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PostPosted: Sat Jul 01, 2017 3:53 pm 
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Joined: Sat Jul 01, 2017 3:33 pm
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Location: Englewood, CO, US
I'm trying out a Lehmann Audio Decade phono preamp and comparing it to my internal Spectral DMC-20 internal phono stage. I'm trying to optimize capacitance setting for the best comparison, but unsure how to do so. I have an older Transfiguration Phoenix cartridge, internal impedance 4 Ohms, recommended Load impedance > 4 Ohms, output Voltage 0.4 mV.
Options for the Decade include 1 nF, 220 pF, 100 pF capacitance (not sure what default is) and 100 Ohm, 1 kOhm, and 47 kOhm resistance. I started with 100 Ohm resistance and tried each of the capacitance values. There is a significant sound difference with each setting, different aspects better/worse with each one. What I'm looking for is the most accurate, natural reproduction of the sound. I'm presuming there's an optimal time constant, but I'm not sure what that is and how I can best achieve it. So far the 220 pF and 1 nF are sounding best, but I haven't done a prolonged evaluation. There's also the option of changing the impedance to any desired value by placing an internal resistor (which I may do if I purchase the unit, but not now).
I'm looking for the best place to start to get a rapid and accurate evaluation of the unit, as I have it on loan for about 4 days.


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PostPosted: Sat Jul 01, 2017 4:40 pm 
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Joined: Sat Jun 23, 2012 9:12 am
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Location: Windsor, CO, US
First off, moving coil cartridges do not "see" capacitance, so it doesn't matter. Only moving magnet carts need this setting. It "should" have no sonic effect one way or the other.

That being said, MC carts do need the resistance set correctly. On your cart, I would recommend, since you don't have a lot of choices, the 100ohm setting. Your cart will do better with a setting of between 5 and 50 ohms, but if you don't have it, get as close as you can.

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PostPosted: Sun Jul 02, 2017 10:30 am 
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Joined: Sat Jul 01, 2017 3:33 pm
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Location: Englewood, CO, US
After several hours of listening to the Decade, my impression is that it is more forward and has more intrinsic character than my Spectral DMC 20 phono stage. It's maybe a little more "engaging" initially, but less "natural". I can hear the electronics more, and I'm not sure if it's the electronics of the recording or the electronics of the unit itself. I'm not sure if going lower in impedance would bring out a more natural sound, or if this is just intrinsic to the unit.


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PostPosted: Sun Jul 02, 2017 8:49 pm 
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Joined: Sat Jul 01, 2017 3:33 pm
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Location: Englewood, CO, US
I had previously increased the VTF of my cartridge (2.13) to improve transients, but I think this may have been due more to the electronics than the cartridge. Further listening allowed me to back down on the VTF of my cartridge (2.008, recommended 2.0), maintain the transients, and open up the soundstage. The heavy electronic sound I was hearing previously is essentially gone) I still have the Impedance set to 100 Ohms, and the capacitance now set to 220 pF. I'm thinking a lower resistor may help even more, but this is sounding really nice!


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PostPosted: Mon Jul 03, 2017 6:12 am 
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Joined: Sat Jun 23, 2012 9:12 am
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Location: Windsor, CO, US
Glad you are getting close. If you do want to play with loading, I would try whatever resistor gets you to around 40 ohms. This will smooth out the cart and open it up a bit. It will not, however, change the overall intrinsic sound of the phono stage.

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PostPosted: Mon Jul 03, 2017 2:06 pm 
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Joined: Fri Feb 12, 2016 12:15 pm
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Location: Marietta, GA, US
One easy way to get quickly changeable phono loading, is to get a pair of RCA Y-adapters (two female, one male), some RCA plugs, and various value resistors (1/2 watt is usually a good choice). Plug in the Y-adapters inline with the turntable cables, solder in resistors across the RCA plugs (from "hot" to ground), and plug in the new RCA plug/resistor units, so that with the new resistor in parallel with the 100 ohms in the existing preamp loading, you get the desired overall impedance.

For example- with 100 ohms in the unit, if you soldered a 100 ohm resistor across the added RCA plug and put that in as described above, you'd get a composite of 50 ohms. A 68 ohm resistor would give very close to a total of 40 ohms, and a 33 ohm resistor would get very close to 25 ohms.

There used to be a kit that did this- it had the Y-adapters, and various RCA plugs with the resistors pre-wired, in various values. However, I've not see one available in years. Too bad, as it was a pretty darn clever idea, IMHO.

Regards,
Gordon.


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PostPosted: Mon Jul 17, 2017 7:01 am 
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Joined: Thu Jun 18, 2015 7:41 am
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Location: Evanston, IL, US
db Audio is the company that sold kits with y-adapters and resistors and capacitors for experimenting with moving coils and moving magnet cartridges. I don't know if the company is still in business but you can sometimes find one of these kits online: I got a capacitor kit on eBay a couple years ago.


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PostPosted: Tue Jul 18, 2017 4:42 pm 
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Joined: Sat Jun 23, 2012 9:12 am
Posts: 141
Location: Windsor, CO, US
Here are those db systems loading kits:

http://store.acousticsounds.com/d/60044 ... ccessories

http://store.acousticsounds.com/d/60043 ... ccessories

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