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Ultrasimple mm/mc RIAA preamp 2
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Old 21st November 2020, 12:41 AM   #411
wrenchone is offline wrenchone  United States
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Your fet feedback capacitance will be multiplied by the gain of the first stage without a cascode - that's a lot of capacitance.
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Old 21st November 2020, 08:48 PM   #412
wrenchone is offline wrenchone  United States
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I looked up the feedback capacitance of the LSK170, and it's about 5 pF. An input stage gain of 40X would make the amplified Miller capacitance dominant, and make it impossible to properly load many MM cartridges.
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Old 22nd November 2020, 01:23 AM   #413
andyr is offline andyr  Australia
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Quote:
Originally Posted by wrenchone View Post
I looked up the feedback capacitance of the LSK170, and it's about 5 pF. An input stage gain of 40X would make the amplified Miller capacitance dominant, and make it impossible to properly load many MM cartridges.
That indeed makes a lot of sense, wrenchone.

I'd appreciate if you could now confirm something that leads on from your statement.

I have included 3 cct attachments:

#1 is the basic 1st stage for a MM phono stage. The 1K 'gain resistor' between the Drains of the 2SK170s and the +ve rail delivers about 40x gain.

So this has a heavy Miller capacitance load on the cartridge.

#2 has added a CCS between the Drains of the 2SK1I70s and the +ve rail. My understanding is that the CCS reduces distortion (compared to #1) - but keeps the same high level of Miller capacitance, right?

#3 has a 2n4401 as a cascode on the 2SK170s. (Have I drawn this correctly?)

So #3 is what needs to be used for the 1st stage of an MM phono stage, to avoid cap-loading the cart - right?

If that is so, can you advise what voltage the Base of the cascode transistor needs to sit at?

(I had assumed about half way between the +ve DC rail and Gnd - hence the value given to the orange-highlighted resistors.)

Thanks,
Andy
Attached Images
File Type: jpg MM 1st Stage - Basic.jpg (261.1 KB, 93 views)
File Type: jpg MM 1st Stage - with CCS.jpg (270.8 KB, 92 views)
File Type: jpg MM 1st Stage - CCS plus Cascode.jpg (283.4 KB, 92 views)
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Old 23rd November 2020, 06:46 PM   #414
wrenchone is offline wrenchone  United States
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#2 and #3 are not drawn properly. In order to ease the requirements for the supplemental current source, there should be a cascode for #2 and the current source should feed the drain of the gain transistor (emitter of the cascode) so that the current source is dumping into a relatively constant voltage. The load resistor should be referred to Vcc, not ground. Thie circuit starts to look like a folded cascode that's been unfolded. This is shown in the redrawn circuit on the left.
The circuit on the right shows a folded cascode with split supplies to accommodate the needs of the 2SK/LSK 170. The gate leakage on these transistors is such that it increases exponentially at a relatively low voltage .Running them at a Vds of 4-5 volts is a compromise between keeping the gate leakage to a relatively low value and having enough Vds to reduce the parasitic capacitance of the device. Splitting the supplies as shown in the circuit on the right allows running the input fet at a relatively low voltage, while allowing enough output signal swing for dynamic range and overdrive tolerance.

Other fets are not as restrictive as the @SK/LSK170 regarding drain voltage vs. gate leakage. A classic example id the PN4393, originally slated for switch/chopper duty, but also can be utilized a a fairly low noise amplifier.
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File Type: gif Casc.GIF (95.7 KB, 67 views)
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Old 23rd November 2020, 11:22 PM   #415
bepone is offline bepone  Croatia
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in progress cascode version with BC550, im developing some pcbs.. need to find some acid tomorrow
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Old 24th November 2020, 04:31 AM   #416
andyr is offline andyr  Australia
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Thanks very much for the re-drawn versions of my #2, wrenchone.

Quote:
Originally Posted by wrenchone View Post
#2 and #3 are not drawn properly.
I'm not surprised #3 is drawn wrongly - that was my uneducated attempt to add a cascode to #2.

#2, however, is a real circuit - which actually sounds very good.

Quote:
Originally Posted by wrenchone View Post
In order to ease the requirements for the supplemental current source, there should be a cascode for #2.
My fundamental reason for wanting to add a cascode was to zero-ise the Miller capacitance of this gain stage (otherwise, as you pointed out above, it makes it hard to correctly cap-load a MM cart if it is the 1st gain stage in a phono stage).

But it's nice to hear that it helps the CCS, too.

Quote:
Originally Posted by wrenchone View Post
and the current source should feed the drain of the gain transistor (emitter of the cascode) so that the current source is dumping into a relatively constant voltage. The load resistor should be referred to Vcc, not ground. The circuit starts to look like a folded cascode that's been unfolded. This is shown in the redrawn circuit on the left.
In your redrawn circuit on the left:

Q1. Q1 is the CCS transistor and Q2 is the cascode transister - right?

Q2. When you say "The load resistor should be referred to Vcc, not ground. " - I presume the "load resistor" is R5?

Thanks,
Andy
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Old Yesterday, 02:01 AM   #417
wrenchone is offline wrenchone  United States
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R4 is the load resistor for the circuit on the left, and R8 for the right.
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Old Yesterday, 02:32 AM   #418
andyr is offline andyr  Australia
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Quote:
Originally Posted by wrenchone View Post
R4 is the load resistor for the circuit on the left
Aah, thanks very much!

fyc, I've attached the drawing of your 2x cct alternatives:

Q1: so if I set R4 to 1K ... should I get a gain similar to what I did in my cct #1 (also attached fyc - ie. about 40x).

Q2: and R5 & R6 should be set so that the Base of Q2 is at about the 15v point? (Say, 3K3 each?)

Regards,
Andy
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