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Old 30th August 2013, 03:41 AM   #1661
phil g is offline phil g  New Zealand
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This my first post so please forgive me if this is wrong place to ask this question.
I recently put together one of the LM4780 boards from Audiosector (to use in stereo for testing before finishing the whole kit).
I have tried three different potentiometers but there seems to be very little attenuation and certainly not all the way down to cut off.
The first two, a 10k and 20k (with a "B" designation I assume for "Linear") the attenuation is faint at best and not down to silence.
The third pot, is one retrieved from my old Technics amp with the makings of 100k Ohm B. This one does noticeably attenuate but not to cut off. To help make it clear what I am saying, the volume goes from loud to less loud.
Power is from 18v +- rails and rectifiers to 26.5v and 25v.
Thank you for any assistance on this question.
Phil
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Old 30th August 2013, 09:26 AM   #1662
AndrewT is offline AndrewT  Scotland
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You have probably wired up the pot incorrectly.

The wiper and one end go to the Amplifier/Receiver input.
The two ends go the the Source output.
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Old 30th August 2013, 08:57 PM   #1663
phil g is offline phil g  New Zealand
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Thank you Andrew, I will look at the wiring of the pot more closely again.
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Old 9th September 2013, 04:38 PM   #1664
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Default My latest gainclone

I thought someone may be interested in my latest, and last gainclone. It is rather unusual as to keep all the ac away from the inputs and still have an isolated speaker protection system, I have mounted the speaker protection board in the psu box, therefore the speaker sockets are on the psu box. This also gave me room in the amp box to fit a phono board for my vinyl.
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File Type: jpg rp3.jpg (883.4 KB, 177 views)
File Type: jpg rp4.jpg (767.3 KB, 180 views)
File Type: jpg rp5.jpg (871.0 KB, 151 views)
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Old 9th September 2013, 04:51 PM   #1665
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Here is a pic of the completed amp and psu. It sounds great.
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Old 17th September 2013, 08:39 AM   #1666
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Default Need some help with audiosector kit

I just put together an my Audiosector LM3875 kit with a PSU I had built from Chipamp.com. I used a 24 volt transformer, and have 33.6 v in both rails. I used a 10 uf electrolytic cap in place of r1 for DC coupling. I used a 20k potentiometer that I had in my last Chip amp that I busted up to build this.

So in 1 channel I'm getting about 66 mV of DC offset, and in the other about 115 mV. There is no difference when I move the volume pot from full off to full on.

I noticed a couple of little breaks in one of the 1500 uF caps, like maybe I brushed my soldering iron against it a couple of times, on the side with the lower DC offset.

I'm not sure how to troubleshoot. I'm thinking the potentiometer isn't working since it doesn't change the offset, so maybe I should bypass it and see what I get.

Any suggestions?

Another question I have is whether it is OK to install an LED between V+ and PG+ coming off of the power supply.

Thanks in advance for any help.
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Old 17th September 2013, 09:48 AM   #1667
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I can't see any problem installing a led. Some people fit one for pos and one for neg rails. The difference in offset is quite large. You could try replacing R1 and see what the difference is. The tranny and voltage are good. 20k pot should be LOG. Not sure what you mean by no difference in the pot. Is the volume changing. If not I would check the wiring and earthing on both channels.
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Old 17th September 2013, 04:32 PM   #1668
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Thank you.

In Peter's instructions he says that the offset should change when the volume is increased or decreased, and mine does not. I haven't hooked it up to speakers yet, so I'm not sure if the volume changes. I don't have R1 installed, and instead have a 10 uF electrolytic capacitor installed for DC blocking. Maybe I should install R1 instead.

Josh
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Old 17th September 2013, 05:07 PM   #1669
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Peter often reads these posts, so maybe he will come and help. I would certainly re-install R1 unless Peter has suggested otherwise. I would re-check the wiring to the pot, I think the offset should be less than 50mv.
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Old 18th September 2013, 07:46 AM   #1670
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Hi All.

I pulled out the little caps from the R1 positions and installed the resistors. My volume pot is working ok. With nothing connected to the amp, on one side I get 10 mV to 37 mV offset depending on the volume, and on the other side I get 46 mV to 86 mV offset depending on the volume. Does this indicate a problem? Is the side with the higher offset destined to sound worse? I listened to it briefly in the middle of the night here at low volume, so I know there is sound, but I haven't been able to judge it too much yet. If the channel with the higher offset is a problem, can anyone suggest any trouble shooting steps to take?

On a separate note, I am using an Alpha 20K dual Audio pot, which I think cost about $3. I am thinking about getting an Alps Blue Velvet from PartsConnexion. Anyone have anything to say about how much of a difference that might make and what that difference would be?

Thanks for any responses!
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