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Latent Load on USB/Line-out Decks (and...

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    Posted: 10 Dec 2017 at 8:47am
Latent Load on USB/Line-out Decks (and other complaints)

Dull, or sound lacking in colour, or suppressed highs in-fact, when using a USB/Line-out turntable's direct cartridge output, is quite common.

It will differ between manufacturers I guess, but I discovered this problem using an Audio Technica PL120 turntable a few years ago.

The turntable featured its own preamp and you had the choice of line-out, not requiring a separate phono preamp, or cartridge-out, where you'd need a separate phono preamp.

There was a changeover switch between the two, and when using it in cartridge-out mode with an external phono preamp it didn't sound right. No matter which phono preamp I used, the turntable's line-out sounded better.

I eventually realised what was happening. The changeover switch was switching between the inbuilt preamp's output and the cartridge output...

It wasn't switching its preamp out of circuit, and so the 47k load was still there across the cartridge.

When connected to an external phono preamp the cartridge had two 47k loads in parallel!

If you load down the output of a cartridge with two 47ks in parallel you get 23.5k, and with a moving magnet cartridge that rolls off the highs. This can be proven mathematically but I shall not bore you with the details here.

If the manufacturer had used a 4-pole changeover switch to completely disconnect the built-in phono preamp electronics, this problem would not arise, but AT had used a 2-pole changeover switch, so it could only switch the output.

I would have thought AT would have had the sense to know? Obviously not. I disconnected the built-in phono stage input completely and it sounded quite good for what the turntable is...

Around the same time I saw a 5-star review for it in WHF. Either AT had suddenly changed the switching arrangement or WHF hadn't actually listened to it.

However, since then I've had complaints against my phono stages from people using this and similar turntables. On each occasion where the turntable has been swapped for a normal one, the complaint has been withdrawn, the reasoning understood, and the owner has been very pleased with our phono stage.

But there are a small number of people who just don't get it. Some argue that on using a different phono stage they don't have this problem. All I have to say to them is that physics is physics, and therefore the other phono preamp simply cannot have 47k loading (more like 470k or even 1 meg).

As USB turntables have a preamp first before the digitisation, then the same problem could occur. I am not able to go out and buy all these turntables to test this, so it is up to the customer to find out, but often a reseller, when asked, will simply not know, especially a "box shifter".

But in my opinion anybody buying a preamped or USB turntable isn't bothered about high fidelity - they are just toys! - and the hi-fi press should not abet such manufacturers, and stick to promoting high fidelity sound!

There now follows a list of other complaints:

Line-out turntable into phono preamp complaint: "your phono stage sound is highly distorted and damaging my speakers"

Solution: do not use line out into the input of a phono stage because you have two phono stages one after the other, and with 'phono stage squared' you will get 100 times more bass which will clip like hell!

Phono stage into amplifier disc input complaint: "your phono stage sound is highly distorted and damaging my speakers" (heard that before?)

Solution: Use a line or aux input. The disc input is a phono stage and by driving one phono stage into another you have.... (see above)

Reflex M and Elevator EXP complaint: "distortion and dull sound, improves when Elevator EXP set to 47k"

Solution: The (MC) cartridge first goes to the Elevator EXP, then the Reflex M. Reply: "Oh, I thought it was the other way round. Now it sounds much better"

etc etc etc Cry

That none should be able to buy or sell without a smartphone and the knowledge in how to use apps
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Post Options Post Options   Thanks (0) Thanks(0)   Quote morris_minor Quote  Post ReplyReply Direct Link To This Post Posted: 10 Dec 2017 at 1:09pm
WHF=WTF LOL
Bob

Majestic DAC/pre-amp
Accession MC/Enigma, Accession MM, Reflex M, Elevator EXP, Era Gold V
Solo ULDE, Novo, Lautus USB and digital, Libran balanced, CuSat50
2 x Proprius + Spatia/Spatia Links
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