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2:1 Step Up Device for low output cartridges.

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GrahamD View Drop Down
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    Posted: 17 May 2017 at 1:13am
Many excellent cartridges , both MM and HOMC, have outputs of only around 2 to 2.5 mV.
While this is generally fine and just require a bit more tweaking of the volume control to achieve acceptable listening levels, I find this can sometimes lead to increased noise floor and is simply a bit annoying. Other cartridges ,such as the Ortofon 2M series have outputs of 5mV and only about half volume is required on the volume control for similar listening levels and a totally silent noise floor.
My question for you circuit gurus is, can a simple step up circuit be produced that will give a 2:1 gain to overcome this. Obviously it would have to match both the cartridge and preamp and not introduce any noise or distortion.
Any suggestions ?
 
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Graham Slee View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 17 May 2017 at 7:01am
Like all equipment, phono stages have what's called a signal to noise ratio. All audio circuits produce noise (like white noise) because of the way physics is.

The noise is (relatively) steady state in that it doesn't increase or decrease, and is stated in the specifications as being so many decibels below the signal level. The signal level I use is 0dBU which is 775mV, so for example, in the Accession MM it is 66dB below this level. This is measured on an Audio Precision audio analyser.

This doesn't tell us much, but if we add on the gain of the stage - 41.5dB - we get -107.5dB, and this is the equivalent input noise.

Now we can do comparisons with cartridges, but first we need to find their outputs in dB...

if a cartridge outputs 5mV it is so much lower than the 775mV I mentioned earlier. It is 775/5 lower.

It is 155 times lower which is -43.8dB, How did we arrive at that? We took the log of 155 and multiplied it by 20 - that's how it's done - dead simple.

For a 2mV cartridge it is therefore 775/2 = 387.5. Find the log: 2.58827... and multiply by 20 gives 51.76dB.

So we have one cartridge doing -43.8dB and the lower output one doing 51.76dB, a difference of 8dB to the nearest whole dB.

Let's now take each away from the equivalent input noise...

107.5 - 43.8 = 63.7
107.5 - 51.76 = 55.74

These are decibels and you'd be hard pressed to hear a fraction of a dB because the very definition of a decibel is... well, looking around the web it seems the world has forgotten... a decibel is the smallest difference in sound pressure the human ear can detect.

So we'll round them so that the high output cartridge is 64 dB more than the noise, and the low output cartridge is 56 dB more than the noise. 8dB difference proved.

So, by turning up the volume to compensate we effectively put 8dB more noise into the system. 8dB isn't far off the 10dB we need for something to sound twice as loud, so the noise using the lower output cartridge should sound almost twice as loud as if we were using the high output cartridge.

But you're saying you get a "totally silent noise floor" with the high output cartridge and you find using the lower output cartridge "a bit annoying"?

That doesn't add up to me, and simply by making a 2:1 step up device isn't going to remove noise.

The only thing I can suggest is that additional noise is produced when turning up the volume, either that, or the speakers (or headphones) emphasize the noise?

Can you tell us more about your set-up?

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GrahamD View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote GrahamD Quote  Post ReplyReply Direct Link To This Post Posted: 17 May 2017 at 10:14am
Sorry if you misunderstand me.
The issue is not noise but the fact that I have to turn the volume up so far with the lower output cartridge to achieve the same listening level.
For example the Ortofon 2M Blue usual volume control position is 1/3 to 1/2 volume.
For my Denon DL110 it is closer to 3/4 volume.
It's not really a big deal and sure I can just turn the volume up, but I am asking if it would be possible to DIY a simple 2:1 gain circuit  to allow the Denon to match the Ortofon volume.
I enjoy fiddling around with this sort of thing and have been accused of solving problems that don't exist.
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tg View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote tg Quote  Post ReplyReply Direct Link To This Post Posted: 17 May 2017 at 10:41am

This thread covers the issue of perceived greater surface noise from the DL-110 and offers a method to reduce that that does not involve output boosting.

It might prove of interest to have a read of it.




Edited by tg - 17 May 2017 at 10:44am
Tony G
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Graham Slee View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 17 May 2017 at 11:53am
Originally posted by GrahamD GrahamD wrote:

...and have been accused of solving problems that don't exist.


Well, this one definitely exists...

Easiest way is to attenuate the loudest one. A double pole switch and four resistors (two per channel) between phono stage and amp input would do that.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Drewan77 Quote  Post ReplyReply Direct Link To This Post Posted: 17 May 2017 at 5:15pm
This isn't an answer to GrahamD's question but it gives me a chance to praise the Majestic/Accession for the way they solve the same issue in my system.

My preamp has two analogue inputs - one balanced XLR, the other standard RCA. 

One turntable goes via the Accession variable output directly into the preamp (RCA).

The other via a Reflex M using the Majestic analogue input & then into the preamp (balanced, using the Majestic variable output). The Majestic also handles various other digital devices.

With the volume dials on both Slee devices set by ear, all inputs, digital and analogue remain at the same volume level when switching.

Perfect! Big smile
Older than I once was, younger than I'll be
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GrahamD View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote GrahamD Quote  Post ReplyReply Direct Link To This Post Posted: 17 May 2017 at 9:56pm


Easiest way is to attenuate the loudest one. A double pole switch and four resistors (two per channel) between phono stage and amp input would do that.
[/QUOTE]
 
Ah yes, I've already done that with a DIY L Pad to the CD player input to the amp.
Now I would like to do the opposite ( amplify rather than attenuate) the Denon HOMC output.
If this is going to be more trouble than it's worth I'll forget about it and just turn the volume up.
 
Cheers
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