phono load resistors

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Gerry
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#16

Post by Gerry »

Graeme

Not sure if you've found the Hagerman site which has a calculator which provides the parallel value dependent on the phono stage and turns ratio.
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#17

Post by Neal »

Many moons ago I tried primary loading...didn't like it, I likened it to a loudness switch, others also thought it sounded harsh but some liked it.

i was wondering who is still using it? Prompted by a thread on AoS I've gone back and tried it again but this time I've also looked at the secondary and snubbed the SUT ringing with a Zobel...it works, well it seems to so far.

The sound is better balanced than I recall and it has greater 'weight' and detail than the secondary loading I've been using. Technically it seems the correct way to load a SUT but without a signal generator and 'scope to get the values for the Zobel you will be in the dark. Also changing the cartridge or load or SUT mean you need to recalculate the Zobel (ideally)...
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#18

Post by Neal »

Oh, yeah, before and after 'scope shots:

Image

Image

Roll off is above 100KHz but ringing has gone, no brightness or hardness.
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#19

Post by Andrew »

Excellent stuff Neal, proper science :) I have always meant to do this. What values did you use for the zobel? What was the cart!? And what did you use for the test signal?
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#20

Post by Neal »

The Zobel ended up at 1K1 res and 1n0 cap. -3db out at 145Khz.

It was a bit of compromise for the two carts I use, a Kontra B with 5Ohms o/p and a DL103R at 14Ohms. I ended up with those values that seemed to suit both carts. The SUT are Lundahl 9206 and I loaded the primary with 100R and the secondary with 1Meg, so the reflected load was about 90R plus 10R from the primary winding of the SUT.

Test setup was as per Morgan Jones, use the 50 Ohm output of a sig gen into a potential divider where the lower resistor is approx. the same as the cartridge R output and the upper R 100x that value.

Connect the PD to the Phono and feed a 1Khz Sq Wave in for a 100mV output from the SUT and measure. I found having the phono switched on (thinking about millar capacitance) made virtually no different to having it switched off during testing so it remained off.

I used a variable 5K pot and just substituted small value caps to get a good response then measured the pot out of cct and selected the nearest fixed resistor. The resistor damped the peak and the cap controlled the roundness of the SW.

Using a sine wave sweep for the Kontra B with low o/p res (5R) -3db was 66Khz and for the Denon 103R (14R) -3db was 47Khz.

Not a bad result and listening so far has been good...I also have a standard 103 so will try to measure with a 40R o/p impedance and a 330R load but the Zobel will have to be tweaked to accommodate it.
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#21

Post by Greg »

Oh, I would so like to be able to get my head around this but I simply fail. Neal, it would be good for us at Owston if we could have a talk so you can instruct me accordingly. I hope so.
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#22

Post by Neal »

Sure Greg, no problem.

Background to it is with secondary loading you are really loading the SUT and changing its behavior rather than loading the cart (you still are loading the cart but the effect is greater for the SUT). For instance when loaded with 4k7 on the secondary, the SUT is well damped with no ringing... I used this setting with my the Kontra B...the SUT is damped and the cart sees 47R+10R (primary R) as its load....but the gain is greatly reduced, meaning the SN ratio goes up.

Load it with 10K (100R+10R reflected) gain increases but ringing starts to appear. Load it with 33K for the DL103 to see 330R+10R and the ringing has increased substantially, it can be heard as a hardness and increased brightness in the presentation.

So by loading the primary the idea is you set the cart load sort of independently of the SUT loading on the secondary (sort of as they do still interact). Theres a good write up on the Rothwell site but it stops short of actually giving details on how to do the above:

http://www.rothwellaudioproducts.co.uk/ ... xplai.html

At the moment I'm using 100R on the primary for both Kontra B and DL103R and the Zobel set as my last post to banish the ringing. Will try and measure for the standard 103 later...
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#23

Post by Neal »

A 340pF cap (220+120) and 1K1 res works OK for the DL103, so this means its possible to make the loading and Zobel switchable for low and high o/p impedance carts...
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#24

Post by Andrew »

Cool Neal, what happens if you ater the frequency of the square wave? I realize yo'l have 1kHz plus all the major harmonics in the square wave but I'd be intersted what pahens at 10kHz. Sometime when I have completed the DHT Heaters I shall have to try this and get a spectrum of that ringing.

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#25

Post by Neal »

It does surprisingly well Andrew:

Image

It's getting out of shape but still remarkably intact...
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#26

Post by Andrew »

That's not too bad at all is it? I'd put the sloping edges down to the very high order harmonics in the 10kHz waveform being attenuated by the low pass filter of the step up, wouldn't you?

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#27

Post by Andrew »

Neal wrote:A 340pF cap (220+120) and 1K1 res works OK for the DL103, so this means its possible to make the loading and Zobel switchable for low and high o/p impedance carts...
Hi Neal,

Is this a 103 or a 103R?

thanks,

Andrew
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#28

Post by Neal »

Andrew wrote:
Neal wrote:A 340pF cap (220+120) and 1K1 res works OK for the DL103, so this means its possible to make the loading and Zobel switchable for low and high o/p impedance carts...
Hi Neal,

Is this a 103 or a 103R?

thanks,

Andrew
Hi Andrew, thats for the 103 on my SUT, for the 103R a 1n0 + 1K1 res works well. Any other SUT will need different values.

The SUT only has so much bandwidth so there will be a roll off at some point, I'm surprised the 10Khz passed as well as it did.
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Paul Barker
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#29

Post by Paul Barker »

Sorry to pick up fagends Neal but do you have a SW test record? or is it worked out with generator of same source resistance as cart?

Edit: Ignore this I found the answer.
Last edited by Paul Barker on Fri Mar 29, 2013 9:17 am, edited 1 time in total.
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#30

Post by Paul Barker »

Thanks Neal I found the method.
Neal wrote: Test setup was as per Morgan Jones, use the 50 Ohm output of a sig gen into a potential divider where the lower resistor is approx. the same as the cartridge R output and the upper R 100x that value.

Connect the PD to the Phono and feed a 1Khz Sq Wave in for a 100mV output from the SUT and measure. I found having the phono switched on (thinking about millar capacitance) made virtually no different to having it switched off during testing so it remained off.

I used a variable 5K pot and just substituted small value caps to get a good response then measured the pot out of cct and selected the nearest fixed resistor. The resistor damped the peak and the cap controlled the roundness of the SW.

Using a sine wave sweep for the Kontra B with low o/p res (5R) -3db was 66Khz and for the Denon 103R (14R) -3db was 47Khz.

Not a bad result and listening so far has been good...I also have a standard 103 so will try to measure with a 40R o/p impedance and a 330R load but the Zobel will have to be tweaked to accommodate it.
Maybe there is a SW record out there somewhere?
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