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Phono Preamp Pt2: MC

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Post Options Post Options   Thanks (0) Thanks(0)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 22 Feb 2017 at 7:29am
The problem with the aforementioned designs is they easily pick-up interference - they do not have the immunity currently demanded by legislation.

It was noticed that on the introduction of RoHS compatible components that the incidence of interference increased. This might have been coincidental with an increased proliferation in radio frequency devices, and there are indications of relaxation of rado frequency legislation about that time.

However, I am convinced that the "removal of (alleged) harmful substances" (RoHS) had some unknown effect, because the sound quality also changed, leading me to rework virtually everything I had designed prior to that date. It did happen and I have the "scars" to prove it! The sound quality became quite dry and brittle, very much like tin-only soldered joints look and feel! And so I do believe the materials used have an influence on the sound, but I am at a loss to describe why.

But back on track, those circuits neither perform well interference wise or sonically today.

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Post Options Post Options   Thanks (0) Thanks(0)   Quote DogBox Quote  Post ReplyReply Direct Link To This Post Posted: 13 May 2019 at 12:35pm
Feb. 2017... Been a while in this lonely place to have had no entries. I could quite possibly lodge here and no-one would ever know! But I 'am' tempted to get a circuit board from ESP for Proj. 13; use the parts Graham described; make sure all my leads are as best as I can make them and just 'see if' that set-up will be passionate about driving a MC cartridge for myself? What could possibly go wrong...? 
I'm sure the Genera will do its bit... Approve 

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Post Options Post Options   Thanks (0) Thanks(0)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 13 May 2019 at 4:45pm
I wouldn't.

I'd take a look at this: http://sound.whsites.net/p25_fig1.gif


And is from Doug Self's book Small Signal Audio Design.

And I'd ignore everything to the right of R7.

I'd then flip it horizontally to use NPN transistors and a positive supply ensuring that all polarities are correct (18V from the Genera op amp supply).

There's too much gain so I'd make R3 5k6 and R2 1k and maybe adjust R2 such that there's about 5V collector voltage.

Then place a 470R resistor in series with C1, and make R1 a few k (3k3 possibly).

The ac output is from the collectors and this needs to be something like 10u (pos end to collectors obviously) followed by a 10k resistor to ground.

Sketch it, scan it, upload it to here and I'll check it.

Should be able to build it on a tag-strip.

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Post Options Post Options   Thanks (0) Thanks(0)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 14 May 2019 at 8:50pm
So many bad things have been written about a single transistor MC head amp that it must sound ludicrous me suggesting it.

A single transistor stage distorts badly, but only where there is a "useful" signal swing. If we just want a few millivolts from a few microvolts the collector isn't going to move out of linearity by much if any.

By making the collector resistor voltage large, say 2:1 over collector emitter voltage, and making the collector resistor say 1/10th the following MM stage's load impedance, it's going to be reasonably linear.

Then we can use negative feedback collector to base (as part of base bias), and Rin (virtual earth input resistor) suitable for a high(ish) MC cartridge load, and by doing so reduce distortion further.

The transistor voltage needs to be about 5 times the largest incoming peak signal voltage, and here we need look at frequencies up to say 100kHz where ultrasonic noise lives, this being due to wear of the vinyl.

If the cartridge outputs 500uV at 1kHz it will output 707uV peak and at 100kHz 70.7mV. With a gain of 10 that is 707mV, and so 3.5 volts collector to base looks good.

Now let's add some to make up for saturation voltage. 1.5V will do, so collector voltage needs to be around 5V.

The collector resistor can drop 10V making the supply 15V. We have 18V so we can lose 3V in a supply decoupling filter.

So we don't compromise noise of the RIAA stage we ought to keep the collector resistor voltage noise low, and a 2.2k resistor will contribute 8.8nV per square root of frequency, which is better than the average FET phono stage input.

Collector current is therefore 4.5mA, or if we use three paralleled transistors for low noise, 1.5mA each.

Paralleled transistors reduce base spreading resistance reducing noise by perhaps 3dB. As we don't need loads of beta a power transistor could do the same.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote DogBox Quote  Post ReplyReply Direct Link To This Post Posted: 15 May 2019 at 3:27am
Ok, "You Wouldn't"... I'm not one to rebuke your advice as I do gratefully accept it. However, isn't all we need just a "boost" in signal voltage for the Genera to take over - which it does so well? 
Keeping our basics of using 18V which the Genera gets, can't we just get the Low MC Cartridge output signal range increased and let the Genera do the rest? If just a Power transistor will be better for our needs instead of using Fets, surely there is a NPN model to increase our signal without adding  too much noise in the process? 
Without borrowing from Mr Elliot who borrowed from Mr Self, Why can't we just use proven design layout resulting in a "Step-Up" arrangement in front of the Genera? 
 
Just borrowing the "idea" from a well publicized and free schematic. And it uses a Fet that we don't want as they are "noisier", yes? 
Weren't you well on the way when you just couldn't get that extra 2dB level - 22Hz - 22kHz? 
Graham, You (and by effect, we) are blessed with having "Golden Ears"... However, if you had my hearing it would probably drive you nuts! Plus, many who would love to make something like this probably haven't got Golden Ears either! Nor is their Hi-Fi to the standard that you are able to enjoy. Maybe to a lot of one's, that 2dB won't be very noticeable - even IF you know its there. 
I still think I have a lot of Mr Elliots writings printed out for reading [that I haven't got round to..] but I really like the way YOU do things. You're a Good Man, Charlie Brown! 

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Post Options Post Options   Thanks (0) Thanks(0)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 15 May 2019 at 8:04am
Eh?

"Why can't we just use proven design layout resulting in a "Step-Up" arrangement in front of the Genera?"

I just did!

"Graham, You (and by effect, we) are blessed with having "Golden Ears"... However, if you had my hearing it would probably drive you nuts!"

So why are you bothering with moving coil?

"Weren't you well on the way when you just couldn't get that extra 2dB level - 22Hz - 22kHz?"

Rhetorical question maybe?

Steve, my previous post was to illustrate something very easy to build (on a bit of tag-strip) which costs pence, and I was simply going through the mental process of what goes into the design.

As for proven, I am not going to say where it's used, but it's the biggest gift you'll ever get on the internet, or anywhere else for that matter.

But I'm sensing that whatever I offer, those reading see others as being of superior intelligence, obtaining superior results, so it's probably best you talk with them.
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Post Options Post Options   Thanks (2) Thanks(2)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 15 May 2019 at 8:20pm
Moving Coil Step-Up Using One Transistor
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