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Old 27th April 2011, 04:25 PM   #51
sbelyo is offline sbelyo  United States
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Hi Pete..

I have a question for you, on you website it says that a 5K primary could be used. This would increase the gain. About how much? and could you change the value of the volume pot to get more range out of the knob?

The reason why I ask is because I have a pair of electra prints that are 5K Primary @ 65ma 3W Secondaries are 32, 120, 300, 400 for headphones

I was going to sell these since I decided not to finish the project I had them wound for. But it looks like they might work for this amp.

What do you think?
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Old 27th April 2011, 06:06 PM   #52
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Yes the hum is there with the volume all the way down. The transformers are positioned per your drawing of a Hamond chassis. The choke about an inch or so to the right of the power transformer.
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Old 1st May 2011, 09:26 PM   #53
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Ok
I disconnected the choke and installed a resistor. Still hummed, so reconnected the choke. Hum does not change with volume pot. Click the image to open in full size.
If anyone else decides to use a Hammond chassis, get sockets without a shoulder. Install the board then the sockets. Otherwise you will have to drill holes large to get the board to go in. It doesn't look too bad, but I will be getting a new chassis and doing it this way.
Al
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Old 1st May 2011, 09:36 PM   #54
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Could it be that I need JP1?
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Old 3rd May 2011, 05:26 PM   #55
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Paging Pete. Should I try dc on the filaments?
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Old 2nd August 2011, 07:14 PM   #56
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Hi Apurcell, late to reply I know, but I'm finishing up mine and I found that I needed to put a jumper across JP1 to connect signal ground to AC ground to eliminate noise. I initially thought the problem had to do with the pot not being properly grounded as the noise is mostly present at 50% on the pot, so I ran a wire from the chassis ground to one of the rear screws on the Alps, but that didn't offer any decrease in noise.
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Old 2nd August 2011, 07:32 PM   #57
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Thanks for the response. I will give that a try.
Al
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Old 3rd August 2011, 01:05 AM   #58
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Sorry I didn't see these posts - you should have sent me email!

If the hum is still there with the volume all the way down and with no source connected, I suspect you are hearing magnetic coupling from the power transformer to the OPTs. This will be worse if your line voltage is high, and/or 50Hz, and/or if you are using very high efficiency headphones or IEMs. I can just barely hear it on my amp with IEMs, maybe or maybe not with Grados. Not at all with AKG K701 or Senn HD600's.

The only way to tell for sure would be to try moving the transformers apart. Maybe unbolt one and tilt/rotate it as much as you can without extending the wires, and see if the hum changes in that channel.

If that is the problem, I'm afraid the only solution is to move them further apart.

On the ground jumper: if your source uses a 2-wire AC cord and there is no other "real" ground connection in the system, things do work better with that jumper in place. This is caused by the EMI filter in the source, which sends a bunch of AC line current into the signal "ground".

Pete
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Old 3rd August 2011, 01:11 AM   #59
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Quote:
Originally Posted by sbelyo View Post
Hi Pete..

I have a question for you, on you website it says that a 5K primary could be used. This would increase the gain. About how much? and could you change the value of the volume pot to get more range out of the knob?

The reason why I ask is because I have a pair of electra prints that are 5K Primary @ 65ma 3W Secondaries are 32, 120, 300, 400 for headphones

I was going to sell these since I decided not to finish the project I had them wound for. But it looks like they might work for this amp.

What do you think?
Not sure if I already answered you via email on this... in any case, changing from 8k to 5k won't make a big gain difference... the higher Z secondaries will make a bigger difference (I was using 8k:8). I think you'd be fine using 5k:32, that'll be about 4dB more gain than 8k:8.

Pete
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Old 3rd August 2011, 04:58 AM   #60
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Pete,
Thanks for the response. I wasn't in to big of a hurry on it. I have been working on wedding plans for this Saturday. It's going to be a couple of weeks before I get to play with this.
Al
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