Flat Phono Preamp Based on John's P10 and 2SK389

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mediatechnology
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Re: Flat Phono Preamp Based on John's P10 and 2SK389

Post by mediatechnology »

mediatechnology wrote:
I found (by accident) that simply steering the warp from Side to Mid made a big difference in listenability.

You gonna explain to us mere mortals what dis magic trick is? :o
OK, made me look.
I stumbled into the magic trick here: viewtopic.php?f=6&t=622&start=18

A simpler example is the LR+/-S Width Control with a LPF in Side: viewtopic.php?f=7&t=731
In the second post is a simple passive EE filter.
The same process that steers Bass-to-mono as a mastering EE also steers warp to mono on playback.
The warp information is still there but, particularly on 'phones, having warp in side is uneasy-sounding.
Warp steered to mono seems to benefit from masking.

In the MS domain it is possible to HPF Side with high-order and a relatively high cutoff to aggressively filter warp without being too intrusive.
Eric, one of our members, suggested this approach and may be using it. viewtopic.php?f=6&t=112&p=5684
I haven't tried it but I need to.
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mediatechnology
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Re: Flat Phono Preamp Based on John's P10 and 2SK389

Post by mediatechnology »

I've been trying the Flat front end with various input terminations and have been grappling with AC leakage current from the turntable.
Off or on, as soon as the turntable is plugged in, the noise floor rises 10-20 dB.
Line cord polarity does make a small difference but what was odd is that the SP-15 uses a 20 kHz switching power supply to lower leakage current.

After downloading the operation manual I discovered a "second" ground connection in addition to the tonearm ground lead.
I don't recall having looked at the underside of this turntable in 20 years if at all.
I had completely forgotten about it if I had seen it.

Image
Technics SP-15 Turntable Frame Ground

Grounding problem solved.

Balanced Wiring

I want to try balance shielded twisted pair wiring.
The tonearm is wired "5 wire" having twisted pair for the cartridge leads with an overall shield.
It's fully balanced up to the male DIN plug.
The harness is unbalanced RCA.

I've ordered a 240 degree 5 Pin female DIN plug to build and test a fully-balanced wiring configuration.

Some of the female DINs and harnesses are esoterica and are quite expensive: http://www.ebay.com/itm/JELCO-XLR-TONEA ... 2a3989abcf
I found a relatively inexpensive Lumberg from Allied: https://www.alliedelec.com/lumberg-kv-50-6/70151666/
My hope is to remove the Lumberg's pin block from the body and use it.

With the newly discovered frame ground the "unbalanced" connection, which is really balanced up to the RCA harness and balanced at the preamp input, has hum that's now in the noise floor.
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JR.
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Re: Flat Phono Preamp Based on John's P10 and 2SK389

Post by JR. »

Nice... I vaguely recall some carts that had a metal grounding tab bonding one channel ground to the tone arm shell.

Quiet is good...

JR
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ricardo
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Re: Flat Phono Preamp Based on John's P10 and 2SK389

Post by ricardo »

mediatechnology wrote:In the MS domain it is possible to HPF Side with high-order and a relatively high cutoff to aggressively filter warp without being too intrusive.
Eric, one of our members, suggested this approach and may be using it. viewtopic.php?f=6&t=112&p=5684
I haven't tried it but I need to.
I've always done this in the stereo versions of my Powered Integrated Super Sub technology.

Most warps are up/down so this has many beneficial effects. Note that measured xtalk is degraded but not the audible stereo image on speakers. It looks like its very beneficial on headphones too but I haven't tested this personally. The stuff I did was always for speakers.
I've ordered a 240 degree 5 Pin female DIN plug to build and test a fully-balanced wiring configuration.

Some of the female DINs and harnesses are esoterica and are quite expensive: http://www.ebay.com/itm/JELCO-XLR-TONEA ... 2a3989abcf
I found a relatively inexpensive Lumberg from Allied: https://www.alliedelec.com/lumberg-kv-50-6/70151666/
My hope is to remove the Lumberg's pin block from the body and use it.
I used to have a stash of 240 degree DINs cos they were used to connect to various bits of old B&K kit ... but these might be even more Unobtainium.

Might be better replacing the 240 insert in your pickup arm with a 180 insert so you can use easily available DINs.
Nice... I vaguely recall some carts that had a metal grounding tab bonding one channel ground to the tone arm shell.
I think all the SHUREs from M44-7 onwards had this.
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mediatechnology
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Re: Flat Phono Preamp Based on John's P10 and 2SK389

Post by mediatechnology »

A Fully-Balanced DIN Tonearm harness for the Balanced-Input Flat/RIAA Phono Preamplifier.

I received the Lumberg KV50/6 240 degree 5 pin DIN a day early from Allied Electronics and built a fully-balancedtone arm harness.

Allied has the Lumberg KV50/6 for about $9: http://www.alliedelec.com/lumberg-kv-50-6/70151666/
All Electronics currently have them surplus for around $3: http://www.allelectronics.com/make-a-st ... ing/1.html
Based on the prices of DIN females specifically made for tonearms the above seem very reasonable.

The connector pin block has to be modified slightly to be a DIN tonearm connector.
It takes a couple of minutes at most.

Image
Lumberg KV50/6 5 Pin 240 Degree Female Locking DIN Connector

The pin block has to be removed from the outer metal shell which will be discarded.
Tabs extending from the back of the pin block interlock into an insulating plastic shell that is also not used.
The tabs interfere with insertion of the connector body into the tone arm.
Filing them off to make the end of the pin block round allows clearance.
Apparently this is what makes DIN tone arm connectors "special."

Image
Lumberg KV50/6 5 Pin 240 Degree Female Locking DIN Connector Modified Pin Block

Once the pin block has had the plastic tabs filed down with the body completely circular it can be used to make balanced phono cables:

Image
Lumberg KV50/6 5 Pin 240 Degree Female Locking DIN Connector Modified Pin Block With Leads

The assembly slips into the Audio Dynamics tonearm.

I realize that the Brand-Rex cable isn't made of unobtainium: I could have paid $229 if I wanted unobtainium.

This is a comparison of unbalanced coaxial RCA cables to fully-balanced shielded twisted pair cables using a Stanton 681 dummy cart mounted in a tonearm:

Unbalanced RCA connections with a grounded differential input:

Image
Balanced Flat Phono Preamp with RIAA EQ Applied, Noise Floor, Unbalanced coaxial RCA Connections, Grounded Differential Input, Stanton 681

Balanced shielded twisted pair connections with a fully differential input:

Image
Balanced Flat Phono Preamp with RIAA EQ Applied, Noise Floor, Balanced Shielded Twisted Pair Connections, Fully Differential Input, Stanton 681

The difference in "hash" between 1-10 kHz is significant.
The 60 Hz hum is about 6 dB less.
The performance with the grounded differential instrumentation amp input is good, but given the modest investment to make a balanced phono preamp cable I'd say the improvement from a fully-balanced connection is worth it.

Tomorrow I'll try the fully-balanced harness on the actual turntable.
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Re: Flat Phono Preamp Based on John's P10 and 2SK389

Post by JR. »

Besides the hum spikes, that noise floor looks different in character. I would be curious subjectively about what the hiss (not hum) difference sounds like? Surely well below any surface noise so probably moot.

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mediatechnology
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Re: Flat Phono Preamp Based on John's P10 and 2SK389

Post by mediatechnology »

I used my balanced tonearm as a hum sniffer.
I haven't found the exact cause but I have found the area of the room where the hum spike occurs.
I can rotate the arm within the field and get deep nulls.
Unfortunately both channels do not null at the same time.
I think I have a saturated linear wall-wart powering something or a UPS with saturated magnetics directly below me creating a field with a strong odd-order.
Besides the hum spikes, that noise floor looks different in character. I would be curious subjectively about what the hiss (not hum) difference sounds like? Surely well below any surface noise so probably moot.
The area from 2-10 kHz seems to be high-order line harmonics from LED lighting or PC SMPS.
There also seems to be less RF so perhaps some of the difference may be RF rectification folded down into the audio band.
Or, it could be "all of the above."

With the unbalanced RCA coaxial connection I can see the midrange noise rise when I power up the LED overheads.
The workbench LEDs aren't a problem but if I probe the base of them with the "hum sniffer" I definitely see a rise in the midrange band.
(They happen to be the same brand as the overheads.)

The balanced connection seems to reject the very high-order hash well.
It's hard to A-B but the fully balanced noise floor sounds smoother.
Less buzz and more B-flat hum.
It may well be below record surface noise...

I did discover that my balanced link between remote preamp and EQ board were actually terminated SE.
Though at the moment they're less than a foot apart changing it also lowered the high-order hum spikes.
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Re: Flat Phono Preamp Based on John's P10 and 2SK389

Post by mediatechnology »

This is the balanced noise floor with an AT-96E mounted on the SP-15.

Image
Balanced Flat Phono Preamp with RIAA EQ Applied, Noise Floor, Balanced Shielded Twisted Pair Connections, Fully Differential Input, Audio Technica AT-96E

The RIAA Output gain was calibrated to +4 dBu with 3.54 cm/s rms with left/right-only modulation.
With both channels modulated the equivalent lateral modulation is 5 cm/s rms.
The AT-96 output is rated at 4.2 mV at 5 cm/s lateral.
The gain at 1 kHz is approximately 49 dB.
0 dBu in the FFT is roughly +8 dBu.

FWIW the surface noise on the CBS STR-100's extended lead-out is approximately 20 dB higher.
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Re: Flat Phono Preamp Based on John's P10 and 2SK389

Post by Gold »

Looking good Wayne. This is something I've wanted for a long time. Seeing the measurements is enlightening. Pretty much confirms what I've suspected for a while. Reducing the hash that creeps into every setup will do far more to improve sound quality than another zero or two in the THD or noise floor specs.
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Re: Flat Phono Preamp Based on John's P10 and 2SK389

Post by JR. »

While better is always better, if it's 20 dB below the surface noise any incremental improvements will probably be subtle.

JR
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