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Old 20th November 2009, 02:44 AM   #1
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Default 1st Pass at 1st Phono Preamp; Comments?

Hi All,

I've built a couple of projects from scratch, and for 2010 I'd like to try my hand at a phono preamp. Since it's my first go, I'd appreciate it if I could get some comments, critques, "yeah, but"'s, or what have you. Here's my first shot at drawing something up:

Click the image to open in full size.

A couple comments on the design. The gain in the first stage is a little low for what you might expect out of a cascode but in total I figured gain is ~42db, which is about what I'm looking for. And, the 4.7k load resistor lets me try a low-value RIAA network. Gary Pimm has used it in his preamp and liked it, so I figured I'd give it a try here.

I used LTspice to mess with the RIAA components and figure out some DC operating points. There's several 6DJ8/ECC88 models floating around, so I tried more than a couple to get some sense of how the design would wander from tube to tube. FWIW, the RIAA looks to be pretty solid, and the resistor values come right off of Mouser's IRC catalog page.

I tried to keep the bias at >1.7V to stay away from any possibility of drawing grid current and mucking up the RIAA. Consequently, the 6DJ8 are running at around 10mA.

I'm planning on running both channels off of a single, well-regulated B+. I'm hoping the LC (one for each channel) will provide sufficient PSRR/decoupling for the cascode stage, and the mu-follower should have enough PSRR to hold its own. Or am I off-base here?

The final coupling capacitor is a little on the large size, but the next stage has a 10k input impedence. As near as I can tell, the mu-follower will be okay with the 10k impedence.

Any feedback for me?

Regards,
John
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Old 22nd November 2009, 07:38 PM   #2
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Hi All,

I'm assuming the deafening silence equates to a collective "Meh", so here's another take -- hybrid cascode input stage and a GPimm CCS-loaded output stage, using the mu-follower 'out':

Click the image to open in full size.

FWIW, I messed around in LTspice to see what kind of gain I could expect with the 4.7k anode resistor, and it's spitting out 47dB for the preamp in total. Is that believable, or will gain vary considerably depending on the specific examples I acquire? I don't have any experience with the 2SK170 or for that matter much SS in general, so if anyone has any helpful hints/ construction tips, I'm all ears.

Better/worse than my previous schematic? Decent probability of building something worthwhile?

Regards,
John
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Old 22nd November 2009, 08:30 PM   #3
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Dude,

Contact Allen Wright at VSE and run your ideas past him. AW is the "guru" of cascode 1st gain block phono preamps. Also, VSE sells properly matched pairs and quads of low noise, high gfs, JFETs. Matched "sand" is very important in the hybrid cascode case.
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Old 23rd November 2009, 03:22 PM   #4
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Thanks Eli. I had spent a good bit of time going over the schematics Allen has posted, but I suppose not enough has sunk in.

Regards,
John
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Old 23rd November 2009, 03:41 PM   #5
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John,
1/ 4k7 is very low value for a tube anode load R, more likelyto be seen in a ss amp. We use 25k and even that's low compared to most others. This iwhy youhave such low gain. Set the anode v of your lower tube to around 60V, and the anode of the upper tube to around 130V. Do this by changing the cathode R.

2/ The hybrid will give you even more gain, but again 4k7 is WAY too low an anode load R. Try 25k and in this hybrid's case, the grid of the tube should be at around+7V and it's anode around 70V. With a 260V B+, this will allow you a lot of running current, hence good gain and low noise. This two stage design should be able tohandle MCs with no problems.

3/ Your RIAA network is very low impedance - I'd be looking at (at least) 10x higher impedance there. I.E 10x the R values and 1/10 x the cap values.

This will help put the gain up as well.

4/ The 2SK170 doesn't change gain miuch unit to unit, but matched pairs are advised if you want to do it right.

Regards, Allen
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Old 23rd November 2009, 10:10 PM   #6
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Thanks a lot for the feedback Allen; I certainly appreciate it.

I recognize that the low anode resistor is responsible for the low gain out of the first stage in either schematic. If you'll indulge a small thought experiment, setting aside the issue of low gain, what are the repurcussions of a small anode resistor on a cascode? Will distortion be remarkably higher for 4.7k vs. 25k resistor?

Also, using a 2SK170BL in the first stage, what are the implications for overload? It looks like for most reasonable values of source resistors, bias works out to be .2-.3V. Is that going to be sufficient for your typical MM cartridge, with up to 5mV?

Regards,
John
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Old 23rd November 2009, 10:32 PM   #7
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A cascode circuit has extremely high output impedasnce, effectively it's a CCS. So whatever value of resistor it drives makes little/no difference to distortion. And from my measuremets, the distortion is extremely low.

A jfet does not run into gate current until the gate goes +0.7V positive. So with 0.2 to 0.3V neg already - you have NO worries about overload at the input, even from a hot MM cart.

But you shouldn't need a jfet to get enough gain for a MM, with a decent value of anode R you'll be right there all tube.

Regards, Allen
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Old 23rd November 2009, 10:45 PM   #8
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Thanks so much Allen; I really appreciate the help.

Regards,
John
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Old 23rd November 2009, 11:41 PM   #9
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I'll see Allen's load resistor and raise you a CCS. With a high gain tube in the first hole and a CCS plate load, you'll have enough oomph to get a decent signal through the all-in-one-go RIAA network and into the next stage.
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Old 24th November 2009, 01:37 AM   #10
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Thanks SY - a 6GK5 perhaps? I know Eli is a fan... As an aside, I'm looking forward to reading about your phono preamp project once you get the schematic and write-up together.

Regards,
John
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