Yamaha A-720 amp and the loudness control - diyAudio
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Old 25th June 2012, 05:54 PM   #1
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Default Yamaha A-720 amp and the loudness control

I pulled this one out of the "to be done sometime in the future" area of my basement "laboratory" and just hooked up earphones and the sound was really "thin" unless I pulled down the treble and raised the bass. I felt the loudness control and it was loose and took the chassis half apart and found the damn thing is just split in two. The securing nut was loose and I figger with use from the prev. owner it just fatigued. But I digress. It's one of those 4 pronged pots with a fixed tap. I can get the original part aus Deutschland fur 40 euros, frankly more than I wanted to spend to resurrect this amp but I might do it as it's nice with both mm and mc phonos. What exactly did the loudness thing do? Can I fix up a fixed resistance solution to bypass the need for such a control? Does anyone know of a source for the 300k pot with the fixed tap? Is it easier to find world peace than this part? Yamaha hasn't stocked the part in 10 years and doesn't have the specs for it either (other than the schematic/parts list).
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Old 25th June 2012, 06:08 PM   #2
Mooly is offline Mooly  United Kingdom
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A circuit is always best so we can see exactly what is involved but normally the loudness tapping can be left disconnected.

If you search for circuit diagrams for older amps that had a loudness function you will see they all follow the same basic pattern.

Post #1 here is one such,
Help with DC on volume pot
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Old 25th June 2012, 08:06 PM   #3
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I have been working on a Yamaha AX 730 recently. I hope it is similar. Please have a look at the jpg. The AX 730, like most Yamahas I have seen, has a volume control and a loudness control. VR501 is volume, VR 502 is loudness.
You can make the amplifier work without VR502 by connecting the left pin (in the sch.) of R599 to where the top of the pot. has been. C561 and R597 are shunted with 470 ohms that way. Likewise in the other channel of course. This is like turning loudness all the way down. Linear. I always liked the Yamahas best that way.
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File Type: jpg loudness.JPG (89.7 KB, 144 views)
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Old 25th June 2012, 11:55 PM   #4
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Actually not similar at all. There is no on/off switch for the loudness. There is tone control disable but I don't think it affects the loudness circuit. I'm trying to take a snapshot of the pdf but I'm just too dumb to get it right! Here is a URL to the service manual. The pot is located on the schematic to the right of the DC servo real time EQ amp box.
http://akdatabase.org/AKview/albums/...%20Service.pdf
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Old 26th June 2012, 12:27 AM   #5
jaycee is offline jaycee  United Kingdom
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It looks very similar. Interesting how they use a seperate pot to implement it.

I would fit a regular 3 terminal pot and bypass the loudness circuitry entirely. I'd do this by removing C201 C202 R203 R204 R205 R206, and installing a bridge across where C201/R203 and C202/R204 were. You can then replace the volume control in a standard configuration with a dual 50K log pot such as an ALPS Blue Velvet. Alternatively a 47K dual log pot will work.
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Old 26th June 2012, 01:08 AM   #6
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WOW! tmi!. For all that, I'd just go and order the part from Germany and run with it! Thanks for the information.
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Old 26th June 2012, 06:47 AM   #7
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Ok, so you need to bridge the RC (68k - 470p) to have a flat response.
Just to avoid confusion: SW504 in the AX 730 and SW 201 in the A 720 are the "mono" switches. You cannot switch loudness control off in either of the sets.
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Old 26th June 2012, 10:48 PM   #8
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I've decided to spend the 40E and get the original pot from Germany. Now just trying to find out if I really have to pay the VAT! Thanks for the help!
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Old 27th June 2012, 12:28 AM   #9
jaycee is offline jaycee  United Kingdom
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The changes i suggested are not exactly hard to make, but it's your call
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Old 27th June 2012, 01:24 AM   #10
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Jaycee: Since you're in the UK, perhaps you can answer the question of whether or not if I import from the EU am I required to pay the VAT?
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