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Post by freddypipsqueek on May 16, 2019 21:44:22 GMT
A quick question - I think I know the answer but I wanted to be sure.
I now use a DAC with a digital volume control. It is a permanent feature - turn it all the way up and you get what would have been a 2v fixed output - Its a Benchmark DAC2 D.
I have one amp which I intend to use with it permanently - A Ben Duncan HP02 (battery powered) which works lovely with K701s.
If I wire out the volume control on the amp is it simply a case of finding the highest level I intend listening too which I find by turning the DAC to maximum and then set the amps volume control - and then measuring the resistance created by the pot and buying two resisters to match it and replacing the pot with them.
I am thinking that doing this will both avoid having two volume controls and also improve the sound by wiring out the alps pot with two decent resisters.
Do I need resisters with a high watt handling ?
Many Thanks.
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pingu
Been here a while!
Reallising what has been taken on with the GB
Posts: 332
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Post by pingu on May 17, 2019 0:48:06 GMT
Hi Freddy,
Sounds good to me. The volume pot is never high wattage so 1w will do, it’s my understanding the pot is on the input before the amp so only has low wattage signal. A couple of quality resists will do the job fine. If I’m wrong some will no doubt correct me
All the best Martin
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Post by freddypipsqueek on May 17, 2019 15:22:05 GMT
Thanks Martin. Hopefully Mike will add a word and then I'll order the bits etc.
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Post by PinkFloyd on May 19, 2019 19:07:55 GMT
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Post by freddypipsqueek on May 20, 2019 14:44:52 GMT
Mike. Thanks for your input. No need to apologize. If the right resister is around the £10 mark I try the Charcrofts. There isn't much room - it's a mini pot (i think the same as the XCans) but there a cm or so either side. I'll open her up and order the bits.
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Post by freddypipsqueek on May 20, 2019 16:40:59 GMT
I know this is a stupid question but I only need a resister between the input (Pin 1) and output (pin 2) ??. I assume I can forget about any impedance the other side of the wiper to earth. I have opened her up and it is a 10k pot marked Mexico 9731x and then a long serial number. The max I need is about 2 o'clock on the dial which needs a 4k3 resistor - which is £9.20 + vat etc which is quite do-able. Ta A.
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Post by PinkFloyd on May 20, 2019 19:52:58 GMT
I know this is a stupid question but I only need a resister between the input (Pin 1) and output (pin 2) ??. I assume I can forget about any impedance the other side of the wiper to earth. I have opened her up and it is a 10k pot marked Mexico 9731x and then a long serial number. The max I need is about 2 o'clock on the dial which needs a 4k3 resistor - which is £9.20 + vat etc which is quite do-able. Ta A. Correct.
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Post by freddypipsqueek on May 22, 2019 20:49:38 GMT
All done. Bit messy due to the 5 pin XLR input for the amp breaking at the same time !!. I have to say the Charcrofts sound a bit thin with the AKG but switching to HD600 and its better (but still lacking a bit of bass) so I am guessing they need to break in. I'll report back in a couple of days - maybe a week. A.
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Post by freddypipsqueek on May 30, 2019 14:01:41 GMT
Well I think I’ve made a complete hash of this.
I found that having replaced the pot with a resister(s) there was distortion and a lack of bass after about 3 to 4 minutes. As the unit is battery driven I assumed it would be the batteries and that it was being overloaded as I had picked too high an output. Higher value resisters made no difference however and in fact made it worse. I also removed and recharged the batteries separately and found they were fine. I have tried a few options (including putting the volume pot back) but it’s now distorting quite badly whatever I do – its as though I have created a short to earth (somewhere)and the power in the amp cannot keep up with demand. Oddly for a few seconds the playback is lovely but then . . .
I am also a bit embarrassed to say I have also managed to lift some of the pads from where the pots sit such that it’s all becoming a bit of a mess . . . .
I have sent an email to Ben Duncan who designed the amp asking for some comments and a copy of the schematic but I think it’s been knackered beyond repair.
The more I think about it, the more I think the pot does more than act as a variable resister between the input and output. It also creates a resistance to earth but placing a resister in the path (to match that) results in no volume.
Overall I have no idea where I may have gone wrong. I thought it would be one of the simplest bits of modification I’ve ever done. It probably a timely reminder that it’s all beyond me !!!
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Post by freddypipsqueek on May 30, 2019 14:39:15 GMT
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Post by freddypipsqueek on May 30, 2019 18:41:15 GMT
I've discounted the XLR connections by using an unbalanced set as directed by Ben Duncan (a few years ago he gave me instructions).
I am tempted to wire up the +12v power rail to a linear supply ??.
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Post by freddypipsqueek on Jun 1, 2019 14:32:16 GMT
Further update.
I do not believe the batteries are at fault. They go through a switch (which switches between the charger and output) and are 13.2v when the amp is off (obviously the charger is not plugged in) and drop top 11.53 when the amp is on - the amp needs a minimum current of 12v so that may explain the lack of sound.
Having said this I now suspect I have blown both of the SSM2017 (pre) op amps. When I disconnected the volume control I initially soldered the resisters between input and earth - I mistakenly assumed the pin would have been correctly used. I assumed I had done no damage as the amp fired up fine. Whilst this would not explain the sound being initially OK and then failing the SSM2017 are known to fail and were withdrawn from the market as a consequence. As 3 off THAT1510's (the correct pin replacement) are only £10 I have bought them to try. I have checked everything else and am left with the opamps being the problem - the SSM2017 are the (only) ones the input side of the volume pot.
Further(perhaps boring)updates to follow.
P.S. I will be bidding for an X-Can soon on the basis that it would make an interesting replacement. I am however very loathed indeed to bin this amp - as I would be any.
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Post by freddypipsqueek on Jun 6, 2019 8:28:59 GMT
No luck changing the SSM2017s for THAT1510s so back to the drawing board. I had discounted the battery but there seems no harm trying a 12v power supply.
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Post by freddypipsqueek on Jun 6, 2019 15:42:14 GMT
Given up - Whilst messing about with it its damaged the K701 drivers - I had been using a duff pair of headphones - but forgot - after fitting the THAT1510s. The left driver is all be but gone and both lack bass. As this amp was a match for the K701s there is little point continuing with it. A.
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