a schematic for Proton AM300, an integrated amp which some say has a similar power section. I have no idea if the circuits are similar in other respects.

schematic
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Old 20th February 2008, 12:54 AM   #5
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Hello bonjonno, for some reason the schematic came through as a very illegible thumbnail size pic. I did a quick search and there are other posts in the forum where people said they had copies of the schematic for this or a very similar amp. You could find that post and email the person asking if they still have the schematic. The other route you could take is to get a stereo volume potentiometer (pot), about 10k to 50k log scale, and wire that in between the output of your sound card and the power amp input of the Proton. Then you wouldn't have to worry about getting into the innards of the amp. This sort of volume control is pretty easy, hook the wiper terminal of the pot to the input of the power amp, hook one end of the pot to a common ground and then hook the other end of the pot to the output of the sound card. This will give a decent attenuation of the signal from the sound card. I would suggest mounting the pot in a metal case. This will act as a common ground that you can wire to. Hope this helps.

Peace,

Dave
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Old 20th February 2008, 01:24 AM   #6
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Just one question:Is the input selector electronic based or mechanical switches based???
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Old 20th February 2008, 01:50 AM   #7
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Dave

How strange; I got that schematic link from another post on this forum in fact and when I tried it in the preview it linked to the full image.

The input selector is mechanical but the insides of this thing are really cramped and getting at anything is a real puzzle. Multiple layers of pcb's and everything is screwed into everything else. I remember now trying to get to the selector switch years ago, what a hassle. The front panel control is connected to a long rod that runs the switch deep inside of everything else. Even the volume pot which is close to the front panel has a number of small boards surrounding it and has poor access.

So I really appreciate your suggestion for an external volume pot. I'll try that.

Best, Jon
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Old 20th February 2008, 02:01 AM   #8
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Hello Jon, Not a problem on the external pot suggestion. I'm kind of a lazy sort that way, and I also wasn't sure how comfortable you are with tearing around inside of the Proton. Not knowing that receiver, I can't give any specific recommendations about that, so the external pot idea seemed best. You could use a lot of different values of pot, but this region is probably the most likely to match in a friendly way with your amp. You also want to make sure it's a log pot, otherwise it won't sound like a smooth increase of volume when you turn the volume up. This has to do with the fact that the human ear has a logarithmic behavior to sound intensity. There are ways of making a linear pot behave like a log pot, but that adds a couple more parts, and then I would have to hunt down a schematic to show you how to wire it up. If you get the parts and have any questions about wiring, drop another line. I'm happy to help, as I'm sure many others are. There are people here with much more knowledge about audio electronics than I.

Peace,

Dave

P.S. Another possibility is to get two mono log pots of the same value and then you end up with separate volume controls for each channel. This allows you to set the volume in a balanced manner without having a balance control
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Old 20th February 2008, 02:09 AM   #9
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Dave

Thanks again. All this is great info.

I actually tracked down a guy thru a 4 year old post (!) and he may have the schematic. I'm waiting to hear.

My posted image link above has a .th in it; my error. That's why it shows a thumbnail. But I can't seem to find a way to edit it. the correct link is am300

Best, Jon
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Old 20th February 2008, 02:55 AM   #10
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Hello Jon, If you get the schematic for the D940, I would be interested in getting a copy to look at. If it's anything like the AM300 schematic you posted, it would be easier to just do the external volume control like I suggested. The usage of semi-custom IC's for controlling input selection, volume and tone makes for a lot of bouncing between the schematic and the datasheets for the IC's. It's doable, but kind of a pain in the buttocks. Besides, if you were to tap right after the input switching, you still have the gain stages I seen shown as opamps in the schematic. Since you say you get plenty of gain out of the PC soundcard, you really don't need this extra amplification in your signal path. Just more of a headache to deal with.

Peace,

Dave
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